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Organic materials for linear and nonlinear optical microdevices

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Organic materials for linear and nonlinear optical microdevices

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Organic Materials for Linear and Nonlinear Optical Microdevices

By

Jinghan He

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
December 2022

Copyright 2022 Jinghan He
ii

Acknowledgements
The journey of PhD is never a straightforward pathway. It is in the middle of nowhere
where failures happen constantly. It could be desperate and painful because there is no guide, no
light traffic, and no scenery in the beginning. Nonetheless, the fascination for many people to join
this journey is that one can grow stronger enough to design and paint a novel and unique scenery.
The accumulation of the new research will eventually lead to a better human life. In my past five
years, I came from East Asia to North America for the first time in my life, and a lot of things
happened around me. This PhD journey is indeed a turning point in my life as I gradually became
more and more mature, and less anxious about handling problems. I am fortunate to get supports
from everyone around me in my PhD study. My journey could not be more meaningful without all
supports from you. The historic Covid-19 pandemic happened during my last two years and made
things even more difficult for everyone around the world. My PhD moments will be the invaluable
treasure to support me in the future.
First, I would like to express my gratitude to my PhD advisor Prof. Andrea M. Armani for
her support in my entire PhD research. I still remember when I first contacted her in March 2017
for the inquiry on the perspective PhD position. She is always supportive, treating students equally,
and the best advisor to consult with students’ concerns not only on research but also on personal
matters. I also would like to thank other professors during my PhD study. Thank Prof. Wei Wu for
his kind assistance in my stretchable optics project and thank Prof. Mark E. Thompson for my
chemistry rotation program in my first year.
I cannot accomplish my PhD research so far without kind assistances from my lab
colleagues around me. Dr. Andre Kovach, my most frequent research collaborator, was my mentor
and the great lab mate to get access to lab resources. He is also a good friend of me and treated me
iii

nicely for non-research activities. Dr. Dongyu Chen, who was a senior grad student when I joined
the group, was an expert in optics and simulations, and very nicely approachable. Dr. Hyungwoo
Choi, who was another senior grad student, was an expert in the resonator setup. I was so grateful
to learn a lot of resonator testing from him. Prof. Xiaoqin Shen, who was a former postdoc in the
group, was a good person for sharing his research ideas and personal suggestions, which were
greatly beneficial to me. Dr. Yingmu Zhang is a great collaborator in organic synthesis, and I was
appreciated by her skills in cross-coupling reactions. Dr. Rene Zeto is an expert in microscopy and
nanoparticles. Dr. Patrick Saris has solid chemistry knowledge and great choice for the help in
synthesis. Kylie Trettner and Yasaman Moradi are current lab colleagues, and they are very nice
to talk with. Raymond Yu is a great young talent in optics. Tian Sang and Jiaqi Yuan are eager to
learn optics and have strong attitude in upcoming research. Jingxin Zhang is a good at device
fabrication. Armando Urbina, the lab “Chief Fun Officer”, is a good person to bring fun to the lab.
Brian Hernandez was my undergrad mentee, and he was smart to pick up organic synthesis and
molecular modeling. William Wang was a high school student, working on the stretchable grating
project with me during 2021. He was ambitious, smart, and eager in science. I wish everyone all
the best and the brightest future.
I also would like to thank all other previous and current members of the lab: Dr. Alexa
Hudnut, Dr. Victoria Sun, Dr. Samantha McBirney, Dr. Soheil Soltani, Dr. Vinh Diep, Dr. Haijie
Zuo, Dr. Meghan Barrett, Clayton Cheung, Luciana Custer, Sandra Lara Galindo, Marko Lilic,
Lexie Scholtz, Fakhar Singhera, Ruojiao Sun, Debasmita Banerjee, Arynn Gallegos, Kristina
Kaypaghian, Daniel Cummins, Brock Hudnut, Omar Garcia, Amber Chow, Dania Duran, Mike
Shao, Hsin Pan, Goldie Roth, Mark Veksler, Sydney Fiorentino, Vincent Fu, Ernesto Ortiz
iv

Zamora, Megan Phi, Hari Sridhara, Shakthi Srinivasan, and everyone else who I have not included
here. I wish all of you for your best and continuous success in the future.
Additionally, I would like to thank my friends whom I met outside of Armani Group or
USC. Many thanks to Rodrigo Elizalde Segovia, Justin Overhulse, Jeremy Intrator, Jeff Wang,
Misaki Saito, Devang Dasani, Samer Idres, Phong Ho, Tina Xu, Renzhe Zhang, Yuchen Huang,
Yingge Zhou, and many others not listed here. I wish the best to all of you in the future.
Last but not the least, I would like to express my deepest gratitude to my parents for
supporting me as always. When I brought about pursuing the PhD program in the United States
for at least five years, I was fortunate and grateful to get their countless spiritual supports. I realize
that life in the United States is an entirely different experience for me, compared to my past years
in China and Japan. The culture and life transitions were not easy for me. When I had difficulties,
my parents encouraged me to overcome. I cannot achieve so far without your supports. Thank you
again my parents! Fight on!

v

Table of Contents
Acknowledgements .....................................................................................................................ii
List of Tables .............................................................................................................................. x
List of Figures ............................................................................................................................ xi
Abstract ................................................................................................................................. xxxi
Chapter 1. Overview ................................................................................................................... 1
1.1 Introduction.................................................................................................................... 1
1.2 Thesis Overview ............................................................................................................ 3
References ........................................................................................................................... 5
Chapter 2. Background ............................................................................................................. 13
2.1 Organic Photoswitches for Optical Sensing .................................................................. 13
2.2 Whispering-Gallery Mode (WGM) Optical Microresonators ........................................ 20
2.3 Organic Materials with Aggregation-Induced Emission ................................................ 26
2.4 Stretchable Polymers for Optical Micro-Devices .......................................................... 31
2.5 Nanoimprint and Soft Lithography for Optical Micro-Devices ..................................... 42
References ......................................................................................................................... 47
Chapter 3. Tunable Photoswitchable Optical Microcavities with a Self-assembled
Monolayer of Azobenzene ........................................................................................................ 63
3.1 Introduction.................................................................................................................. 63
3.2 Azobenzene Characterization ....................................................................................... 65
3.3 Computational Modeling .............................................................................................. 68
3.4 Device Fabrication ....................................................................................................... 73
3.5 Testing Setup for Optical Characterization ................................................................... 75

vi

3.6 Reversible Photoswitching ........................................................................................... 79
3.6.1 Quality Factor Characterization .......................................................................... 80
3.6.2 Reversibility and Sensitivity ............................................................................... 83
3.7 Broadband Optical Spectrum Tuning ............................................................................ 88
3.8 Summary ...................................................................................................................... 95
References ......................................................................................................................... 97
Chapter 4. Reversibly Tunable Optical Diffraction Grating Using Poly(acrylic
acid)/Polyethylene Oxide Stereocomplex ................................................................................ 105
4.1 Introduction................................................................................................................ 105
4.2 Fabrication of Polymer Diffraction Grating by PAA/PEO Stereocomplex .................. 107
4.2.1 Polymer Film Preparation ................................................................................. 107
4.2.2 Characterization of Polymer Film ..................................................................... 112
4.2.3 Fabrication of Si Master Grating....................................................................... 113
4.2.4 Fabrication of Stretchable Polymer Grating ...................................................... 114
4.2.5 Surface Morphology Analysis of PAA/PEO Polymer Diffraction Grating ........ 115
4.3 Mechanical Characterization of PAA/PEO Polymer Film ........................................... 117
4.4 Computational Modeling on Optical Diffraction Grating by PAA/PEO
Stereocomplex ................................................................................................................. 121
4.5 Optical Characterization on Reversibly Tunable PAA/PEO Diffraction Grating ......... 123
4.6 Summary .................................................................................................................... 133
References ....................................................................................................................... 135
Chapter 5. Multifunctional Photoresponsive Organic Small Molecules by Aggregation-
Induced Emission.................................................................................................................... 139

vii

5.1 Introduction................................................................................................................ 139
5.2. Synthesis of NAI-TPE-PyS ....................................................................................... 142
5.2.1 Materials and Synthesis Overview .................................................................... 142
5.2.2 Synthesis of 1,8-Naphthalimide (Compound 1) ................................................ 143
5.2.3 Synthesis of 2-(10-Bromodecyl)-1H-Benz[de]Isoquinoline-1,3(2H)-
Dione (Compound 2) ................................................................................................ 144
5.2.4 Synthesis of 1-(4-Bromophenyl)-2,2-Bis(4-Hydroxyphenyl)-1-
Phenylethene (Compound 3) ..................................................................................... 145
5.2.5 Synthesis of 4'-(4,4,5,5-Tetramethyl-1,3,2-Dioxaborolan-2-Yl)-[1,1'-
Biphenyl]-4-Carbaldehyde (Compound 4) ................................................................. 147
5.2.6 Synthesis of 4,4'-(2-(4-Bromophenyl)-2-Phenylethene-1,1-Diyl)-
Diphenol (Compound 5)............................................................................................ 148
5.2.7 Synthesis of 4''-(2,2-Bis(4-Hydroxyphenyl)-1-Phenylvinyl)-[1,1':4',1''-
Terphenyl]-4-Carbaldehyde (Compound 6) ............................................................... 149
5.2.8 Synthesis of 4''-(2,2-Bis(4-((10-(1,3-Dioxo-1H-Benzo[de]Isoquinolin-
2(3H)-Yl)Decyl)Oxy)Phenyl)-1-Phenylvinyl)-[1,1':4',1''-Terphenyl]-4-
Carbaldehyde (NAI-TPE-CHO, Compound 7) .......................................................... 151
5.2.9 Synthesis of 3-(4-Methylpyridin-1-Ium-1-Yl)Propane-1-Sulfonate
(Compound 8) ........................................................................................................... 153
5.2.10 Synthesis of 3-(4-(2-(4''-(2,2-Bis(4-((10-(1,3-Dioxo-1H-
Benzo[de]Isoquinolin-2(3H)-Yl)Decyl)Oxy)Phenyl)-1-Phenylvinyl)-[1,1':4',1''-
Terphenyl]-4-Yl)Vinyl)Pyridin-1-Ium-1-Yl)Propane-1-Sulfonate (NAI-TPE-PyS) ... 154
5.3. Synthesis of Other Molecules .................................................................................... 156

viii

5.3.1 Synthesis Overview .......................................................................................... 156
5.3.2 Synthesis of 4,4'-(2-(4-Bromophenyl)-2-Phenylethene-1,1-
Diyl)Bis(Methoxybenzene) (Compound 11) .............................................................. 157
5.3.3 Synthesis of 4-(Ethyl(2-Hydroxyethyl)Amino) Benzaldehyde (Compound 14) . 158
5.3.4 Synthesis of 2-(Ethyl(4-Vinylphenyl)Amino)Ethan-1-Ol (Compound 15) ........ 158
5.3.5 Synthesis of (E)-4,4'-(2-(4-Bromophenyl)-2-(4-(4-(Ethyl(2-
Hydroxyethyl)Amino)Styryl)Phenyl)Ethene-1,1-Diyl)Diphenol (Compound 16) ...... 159
5.3.6 Synthesis of 4,4'-(2-(4-Bromophenyl)-2-Phenylethene-1,1-Diyl)Bis((2-
Bromoethoxy)Benzene) (Compound 17) ................................................................... 161
5.3.7 Synthesis of 2-(2-Hydroxyethyl)-1H-Benzo[de] Isoquinoline-1,3(2H)-
Dione (Compound 18)............................................................................................... 162
5.3.8 Synthesis of (Z)-2-(2-(2-(4-(2-(4-Bromophenyl)-1,2-
Diphenylvinyl)Phenoxy)Ethoxy)Ethyl)-1H-Benzo[de] Isoquinoline-1,3(2H)-
Dione (Compound 19)............................................................................................... 163
5.3.9 Synthesis of Tetrakis(decyl) Perylene-3,4,9,10-Tetracarboxylate
(Compound 21) ......................................................................................................... 165
5.3.10 Synthesis of 1H,3H-Perylo[3,4-cd]Pyran-8,9-Dicarboxylic Acid, 1,3-
Dioxo-, 8,9-Didecyl Ester (Compound 22) ................................................................ 166
5.4. Results and Discussions ............................................................................................ 167
5.4.1 Molecular Design and Computational Modelling .............................................. 167
5.4.2 Spectroscopic Characterization ......................................................................... 169
5.4.3 Aggregation Induced Emission Properties ........................................................ 178
5.4.4 Nonlinear Optical Properties............................................................................. 180

ix

5.4.5 Photoconductivity Characterization .................................................................. 184
5.4.6 Cyclic Voltammetry Measurements .................................................................. 186
5.4.7 Cytotoxicity of NAI-TPE-PyS .......................................................................... 188
5.5 Summary .................................................................................................................... 192
References ....................................................................................................................... 193
Appendices ............................................................................................................................. 202
Appendix A. Magnetic Polymers ..................................................................................... 202
Appendix B. Mechanochromic Stretchable Devices ......................................................... 205
References ....................................................................................................................... 213

x

List of Tables
Table 3.3.1 - Results from DFT calculations. ............................................................................ 70
Table 3.6.1 - Q0,1300 and Qloaded,410 of [CH3:Aazo = 10:1] device on different Aazo isomers. ...... 81
Table 4.2.1 - Summary of methods for compressing into PAA/PEO films. .............................. 111
Table 4.5.1 - The relative diffraction efficiencies, absolute diffraction angles and changes
in diffraction angles of each order at various wavelengths. Diffraction efficiencies were
measured at 0% strain. Reprinted with permission from Chapter 1 Reference [61] © The
Optical Society. ...................................................................................................................... 128
Table 5.4.1 - Detailed photophysical data of NAI-TPE-PyS in selective solvents.
Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry. ..... 176
Table 5.4.2 - The responsivity studies of our molecules. Reprinted with permission from
Chapter 1 Reference [66] © Royal Society of Chemistry. ........................................................ 186
Table 5.4.3 - Viability of MCF7 cell line................................................................................. 191
Table 5.4.4 - Viability of U2OS cell line ................................................................................. 191

xi

List of Figures
Figure 2.1.1 – The most common organic photoswitches and their reversible
conformation changes upon light or thermal energy. Azobenzenes and stilbenes undergo
trans-to-cis isomerization, and diarylethenes and spiropyrans undertake ring
opening/closing isomerization. Reprinted with permission from Chapter 2 Reference [2]
© Royal Society of Chemistry................................................................................................... 13
Figure 2.1.2 – Standard chemical structures and UV/Vis absorption spectra of three
azobenzene derivatives: basic azobenzene, aminoazobenzene and pseudo-stilbene.
Reprinted with permission from Chapter 2 Reference [12] © Royal Society of Chemistry.
................................................................................................................................................. 15
Figure 2.1.3 – Proposed mechanisms for the trans-to-cis isomerization of azobenzene.
Reprinted with permission from Chapter 2 Reference [14] © Royal Society of Chemistry.
................................................................................................................................................. 16
Figure 2.1.4 – Molecular structures of ReAzoC in trans- and cis- isomeric conformations
on the oxide substrate. Reprinted with permission from Chapter 2 Reference [21] ©
American Chemical Society. ..................................................................................................... 17
Figure 2.1.5 – (a) Chemical structures of alkanethiol-terminated azobenzene (Az11) and
1-dodecanethiol (C12); (b) Az11 monolayer on Au substrate with thermal isomerization
at various conditions. Reprinted with permission from Chapter 2 Reference [22] ©
American Chemical Society. ..................................................................................................... 18
Figure 2.1.6 – Optical microscope images of the silica microsphere resonator (a) before
and (b) after depositing with 2 wt% EO-doped PVA coating. (c) Schematic illustration

xii

of the reversible photoisomerization of the EO molecule. Reprinted with permission from
Chapter 2 Reference [23] © Elsevier. ........................................................................................ 19
Figure 2.1.7 – Top-view optical microscope images of a bR-coated silica microsphere
positioned between two optical tapered fibers containing four ports. (a) ambient
illumination on the trans-bR-coated silica microsphere with a CCD camera. (b) The cis-
bR-coated microsphere excited with a pump 532 nm laser from Port 1. The image was
also taken with a CCD camera. (c) IR image of the trans-bR-coated microsphere pumped
with the probe 1311 nm laser from Port 1. The photoisomerization working wavelength
532 nm was not pumped. (d) IR image of the cis-bR-coated microsphere pumped at both
1311 nm and 532 nm from Port 1. The near-IR probe wavelength was coupled to the
device to propagate to Port 3. Reprinted with permission from Chapter 2 Reference [24]
© AIP Publishing. ..................................................................................................................... 20
Figure 2.2.1 – Picture of whispering gallery architectures in the world: (a) St. Paul
Cathedral in London and (b) Temple of Heaven in Beijing. ....................................................... 21
Figure 2.2.2 – (a) Illustration of the WGM microresonator where light with resonant
wavelength is confined and traveling along the periphery of the resonator by total internal
reflection. (b) Total internal reflection at the boundary interface with an incident angle
greater than the critical angle (θc) according to Snell’s law. ...................................................... 21
Figure 2.2.3 – Rendering of common optical microcavity geometries: microcylinders,
microspheres, microtoroids, microdisks, and microring resonators. Reprinted with
permission from Chapter 2 Reference [35] © The Optical Society. ........................................... 22

xiii

Figure 2.2.4 – DASP-coated microsphere resonators containing nonlinear optical
parametric oscillation behavior. Reprinted with permission from Chapter 2 Reference
[46] © American Association for the Advancement of Science. ................................................ 22
Figure 2.2.5 – The surface coating chemical structures and the Raman emission
intensities of (a) bare silica, (b) methyl-coated silica, and (c) dimethyl-coated silica
microsphere resonators at various coupled laser power and two different polarization
states. (d) Vector decomposition of the horizontal and vertical components of the Si–O–
Si mode with respect to the toroidal cavity surface. Reprinted with permission from
Chapter 2 Reference [37] © Springer Nature. ............................................................................ 23
Figure 2.2.6 – Optical Q measurement setup consisting of a tunable narrow linewidth
laser, tapered fiber, photodetector, oscilloscope, function generator, camera, and a computer. ... 24
Figure 2.3.1 – (top) Aggregation-caused quenching occurs in conventional organic
fluorescent dye Fluorescein; (bottom) aggregation-induced emission in
tetraphenylethylene. Reprinted with permission from Chapter 2 Reference [57] © Royal
Society of Chemistry. ................................................................................................................ 27
Figure 2.3.2 – Examples of heteroatom-containing AIE luminogens whose AIE
phenomena are mainly caused by RIR process. Reprinted with permission from Chapter
2 Reference [58] © John Wiley and Sons. ................................................................................. 28
Figure 2.3.3 – Suzuki cross-coupling reaction on the synthesis of Th
4+
sensitive TPE.
Reprinted with permission from Chapter 2 Reference [59] © John Wiley and Sons. .................. 29
Figure 2.3.4 – (a) Fluorescence spectra of TPE-PDA (10 µM) at a water volume fraction
of 30% upon the addition of 1 equivalent of metal ions (excitation wavelength at 225nm);
(b) image of the TPE-PDA (10 µM)/metal ions (10 µM) mixtures at taken under UV

xiv

illumination (excitation wavelength at 365nm). Reprinted with permission from Chapter
2 Reference [59] © John Wiley and Sons. ................................................................................. 29
Figure 2.3.5 – Examples of TPE-bearing conjugated polymers where AIE effective
groups are located in the backbones. Reprinted with permission from Chapter 2
Reference [65] © Elsevier. ........................................................................................................ 30
Figure 2.3.6 – Examples of TPE-functionalized PNIPAM copolymers by controlled
radical copolymerization. Reprinted with permission from Chapter 2 Reference [67] ©
Royal Society of Chemistry. ..................................................................................................... 31
Figure 2.4.1 – Bare PMMA polymer vs PMMA/boron nitride nanosheets composites: (a)
transmission at various wavelengths; (b) elastic modulus; (c) mechanical strength.
Reprinted with permission from Chapter 2 Reference [100] © John Wiley and Sons. ................ 33
Figure 2.4.2 – The optical transmission spectra of polyester and polyarylate at visible and
near-IR wavelengths. Reprinted with permission from Chapter 2 Reference [102] ©
Springer Nature. ........................................................................................................................ 34
Figure 2.4.3 – Scheme of the self-healing process in the polymer composites. Red large
circles indicate the microcapsules containing reactive monomers. Smaller red circles
indicate the catalysts for the monomer crosslinking reactions. (a) The crack starts to break
microcapsules; (b) The reactive healing monomers get released by the crack; (c)
polymerization of the healing agent undergoes to heal the crack. Reprinted with
permission from Chapter 2 Reference [107] © Springer Nature. ................................................ 36
Figure 2.4.4 – Schematic illustration on the process of dynamic bonds association and
dissociation to perform the intrinsic self-healing polymers. Reprinted with permission
from Chapter 2 Reference [108] © Royal Society of Chemistry. ............................................... 37

xv

Figure 2.4.5 – An overview of dynamic bonds for implementing the intrinsic self-healing
polymers. Reprinted with permission from Chapter 2 Reference [108] © Royal Society
of Chemistry. ............................................................................................................................ 38
Figure 2.4.6 – The direct observation on the self-healing process of the PAA-PEO
polymer film with an optical microscope. (a) The scratched film; (b−d) the scratched film
after being immersed in pH 2.5 water for (b) 10 s, (c) 20 min, and (d) 30 min. Reprinted
with permission from Chapter 2 Reference [121] © American Chemical Society. ..................... 39
Figure 2.4.7 – (a, b) Photos of the PAA-PEO film before (a) and after (b) being cut into
two pieces. Photos of the 1 h healed PAA-PEO film sample that was previously cut into
two pieces, captured before (c) and after (d) being stretched to a 175% strain. (e)
Stress−strain curves of the intact and 1, 6, 12, and 24 h healed samples that were
previously cut into two pieces. Reprinted with permission from Chapter 2 Reference
[122] © American Chemical Society. ........................................................................................ 40
Figure 2.4.8 – (a) Schematic of the diffraction measurement, showing of a metal grating
on PDMS substrate; (b) a photograph showing the actual experimental apparatus for
stretching and measuring the tunable gratings; (c) a series of the laser diffraction spots
(0th order and 1st order) presented by deforming the diffraction pitch from 0% strain up
to 66.7% strain, then retreating the deformation back to 0% strain. Reprinted with
permission from Chapter 2 Reference [123] © Springer Nature. ................................................ 41
Figure 2.5.1 – Scheme of NIL vs photolithography. Reprinted with permission from
Chapter 2 Reference [126] © Canon. ........................................................................................ 43
Figure 2.5.2 – (a) Cross-sectional SEM image of one-dimensional grating fabricated with
V-shaped Si master mold; (b) cross-sectional SEM image of one-dimensional grating

xvi

fabricated with the Si master mold without line edge smoothing. Reprinted with
permission from Chapter 2 Reference [127] © Springer Nature. ................................................ 43
Figure 2.5.3 – Fabrication scheme of Si master grating using UV-based NIL. ........................... 45
Figure 2.5.4 – (a) a master array of SiO2/Si microtoroids on the wafer; (b) the
microtoroidal master was coated to cure the PDMS stamp; (c) backfill another PDMS
batch and peel-off master; (d) top-down view of one microtoroidal PDMS resonator.
Reprinted with permission from Chapter 2 Reference [131] © The Optical Society. .................. 45
Figure 2.5.5 – (a) The master microring resonator fabricated using direct laser writing on
crosslinked SU-8 (Grey). (b) First soft lithography step of replication of the master device
into non-crosslinked SU-8 (Yellow). (c) Thermal Reflow from (b) to reduce the surface
roughness. (d) Second soft lithography step of replication of the reflowed resonator into
sol-gel (Blue). Reprinted with permission from Chapter 2 Reference [132] © Springer Nature.. 46
Figure 3.1.1 - (a) Rendering of the trans state of the aminoazobenzene derivative free
molecule and (b) the cis state of the molecule. Blue light initiates the trans-to-cis
conformation change, and heat reverts the process. (c) SEM image of coated microtoroid.
(d) Rendering of coated microtoroid, with the molecule enlarged for clarity in (a).
Reprinted with permission from Chapter 1 Reference [46] © The Optical Society..................... 65
Figure 3.2.1 - (a) UV-Vis absorption spectra of both trans and cis isomer states. The
wavelengths used to initiate photo-isomerization in the present work are indicated by
vertical lines. (b) Spectroscopic ellipsometry results over the spectral range of the 1300
nm probe laser. (c) Measured refractive index for each step: before exposure, after
exposure, and after heating for different time periods. Reprinted with permission from
Chapter 1 Reference [46] © The Optical Society. ...................................................................... 66

xvii

Figure 3.2.2 - XPS results verifying the two steps in the surface functionalization process.
The Cl 2p which is present in both CMPS samples indicates successful silanization, and
the N 1s indicates successful attachment of the Aazo group. The C 1s is likely due to the
carbon tape used to hold down the coated wafer sample. Reprinted with permission from
Chapter 1 Reference [45] © The Optical Society. ...................................................................... 68
Figure 3.3.1 - (a)-(d) Renderings of the different Aazo molecules simulated using DFT
along with the different isomerization states. Reprinted with permission from Chapter 1
Reference [45] © The Optical Society. ...................................................................................... 70
Figure 3.3.2 - (a), (b) COMSOL plots of optical mode profiles of both trans and cis states
for a device with major and minor diameters of 50 𝜇m and 7 𝜇m. (c) Simulated total
optical mode volume and (d) mode volume in the thin Aazo film as a function of minor
radii for both cis and trans states. The major radius was fixed at 25 𝜇m. (e) Simulated
total optical mode volume and (f) mode volume in the thin film as a function of major
radii for both cis and trans states. The minor radius was fixed at 3.5 𝜇m. For a given
geometry, the trans state has a larger interaction with the optical field than the cis state.
Reprinted with permission from Chapter 1 Reference [46] © The Optical Society..................... 72
Figure 3.4.1 - Rendering of surface functionalization scheme showing attachment of the
linker molecule (CMPS) with subsequent Aazo attachment. Reprinted with permission
from Chapter 1 Reference [45] © The Optical Society. ............................................................. 74
Figure 3.5.1 – The first version of the testing setup for the optical characterization on
Aazo-coated Silica microtoroid devices. It consists of a tapered optical fiber to couple
the probe wavelength 1300 nm, a tunable 450 nm blue laser to conduct Aazo
photoisomerization, a 405 nm lamp to provide heating and a sideview camera. ......................... 75

xviii

Figure 3.5.2 – The revised optical device characterization. (a) Optical device
characterization testing setup and photoisomerization system with all components
labeled. (b) Example of a Q spectrum at 1300 nm. Inset is a side view optical microscope
image of an Aazo-coated device coupled by an optical taper. (c) Example of a Q spectrum
at 410 nm. Reprinted with permission from Chapter 1 Reference [45] © The Optical Society. .. 76
Figure 3.5.3 – (a) The fluctuation on the room temperature of the lab space where the
optical testing setup was located; (b) the cavity resonant wavelength Δλ 1300 measured at
1300 nm with respect to the testing time. The testing device was a methyl-coated silica
microtoroid with Q0 = 3.9 × 10
6
. (a) and (b) were experimentally conducted at the same
time. (c) The fluctuation on the room temperature of the lab space where the optical
testing setup was located, tested on another day; (d) Δλ 1300 measured at 1300 nm with
respect to the testing time. The testing device was a methyl-coated silica microtoroid with
Q0 = 6.6 × 10
5
. (c) and (d) were experimentally conducted at the same time. ............................. 78
Figure 3.6.1 – Chemical structure of MeOAzo. ......................................................................... 79
Figure 3.6.2 – (a) The cavity resonant wavelength Δλ 1300 measured at 1300 nm with
respect to the testing time. The testing device was a MeOAzo-coated silica microtoroid
with Q0 = 2.3 × 10
5
. (b) The fluctuation on the room temperature of the lab space where
the optical testing setup was located. (a) and (b) were experimentally measured at the
same time.................................................................................................................................. 80
Figure 3.6.3 - Optical cavity quality factors. (a) Q0,1300 and (b) Qloaded,410 of Aazo-coated
devices with different ratios. Each data point is a unique device, and the error is the error
in the fitting of the linewidth. (c) Q0,1300 and (d) Qloaded,410 with respect to the input power
of 410 nm laser. The measurements in part (c) and (d) were performed on the same Aazo-

xix

coated SiO2 microtoroid device. Reprinted with permission from Chapter 1 Reference
[45] © The Optical Society. ...................................................................................................... 81
Figure 3.6.4 - (a) SEM image of Aazo-coated microtoroid resonant cavity. (b,c)
Rendering of the region indicated by the box in (a) showing the reversible Aazo trans to
(c) cis photoisomerization process. Reprinted with permission from Chapter 1 Reference
[45] © The Optical Society. ...................................................................................................... 83
Figure 3.6.5 - Tracing 𝜆 1300 shifts on two different [CH3:Aazo = 10:1] Aazo-
functionalized optical cavities upon coupling 410 nm laser. (a) One cycle of reversible
𝜆1300 shift. When the device was coupled by the 410 nm light (Pcirc = 420.86 mW), 𝜆1300
blueshifted 7.8 pm. Upon exposure to the 10.6 𝜇m CO2 laser, 𝜆 1300 immediately red-
shifted; ultimately, it returned to the initial 𝜆1300. (b) Repeated photoisomerization of
Aazo on-device. The black, red and blue regions are the 1st, 2nd and 3rd cycle,
respectively. The corresponding blueshifts for each cycle were 4.5 pm (Pcirc = 127.36
mW), 3.6 pm (Pcirc = 96.33 mW), and 3.0 pm (Pcirc = 92.35 mW), respectively. (c) Two
reversible cycles on the same device as with similar 410 nm input power and 𝛥𝜆 : black
curve was the first study and red curve was tested after 6 months. Reprinted with
permission from Chapter 1 Reference [45] © The Optical Society. ........................................... 84
Figure 3.6.6 - 𝛥𝜆 as a function of Pcirc across various Aazo surface concentration. (a) For
a given surface density of Aazo, as Pcirc increases, 𝛥𝜆 also increases. The slopes of the
linear fits are the optical responses 𝛥𝜆/Pcirc of devices which are characteristic to the
surface density of Aazo. The error of each data point (each reversible cycle) was
determined by the Gaussian fit on the histogram of the data points (N > 500) in the
baseline. Inset: The magnitude of 𝛥n1300 behaves in a similar manner to 𝛥𝜆 with respect

xx

to Pcirc among varied [CH3:Aazo] ratios. The error is directly related to the error in the
primary data set. (b) Optical responses 𝛥𝜆 /Pcirc with respect to [CH3:Aazo] ratios.
Negative values indicate the reduction of Δλ by 410 nm. The error was the error in the
linear fitting in part (a). (c) Using the optical cavity refractive index changes from (a,
inset) and setting Pcirc = 200 mW, the experimental results on change of optical cavity
refractive index (black curve) can be compared to the change of refractive index from
ellipsometry studies (red curve). The general trend of two curves is also similar. The error
in the black curve was the error in the linear fit of 𝛥n1300 when setting Pcirc = 200 mW.
The error in the red curve was taken from the average error of 𝛥n1300 (ellipsometry)
among three wafers on each ratio. Reprinted with permission from Chapter 1 Reference
[45] © The Optical Society. ...................................................................................................... 86
Figure 3.7.1 - (a) Rendering of the characterization setup for broadband optical spectrum
tuning. (b) Representative transmission spectrum of a coated microtoroid on resonance
at 1300 nm (Q ~ 4.4 × 10
6
). Reprinted with permission from Chapter 1 Reference [46] ©
The Optical Society................................................................................................................... 89
Figure 3.7.2 - (a) Real-time tracking of the 1300 nm resonance peak tuning due to photo-
isomerization upon exposure to either the 410 nm or 450 nm laser. (b) Broadband
transmission spectra tracking the shifting resonance peak. (c) Sensitivity analysis using
of coated device with varying input powers for both the 410 nm and 450 nm pumps.
Reprinted with permission from Chapter 1 Reference [46] © The Optical Society..................... 91
Figure 4.2.1 - Three-dimensional illustration of PAA/PEO stereocomplex, containing
intermolecular hydrogen bonds formed between the carboxylic acid group in PAA and
oxygen in PEO. ....................................................................................................................... 108

xxi

Figure 4.2.2 - Outline of the material synthesis and the fabrication of the PAA/PEO
polymer stereocomplex diffraction grating. Reprinted with permission from Chapter 1
Reference [61] © The Optical Society. .................................................................................... 109
Figure 4.2.3 - (a) PAA/PEO polymer stereocomplex film on a piece of glass slide; (b)
optical microscope image of PAA/PEO polymer stereocomplex. Reprinted with
permission from Chapter 1 Reference [61] © The Optical Society. ......................................... 110
Figure 4.2.4 - UV-Vis spectrum of PAA/PEO stereocomplex film. Reprinted with
permission from Chapter 1 Reference [61] © The Optical Society. ......................................... 112
Figure 4.2.5 – Sample image of Si master gratings. ................................................................. 113
Figure 4.2.6 – The blueprint of ISO 37-4 Type dumbbell molding stamp die. The whole
size of the die model is 35 × 6 mm
2
, and the dimension for the middle rectangular region
is 12 × 2 mm
2
. Reprinted with permission from Chapter 4 Reference [29] © Dumbbell
Co., Ltd................................................................................................................................... 114
Figure 4.2.7 – SEM images of (a) Si master grating and (b) PAA/PEO replicated polymer
grating. (c) Optical microscope image of Si master with patterned dots. (d) SEM image
of PAA/PEO replicated polymer film with patterned dots. Reprinted with permission
from Chapter 1 Reference [61] © The Optical Society. ........................................................... 115
Figure 4.2.8 - SEM images of the PAA/PEO polymer grating. (a,b) Vertical direction
view at 0% strain; (c,d) 45-degree tilted view at 0% strain; (e,f) vertical direction view at
50% strain; (g,h) 45-degree tilted view at 50% strain. Reprinted with permission from
Chapter 1 Reference [61] © The Optical Society. .................................................................... 116

xxii

Figure 4.3.1 - Image of (a) 3342 Single Column Universal Testing Systems; (b) a pair of
pneumatic film clamps in Instron; (c) PAA/PEO film being stretched by Instron where
the strains were applied. .......................................................................................................... 117
Figure 4.3.2 - Stress-strain curve of PAA/PEO polymer stereocomplex at the stretching
rate of 50% min
-1
. Reprinted with permission from Chapter 1 Reference [61] © The
Optical Society. ...................................................................................................................... 118
Figure 4.3.3 - The cyclic loading-unloading test results from (a) Day 1 and (b) Day 2,
with the polymer film being relaxed between those days. The onset strain in each
hysteresis loop cycle for (c) Day 1 and (d) Day 2. The cyclic curves of the PAA/PEO
polymer film recovering after resting 1 day. Reprinted with permission from Chapter 1
Reference [61] © The Optical Society. .................................................................................... 120
Figure 4.4.1 – FEM simulation results on diffraction order distances on PAA/PEO
polymer grating at 633 nm. Distances between: (a) 0th and 1st order; (b) 0th and 2nd
order; (c) 0th and 3rd order. .................................................................................................... 122
Figure 4.4.2 – FEM simulation results on diffraction order distances on PAA/PEO
polymer grating at 1064 nm. Distances between: (a) 0th and 1st order; (b) 0th and 2nd order. . 123
Figure 4.5.1 – (a) Testing setup for the polymer grating consists of a 633 nm or 1064 nm
laser, a universal tensile test machine (Instron) to apply continuous strains, a beam
splitter, a receiving screen, and a camera to capture diffraction pattern images. Inset:
Illustration of the grating structure and generated diffraction pattern. The diffraction
orders (0th, 1st, 2nd, and 3rd) are indicated. (b) Image of the actual testing setup for the
diffraction measurements. Reprinted with permission from Chapter 1 Reference [61] ©
The Optical Society................................................................................................................. 124

