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Porphyrin based near infrared‐absorbing materials for organic photovoltaics

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Porphyrin based near infrared‐absorbing materials for organic photovoltaics

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Porphyrin Based Near Infrared-Absorbing
Materials for Organic Photovoltaics

by
Qiwen Zhong
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)

May, 2014
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Dedication

Dedicated to My Parents:
My Dad Mr. Weixiong Zhong & My Mom Mrs. Meilan Chen

献给我亲爱的爸爸妈妈
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Acknowledgement
I would like to express my most sincere gratitude to my PhD research advisor
Prof. Mark E. Thompson for his guidance throughout my time at USC. I am truly
grateful and proud to be able to work with and learn from such a great chemist and kind
mentor.
I would like to thank all the committee members of my screening exam,
qualifying exam and dissertation, including Prof. Hanna Reisler, Prof. Richard Brutchey,
Prof. Chongwu Zhou, Prof. Barry Thompson and Prof. Noah Malmstadt.
I would also like to thank my dear friends: Dr. Slava Diev, Dr. Marco Curreli, Dr.
Lincoln Hall, Dr. Cong Trinh, Dr. Rui Zhang, Dr. Kathryn Allen, Sarah Rodney, Yan
Song and Niki Bayat. I wouldn't have made it through graduate school without their
support and encouragement.
Most importantly, I would like to thank my family. I wouldn't have become the
person I am, or accomplished what I have accomplished without my parents, Mr.
Weixiong Zhong and Mrs. Meilan Chen. Their enduring and unconditional love, their
faith in me, has always given me strength and hope. I am also very thankful to have met
my husband, my life companion, Aaron George, during my time in graduate school.
Last but not least, to the people who have given me help, who have believed and
continue to believe in me, I would like to say thank you. It has been a great journey with
all of you.

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Table of Contents
Dedication ................................................................................................................................... ii
Acknowledgement .................................................................................................................. iii
List of tables ............................................................................................................................... vi
List of figures ............................................................................................................................. vi
Abstract ..................................................................................................................................... xii
Chapter 1 Introduction ....................................................................................................... 1
1.1 Porphyrins ................................................................................................................... ............................. 1
1.1.1 Structure and nomenclature .................................... ............................................................... .. 1
1.1.1 Aromaticity and me talloporphyrins ............................. ......................................................... 2
1.1.3 Porphyrin absorption spectra ................................. ............................................................... .. 4
1.1.4 Ex c ited state processes ...................................... ............................................................... .......... 6
1.2 Porphyrin as near‐infrar ed absor ber in organic photovoltaics . . ........................................ 8
1.3 Porphyrin‐single walled ca rbon nanotube hybrids .............. ................................................ 11
Chapter 1 re ference:........................................... ............................................................... .............................. 14
Chapter 2  ‐extended perylenyl porphyrins ........................................................... 16
2.1  ‐ex t ended porphyrins .......................................... ............................................................... ............. 16
2.1.1 Stru c tures of  ‐ex tend e d porphyrins ................................................................................. 16
2.1.2 Photophysic a l properties of  ‐ex t ended porphyrins .......................................... ......... 18
2.1.3 Synthesis of  ‐ex tended porphyrins via intramolecular ox i d a tive coupling ..... 22
2.2 Results and discussion ........................................ ............................................................... ............... 24
2.2.1 Synthesis of peryl enyl porphyrins ............................. .......................................................... 24
2.2.2 Photophysical and electroche mical characterizations ........... ..................................... 29
2.2.3 Structural characterizations of perylenyl porphyrins ................................................. 33
2.3 Conclusion .................................................... ............................................................... ........................... 42
Chapter 2 re ferences: ......................................... ............................................................... ............................. 43
Chapter 3 Pyrazoloporphyrins ..................................................................................... 45
3.1 Carbaporphyrins ............................................... ............................................................... ................... 45
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3.1.1 Structures of ca rbaporphyrins ................................. ............................................................. 4 5
3.1.2 Photophysical properties of carbaporphyrins ................... ............................................. 46
3.1.3 Brief s ummary of porphyrin and car b aporphyrin oid synthesi s ... .......................... 49
3.2 Results and discussion ........................................ ............................................................... ............... 51
3.2.1 Synthesis of pyrazoloporphyrin ................................ ........................................................... 51
3.2.2 Photophysical char acterization ................................ ............................................................ 56
3.2.3 Electrochemical and theore tical characterization .............. .......................................... 59
3.2.4 Device fabri cation ............................................ ............................................................... ............ 62
3.3 Conclusion .................................................... ............................................................... ........................... 65
Chapter 3 re ferences: ......................................... ............................................................... ............................. 65
Chapter 4 Porphyrin modified SWNT hybrids ........................................................ 68
4.1 Introduction .................................................. ............................................................... ......................... 68
4.2 Results and discussion ........................................ ............................................................... ............... 71
4.2.1 Preparation of por phyrin modified SWNT s hybrids ............... ..................................... 71
4.2.2 Modification ratio estimation: ................................ ............................................................... 79
4.2.3 Photophysical char acterizations ............................... ........................................................... 82
4.3 Conclusions ................................................... ............................................................... .......................... 90
Synthesis & Ex peri mental Met hods: ............................. ............................................................... ............. 91
Chapter 4 re ferences: ......................................... ............................................................... ............................. 93
Future Outlook ....................................................................................................................... 97
Bibliography .......................................................................................................................... 101
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List of tables
Table 2.1 Electrochemical potentials of compounds 1, 1m, 2, 2m and 2d