xxiii

Figure 4.5.2 - The diffraction patterns of PAA/PEO polymer grating by various incident
wavelengths. It turns out that 633 and 1064 nm are the best incident wavelengths because
there are scatterings or no diffractions at other wavelengths due to our set-up with a
specific IR card as the receiving screen and a specific beam splitter that works at a certain
wavelength. Reprinted with permission from Chapter 1 Reference [61] © The Optical
Society. ................................................................................................................................... 125
Figure 4.5.3 – The relative phosphor sensitivity of the IR card used in the testing setup
with respect to the sensing wavelength. Reprinted with permission from Chapter 4
Reference [30] © Thorlabs. ..................................................................................................... 126
Figure 4.5.4 - The diffraction patterns generated at (a) 633 nm and (b) 1064 nm during
one stretching-and-relaxing cycle are shown. The cycle started at 0% strain, increased to
a maximum of 70% strain, and returned to 0% strain. Scale bar is 5 mm. Reprinted with
permission from Chapter 1 Reference [61] © The Optical Society. ......................................... 127
Figure 4.5.5 - Measurements and FEM simulations of strain-induced changes in the
spacing between diffracted orders at 633 nm. Measurements are with respect to the 0th
order. (a) 0th-1st order, stretching; (b) 0th-1st order, relaxing; (c) 0th-2nd order,
stretching; (d) 0th-2nd order, relaxing; (e) 0th-3rd order, stretching; (f) 0th-3rd order,
relaxing. Reprinted with permission from Chapter 1 Reference [61] © The Optical Society. ... 129
Figure 4.5.6 - Studies on the sequential diffraction order distances of the polymer grating
between X-th (X indicated on the top right of every figure) orders at 633 nm in 5
individual cycles. (a,b,c) Cycle 1, (d,e,f) Cycle 2, (g,h,i) Cycle 3, (j,k,l) Cycle 4 and
(m,n,o) Cycle 5 of the sequential diffraction order distances when stretching and relaxing.
Reprinted with permission from Chapter 1 Reference [61] © The Optical Society................... 131

xxiv

Figure 4.5.7 - Measurements and FEM simulations of strain-induced changes in the
spacing between diffracted orders at 1064 nm. Measurements are with respect to the 0th
order. (a) 0th-1st order, stretching; (b) 0th-1st order, relaxing; (c) 0th-2nd order,
stretching; (d) 0th-2nd order, relaxing. Reprinted with permission from Chapter 1
Reference [61] © The Optical Society. .................................................................................... 132
Figure 5.1.1 - Chemical structure of NAI-TPE-PyS with TPE-module in red and NAI-
module in blue. Reprinted with permission from Chapter 1 Reference [66] © Royal
Society of Chemistry. .............................................................................................................. 140
Figure 5.2.1 - Synthetic scheme of NAI-TPE-PyS. Reprinted with permission from
Chapter 1 Reference [66] © Royal Society of Chemistry. ........................................................ 142
Figure 5.2.2 -
1
H NMR of Compound 1. ................................................................................. 143
Figure 5.2.3 -
1
H NMR of Compound 2. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 144
Figure 5.2.4 -
13
C NMR of Compound 2. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 145
Figure 5.2.5 -
1
H NMR of Compound 3. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 146
Figure 5.2.6 -
13
C NMR of Compound 3. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 146
Figure 5.2.7 -
1
H NMR of Compound 4. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 147
Figure 5.2.8 -
13
C NMR of Compound 4. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 148

xxv

Figure 5.2.9 -
1
H NMR of Compound 5. ................................................................................. 149
Figure 5.2.10 -
1
H NMR of Compound 6. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 150
Figure 5.2.11 -
13
C NMR of Compound 6. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 151
Figure 5.2.12 -
1
H NMR of Compound 7. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 152
Figure 5.2.13 -
13
C NMR of Compound 7. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 152
Figure 5.2.14 -
1
H NMR of Compound 8. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 153
Figure 5.2.15 -
13
C NMR of Compound 8. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 154
Figure 5.2.16 -
1
H NMR of NAI-TPE-PyS. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 155
Figure 5.2.17 -
13
C NMR of NAI-TPE-PyS. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry. ........................................................................ 155
Figure 5.3.1 - Synthetic scheme of different molecules in Chapter 5. ...................................... 156
Figure 5.3.2 -
1
H NMR of Compound 11................................................................................. 157
Figure 5.3.3 -
1
H NMR of Compound 14................................................................................. 158
Figure 5.3.4 -
1
H NMR of Compound 15................................................................................. 159
Figure 5.3.5 – The predicted
1
H NMR of Compound 16 using MestReNova Program. ............ 160

xxvi

Figure 5.3.6 -
1
H NMR of the synthesis on Compound 16. This reaction was unsuccessful
with numerous unknown proton peaks and unmatched multiplet integrals with the
predicted
1
H-NMR spectrum. .................................................................................................. 161
Figure 5.3.7 -
1
H NMR of Compound 17................................................................................. 162
Figure 5.3.8 -
1
H NMR of Compound 18................................................................................. 163
Figure 5.3.9 – The predicted
1
H NMR of Compound 19 using MestReNova Program. ............ 164
Figure 5.3.10 -
1
H NMR of the synthesis on Compound 19. This reaction was
unsuccessful with several unknown proton peaks and unmatched multiplet integrals with
the predicted
1
H-NMR spectrum. ............................................................................................ 164
Figure 5.3.11 -
1
H NMR of Compound 21. .............................................................................. 165
Figure 5.3.12 – The predicted
1
H NMR of Compound 22 using MestReNova Program. .......... 166
Figure 5.3.13 -
1
H NMR of the synthesis on Compound 22. This reaction was
unsuccessful with unknown proton peaks and unmatched multiplet integrals with the
predicted
1
H-NMR spectrum. .................................................................................................. 167
Figure 5.4.1 - TPE-PyS model compound: (a) chemical structure; (b) optimized ground
state geometry; (c) HOMO (red solid) and LUMO (blue mesh) isosurfaces (0.05 Å
3
);
natural transition orbital iso-surfaces (0.05 Å
3
) for hole (red solid) and electron (blue
mesh) wavefunctions: (d) S1, (e) S2, and (f) S3. Reprinted with permission from Chapter
1 Reference [66] © Royal Society of Chemistry. ..................................................................... 169
Figure 5.4.2 - UV-Vis absorption measurements of several synthetic precursors, the final
product both in DCM and in PBS, and the final product deposited as a thin film on quartz
substrate. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry. .............................................................................................................................. 171

xxvii

Figure 5.4.3 - Absorption spectra of NAI-TPE-PyS in different solvents. Concentration:
10 µM. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry. .............................................................................................................................. 171
Figure 5.4.4 - Emission spectra of NAI-TPE-PyS in different solvents. Concentration: 10
µM, λex = 390 nm. Reprinted with permission from Chapter 1 Reference [66] © Royal
Society of Chemistry. .............................................................................................................. 172
Figure 5.4.5 - Photographic images of NAI-TPE-PyS emission in different solvents at λex
= 365 nm. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry. .............................................................................................................................. 172
Figure 5.4.6 - Emission spectra of NAI-TPE-PyS at λex = 350 nm (black squares) and λ ex
= 390 nm (red circles) in PBS buffer (concentration: 10 µM, containing 1% DMSO).
Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry. ..... 173
Figure 5.4.7 - Optical absorption and emission properties of NAI-TPE-PyS. (a) Images
of fluorescence from NAI-TPE-CHO and NAI-TPE-PyS solutions and solid powder
(λex = 365 nm). (b) Normalized absorption spectra (solid symbols) and emission spectra
(hollow symbols) of NAI-TPE-PyS in DCM or PBS or in solid state, and NAI-TPE-CHO
at λex = 390 nm. Reprinted with permission from Chapter 1 Reference [66] © Royal
Society of Chemistry. .............................................................................................................. 174
Figure 5.4.8 - Lippert-Mataga plot for NAI-TPE-PyS in different solvents as a function
of solvent polarity. (Df: orientation polarizability; nabs-nem: Stoke shifts). Reprinted with
permission from Chapter 1 Reference [66] © Royal Society of Chemistry. ............................. 175
Figure 5.4.9 - Photographic images of NAI-TPE-PyS emission in PBS buffer with
different pH values λex= 365 nm (Concentration: 10 µM, containing 1% DMSO). ................... 177

xxviii

Figure 5.4.10 - Emission spectra of NAI-TPE-PyS in PBS buffer solutions with different
pH values. Concentration: 10 µM, containing 1% DMSO. λex= 390 nm. .................................. 177
Figure 5.4.11 - Plot of fluorescence peak intensity (I/I0) vs. pH values. The data was
extracted from Figure 5.4.10. .................................................................................................. 178
Figure 5.4.12 - AIE behavior of NAI-TPE-PyS. (a) Plot of relative PL intensity versus
THF fraction. Inset: PL spectra of NAI-TPE-PyS in DMSO/THF mixtures with different
THF fractions (fTHF). Concentration of NAI-TPE-PyS is 20 µM; λex = 390 nm. (b) Plot of
PL intensity versus NAI-TPE-PyS concentration in water. Inset: PL spectra of aqueous
solutions of NAI-TPE-PyS at concentrations ranging from 0.01 µM to 80 µM (λ ex = 390
nm). Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry. .............................................................................................................................. 179
Figure 5.4.13 - Particle size distribution of NAI-TPE-PyS aggregates in DMSO/THF
mixture with a 99% THF fraction. Concentration: 50 µM. Reprinted with permission
from Chapter 1 Reference [66] © Royal Society of Chemistry. ............................................... 180
Figure 5.4.14 - Phasor plots of NAI-TPE-PyS in DMSO with concentration of 10 mM. .......... 181
Figure 5.4.15 - Phasor plots of NAI-TPE-PyS in DMSO with concentration of 100 µM. ......... 182
Figure 5.4.16 - Phasor plots of pure DMSO used as a control. ................................................. 182
Figure 5.4.17 - TCSPC histogram of NAI-TPE-PyS under 750 nm excitation. Red:
transient decay trace; Blue: exponential fit line; Inset: exponential fit parameters.
Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry. ..... 183
Figure 5.4.18 - Photoconductivity of NAI-TPE-PyS. (a) current vs. voltage
measurements of the device. Inset: Schematic of NAI-TPE-PyS-coated two-terminal
device. SiO2:2 µm; Ti: 5 nm; Au: 100 nm. Device channel size: length × width = 200 µm

xxix

× 5000 µm. (b) current vs. time measurements of the device at different incident optical
powers. (c) Responsivity vs. voltage of the device at different incident optical powers.
Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry. ..... 185
Figure 5.4.19 - I-t curve of the device at fixed voltage of 10 V upon light illumination
with 19 mW optical power. Reprinted with permission from Chapter 1 Reference [66] ©
Royal Society of Chemistry. ................................................................................................... 185
Figure 5.4.20 - Cell set up with a glassy carbon working electrode, platinum as counter
electrode, and Ag/AgCl as the reference electrode................................................................... 186
Figure 5.4.21 - (a) CV data for NAI-TPE-PyS in solution in the potential range of -1.5 to
1.6 V as a function of various scan rates from 10 to 100 mV s
-1
and (b) from 0 to 1.6 V.
(c) Log of the peak current (i) vs. log of the scan rate (v) for the data shown in Figure
5.4.21a,b. (d) CV curves of NAI-TPE-PyS in solution at 20 mV s
-1
for 20 cycles. ................... 187
Figure 5.4.22 - Viability of MCF7 cell line (black) and U2OS cell line (red) treated with
concentrations of NAI-TPE-PyS from 0.01 µM to 50 µM. Inset: Comparison of three
control measurements (media, 1% DMSO, 200 µM TMX) with three concentrations of
NAI-TPE-PyS. ........................................................................................................................ 190
Figure A.1 – Syntheses of PDMAEMA and the corresponding polymeric salts using BIBN. ... 203
Figure A.2 – Syntheses of EBIB-PDMAEMA and the corresponding polymeric salts
using EBIB. Reprinted with permission from Appendices Reference [5] © Royal Society
of Chemistry. .......................................................................................................................... 204
Figure A.3 – GPC curve of EBIB-PDMAEMA. ...................................................................... 204
Figure A.4 – Pictures of PDMAEMA-based polymers sticking on the magnet: (a)
PDMAEMA-bzl-Fe; (b) PDMAEMA-CH3-Fe; (c) EBIB-PDMAEMA-CH3-Fe. ..................... 205

xxx

Figure B.1 – Synthetic scheme of 4-SuTPE. Reprinted with permission from Appendices
Reference [19] © Taylor & Francis Online.............................................................................. 206
Figure B.2 – (a) Chemical structure of 4-SuTPE with distinct protons; (b)
1
H-NMR
spectrum of 4-SuTPE. ............................................................................................................. 206
Figure B.3 – Pictures of (a) PAA/PEO dumbbell film and (b) PAA/PEO/Rhodamine
dumbbell film under room light. Pictures of (c) PAA/PEO dumbbell film and (d)
PAA/PEO/Rhodamine dumbbell film under 365 nm. .............................................................. 208
Figure B.4 – (a) Testing setup of the mechanical elongation of the
PAA/PEO/chromophore film; (b) zoomed-in picture of the clamp region of the Instron. ......... 208
Figure B.5 – (a) Fluorescent images of (a) PAA/PEO/Rhodamine film and (b)
PAA/PEO/SuTPE film deformed at various strains. The excitation wavelength was 365 nm. .. 209
Figure B.6 – (a) Schematic illustration of PDMS stamp and silicon master; optical
microscope images of (b) silicon master with micropillars and (c) PDMS stamp with
microholes. ............................................................................................................................. 210
Figure B.7 – (a) Customized mechanical clamp to induce mechanical deformation on
PDMS/TPE microstructures on the fluorescent microscope; (b) PDMS/TPE microdevice
being clamped; (c) Illumination on the clamped PDMS/TPE microdevice from the
fluorescence microscope. ........................................................................................................ 211
Figure B.8 – Fluorescence images of PDMS/TPE microstructures at (a) 10 mm, 0%
strain; (b) 15 mm, 50% strain; (c) 20 mm, 100% strain. (d) Fluorescence intensities at
various mechanical strains. The fluorescent excitation wavelength for TPE chromophore
was 365 nm. The blue bars indicate the distances measured for the microstructures at
various strains. ........................................................................................................................ 212

xxxi

Abstract
Over the past century, optical devices have catalyzed numerous fields of study and enabled
many technologies including lasers, spectroscopy, optical sensing, and displays. Many of these
advances relied on innovations in both device design and in optical materials. Therefore, as
researchers look towards the future, pursuing the design and synthesis of new material systems by
leveraging the structure-property relationship is a promising field. However, any new material can
present new fabrication challenges due to incompatibilities with existing techniques.
Two general classes of fabrication include bottom-up and top-down methods. The bottom-
up method is taken into the consideration where the design of the optical device starts from the
molecular level in order to meet the physical and chemical properties in the macroscopic view.
When using organic materials, this method is commonly used. In contrast, the top-down technique
starts with a bulk material and removes or deposits additional layers. This method requires
compatibility with harsh chemicals. Therefore, it is typically only used with robust substrates,
including silicon.
In this thesis, I develop several new optical devices by first designing and optimizing new
functional and multi-functional optical organic materials and then combining them with different
device platforms. By using optically and mechanically tunable materials, I demonstrated reversibly
tunable on-chip whispering-gallery mode optical microresonators and diffraction gratings. In
addition, a molecular device consisting a multi-functional compound is also designed and
synthesized.
The first project leveraged the ultra-high quality factors (Q) of on-chip SiO2 microtoroid
resonators (Q > 10
6
). The top surface of the microresonator was treated in a series of chemical
vapor deposition steps to form a monolayer of azobenzene where the photoisomerization induces
xxxii

the change of the cavity refractive index. The azobenzene-coated SiO2 microresonators were tested
to show reversible tuning on the cavity resonant wavelength as well as the shift of the free spectral
range upon excitation by blue or mid-IR light. The device shows consistent optical performances
after being stored in air for 6 months, and the experimental results agree with finite element method
and density functional theory modeling.
The second optical device studied is the stretchable optical diffraction grating fabricated
by the novel polymer stereocomplex of poly(acrylic acid) and polyethylene oxide. This type of
polymer film exhibits above 800% strain when being stretched, and the mechanical performances
were recovered given sufficient relaxation time. The optical transmittance is above 80% in the
desired visible and near-IR wavelength range. The polymer grating was fabricated by replica
molding a silicon master grating. The tunable diffraction behavior of the polymer grating aligns
well with finite element method modeling.
The multifunctional molecular devices were synthesized with multi-step cross-coupling
reactions, consisting of a 'sensing' module with tetraphenylethylene and pyridinium salt, and a
'modulate' module with naphthalimide. This type of molecular device presents aggregation-
induced emissive properties where the emission intensity increases in aggregated solid state as
compared to the solution state. The photophysical properties and the photoconductivity were
studied in a variety of different solvents and in solid state, and the results agree with the simulations
by density functional theory study.
1

Chapter 1. Overview
1.1 Introduction
Optically active organic small molecules have demonstrated the optical tunability by their
reversible molecular conformations when being exposed at certain wavelengths [1–4].
Azobenzenes [5–7], stilbenes [8–10], diarylethenes [11–13] and spiropyrans [14–16] are the most
extensively studied organic photoswitches, and azobenzenes are the most ubiquitous
photoswitches among them. The azobenzene derivatives have been used in the productions of
optical switches [17,18], waveguides [19,20], holography [21,22], light shutters [23,24], memory
elements [25,26], storage of solar energy [27,28] and so on.
In the past decade, many different types of optical devices have been created, aiming to
achieve the optical tunability, such as optical microresonators, field-effect transistors, photonic
crystals, metasurface materials, and hybrid devices [29–34]. Whispering-gallery mode optical
microresonators have demonstrated a number of applications in lasers [35–37], on-chip frequency
combs [38–40] and spectroscopy [41,42] owing to the ultra-high quality factor (>10
7
–10
8
)
resulting in very long photon lifetime [43,44]. Recently, a method to self-assemble an azobenzene
monolayer on the surface of silica microtoroid resonators to tune the cavity resonant wavelength
was developed, leading to the reversible optical spectrum tuning. The stability of this organic-
modified microtoroid resonator can last at least six months without device degradation [45,46].
Diffraction gratings are the accurate optical elements containing a series of discrete
periodic patterns in micro- and nano-scale. In the field of tunable optical diffraction gratings,
previous reports were either electrical [47–49] or thermal tuning [50–52] of the optical properties.
Nowadays, an emerging and facile approach is to induce the mechanical deformation of polymers
as the flexible substrates. Contrasting to the conventional crystalline materials, a number of
2

polymers can exhibit large and reversible elastic responses to the mechanical strains [53–55].
There are several reports on presenting polydimethylsiloxane (PDMS) as the flexible substrate in
tuning the optical diffraction grating [56–60]. The polymer stereocomplex of poly(acrylic acid)
(PAA)/polyethylene oxide (PEO) can be mixed to perform replica transfer molding to obtain a
stretchable and deformable grating, with the elongation capability nearly tenfold than PDMS.
PAA/PEO polymer grating also exhibits reproducible mechanical deformations and diffraction
patterns given sufficient relaxation [61].
Fluorescent compounds based on synthetic organic molecules are powerful tools to
visualize biological events in living cells and organisms, as well as applications in organic light-
emitting diodes [62–64]. Tetraphenylethylene (TPE) is one of the representative types of
aggregation-induced emission (AIE) type fluorophores that show stronger emissions in aggregated
states due to their characteristic restrictions of intramolecular motions that prohibits the
nonradiative energy dissipation [65]. Multifunctional materials typically bear multiple functional
groups in the skeleton of the same molecule. Recently, TPE and naphthalimide group was
covalently synthesized together as a multifunctional molecular device, proved by various
photophysical properties, such as AIE and photoconductivity across different solvents and solid
state [66].

3

1.2 Thesis Overview
Chapter 2 includes a general background on optical devices and materials used in this
thesis. Whispering-gallery mode optical microresonators, optical diffraction gratings, and
multifunctional molecular devices are discussed. Material designs on tuning the optical devices
are also reviewed, including organic photoswitches, stretchable polymer materials, synthetic
designs on multifunctional molecular devices.
Chapter 3 introduces the tuning of the optical resonant performance of the silica
microtoroid resonator by self-assembled azobenzene monolayer functionalized on the surface of
the optical device. The microresonator can be quantitatively tuned by introducing a non-responsive
spacer silane, verified ellipsometry measurements. The experimental optical tuning range aligns
well with finite element method modeling, and the tuning speed and kinetics are analyzed. This
type of organic monolayer-modified microtoroid resonator also exhibits optical free spectral range
(FSR) tuning, capable of tuning to 0.67 FSR for around 4.2 nm.
Chapter 4 demonstrates a stretchable polymer diffraction grating made by a polymer
sterecomplex of poly(acrylic acid) and polyethylene oxide, fabricated by the transfer replica
molding. The basic optical and mechanical performances of the polymer film are investigated to
validate high optical transmittance at the desired optical wavelengths and high mechanical
stretchability over 800% strain. The polymer grating was systematically applied with mechanical
strains in order to probe the diffraction behaviors at various incident wavelengths, and the
experimental observations matches well with the simulation results.
Chapter 5 discussed on the synthesis of novel multifunctional organic molecular devices,
consisting of three components: photoconductive 'modulate' module – naphthalimide (NAI);
fluorescent 'sensing' module – tetraphenylethylene (TPE); electron acceptor moiety pyridinium
4

salt (PyS). The photophysical performances, including aggregation-induced emission and
photoconductivity studies, were performed in a variety of solvents. The 'sensing' and 'modulate'
dual modules can be separately controlled via different incident wavelengths. The experimental
results fit with the density functional theory modeling.
5

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13

Chapter 2. Background
2.1 Organic Photoswitches for Optical Sensing
A molecular photoswitch processes a chromophore that can interconvert between two or
more stable conformations. Such isomerization process can be triggered by light, thermal energy
or natural decay. Figure 2.1.1 shows the most common organic photoswitches that have been
studied in the past few decades. Azobenzenes and stilbenes generally undergo trans-to-cis
isomerization, while diarylethenes and spiropyrans can interconvert between open and closed
states [1,2]. Among the molecular candidates, azobenzene is one of the most ubiquitous organic
photoswitches owing to its versatile synthetic capabilities and the tunability of structure-function
relationship. These attributes of azobenzene facilitated rapid developments for the organic
photoswitches in the areas of molecular machines [3,4], liquid crystals [5,6], biological sensing
[7,8], photoswitchable adhesives [9,10], and so on [11]. In this section, the basic properties, surface
functionalization, and optical applications of azobenzene will be discussed.

Figure 2.1.1 – The most common organic photoswitches and their reversible conformation changes upon light or thermal energy.
Azobenzenes and stilbenes undergo trans-to-cis isomerization, and diarylethenes and spiropyrans undertake ring opening/closing
isomerization. Reprinted with permission from Chapter 2 Reference [2] © Royal Society of Chemistry.
14

There are mainly three types of azobenzenes in general: the original azobenzene,
aminoazobenzene-type, and pseudo-stilbene-type. Their molecular structures and feature UV-Vis
spectra are shown in Figure 2.1.2 [12]. In the black curve of UV-Vis spectra, there are two types
of electron transitions in azobenzene when being excited: π–π* transition and n–π* transition. The
stronger absorption band π–π*, namely the S2 (second-order excited singlet state) ← S0 (ground
state) transition in the UV-region (~320 nm), refers to the delocalized π-bonding in the azobenzene
moiety. The weaker absorption intensity band n–π*, namely the S1 (first-order excited singlet state)
← S0 transition in the visible region (~450 nm), forms due to the presence of lone pairs at the
nitrogen atoms. Upon trans-to-cis isomerization, the intensity of π–π* absorption band decreases,
whereas the intensity of n–π* absorption band increases [13,14]. On top of the pristine azobenzene,
the aminoazobenzene has an extra electron-donating amine group attached to one azobenzene
aromatic ring. This increases the electron density of the aminoazobenzene and assists the
delocalization of π electrons in the aminoazobenzene, compared to the original azobenzene. As a
result, the π–π* and n–π* transition overlap in the UV-Vis spectra, with a typically red-shifted
wavelength from the π–π* band of azobenzene (red curve in Figure 2.1.2).
The pseudo-stilbene-type azobenzene was named due to the structural similarity with the
stilbene molecule, by simply substituting C=C double bond with N=N double bond. In addition,
the pseudo-stilbene-type azobenzene contains an electron-withdrawing nitro group on one
azobenzene phenyl ring and an electron-donating amine (or derivative) group on the other
azobenzene phenyl ring. Such unique structural design expedites a “push-pull” electron
configuration where the delocalization of π electrons in the molecule will be even more facilitated
than the aminoazobenzene-type. This considerably decreases the energy bandgap between the
excited state and the ground state in the pseudo-stilbene-type azobenzene, resulting in a more
15

redshifted absorption peak in the UV-Vis spectra (blue curve in Figure 2.1.2). The absorption
wavelength of azobenzene, therefore, can be tuned from UV light to visible light, as long as the
modifications of the chromophores satisfy the requirements of organic synthesis [15].

Figure 2.1.2 – Standard chemical structures and UV/Vis absorption spectra of three azobenzene derivatives: basic azobenzene,
aminoazobenzene and pseudo-stilbene. Reprinted with permission from Chapter 2 Reference [12] © Royal Society of Chemistry.
Several possible mechanisms of the azobenzene photoisomerization between trans- and
cis-state are illustrated in Figure 2.1.3 [14]. The first rotational pathway begins with the transition
from the N=N bond to N-N bond to initiate the free rotation of trans-azobenzene. Rotation changes
the C–N–N–C dihedral angle while the C–N–N angle remains fixed at ~120°

[16]. In the inversion
mechanism, which remains fixed at 0° at first, one C–N=N angle starts to increase to 180°. At this
point, there is a transition state containing one sp hybridized azo-nitrogen atom. Then the C–N=N–
C dihedral angle starts to finish forming cis-azobenzene [17]. For the concerted isomerization,
both N=N–C bond angles increase to 180°, generating a linear transition state [18]. In the
16

inversion-assisted rotation, the beginning process is similar to the rotation mechanism where the
N=N double bond becomes N-N bond. After that, large changes in the C–N=N–C dihedral angle
and smaller but significant changes in the N=N–C angles occur simultaneously. The transition
state formed in concerted inversion has no net dipole moment, whereas the other three pathways
possess polar transition states [19]. To explain the actual experimental observations, it is advisable
to consider multiple pathways for the photoisomerization mechanisms.

Figure 2.1.3 – Proposed mechanisms for the trans-to-cis isomerization of azobenzene. Reprinted with permission from Chapter 2
Reference [14] © Royal Society of Chemistry.
It is intriguing to control the surface optical property by surface functionalization with
azobenzene molecules, as the oriented molecular monolayers can be reconfigured through the
external stimuli to affect the physical properties for the potential energy or electronics applications
[20]. A couple of examples are mentioned below, in terms of the surface functionalization on solid
substrates. A metal-organic complex is designed with a Re-bipyridine group, an azobenzene group,
and a carboxylic acid linker, as illustrated in Figure 2.1.4 [21]. The self-assembled monolayer of
this molecule, namely ReAzoC, was prepared using the Langmuir-Blodgett method to immobilize
17

on the oxide substrates. During the UV light exposure, up to 50% of the tethered ReAzoC
molecules can undergo the trans-to-cis photoisomerization.
Another example of chemically modified azobenzene is shown in Figure 2.1.5a. Moldt et
al. synthesized an azobenzene modified with a terminal alkanethiol group (Az11) to elongate the
molecular length to reduce the steric hindrance at the interface when photoisomerization occurs.
The azobenzene monolayer is tethered on a metal gold substrate. Besides, 1-dodecanethiol (C12)
was also introduced as a co-existing spacer group to further reduce the steric hindrance. In this
study, up to 74% of azobenzene on the gold surface can be tuned via trans-to-cis
photoisomerization, while the rest portion of the molecular transition was limited by the rapid
thermal decay back to the trans-isomer. Such fast thermal isomerization decay was demonstrated
by evacuating the sample under vacuum. In contrast, the natural decay under the ambient condition
will take more than 1 day [22].

Figure 2.1.4 – Molecular structures of ReAzoC in trans- and cis- isomeric conformations on the oxide substrate. Reprinted with
permission from Chapter 2 Reference [21] © American Chemical Society.
18

Figure 2.1.5 – (a) Chemical structures of alkanethiol-terminated azobenzene (Az11) and 1-dodecanethiol (C12); (b) Az11
monolayer on Au substrate with thermal isomerization at various conditions. Reprinted with permission from Chapter 2 Reference
[22] © American Chemical Society.
In the optical applications, azobenzene coating on the optical micro-devices can be
expected as a potential add/drop filter. Li and his coworkers have reported an example of using an
ethyl-orange azobenzene (EO) doped poly(vinyl alcohol) coating to deposit on the surface of
optical microsphere resonators (Figure 2.1.6) [23]. The EO/polymer-coated microsphere shows an
increase of about 8.45 µm in the device diameter compared to that in the bare silica microsphere.
The cavity resonant wavelength could be tuned by the 532 nm excitation laser, and the
experimental results indicate that the azobenzene-coated microsphere has a linear resonance
wavelength tunability up to 0.158 nm/(mW mm
-2
). The tunability is dependent on the EO
concentration and pump laser power densities, which agrees with the theoretical analysis. This
coating method results in the thickness of azobenzene in the micron order. Although it induces
19

apparent thickness change on the resonator, the device sensitivity and the quality factor of the
microresonator will be limited at this thickness scale. Hence, the improvement on the device
coating with organic photoswitches at monolayer scale is highly desired.

Figure 2.1.6 – Optical microscope images of the silica microsphere resonator (a) before and (b) after depositing with 2 wt% EO-
doped PVA coating. (c) Schematic illustration of the reversible photoisomerization of the EO molecule. Reprinted with permission
from Chapter 2 Reference [23] © Elsevier.
Topolancik and Vollmer’s team developed a photochromic protein bacteriorhodopsin (bR)
molecule to coat on silica microsphere resonators. This molecule can be excited at 532 nm, leading
to the trans-to-cis isomerization occurs at the C=N double bond where the trans-isomer loses one
equivalent of proton to form the cis-isomer. In this way, during the optical testing in Figure 2.1.7,
when the excitation of 532 nm on the photoswitch coating occurs, the probe wavelength 1550 nm,
originally propagating from Port 1 to Port 2, can be coupled into the coated-device to give optical
feedbacks from Port 1 to Port 3 [24]. This study could be further improved to become a potential
wavelength-dependent add/drop optical switch in the future.
20

Figure 2.1.7 – Top-view optical microscope images of a bR-coated silica microsphere positioned between two optical tapered fibers
containing four ports. (a) ambient illumination on the trans-bR-coated silica microsphere with a CCD camera. (b) The cis-bR-
coated microsphere excited with a pump 532 nm laser from Port 1. The image was also taken with a CCD camera. (c) IR image of
the trans-bR-coated microsphere pumped with the probe 1311 nm laser from Port 1. The photoisomerization working wavelength
532 nm was not pumped. (d) IR image of the cis-bR-coated microsphere pumped at both 1311 nm and 532 nm from Port 1. The
near-IR probe wavelength was coupled to the device to propagate to Port 3. Reprinted with permission from Chapter 2 Reference
[24] © AIP Publishing.
2.2 Whispering-Gallery Mode (WGM) Optical Microresonators
In the whispering gallery architectures at St. Paul Cathedral in London or Temple of
Heaven in Beijing (Figure 2.2.1), one can hear the whispering from the other at different locations.
Inspired by this unique phenomenon of whispering gallery, whispering gallery mode (WGM)
microresonators are one class of optical microcavities that can confine a certain wavelength of
light inside the optical devices [25]. In WGM microresonators, the optical wave can travel at the
periphery of the device, governed by the total internal reflection of the light wave when the incident
angle is greater than the critical angle in the higher index media (Figure 2.2.2) [26].
21

Figure 2.2.1 – Picture of whispering gallery architectures in the world: (a) St. Paul Cathedral in London and (b) Temple of Heaven
in Beijing.

Figure 2.2.2 – (a) Illustration of the WGM microresonator where light with resonant wavelength is confined and traveling along
the periphery of the resonator by total internal reflection. (b) Total internal reflection at the boundary interface with an incident
angle greater than the critical angle (θc) according to Snell’s law.
Various geometries of WGM microresonators are summarized in Figure 2.2.3, including
microspheres [27,28], microdisks [29,30], microtoroids [31,32], microrings [33,34], and so on. All
these types of microresonators have achieved ultra-high qualify factors (Q) at least 10
6
. The
microspheres have the highest average Q in the range of 10
9
, however, they lack the integration
capability for the practical integrated optics. The microtoroids and microdisks have Q factors in
the range of 10
8
. The Q of microring resonators, which are fully integrated on the wafer, can also
exceed 10
8
[35]. As a result, the WGM resonators have been studying both linear and nonlinear
optical processes for diverse applications, such as on-chip lasers [36,37], sensors [38,39], and
frequency comb generation [40,41].
22

Figure 2.2.3 – Rendering of common optical microcavity geometries: microcylinders, microspheres, microtoroids, microdisks, and
microring resonators. Reprinted with permission from Chapter 2 Reference [35] © The Optical Society.
The representative materials of microcavities are dielectric materials, such as silica [23,31],
silicon [42,43], silicon nitride [34,44], silicon oxynitride [32,45], etc. To enhance the versatility of
optical tuning, organic coating is desired to deposit on the top surface of the resonator cavities.
Here are two examples reported by Armani group to apply organic monolayer to surface
functionalize the optical microdevices.

Figure 2.2.4 – DASP-coated microsphere resonators containing nonlinear optical parametric oscillation behavior. Reprinted with
permission from Chapter 2 Reference [46] © American Association for the Advancement of Science.
Figure 2.2.4a demonstrated a monolayer of 4-[4- diethylamino(styryl)]pyridinium (DASP)
coated on a silica microsphere, with the thickness around 1.8 nm. DASP is a nonlinear organic
molecule and exhibits a nonlinear index (n2) of 2.54 × 10
-17
m
2
/W, three orders of magnitude larger
23

than that of silica [46]. The bare silica microsphere shows stimulated Raman scattering in Figure
2.2.4b, with the Q of 1.0 × 10
8
. However, when the DASP monolayer is attached to the silica
microsphere, the nonlinear phenomenon Kerr comb can be generated around the pump wavelength
1550 nm, despite that the Q of the DASP-coated device has decreased a bit to 0.7 × 10
8
. This
surface coating chemistry method presents a promising way of nonlinear optical switches.
Another example is by utilizing organic methyl or dimethyl groups to surface functionalize
silica microtoroid resonators to make organic stimulated Raman scattering lasers. In Figure 2.2.5,
the surface structures of bare silica, methyl-coated silica, and dimethyl-coated silica microsphere
resonators are listed. With methyl or dimethyl attached on the silica surface, the vibrational mode
of Si-O-Si plays important roles in enhancing the stimulated Raman scattering emissions. In other
words, given a specific polarization state, the laser threshold of the stimulated Raman scattering
will be drastically decreased in the coated devices than the bare silica devices [37].

Figure 2.2.5 – The surface coating chemical structures and the Raman emission intensities of (a) bare silica, (b) methyl-coated
silica, and (c) dimethyl-coated silica microsphere resonators at various coupled laser power and two different polarization states.
(d) Vector decomposition of the horizontal and vertical components of the Si–O–Si mode with respect to the toroidal cavity surface.
Reprinted with permission from Chapter 2 Reference [37] © Springer Nature.
24

The value of Q accounts for the performance of the optical resonators in confining photon
in the cavity, and governed by various optical loss mechanisms based on Equation 2.2.1 [28]:
𝑄
&'&()
*+
=𝑄
-(.
*+
+𝑄
00
*+
+𝑄
1(&
*+
+𝑄
2'3&
*+
+𝑄
2'45)
*+
=𝑄
63&
*+
+𝑄
78
*+
(2.2.1)
where Qrad is the radiation loss or bending loss due to the curvature of the resonator, Qss is the
surface scattering loss because of defects or surface roughness, Qmat is the material absorption loss
caused by the material from which a resonator made, Qcont is the contamination loss by
contamination of a device, and Qcoupl is the coupling loss due to coupling light into the cavity. In
addition, the first four variables (Qrad, Qss, Qmat, and Qcont) in Equation 2.2.1 are the intrinsic loss
(Qint) from the resonator, whereas the last term (Qcoupl) is extrinsic loss to the cavity (Qex) due to
coupling light into the resonators.

Figure 2.2.6 – Optical Q measurement setup consisting of a tunable narrow linewidth laser, tapered fiber, photodetector,
oscilloscope, function generator, camera, and a computer.
The schematic of Q measurements is shown in Figure 2.2.6. All experimental data and
images are recorded with a computer integrated with a function generator and an oscilloscope. The
tunable laser communication port (PCI GPIB) and a function generator are connected to a tunable
laser, and they are used to precisely tune the resonant wavelength of the resonator. An oscilloscope
with a high-speed digitizer collects the transmission data from a photodetector to calculate the Q
25

factor of the resonator. The tapered optical fibers are obtained by slowly pulling a single mode
optical fiber for each wavelength while it is heated with a hydrogen torch, followed by aligning
the taper to the toroid with 3-axis nano-positioners. The output from the tapered fiber is sent to the
photodetector.
In the actual optical experiments, loaded Q (Qloaded) can be obtained experimentally from
a transmission spectrum fitted to a Lorentzian according to Equation 2.2.2 [47]:
𝑄
)'(.7.
=
9
:9
(2.2.2)
where λ is a resonant wavelength of a resonator, Δλ is the full-width at half maximum value of the
resonant peak. Based on Equation 2.2.2, the Qloaded has a linear relationship with respect to the
coupling percentage. In other words, as the coupling percentage decreases, coupling loss decreases
causing the Qloaded increases. Therefore, by plotting the Qloaded as a function of the coupling
percentage and fitting linearly, Qint can be obtained at the y-intercept point, where the extrinsic
(coupling) loss becomes zero.
Due to the nature of high Q, the WGM resonator can substantially buildup the coupled
laser power, namely circulating power (Pcirc), within the microcavity according to Equation 2.2.3
[48]:
𝑃
26-2
=
9<
=>?
@
A
3B
C
(+EC)
A
𝑃
6354&
(2.2.3)
where Qint is the intrinsic Q factor, and K is defined by the ratio of intrinsic photon lifetime to
photon lifetime by coupling (K = Qint/QCoupl). Pcirc is directly proportional to the Qint of the devices,
indicating that resonators with the ultra-high Q factor act as an amplifier enhancing the circulating
power inside the cavities substantially.
The free spectral range (FSR) of a resonator is the spacing of the wavelength (or frequency)
between two successive fundamental mode peaks [49]. The modes which determine the FSR are
26

the sequential modes that have the same transverse mode, either transverse electric (TE) or
transverse magnetic (TM) mode.
𝛥𝜐=
2
H@3B
(2.2.4)
𝛥𝜆=
9
A
H@3B
(2.2.5)
where λ is the resonant wavelength inside the microcavity, n represents the refractive index, and R
is the radius of the resonator. Equation 2.2.4 is based on the frequency domain, while Equation
2.2.5 is in the wavelength domain. Using Equation 2.2.5, the theoretically calculated FSR can be
compared with the experimentally obtained FSR.
2.3 Organic Materials with Aggregation-Induced Emission
The fundamental studies of chemical and biological species tend to require the non-
invasive imaging tools to provide visual information at the material interface or in the living
systems at the molecular level. Photoluminescent fluorophores play important roles in monitoring
the interactions between the species and the surrounding environment. Indeed, many organic
luminophores show very different light-emitting behaviors in dilute and concentrated solutions
[50]. The conclusions drawn from the studies in the dilute solution, however, cannot commonly
be extended to the ones studied in the concentrated solutions. Typically, the luminescence of the
organic small molecule-based fluorophore is often weakened or quenched at high concentrations,
a phenomenon widely known as aggregation-caused quenching (ACQ) [51]. This detrimental
effect has been the major obstacle for the developments in the biological sensing and organic light-
emitting diodes applications [52,53].
Fortunately, scientists have come up with an alternative strategy to overcome the ACQ
issue. The aggregation-induced emission (AIE) effect has become a very effective strategy to
construct efficient solid-state emitters for applications in high performance optical materials,
27

optoelectronic devices and mechanochromic sensing. In the aggregate state, the intramolecular
rotation and the vibration of the fluorescent dye is greatly suppressed by the physical conformation
constraints or the interactions from the surrounding molecules, which restricts the non-radiative
decay channels and results in high luminescence [54–56].

Figure 2.3.1 – (top) Aggregation-caused quenching occurs in conventional organic fluorescent dye Fluorescein; (bottom)
aggregation-induced emission in tetraphenylethylene. Reprinted with permission from Chapter 2 Reference [57] © Royal Society
of Chemistry.
AIE typed fluorescent dyes show superior improvements in the fluorescent intensity in the
solid-state conditions. Shown in Figure 2.3.1, the conventional fluorophores, such as Fluorescein,
show stronger emission in solution state. However, the emission in the solid state is quenched due
to π–π stacking interactions in the molecules. As opposed to Fluorescein, tetraphenylethylene
(TPE) has a propeller-like molecular configuration with four phenyl rings conjugated via a C=C
bond, which leads to the restriction of intramolecular motion (RIM). Besides, such configuration
resists the π–π stacking interactions in the molecules. The contribution of these two effects turn on
the fluorescence of TPE upon aggregation formation. As a result, TPE is highly emissive in solids
[57]. Considering the fact that the photoactive materials are desired to be fabricated into devices,
the highly emissive fluorescence in aggregates is preferred in the molecular design stage.
28

Heteroatom-bridged AIE compounds have attracted much attention owing to their unique
electronic structures and optoelectronic properties. Here are some representative examples of
heteroatom-embedded organic small molecular AIE dyes summarized by Tang et al. (Figure 2.3.2)
[58]. The main contribution to the AIE effect of these luminogens is the restricted intramolecular
rotation. The circular arrows in Figure 2.3.2 indicate the molecular rotation which was suppressed
when the molecules were aggregated.

Figure 2.3.2 – Examples of heteroatom-containing AIE luminogens whose AIE phenomena are mainly caused by RIR process.
Reprinted with permission from Chapter 2 Reference [58] © John Wiley and Sons.
29

Figure 2.3.3 – Suzuki cross-coupling reaction on the synthesis of Th
4+
sensitive TPE. Reprinted with permission from Chapter 2
Reference [59] © John Wiley and Sons.
TPE-active organic small molecules can also be tuned with various functional groups
aimed for the identification of different species as the chemical sensors, particularly metal ions
[60–62]. As an example, thorium is a symbolic element of an actinide metal that has radioactivity
and is widely dispersed over the earth’s crust, and it often coexists with lanthanides and other
transition metals. Nonetheless, thorium is highly toxic due to its radioactivity, and human exposure
to thorium may increase the risk of lung cancer, bone tumors, and it is known to be mutagenic
[63]. Wen et al. synthesized a novel fluorescent sensor for Th
4+
by cross-coupling 2,6-
pyridinedicarboxylic acid (PDA) with TPE in Figure 2.3.3. This TPE-PDA sensor presents
excellent selectivity on detecting Th
4+
among lanthanides, transition metals, and alkali metals
under UV light (Figure 2.3.4) [59].

Figure 2.3.4 – (a) Fluorescence spectra of TPE-PDA (10 µM) at a water volume fraction of 30% upon the addition of 1 equivalent
of metal ions (excitation wavelength at 225nm); (b) image of the TPE-PDA (10 µM)/metal ions (10 µM) mixtures at taken under
UV illumination (excitation wavelength at 365nm). Reprinted with permission from Chapter 2 Reference [59] © John Wiley and
Sons.
30

In addition to synthesis of AIE small molecules by various coupling reactions, studies have
also explored the field of macromolecular AIE materials. In general, the AIE polymers can be
categorized based on the locations of the AIE effective groups. For instance, shown in Figure 2.3.5,
the AIE functional groups were embedded in the main backbones of the polymers. The
polymerization of the AIE luminogen monomers were conducted by the coupling reaction with the
vinylene linkages [64]. Because the intramolecular rotations become more difficult when the TPE
or other types of luminogens are connected by a polymer chain, the AIE effects have been
frequently observed in polymer systems.

Figure 2.3.5 – Examples of TPE-bearing conjugated polymers where AIE effective groups are located in the backbones. Reprinted
with permission from Chapter 2 Reference [65] © Elsevier.
Another conventional type of AIE active polymers is the pendant or side-chain AIE
polymer. In Figure 2.3.6, there is an example of the copolymer including the poly(N-
isopropylacrylamide) (PNIPAM) part and the AIE effective TPE polymer part. PNIPAM has
drawn much attention in recent years and has been extensively investigated due to its intriguing
thermo-responsive property. Such polymer undergoes conformational transition from the soluble
hydrated coil to the insoluble gel globule in water with lower critical solution temperature (LCST)
at around 32
o
C [66]. Therefore, this type of TPE-labeled PNIPAM copolymer can be potentially
used as a fluorescent thermometer with AIE mechanism [67]. To conduct the controlled radical
31

copolymerization, the alkene functionalized-TPE can be synthesized by the copper-catalyzed
alkyne-azide click reaction. The amount of incorporated TPE was tunable in a certain range.

Figure 2.3.6 – Examples of TPE-functionalized PNIPAM copolymers by controlled radical copolymerization. Reprinted with
permission from Chapter 2 Reference [67] © Royal Society of Chemistry.
2.4 Stretchable Polymers for Optical Micro-Devices
Materials are the fundamental chemical and physical components to formulate the
structures for different purposes. According to the chemical composition and material properties,
materials can be cataloged into metals, ceramics and polymeric materials. Here are some
representative scientific reports made by the inorganic optical materials in the past decades. Silicon
is one of the most ubiquitous inorganic materials that has been studied over the past few decades
with continuous innovation in the integrated photonics, due to its cost-effectiveness, small
footprint and the complementary metal-oxide semiconductor (CMOS) compatibility [68–70]. The
applications of silicon optics range from nonlinear optics [71], optical parametric oscillators [72],
optomechanics [73], sensing [74], and so on. Silica, a characteristic dielectric material, has been
recognized with remarkable optical transparency, or low optical loss, over a wide range of
wavelengths between the near infrared and the ultraviolet. Silica glass has been utilized as fibers
for worldwide optical telecommunications and as photomasks and lenses for microlithography
32

with ultraviolet light [75]. Many research efforts have been made to improve the optical
performance of silica glass. For instance, due to its symmetric structure and property, it does not
exhibit second order nonlinear optical performances. As a result, the structural asymmetry is
necessary to induce the second order nonlinearity [35,76]. In the field of surface chemistry, thanks
to the advance developments in the siloxane chemistry, the chemical modification on the silica
substrate or silica nanoparticles has enabled unprecedented applications, such as biosensing
[77,78] and photodynamic therapy [79,80].
GaAs is another typical semiconductor material that has been reported in the applications
for nanowire lasers [81], optical waveguides [82] and nanostructured solar cells [83] as it presents
an excellent optical transmission window across the entire solar spectrum. Graphene oxide is a
thin sheet of graphite that processes nonlinear optical properties [84], fluorescence over a broad
range of wavelength [85], and biosensing [86] due to its heterogeneous electronic structure by the
oxygen-containing functional groups [87].
There are some excellent attributes in the polymeric materials compared to inorganic
ceramics and metals, such as mechanical stretchability, light-weighted, cost-effective high-volume
manufacturing, the tunability of the structure-performance relationship, etc. There have been
enormous developments in optics with the use of polymeric materials in the past few decades [88–
92]. Furthermore, optical losses, an expression of the loss during the transmission of light, can be
significantly suppressed with the use of polymeric materials owing to their unique glass transition
states. Upon annealing the materials above their glass transition temperature (Tg), optical polymers
can be annealed to improve performance [89]. The most ubiquitous optical polymers are
poly(methyl methacrylate) (PMMA) [93], polystyrene (PS) [94], polycarbonate (PC) [95],
polyurethane (PU) [96], epoxy resin [97], etc. PMMA is one of the most studied organic optical
33

polymers owing to its refractive index of 1.489 [98] which is similar to that of silica glass (1.46)
[99]. With such similarity in the refractive index, PMMA is commonly referred to as “organic
glass”.
Zhi et al. studied PMMA/boron nitride nanosheets composites, demonstrating that even
when boron nitride nanosheets are intercalated into the PMMA matrix, the change in the optical
transmittance is almost negligible. The high optical transmission of PMMA/boron nitride
nanosheets composites is still above 90% for the wavelengths higher than 600 nm (Figure 2.4.1a).
On the other hand, the mechanical performances of the composites were improved from the blank
PMMA (Figure 2.4.1b,c) [100].

Figure 2.4.1 – Bare PMMA polymer vs PMMA/boron nitride nanosheets composites: (a) transmission at various wavelengths; (b)
elastic modulus; (c) mechanical strength. Reprinted with permission from Chapter 2 Reference [100] © John Wiley and Sons.
Liu and his coworkers discovered another intriguing property on PMMA. They found that
by annealing PMMA above its Tg (104 ℃), the annealing does not repair the rupture because the
high temperature also facilitates enlarging the crack. Healing by methanol is one way to heal, but
the healing process takes more than 1 day [101].
Figure 2.4.2 shows the optical transmission spectra of other optical polymers, such as
aliphatic polyester and aromatic polyarylate in the wavelength range of visible and near-IR. In the
range of ~500-1600 nm, both types of polymers exhibit high optical transmittance at least 90%.
34

At above ~1600 nm, polyarylate maintains high transmittance until 2500 nm, while the
transmittance of polyester gradually decreases to less than 50% [102].

Figure 2.4.2 – The optical transmission spectra of polyester and polyarylate at visible and near-IR wavelengths. Reprinted with
permission from Chapter 2 Reference [102] © Springer Nature.
On the other hand, there are still a few drawbacks in the utility of polymers. One of the
main issues working with polymeric materials is that the mechanical strength of the polymeric
materials is generally less competitive than other types of materials [103]. As a result, the resilience
or the lifespan of polymers needs to be improved before being considered as a stretchable or
deformable substrate.
To address this issue, researchers have come up with a smart solution by using the bottom-
up synthetic architecture to afford the polymeric materials that can heal damages from the
molecular level. When the cracks occur in the polymeric material systems, the macroscopic cracks
can be healed under certain conditions. More specifically, in the microscopic perspective, the
35

healing process occurs either by releasing the healing agents in the polymer media to cure the
ruptures, or by featuring weak dynamic molecular bonds, a series of preferentially sacrificial but
self-repairing chemical groups, into the polymer systems. Since the healing process does not
require the broken material interface to be jointed constantly by means of labor force, such unique
healable polymers are therefore termed “self-healing” polymers.
The first classic example of the self-healing polymers is the self-healing polymer
composites [104–106]. One of the most representative scientific reports is published by White et
al [107]. According to the authors, they designed a system consisting of the polymer media and
microcapsules where the healing agents named dicyclopentadiene can undertake ring-opening
metathesis polymerization with the Grubbs Catalyst (Figure 2.4.3a). Once there are cracks existing
in the materials, the healing agents will be released from the microcapsules (Figure 2.4.3b). With
the Grubbs Catalyst, the healing monomers will be polymerized to fill the voids caused by the
ruptures (Figure 2.4.3c). The major drawback in such self-healing polymeric systems is that the
curing reagents are generally one-time-use. When the healing reagents in the materials are used
up, it is impossible to heal the subsequent material failures anymore. This type of material system
greatly suppresses the longevity of the materials for further practical applications. Not only
because of the concentration limit of the healing agents in the polymer composites, but also the
fact to compromise the balance between the polymer performance and the healing fillers. Even
though this research is still of great significance as it is marked as the pioneer research in the self-
healing material field.
36

Figure 2.4.3 – Scheme of the self-healing process in the polymer composites. Red large circles indicate the microcapsules
containing reactive monomers. Smaller red circles indicate the catalysts for the monomer crosslinking reactions. (a) The crack
starts to break microcapsules; (b) The reactive healing monomers get released by the crack; (c) polymerization of the healing agent
undergoes to heal the crack. Reprinted with permission from Chapter 2 Reference [107] © Springer Nature.
To fulfill the repeatable performance of the self-healing process, nowadays, the second
type of the self-healing materials are of particular scientific interest. This type of polymers is also
termed as the intrinsic self-healing polymers where the use of dynamic bonds (DBs) are unique.
An overview scheme of the self-healing process on the DB-based polymers is shown in Figure
2.4.4 [108]. When the polymeric material films or gels were scratched or damaged by external
interactions, DBs were dissociated preferentially in advance to prevent the stress accumulation
around the crack, acting as a “sacrificial” role to protect the polymer backbones. This is due to the
fact that the reversible DBs are typically weaker than covalent polymer backbones.
37

With the external treatments (i.e., temperature, light, solvent, pH or humidity) to improve
the mobility of the polymer chains, the dissociated DBs can rejoin together in the rupture region,
leading to heal the rupture intrinsically [109]. In other words, the microscopic dynamic molecular
bonds will lead to the macroscopic self-healing phenomenon. In this way, intrinsic self-healing
polymers are the better candidates for multiple usages.

Figure 2.4.4 – Schematic illustration on the process of dynamic bonds association and dissociation to perform the intrinsic self-
healing polymers. Reprinted with permission from Chapter 2 Reference [108] © Royal Society of Chemistry.
An overview of various DBs in self-healing polymer gels is summarized in Figure 2.4.5
[108]. Noncovalent bonds include hydrophobic interaction [110], hydrogen bonds [111], host-
guest interactions [112], metal-ionic interactions [113], crystallization [114] and supramolecular
interaction [115]. These noncovalent bonds are representative examples to form self-healing
physical gels in the polymer media by the physical rearrangements of oligomers, polymer chains
or pendant groups alongside with the external mechanical deformations or damages. In the bottom
half of Figure 2.4.5, several characteristic dynamic covalent bonds are also listed, such as boronate
ester complexation [116], disulfide bond [117], imine bond [118], acylhydrazone bond [119] and
Diels-Alder reaction [120]. Contrary to the noncovalent bonds, these covalent bonds do form new
38

chemical bonds when the related chemical groups are rearranged and re-jointed; thus, such self-
healing gels can also be called chemical self-healing gels.

Figure 2.4.5 – An overview of dynamic bonds for implementing the intrinsic self-healing polymers. Reprinted with permission
from Chapter 2 Reference [108] © Royal Society of Chemistry.
Among the DBs mentioned above, hydrogen bonds are of great research interest due to
several reasons. First, the choices to form hydrogen bonds are not limited to certain atoms or
groups, as long as the hydrogen donors and the hydrogen acceptors are incorporated in the
molecules. There is flexibility in the molecular design of hydrogen donor and acceptor. In other
types of non-covalent or covalent bonds, the bond reaction can be achieved only by the required
functional groups. Therefore, for other common bonds, some mandatory groups or reactions need
to be synthesized into the molecules before the self-healing process can be considered initially.
On the other hand, owing to the flexibility in the chemical moiety of hydrogen bonds, the
hydrogen bonds can be utilized to achieve the reversible three-dimensional network structures with
39

physical gels. More specifically, most common DBs are used to obtain chemically reversible
networks where DBs result from the linkages between the crosslinkers and the polymer backbones
with synthetic chemistry. However, simply blending dual homopolymers bearing hydrogen bond
donors and acceptors without synthetic tactics might be an alternative strategy.
Wang et al. reported a layer-by-layer assembly of poly(acrylic acid) (PAA) and
poly(ethylene oxide) (PEO) on substrates to produce the polymer films that can conveniently heal
the damages and are capable of being erased from substrates on demand [121]. The self-healing
process was monitored with an optical microscope (Figure 2.4.6). To start with, PAA-PEO film
was scratched as shown in Figure 2.4.6a. The scratches in the polymer films can start to be healed
by immersing the films in acidic water (pH = 2.5) for 10 s (Figure 2.4.6b). After 20 min immersing,
more than half of the scratched areas were recovered (Figure 2.4.6c). Given sufficient immersing
in acidic water for 30 min, the apparent scratches will not be seen anymore (Figure 2.4.6d).

Figure 2.4.6 – The direct observation on the self-healing process of the PAA-PEO polymer film with an optical microscope. (a)
The scratched film; (b−d) the scratched film after being immersed in pH 2.5 water for (b) 10 s, (c) 20 min, and (d) 30 min. Reprinted
with permission from Chapter 2 Reference [121] © American Chemical Society.
40

Figure 2.4.7 – (a, b) Photos of the PAA-PEO film before (a) and after (b) being cut into two pieces. Photos of the 1 h healed PAA-
PEO film sample that was previously cut into two pieces, captured before (c) and after (d) being stretched to a 175% strain. (e)
Stress−strain curves of the intact and 1, 6, 12, and 24 h healed samples that were previously cut into two pieces. Reprinted with
permission from Chapter 2 Reference [122] © American Chemical Society.
The “cut-and-heal” process of the standalone PAA-PEO polymer film was further studied
as the proof-of-concept for the self-healing polymers [122]. In this case, the polymer film was
firmly cut into two pieces (Figure 2.4.7a,b), compared to the previous milder scratch studies as the
way it was damaged. The healing process took place when the sample was brought into contact
together and incubated in an environment with the relative humidity above 90% at room
temperature (Figure 2.4.7c). The appearance of the healed sample was quite similar to the one
before damage (Figure 2.4.7d). The quantitative mechanical analysis (tensile tests) of the healed
sample showed the stress-strain curves in Figure 2.4.7e. As seen, as the healing time increases,
both the maximum stress and strain will recover following the curve of the original sample. Given
sufficient healing time for about 24 h, the mechanical performance completely recovered as
original behaviors.

41

Figure 2.4.8 – (a) Schematic of the diffraction measurement, showing of a metal grating on PDMS substrate; (b) a photograph
showing the actual experimental apparatus for stretching and measuring the tunable gratings; (c) a series of the laser diffraction
spots (0th order and 1st order) presented by deforming the diffraction pitch from 0% strain up to 66.7% strain, then retreating the
deformation back to 0% strain. Reprinted with permission from Chapter 2 Reference [123] © Springer Nature.
Another example of stretchable polymer is polydimethylsiloxane (PDMS), which is a
silicone type polymer consisting of Si-O bonds with two methyl groups on Si. PDMS is
mechanically flexible and optically transparent across visible and near-infrared wavelengths. The
preparation of PDMS is readily straightforward by mixing the base monomer and the curing
catalyst to obtain a flexible elastomer. In Figure 2.4.8, PDMS was utilized as a flexible substrate
for a micro/nano-patterned metal diffraction grating [123]. The authors monitored the diffraction
spots of the metal/PDMS hybrid grating at the center 0th order and the following first-order
diffraction, while applying the mechanical deformation strain cycle from 0% unstretched state up
to 66.7% strain and then retreated to 0% strain. The diffraction distance between the 0th order and
1st order linearly decreases at first and then linearly recovers to the original distance.
42

2.5 Nanoimprint and Soft Lithography for Optical Micro-Devices
In the last two decades, nanoimprint lithography (NIL) has been recognized as an emerging
cost-effective technique for depicting micro/nano-order features, particularly on a soft and flexible
substrate. There are two kinds of NILs depending on the curing mechanism. The first report of
thermal-NIL was by Chou and his coworkers [124], and the first report of ultraviolet (UV)-NIL
was by Haisma and his coworkers [125]. The comparison between NIL and common
photolithography is shown in Figure 2.5.1 [126]. While applying a photoresist layer directly on
the wafer in photolithography, the resist can be applied as droplets of liquid resin to the wafer
surface in NIL. In the patterning step, the photolithography is going to use a photomask, which
contains micro/nano-patterns for the integrated circuits designs, to selectively expose UV light on
the photoresist. Depending on the chemical properties of photoresist, once the exposed photoresist
layer reacts with UV light, the solubility of the exposed photoresist layer will be changed compared
to unexposed photoresist layer. Then the development after the UV exposure can create the desired
patterning in the photoresist layer. As for NIL, a pre-made mask with the patterned features will
be applied on top of the resist, before curing the resist either by heat or UV light. Finally, the mask
will be demolded from the resist layer to obtain the micro/nano-patterned resist layer before
implementing reactive-ion etching to transfer the patterned features from the resist to the substrate.

43

Figure 2.5.1 – Scheme of NIL vs photolithography. Reprinted with permission from Chapter 2 Reference [126] © Canon.
A characteristic example of NIL developed by Wu et al. on the smoothed periodical
diffraction gratings with a pitch of 300 nm by NIL on silicon wafers. Figure 2.5.2a exhibits the
improved smoothness on the side walls of one-dimensional diffraction grating, in contrast with the
unsmoothed one in Figure 2.5.2b [127]. The authors processed with a V-shaped PDMS/curing
resist master template, and controlled the etching profile, duty cycles and the etching time to
improve the etching accuracy and smoothness.

Figure 2.5.2 – (a) Cross-sectional SEM image of one-dimensional grating fabricated with V-shaped Si master mold; (b) cross-
sectional SEM image of one-dimensional grating fabricated with the Si master mold without line edge smoothing. Reprinted with
permission from Chapter 2 Reference [127] © Springer Nature.
44

In addition to NIL, another promising strategy to fabricate micro/nanostructures is the soft
lithography. There are some similarities and differences between NIL and soft lithography. For the
similarities, in soft lithography, there is also a pre-prepared soft stamp considered as the master
mold to replicate the micro/nanostructures. The differences reside in the processes after the
molding. NIL will still use the reactive ion etching to transfer the patterned micro/nanostructures
onto the substrate wafer, and this must be performed in the cleanroom environment. On the other
hand, once the master is available, most of the fabrication tasks in soft lithography can be continued
outside a cleanroom with the use of only the replica transfer molding procedure. At the current
stage of development, soft lithography still depends on the use of photolithography (or e-beam
lithography) to generate the master [128].
Soft lithography offers access to a broader range of materials, as well as experimental
simplicity and flexibility in forming various patterns [129]. The availability of fabricating
relatively large devices, such as the microfluidic devices, can be conducted in a conventional wet
lab. This is especially helpful when the fabrication cost is a critical demand [130].
An overall fabrication protocol of the silicon grating master for soft lithography is shown
in Figure 2.5.3. A lift-off underlayer and a layer of UV nanoimprint resist are deposited on a silicon
wafer. The pre-prepared PDMS stamp with nanopatterned structures will be placed on top of the
coated wafer, in contact with the nanoimprint resist before conducting the UV curing. After the
resist is cured with stamped structures, the stamp will be demolded. A series of reactive ion etching
steps will be performed next to remove the residues of resist and underlayer in sequence. The
conformal deposition of a hard metal mask (i.e., Cr) can be applied via e-beam evaporation before
lifting of the underlayer. The Cr mask will be regarded as the protective layer for the reactive ion
etching on the Si substrate to obtain the final Si master.
45

Figure 2.5.3 – Fabrication scheme of Si master grating using UV-based NIL.
Here are a couple of examples of optical micro-devices fabricated using soft lithography.
The first case is the replicated soft optical microtoroid resonators replicated by the PDMS molding.
Figure 2.5.4 illustrated the process of such soft lithography [131]. To start with, an array of
microtoroid resonator devices were fabricated on a SiO2/Si wafer (Figure 2.5.4a) [31]. Then, the
obtain the microtoroidal stamp, the PDMS slurry was casted on top of the device master to cure
the PDMS stamp (Figure 2.5.4b). By backfilling with another batch of the PDMS slurry, curing,
and peeling off, the final polymer optical devices are formed (Figure 2.5.4c and Figure 2.5.4d).
The quality factor of PDMS devices can reach to the 10
6
range.

Figure 2.5.4 – (a) a master array of SiO 2/Si microtoroids on the wafer; (b) the microtoroidal master was coated to cure the PDMS
stamp; (c) backfill another PDMS batch and peel-off master; (d) top-down view of one microtoroidal PDMS resonator. Reprinted
with permission from Chapter 2 Reference [131] © The Optical Society.
46

Scheuer and his coworkers reported another example of using dual-step soft lithography to
make microring optical microresonators with quality factor around 3 × 10
6
[132]. In Figure 2.5.5a,
the first step was the fabrication of the master device by two-photon polymerization of SU-8 in the
focus of a fs-pulsed laser beam [133]. After obtaining the negative PDMS stamp, the first soft
lithography was conducted to form a backfilled non-crosslinking SU-8 replica microring device
(Figure 2.5.5b). Then, to reduce the surface roughness, a thermal reflow step (Figure 2.5.5c) was
performed. Finally, the second soft lithography step was implemented to replicate molding the
device with the UV-curable sol-gel (Figure 2.5.5d).

Figure 2.5.5 – (a) The master microring resonator fabricated using direct laser writing on crosslinked SU-8 (Grey). (b) First soft
lithography step of replication of the master device into non-crosslinked SU-8 (Yellow). (c) Thermal Reflow from (b) to reduce
the surface roughness. (d) Second soft lithography step of replication of the reflowed resonator into sol-gel (Blue). Reprinted with
permission from Chapter 2 Reference [132] © Springer Nature.

47

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63

Chapter 3. Tunable Photoswitchable Optical Microcavities with a
Self-assembled Monolayer of Azobenzene
3.1 Introduction
Photoactive materials have rapidly gained prominence in a range of fields including
photonics [1–4], plasmonics [5–7], electronics [8–10], and even drug delivery [11–13]. This broad
impact is possible because of the diversity of mechanisms available for light-triggerable materials
such as photoisomerization, photopolymerization, photo-induced ring-opening, and photo-
oxidation. Importantly, many of these material reactions are reversible. Traditional
photoswitchable organic functional groups include azobenzenes [14], spiropyrans [15],
diarylethenes [16,17], and a whole host of their derivatives.
Optical resonant cavities are a fundamental element of on-chip integrated optical circuits,
serving as amplifiers, filters, and buffers [18–24]. These devices have two key features that
differentiate them from other components: the ability to isolate and to store pre-defined or resonant
wavelengths (l0). In many cases, it is desirable to change the l0, for example, tuning an add-drop
filter or encoding an optical signal. Because the l0 is governed, in part, by the device refractive
index, a common strategy is to leverage the electro-optic effect [25–27]. However, many optical
cavities are fabricated from materials like silica with low to negligible electro-optic coefficients.
Additionally, while the electro-optic effect can achieve fast tuning over small wavelength ranges,
performing large shifts, comparable to the free spectral range of the cavity, is challenging. Recent
efforts have explored using the thermo-optic effect or the photo-acoustic effect to accomplish
larger range tuning [28–32]. However, these control mechanisms are susceptible to cross-talk with
adjacent optical components, thus decreasing the density of the optical circuit. Using the organic
64

photoswitchable materials could be an alternative strategy to achieve large wavelength range
optical spectrum tuning.
Azobenzenes have become the most ubiquitous, well-characterized, and implemented
organic photoswitch due to its simplicity as a photoswitchable molecule. Consisting of two
benzene rings joined by a nitrogen-nitrogen double bond, azobenzenes undergo a light-mediated
isomerization that is fully recoverable by either light or heat. Azobenzenes will typically
experience a trans-to-cis isomerization when exposed to UV or blue light and will revert to the
thermodynamically preferred trans isomer through application of heat or isolation in a dark
environment when decayed (Figure 3.1.1 (a, b)). The specific triggering wavelength or absorption
band of these transitions can be tuned by modifying the functional groups on either of the benzene
rings, and the refractive index of the azobenzene is dependent on the isomerization.
Given their predictable response, they are ideally suited to create an all-optically controlled
switch. However, fabricating a monolithic optical device comprised solely from azobenzene while
maintaining the photoswitching functionality is challenging. Although spinning coating of
azobenzene is one possible approach to introduce azobenzene on the optical device, there is the
thickness limit on the azobenzene coating with such method for thin coating. In this work, we
combine integrated photonics with optically switchable organic molecules to create an optically
controlled integrated device.
A silica toroidal resonant cavity is functionalized with a monolayer of an azobenzene
derivative. After functionalization, the loaded cavity Q is above 10
6
. When 410 nm or 450 nm light
is coupled into a resonance mode, the azobenzene isomerizes from trans-isomer to cis-isomer,
inducing a refractive index change. Because the resonant wavelength of the cavity is governed by
the index, the resonant wavelength changes in parallel. At the probe wavelength of 1300 nm, the
65

wavelength shift is determined by the duration and intensity of the 410 nm or 450 nm light and the
density of azobenzene functional groups on the device surface, providing multiple control
mechanisms. Using a CO2 laser focused onto the resonator, the molecule is reversibly flipped
between molecular conformations, inducing a refractive index change which results in a shift in
the resonant wavelength. The magnitude of the shift scales with the relative surface density of
azobenzene. To investigate reproducibility and stability of the organic monolayer, three switching
cycles are demonstrated, and the performance is consistent even after a device is stored in air for
6 months. Using this photoswitchable device, resonance frequency tuning is as far as 67% of the
cavity’s free spectral range in the near-IR is demonstrated. The kinetics of the tuning agree with
spectroscopic and ellipsometry measurements coupled with finite element method calculations.

Figure 3.1.1 - (a) Rendering of the trans state of the aminoazobenzene derivative free molecule and (b) the cis state of the molecule.
Blue light initiates the trans-to-cis conformation change, and heat reverts the process. (c) SEM image of coated microtoroid. (d)
Rendering of coated microtoroid, with the molecule enlarged for clarity in (a). Reprinted with permission from Chapter 1 Reference
[46] © The Optical Society.
3.2 Azobenzene Characterization
Several characterization measurements are performed on the aminoazobenzene (Aazo)
compound before and after surface attachment to verify the photoswitchable behavior and the
optical properties. The optical absorbance of a 39.5 𝜇M Aazo solution in tetrahydrofuran is
66

measured from 300 nm–1400 nm before and after exposure to 450 nm laser light (~1.6 W/cm
2
).
As seen in the spectra (Figure 3.2.1(a)), the absorption maximum occurs at 450 nm and the
absorption value decreases after exposure due to the π–π* transition of the molecule [33].
Additionally, there is minimal absorption in the near-IR in either state.