List of figures
Figure 1.1 Fischer (left) and IUPAC (right) nomenclature for porphyrins.
Figure 1.2 Delocalization pathway in porphyrin according to Vogel's [18] annulene
model.
Figure 1.3 Elements known to form complexes with porphyrins. Metals found in
nature are shown in red.
Figure 1.4 Gouterman's four-orbital model.
Figure 1.5 Typical absorption spectrum of porphyrins.
Figure 1.6 Illustration of the excited electronic and vibrational states of a molecule.
Figure 1.7 Absorption (blue) and emission (red) bands of a molecule following the
Franck-Condon progression.
Figure 1.8 Typical device structure for lamellar OPVs; Current-voltage curve of OPV
devices under dark and illuminated conditions.
Figure 1.9 Structure of perylene-bridged porphyrins before and after extension;
absorption coefficient of the corresponding porphyrins and solar photon flux derived
from ASTM G173-03 AM 1.5G spectral irradiance.
Figure 1.10 a) SWNT: wrapping of a graphene sheet into a cylinder; b) hexagonal
lattice with vectors a
1
, a
2
, C
h
and T, and chiral angle .
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vii
Figure 1.11 a) Absorption spectrum of SWNTs and b) corresponding transitions in the
DOS diagram showing van Hove singularities and the bands involved.
Figure 2.1 Fusion types of porphyrin.
Figure 2.2 Direct meso- fusion with PAHs.
Figure 2.3 UV-vis-NIR absorption spectra of -meso- fused porphyrin tapes.