Figure 3.2.1 - (a) UV-Vis absorption spectra of both trans and cis isomer states. The wavelengths used to initiate photo-
isomerization in the present work are indicated by vertical lines. (b) Spectroscopic ellipsometry results over the spectral range of
the 1300 nm probe laser. (c) Measured refractive index for each step: before exposure, after exposure, and after heating for different
time periods. Reprinted with permission from Chapter 1 Reference [46] © The Optical Society.
The optical response of the bound Aazo layer is analyzed using spectroscopic ellipsometry
from 1000–1700 nm (V-VASE, J.A. Woollam Co.). The refractive index is determined using a
five parameter Sellmeier equation and the fits agree well with literature. The results, zoomed in
around the probe wavelength of 1300 nm, are shown in Figure 3.2.1(b). The scans show a 0.0023
decrease in the refractive index of the control wafers after exposure to 450 nm light, indicating a
trans-to-cis isomerization. The method used to attach the monolayer to the wafer is detailed in the
next section.
67

Furthermore, the time dependent kinetics were also measured using ellipsometry on a
separate coated and uncoated silica wafer (Figure 3.2.1(c)). The refractive index at 1300 nm was
measured before and after a one-minute exposure to 450 nm laser light and at distinct time points
with heat being applied via a hotplate. Noticeably, adding heat accelerates the recovery process,
reducing the time to full recovery to about 6-8 hours from over 24 hours without heat, it is still a
considerably slow process. As expected, an uncoated silica control wafer showed little to no
variation in refractive index when undergoing the same thermal treatments.
The covalent attachment and optical response of the Aazo layer is characterized using two
methods. The surface chemistry is first confirmed by X-Ray photoelectron spectroscopy (XPS).
All samples were prepared on SiO2/Si wafers and stored in a desiccator before measurements to
minimize the contaminations from the moisture. XPS data was taken using an AXIS Ultra
photoelectron spectrometer (Kratos Analytical Ltd) equipped with Al Kα X-ray source (1486.6
eV). Survey scans were performed 5 times for each sample across 1200–0 eV with the analyzer
pass energy of 160 eV. For N 1s, high resolution scans were conducted 15 times for each sample
ranging 410–390 eV with the analyzer pass energy of 40 eV. The XPS results in Figure 3.2.2
verified the success of the surface functionalization of Aazo on the SiO2/Si wafers. For the initial
sample, there are O 1s peak at 530 eV, C 1s peak at 282 eV, and Si 2s peak at 152 eV. For the
CMPS-functionalized sample, there is an additional Cl 2p peak at 197 eV, showing the success of
the CVD process. Lastly, for the Aazo-coated sample, a small N 1s at 396.7 eV was confirmed by
a high-resolution scan, demonstrating that Aazo is functionalized on the control substrates. The
trend of peak intensities correlates to our previously reported work.
68

Figure 3.2.2 - XPS results verifying the two steps in the surface functionalization process. The Cl 2p which is present in both CMPS
samples indicates successful silanization, and the N 1s indicates successful attachment of the Aazo group. The C 1s is likely due to
the carbon tape used to hold down the coated wafer sample. Reprinted with permission from Chapter 1 Reference [45] © The
Optical Society.
3.3 Computational Modeling
The resonance shift due to the isomerization of the attached molecule on the surface will
be due to two dominant effects: 1) a local refractive index change due to a change in the Debye
potential between isomers and 2) a slight change in device radius as the molecule changes size
from one isomer to the next. Mathematically, we can express as Equation 3.3.1:
𝛥𝜆=𝜆K
:3
3
+
:B
B
L (3.3.1)
where Δl is the change in resonant wavelength, l is the resonant wavelength, Δn is the change in
refractive index, ΔR is the change in cavity radius, n is the effective refractive index and R is the
cavity radius. Upon analysis, it becomes evident that it is the relative, not absolute, change in n or
R that governs Δl. Therefore, the larger the device size, the more difficult it is for ΔR to play a
dominant role in inducing a wavelength change. In contrast, Δn is diameter independent. Previous
work demonstrated photoswitching through geometric modulation (ΔR). However, this approach
required extremely thick layers of azobenzene which degraded the Q factor [34]. Other interesting
69

work has leveraged the switching ability of bacteriorhodopsin layers integrated onto silica
microspheres to produce switches and even realize photonic circuitry [35–37].
To calculate a theoretical resonant wavelength shift, both the ΔR and Δn terms must be
considered. ΔR is simply the change in the Aazo length (0.3-0.5 nm). Thus, the ΔR/R term is
between 1E-5 to 3E-5. To calculate the index change, density functional theory (DFT) modeling
of the molecular layer must be combined with finite element method modeling results of the optical
mode. This approach allows for the effective refractive index (neff) term to be approximated
according to Equation 3.3.2:
𝑛
7NN
=𝛼𝑛
2(P6&Q
+𝛽𝑛
)(Q7-
+𝛾𝑛
(6-
(3.3.2)
where 𝛼, 𝛽, and 𝛾 represent the portions of the optical field inside the cavity, inside the layer of
Aazo, and outside the cavity, respectively [38]. These values can be obtained using finite element
method (FEM) simulations. The refractive index of the layer in the cis and trans states is
approximated using DFT, and the value for silica is taken from the COMSOL library.
To gain insight into the relative magnitude of the ΔR/R and Δn/n terms, DFT calculations
were performed using Q-Chem 5.1 software (Q-Chem, Inc.). The gas phase ground state molecular
geometry was optimized at the B3LYP/6-31G** level of theory [39,40] for two pairs of free
standing Aazo molecules before calculating their dynamic polarizabilities at 1300 nm by time-
dependent DFT. In this study, Aazo and CMPS-Aazo in both trans- and cis-isomers were
considered in Figure 3.3.1 (a,b). A similar molecule, 4-[4-(N,N-dimethylamino
phenyl)azo]pyridine (MAP), was used as a standard. MAP has the exact structure as Aazo except
for two methyls on the amine group.
70

Figure 3.3.1 - (a)-(d) Renderings of the different Aazo molecules simulated using DFT along with the different isomerization states.
Reprinted with permission from Chapter 1 Reference [45] © The Optical Society.
Table 3.3.1 - Results from DFT calculations.
Type of Aazo Polarizability/Å
3
Length/Å N/Å
-3
n L Δn
trans-Aazo 110.576 13.33 1/219.474 2.708
0.201
cis-Aazo 92.609 9.60 1/220.261 2.507
trans-CMPS-Aazo 213.677 20.68 1/395.745 2.790
0.186
cis-CMPS-Aazo 182.415 15.26 1/396.532 2.604
The length of each molecule was estimated by measuring the distance between the two
ends of each Aazo in the optimized ground state geometry. From Table 3.3.1, the length of trans-
Aazo and trans-CMPS-Aazo are 13.33 Å and 20.68 Å, respectively. The length of MAP and
CMPS-MAP are previously reported as 10 Å and 18 Å, respectively [41]. This shows that the sizes
of MAP and Aazo are very similar, further providing support for using MAP as the reference
molecule.
Because the radius of the device is unaffected by the circulating optical field at low input
powers, the only contributor to the ΔR/R term is the change in the length of the molecule. Based
on the calculations, as the molecule switches from trans- to cis-, the length change (ΔR) is ~3-5 Å.
As mentioned previously, the devices used in the present work have radii on the order of 20–30
𝜇m. Therefore, the ΔR/R contribution to Δl will be approximately 1E-5.
71

The index of refraction for each Aazo was estimated by combining the DFT results with
the Lorenz Model [42] that describes the relationship between polarizability and index of refraction
in Equation 3.3.3:
𝑛
T
= U1+4𝜋𝑁𝑝 (3.3.3)
where 𝑛
T
is the calculated refractive index of the specific Aazo, 𝑁 is the average number of Aazo
molecules per unit volume, and 𝑝 is the mean dynamic polarizability at 1300 nm in all directions.
𝑁 was calculated as the inverse of unit Aazo volume which was estimated using Connolly solvent-
excluded volume.
Based on the results calculated using Equation 3.3.3 and summarized in Table 3.3.1,
several important conclusions can be drawn. First, as the molecule photoswitches from trans- to
cis-, the refractive index decreases, and the Δ𝑛 is approximately 0.2. This decrease will induce a
blueshift in 𝜆. However, to calculate the Δ𝑛/𝑛 term, it is necessary to determine the strength of
the optical field interaction with the photoswitchable molecular layer.
The optical field distribution in the toroidal optical cavities is modeled using FEM software
(COMSOL Multiphysics v5.4). The simulations are performed at 1300 nm. The mesh size in the
simulations is < 𝜆/10 to assure accuracy. The refractive index of silica is set to literature values of
1.4469 (1300 nm). The refractive index of Aazo is set to values as determined by the DFT as
mentioned previously. The device geometry (major, minor diameters, disk thickness, and edge
angle) is determined by measuring the specific device tested using optical microscopy and SEM.
Based on these parameters, the distribution of the optical field in the devices (Figure 3.3.2) is
determined by controlling the azimuthal mode order (M) in the cavity and the relative amount of
the field in the resonator, Aazo layer, and air is determined by finding the mode volume in each of
the respective portions of the physical system. Additionally, cavities with varying major and minor
72

radii are modeled to determine any significant changes in mode volumes due to the isomerization,
as seen in Figures 3.3.2 (c)-(f). The total mode volume values largely agree with previously
published reports [43]. As expected, there is not a large change in total mode volume due to the
change in the thin molecular layer. However, there is an appreciable change in the mode volume
within the thin, higher index layer. In the trans state, more of the optical mode resides in the thin
layer due to the larger refractive index, Figures 3.3.2 (d) and (f). However, the actual change is
small compared to that of the entire cavity system, ensuring that the majority of the optical mode
remains relatively unperturbed, allowing the device to maintain a high Q. Based on these values,
it is anticipated that the Δ𝑛
7NN
/𝑛 term will vary between 4.1 × 10
-4
and 5.6 × 10
-4
.

Figure 3.3.2 - (a), (b) COMSOL plots of optical mode profiles of both trans and cis states for a device with major and minor
diameters of 50 𝜇m and 7 𝜇m. (c) Simulated total optical mode volume and (d) mode volume in the thin Aazo film as a function of
minor radii for both cis and trans states. The major radius was fixed at 25 𝜇m. (e) Simulated total optical mode volume and (f)
mode volume in the thin film as a function of major radii for both cis and trans states. The minor radius was fixed at 3.5 𝜇m. For a
given geometry, the trans state has a larger interaction with the optical field than the cis state. Reprinted with permission from
Chapter 1 Reference [46] © The Optical Society.
73

3.4 Device Fabrication
The fabrication of silica microtoroids is shown in this section. Silicon wafers with 2 𝜇m of
thermal oxide are cleaned thoroughly with acetone, ethanol, and isopropanol, then treated with
hexamethyldisilazane (HMDS, from Sigma Aldrich) via vapor deposition to improve the adhesion
between the wafer and the photoresist. Then, Shipley 1813 photoresist is spin-coated on all wafers
with a spinning speed of 500 rpm for 5 seconds followed by 3000 rpm for 50 seconds. The wafers
are soft baked at 95 °C for 2 minutes. The dose of UV exposure used in the photolithography step
is 80 mJ/cm
2
, and the diameter of the circular pads patterned is 150 µm. Then, Microposit MF-321
developer (from DOW) is used to remove the exposed photoresist. After thorough washing with
DI water and drying by a nitrogen air gun, samples are hard baked at 120 °C for two minutes.
Buffered oxide etchant (from Transene Co.) is used to etch the samples. The complete,
exposed oxide layer is etched. It takes about 20 minutes to etch the 2 𝜇m thick SiO2 layer. After
BOE etching, the remaining photoresist on the wafer is removed using acetone, isopropyl alcohol,
and DI water. The wafer was baked and followed by the pulsed XeF2 dry etching is used to etch
the silicon substrate to undercut the SiO2. Before starting etching, the samples are cleaned and
dried to avoid any contamination. The etching pressure is 2800 mTorr. Each XeF2 pulse lasts for
80 seconds. The total number of pulses is adjusted according to the sample loading in the chamber.
The last step is to smooth the surface of the devices using a high-power CO2 laser
(SYNRAD 48-2KAM) to reflow the cavities. The maximum output power is 25 W. For SiO2
cavities, we can either increase the power gradually or set the output power to a fixed amount and
pulse the laser intensity. After reflowing, the diameter of the cavity shrinks to about 60 𝜇m with a
minor diameter around 6 𝜇m.
74

Figure 3.4.1 - Rendering of surface functionalization scheme showing attachment of the linker molecule (CMPS) with subsequent
Aazo attachment. Reprinted with permission from Chapter 1 Reference [45] © The Optical Society.
An overview of the process to form the organic photo-responsive monolayers is shown in
Figure 3.4.1. The initial silica microcavities are first treated by O2 plasma to generate a dense layer
of hydroxyl groups on the silica surface. Then, a [4-(chloromethyl)phenyl]trichlorosilane (CMPS)
coupling agent (Sigma, 97%) and a trichloromethyl silane (TCMS, Sigma) are deposited on the
surface of the plasma-treated silica microtoroids using chemical vapor deposition at room
temperature for 8 min, yielding a grafted microcavity containing CMPS and TCMS [44]. TCMS
offers the methyl spacer molecule to give the 4-(diethylamino)azobenzene (Aazo, Sigma)
molecules more space on the surface to isomerize without steric hinderance. The [CH3:Aazo] ratio
used in this chapter was 1:1, 3:1, 5:1, 7:1 and 10:1 (v:v). Faster switching can be achieved by
lowering the overall steric hinderance to the Aazo molecules on the surface by increasing the
amount of spacer molecules. This, however, will come at a cost of tuning range and sensitivity to
input pump powers. The utility of spacer molecules to optimize isomerization and switching times
has been studied previously [45,46]. The Aazo solution (~8.0 mM) in tetrahydrofuran is then drop-
casted onto the grafted microcavities, followed by heating to 110 °C under vacuum for 20 min.
The devices are cooled to room temperature, rinsed thoroughly with chloroform and acetone, and
dried under vacuum at 110 °C for 2 min, yielding a grafted ~2 nm Aazo layer on the surface [47].
Control samples consisting of Aazo-functionalized SiO2 on Si wafer are prepared using the same
procedure for spectroscopic analysis.
75

3.5 Testing Setup for Optical Characterization
This section will discuss the designs and developments of the testing setups for the optical
characterization of Aazo-coated microcavities, as well as the issues occurred during the
developments and how to address the problems.
The first version of the optical testing setup is shown in Figure 3.5.1. In the beginning of
this research project, the switching light sources were implemented with a pair of tunable 450 nm
diode laser (Thorlabs, 15 mW) and 405 nm lamp (Dymax). 450 nm was used to induce the
photoisomerization of Aazo for the molecular switching, and 405 nm is to perform the heating to
recover the cavity wavelength shifts. A couple of drawbacks on this design were raised from the
following aspects.

Figure 3.5.1 – The first version of the testing setup for the optical characterization on Aazo-coated Silica microtoroid devices. It
consists of a tapered optical fiber to couple the probe wavelength 1300 nm, a tunable 450 nm blue laser to conduct Aazo
photoisomerization, a 405 nm lamp to provide heating and a sideview camera.
Firstly, 450 nm laser is not optically coupled into the Aazo-coated cavity. Instead, the 450
nm laser was illuminated with an incident angle on top of the cavity, which substantially reduces
the effective coupling of the photoswitching light into the Aazo-coated cavity. Secondly, although
the use of 405 nm is initially intended to heat the Aazo-coated device to erase the cavity resonant
wavelength shift, 405 nm is not an ideally wavelength for heating the Aazo-coated as the
76

absorption range of Aazo covers at 405 nm (Figure 3.2.1), which is intended to induce the Aazo
photoisomerization. Therefore, for the following revision on the testing setup, another wavelength
that does not crosstalk with the absorption range of Aazo is considered necessary. Besides, in this
initial setup, the side-view camera is close to the chip stage. In this case, the light or the heat
generated from the camera may interfere with the photoisomerization or the decay process of
surface functionalized Aazo. Therefore, the positions of the optical components were required to
be realigned.

Figure 3.5.2 – The revised optical device characterization. (a) Optical device characterization testing setup and photoisomerization
system with all components labeled. (b) Example of a Q spectrum at 1300 nm. Inset is a side view optical microscope image of an
Aazo-coated device coupled by an optical taper. (c) Example of a Q spectrum at 410 nm. Reprinted with permission from Chapter
1 Reference [45] © The Optical Society.
The improved and current version of the testing setup is shown in Figure 3.5.2. The two
fiber coupled lasers were combined into a single fiber using a 90% (410 nm or 450 nm):10% (1300
nm) coupler. The output from the coupler was sent into a tapered optical fiber waveguide capable
of simultaneously coupling 1300 nm and 410 (450) nm light into the cavity. On the other hand,
using a single waveguide resulted in a decreased efficiency at both wavelengths: approximately
15% at 410 (450) nm, and about 30-40% at 1300 nm. This is because when using a single taper
for both wavelengths, it is required to keep the taper at a certain coupling position for those
wavelengths; thus, it is not optimal for either wavelength. The optical taper waveguide was
fabricated by pulling a coating-stripped optical fiber (single mode fiber F-SMF-28, Newport) on a
77

two-axis stage controller (Sigma Koki) while being heated by a hydrogen torch. The coupling
distance between the taper and the microtoroid cavity was precisely controlled by a 3-axis
nanopositioning stage. Photodetectors (Thorlabs) were used to detect the optical signals,
displaying each signal on an oscilloscope processed with a high-speed digitizer [48].
Previous work demonstrated that by adding thermal energy to the system, it is possible to
accelerate the reverse isomerization [49,50]. Therefore, to assist in the reverse isomerization, we
use an alternative light source, CO2 laser, to indirectly heat the molecule on the device. The 10.6
𝜇m CO2 laser (48-1KAM, Synrad) was guided by a gold coated copper mirror (Electro Optical
Components, EOC) and focused by a ZnSe lens (Thorlabs) onto the microtoroid samples. The CO2
laser is highly absorbed by silica, generating thermal energy. As a result, the resonant wavelength
shifts due to a combination of the thermo-optic effect of silica (approximately increase 4.64 °C
according to the resonant wavelength shift from the thermo-optic coefficient equation [51]) and
the molecular switching. Therefore, the cavity must thermally equilibrate in the dark to completely
return to its initial position. The incident power used from the CO2 laser was approximately ~0.22
W with a spot size of approximately 100 𝜇m, which is over an order of magnitude lower than the
power used to reflow the cavity.
All optical cavity measurements are performed at room temperature under ambient
conditions (room temperature, pressure, air) with one exception. All measurements were
performed in the dark, to reduce the effect of room light on the Aazo group. When not being tested,
all devices are stored under ambient conditions. To reversibly switch the Aazo between trans and
cis isomerization states, first, the blue 410 or 450 nm laser initiated the photoswitching behavior
from trans-Aazo to cis-Aazo. The 1300 nm resonant wavelength was tracked in real-time as soon
as the blue laser started to couple light into a microtoroid sample.
78

After resolving the issues on designing the practical optical testing setup, another issue
appeared during the measurements. From Figure 3.5.3, by merely loading the control sample, the
methyl-coated silica microtoroid device, the cavity resonant wavelength Δλ shifts along with the
temperature fluctuation. This was found consistent across different microcavity devices and testing
days until the room temperature fluctuation was resolved. The slight offset between Δλ and room
temperature was due to the stabilization of the resonant cavity. Therefore, it is crucial to exclude
the impact of temperature fluctuation around the testing setup and the microtoroids. This also
indirectly proved the ultrahigh sensitivity of environmental temperature with ultrahigh-Q based
microcavity devices.

Figure 3.5.3 – (a) The fluctuation on the room temperature of the lab space where the optical testing setup was located; (b) the
cavity resonant wavelength Δλ 1300 measured at 1300 nm with respect to the testing time. The testing device was a methyl-coated
silica microtoroid with Q 0 = 3.9 × 10
6
. (a) and (b) were experimentally conducted at the same time. (c) The fluctuation on the room
temperature of the lab space where the optical testing setup was located, tested on another day; (d) Δλ 1300 measured at 1300 nm
with respect to the testing time. The testing device was a methyl-coated silica microtoroid with Q 0 = 6.6 × 10
5
. (c) and (d) were
experimentally conducted at the same time.
79

3.6 Reversible Photoswitching
In this section, reversible photoswitching of the covalently attached azobenzene will be
presented in detail. The first azobenzene, 4-[2-(4-Methoxyphenyl)diazenyl]pyridine (MeOAzo),
was used in the initial preliminary study. Aazo-coated SiO2 microcavities will be discussed with
quality factor characterizations and the reversible photoswitching in the lateral part.

Figure 3.6.1 – Chemical structure of MeOAzo.
MeOAzo (Figure 3.6.1) was obtained from our previous lab member Dr. Xiaoqin Shen.
His synthesis of MeOAzo was conducted according to a previously reported literature [52]. The
surface functionalization of MeOAzo on silica microtoroid resonators were performed in a similar
manner in Figure 3.4.1. The MeOAzo and methyl spacer was deposited by 1:1 (v:v). The MeOAzo-
coated device was tested by coupling with the tunable 410 nm laser for first 10 min, followed by
leaving the device in dark for additional 40 min. The cavity resonant wavelength shift Δλ and the
room temperature fluctuation were shown in Figure 3.6.2. Δλ continued to redshift throughout the
whole experiment where the device was interacted with 410 nm and then left under dark. At the
same time, the room temperature decreased slightly when the 410 nm was coupled to the device.
Under dark, the room temperature was relatively stable. Due to the redshifted net Δλ, It can be
concluded that the photoisomerization of MeOAzo on the microtoroid will not induce the blueshift
of Δλ. In the rest of this section, Aazo will be discussed.
80

Figure 3.6.2 – (a) The cavity resonant wavelength Δλ 1300 measured at 1300 nm with respect to the testing time. The testing device
was a MeOAzo-coated silica microtoroid with Q 0 = 2.3 × 10
5
. (b) The fluctuation on the room temperature of the lab space where
the optical testing setup was located. (a) and (b) were experimentally measured at the same time.
3.6.1 Quality Factor Characterization
In terms of the coupling mechanisms at varied wavelengths, at 1300 nm, light from a
tunable, narrow linewidth laser (Velocity series, Newport) is coupled evanescently into the cavity
[53]. The transmission vs 𝜆 spectra were fit to a Lorentzian, and the loaded Q1300 was calculated
by Q = 𝜆/𝛿𝜆, where 𝛿𝜆 is the spectral linewidth. By varying the coupling strength, the intrinsic
Q1300 (Q0,1300) was calculated using a coupled cavity modeled.
At 410 nm or 450 nm, light from a tunable, narrow linewidth laser (Velocity series,
Newport) is coupled into the optical cavity. However, because phase and index matching criteria
are unable to be continuously met from 400–1300 nm, it is not possible to evanescently couple at
both wavelengths in a single taper. Therefore, a different coupling mechanism is used for the blue
81

lasers, specifically, Rayleigh scattering. The loaded Q410 (Qloaded,410) was calculated using the same
method described previously. Unfortunately, using this coupling mechanism, it is not possible to
vary the coupling strength. Therefore, an intrinsic cavity Q at 410 nm cannot be determined.
In the present system, the thermal damage threshold of the organic monolayer is governed
by the bond strength between the benzylchlorosilane linker molecule and the oxygen groups on
the surface. The thermal degradation of a typical siloxy bond begins at 200 °C. This roughly
corresponds to a thermal damage threshold of 200–300 °C. For comparison, the damage threshold
of silica is about 1100 °C [54].

Figure 3.6.3 - Optical cavity quality factors. (a) Q 0,1300 and (b) Q loaded,410 of Aazo-coated devices with different ratios. Each data
point is a unique device, and the error is the error in the fitting of the linewidth. (c) Q 0,1300 and (d) Q loaded,410 with respect to the
input power of 410 nm laser. The measurements in part (c) and (d) were performed on the same Aazo-coated SiO 2 microtoroid
device. Reprinted with permission from Chapter 1 Reference [45] © The Optical Society.
Table 3.6.1 - Q 0,1300 and Q loaded,410 of [CH 3:Aazo = 10:1] device on different Aazo isomers.
Wavelength/nm Aazo Isomer Q 0,1300 or Q loaded,410/×10
6

1300 trans 7.68 ± 0.05
1300 cis 7.40 ± 0.03
410 trans 3.14
410 cis 2.93
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Both Q0,1300 and Qload,410 were determined on three different devices for each coating ratio.
The Q0,1300 are plotted in Figure 3.6.3(a), and the values range from ~10
6
to ~10
7
. The Q0,1300 values
of the control device (CH3 coated) are similar to previous works using silanization processes
[55,56]. On the other hand, the Qloaded,410 of various devices were plotted in Figure 3.6.3(b), ranging
from ~10
5
to ~10
6
. For all devices, the Qloaded,410 is less than an order of magnitude lower than the
corresponding Q0,1300 on the same device. The relatively small variations between various Aazo-
coated and CH3 control devices are evidence of the uniformity of the molecular surface
functionalization at both Q0,1300 and Qloaded,410. Table 3.6.1 summarizes the quality factors of
[CH3:Aazo = 10:1].
Additionally, given the important role of Q at 410 nm and at 1300 nm both in inducing and
in detecting the switching behavior, the dependence of Q as a function of 410 nm input power was
also investigated. As can be observed in Figure 3.6.3 (c) and (d), the quality factor at both
wavelengths is independent of the input power of the 410 nm laser over the range used in this
work. The input power at 1300 nm was held constant throughout the measurements.
The Qloaded,410 exhibits at least 10
5
, indicating high Q modes are excited in our whispering
gallery mode system, allowing for a resonantly enhanced absorption of 410 nm light by the Aazo
monolayer on-device. However, given these Q values and the observed low coupling efficiency,
the coupling losses are the dominant losses in this system. Therefore, due to the uncertainty in
coupling losses at 410 nm, the conventional approximation that K = 1 should not be applied by
default. Nonetheless, the loaded Q values at 410 nm are less than an order of magnitude lower than
the intrinsic Q values at 1300 nm. Given this data, the coupling losses cannot be significant.
Therefore, for the Pcirc calculations in the following contents, the conventional approximation was
applied.
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3.6.2 Reversibility and Sensitivity
The reversibility and sensitivity of Aazo-coated silica microtoroid resonators were
systematically studied. The experimental design was shown in Figure 3.6.4. Aazo photoisomerizes
from the thermodynamically stable trans-isomer to thermodynamically unstable cis-isomer at 410
nm and reverts to the trans state by heating with a 10.6 𝜇m CO2 laser.

Figure 3.6.4 - (a) SEM image of Aazo-coated microtoroid resonant cavity. (b,c) Rendering of the region indicated by the box in (a)
showing the reversible Aazo trans to (c) cis photoisomerization process. Reprinted with permission from Chapter 1 Reference [45]
© The Optical Society.
First, the entire photoswitching cycle is shown in Figure 3.6.5(a) for the [CH3:Aazo = 10:1]
device with the three phases indicated. Initially, the 410 nm laser is coupled into the device. This
initiates a trans-to-cis Aazo photoisomerization, inducing a blue-shift in the resonant cavity
wavelength (𝜆1300) of approximately 7.8 pm (Pcirc = 420.86 mW). It took around 10 min to stabilize
the blueshift. This timeframe is significantly longer than in solution; however, the two results
should not be directly compared for a couple reasons. First, the laser excitation wavelength and
the incident optical power are different in the two measurements. Second, the molecules are
anchored to the device surface, which reduces their mobility.
84

Figure 3.6.5 - Tracing 𝜆 1300 shifts on two different [CH 3:Aazo = 10:1] Aazo-functionalized optical cavities upon coupling 410 nm
laser. (a) One cycle of reversible 𝜆 1300 shift. When the device was coupled by the 410 nm light (P circ = 420.86 mW), 𝜆 1300 blueshifted
7.8 pm. Upon exposure to the 10.6 𝜇m CO 2 laser, 𝜆 1300 immediately red-shifted; ultimately, it returned to the initial 𝜆 1300. (b)
Repeated photoisomerization of Aazo on-device. The black, red and blue regions are the 1st, 2nd and 3rd cycle, respectively. The
corresponding blueshifts for each cycle were 4.5 pm (P circ = 127.36 mW), 3.6 pm (P circ = 96.33 mW), and 3.0 pm (P circ = 92.35
mW), respectively. (c) Two reversible cycles on the same device as with similar 410 nm input power and 𝛥𝜆 : black curve was the
first study and red curve was tested after 6 months. Reprinted with permission from Chapter 1 Reference [45] © The Optical
Society.
Then, the 410 nm laser was replaced by the CO2 laser. Due to the high optical absorption
of silica at 10.6 𝜇m, the device surface becomes hot within a couple of seconds. As a result, the
thermal energy provided by the CO2 laser reversed the cis-Aazo back to trans-Aazo, allowing a
transient redshifted 𝜆1300. After the CO2 laser was turned off, 𝜆1300 can equilibrate in the dark and
return to its initial 𝜆 1300. As seen in Figure 3.6.5(a), the net shift for this entire cycle was
approximately zero, indicating the complete reversibility of the photoisomerization process.
A common concern when using organic molecules for optical applications is the molecular
stability (or reproducibility of the response), particularly when they are operated and stored in
ambient environments (environments containing oxygen). This concern is further compounded
when they are exposed to high intensity optical fields, such as the ones used in the present work.
To study this potential degradation mechanism, a second device with the same ratio
[CH3:Aazo = 10:1] is fabricated and is cycled multiple times in air. On this device, the results from
three photoswitching events which were conducted sequentially are shown in Figure 3.6.5(b). 𝛥𝜆
scales with Pcirc, and the net shift for each cycle is negligible, indicating complete reversibility of
the photoisomerization process with minimal loss of response. This qualitative reproducibility of
85

three cycles is notable as it confirms two important technical considerations. First, the surface
density of Aazo on-device is sufficiently optimized to allow for free movement of the Aazo, and
second, the optical and thermal powers being used to induce the switching do not damage the
molecule. Due to the testing procedure used, which required measuring the power in the blue laser
before and after each measurement, the noise and total Pcirc changed slightly in each cycle, as
indicated in Figure 3.6.5(b).
In a complementary study, the same device was analyzed twice over a timespan of six
months. During the interim period, the device was stored in an ambient environment (room
pressure, temperature, and air). As can be seen in Figure 3.6.5(c), even after 6 months of storage,
the device response did not noticeably change, and the performance is identical within the error of
the measurement. This result provides further evidence that the surface chemistry protocol
developed to attach the Aazo monolayer is environmentally stable, and the device generates a
reproducible response.
The reason for the observed stability is rooted in the fundamental photoswitching
mechanism. Unlike many organic molecules used in photophysics measurements, Aazo does not
sensitize singlet oxygen which could then react with the molecule, leading to degradation. Instead,
Aazo is changing its conformation, which is a physical change on ultrafast timescales, many orders
of magnitude faster than other degradation mechanisms may act. Previous work studying the
fundamental photochemistry of various Aazo compounds observed high photostability in solid
state. For example, for one type of azopyridine derivative, even after exposure to 50,000 cycles of
UV pulses (355 nm, 10 mJ per pulse), the absorbance of the molecule was unchanged [57,58]. In
our previous work involving a surface bound Aazo compound, individual measurements were up
86

to 34 hours long, and a total of 8 measurements were performed on the same device with no
degradation in response observed [49]. Thus, the cumulative exposure was ~2500 J/cm
2
.
To investigate the effect of the surface density of Aazo on performance, similar
measurements to those in Figure 3.6.6 were performed with all [CH3:Aazo] ratios. Additionally,
to more thoroughly understand the dependence of the generated response on the circulating optical
power, the amount of Pcirc was changed to study the response and sensitivity. The data was taken
continuously in a similar manner to Figure 3.6.5(b) where each data point represents an entire
switching cycle, except the input power was varied. Notably, all data for a single surface density
is taken on the same device. The optical response is defined as the 𝜆 1300 blueshift for a given
amount of circulating power (𝛥𝜆/Pcirc). In each [CH3:Aazo] ratio, at least five (Pcirc, 𝛥𝜆) data points
were taken before determining the optical response of the system from the slope of the data. In
performing this analysis, it is important to account for variations in device performance and size
among the different cavities. For this reason, the conventional strategy is to use Pcirc instead of the
circulating intensity when comparing the optical response between devices.

Figure 3.6.6 - 𝛥𝜆 as a function of P circ across various Aazo surface concentration. (a) For a given surface density of Aazo, as P circ
increases, 𝛥𝜆 also increases. The slopes of the linear fits are the optical responses 𝛥𝜆/P circ of devices which are characteristic to the
surface density of Aazo. The error of each data point (each reversible cycle) was determined by the Gaussian fit on the histogram
of the data points (N > 500) in the baseline. Inset: The magnitude of 𝛥n 1300 behaves in a similar manner to 𝛥𝜆 with respect to P circ
among varied [CH 3:Aazo] ratios. The error is directly related to the error in the primary data set. (b) Optical responses 𝛥𝜆/P circ with
respect to [CH 3:Aazo] ratios. Negative values indicate the reduction of Δλ by 410 nm. The error was the error in the linear fitting
in part (a). (c) Using the optical cavity refractive index changes from (a, inset) and setting P circ = 200 mW, the experimental results
on change of optical cavity refractive index (black curve) can be compared to the change of refractive index from ellipsometry
studies (red curve). The general trend of two curves is also similar. The error in the black curve was the error in the linear fit of
𝛥n 1300 when setting P circ = 200 mW. The error in the red curve was taken from the average error of 𝛥n 1300 (ellipsometry) among
three wafers on each ratio. Reprinted with permission from Chapter 1 Reference [45] © The Optical Society.
87

A summary of the trans-to-cis induced shift is shown in Figure 3.6.6 (a). Experimentally,
it is limited by two factors: Pcirc of the 410 nm laser and the scanning range of the testing set-up
over which the resonant wavelength can be monitored. Based on experimental results, for a given
[CH3:Aazo] ratio, the device is operating in the exhibited a linear response to the input power range
throughout the entire series of Pcirc investigated, indicating that higher Pcirc would continue to
generate a larger. Based on classic sensor theory, the device response will saturate at some point
[59]. However, this range is outside of our testing set-up capabilities. Additionally, this linearity
in response indicates that the monolayer is not degrading even as the device is being cycled a
higher number of times and with higher input powers. Because there is minimal response in the
CH3 control, the entire shift can be solely attributed to the Aazo photoisomerization. Therefore, it
is anticipated that the surface density of photoswitchable Aazo groups would modify the 𝛥𝜆/Pcirc
of the cavity. The Aazo switching was not impeded in any surface density studied.
In order to directly determine the effect of Pcirc and [CH3:Aazo] ratio on the change of the
cavity refractive index at 1300 nm (Dn1300), Dn 1300 vs Pcirc curves at various [CH3:Aazo] ratio were
plotted as Figure 3.6.6(a) inset, where Dn1300 = Dneff/1300 nm. Since neff and 1300 nm are constants,
Dn1300 vs Pcirc curves exhibit the same behaviors as vs Pcirc curves.
To visualize the optical response on each [CH3:Aazo] ratio on the cavity, Figure 3.6.6 (b)
was plotted. As seen, 𝛥𝜆/Pcirc decreases as the surface density of Aazo in [CH3:Aazo] decreases,
demonstrating the ability to control the surface density of the Aazo group through the integration
of the CMPS. As expected, the control CH3-coated device does not show any response, and the
optical response extends to nearly -0.3 pm/mW for Aazo-coated devices.
The 𝛥𝜆 in Figure 3.6.6 (b) blueshifted a few tens of pm, which is lower than FEM
simulation prediction of approximately -130 pm. However, as mentioned, the FEM results should
88

be viewed as an upper bound because the calculations most likely over-approximate both the
density of Aazo molecules and the switching efficiency. A more accurate model system is the
ellipsometry results. Because the ellipsometry data was acquired with a fixed power blue laser
operating at 450 nm, it is not possible to quantitatively compare the resonant cavity and
ellipsometry results over the entire power range investigated. However, the general trends can be
compared. To perform this analysis, the responsivity values from Figure 3.6.6(b) are used to
calculate a wavelength shift assuming a Pcirc of 200 mW. Using the previously defined expression,
Dn1300 is computed. While this approach most likely also results in an over-approximation for the
higher Aazo surface densities, using a lower value would result in null or even negative responses
for lower surface densities, which is not physically realizable.
Using these values, the resonant cavity index change values are plotted alongside the
ellipsometry index change values (Figure 3.6.6(c)). As can be seen, the trend in response is similar.
This provides further evidence that the blueshift can be solely attributed to the photoswitchable
Aazo and, by varying the density of Aazo on the surface and the magnitude of the optical response
can be controlled. Furthermore, Dn1300 (ellipsometry) has an excellent agreement with previous
FEM simulations. While the CH3-coated sample indicates a negligible positive change on Dn1300
(ellipsometry) upon blue laser exposure, all Aazo-coated samples show the reduction of Dn1300
(ellipsometry) up to -6.0E-4. This is within the same order of magnitude of FEM 𝛥neff (-2.1E-4),
and it further strengthens that FEM is qualitatively reasonable despite an over-estimation.
3.7 Broadband Optical Spectrum Tuning
This section discusses the study of broadly tuning the resonance conditions of azobenzene-
functionalized silica microtoroid resonators over multiple linewidths of the cavity resonance.
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The optical testing setup (Figure 3.7.1) for the broadband optical spectrum tuning is similar
to the setup for reversible photoswitching shown in Figure 3.5.2. The same designs and principles
will not be included again in this section, but some minor changes and the specific design
principles for the broadband optical spectrum tuning will be noted in the following.