Figure 2.4 Schematic molecular structures of triply linked Zn(II) porphyrin arrays
and schematic diagram of energy relaxation dynamics of TBn after photoexcitation to the
S
1
state.
Figure 2.5 Bis-pyrenyl-porphyrins
Figure 2.6 Ar = 3,5-di-tert-butyl-phenyl; i a) TFA, CH2Cl2, followed by DDQ; b)
Zn(OAc)2, CH2Cl2, MeOH, Yield 50%; ii NBS, CH2Cl2-pyridine, -10 °C, yield = 80-
95%; iii Pd(PPh3)4, Cs2CO3, pyridine, toluene, 110 °C, 10 - 20h
Figure 2.7 Annulation of a perylene ring to a porphyrin core
Figure 2.8 Annulation of two porphyrin rings to a perylene core.
Figure 2.9 UV-vis absorption spectra of 1, 1m, 2, 2m and 2d in CH
2
Cl
2
.
Figure 2.10 Emission spectra of 1, 1m, 2, 2m and 2d in CH
2
Cl
2
.
Figure 2.11 Electochemical characterization of perylenyl porphyrins by cyclic
voltammetry. * peaks from internal standard ferrocene. Fc
+
/Fc = 0 V; ** peaks from
internal standard decamethylferrocene. dcFc
+
/dcFc = -0.54 V vs. Fc
+
/Fc.
Figure 2.12 a)
1
H NMR of a mixture of 3,9 and 3,10-Br
2
-perylene in CDCl
3
at room
temperature; b)
1
H NMR of single isomer 3,9-Br
2
-perylene in DMSO at 90 °C.
Figure 2.13 a)
1
H NMR and b) gCOSY spectra of anti-2 in chloroform-d/pyridine-d
5
at
58 °C.
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viii
Figure 2.14 a)
1
H NMR and b) gCOSY spectra of 1m in chloroform-d/pyridine-d
5
at
58 °C.
Figure 2.15 a)
1
H NMR spectra of anti-2m in chloroform-d/pyridine-d
5
at 58 °C; b)
comparison of 2m isomer mixture and anti-2m.
Figure 2.16
1
H NMR spectra of anti-2d in chloroform-d/pyridine-d
5
at 58 °C.
Figure 3.1 Examples of carbaporphyrinoid systems with highlighted delocalization
pathway.
Figure 3.2 Color and absorption spectra of NCP in CH
2
Cl
2
(inner-3H form, left) and
in DMF (inner 2H-form, right).
Figure 3.3 Common routes for porphyrin synthesis.
Figure 3.4 Acid catalyzed condensation followed by oxidation for free-base
pyrazoloporphyrin pz-por.
Figure 3.5 Synthesis of the diformylpyrazole (pz) precursor.
Figure 3.6 Knorr pyrrole synthesis and tripyrrane (tpy) precursor synthesis.
Figure 3.7 Metalation of free-base pz-por.
Figure 3.8 Thermal gravimetric analysis of Pdpz.
Figure 3.9 Absorption spectra of Pdpz in CH
2
Cl
2
solution and in a vapor deposit thin
film on glass.
Figure 3.10 Transient spectra for Pdpz in THF following photoexcitation at 520 nm.
Figure 3.11 Excited state kinetics following photoexcitation of Pdpz.
Figure 3.12 a) Absorption spectra of S
1
and T
1
and b) time profile of normalized
population from global fitting.
Figure 3.13 Electrochemical characterization of Pdpz by CV (a) and DPV (b).
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* peaks from internal standard decamethylferrocene dcFc
+
/dcFc = -0.54 V vs. Fc
+
/Fc.
Figure 3.14 Theoretical calculations on regular Pd porphyrin and Pdpz.
Figure 3.15 Spin density of Pdpz triplet.
Figure 3.16 a) ITO/Pdpor(190 Å) before annealing and b) after annealing; c)
ITO/MoO
3
(150 Å)/Pdpor(190 Å); d) and e) ITO/Pdpor(190 Å)/C
60
(400Å) annealed
before and after C
60
deposition, respectively; f) ITO/MoO
3
(150 Å)/Pdpor(190 Å)/
C
60
(400 Å), non-annealed.
Figure 3.17 Current-voltage curve of device ITO/Pdpz(190 Å)/C
60
(400 Å)/BCP (100
Å)/Al(1000 Å), with J
sc
= 0.12 mA/cm
2
, V
oc
= 0.159 V, FF= 0.29 and PCE= 0.005;
EQE and normalized porphyrin Pdpz thin film, C
60
thin film, and device absorption.
Figure 4.1 Structures of non-fused bis-pyrenyl porphyrin 1, mono-fused pyrenyl
porphyrin 2, and doubly-fusedpyrenyl porphyrin 3 including the isomers anti-3 and syn-
3. Ar indicates 3,5-di-tert-butylphenyl groups.
Figure 4.2 Absorption spectra of (A) non-fused bis-pyrenyl porphyrin 1 and 1-SWNT
hybrids in DMF. Insert: porphyrin Q-band at 560 nm; (B) Mono-fused porphyrin 2,
CoMoCAT SWNTs and 2-SWNT hybrids in DMF; (C) Doubly-fused porphyrin 3,
CoMoCAT SWNTs and 3-SWNT hybrids in DMF.
Figure 4.3 (A) TEM characterization of the SWNT sample before porphyrin 3
modification and (B) after modification.The black scale bar in both images indicates 200
nm.
Figure 4.4 Absorption spectra of the supernatant. (A) Samples with different 3 to
SWNT weight ratios. Black arrows indicate the disappearance of absorption features
from free porphyrin 3. (B) Samples with a mixture of porphyrins 1, 2 and 3 (red: initial
x

x
porphyrin mixture absorption; blue: supernatant after sonication with SWNTs and
centrifugation). Insert: supernatant absorption from 400–600 nm plotted on an expanded
scale.
Figure 4.5 Absorption spectra of 3-SWNT hybrids (A) after one and three cycles of
washing, filtering and re-dispersing, and (B) with pyridine (py) and dodecylamine (dda)
as coordination ligands.
Figure 4.6 Top: NMR spectrum of 3 before modification with SWNTs; Bottom:
NMR spectrum of the filtrate following the addition of SWNTs.
Figure 4.7 (A)& (B) Comparison of the -system size and shape of anti-3 and syn-3
with that of the SWNT along the SWNT’s long axis. (C) & (D) optimized structures of
model porphyrins anti-3 and syn-3 without the 3,5-di-tert-butylphenyl groups, calculated
at the B3LYP/6-31G*/LANL2DZ level.
Figure 4.8 Emission spectra of 3 and 3-SWNT hybrids in DMF.
Figure 4.9 (A)&(B) Transient spectra measured for 3-SWNT hybrids in DMF
following photoexcitation at 800 nm. (C) Temporal slices that correspond to the
relaxation of the photobleach of the Q-band of 3 (black squares) and the induced
absorption attributed to the cation of 3 (red circles). The dash dot blue lines correspond
to biexponential fits with time constants of 260 fs and 6 ps.
Figure 4.10 Absorption spectra of the porphyrin 3 cation measured through
electrochemical oxidation.
Figure 4.11 (A) Spectral slices showing the evolution of transient absorption spectra of
3-SWNT hybrids in the NIR spectral range following photoexcitation at 850 nm. (B)
Temporal slices showing the relaxation of the photobleach of the SWNT (black squares)
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and 3 cation induced absorption (red circles). The sign of the SWNT photobleach has
been inverted to enable better comparison with the 3 cation induced absorption profile.
After 1 ps, these features show similar relaxation profiles. (C) Comparison of the
absorption spectrum of 3-SWNT hybrids and the transient spectrum measured at a time
delay of 1 ps.

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Abstract
The conservation and transformation of energy is essential to the survival of
mankind, and thus concerns every modern society. Solar energy, as an everlasting source
of energy, holds one of the key solutions to some of the most urgent problems the world
now faces, such as global warming and the oil crisis. Advances in technologies utilizing
clean, abundant solar energy, could be the steering wheel of our societies. Solar cells,
one of the major advances in converting solar energy into electricity, are now capturing
people’s interest all over the globe. While solar cells have been commercially available
for many years, the manufacturing of solar cells is quite expensive, limiting their broad
based implementation. The cost of solar cell based electricity is 15-50 cents per kilowatt
hour (¢/kwh), depending on the type of solar cell, compared to 0.7 ¢/kwh for fossil fuel
based electricity. Clearly, decreasing the cost of electricity from solar cells is critical for
their wide spread deployment. This will require a decrease in the cost of light absorbing
materials and material processing used in fabricating the cells.
Organic photovoltaics (OPVs) utilize organic materials such as polymers and
small molecules. These devices have the advantage of being flexible and lower cost than
conventional solar cells built from inorganic semiconductors (e.g. silicon). The low cost
of OPVs is tied to lower materials and fabrication costs of organic cells. However, the
current power conversion efficiencies of OPVs are still below 15%, while convention
crystalline Si cells have efficiencies of 20-25%. A key limitation in OPVs today is their
inability to utilize the near infrared (NIR) portion of the solar spectrum. This part of the
spectrum comprises nearly half of the energy in sunlight that could be used to make
electricity.
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The first and foremost step in conversion solar energy conversion is the
absorption of light, which nature has provided us optimal model of, which is
photosynthesis. Photosynthesis uses light from the sun to drive a series of chemical
reactions. Most natural photosynthetic systems utilize chlorophylls to absorb light energy
and carry out photochemical charge separation that stores energy in the form of chemical
bonds. The sun produces a broad spectrum of light output that ranges from gamma rays
to radio waves. The entire visible range of light (400-700 nm) and some wavelengths in
the NIR (700-1000 nm), are highly active in driving photosynthesis. Although the most
familiar chlorophyll-containing organisms, such as plants, algae and cyanobacteria,
cannot use light longer than 700 nm, anoxygenic bacterium containing
bacteriochlorophylls can use the NIR part of the solar spectrum. No organism is known
to utilize light of wavelength longer than about 1000 nm for photosynthesis. NIR light
has a very low-energy content in each photon, so that large numbers of these low-energy
photons would have to be used to drive the chemical reactions of photosynthesis. This is
thermodynamically possible but would require a fundamentally different molecular
mechanism that is more akin to a heat engine than to photochemistry.
Early work on developing light absorbing materials for OPVs was inspired by
photosynthesis in which light is absorbed by chlorophyll. Structurally related to
chlorophyll is the porphyrin family, which has accordingly drawn much interest as the
potential light absorbing component in OPV applications.
In this dissertation, the design and detail studies of several porphyrin-based NIR
absorbing materials, including extended perylenyl porphryins and pyrazole-containing
carbaporphyrins, as well as porphyrin modified single-walled carbon nanotube hybrids,
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will be presented, dedicating efforts to develop novel and application-oriented materials
for efficient utilization of sustainable solar energy.
1