Figure 3.7.1 - (a) Rendering of the characterization setup for broadband optical spectrum tuning. (b) Representative transmission
spectrum of a coated microtoroid on resonance at 1300 nm (Q ~ 4.4 × 10
6
). Reprinted with permission from Chapter 1 Reference
[46] © The Optical Society.
Instead of focusing on using the 410 nm laser for inducing the blueshifted photoswitching
to tune the cavity resonant wavelengths, in this study, two different blue pump lasers are used to
photoswitch Aazo: a 410 nm narrow linewidth tunable laser (Newport, Velocity series) and a 450
nm diode laser (Thor Labs). This pair of lasers allowed several effects to be investigated and
compared. While both sources fall within the absorption range of the Aazo group, 450 nm is at
the maximum absorption of the Aazo. Additionally, the 410 nm is a narrow-linewidth source,
allowing for investigation of resonant-enhancement of the index change. Specifically, in one
measurement, both the 410 nm and the 1300 nm lasers were tuned to be simultaneously on-
resonance. In the second measurement, only the 1300 nm laser was on-resonance. In the case of
the 450 nm laser, the laser linewidth is very large (roughly 1000 times) compared to the linewidth
of the cavity being tested, and the resonant cavity free-spectral range (FSR) is also small at 450
nm (~1.6 nm). As such, the laser is statistically likely to match a resonant wavelength and continue
90

to do so even as the refractive index of the cavity changes. Therefore, measurements acquired
using the 450 nm laser represent the maximum shift expected.
In this study, the [CH3:Aazo] surface ratio is set at 1:1 to maximize the relative surface
density of Aazo to facilitate broadband spectrum tuning by the Aazo photoisomerization. By
scanning across a series of wavelengths, the resonant wavelength is identified, and the spectrum
is recorded. By fitting this spectrum to a Lorentzian, the linewidth (𝜆) is determined, and the loaded
cavity Q at either 410 nm or 1300 nm is calculated. Typical loaded quality factors of the
functionalized devices at 410 nm are in the mid-high 10
5
. To determine the intrinsic cavity Q at
1300 nm, the amount of coupled power is varied, and a coupled cavity model is used [60]. Typical
intrinsic Q factors at 1300 nm are in the mid-10
6
range (Figure 3.7.1(b)). To minimize nonlinear
effects which may distort the spectrum and any subsequent analysis, the input power from the 1300
nm laser is kept below approximately 10 𝜇W, and the coupling is kept relatively constant. The
power being coupled into the cavity from the 410 nm laser is ~200 𝜇W and from the 450 nm laser
is ~1.5 mW.
Two different approaches are used to monitor the resonant wavelength. The first method
relies on the tunable laser’s ability to continuously raster across a narrow wavelength range,
allowing for real-time monitoring and tracking of the peak position and transmission. This method
allows for a single resonance peak to be monitored with high temporal and spatial resolution [61].
However, the precision comes at the cost of working range, and large shifts are unable to be
measured. The second approach solves this challenge by leveraging the broad tunability of the
1300 nm laser. By scanning over the entire wavelength range of the laser, the entire resonance
spectrum can be recorded, and all resonant peaks of the cavity identified and tracked. However,
91

while this method solves the challenge associated with working range, the temporal and spatial
resolution are reduced. Therefore, most results are acquired using the first method.

Figure 3.7.2 - (a) Real-time tracking of the 1300 nm resonance peak tuning due to photo-isomerization upon exposure to either the
410 nm or 450 nm laser. (b) Broadband transmission spectra tracking the shifting resonance peak. (c) Sensitivity analysis using of
coated device with varying input powers for both the 410 nm and 450 nm pumps. Reprinted with permission from Chapter 1
Reference [46] © The Optical Society.
Figure 3.7.2 (a) presents a series of broadband turning on the cavity resonant wavelength
of the Aazo-coated microtoroidal cavity with various controls. There are several points worthy of
note. First, without the Aazo functional group, the resonant wavelength does not shift within the
noise of the measurement. This stability indicates that any red or blue shifts measured in Aazo-
functionalized devices can be directly attributed to the presence of the Aazo, and no other
competing optical effects, such as the thermo-optic effect.
Second, when the 410 nm laser was tuned off-resonance with 200 µW of input power, a
small fraction of blue light was still scattered into the Aazo functionalized cavity, and a probe
resonant wavelength shift of 180 pm (0.0125 FSR) was observed (Figure 3.7.2(a)). In contrast, if
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the 410 nm was tuned on-resonance, the 1300 nm resonance shift increased by nearly an order of
magnitude from 180 pm to 1.3 nm. This increase is due to the resonant enhancement of the 410
nm light intensity. However, the 410 nm laser does not coincide with the maximum absorption of
the Aazo molecule, limiting the full extent to which isomerization can take place. As a result, the
maximum observable shift is about 1.3 nm, which corresponds to 0.17 FSR, even with Q’s in the
high 10
5
range at 410 nm.
To maximize the Aazo absorption during photoswitching, the 410 nm narrow linewidth
tunable laser is replaced with a 450 nm diode laser (~1 nm full width at half maximum). This
approach allows for continuous pumping of the resonance wavelength. However, using this fixed
wavelength laser system, it is not possible to measure the cavity Q at 450 nm. With this change,
the shift increases to 4 nm (0.67 FSR), which is roughly a half order of magnitude further
improvement. This additional increase is likely due to the higher optical intensity of the 450 nm
laser, the higher optical absorption of the azobenzene at 450 nm as compared to 410 nm, and the
ability to maintain resonant excitation. This shift corresponds to about 40% of that specific cavity’s
FSR at the probe wavelength (~1300 nm).
Moreover, to further verify these measurements, a series of broadband scan spectra are
acquired, and the wavelength shift of a single peak is calculated. Characteristic results are shown
in Figure 3.7.2(b). Over the course of about 33 hours, the resonance peaks shift about 4.2 nm,
which corresponds to about 0.67 FSR of that device with about 1 nm of tuning within the first two
hours. This represents a large contrast in the switching behavior. While the resonance peak itself
is about 5 pm wide at half maximum, the total tuning range is about three orders of magnitude
larger. Although switching contrast in our devices is large, it is still limited by the quality of the
coupling at the pump wavelength, which is roughly 15% as we are unable to achieve critical
93

coupling. This presents a possible area of improvement for future work whereby contrast and
switching speed could likely be increased by improvements in coupling through waveguide
engineering. Broadly scanned resonance data, plotted alongside the other peak tracking results in
Figure 3.7.2, largely agree and present very similar kinetics with each other. As mentioned, the
inherent disadvantage of this method presents itself as poor spatial and temporal resolution as
compared to continuously tracking the resonance peak. Nevertheless, this method provides an
important validation on continuous peak tracking.
In Figure 3.7.2(c), the maximum blueshifts were analyzed with respect to the coupled blue
laser power at four different conditions. When blue laser (410 nm or 450 nm) was coupled to Aazo-
coated devices, the maximum blueshifts linearly increase as the laser power increases. The extent
of shifting at 450 nm is larger than when molecular layer is triggered using 410 nm. This can be
attributed to the fact that 450 nm laser offers higher laser power and better absorption for Aazo.
When blue laser was coupled to devices without Aazo surface functionalization, no blueshift is
observed. This proves that the blueshift was solely driven by the Aazo photoisomerization.
Using Equation 3.3.1 and assuming that the radius change is negligible, the approximate
refractive index change was calculated based on the resonant wavelength shift results and
compared to the refractive index change measured via ellipsometry. The resonance shift yields a
refractive index change of ~0.0028 which is roughly the refractive index change measured by
ellipsometry of ~0.0023. Any discrepancy is most likely due to the much longer exposure during
resonant cavity testing and the resonantly enhanced exposure to the 450 nm laser light as well as
error in the spectroscopic ellipsometry measurement.
Analysis of the noise of our time dependent measurement was done to determine how the
intrinsic noise from the pump lasers contributes to the probe sensing. While coupling 410 nm or
94

450 nm light into the cavity does increase the noise of the 1300 nm peak shift measurement
slightly, the noise level is always similar to or less than the linewidth of the cavity, with a maximum
noise level of around 3.4 pm at three standard deviations. This value is far below the
photoisomerization-induced resonance peak shift, which is three orders of magnitude larger.
Additionally, the noise distribution is Gaussian, indicating that the cause of the noise is not due to
any non-linear optical processes.
As shown in Figure 3.7.2(a), the overall temporal response of the Aazo-coated device is,
as expected, nonlinear. This response mirrors the kinetics of the molecule in solution which are
also temporally non-linear, albeit with a much faster response. The slower response of the Aazo
when grafted onto a surface, in comparison to a liquid environment, is expected since the motion
of the molecule is restricted. The grafting anchors half of the molecule’s possible isomer transitions
and steric hinderance can play a role due to closely packed neighboring molecules. In solution,
isomerizations can usually take place in a few minutes, given favorable solvent conditions and low
enough solute concentrations. However, while the tuning speed is slow, it compensates for this
performance in the range of tunability and power sensitivity.
Depending on the input wavelength, the device response is governed by a balance of device
Q and Aazo absorption. For example, at 410 nm, the tuning sensitivity is roughly 7.93 nm/mW of
input power. This level of optical sensitivity is likely only achieved through the resonantly
enhanced absorption of blue light within the device. By moving to 450 nm, where the optical
absorption of the Aazo is higher, the sensitivity decreases to 2.79 nm/mW of input power. This
decrease is most likely due to poor excitation of optical modes. The device shows a linear response
at both wavelengths, indicating no competing nonlinear effects. Moreover, the lasers are limited
in their max power output and therefore limits our ability to observe any saturation of sensitivity.
95

Although the cavity can be pumped by the 450 nm laser with an order of magnitude more power
than the 410 nm laser, it possesses a much larger linewidth as stated previously. This will likely
cause multiple resonances to be excited within the spectral bandwidth of the laser decreasing the
efficiency of excitation.
Lastly, the surface chemistry demonstrated in this study creates a robust photoswitch
consisting of covalently attached azo moieties. All data taken in Figure 3.7.2(b) was performed on
the same device, repeatedly insulting the device with high intensity circulating optical fields. For
reference, the circulating intensities in a device used in this work at 410 nm can reach roughly 9
MW/cm
2
given a quality factor of about 7.7 × 10
5
. The linear response indicates that the Aazo
groups did not degrade throughout the numerous cycles, despite these high optical fields. This
stability is to be expected given that the thermal damage threshold is essentially dictated by the
strength of the linking siloxy bond on the surface. Thermal degradation of a typical siloxy bond
begins around 200 °C [62,63]. Thermal degradation of silica, by comparison, begins around 1100
°C, making our overall device system very robust.
3.8 Summary
By merging photoswitchable organic molecules with integrated optical devices, all-optical
control of the resonant wavelength of integrated optical resonant cavities is demonstrated. Using
a highly controlled process to attach the Aazo layer, the cavity Q at both 410 nm and 1300 nm is
maintained, allowing low input power for the reversible resonant wavelength modulations. To
further increase the dynamic range, a competitive surface chemistry approach was developed using
chemical vapor deposition which enabled two molecules to be simultaneously anchored to the
device surface, providing control over the density of the photoswitchable Aazo groups. A single
waveguide was used to simultaneously excite the visible resonance used for tuning the near-IR
96

resonant modes excite resonant modes in the visible and the near-IR. The visible resonance was
used to photoswitch the Aazo groups while the near-IR resonance probed the state of the
molecules. The magnitude of the optical response of the laser-induced photoswitching is governed
by the surface density of Aazo groups and the circulating power of the 410 nm laser. The change
of ellipsometry in refractive index measured by ellipsometry at 1300 nm (𝛥n1300 (ellipsometry))
with respect to [CH3:Aazo] ratio correlates to that of the cavity index change with respect to
[CH3:Aazo] ratio at 1300 nm (𝛥n1300). The observed switching behavior is stable over multiple
switching cycles and is conserved, even after 6 months of storage in an ambient environment.
Lastly, the broadband optical spectrum tuning is also achieved by optimizing the Aazo surface
density, blue laser power and wavelength, as well as the surface coverage of by the Aazo. The
extent of broadband tuning reached about 0.67 FSR, which is promising in our hybrid
inorganic/organic optical microcavities.

97

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Chapter 4. Reversibly Tunable Optical Diffraction Grating Using
Poly(acrylic acid)/Polyethylene Oxide Stereocomplex
4.1 Introduction
Optomechanical systems are photonic devices and architectures that can respond to
external stimuli or internal mechanical components to change their optical properties or
configurations. The first known optomechanical system relied on electrically controlled micro-
positioners, and more recent work has explored integrating reconfigurable materials including
memory materials and stimuli responsive materials with integrated optical elements [1–8]. These
functional materials are capable of controlling the optical properties of the device, and at the same
time reducing the weight and power requirements of the system, two key metrics for many
applications.
Various types of optical components are incorporated into this category, such as lenses,
mirrors, and diffraction gratings. Among them, optical diffraction gratings are significant optical
elements that can split the incident light beam into an array of different wavelengths. The formation
of the optical diffraction patterns resides with the incident wavelength and the geometric features
of the periodic micro/nanostructures. With the continuous studies in the miniaturization of the
dimensions and the features in the micro/nanoscale, the applications of the optical diffraction
gratings have progressed from static optical elements into dynamic tunable surfaces for sensing
and detection applications in integrated optics, spectroscopy, and telecommunications [9–11].
Nonetheless, there is still room for further improvements, such as the reversible tunability
of the size and features on the diffraction grating. While static diffraction gratings show a fixed
characteristic optical performance, once the periodic features of the diffraction grating are defined,
the space for the tunability of the grating features is greatly suppressed. The traditional strategy
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for creating a tunable grating is by using electrically or thermally tuning of the material properties,
such as refractive index or mechanical shape. Currently, a promising approach is by utilizing
mechanical deformation of polymer substrates. Contrary to conventional crystalline materials,
some polymers can exhibit a large, reversible elastic response to mechanical strain [12–15].
Therefore, when applying the tensile strain to the patterned optical structures, they are able to be
deformed, resulting in the tuning of the optical diffraction [16–19].
This type of dynamic diffraction grating where the grating features will be flexible under
certain external stimuli is getting more and more attention. To address the tunability issue with
reduced scattering, hard metal lines have been patterned on a stretchable polymer substrate.
Several researchers have reported their methods for designing stretchable diffraction patterns by
combining metal micropatterns with polydimethylsiloxane (PDMS), a facile and stretchable
polymer substrate [20,21]. This strategy improved the tunability of the optical diffraction gratings,
but it raised another issue as well. The metal/polymer hybrid stretchable diffraction grating is in a
heterogeneous state, where the miscibility or the compatibility must be satisfied, especially at the
interface. To address this issue, a third sacrificial layer may be required. Besides, the typical strain
of a common 10:1 (base: curing agent) PDMS is around 100% [22–25], which is low for an
elastomeric material.
In our studies, we proposed that a homogenous material structure is a more compatible and
facile pathway to prepare a tunable optical diffraction grating. Poly(acrylic acid)
(PAA)/Polyethylene oxide (PEO) polymer stereocomplex is a great candidate that has excellent
stretchability of over 1400% and exceptional optical transparency [26]. In this thesis chapter, we
will focus on the novel fabrication of the PAA/PEO homogeneous polymer grating and its physical
characterization to demonstrate the potential application as a reversible optical stress sensor. The
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polymer stereocomplex of PAA/PEO can be obtained by directly mixing the dual aqueous
solutions in the acidic conditions, followed by moderate centrifugation. Nanoimprint compression
greatly accelerates the transfer replica molding of the diffraction grating features from the silicon
master to the polymer film, which exhibits the optical transmittance at or above 80% from 500 nm
to 1400 nm and stretchability over 800% strain with reversibility under 70% strain. Scanning
electron microscopy (SEM) confirms the response of the diffraction grating when being exposed
to external mechanical stimuli. Our experimental observations align well with the theoretical
simulations of the diffraction grating patterns with respect to the applied stimuli.
4.2 Fabrication of Polymer Diffraction Grating by PAA/PEO Stereocomplex
This section is devoted to the studies of the PAA/PEO stereocomplex-based polymer
grating. To begin with, the sample preparation of the PAA/PEO polymer film is included, as well
as the characterizations of the optical and mechanical performances. Then, the fabrication of the
polymer grating and the computational simulation of the polymer grating will also be included.
4.2.1 Polymer Film Preparation
To prepare a transparent stretchable film from polymer pellets or powders, it is advisable
to mix the polymers in a solution efficiently. Both PAA and PEO are all solids or powders at
ambient temperature, respectively; therefore, it requires molecular interactions between these two
polymers in order to form films. As shown in Figure 4.2.1, intermolecular hydrogen bonds can be
formed between the carboxylic acid in PAA and oxygen in PEO. This leads to the physical three-
dimensional network structures in the polymer film. The molar ratio of COOH groups in PAA
over oxygen segment in PEO is ideally 1:1 to facilitate the creation of intermolecular hydrogen
bonds, resulting in a uniform and robust network. The stretchability of the polymer film is
significantly enhanced with the assist of such robust structure, which will be verified in the
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mechanical characterization section. Other than the mechanical merit of the polymer
stereocomplex, both PAA and PEO can be commercially available at modest prices. Besides, both
PAA and PEO are biodegradable and environmentally friendly, and they can be dissolved in water;
thus, for the following sample solution preparations, the deionized water will be used as the
solvent.

Figure 4.2.1 - Three-dimensional illustration of PAA/PEO stereocomplex, containing intermolecular hydrogen bonds formed
between the carboxylic acid group in PAA and oxygen in PEO.
An overview of the fabrication process of the polymer diffraction grating is shown in Fig.
4.2.2. The first half contains the preparation of PAA/PEO polymer film, and the second half is to
stamp the polymer film into a micro/nano-patterned diffraction grating. For the film preparation,
the key goal is to form hydrogen bonds between PAA and PEO molecular chains as mentioned.
Once polymer stereocomplex pellets are formed, the molecular chains of PAA and PEO will
convert into a three-dimensional network structure, which will transform from a water-soluble
109

state to a water-insoluble state. The precipitated pellets will be collected to compress into a uniform
film. Experimentally, the uniform and self-supporting PAA/PEO polymer stereocomplex film
were formed using procedures similar to previous works [15,27]. PAA (Mw = 240 000, BeanTown
Chemical) and PEO (Mw = 1 000 000, Alfa Aesar) were used as received.

Figure 4.2.2 - Outline of the material synthesis and the fabrication of the PAA/PEO polymer stereocomplex diffraction grating.
Reprinted with permission from Chapter 1 Reference [61] © The Optical Society.
Typically, 50 mL PAA aqueous solution (1.8 mg/mL) is prepared in a 100 mL beaker,
followed by preparing 50 mL PEO aqueous solution (2.9 mg/mL) in another 100 mL beaker. To
improve the solubility of the polymers in deionized water, the temperature of both solutions was
ramped up to 30 °C and these two polymer solutions were mixed for around 1 h until polymers
were fully dissolved in water.
Then, the pH of both solutions was tuned by adding hydrochloric acid (VWR, 36%, ~100
µL) to obtain a pH of 2.8, which is an optimal pH to avoid the side dimerization and ionization of
110

the carboxylic groups [28]. In order to maximize the formation of the intermolecular hydrogen
bonds, it is crucial to avoid these side reactions. Next, the PAA and PEO solutions were slowly
mixed into a third 250 mL beaker via a syringe pump at a rate of 5 mL/min. This leads to the
efficient precipitation of the PAA/PEO pellets which were further collected via centrifugation. To
perform the centrifugation of the polymer pellets, the mixed suspension solutions were collected
in Falcon tubes with 7800 relative centrifugal force at 0 °C for 30 min. The supernatant was
disposed to acquire the PAA/PEO pellets.

Figure 4.2.3 - (a) PAA/PEO polymer stereocomplex film on a piece of glass slide; (b) optical microscope image of PAA/PEO
polymer stereocomplex. Reprinted with permission from Chapter 1 Reference [61] © The Optical Society.
During the step of compressing into a PAA/PEO film (Figure 4.2.3), the most important
factor to obtain a uniform film is the peeling-off process without sample damage. In other words,
reducing the adhesion between the compressing substrate and polymer film matters. Various
methods were tried to explore the optimal protocol. The experimental protocols were considered
in the following aspects: (1) compressing substrates, which were used as a pair to transfer the
compressing pressure to the polymer; (2) compressing temperature; (3) compressing time. The
methods were summarized in Table 4.2.1.

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Table 4.2.1 - Summary of methods for compressing into PAA/PEO films.
Compressing substrates Temperature (°C) Time Film condition after peel-off
PTFE plates room temperature 1 h Good
Al plates + PTFE films room temperature 1 day Good
PTFE films room temperature 1 day Not good
SiO 2 80 (hot press) 10 min Not good
Si wafers room temperature 1 day Not good
Si wafers treated with hydrophobic silane room temperature 1 day Not good
Al plates room temperature 1 day Not good
Therefore, the first two methods in Table 4.2.1 gives good film quality. The first method
takes a shorter time to compress well, probably due to the thicker thickness of PTFE plates than
that of PTFE films, which leads to a more uniform compression. The detailed optimal protocol is
presented here. To obtain a uniform polymer film (Figure 4.2.3), the collected PAA/PEO
stereocomplex pellets were placed between a pair of 100 × 100 mm
2
polytetrafluoroethylene
(PTFE) sheets and compressed by aluminum blocks (~5 kPa) for one day at room temperature
before peeling off. The use of PTFE sheets and the amount of pressure by the weight blocks were
particularly important to reduce the peeling-off stiction. If the stiction is too high, the peel-off
process will damage the compressed PAA/PEO film. Several other types of sheets were also
considered, such as hydrophobic-treated silicon wafers and glass slides. Overall, PTFE sheets
presented the best peeling-off capabilities. On the other hand, after compressing the film for the
first 30 min to 1 h, there is some moisture, which is the remaining supernatant, left within the
compressed polymer film. It will be advisable to remove the moisture either by the compressed air
flow or the nitrogen flow. Before imprinting the grating features, the basic optical and mechanical
properties of the film were measured.
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4.2.2 Characterization of Polymer Film
In terms of the characterization of the polymer film, the first study is the optical
transmission where the most fundamental optical response of the polymer film will be explored.
The high optical transparency (~80%) is a prerequisite for the choice of polymer materials. The
transmission of the PAA/PEO polymer film was measured using the wavelength scan mode in the
LAMBDA 950 UV-Vis Spectrophotometer (PerkinElmer). Before experimental measurements, a
clean glass slide was used as the substrate to hold the PAA/PEO film; thus, in the actual
measurements, the background absorption of the glass slide has to be subtracted. The polymer
film/glass slide mixture is placed inside the spectrophotometer across the optical path to measure
the transmittance at both ultraviolet and visible wavelengths.
The UV-Vis spectrum is presented in Figure 4.2.4. The transmission of PAA/PEO polymer
film is indicated at or above 80% from 500 nm to 1400 nm, which is appropriate as a transparent
polymer film. Using the transmission results as the guide, subsequent diffraction measurements
were performed at 633 nm and 1064 nm. At this pair of wavelengths specifically, the transmission
is 82% and 85%, respectively.

Figure 4.2.4 - UV-Vis spectrum of PAA/PEO stereocomplex film. Reprinted with permission from Chapter 1 Reference [61] ©
The Optical Society.
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4.2.3 Fabrication of Si Master Grating
To begin with, a 100 nm lift-off underlayer (I-ULP 3.5% concentration, EZImprinting Inc.)
was spin-coated on a Si wafer (WRS Materials) and baked at 120 °C for 5 minutes, which served
as the adhesive layer. Then, another 100 nm UV nanoimprint resist (I-UVP 4.1% concentration,
EZImprinting Inc.) layer was spin-coated onto the lift-off underlayer, and a subsequent
nanoimprint lithography step was performed to form the grating pattern. Reactive ion etching (RIE,
Oxford PlasmaPro 100) was performed to etch the residual UV imprint resist layer and the lift-off
underlayer. Then, a 30 nm Cr layer was deposited by e-beam evaporation (Temescal BJD-1800 E-
Beam Evaporator). To remove the lift-off underlayer and the layers above, a hot acetone bath was
utilized, and the patterned Cr etching mask remained on the Si substrate, which was subsequently
etched to 1 µm deep by RIE and removed the Cr etching mask. Lastly, to obtain the hydrophobic
Si master grating, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (GELEST Inc.) was
deposited on the Si grating by chemical vapor deposition at room temperature for 30 min under
the vacuum. Figure 4.2.5 shows a sample image of Si master gratings.

Figure 4.2.5 – Sample image of Si master gratings.
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4.2.4 Fabrication of Stretchable Polymer Grating
To obtain the stretchable polymer diffraction grating, there are two steps to process from
the pristine PAA/PEO polymer film. Firstly, the polymer film was molded into a dumbbell-shaped
standard film using the ISO 37-4 type dumbbell stamp die (Figure 4.2.6).

Figure 4.2.6 – The blueprint of ISO 37-4 Type dumbbell molding stamp die. The whole size of the die model is 35 × 6 mm
2
, and
the dimension for the middle rectangular region is 12 × 2 mm
2
. Reprinted with permission from Chapter 4 Reference [29] ©
Dumbbell Co., Ltd.
This type of molded film is not only used as a standard tensile specimen for the subsequent
mechanical characterization of the polymer properties, but also as a precursor to fabricate the
stretchable polymer diffraction grating. The second molding process was able to accurately
replicate the micro and nanoscale features of the lithographically fabricated lines and dots over a
large area (Figure 4.2.7). The line-patterned structures were regarded as the polymer diffraction
grating in the following sections.
115

Figure 4.2.7 – SEM images of (a) Si master grating and (b) PAA/PEO replicated polymer grating. (c) Optical microscope image of
Si master with patterned dots. (d) SEM image of PAA/PEO replicated polymer film with patterned dots. Reprinted with permission
from Chapter 1 Reference [61] © The Optical Society.
The characterization for the surface morphologies of the polymer gratings will be discussed
in detail in the later section. During the nanoimprint replica molding step, the nanofabricated Si
master grating was oriented perpendicular to the long axis (vertical) of the stamp die and covered
upside down onto the dumbbell-shaped polymer film. In this way, the changes on the pitch and
periodicity of the polymer grating will be uniformly distributed across the grating during
deformation and quantitatively controlled along the external strains.
4.2.5 Surface Morphology Analysis of PAA/PEO Polymer Diffraction Grating
Prior to the mechanical and optical characterization of PAA/PEO polymer diffraction
grating, it is popular to explore the surface morphology of the polymer grating before stretching
116

and after stretching. Scanning Electron Microscopy (SEM) was used as it offers precise surface
textures and conformations. SEM images of the stretched and unstretched polymer gratings were
acquired with a freshly made sample using a FEI Helios G4 P-FIB using a beam current of 1.6 nA
and an accelerating voltage of 10 kV. Before imaging, a thin layer of platinum was sputter coated
on top of the PAA/PEO polymer grating for 15 seconds to reduce charging. All SEM Images are
taken from the vertical direction and when the sample is tilted at 45°. The alignment in the vertical
direction facilitates the comparison of the periodicity of the diffraction grating when stretched and
unstretched. The tilted images can probe the surface texture and roughness.
As seen in Figure 4.2.8, the initial periodicity of the grating at an unstretched state (0%
strain) is about 10 µm (Figure 4.2.8(a,b)), and the surface has minor imperfections (Figure
4.2.8(c,d)). The polymer grating accurately replicated the negligible surface roughness of the Si
master grating. However, because of its highly elastic nature, the material had difficulty supporting
deep and narrow trenches. As a result, in Figure 4.2.8b, the diffraction grating lines are slightly
non-uniform. To improve this, the material could be slightly stiffened. In fact, in the process of
reducing the hysteresis, this increase would most likely occur.

Figure 4.2.8 - SEM images of the PAA/PEO polymer grating. (a,b) Vertical direction view at 0% strain; (c,d) 45-degree tilted view
at 0% strain; (e,f) vertical direction view at 50% strain; (g,h) 45-degree tilted view at 50% strain. Reprinted with permission from
Chapter 1 Reference [61] © The Optical Society.
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When the polymer grating is being stretched at 50% strain, which is the maximum possible
inside the SEM chamber, the period of the grating is increased to around 15 µm (Figure 4.2.8(e,f)),
and the surface roughness or number of imperfections increases (Figure 4.2.8(g,h)). While such an
increase could potentially be an explanation for the relatively low diffraction efficiency, it can still
be concluded that the strain applied on polymer grating is uniformly distributed across the whole
device with consistent dimensional deformation. This circumstance laid fundamental designs on
the optical characterizations with respect to external mechanical deformations.
4.3 Mechanical Characterization of PAA/PEO Polymer Film
The purpose of the mechanical analysis is to measure the elasticity of the PAA/PEO film
as the ability to reversibly deform is a prerequisite to be considered as a reversibly tunable grating
material. In this section, the mechanical performances of the polymer film will be experimentally
studied with a tensile tester called 3342 Single Column Universal Testing Systems (Instron Corp.,
Figure 4.3.1).

Figure 4.3.1 - Image of (a) 3342 Single Column Universal Testing Systems; (b) a pair of pneumatic film clamps in Instron; (c)
PAA/PEO film being stretched by Instron where the strains were applied.
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The Intron Tensile Tester consists of the following components: (1) a guide column that
navigates the testing sample to be stretched or compressed; (2) 500 N loading cell which acts as
the mechanical transducer to communicate with the terminal pad; (3) a pair of pneumatic clamps
to mount the dumbbell-shaped polymer film with the aim of reducing the sample damage; (4) a
terminal pad with an integrated computer containing the Instron operating program.
There are two methods to experimentally study the mechanical properties of the polymer
film. The first one is by plotting the stress-strain curve of the polymer film. The stress-strain curve
is the first source to depict the insight into the mechanical performance of the sample with Young’s
Modulus, elongation at break, yield point, maximum strength, toughness, etc. The stress-strain
curve of PAA/PEO stereocomplex film is shown in Figure 4.3.2. Young’s Modulus of PAA/PEO
film is 4.1 MPa, which can be determined at the linear region in the initial strain range (up to ~70%
strain). This value is used in the following mechanical simulation in Section 4.4. The elongation
at break is also determined from the data in Figure 4.3.2. It turns out that the polymer film can
withstand elongation above 800% strain before the permanent damage. This result demonstrates
superior elasticity of the polymer, nearly tenfold larger than the stretchability of the previously
used 10:1 PDMS.

Figure 4.3.2 - Stress-strain curve of PAA/PEO polymer stereocomplex at the stretching rate of 50% min
-1
. Reprinted with
permission from Chapter 1 Reference [61] © The Optical Society.
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However, there are some limitations of the stress-strain measurement shown in Figure
4.3.2. First, although it expresses the material performances in general, it cannot display the
reversibility of the stretch/relax when exposed to multiple deformation cycles. This information is
crucial when evaluating the suitability of the polymer.
The second method to characterize the material is the cyclic measurement. This method
conducts a series of continuous cyclic loading-unloading measurements on the polymer film to
supplement the mechanical responses by providing the hysteretic response, onset strain to deform
and the sample recovery insight. Since polymeric materials are visco-elastic, there is a tendency
of delaying the response to the external mechanical strain. Thus, it is desirable to investigate the
extent of delaying the response by plotting the “hysteresis loop”. Usually, the mechanical behavior
of the forward stretching step does not overlap with that of the following relaxing step, and this is
where the hysteresis loop arises. On the other hand, the onset strain can be interpreted as another
useful parameter. All measurements were performed at a stretching rate of 50% strain/min under
ambient conditions, and sample films were stored at ambient conditions when not being tested.
As mentioned earlier in Figure 4.3.2, the linear region is up to ~70% strain. When the
polymer film is being stretched to higher strains, plastic deformation is likely to occur, leading to
the permanent degradation of the sample. A series of mechanical strain cycles will be applied
across the PAA/PEO grating for the optical characterization. It is suitable to perform the following
optical experiments in the linear elastic region to prevent the permanent deformation.
The cyclic test results were shown in Figure 4.3.3. Figure 4.3.3a lists five continuous cycles
of stretching the polymer film from unstretched state (0% strain) up to 70% strain, the maximum
in the elastic region, and relaxing the polymer film back to 0% strain. It should be noted that all
five cycles were performed iteratively without any recovery time in between the measurements.
120

PAA/PEO film exhibits classic features of a viscoelastic material, or noticeable hysteretic behavior
with significant energy loss after the first cycle. This is anticipated because of the dynamic
hydrogen bonds in the PAA/PEO stereocomplex which can lead to strain accumulation and
deformation of the sample.

Figure 4.3.3 - The cyclic loading-unloading test results from (a) Day 1 and (b) Day 2, with the polymer film being relaxed between
those days. The onset strain in each hysteresis loop cycle for (c) Day 1 and (d) Day 2. The cyclic curves of the PAA/PEO polymer
film recovering after resting 1 day. Reprinted with permission from Chapter 1 Reference [61] © The Optical Society.
One unique feature of the viscoelastic material used in this work is its self-healing
capability. Given sufficient time (~1 day), the material’s mechanical response is fully recovered
(Figure 4.3.3b). This self-healing capability makes this material system more advantageous on the
material lifespan than conventional metal-based gratings or gratings based on stacked thin films.
On the other hand, for Figure 4.3.3 (a or b), the hysteresis loops are drifting to the larger
strain regions. To evaluate the onset strain at each cycle for both Day 1 and Day 2, the onset strains
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are plotted in Figure 4.3.3 (c and d), respectively. As seen, the onset stain is increasing with the
increased number of cycles, ramping up to 12% strain and 16% strain for Day 1 and Day 2,
respectively. To conclude, the PAA/PEO stereocomplex offers excellent stretchability over 800%
strain. Even if the polymer film is performed with continuous stretch and relax cycles, the
mechanical performances can be recovered given efficient relaxation. One strategy to overcome
the hysteretic concern is by improving the mechanical strength of the material with tunable
physical or chemical structures to enhance the strain of the elastic region.
4.4 Computational Modeling on Optical Diffraction Grating by PAA/PEO Stereocomplex
Before implementing the optical characterization on the polymer diffraction grating, it is
valuable to study the theoretical diffraction patterns with respect to the applied mechanical strain.
In this study, the theoretical modeling of the polymer diffraction grating was conducted using the
finite element method (FEM) modeling (COMSOL Multiphysics; COMSOL, Inc.). Mechanical
and optical properties of PAA/PEO stereocomplex are used in order to establish the modeling unit.
The Young's modulus of PAA/PEO polymer is set to 4.1 MPa based on our experimental
measurements in Section 4.3.
To simulate the optical properties of the PAA/PEO grating, a single cell was modeled using
periodic boundary conditions in the in-plane dimensions and perfectly matched layers in the out-
of-plane dimensions. PAA/PEO polymer stereocomplex is considered as the Cauchy model with
Cauchy parameters A = 1.446, B = 0.01. The initial grating sizes, including the pitch, linewidth
and height of the polymer grating, were set to 8.3 µm, 500 nm, and 1 µm, respectively. When
modeling the polymer grating at each strain, each single cell dimension and the size of the grating
are redefined based on our mechanics simulation results. A single cell of the polymer grating is
considered as a steady state where the dimension of the single cell is rearranged as strain is applied.
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The dimension of the cell and the size of the grating under each strain are obtained using solid
mechanics simulation in COMSOL Multiphysics.
The direct output from the COMSOL simulation is the diffraction angle. According to
Equation 4.4.1, the diffraction order distances can be calculated:
𝑡𝑎𝑛𝜃
1
=
a
b
c
(4.4.1)
where θ is the diffraction angle at mth order. Ym stands for the diffraction order distance between
the mth order and the center 0th order. D is the distance between the diffraction grating and the
receiving screen. D is a given value of 68.78 mm.