1

Chapter 1 Introduction

1.1 Porphyrins
1.1.1 Structure and nomenclature
Porphyrins, a large class of deeply colored organic compounds of natural or
synthetic origin, are aromatic macrocycles consisting of four pyrrole units linked together
by four methine bridges. The macrocyclic structure of porphyrin was first proposed by
Küster
1
in 1912 and confirmed by Hans Fischer,
2
the father of modern porphyrin
chemistry, when the first porphyrin heme was successfully synthesized by Fischer in
1929. The porphyrin core consists of four nitrogen atoms from the pyrrole units,
providing a cavity that can coordinate to a metal center. The porphyrin periphery has two
types of positions, i.e. eight  positions from the pyrrole fragments and four meso
positions from the bridging carbon atoms. Fischer proposed the first nomenclature
system for porphyrins, in which carbons were given a number 1 to 8, while
meso carbons were given a Greek lower case letter, - . This numbering system is
straightforward for simple porphyrins in early studies, but as the complexity of a
porphyrin derivative increases, the system becomes unwieldy and contradictory. The
IUPAC nomenclature introduced in 1979 is more systematic, where all the atoms in the
macrocycle are numbered, allowing substituents on any of the atoms of the macrocycle to
be accurately recorded in more complicated porphyrins.
3

2

2

Figure 1.1 Fischer (left) and IUPAC (right) nomenclature for porphyrins.

1.1.1 Aromaticity and metalloporphyrins
The word porphyrin has its origins in the classical world of ancient Greece, where
porphura was used to describe the color purple, a color only used for the clothes of
royalty and high priests. Porphyrins display intense color, a characteristic directly related
to the aromatic system, which is most evidently manifested in the
1
H NMR spectra by
the downfield shifts of peripheral protons and the strongly upfield shifts of the inner NH's
(usually negative on the  scale). Porphyrin aromaticity is most frequently described in
terms of the [18] annulene model, proposed by E. Vogel.
4
According to Vogel’s model, a
delocalization pathway is distinguished in the macrocycle, which is aromatic acording to
the traditional Hückel [4n+2] rule. The porphyrin is thus viewed as a bridged
diaza[18]annulene. However, the description of aromaticity in porphyrin and related
porphyrinoids still remains a debate until now.
5-6

Figure 1.2 Delocalization pathway in porphyrin according to Vogel's
[18] annulene model.
N
NH N
HN
1
23
4
5
6
7
8