Figure 4.4.1 – FEM simulation results on diffraction order distances on PAA/PEO polymer grating at 633 nm. Distances between:
(a) 0th and 1st order; (b) 0th and 2nd order; (c) 0th and 3rd order.
Although diffraction angles are intrinsic properties and they are independent from the
design of the testing setup, the use of the diffraction order distances will be more directly
comparable with the experimental observations. In Figure 4.4.1, the diffraction distances at various
diffraction orders are charted at 633 nm as the incident wavelength. Black and red curves represent
the diffraction order distances at various strains in the stretching and relaxing step, respectively.
For all diffraction orders at both stretching and relaxing, as the strain increases, the diffraction
distances decrease. One point has to be noted is that, only one cycle of stretching and relaxing is
modeled because the stretching or the relaxing step is identical in each cycle of the simulations in
COMSOL.
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Similarly, in Figure 4.4.2, the diffraction distances at 1064 nm were also modeled. The
trend of the simulation at 1064 nm correlates to that at 633 nm. At a larger wavelength of 1064
nm, the diffraction order distances at the same order are larger than the corresponding order at 633
nm.

Figure 4.4.2 – FEM simulation results on diffraction order distances on PAA/PEO polymer grating at 1064 nm. Distances between:
(a) 0th and 1st order; (b) 0th and 2nd order.
4.5 Optical Characterization on Reversibly Tunable PAA/PEO Diffraction Grating
In this section, the diffraction patterns on the PAA/PEO polymer grating are studied
experimentally and quantitatively. The previous theoretically simulated diffraction pattern changes
are also included to compare with the experimental results. To experimentally study how the
PAA/PEO polymer grating adaptively responded to the external mechanical strains, the strains on
the polymer grating specimen were precisely integrated into the Instron universal tensile test
instrument (Instron 3342, stretching rate 50% strain/min). The testing setup in this section was a
modification on Figure 4.3.1 in Section 4.3 where several optical components were precisely
mounted around the Instron (Figure 4.5.1).
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Figure 4.5.1 – (a) Testing setup for the polymer grating consists of a 633 nm or 1064 nm laser, a universal tensile test machine
(Instron) to apply continuous strains, a beam splitter, a receiving screen, and a camera to capture diffraction pattern images. Inset:
Illustration of the grating structure and generated diffraction pattern. The diffraction orders (0th, 1st, 2nd, and 3rd) are indicated.
(b) Image of the actual testing setup for the diffraction measurements. Reprinted with permission from Chapter 1 Reference [61]
© The Optical Society.
The first optical component in the modified testing setup was a tunable laser, which serves
as the incident wavelength light source to induce the diffraction pattern. To guide the laser beam
on the surface of the polymer diffraction accurately, a collimator was coupled with the beam with
an optical patch cable. A lens was mounted between the collimator and the polymer grating to
facilitate focusing the laser beam spot on the surface of the polymer grating. The height of the
collimator, lens and the polymer grating sample was well aligned at the same point to ensure the
laser beam was sufficiently guided to the sample.
At this point, a charge-coupled device (CCD) camera was supposed to be mounted behind
the diffraction grating to capture the diffraction patterns. However, owing to the limited space to
mount the camera properly along the laser beam direction, the CCD camera was forced to be
mounted perpendicularly to the laser beam direction. To supplement the beam guidance towards
the CCD camera, a ZnSe beam splitter (25 mm diameter, Thorlabs) was mounted behind the
polymer grating. The advantage of such a vertically aligned beam splitter was that it cannot only
split the laser beam one way to the CCD camera, but also split the beam to the opposite direction
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of the camera where a receiving screen (IRC3 IR phosphor Card, Thorlabs) is set to receive the
diffraction patterns. The polymer grating sample was mounted on the Instron by a pair of
pneumatic clamps to apply the force to induce the mechanical strains.
Next, the choice of the incident wavelength was studied. Given the large optical
transparency window shown in Figure 4.2.4, initial diffraction measurements using several lasers
in the lab were performed. The wavelengths used were 633 nm, 980 nm, 1064 nm, 1330 nm and
1550 nm, and the results are shown in Figure 4.5.2. It is important to note that these initial
exploratory measurements were performed with different lasers with different output powers. Due
to the intrinsic laser type, the 633 nm and 1550 nm had the highest output power.

Figure 4.5.2 - The diffraction patterns of PAA/PEO polymer grating by various incident wavelengths. It turns out that 633 and
1064 nm are the best incident wavelengths because there are scatterings or no diffractions at other wavelengths due to our set-up
with a specific IR card as the receiving screen and a specific beam splitter that works at a certain wavelength. Reprinted with
permission from Chapter 1 Reference [61] © The Optical Society.
As seen, the diffraction patterns at 633 nm and 1064 nm showed the best quality at visible
and near-IR, respectively. Other than the laser output power, the sensitivity of the receiving screen,
a specific IR phosphor card, also played a role in the results.
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Figure 4.5.3 – The relative phosphor sensitivity of the IR card used in the testing setup with respect to the sensing wavelength.
Reprinted with permission from Chapter 4 Reference [30] © Thorlabs.
In Figure 4.5.3, the phosphor card has an optimized sensitivity response from ~800 nm-
1200 nm; hence, when the wavelength is larger than 1200 nm, the laser beam sensitivity drastically
decreases, resulting in the fact that there is no apparent diffraction pattern at 1300 nm and 1550
nm. When considering the effects of both laser output power and detector card sensitivity on
producing the diffraction patterns, one would expect a strong signal at 633 nm, 980 nm and 1550
nm in Figure 4.5.2. At 980 nm, there is a little more unwanted scattering than at 1064 nm. In
general, the present work focused on analyzing the diffraction at 633 nm and 1064 nm, to balance
laser power, sensor card responsivity, and wavelength range.
One cycle of stretching-and-relaxing the polymer grating was conducted, and the
summarized diffraction pattern images were taken at 633 nm and 1064 nm in Figure 4.5.4. In the
stretching step, the increment of the strain is 10% with the stretching speed of 50% strain/min.
After reaching 70% strain, the polymer grating stopped being stretched and began being relaxed.
In the relaxing step, the increment of strain is -10% along with the speed of -50% strain/min.
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Figure 4.5.4 - The diffraction patterns generated at (a) 633 nm and (b) 1064 nm during one stretching-and-relaxing cycle are shown.
The cycle started at 0% strain, increased to a maximum of 70% strain, and returned to 0% strain. Scale bar is 5 mm. Reprinted with
permission from Chapter 1 Reference [61] © The Optical Society.
For both wavelengths, all subsequent diffraction distances to the center 0th-order decrease
with the increase in the mechanical strain. This corresponds to the trend seen in Section 4.4 as the
periodicity (slit) increases, the smaller the diffraction distances to the 0th-order become. Contrary
to the visible subsequent diffraction orders at 633 nm in Figure 4.5.4a, there are patterns missing
at 1064 nm in Figure 4.5.4b. This phenomenon could be caused by the limited laser output power
at 1064 nm. If a higher laser power is provided, this drawback could be solved.
The relative diffraction efficiency of each order at 0% strain is listed in Table 4.5.1. The
relative diffraction efficiency was calculated by dividing the diffraction intensity of a specific order
over 0th-order as the benchmark. The diffraction efficiency is governed by a combination of the
surface roughness of the grating and the wavelength. The relatively lower diffraction efficiencies
in the subsequent diffraction orders were likely induced by the surface imperfection while
stretching the polymer grating in Figure 4.2.7. All relative diffraction efficiencies were analyzed
in the diffraction pattern images, using a custom computer program Python, which has been
uploaded to Armani Lab GitHub [31].
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Table 4.5.1 - The relative diffraction efficiencies, absolute diffraction angles and changes in diffraction angles of each order at
various wavelengths. Diffraction efficiencies were measured at 0% strain. Reprinted with permission from Chapter 1 Reference
[61] © The Optical Society.
Wavelength/nm Diffraction Order
Relative
Diffraction
Efficiency/%
Absolute
Diffraction Angle
at 0% strain
Change in
Diffraction Angle
633 1st-order 41 3.8° 0.9°
633 2nd-order 15 7.5° 2.4°
633 3rd-order 10 11.2° 3.5°
1064 1st-order 29 7.4° 0.8°
1064 2nd-order 18 14.8° 1.7°
As a supplement to demonstrating the tunability of the diffraction grating, the diffraction
angles were also listed in Table 4.5.1, which is independent on the grating testing setup. The
absolute diffraction angle at 0% strain indicates that the diffraction angle increases as the
diffraction order increases at a given wavelength. The changes in diffraction angles at a certain
diffraction order were calculated by subtracting the diffraction angles at 70% strain from the ones
at 0% strain on the same diffraction order. The changes in diffraction angles are agreeable on the
trend where the maximum diffraction angle increases as the diffraction order increases.
For the quantitative analyses on the diffraction pattern distances over multiple
stretching/relaxing cycles, data was taken at discrete (steady state) strains at both 633 nm and at
1064 nm. Throughout all measurements, the incident angle of the laser beam and the separation
distance (D = 68.78 mm) between the grating and the detector card are constant. Thus, the
separation distance between diffracted patterns for the two wavelengths is a direct indicator of the
reversible tunability of the optical diffraction grating.
Figure 4.5.5 summarized the data of the diffraction pattern distances directly measured on
the receiving screen. The values of the diffraction distances, up to three orders of diffraction, were
analyzed from the images with Python. The polymer grating was cyclically stretched-and-relaxed
for five times, mimicking the mechanical cyclic measurements in Section 4.3. Each cycle was
analyzed individually, and the plots were categorized based on stretch/relax and the diffraction
129

order. As a comparison, the FEM modeling results in Section 4.4 were also included in Figure
4.5.5 as the red dashed curves.

Figure 4.5.5 - Measurements and FEM simulations of strain-induced changes in the spacing between diffracted orders at 633 nm.
Measurements are with respect to the 0th order. (a) 0th-1st order, stretching; (b) 0th-1st order, relaxing; (c) 0th-2nd order, stretching;
(d) 0th-2nd order, relaxing; (e) 0th-3rd order, stretching; (f) 0th-3rd order, relaxing. Reprinted with permission from Chapter 1
Reference [61] © The Optical Society.
As the polymer grating was stretched from 0% to 70% strain, the diffraction order distances
of 0th-1st order, 0th-2nd order, and 0th-3rd order changed by ~1.5 mm, ~3 mm, and ~4.5 mm,
respectively (Figure 4.5.5(a,c,e)). During relaxation, these values systematically decreased by
~0.5-1 mm (Figure 4.5.5(b,d,f)). These correspond to a change in diffraction angle of 0.9°, 2.4°
and 3.5°, where the absolute angle values at 0% strain were 3.8°, 7.5° and 11.2°, respectively. On
130

the other hand, theoretically, there is a method of increasing the tunability of the diffraction angles
by reducing the periodicity of the polymer/master grating. For instance, if the periodicity is only 3
µm, the absolute diffraction angles at 0% strain can be enhanced up to 40° at 633 nm. However,
the smaller the grating pitches or features, the more difficult to obtain a uniform and intact
diffraction grating with a soft polymer. As a result, in this study, the initial periodicity in the
polymer grating is set at 10 µm as a constant.
The nonlinear optical responses in diffraction distances can be attributed to the nonlinear
viscoelastic mechanical behavior of the material (Figure 4.3.3). This brings worthy discussions in
terms of such unique features. First, in Figure 4.5.5(a,c,e), for the second and almost all subsequent
stretching cycles, the first strain step (black curve, typically from 0% to 10%) actually increases
or has a negligible impact on various diffraction order separations. This seems to be contradictory
to how the diffraction pattern would change, especially when performing on a fully elastic material.
Moreover, at the most relaxation steps, the diffraction order separations do not fully return to the
initial starting point of the corresponding stretching step. Such behavior was also observed in the
previous mechanical test results. It can be interpreted that both results are due to an accumulation
of temporary structural damage in the material, also referred to as energy loss, which decreases the
elasticity of the material. Such structural damage is not permanent as it is supported by the
material’s ability to recover its mechanical behavior after resting for 24 hours. In other words, once
the polymer grating is treated with sufficient material resting after mechanical tests, the
performances will be repeatable as the initial conditions.
131

Figure 4.5.6 - Studies on the sequential diffraction order distances of the polymer grating between X-th (X indicated on the top
right of every figure) orders at 633 nm in 5 individual cycles. (a,b,c) Cycle 1, (d,e,f) Cycle 2, (g,h,i) Cycle 3, (j,k,l) Cycle 4 and
(m,n,o) Cycle 5 of the sequential diffraction order distances when stretching and relaxing. Reprinted with permission from Chapter
1 Reference [61] © The Optical Society.
The data in Figure 4.5.6 was the supplement to Figure 4.5.5 regarding the following
aspects: firstly, instead of the diffraction distances to the center 0th order, the individual sequential
distances were compared. Secondly, all figures in Figure 4.5.6 were plotted by each cycle, where
each chart only contains one cycle of stretching-and-relaxing at a specific distance order. In
stretching steps, the diffraction order distances of most data points started at around 4-4.5 mm at
0% strain and finished at around 3 mm at 70% strain. In the relaxing steps, the diffraction order
distances of data started at around 4 mm at 0% strain and finished at around 3 mm at 70% strain.
Therefore, the optical behavior of the polymer grating is consistent throughout the experiments.
132

Figure 4.5.7 - Measurements and FEM simulations of strain-induced changes in the spacing between diffracted orders at 1064 nm.
Measurements are with respect to the 0th order. (a) 0th-1st order, stretching; (b) 0th-1st order, relaxing; (c) 0th-2nd order, stretching;
(d) 0th-2nd order, relaxing. Reprinted with permission from Chapter 1 Reference [61] © The Optical Society.
Then, near-IR 1064 nm was also used as the incident wavelength to produce the diffraction
patterns. As seen in Figure 4.5.4, since the diffraction patterns were not visible at strains above
20%, the direct visual observations were conducted from 0% through 20% strain for the stretching-
and-relaxing experiments. Sequential diffraction measurements were investigated up to the second
order in 1064 nm. This is due to the significant increase in diffraction order spacing at 1064 nm
and the limited size of the receiving card. The results of the diffraction order distances measured
at 1064 nm were shown in Figure 4.5.7.
As the polymer grating was stretched from 0% to 20% strain, the diffraction order distances
of 0th-1st and 0th-2nd changed by ~0.8 mm and ~2.0 mm, respectively. These correspond to a
change in diffraction angle of 0.8° and 1.7°, or absolute diffraction angle values of 7.4° and 14.8°
133

at 0% strain. These results were also included in Table 4.5.1. There are some similarities and
differences in comparison with the results taken using 633 nm. For the similarities, the
accumulated strains in the polymer during the continuous cycling also limited the ability of the
grating to fully recover. This was particularly noticeable during the first cycle. Likewise, if the
grating is treated with full relaxation, the optical behavior will be fully recovered. Another
similarity is that the experimental observations at 1064 nm closely follows on the FEM modeling
results with experimental errors at the same incident wavelength. As for the differences, it can be
noticed that the absolute diffraction angles at 1064 nm were larger than those at 633 nm when the
incident wavelength and the diffraction order are fixed. Meanwhile, the change of diffraction
angles at 1064 nm was a little smaller or in the same range compared to the corresponding results
at 633 nm.
4.6 Summary
In summary, a homogenous deformable polymer grating using a combination of
stereocomplex elastomer and nanoimprint lithography has been demonstrated. This work covers a
wide range of topics, including polymer material preparation, grating fabrication, device modeling,
as well as mechanical and optical characterization. Our approach provides a path for fabricating
optical devices from the polymer stereocomplexes with exciting self-healing properties.
The polymer film of PAA/PEO stereocomplex is easily accessible by mixing the aqueous
solutions of each polymer at a desired pH to precipitate the pellets, which were subsequently
compressed into uniform films. The hydrogen bonding between the polymer chains plays an
important role in forming the three-dimensional structure, which offers excellent material
elasticity. The replica molding accurately transferred the micro-scale structures of the silicon
grating template into PAA/PEO polymer stereocomplex. The characterization of the material on
134

fundamental optical and mechanical properties demonstrates its use as a diffraction grating, proved
by high optical transparency throughout most wavelength ranges and a high percent elongation
before failure (800%).
Also, due to the intermolecular dynamic hydrogen bonds in the PAA/PEO stereocomplex,
the polymer can be reversibly loaded and unloaded at lower strains in the elastic region within
70% strain. Given sufficient recovery time, the mechanical performances of the polymer can be
recovered to the initial state. This combination of the optical and mechanical behaviors makes it
ideally suited for a deformable optical element. As a result, at both visible and near-IR
wavelengths, the diffraction order distances of PAA/PEO grating agreed with the theoretical
simulations.
This study opens novel pathways in merging materials research and optics. The novel
material designs will greatly benefit the improvements in the optics field, enabling potential
developments of optical devices and related applications, including tunable or flexible
spectroscopy systems, wearable sensors and optical filters. Future studies may lie in the continuing
optimizations of the material designs, integrations of optically active components and fabrications
of more complex optical elements.

135

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139

Chapter 5. Multifunctional Photoresponsive Organic Small
Molecules by Aggregation-Induced Emission
5.1 Introduction
The development of organic and inorganic fluorescent molecules and particles for
biomedical and clinical studies began over two centuries ago and have enabled critical discoveries.
For instance, fluorophores, the molecules with the fluorescence capabilities, have revealed internal
cell structures and functions, been used as indicators for detecting biological toxins, and have
served as the active layer in many optoelectronic devices [1–6]. Inspired by the potential
applications in those optoelectronic devices, the organic small molecules containing fluorophores
can be designed to meet the desired structure-property relationship.
Optical and mechanical responding signals are powerful tools to probe for biodetection and
biomolecular motors [7–11]. Given their length scale, the potential applications of molecular
devices extend far beyond biological systems into optoelectronics and quantum devices. For
example, recent work has explored stimulated emission materials, organic solid-state lasers, and
frequency upconversion [12–18]. However, to fulfill a molecular device capable of executing
multiple functions requires merging various functional groups into a single molecular structure.
One of the key challenges in designing multifunctional molecules is reducing the cross-
talk between the different excitation mechanisms. If the cross-talk occurs, this type of interference
can reduce the overall system efficiency and performance [19,20]. With recent advances in
machine learning and computational design algorithms, material chemists are able to optimize
molecular designs accelerating experimental efforts [21–23].
In the present work, we developed an all-optical multifunctional molecular device designed
to simultaneously sense-and-modulate electric fields. The 'sensor' module is derived from
140

tetraphenylethylene (TPE), which is a two-photon (2p) fluorophore that is excited in the near-IR
(NIR). The 'modulator' module is an organic photoconductor, naphthalimide (NAI), whose
resistivity can be tuned using an ultraviolet source. The non-interacting alkyl chain does not affect
the photophysical properties of the modules but rather reduces Dexter energy transfer between
modules by physical spacing. The multifunctional molecule, named as NAI–TPE-PyS, is
comprised of two non-interacting modules that are tethered by a non-interacting spacer (Figure
5.1.1). PyS is a strong electron acceptor, improving the charge transfer from the TPE module.
The TPE and NAI modules were judiciously chosen for low overlap of their absorption
and emission spectra to reduce Foerster resonant energy transfer. The precise chemical structure
of the molecule is further optimized using density functional theory (DFT) as part of this work.
The tether enforces colocalization of the modules to reduce the possibility for bulk phase
separation of the modules, which could occur for a blend of the two components under the
processing conditions for various applications. The entire system is optically controlled, including
signal read-out, and the two modules can be operated simultaneously or individually (Figure 5.1.1).

Figure 5.1.1 - Chemical structure of NAI-TPE-PyS with TPE-module in red and NAI-module in blue. Reprinted with permission
from Chapter 1 Reference [66] © Royal Society of Chemistry.

141

The TPE sensor module relies on the ultrafast photo-induced electron transfer (PET)
process to allow large changes in the electric field to be detected. Unlike voltage sensitive dyes
[24–28], PET molecules show superb signal-to-noise ratio, sensitivity, and response speed [29–
33]. This performance is inherent to the ultrafast PET process, which is an intramolecular electron
transfer through its donor-spacer-acceptor backbone upon exposure to excitation sources,
competing with the radiative fluorescent emission [34,35]. In addition, TPE exhibits a unique
behavior known as aggregation-induced emission (AIE). In an AIE material, the emission signal
in the condensed state is increased due to restricted intramolecular rotation (RIR) [36,37]. Among
PET and AIE materials, TPE is unique because it exhibits both behaviors.
As mentioned, NAI is a photoconductor [38–41]. This type of molecule allows the
resistivity to be modulated with light. Although organic photoconductors have been widely used
in optoelectronics [42–47], their integration into a multifunctional molecule is underexplored [48].
Moreover, an organic molecular device with both electric field sensor and modulator capabilities
has yet to be demonstrated.
142

5.2. Synthesis of NAI-TPE-PyS
5.2.1 Materials and Synthesis Overview

Figure 5.2.1 - Synthetic scheme of NAI-TPE-PyS. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry.
All commercially available chemical reagents and solvents including 1,8-naphthalic
anhydride, ammonia, 1,10-dibromodecane 4-bromoiodobenzene, 4-Formylphenylboronic acid,
bis(pinacolato)diboron, tetrakis(triphenylphosphine)palladium (Pd(PPh3)4), [1,1′-
Bis(diphenylphosphino)ferrocene]dichloropalladium(II) (Pd(dppf)Cl2), 4-Picoline, 1,3-
propanesulton, piperidine, potassium carbonate (K2CO3), potassium acetate (KOAc), anhydrous
sodium sulfate (Na2SO4), N,N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO),
dichloromethane (DCM), chloroform, Tetrahydrofuran (THF), methanol (MeOH), ethanol
(EtOH), ethyl acetate, hexane, acetonitrile (ACN) were either purchased from VWR or Sigma
Aldrich. Unless otherwise noted, they were used without further purification. 1,8-Naphthalimide
143

(1) and 4,4′-(2-(4-Bromophenyl)-2-phenylethene-1,1-diyl)-diphenol (5) were synthesized based
on previously published protocols with modifications, and those previously known structures were
proved by
1
H NMR spectra shown in the following contents. For all novel molecules in this study,
1
H and
13
C NMR spectra were recorded on a Varian Mercury 400 MHz spectrometer with 96-
spinner sampler changer using either deuterated chloroform or DMSO as solvent, as indicated.
Figure 5.2.1 shows the overview of the entire synthesis process for the NAI-TPE-PyS
molecule. Compound 2, Compound 6, and Compound 8 are combined to form the desired product.
5.2.2 Synthesis of 1,8-Naphthalimide (Compound 1)
Compound 1 was synthesized according to the previous published procedure [49]. Into a
100 mL round-bottom flask was added 1,8-naphthalic anhydride (10.0 g, 50.5 mmol) and ammonia
solution (40 mL, 28% NH3 in H2O). The reaction was stirred at 100 °C for 12 h. Ice water was
added to precipitate yellowish solids, which were filtrated and dried in vacuo to afford Compound
1.
1
H NMR (400 Hz, CDCl3): δ 11.68 (s, 1H), 8.46-8.37 (m, 4H), 7.83 (dd, J = 8.1, 7.4 Hz, 2H).

Figure 5.2.2 -
1
H NMR of Compound 1.
144

5.2.3 Synthesis of 2-(10-Bromodecyl)-1H-Benz[de]Isoquinoline-1,3(2H)-Dione (Compound
2)
To synthesize Compound 2, into a 250 mL double-neck round-bottom flask was added 1
(2.0 g, 10.1 mmol) and dry N,N-Dimethylformamide (DMF, 80 mL). The mixture was stirred at
60 °C for 12 h to make 1 fully dissolved. Then, K2CO3 (2.79 g, 20.2 mmol) and 1,10-
Dibromodecane (15.15 g, 50.5 mmol) were added, and the system was allowed to react at 60 °C
for another 24 h. After finishing, K2CO3 was removed by filtration, and the solvent was removed
under vacuum. The crude product was further purified by silica column chromatography with ethyl
acetate: hexane (1:9/v:v) as eluent to yield a white solid.
1
H NMR (400 Hz, CDCl3): δ 8.61 (d, J = 7.2 Hz, 2H), 8.21 (d, J = 8.1 Hz, 2H), 7.76 (t,
2H), 4.18 (t, 2H), 3.40 (t, J = 7.0 Hz, 2H), 1.84 (p, J=7.0 Hz, 2H), 1.74 (p, J=7.5 Hz, 3H), 1.47 =
1.24 (m, 12H).
13
C-NMR (101MHz, CDCl3): δ 164.20, 133.83, 131.58, 131.16, 128.16, 126.91,
122.77, 40.47, 34.04, 32.83, 29.38, 29.34, 29.27, 28.71, 28.15, 28.09, 27.09.

Figure 5.2.3 -
1
H NMR of Compound 2. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
145

Figure 5.2.4 -
13
C NMR of Compound 2. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
5.2.4 Synthesis of 1-(4-Bromophenyl)-2,2-Bis(4-Hydroxyphenyl)-1-Phenylethene
(Compound 3)
Into a 250 mL round-bottom flask were added 4-Bromoiodobenzene (20 mmol, 5.66 g), 4-
Formylphenylboronic acid (20 mmol, 3.0 g), Pd(PPh3)4 (0.52 mmol, 600 mg) and K2CO3 (7.0g,
50 mmol). The flask was fitted on the Schlenk line, vacuum evacuated, and refilled with nitrogen
alternately three times. A mixing solvent (dioxane/water: 80 mL/20 mL) was bubbled with
nitrogen for 30 min and then transferred to the flask through a canula. The mixture was then
allowed to react for 12 hours at 80 °C. After cooling to the room temperature, the mixture was
poured into water and extracted with DCM three times. The combined organic part was dried with
sodium sulfate, and the solvent was removed by vacuum. The obtained solid was then purified by
column chromatography with eluent ethyl acetate: hexane (1:10/v:v) to give a white powder.
1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0
p p m
2 7 . 0 9
2 8 . 0 9
2 8 . 1 5
2 8 . 7 1
2 9 . 2 7
2 9 . 3 4
2 9 . 3 8
3 2 . 8 3
3 4 . 0 4
4 0 . 4 7
1 2 2 . 7 7
1 2 6 . 9 1
1 2 8 . 1 6
1 3 1 . 1 6
1 3 1 . 5 8
1 3 3 . 8 3
1 6 4 . 2 0
146

1
H NMR (400 MHz, CDCl3): δ 10.07 (s, 1H), 7.95 (s, 2H), 7.71 (s, 2H), 7.62 (d, J = 8.6
Hz, 2H), 7.51 (d, J = 8.6 Hz, 2H).
13
C-NMR (101MHz, CDCl3): δ 191.76, 145.88, 138.61, 135.45,
132.18, 130.35, 128.90, 127.49, 122.95.

Figure 5.2.5 -
1
H NMR of Compound 3. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.

Figure 5.2.6 -
13
C NMR of Compound 3. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0 2 2 0 2 3 0
p p m
1 2 2 . 9 5
1 2 7 . 4 9
1 2 8 . 9 0
1 3 0 . 3 5
1 3 2 . 1 8
1 3 5 . 4 5
1 3 8 . 6 1
1 4 5 . 8 8
1 9 1 . 7 6
147

5.2.5 Synthesis of 4'-(4,4,5,5-Tetramethyl-1,3,2-Dioxaborolan-2-Yl)-[1,1'-Biphenyl]-4-
Carbaldehyde (Compound 4)
Compound 3 (1.0 g, 3.8 mmol), bis(pinacolato)diboron (1.5 g, 5.7 mmol), KOAc (1.2 g,
11.4 mmo) and Pd(dppf)Cl2 (150 mg, 0.2 mmol) were added to a 250 mL Schleck flask with a stir
bar. The flask was pumped under vacuum and refilled with N2 three times before 50 mL degassed
DMSO was transferred to the system. The solution mixture was then heated at 100 °C overnight
under N2. After cooling to room temperature, the mixture was poured into 200 mL of DI water,
extracted with DCM twice and then washed with water three times. The combined organic layer
was dried by anhydrous Na2SO4 and the organic solvent was pumped out. The crude product was
then purified by column chromatography on silica gel with ethyl acetate: DCM (1:9/v:v) to give
compound 4.
1
H NMR (400 MHz, CDCl3): δ 10.07 (s, 1H), 7.95 (s, 2H), 7.71 (s, 2H), 7.62 (d, J = 8.6
Hz, 2H), 7.51 (d, J = 8.6 Hz, 2H), 1.35 (s, 12 H).
13
C-NMR (101MHz, CDCl3): δ 191.88, 146.97,
142.24, 135.42, 130.23, 127.79, 126.62, 83.97, 83.48, 25.01, 24.87.

Figure 5.2.7 -
1
H NMR of Compound 4. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
148

Figure 5.2.8 -
13
C NMR of Compound 4. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
5.2.6 Synthesis of 4,4'-(2-(4-Bromophenyl)-2-Phenylethene-1,1-Diyl)-Diphenol (Compound
5)
Compound 5 was synthesized according to previously reported literature [50]. Into a 500
mL two-necked round-bottom flask was added 4,4'-dihydroxybenzophenone (3.21 g, 15 mmol),
4-bromobenzophenone (7.80 g, 30 mmol) and zinc dust (8.82 g, 135 mmol). The flask was
vacuumed and purged with nitrogen for three times. Afterward, 200 mL of THF was injected into
the flask, followed by cooling down to −78 °C with an acetone/dry ice bath. TiCl4 (6.74 mL, 67.5
mmol) was added into the mixture in a dropwise way. The reaction was then refluxed overnight
under nitrogen conditions. After the solution cooled to room temperature, hydrochloric acid (1 M)
was added to the reaction mixture to adjust the pH to 2. The mixture was then extracted with DCM
and dried with anhydrous sodium sulfate. The crude product was purified by silica column
chromatography, using hexane and ethyl acetate (5:2/v:v) as the eluent to give a white solid.
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 2 0 0 2 1 0
p p m
2 4 . 8 7
2 5 . 0 1
8 3 . 4 8
8 3 . 9 7
1 2 6 . 6 2
1 2 7 . 7 8
1 3 0 . 2 3
1 3 5 . 4 2
1 4 2 . 2 4
1 4 6 . 9 7
1 9 1 . 8 8
149

1
H NMR (400 MHz, DMSO-d6): δ 9.39 (s, 1H), 9.35 (s, 1H), 7.31 (d, J = 8.5 Hz, 2H), 7.31-
7.03 (m, 4H), 6.93 (d, J = 6.7 Hz, 2H), 6.85 (d, J = 8.5 Hz, 2H), 6.79-6.67 (m, 4H), 6.59-6.44 (m,
4H).

Figure 5.2.9 -
1
H NMR of Compound 5.
5.2.7 Synthesis of 4''-(2,2-Bis(4-Hydroxyphenyl)-1-Phenylvinyl)-[1,1':4',1''-Terphenyl]-4-
Carbaldehyde (Compound 6)
4 (2.3 mmol, 0.7 g), 5 (1.9 mmol, 0.84 g), Pd(PPh3)4 (0.19 mmol, 200 mg) and K2CO3 (7.6
mmol, 1.05 g) were added into a 100 mL round-bottom flask. The flask was fitted on the Schlenk
line, vacuumed and refilled with nitrogen alternately three times. A mixing solvent (dioxane/water:
40 mL/10 mL) was bubbled with nitrogen for 30 min and then transferred to the flask through a
canula. The mixture was then allowed to react for 24 hours at 100 °C. After cooling to room
temperature, the mixture was poured into DI water and the pH was adjusted to about 5. Then, the
150

system was extracted with DCM and washed with water three times. The organic solvent was then
removed, and the crude solid was recrystallized by hexane/DCM to give the pure product.
1
H NMR (400 MHz, CDCl3): δ 10.06 (s, 1H), 7.96 (d, J = 8.1 Hz, 2H), 7.79 (d, J = 8.4 Hz,
2H), 7.68 (s, 4H), 7.41 (d, J = 8.2 Hz, 3H), 7.12 (d, J = 2.2 Hz, 4H), 7.06 (s, 2H), 6.93 (dd, J =
16.6, 8.5 Hz, 4H), 6.59 (t, J = 9.0 Hz, 4H).
13
C-NMR (101MHz, CDCl3): δ 191.91, 154.22, 154.14,
146.48, 144.10, 143.74, 140.85, 140.34, 138.79, 137.56, 136.40, 135.16, 132.78, 131.90, 131.42,
130.32, 127.75, 127.64, 127.46, 127.37, 126.22, 126.13, 114.68, 114.56.

Figure 5.2.10 -
1
H NMR of Compound 6. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
151

Figure 5.2.11 -
13
C NMR of Compound 6. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
5.2.8 Synthesis of 4''-(2,2-Bis(4-((10-(1,3-Dioxo-1H-Benzo[de]Isoquinolin-2(3H)-
Yl)Decyl)Oxy)Phenyl)-1-Phenylvinyl)-[1,1':4',1''-Terphenyl]-4-Carbaldehyde (NAI-TPE-
CHO, Compound 7)
Into a 250 mL two-necked round-bottom flask was added K2CO3 (248.8 mg, 1.8 mmol), 2
(500 mg, 1.2 mmol) and 6 (163 mg, 0.3 mmol). The flask was vacuumed and purged with dry N 2
three times. Then DMF (15 mL) was added and the reaction was stirred overnight at 70 °C. After
cooling to room temperature, the mixture was poured into water, extracted with dichloromethane
(DCM), washed with distilled water several times and dried with anhydrous magnesium sulfate.
The crude product was purified by silica column chromatography with hexane and ethyl acetate
(gradient to 1:1/v:v) as eluent to give 7 as a yellow viscous oil (200 mg, 56 %).
1
H NMR (400 MHz, CDCl3): δ 9.97 (s, 1H), 8.54 – 8.48 (m, 4H), 8.16 – 8.11 (m, 4H), 7.87
(d, J = 8.4 Hz, 2H), 7.74 – 7.63 (m, 7H), 7.60 (s, 4H), 7.32 (d, J = 8.4 Hz, 2H), 7.07 – 6.97 (m,
6H), 6.87 (dd, J = 17.3, 8.8 Hz, 4H), 6.60 – 6.51 (m, 4H), 4.12 – 4.07 (m, 4H), 3.79 (t, J = 6.5 Hz,
9 0 9 5 1 0 0 1 0 5 1 1 0 1 1 5 1 2 0 1 2 5 1 3 0 1 3 5 1 4 0 1 4 5 1 5 0 1 5 5 1 6 0 1 6 5 1 7 0 1 7 5 1 8 0 1 8 5 1 9 0 1 9 5 2 0 0
p p m
1 1 4 . 5 6
1 1 4 . 6 8
1 2 6 . 1 3
1 2 6 . 2 2
1 2 7 . 3 7
1 2 7 . 4 6
1 2 7 . 6 4
1 2 7 . 7 5
1 3 0 . 3 2
1 3 1 . 4 2
1 3 1 . 9 0
1 3 2 . 7 8
1 3 2 . 8 0
1 3 5 . 1 6
1 3 6 . 4 0
1 3 7 . 5 6
1 3 8 . 2 5
1 3 8 . 7 9
1 4 0 . 3 4
1 4 0 . 8 5
1 4 3 . 7 4
1 4 4 . 1 0
1 4 6 . 6 8
1 5 4 . 1 4
1 5 4 . 2 2
1 9 1 . 9 1
152

4H), 1.71 – 1.59 (m, 8H), 1.28 (d, J = 42.0 Hz, 24H).
13
C-NMR (101MHz, CDCl3): δ 191.52,
164.19, 133.83, 132.61, 132.21, 131.56, 131.16, 130.30, 128.13, 127.79, 127.60, 127.42, 127.34,
126.09, 122.73, 113.90, 68.23, 40.48, 32.75, 32.64, 29.38, 29.32, 29.29, 29.28, 28.08, 28.04, 27.09,
26.02, 25.95, 25.78, 25.64.

Figure 5.2.12 -
1
H NMR of Compound 7. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.

Figure 5.2.13 -
13
C NMR of Compound 7. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
153

5.2.9 Synthesis of 3-(4-Methylpyridin-1-Ium-1-Yl)Propane-1-Sulfonate (Compound 8)
4-Picoline (1.8 g, 20 mmol) was dissolved in 15 mL ACN. 1,3-propanesultone (3.7 g, 30
mmol) was added and the reaction mixture was heated to 80
o
C for 4 hours. After completion, the
crude product was precipitated. The solid was filtered and washed with ethyl ether to give
compound 8 (86%) as a white solid.
1
H NMR (400 MHz, DMSO-d6): δ 8.88 (d, J = 6.8 Hz, 2H), 7.94 (d, J = 6.3 Hz, 2H), 4.63
(t, J = 6.9 Hz, 2H), 2.56 (s, 3H), 2.36 (t, J = 7.2 Hz, 2H), 2.17 (q, J = 6.9 Hz, 2H).
13
C NMR (101
MHz, DMSO-d6): δ 159.21, 144.37, 128.78, 59.22, 47.38, 27.69, 21.82.

Figure 5.2.14 -
1
H NMR of Compound 8. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.

154

Figure 5.2.15 -
13
C NMR of Compound 8. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
5.2.10 Synthesis of 3-(4-(2-(4''-(2,2-Bis(4-((10-(1,3-Dioxo-1H-Benzo[de]Isoquinolin-2(3H)-
Yl)Decyl)Oxy)Phenyl)-1-Phenylvinyl)-[1,1':4',1''-Terphenyl]-4-Yl)Vinyl)Pyridin-1-Ium-1-
Yl)Propane-1-Sulfonate (NAI-TPE-PyS)
A mixture solution of 7 (300 mg, 0.25 mmol), 8 (53 mg, 0.25mol), and piperidine catalyst
(0.2 mL) was refluxed in 10 mL dry EtOH under N 2 for 48 hrs. The solution turned deep red. After
cooling to room temperature, the solvent was removed and the crude solid was purified by column
with eluent of DCM: MeOH (10:1/v:v) to give a red solid (83 %). The total yield from the starting
materials is around 7%.
1
H NMR (400 MHz, DMSO-d6): δ 9.21 (d, J = 6.9 Hz, 1H), 8.94 (d, J = 7.2 Hz, 1H), 8.47-
8.41 (m, 10H), 8.29 (s, 2H), 8.20 (d, J = 7.0 Hz, 2H), 7.86-7.66 (m, 8H), 7.64 (d, J = 8.8 Hz, 1H),
7.51 (d, J = 8.5 Hz, 1H), 7.10 (dd, J = 14.4, 7.3 Hz, 4H), 7.03 – 6.95 (m, 6H), 6.84 (dd, J = 22.2,
10.3 Hz, 4H), 6.69 – 6.61 (m, 4H), 4.80 (t, J = 6.9 Hz, 2H), 4.20 (t, J = 7.0 Hz, 4H), 3.81 (t, J =
6.8 Hz, 4H), 2.45-2.41 (m, 2H), 2.22 (t, J = 6.9 Hz, 2H), 1.59-1.52 (m, 10H), 1.26-1.19 (m, 22H).
1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0
p p m
2 1 . 8 2
2 7 . 6 9
4 7 . 3 8
5 9 . 2 2
1 2 8 . 7 8
1 4 4 . 3 7
1 5 9 . 2 1
155

13
C NMR (101 MHz, CDCl3): δ 164.12, 163.99, 133.75, 132.84, 131,10, 131.10, 127.57, 127.09,
126.87, 122,68, 56.55, 40.44, 29.68, 29.43, 29.30, 28.07, 27.09, 26.41, 22.66. Some carbon peaks
in Figure 5.2.17 were not shown as expected due to the large size of NAI-TPE-PyS. For instance,
a few carbons in the pyridine, phenyl bridge and the carbons attaching to the oxygen of TPE.

Figure 5.2.16 -
1
H NMR of NAI-TPE-PyS. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry.

Figure 5.2.17 -
13
C NMR of NAI-TPE-PyS. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry.
156

5.3. Synthesis of Other Molecules
5.3.1 Synthesis Overview

Figure 5.3.1 - Synthetic scheme of different molecules in Chapter 5.
In addition to the synthesis of NAI-TPE-PyS mentioned in Section 5.2, the syntheses of
other types of molecules were also performed in Figure 5.3.1. Some reactions did not proceed.
This section will summarize the experimental results with the synthetic procedures and
1
H-NMR
spectra.
157

5.3.2 Synthesis of 4,4'-(2-(4-Bromophenyl)-2-Phenylethene-1,1-Diyl)Bis(Methoxybenzene)
(Compound 11)
This is a synthesis of a TPE derivative. Compound 11 was synthesized according to the
previous published procedure [51]. Typically, into a 250 mL round-bottom flask were added bis(4-
methoxyphenyl)methanone (Compound 9, 1950 mg, 8.0 mmol, Sigma Aldrich), 4-bromo-
benzophenone (Compound 10, 2500 mg, 9.6 mmol, Sigma Aldrich), and Zn powder (2600 mg, 40
mmol, Sigma Aldrich). Three cycles of Argon purge were then conducted. Anhydrous THF (100
mL) was added to the flask via syringe and the mixture was stirred at 0 °C. Next, TiCl4 (3.5 mL,
Sigma Aldrich) was added dropwise over 30 min. The mixture was then heated to 80 °C for 24 h.
After that, the reaction was cooled down to room temperature and quenched by the addition of 50
mL 10% aq. K 2CO3. The mixture was filtered to remove insoluble Filtration was conducted to
remove insoluble materials and extracted with DCM. The organic layer was dried with MgSO4
and filtered. The solution of the organic layer was evaporated. The crude product was purified by
silica column chromatography with eluent hexane: DCM (1:1/v:v).
1
H NMR (400 Hz, CDCl3): δ 7.21 (d, J = 8.4 Hz, 2H), 7.15-7.05 (m, 4H), 7.04-6.95 (m,
3H), 6.95-6.84 (m, 6H), 6.69-6.59 (m, 3H), 3.75 (d, J = 11.0 Hz, 5H).

Figure 5.3.2 -
1
H NMR of Compound 11.
158

5.3.3 Synthesis of 4-(Ethyl(2-Hydroxyethyl)Amino) Benzaldehyde (Compound 14)
The synthesis of Compound 14 was the first synthetic step to extend the conjugation of
another TPE derivative (Compound 16) in Figure 5.3.1. Compound 14 was synthesized according
to the previous published procedure [52]. In a 250 mL round were added 4-fluorobenzaldehyde
(Compound 12, 6.25 g, 0.05 mmol, Sigma Aldrich), 2-(ethylamino)ethanol (Compound 13, 10.95
g, 0.15 mmol, Sigma Aldrich), K2CO3, and 125 µL [CH3(CH2)7]4N
+
Br
-
(Sigma Aldrich). 100 mL
DMSO was added as the solvent. The mixture was heated to 95 °C for 3 d. After cooling the
reaction mixture, ice water was poured into the mixture to quench the reaction. The mixture was
filtered and purified by silica column chromatography with eluent ethyl acetate: hexane (2:1/v:v).
1
H NMR (400 Hz, CDCl3): δ 9.73 (s, 1H), 7.72 (d, J = 9.1 Hz, 2H), 6.77 (d, J = 9.0 Hz,
2H), 3.87 (t, 2H), 3.59 (t, 2H), 3.53 (q, J = 7.1 Hz, 2H), 1.23 (t, 3H).

Figure 5.3.3 -
1
H NMR of Compound 14.
5.3.4 Synthesis of 2-(Ethyl(4-Vinylphenyl)Amino)Ethan-1-Ol (Compound 15)
Compound 15 was synthesized according to the previous published procedure [53].
Typically, into a 250 mL oven-dried round-bottom flask was added PPh3MeBr (8140 mg, 22.8
159

mmol, Sigma Aldrich). The flask was purged with argon for three cycles. Then, 10 mL anhydrous
THF was added. 1.6 M solution of n-BuLi in hexanes were added via syringe. After stirring the
mixture for 15 min, Compound 14 (3000 mg, 15.2 mmol) was added. After stirring overnight, the
reaction was poured into hexanes. The suspension was filtered through Celite (Sigma Aldrich) and
concentrated under reduced pressure. The residue was taken up in ethyl acetate and filtered through
a thin pad (1-2 cm) of silica.
1
H NMR (400 Hz, CDCl3): δ 7.29 (d, J = 8.8 Hz, 2H), 6.73 (d, J = 8.4 Hz, 2H), 6.62 (dd, J
= 17.6, 10.9 Hz, 1H), 5.53 (dd, J = 17.6, 1.1 Hz, 1H), 5.02 (dd, J = 10.8, 1.1 Hz, 1H), 3.79 (t, 2H),
3.59-3.30 (m, 4H), 1.16 (t, 3H).

Figure 5.3.4 -
1
H NMR of Compound 15.
5.3.5 Synthesis of (E)-4,4'-(2-(4-Bromophenyl)-2-(4-(4-(Ethyl(2-
Hydroxyethyl)Amino)Styryl)Phenyl)Ethene-1,1-Diyl)Diphenol (Compound 16)
The synthesis of Compound 16 was tried using Heck reaction. In a 10 mL round-bottom
flask, Compound 5 (93.1 mg, 0.26 mmol), Pd(OAc)2 (1.0 mg, 0.0045 mmol, Sigma Aldrich), P(o-
Tol)3 (2.43 mg, 0.008 mmol, Sigma Aldrich) were added. The mixture was purged with argon for
160

three times. Then, 2 mL anhydrous triethylamine (Sigma Aldrich) was added. After stirring the
mixture for 10 min, Compound 15 (50.0 mg, 0.26 mmol) was added via cannula transfer. The
reaction was refluxed at 110 °C for 24 h. After that, the reaction was quenched by adding 20 mL
deionized water. The crude compound was purified by silica column chromatography with eluent
hexane and ethyl acetate.

Figure 5.3.5 – The predicted
1
H NMR of Compound 16 using MestReNova Program.
161

Figure 5.3.6 -
1
H NMR of the synthesis on Compound 16. This reaction was unsuccessful with numerous unknown proton peaks
and unmatched multiplet integrals with the predicted
1
H-NMR spectrum.
However, when comparing the
1
H-NMR of the synthesis with the predicted
1
H-NMR of
Compound 16, it can be noted that there are mismatches on the proton peaks as well as the
multiplets. Therefore, the synthesis of Compound 16 was not successful.
5.3.6 Synthesis of 4,4'-(2-(4-Bromophenyl)-2-Phenylethene-1,1-Diyl)Bis((2-
Bromoethoxy)Benzene) (Compound 17)
Compound 17 was synthesized according to the previous published procedure [54].
Typically, into a 250 mL round-bottom flask, Compound 5 (2000 mg, 4.6 mmol) and anhydrous
K2CO3 (5200 mg, 37.2 mmol) were added. The flask was purged with argon for three times. 50
mL acetonitrile was added as the reaction solvent. Then, 3.2 mL 1,2-dibromoethane was added.
Another 50 mL acetonitrile was added. The reaction was refluxed at 80 °C for overnight. The
reaction was worked-up by cooling the mixture to room temperature and filtration. The filtrate was
condensed by rotary evaporation. The crude product was further purified by silica column
chromatography with eluent hexane: ethyl acetate (30:1/v:v).
162

1
H NMR (400 Hz, CDCl3): δ 7.22-7.08 (m, 6H), 7.05-6.96 (m, 3H), 6.95-6.83 (m, 4H),
6.73-6.60 (m, 4H), 4.27-4.19 (m, 4H), 3.66-3.56 (m, 4H).

Figure 5.3.7 -
1
H NMR of Compound 17.
5.3.7 Synthesis of 2-(2-Hydroxyethyl)-1H-Benzo[de] Isoquinoline-1,3(2H)-Dione (Compound
18)
Compound 18 was synthesized according to the previous published procedure [55]. Into a
100 mL round-bottom flask, 1,8-naphthalic anhydride (4000 mg, 20.18 mmol) was added. 40 mL
ethanol was added. 1.3 mL ethanolamine (Sigma Aldrich) was added. The mixture was refluxed
at 80
o
C for 5 h. The reaction mixture was concentrated by evaporating ethanol under vacuo and
cooled to 4°C. The solids were purified by filtration and washed with cold ethanol.
1
H NMR (400 Hz, DMSO-d6): δ 8.46 (dd, J = 7.2, 1.1 Hz, 2H), 8.42 (dd, J = 8.4, 1.1 Hz,
2H), 7.84 (dd, J = 8.4, 7.2 Hz, 2H), 4.80 (t, 1H), 4.13 (t, 2H), 3.60 (q, J = 6.4 Hz, 2H).
163

Figure 5.3.8 -
1
H NMR of Compound 18.
5.3.8 Synthesis of (Z)-2-(2-(2-(4-(2-(4-Bromophenyl)-1,2-
Diphenylvinyl)Phenoxy)Ethoxy)Ethyl)-1H-Benzo[de] Isoquinoline-1,3(2H)-Dione
(Compound 19)
The synthesis of Compound 19 was tried using Williamson Ether Synthesis. Into a 10 mL
round-bottom flask, Compound 17 (50.0 mg, 0.076 mmol), Compound 18 (22.0 mg, 0.091 mmol),
K2CO3 (31.5 mg, 0.228 mmol), KI (1 mg) were added. Nitrogen purge was conducted three times.
2 mL acetonitrile was added. The reaction was refluxed at 85
o
C. The reaction mixture was cooled
to room temperature and filtered to collect the filtrate, which was concentrated under reduced
pressure. The compound was purified by silica column chromatography with eluent DCM: ethyl
acetate (20:1/v:v).
When comparing the
1
H-NMR of the synthesis with the predicted
1
H-NMR of Compound
19, there are unknown proton peaks and unmatched multiplets. Therefore, the synthesis of
Compound 19 was not successful.
164

Figure 5.3.9 – The predicted
1
H NMR of Compound 19 using MestReNova Program.

Figure 5.3.10 -
1
H NMR of the synthesis on Compound 19. This reaction was unsuccessful with several unknown proton peaks and
unmatched multiplet integrals with the predicted
1
H-NMR spectrum.
165

5.3.9 Synthesis of Tetrakis(decyl) Perylene-3,4,9,10-Tetracarboxylate (Compound 21)
In addition to NAI, perylene, another type of organic photoconductive molecules, was also
investigated in this chapter. Perylene is two-equivalent of NAI. The synthesis of Compound 21
was conducted according to a previously reported literature [56]. In a 250 mL round bottom flask,
perylenetetracarboxylic dianhydride (Compound 20, 4000 mg, 10.2 mmol) and 50 mL 5.0 M KOH
aqueous solution were added. The reaction mixture was stirred at 70 °C for 0.5 hour. Then, the
solution was filtered into another 250 mL round-bottomed flask. The pH of the mixture was tuned
between 8 and 9 with 1M HCl. [CH3(CH2)7]4N
+
Br
-
(1670 mg, 3.06 mmol) and KI (340 mg, 2.04
mmol) were added to the reaction before vigorously mixing for 10 minutes. 1-bromodecane (17
mL, 81.6 mmol, Sigma Aldrich) was added. The reaction was refluxed at 105
o
C for 2 h. The
yellow crude product was extracted by chloroform and washed with 15% NaCl aqueous solution
three times. Methanol was added to the chloroform solution dropwise to precipitate the yellow
solid, which was filtered and dried under vacuo at 80 °C.
1
H NMR (400 Hz, CDCl3): δ 8.28 (d, J = 8.0 Hz, 4H), 8.04 (d, J = 7.9 Hz, 4H), 4.32 (t,
9H), 1.79 (p, 9H), 1.50-1.18 (m, 65H), 0.87 (t, 12H).

Figure 5.3.11 -
1
H NMR of Compound 21.
166

As seen in Figure 5.3.11, four decyl groups were confirmed on Compound 21. This is
different from previously studied NAI where monofunctional alkyl group was attached.
5.3.10 Synthesis of 1H,3H-Perylo[3,4-cd]Pyran-8,9-Dicarboxylic Acid, 1,3-Dioxo-, 8,9-
Didecyl Ester (Compound 22)
The synthesis of Compound 22 was conducted according to a previously reported literature
[57]. In a 25mL round-bottom flask, Compound 21 (1000 mg, 1.01 mmol) was added. 2.4 mL
toluene was added to dissolve Compound 21. P-toluenesulfonic acid monohydrate (192 mg, 1.01
mmol) was added. The reaction was stirred at 95 °C for 5 h. The reaction was quenched by cooling
down to room temperature and adding 150 mL methanol. The solids were filtered and dried in
vacuum.

Figure 5.3.12 – The predicted
1
H NMR of Compound 22 using MestReNova Program.
167

Figure 5.3.13 -
1
H NMR of the synthesis on Compound 22. This reaction was unsuccessful with unknown proton peaks and
unmatched multiplet integrals with the predicted
1
H-NMR spectrum.
When comparing the
1
H-NMR of the synthesis with the predicted
1
H-NMR of Compound
22, there are unknown proton peaks and unmatched multiplets. Therefore, the synthesis of
Compound 22 was not successful.
5.4. Results and Discussions
5.4.1 Molecular Design and Computational Modelling
The molecular device is comprised of two covalently coupled modules: a tetraphenylene
(TPE)-based PET dye connected with naphthalimide (NAI)-based photoconductor. As mentioned,
the NAI 'modulate' module was chosen due to its extensive use as an organic photoconductor and
was incorporated without structural modification. The 'sense' module of NAI-TPE-PyS was
designed based on TPE but was modified with PyS to create TPE-PyS, representing the
donorspacer-acceptor system, as required by a PET dye [58].
To assist in this design, the reporter part was modelled in silico-based on a truncated
structure, namely, Me-TPE-PyS (Figure 5.4.1a and b). The alkyl linkers and peryleneimide
168

moieties of NAI-TPE-PyS are separated from electronic communication with TPE-module and
therefore were replaced with methyl groups to reduce the computational cost of the modelling. The
ground state equilibrium geometry and frontier orbital densities of Me-TPE-PyS were calculated
by density functional theory (DFT) in the gas phase at the B3LYP/6-31g* level of theory. With
TPE as an electron donating group and PyS as an electron accepting one, the HOMO density
resides on the TPE moiety with some extension onto the phenylene linker (Figure 5.4.1c, red solid).
The LUMO density resides on the pyridinium and benzylidene moieties with some extension onto
the phenylene linker (Figure 5.4.1c, blue mesh).
The excited states were calculated by TD-DFT with a conductor-like polarizable
continuum medium (dielectric = 78.39, water) at the CAM-B3LYP/6-311
++
G** level of theory.
Figure 5.4.1d–f shows the natural transition orbitals for the lowest three possible singlet states
along with corresponding transition properties. Accordingly, the first singlet excited state (S1) has
a highly allowed vertical transition with an energy of 3.37 eV (368 nm) (Figure 5.4.1d). The dipole
moment of S1 (42.8 D) is 18.9 D more than that of the ground state (29.3 D), suggesting that it may
be moderately solvatochromic. This transition is characterized by a π–π* transition on the
pyridinium and phenylene linker, corresponding to electron transfer from lower lying occupied
orbitals to the LUMO. The second singlet excited state (S2) has a vertical transition energy of 3.84
eV (323 nm) (Figure 5.4.1e). The dipole moment of S2 (37.1 D) is 7.8 D more than that of the
ground state (29.3 D), suggesting that it may be weakly solvatochromic. This transition is
characterized by a localized π–π* transition on the TPE moiety, corresponding to electron transfer
from the HOMO to higher lying unoccupied orbitals. The third singlet excited state (S3) has a
vertical transition energy of 4.14 eV (300 nm) (Figure 5.4.1f). The dipole moment of S3 (81.8 D)
is 52.5 D more than that of the ground state (29.3 D), suggesting that it may be strongly
169

solvatochromic. This transition corresponds to the HOMO to LUMO transition, representing
charge transfer from TPE to PyS. Notably, based on this analysis, solvatochromicity, which is an
indicator of voltage sensitivity, is anticipated.

Figure 5.4.1 - TPE-PyS model compound: (a) chemical structure; (b) optimized ground state geometry; (c) HOMO (red solid) and
LUMO (blue mesh) isosurfaces (0.05 Å
3
); natural transition orbital iso-surfaces (0.05 Å
3
) for hole (red solid) and electron (blue
mesh) wavefunctions: (d) S1, (e) S2, and (f) S3. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry.
5.4.2 Spectroscopic Characterization
In this section, various types of solvents were used for spectroscopic characterizations on
NAI-TPE-PyS. N,N-Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dichloromethane
(DCM), chloroform, Tetrahydrofuran (THF), methanol (MeOH), ethanol (EtOH) were either
purchased from VWR or Sigma Aldrich. Dulbecco's phosphate-buffered saline (DPBS, 1X) was
purchased from Thermo Fisher Scientific.
Due to the combination of the hydrophobic NAI terminals and the hydrophilic pyridinium
inner salt terminal, NAI-TPE-PyS can be readily dissolved in polar aprotic solvents like DMSO
and DMF as well as in low-polarity lipophilic solvents like DCM and chloroform. However, it
shows low solubility in protic solvents like water or PBS buffer. Accordingly, the steady-state
photophysical properties of NAI-TPE-PyS were analyzed both in solutions with different polarity
170

and in solid state thin films using ultraviolet-visible (UV-Vis) and photoluminescence (PL)
spectroscopy.
All UV-Vis absorption spectra were measured on Beckman Coulter Life Science UV/Vis
spectrophotometer, DU 730 with wavelength resolution of 2 nm. Steady-state fluorescence spectra
were recorded on Horiba Scientific Fluoromax-4 spectrofluorometer with excitation slit width of
5 nm and emission slit width of 5 nm. Quantum yield was determined by a Quanta-φ integrating
sphere. Powder sample and aqueous solution of the sample (10 µM, containing 1% DMSO) were
prepared for the measurements.
The optical absorption, steady-state emission, and quantum yield were measured under
several different conditions to understand the role of pH, solvent polarity, and fluorophore
concentration on emission intensity and emission wavelength. Measurements were performed on
both solution and thin film samples. Solution samples in selective solvents were prepared by serial
dilution to give the desired concentrations, and thin film samples were prepared by dropping DCM
solution of NAI-TPE-PyS on Swiss glass slide (size: 25 mm × 75 mm, thick: 1.0 mm) followed by
air-dry evaporation.
TPE is a moderate electron donor, and the pyridinium inner salt is a strong electron
acceptor. Thus, the interaction between these compounds will lower the HOMO-LOMO energy
gap, facilitating the intramolecular charge transfer (CT) process. As a result, the normalized
absorption spectra of the molecule exhibit a broad profile tailing to 500 nm, and absorption
comparison among NAI-TPE-PyS and synthetic precursors in solvents and solid state were shown
in Figure 5.4.2.
171

Figure 5.4.2 - UV-Vis absorption measurements of several synthetic precursors, the final product both in DCM and in PBS, and
the final product deposited as a thin film on quartz substrate. Reprinted with permission from Chapter 1 Reference [66] © Royal
Society of Chemistry.
Figure 5.4.3 and Figure 5.4.4 show the results from a study of the UV-Vis absorption
behavior and of the fluorescence emission behavior of the final product in a range of solvents,
respectively. At shorter wavelengths, around 300–380 nm, the absorption band can be assigned to
the n–π* and π–π* transitions of NAI- and the elongated TPE-conjugation moieties which overlaps
with the absorption of NAI–TPE-CHO precursor. The absorption band at wavelengths longer than
380 nm, which is not present in the NAI-TPE-CHO precursor, can be assigned to the CT transition
from TPE to pyridinium inner salt.

Figure 5.4.3 - Absorption spectra of NAI-TPE-PyS in different solvents. Concentration: 10 µM. Reprinted with permission from
Chapter 1 Reference [66] © Royal Society of Chemistry.
172

Figure 5.4.4 - Emission spectra of NAI-TPE-PyS in different solvents. Concentration: 10 µM, λ ex = 390 nm. Reprinted with
permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
As for the emission spectra of NAI-TPE-PyS in various solvents, the emission peaks
present in the range of 530-590 nm. For the direct color comparison, the photographic images of
each solution were also shown in Figure 5.4.5.

Figure 5.4.5 - Photographic images of NAI-TPE-PyS emission in different solvents at λ ex = 365 nm. Reprinted with permission
from Chapter 1 Reference [66] © Royal Society of Chemistry.
The two modules of NAI-TPE-PyS can be activated under independent optical control
(Figure 5.4.6). When excited at 350 nm, NAI-TPE-PyS displays characteristic narrowed emission
of naphthalene derivatives centered at 400 nm, which indicates that the emission mainly stems
from NAI-based module. When excited at 390 nm, NAI-TPE-PyS emits orange luminescence with
the maximum peak at 585 nm dominated by the emission of TPE-based PET dye, while no
emission of NAI-module was observed. Consequently, the dual-component compound can be
controlled by two wavelengths, where 390 nm is used to excite the TPE-module for voltage
imaging and 350 nm is used to drive the photo-conductive NAI-module for voltage modulation.
173

The difference in energy between these wavelengths (circa 400 meV) is enough to prevent
crosstalk between the two modules.

Figure 5.4.6 - Emission spectra of NAI-TPE-PyS at λ ex = 350 nm (black squares) and λ ex = 390 nm (red circles) in PBS buffer
(concentration: 10 µM, containing 1% DMSO). Reprinted with permission from Chapter 1 Reference [66] © Royal Society of
Chemistry.
To investigate the predicted solvatochromism of the TPE-PyS module from the previous
DFT part, the emission was characterized in several solvents. When excited at 390 nm in PBS, the
NAI-TPE-PyS emitted at 582 nm (Figure 5.4.7). The photoluminescent quantum yield in an
aqueous solution (10 µM, containing 1% DMSO) was measured to be 0.02 (2%). When the NAI-
TPE-PyS was a solid, the quantum yield increased to 8%, providing preliminary evidence of AIE
behavior.
174

Figure 5.4.7 - Optical absorption and emission properties of NAI-TPE-PyS. (a) Images of fluorescence from NAI-TPE-CHO and
NAI-TPE-PyS solutions and solid powder (λ ex = 365 nm). (b) Normalized absorption spectra (solid symbols) and emission spectra
(hollow symbols) of NAI-TPE-PyS in DCM or PBS or in solid state, and NAI-TPE-CHO at λ ex = 390 nm. Reprinted with
permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
This emission wavelength blue-shifts when the solvent is changed to DCM (Figure 5.4.7).
To further confirm this behavior, the emission was characterized over a range of solvents with
varying polarities from chloroform to water, and a linear Lippert–Mataga correlation was
established by plotting Stokes shifts (νabs–νem) against solvent orientation polarizability (Δf)
(Figure 5.4.8) [59], showing a positive solvotochromic effect.
175

Figure 5.4.8 - Lippert-Mataga plot for NAI-TPE-PyS in different solvents as a function of solvent polarity. (Df: orientation
polarizability; n abs-n em: Stoke shifts). Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
The relationship between the Stoke shift (nabs-nem) of the fluorophore and orientation
polarizability f (e, n) can be described by the Lippert-Mataga equation (Equation 5.4.1):
ℎ𝑐(n
(f0
-n
71
)=ℎ𝑐gn
(f0
h
-n
71
h
i+
Hgj
k
*j
l
i
A
(
m
𝑓(𝜀,𝑛) (5.4.1)
where h is Plank’s constant, c is the velocity of light, ƒ is the orientational polarizability of the
solvent, n
(f0
h
-n
71
h
corresponds to the Stokes shifts when ƒ is zero, 𝜇
7
is the excited-state dipole
moment, 𝜇
q
is the ground-state dipole moment, a is the solvent Onsager cavity radius derived from
Avogadro number (N), molecular weight (Mn) and density (d =1.0 g/cm
3
), and e and n are the
solvent dielectric and the solvent refractive index, respectively.
𝜇
7
can be calculated according to Equation 5.4.2:
𝜇
7
=𝜇
q
+r
s2(
m
H
∗u
. (n
vwx
-n
kb
)
.N (e,y)
z{
|
A
(5.4.2)
where 𝜇
q
was estimated around 29.3 D from DFT in the gas phase at the B3LYP/6-31g* level of
theory 31G(d) level, and 𝜇
7
was calculated to be 59.1 D. This large value indicates that the CT-
component of the excited state will be stabilized in polar media [60,61].
176

Table 5.4.1 summarizes the detailed photophysical data of NAI-TPE-PyS in different
solvents including orientation polarizability of selective solvents (Df), maximum absorption
wavelength (labs) and maximum emission wavelength (lem) derived from the UV and FL peaks
and calculated nabs, nem and Stoke shift (nabs-nem). The quantitative analysis of the
solvatochromic behavior of NAI-TPE-PyS will be shown in the lateral part of this section.
Table 5.4.1 - Detailed photophysical data of NAI-TPE-PyS in selective solvents. Reprinted with permission from Chapter 1
Reference [66] © Royal Society of Chemistry.
Solvent Df l abs (nm) l em (nm) n abs (cm
-1
) n em (cm
-1
) n abs-n em (cm
-1
)
DCM 0.218 398 540 25125.63 18518.52 6607.11
chloroform 0.149 410 534 24390.24 18726.59 5663.652
THF 0.210 391 545 25575.45 18348.62 7226.824
DMSO 0.263 386 541 25906.74 18484.29 7422.447
DMF 0.276 387 541 25839.79 18484.29 7355.505
Methanol 0.309 388 541 25773.2 17482.52 8290.678
water 0.32 386 580 25906.74 17241.38 8665.356
PBS _ 398 582 25125.63 17182.13 6607.11
To further investigate the CT process, the related molecule NAI-TPE-CHO was compared.
The maximum emission peak of this molecule was at 420 nm, representing a large bathochromic
shift of 162 nm from the NAI-TPE-PyS compound. This difference can be ascribed to the donor-
spacer-acceptor structure. Such interplay also explains the relatively weak emission of NAI-TPE-
PyS compared to other published TPE-based AIEgens [62], as non-radiative deactivation of the
CT state competes with the radiative process.
Based on the combination of the computational and experimental results, we conclude that
the reporter module will be a voltage sensitive dye. Computationally, the reporter has the proper
ordering of excited state energies and dipole moments to facilitate voltage dependence.
Spectroscopically, the synthesized dye displays the characteristic solvatochromism associated with
voltage sensitive fluorophores due to sensing of the dielectric environment. We expect that it will
177

demonstrate voltage sensitive luminescence when the molecule's motion is restricted, for example,
if the molecule is inserted and aligned in a cellular membrane.
To further investigate the role of pH on emission, the emission spectra of the molecule
were collected in PBS buffer with pH values ranging from 3 to 10 (Figure 5.4.9). Figure 5.4.10
and Figure 5.4.11 show the results from a study of the fluorescence intensity in a range of pH
environments. The pH was controlled by NaOH and HCl aqueous solutions, and the pH values
were verified with a pH meter. The spectra exhibited minimal differences in the emission profile
and the corresponding FL intensity shows subtle fluctuations as well. Hence, the molecule is stable
under the physiological pH range.

Figure 5.4.9 - Photographic images of NAI-TPE-PyS emission in PBS buffer with different pH values λ ex= 365 nm (Concentration:
10 µM, containing 1% DMSO).

Figure 5.4.10 - Emission spectra of NAI-TPE-PyS in PBS buffer solutions with different pH values. Concentration: 10 µM,
containing 1% DMSO. λ ex= 390 nm.
178

Figure 5.4.11 - Plot of fluorescence peak intensity (I/I 0) vs. pH values. The data was extracted from Figure 5.4.10.
5.4.3 Aggregation Induced Emission Properties
The measurement of AIE property requires an environment where the fluorophores form
aggregates or micelles. To rigorously investigate the AIE behavior of the TPE core, two
approaches were used to initiate aggregation in a controlled manner. First, poor or low efficacy
solvents were used. In our studies, DMSO was an ideal solvent for NAI-TPE-PyS, and THF was
an anti-solvent for NAI-TPE-PyS. Thus, by using a mixture of DMSO/THF, aggregates were
induced. Second, the concentration of the NAI-TPE-PyS molecule in water was systematically
increased, resulting in aggregation or micelle formation.
The DMSO solution of NAI-TPE-PyS (20 µM) exhibited weak fluorescence (lex = 390
nm) centered at 545 nm in Figure 5.4.12a. As the volume fraction of the anti-solvent THF was
increased, from 0% to 99%, the emission intensity increased, due to the formation of aggregates.
Finally, at a THF fraction of 99%, the emission intensity of the mixture reached a maximum which
was 3-fold higher than that of the initial DMSO solution (Figure 5.4.12a). Based on the results in
Figure 5.4.12, the introduction of the alkyl NAI terminals did not eliminate the AIE response.
However, it should be noted that, unlike other previously reported TPE-based AIEgens whose
179

emission experiences significant enhancement upon aggregation in solution, the emission intensity
of NAI-TPE-PyS at high THF fraction merely increases three times. This could be attributed to
the long alkyl chains of the NAI-TPE-PyS interfering with the aggregation process, ultimately
limiting the size of the aggregates [60]. Similar increases in emission intensity were observed as
the concentration of NAI-TPE-PyS in water was increased from 0.01 to 80 µM (Figure 5.4.12b).

Figure 5.4.12 - AIE behavior of NAI-TPE-PyS. (a) Plot of relative PL intensity versus THF fraction. Inset: PL spectra of NAI-
TPE-PyS in DMSO/THF mixtures with different THF fractions (f THF). Concentration of NAI-TPE-PyS is 20 µM; λ ex = 390 nm. (b)
Plot of PL intensity versus NAI-TPE-PyS concentration in water. Inset: PL spectra of aqueous solutions of NAI-TPE-PyS at
concentrations ranging from 0.01 µM to 80 µM (λ ex = 390 nm). Reprinted with permission from Chapter 1 Reference [66] © Royal
Society of Chemistry.
Dynamic light scattering (DLS) is used to prove the formation of NAI-TPE-PyS aggregates
in aqueous media. DMSO solution of NAI-TPE-PyS (8 mM) was diluted with DI water to give 50
µM sample solution in DMSO/water mixture with water fraction of 99 % (v%). The measurements
were conducted on cuvette-based DLS instrument DynaProÒNanoStarÒ, WYATT Technology.
The obtained particle size distribution was plotted in Figure 5.4.13 and the diameter of the
nanoaggregates was measured to be approximately 500 nm, analyzed by DYNAMICSÒ software.
180

Figure 5.4.13 - Particle size distribution of NAI-TPE-PyS aggregates in DMSO/THF mixture with a 99% THF fraction.
Concentration: 50 µM. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
5.4.4 Nonlinear Optical Properties
In addition to single photon excitation processes, NAI-TPE-PyS can also undergo two-
photon excitation processes. The emission decay lifetime under two photon excitation was
measured using the multi-photon mode of a Fluorescence Lifetime Imaging Microscope (FLIM)
[63]. Multiphoton FLIM imaging of the solutions were then acquired with Leica SP8 microscope
at the USC Translational Imaging Center. Samples of NAI-TPE-PyS were prepared in DMSO at
nominal concentrations of 100 µM and 10 mM, dropped on an ibidi
®
Micro slide. Pure DMSO
was also used as a control sample. At these concentrations, a significant portion of the solute is
aggregated into suspended solid particles. The samples were excited at λ ex = 750 nm, and the
corresponding emission signals were collected through a yellow-pass filter centered around 580
nm. Lifetime of NAI-TPE-PyS solutions was given by phasor plots transformed from the raw
FLIM images pixel by pixel containing two phasor vectors (G, S) and intensity information. A
customized MATLAB code was written to generate G-S phasor coordinates for phasor plots. As
shown in Figure 5.4.14-5.4.16, the fluorescent background of a DMSO blank falls on the universal
circle of phasor plot, indicating a single exponential decay likely arising from the sample holder.
181

The NAI-TPE-PyS sample produces a multi-exponential decay located inside the circle. This
complex decay time is a weighted average of three species: the fluorescent
background, monomeric NAI-TPE-PyS in solution, and solid aggregates of NAI-TPE-PyS in
suspension. The averaged emission decay times are 0.89 ns and 0.96 ns for 100 µM and 10 mM
solutions, respectively. The comparatively longer average lifetime of the more concentrated
sample is consistent with a greater contribution from the solid aggregates. These results
demonstrate the two-photon excitation and emission of NAI-TPE-PyS using NIR (750 nm). This
ability is a critical stepping stone to allow long-term deep tissue voltage imaging.

Figure 5.4.14 - Phasor plots of NAI-TPE-PyS in DMSO with concentration of 10 mM.
182

Figure 5.4.15 - Phasor plots of NAI-TPE-PyS in DMSO with concentration of 100 µM.

Figure 5.4.16 - Phasor plots of pure DMSO used as a control.
Time-resolved photoluminescence (TRPL) measurements were performed to obtain the
emission decay lifetime of NAI-TPE-PyS/DMSO solution at room temperature using a standard
confocal microscope-based Time-Correlated Single Photon Counting (TCSPC) setup.
183

For this measurement, NAI-TPE-PyS was dropped cast (10 µM, containing 1% DMSO)
on a glass slide and a 750 nm pulsed laser (Spectra-Physics, Mai Tai, Mode-Locked Ti:Sapphire
Laser) with a pulse width of less than 200 fs and repetition rate of 80 MHz was focused on the
sample with a microscope objective. The PL signal from the sample was then passed through two
short pass filters with a cut-off wavelength of 550 nm in order to reject the laser light from the
laser radiation. Finally, the PL photons were detected with a Si Avalanche Photo Diode (Si-APD)
and the PL transient was measured as a function of time. The TCSPC histogram and exponential
decay fit are provided in Figure 5.4.17.

Figure 5.4.17 - TCSPC histogram of NAI-TPE-PyS under 750 nm excitation. Red: transient decay trace; Blue: exponential fit line;
Inset: exponential fit parameters. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
Fluorescence at wavelengths shorter than 550 nm were observed from NAI-TPE-PyS with
a monoexponential excited state decay time of 160 ps. Using the photoluminescent quantum yield
of 0.02 measured previously, the radiative and non-radiative rate constants are calculated to be
1.25 × 10
8
s
−1
and 6.125 × 10
9
s
−1
respectively, which is consistent with a weakly emitting organic
fluorophore. These results confirm the predicted two-photon behavior of NAI-TPE-PyS using NIR
184

excitation (750 nm). This ability is a critical stepping stone in the development of a two-photon
voltage imaging and modulation probe.
5.4.5 Photoconductivity Characterization
The 'modulate'-functionality relies on the NAI-module of the NAI-TPE-PyS. To
characterize the photoconductivity, a two-terminal device was fabricated with Ti/Au electrodes,
and a thin film of NAI-TPE-PyS was deposited (Figure 5.4.18a, inset). Sample preparation is
shown below:
SiO2/Si (2 µm thermal SiO2) substrate was cleaned with acetone, isopropanol and
deionized water. A combination of photolithography, metal deposition, and lift-off were performed
to pattern the electrical pad. Photoresist (AZ 5214) was spuncoat for 60 seconds at 3000 rpm, and
a standard template mask with channel dimensions of 200 µm (length) × 5000 µm (width) was
used as the electrode pattern. 5 nm Ti and 100 nm Au was deposited using e-beam evaporation
(Temescal, SL1800), and the residual photoresist was lifted-off, completing the electrode
fabrication. A thin film of NAI-TPE-PyS was spun-coat on the device from a 6 wt% chloroform
solution for 30 s with 500 rpm.
Current vs. voltage measurements were performed both in dark and under light (350 nm)
illumination. Photoconductivity of the device and I−t measurements were characterized by a
Semiconductor Parameter Analyzer (Keysight B1500a). UV light source was provided by
DYMAX LED DX-100 with lex = 350 nm. As shown in the I–V curves in Figure 5.4.18a, the
device was insulating in the dark. Upon illumination with 19 mW of 350 nm light, photo-induced
current was observed clearly. This increase is attributed to the electron-hole pair generation and
dissociation within the NAI-module. The stability and real-time change in photo-induced
conductance of the device were further studied based on current vs. time measurements at a fixed
185

voltage of 10 V. From the I–t curves in Figure 5.4.18b, photo-induced current can be clearly
distinguished from the device. The photocurrent increased or decreased sharply as the light was
turned on and off, and the dark current was stable. The device also displayed an increase in
responsivity as the optical power increased, demonstrating the power-dependent photoconduction
behavior of NAI-module (Figure 5.4.18c). It is noteworthy that the photocurrent remained stable
over the course of these measurements which were performed in an ambient environment (Figure
5.4.19 and Table 5.4.2).

Figure 5.4.18 - Photoconductivity of NAI-TPE-PyS. (a) current vs. voltage measurements of the device. Inset: Schematic of NAI-
TPE-PyS-coated two-terminal device. SiO 2:2 µm; Ti: 5 nm; Au: 100 nm. Device channel size: length × width = 200 µm × 5000
µm. (b) current vs. time measurements of the device at different incident optical powers. (c) Responsivity vs. voltage of the device
at different incident optical powers. Reprinted with permission from Chapter 1 Reference [66] © Royal Society of Chemistry.

Figure 5.4.19 - I-t curve of the device at fixed voltage of 10 V upon light illumination with 19 mW optical power. Reprinted with
permission from Chapter 1 Reference [66] © Royal Society of Chemistry.
186

Table 5.4.2 - The responsivity studies of our molecules. Reprinted with permission from Chapter 1 Reference [66] © Royal Society
of Chemistry.
Wavelength (nm) Incident Optical Power (mW) Responsivity (µA/W)
Dark N/A 0.07
350 8 0.16
350 12.5 0.22
350 16 0.30
350 19 0.41
5.4.6 Cyclic Voltammetry Measurements
Cyclic voltammetry (CV) measurements of NAI-TPE-PyS were conducted by making a 40
µM solution of the compound in DCM using a glassy carbon electrode as the working electrode, a
Pt wire as the counter electrode, and an Ag/AgCl electrode as a reference electrode. The
experiment was performed in a 0.3 M tetrabutylammonium hexafluorophosphate (TBAPF6)
electrolyte solution at room temperature under the argon atmosphere. Figure 5.4.20 shows the
utilized cell with the working, counter, and reference microelectrodes.

Figure 5.4.20 - Cell set up with a glassy carbon working electrode, platinum as counter electrode, and Ag/AgCl as the reference
electrode.
187

Figure 5.4.21 - (a) CV data for NAI-TPE-PyS in solution in the potential range of -1.5 to 1.6 V as a function of various scan rates
from 10 to 100 mV s
-1
and (b) from 0 to 1.6 V. (c) Log of the peak current (i) vs. log of the scan rate (v) for the data shown in
Figure 5.4.21a,b. (d) CV curves of NAI-TPE-PyS in solution at 20 mV s
-1
for 20 cycles.
Kinetics were analyzed based on a series of CV measurements as a function of scan rate
from 10 to 100 mV s
-1
(Figure 5.4.21a,b). From Figure 5.4.21a, the onset reduction potential of
NAI-TPE-PyS can be determined around -0.4 eV, and the onset oxidation potential is at 0.8 eV.
According to the cyclic voltammograms, the molecule exhibits one quasi-reversible cathodic
reduction peak at half-wave potential (E1/2) of -0.7 V and one irreversible anodic oxidation peak
at E1/2 = 1.2 V vs Ag/AgCl (Figure 5.4.21a). The reduction wave is assigned to the one-electron
reduction of naphthalene imide moiety (two equivalents), forming monoanion species
(NAI/NAI
•−
), while the oxidation wave is assigned to the oxidation of TPE moiety.
188

The minimal shifts of the anodic and cathodic peaks by increasing the scan rates indicates
rapid reaction rates. To further quantify the kinetics redox process, we examined the relation
between the measured current (i) and scan rate (v) denoted by Equation 5.4.3 [64]:
𝑖 =𝑎𝑣
f
(5.4.3)
where b can be determined by the slope of a plot of log (i) vs. log (v) plots for each redox peak. A
value of b equal to 0.5 indicates a process controlled by semi-infinite diffusion, while b is close to
1 indicates a non-diffusion-controlled charge-storage process. From the plot of log (i) vs. log (v)
(Figure 5.4.21c), the b value of the anodic peak is 0.5, indicating that process is controlled by semi-
infinite diffusion.
The HOMO energy level of NAI-TPE-PyS can be calculated from the onset oxidation
potential of the anodic peak denoted by Equation 5.4.4 [65], which gives HOMO(CV) is -5.2 eV.
𝐸

=−[𝐸
'8
'307&
+4.4] (5.4.4)
The LUMO energy level of NAI-TPE-PyS can be calculated from the onset reduction
potential of the anodic peak denoted by Equation 5.4.5 [65], which gives LUMO(CV) is -3.7 eV.
𝐸
T
=−[𝐸
-7.
'307&
+4.4] (5.4.5)
Furthermore, to verify the electrochemical stability of NAI-TPE-PyS, 20 continuous cycled
scans were conducted between -1.5 V and 1.6 V at 20 mV s
-1
. Figure 5.4.21d shows that the anodic
peak remains at the same performance, indicating the redox stability and high reversibility of the
NAI-TPE-PyS.
5.4.7 Cytotoxicity of NAI-TPE-PyS
Lastly, to confirm the suitability of NAI-TPE-PyS as a biocompatible imaging agent, the
cytotoxicity of NAI-TPE-PyS was investigated by performing CellTiter-Glo
®
Luminescent Cell
Viability Assay to study the cytotoxicity of NAI-TPE-PyS on the human breast cancer cell line
189

MCF-7 and the human bone osteosarcoma epithelial cell line U2OS. Both cell lines were were
acquired from American Type Culture Collection (ATCC) and cultured in DMEM (1X, Gibco)
high glucose cell culture medium with 10% fetal bovine serum (Gibco) and Penicillin-
Streptomycin antibiotic solution (GeminiBio) under a humidified atmosphere of 5% CO2 at
37°C. The cell species was confirmed using short tandem repeat (STR) DNA profiling and
Cytochrome C Oxidase I (COI) assays. Approximately 10,000 cells/well for U2OS cell line and
15,000 cells/well for MCF7 cell line were separately seeded in 96 well plates and incubated for 48
hours. Afterwards, various concentrations of NAI-TPE-PyS solutions were added to the cell
medium in each well with the final concentrations ranging from 10 nM to 50 µM (containing 1 %
DMSO). Culture dishes and plates were purchased from Corning Inc. This concentration range
exceeds that which would normally be used in optical imaging. Additionally, control groups with
only media (positive control), 1 % DMSO and 200 µM Tamoxifen (TMX, negative control, Sigma
Aldrich) were also prepared for comparison. After 24 hours incubation, CellTiter-Glo
®
Reagent
(Promega Corporation) were added to the wells based on the CellTiter-Glo
®
protocol, and
luminescence of each well was recorded, which is directly proportional to the number of viable
cells present in culture. By comparing the luminescence of the cell plate wells, the biocompatibility
of the molecule was confirmed. The assay was performed on the two cell lines with three replicates
and each replicate was repeated for four times to reduce the chance of possible errors. Control
samples were prepared as follows: control group with only cells, negative control with only 1 %
DMSO, and positive control with 200 µM TMX in cell medium (TMX was also initially dissolved
in DMSO).
After 24 hr incubation, the viability of both cell lines remains high and, at low to moderate
concentrations, the viability is similar to the control cells that are in media. At low concentrations,
190

the decrease of viability is inevitably caused by the toxicity of DMSO. For MCF-7 cell line, the
decrease of viability (Table 5.4.3) is trivial with the increase of compound concentrations, while
lower concentration of the compound is needed to remain the high viability (Table 5.4.4) of U2OS
cell line (Figure 5.4.22). With the high viability mentioned above, it can be concluded that NAI-
TPE-PyS exhibits negligible cytotoxicity within standard imaging concentrations, making it a
promising multi-functional imaging agent for live cell imaging.

Figure 5.4.22 - Viability of MCF7 cell line (black) and U2OS cell line (red) treated with concentrations of NAI-TPE-PyS from
0.01 µM to 50 µM. Inset: Comparison of three control measurements (media, 1% DMSO, 200 µM TMX) with three concentrations
of NAI-TPE-PyS.

191

Table 5.4.3 - Viability of MCF7 cell line
Viability (%)
MCF7 cell line
Cells
Only
1%
DMS
O
10
nM
100
nM
500
nM
2.5
µM
5 µM
12.5
µM
25
µM
50
µM
200
µM
TMX
Replicate
1
Repeat1 97.28 82.67 91.89 99.57 92.56 82.93 87.25 84.74 83.28 79.23 2.65
Repeat2 100.33 77.49 95.33 88.51 85.93 81.72 86.11 82.53 83.20 71.36 2.28
Repeat3 99.86 72.76 94.34 87.73 86.18 86.09 84.20 80.16 84.06 71.26 2.23
Repeat4 102.53 80.62 89.92 87.07 83.85 80.61 88.18 85.15 84.27 80.24 0.97
Replicate
2
Repeat1 105.67 85.65 89.38 80.48 89.34 83.79 83.01 80.53 91.95 79.85 1.09
Repeat2 98.44 85.04 82.60 84.23 84.93 83.63 82.66 82.04 83.79 77.21 0.86
Repeat3 97.85 84.70 79.55 81.91 81.19 81.78 79.45 77.20 83.92 77.47 0.78
Repeat4 98.03 82.00 76.59 80.98 81.47 83.79 83.12 80.88 78.96 77.19 0.45
Replicate
3
Repeat1 100.48 85.76 96.95 83.95 82.26 82.44 91.83 87.42 75.30 84.84 2.49
Repeat2 100.64 80.43 86.38 88.40 81.83 81.09 87.93 87.24 83.96 82.96 2.10
Repeat3 101.57 84.48 87.06 86.09 84.67 87.01 90.33 86.32 84.58 81.79 2.02
Repeat4 97.31 82.46 86.21 89.50 80.98 91.21 87.59 90.24 86.16 85.57 0.89
Average 100 82.01 88.02 86.54 84.60 83.84 85.97 83.70 83.62 79.08 1.57
Standard Deviation 2.48 3.82 6.27 5.12 3.55 3.00 3.57 3.77 3.91 4.57 0.79
Table 5.4.4 - Viability of U2OS cell line
Viability (%)
U2OS cell line
Cells
Only
1%
DMS
O
10
nM
100
nM
500
nM
2.5
µM
5 µM
12.5
µM
25
µM
50
µM
200
µM
TMX
Replicate
1
Repeat1 103.95 80.36 90.22 84.37 82.15 70.29 71.07 73.58 70.43 62.66 2.36
Repeat2 103.92 78.08 85.98 89.08 81.96 68.85 63.21 67.11 73.77 50.91 1.91
Repeat3 95.82 67.25 84.43 86.00 75.72 84.87 73.77 80.96 64.32 77.02 1.98
Repeat4 96.31 69.47 75.34 73.30 70.56 77.68 70.56 68.84 64.54 65.76 0.86
Replicate
2
Repeat1 102.02 74.94 85.81 80.89 88.68 78.25 76.79 75.08 69.45 76.34 2.32
Repeat2 101.42 67.97 85.60 84.33 90.15 69.87 80.65 74.17 74.58 67.07 1.96
Repeat3 100.08 76.65 85.34 81.33 87.95 87.86 83.32 76.91 72.23 80.11 1.91
Repeat4 96.47 61.85 84.14 67.64 86.79 80.99 75.54 68.55 63.49 56.03 0.82
Replicate
3
Repeat1 103.65 76.74 79.54 78.61 76.77 68.57 69.03 70.99 55.64 44.45 1.09
Repeat2 102.08 66.89 83.45 76.56 79.26 63.61 73.00 57.29 59.74 42.12 0.86
Repeat3 96.92 68.42 77.40 89.07 74.65 77.38 75.97 48.04 59.28 57.95 0.82
Repeat4 97.33 68.90 83.92 74.75 72.16 69.76 69.77 64.76 54.49 49.45 0.46
Average 100 71.46 83.43 80.41 80.57 74.83 73.56 68.86 65.16 60.82 1.45
Standard Deviation 3.23 5.67 4.10 6.66 6.75 7.42 5.42 9.00 6.95 12.86 0.69
192

5.5 Summary
In this chapter, the multifunctional molecular device, NAI-TPE-PyS, has been successfully
designed, synthesized, and characterized. Electric field sensing and modulation abilities are
derived from its two covalently coupled modules: a TPE-derived module connected by an alkyl
chain to an NAI-derived module. To prove the potential of NAI-TPE-PyS as a multifunctional
electric field molecular probe, the photophysical and optoelectronic properties have been
investigated experimentally and theoretically.
The TPE-module is designed for voltage imaging where the TPE core is the donor and is
conjugated with a pyridinium inner salt serving as the acceptor, forming the D–π–A backbone
required by a PET dye. The module exhibits a broad absorption profile and a solvent-dependent
emission owing to the intramolecular charge transfer (CT) between the donor and acceptor. The
CT-component of the excited state has been further confirmed through theoretical calculations. In
addition, the module displays two-photon excited emission upon NIR irradiation. Photo-induced
current is observed when the module is actuated with UV illumination, which results from hole-
electron generation and transportation within the NAI moieties. In addition, preliminary results
show compatibility with a pair of cell types at moderate to high concentrations demonstrating a
high likelihood of its success in future bioimaging experiments.
The study provides a new strategy for developing multifunctional molecules by
synergistically combining the emerging fields of small molecule photoconductors and small
molecule multi-photon fluorophores. In the future, the multifunctional molecule demonstrated here
could prove to be a valuable tool in understanding the complex field of bioelectricity and in
designing integrated optoelectronic quantum circuits.
193

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202

Appendices
Appendix A. Magnetic Polymers
Stimuli-responsive polymers have been drawn into attention owing to their synthetic
tunability and structure-property relationship. Conventional external stimuli include pH,
temperature, light, magnetic field, electric field, chemicals, or ionic strength [1]. Magnetism, the
ability to respond to the externally applied magnetic field, is of particular interest in the fields of
optics, materials, biomedical engineering and chemistry [2–4]. In the material aspect, magnetic
nanoparticles have been extensively studied in the past, with representative examples including
iron oxide and its nanoparticles. In addition, polymeric materials have been considered as another
pathway to develop smart responsive materials that can exhibit varied physicochemical properties
inherent to their structural features. For instance, the biocompatible poly(2-(N,N-dimethylamino)
ethyl methacrylate) (PDMAEMA) exhibits a pH and thermal stimuli-responsive phase-transition
behavior [5].
In this appendix, the synthesis of magnetic polymers will be discussed. In Figure A.1, 2-
(N,N-dimethylamino) ethyl methacrylate (DMAEMA) is considered as the monomer with a
tertiary amine in the pendent group. In first reaction, the atom transfer radical polymerization
(ATRP) reaction of DMAEMA, the polymerization initiator α-Bromoisobutyryl bromide (BIBB),
the ligand 1,1,4,7,10,10-Hexamethyltriethylenetetramine (HMTETA) and the ATRP catalyst
CuBr were also used (yield 92.1%). In the second reaction, the alkane halide iodomethane or
aromatic halide benzyl chloride were used to perform the quaternization reaction where the
pendent tertiary amine was transformed into the quaternary amine, which acts as the precursor for
the subsequent magnetic complexation (yield 70.8%). In the last reaction, Iron (III) chloride
hexahydrate (FeCl3·6H2O) is introduced to the aqueous solution at room temperature to form the
203

paramagnetic polymer (PDMAEMA-CH3-Fe, yield 75.7%). Tetrachloroferrate anion (FeCl4
−
) is
the functional moiety for the paramagnetic susceptibility according to the previous studies [6,7].

Figure A.1 – Syntheses of PDMAEMA and the corresponding polymeric salts using BIBN.
In Figure A.2, the majority of the reaction protocols were similar to the one in Figure A.1,
other than the difference in the reaction initiator, Ethyl α-bromoisobutyrate (EBIB). With the
EBIB-triggered polymerization, EBIB-PDMAEMA-CH3-Fe can be synthesized accordingly, with
a similar yield of ~70%.
Molecular weight (Mn) and polydispersity index (PDI) are the most fundamental properties
to characterize in polymers. Gel permeation chromatography (GPC) was conducted on a Waters
GPC system with refractive index detector and four Waters Styragel columns (Styragel HR1, HR4,
HR4E, HR5E). GPC experiments were performed at room temperature under a tetrahydrofuran
(THF) flow rate of 1 mL/min. Relative molecular weights were determined using polystyrene
calibration standards [8,9]. The GPC curve of EBIB-PDMAEMA is shown in Figure A.3, with Mn
of 5400 g mol
-1
and PDI of 1.14. Since the polymers EBIB-PDMAEMA-CH3 and EBIB-
PDMAEMA-CH3-Fe are ionic, their GPC measurements were not conducted as they may clot the
current GPC columns. Styragel columns are best suited for organic solvents, such as toluene, THF,
and N,N’-dimethylformamide.
BIBB
HMTETA, CuBr
dioxane, 100 ℃
DMAEMA
n
O O
N
Br
O O
N
Br
O
R-X (1. CH
3
I; 2. )
dioxane, 70 ℃, overnight
n
Br
O O
N
R
X
O
Br
FeCl
3
· 6H
2
O
H
2
O, rt, overnight
n
Br
O O
N
R
Br
O
FeCl
3
X
PDMAEMA
Cl
PDMAEMA-CH
3
or
PDMAEMA-Bzl
PDMAEMA-CH
3
-Fe
or
PDMAEMA-Bzl-Fe
204

Figure A.2 – Syntheses of EBIB-PDMAEMA and the corresponding polymeric salts using EBIB. Reprinted with permission from
Appendices Reference [5] © Royal Society of Chemistry.

Figure A.3 – GPC curve of EBIB-PDMAEMA.
After synthesizing the magnetic polymers, the proof-of-concept demonstration of
magnetism was completed using a magnet in Figure A.4. As seen, all polymers after the metallic
complexation reaction can be pulled onto the magnet, showing the paramagnetic phenomenon.
O O
N
DMAEMA
EBIB
HMTETA, CuBr
dioxane, 100 ℃
Br
O O
N
O
O
n
EBIB-PDMAEMA
Br
O O
N
O
O
n
CH
3
I
dioxane, 70 ℃, overnight
EBIB-PDMAEMA-CH
3
FeCl
3
· 6H
2
O
H
2
O, rt, overnight
Br
O O
N
O
O
n
EBIB-PDMAEMA-CH
3
-Fe
I FeCl
3
I
205

Figure A.4 – Pictures of PDMAEMA-based polymers sticking on the magnet: (a) PDMAEMA-bzl-Fe; (b) PDMAEMA-CH 3-Fe;
(c) EBIB-PDMAEMA-CH 3-Fe.
Appendix B. Mechanochromic Stretchable Devices
Many types of organic small molecule and polymer dyes are known to change their
emission color upon the implementation of mechanical deformations [10,11]. This unique
phenomenon is termed mechanochromism, in which mechanical force induces the color change in
the material. Mechanochromism arises from changes in molecular arrangement of the
chromophores, typically stimulating the rearrangements of π–π stacking [12], hydrogen bonds [13]
and other types of interactions [14]. Conventional polymeric mechanochromic materials have been
developed in pressure sensors [15], healthcare [16], and other applications [17,18].
In this appendix, an alternative path for the fabrication of the mechanochromic micro-
devices were studied. As opposed to the traditional strategy to synthesize sophisticated
mechanochromic polymers, this type of device contains a direct hybrid structure of the mechano-
chromophore and the flexible polymeric substrate. The main chromophore utilized was
tetraphenylethylene (TPE) and its derivative owning to the previously mentioned aggregation-
induced emission feature in Chapter 5. The synthetic scheme of the TPE bearing four sulfonate
groups (4-SuTPE) is shown in Figure B.1. In this two-step synthesis, the hydrophobic TPE can be
converted into a water-soluble chromophore.
206

Figure B.1 – Synthetic scheme of 4-SuTPE. Reprinted with permission from Appendices Reference [19] © Taylor & Francis
Online.
The chemical structure and
1
H-NMR spectrum of 4-SuTPE were presented in Figure B.2.
The characteristic doublet peaks of the aromatic protons in the TPE core were located at 7.59 ppm
and 7.28 ppm, respectively.

Figure B.2 – (a) Chemical structure of 4-SuTPE with distinct protons; (b)
1
H-NMR spectrum of 4-SuTPE.
In this study, two geometric types of mechanochromic devices were investigated. The first
one was a dumbbell-shaped device where the flexible polymer was PAA/PEO stereocomplex and
the chromophores used were 4-SuTPE and Rhodamine (negative control). As PAA/PEO offers
207

excellent stretchability mentioned in Chapter 4, the stretching-induced mechanochromism can be
demonstrated using the dumbbell-shaped film.
The preparation of the hybrid PAA/PEO/chromophore film was similar to the polymer
preparation in Thesis Section 4.2.1, with changes noted here. Basically, a 50 mL PAA aqueous
solution (1.8 mg/mL) was prepared in a 100 mL beaker, followed by preparing a 50 mL PEO
aqueous solution (2.9 mg/mL) in another 100 mL beaker. Two aqueous solutions were mixed,
ramped up to 30 °C, and kept for 30 min until all polymers were fully dissolved in water. Then,
the chromophore solutions were also prepared. For 4-SuTPE, it was dissolved in deionized water
at the concentration of 10 mg mL
-1
. Rhodamine was prepared in ethanol at the concentration of 10
mg mL
-1
. Either 4-SuTPE or Rhodamine chromophore solution was added into PAA and PEO
aqueous solutions with 100 µL, respectively. The pH of PAA and PEO solutions were tuned to 2.8
with hydrochloric acid (VWR, 36%, ~100 µL), which is an optimal pH to avoid the side
dimerization and ionization of the carboxylic groups.
Next, the PAA and PEO solutions (containing the chromophore solutions) were slowly
mixed into a third 250 mL beaker via a syringe pump at a rate of 5 mL/min. This leads to the
efficient precipitation of the PAA/PEO/chromophore pellets, which were further collected via
centrifugation. To perform the centrifugation of the hybrid pellets, the mixed suspension solutions
were collected in Falcon tubes with 7800 relative centrifugal force at 0 °C for 30 min. The
supernatant was disposed to acquire the pellets. Lastly, the pellets were compressed into the
uniform films.
208

Figure B.3 – Pictures of (a) PAA/PEO dumbbell film and (b) PAA/PEO/Rhodamine dumbbell film under room light. Pictures of
(c) PAA/PEO dumbbell film and (d) PAA/PEO/Rhodamine dumbbell film under 365 nm.
The dumbbell-shaped PAA/PEO/Rhodamine films are shown in Figure B.3. As a negative
control of mechanochromism, the fluorescent intensity will not be changed when the film is being
mechanically deformed. However, the fluorescence of Rhodamine in the film can still be
confirmed under 365 nm excitation. The mechanical deformation on the PAA/PEO/chromophore
films were tested with the setup in Figure B.4. The room light was off to ensure the excitation only
originated from the 365 nm source. Instron was considered as the accurate mechanical elongation
instrument.

Figure B.4 – (a) Testing setup of the mechanical elongation of the PAA/PEO/chromophore film; (b) zoomed-in picture of the clamp
region of the Instron.
The mechanical deformation experiments with the Instron were conducted as shown in
Figure B.5. The PAA/PEO/Rhodamine film was first stretched up to 50% strain and relaxed back
to 0% strain. As seen in Figure B.5a, the fluorescent intensity of the film, caused by the
209

Rhodamine, remained the same. In the case of PAA/PEO/Su-TPE film, there was slightly reduced
fluorescent intensity as the elongation surpassed 100% strain. The contrast of the fluorescent
reduction was not very noticeable, but this could be due to the limited resolution and the scope of
the camera once the strain was larger than 150%. One possibility was that if this type of film could
be imaged with better quality above 200% or even 300% strain, the contrast of fluorescent intensity
could be improved. Another aspect to improve could be the initial concentration of the
chromophore dye solution. If the initial concentration was larger, the contrast of fluorescent
intensity reduction after stretching could also be enhanced.

Figure B.5 – (a) Fluorescent images of (a) PAA/PEO/Rhodamine film and (b) PAA/PEO/SuTPE film deformed at various strains.
The excitation wavelength was 365 nm.
To further improve the observation contrast, another type of mechanochromic stretchable
devices was developed. In this case, the stretchable substrate used was polydimethylsiloxane
(PDMS) stamps with micro/nano-patterned structures. The chromophore used was TPE. The
210

design illustration of the microstamp is shown in Figure B.6a. The underlying silicon master could
contain either the micropillars or microholes. Since the deposition of TPE, which will be discussed
further in the following paragraph, was conducted with dropcasting to improve the deposition
efficiency of TPE, the silicon master contained micropillars (Figure B.6b). In this way, the
replicated PDMS stamp will have microholes.

Figure B.6 – (a) Schematic illustration of PDMS stamp and silicon master; optical microscope images of (b) silicon master with
micropillars and (c) PDMS stamp with microholes.
The experimental protocol of the PDMS stamp preparation is similar to the conventional
PDMS preparation protocol with minor changes noted here [20]. Typically, the PDMS base and
curing agent were mixed with 20:1 ratio (the total mass is around 6-7 g) before degassing in the
vacuum desiccator at room temperature for at least 30 min. Then, the pre-made silicon master with
patterned micropillars was carefully immersed into the degassed PDMS slurry. The PDMS batch
was thermally cured at 80 °C for 1 h. Finally, the silicon master was nicely peeled off to obtain the
PDMS replica stamp (Figure B.6c). Then, the deposition of TPE was conducted. To begin with,
the preparation of TPE/THF solution was performed. The concentration of TPE in THF was 10
-3

mg mL
-1
. This is an optimized concentration because at higher concentrations of TPE solution, the
PDMS stamp forms a crystalline layer of TPE on the top, which covers the micropatterned
structures. Therefore, higher concentration of TPE solution should be avoided. The 100 µL TPE
211

solution was dropcasted on the top surface of the PDMS stamp with microholes, which was dried
in the vacuum desiccator for 30 min.
To monitor the mechanical deformation of the PDMS/TPE microdevices, the sample batch
was placed on the fluorescence microscope to track the fluorescence intensities at various
elongation strains. However, the Instron could no longer be used to apply strain to the sample on
the microscope. Therefore, a customized clamp was designed, shown in Figure B.7a. The
PDMS/TPE sample was clamped (Figure B.7b) with an initial length. By tuning the knob, the
distance between two clamps (d) can be measured and adjusted quantitatively. The excitation
source and detection (imaging) is provided by the fluorescence microscope (Figure B.7c).

Figure B.7 – (a) Customized mechanical clamp to induce mechanical deformation on PDMS/TPE microstructures on the fluorescent
microscope; (b) PDMS/TPE microdevice being clamped; (c) Illumination on the clamped PDMS/TPE microdevice from the
fluorescence microscope.
The fluorescence images of PDMS/TPE microstructures were presented in Figure B.8. The
excitation wavelength for TPE fluorescence was 365 nm. At the initial d = 10 mm, or 0% strain,
212

fluorescence images were taken at the emission wavelength of 470 nm (Figure B.8a). The
fluorescence intensity decreased by stretching the microstructures to d = 15 mm (50% strain,
Figure B.8b), and the intensity further decreased at d = 20 mm (100% strain, Figure B.8c). The
sample broke at above 100% strain. The fluorescence intensities were mapped in Figure B.8d for
direct comparison. As shown, while the fluorescence intensity decreased, the elongation of the
microstructure features corresponded well to the applied strain.

Figure B.8 – Fluorescence images of PDMS/TPE microstructures at (a) 10 mm, 0% strain; (b) 15 mm, 50% strain; (c) 20 mm,
100% strain. (d) Fluorescence intensities at various mechanical strains. The fluorescent excitation wavelength for TPE
chromophore was 365 nm. The blue bars indicate the distances measured for the microstructures at various strains.

213

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Abstract (if available)

Abstract Over the past century, optical devices have catalyzed numerous fields of study and enabled many technologies including lasers, spectroscopy, optical sensing, and displays. Many of these advances relied on innovations in both device design and in optical materials. Therefore, as researchers look towards the future, pursuing the design and synthesis of new material systems by leveraging the structure-property relationship is a promising field. However, any new material can present new fabrication challenges due to incompatibilities with existing techniques.
Two general classes of fabrication include bottom-up and top-down methods. The bottom-up method is taken into the consideration where the design of the optical device starts from the molecular level in order to meet the physical and chemical properties in the macroscopic view. When using organic materials, this method is commonly used. In contrast, the top-down technique starts with a bulk material and removes or deposits additional layers. This method requires compatibility with harsh chemicals. Therefore, it is typically only used with robust substrates, including silicon.
In this thesis, I develop several new optical devices by first designing and optimizing new functional and multi-functional optical organic materials and then combining them with different device platforms. By using optically and mechanically tunable materials, I demonstrated reversibly tunable on-chip whispering-gallery mode optical microresonators and diffraction gratings. In addition, a molecular device consisting a multi-functional compound is also designed and synthesized.
The first project leveraged the ultra-high quality factors (Q) of on-chip SiO2 microtoroid resonators (Q > 106). The top surface of the microresonator was treated in a series of chemical vapor deposition steps to form a monolayer of azobenzene where the photoisomerization induces the change of the cavity refractive index. The azobenzene-coated SiO2 microresonators were tested to show reversible tuning on the cavity resonant wavelength as well as the shift of the free spectral range upon excitation by blue or mid-IR light. The device shows consistent optical performances after being stored in air for 6 months, and the experimental results agree with finite element method and density functional theory modeling.
The second optical device studied is the stretchable optical diffraction grating fabricated by the novel polymer stereocomplex of poly(acrylic acid) and polyethylene oxide. This type of polymer film exhibits above 800% strain when being stretched, and the mechanical performances were recovered given sufficient relaxation time. The optical transmittance is above 80% in the desired visible and near-IR wavelength range. The polymer grating was fabricated by replica molding a silicon master grating. The tunable diffraction behavior of the polymer grating aligns well with finite element method modeling.
The multifunctional molecular devices were synthesized with multi-step cross-coupling reactions, consisting of a 'sensing' module with tetraphenylethylene and pyridinium salt, and a 'modulate' module with naphthalimide. This type of molecular device presents aggregation-induced emissive properties where the emission intensity increases in aggregated solid state as compared to the solution state. The photophysical properties and the photoconductivity were studied in a variety of different solvents and in solid state, and the results agree with the simulations by density functional theory study.

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Asset Metadata

Creator He, Jinghan (author)

Core Title Organic materials for linear and nonlinear optical microdevices

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Degree Conferral Date 2022-12

Publication Date 08/16/2022

Defense Date 07/18/2022

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag azobenzene,diffraction gratings,OAI-PMH Harvest,optical microresonators,optical polymers,tetraphenylethylene

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Armani, Andrea (committee chair), Dawlaty, Jahan (committee member), Inkpen, Michael (committee member), Wu, Wei (committee member)

Creator Email hejinghan12@gmail.com,jinghanh@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-oUC111376959

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