 20
N
NH N
HN
2
37
8
12
13
17
18
19
1
16
15
14
11
10
9
4
5
6
N
N
H
N
H
N
N
H
N
N
N
H
N
H
N
N
N
H
3

3
Porphyrins form a great number of complexes with metal ions and some
nonmetals. As shown in Figure 1.2 are the elements in the periodic table known to form
complexes with porphyrins, and metals found in nature are marked in red.
7
The
coordinating environment provided by porphyrins is very flexible and can be fine-tuned
to particular oxidation and spin states by varying peripheral substitution and axial
ligands. On the other hand, the reactivity of porphyrins can also be controlled by the
choice of metal to coordinate to the central nitrogen atoms.
3
For example, the position
for electrophilic substitution depends on how electronegative the porphyrin is, and thus
the introduction of divalent central metals produces electronegative porphyrin ligand and
these complexes are substituted on their meso-carbons. On the other hand, metals in
electrophilic oxidation states or the free-base porphyrin tend to deactivate the meso
carbons and activate the positions to electrophilic attack. The metal ion also has effects
on the electrochemistry of porphyrins.
8
Metalloporphyrins containing electroinactive
Cu(II) or Zn(II) metal ions undergo only reactions involving the ring system, but metal-
centered and ring-centered processes are both observed in the case of Fe, Co or Mn
derivatives. Linear relationships are observed between the reversible potentials for both
the first ring-centered oxidation and reduction, and the electronegativity of the divalent
central metal ion. The nature of the central metal also has critical effect on the excited
state processes.
9
For example, the excited states of metalloporphyrins with Zn or Mg are
relatively long, typically several nanoseconds. If transition metals such as Fe, Mn or Cu
are inserted, the excited state lifetimes are shortened by many orders of magnitude due to
ultrafast internal conversion via the unfilled orbitals of the metal.
4

4

Figure 1.3 Elements known to form complexes with porphyrins. Metals
found in nature are shown in red.

1.1.3 Porphyrin absorption spectra
Porphyrin absorption spectra consist of two distinct regions, an intense absorption
around 400 nm, called the B band or Soret band, and two to four much weaker bands in
longer wavelength around 480-700 nm. Theoretical models developed for the porphyrin
absorption spectra include the free electron theory, the cyclic polyene theory and the
simple Hückel theory, etc., which are all unified by Gouterman's four-orbital model.

Figure 1.4 Gouterman's four-orbital model.

In Gouterman's theory,
10
only the two highest occupied molecular orbitals
(HOMOs) and the two lowest unoccupied molecular orbitals (LUMOs) of a porphyrin are
Li B C
Na Al Si P
K Ca Sc Ti V Cr Fe Co Ni Cu Zn Ga Ge As
Rb Sr Y Zr Tc RuThPdAg Cd In Sn Sb Te
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi
Ac
Mg
Mn
Nb Mo
Pr, Eu, Yb, Th
b
1
(a
2u
)b
1
(a
1u
)
c
1
(e
g
) c
2
(e
g
)
N
N
H
N
H
N
x
y
5

5
considered. One-electron transitions between the two degenerate HOMOs (degeneracy
assumed by Gouterman) and the two degenerate LUMOs (from simple Hückel theory),
generates four degenerate electronically excited states.
Configuration interaction then mixes them to give a sum and a difference of
transition dipoles which were originally equal.

The B/Soret band corresponds to the strongly allowed transition while the Q band
corresponds to the forbidden transition.

But because of molecular vibrations within the porphyrin macrocycle which
marginally lifting the degeneracy of the LUMOs, the Q bands become weakly allowed,
showing vibrational fine structure.

Figure 1.5 Typical absorption spectrum of porphyrins.

Metalloporphyrin absorption spectra are divided into two groups depending on
whether the metal center has a closed or open shell of valence electrons. Regular
½(b
1
c
1
+ b
2
c
2
) = B
y
0
; ½(b
1
c
2
+ b
2
c
1
) = B
x
0
½(b
1
c
1
-b
2
c
2
) = Q
y
0
; ½(b
1
c
2
-b
2
c
1
) = Q
x
0
b
1
c
1
-b
2
c
2
= b
1
c
2
-b
2
c
1
= 0
6

6
metalloporphyrin contain closed-shell metal ions (d0 or d10), for example Zn(II), in
which the d dxz, dyzmetal-based orbitals are relatively low in energy. These have
very little effect on the porphyrin to * energy gap in porphyrin electronic spectra.
Hypsoporphyrins are metalloporphyrins in which the metals are of dm, m=6-9, having
filled d orbitals. In hypsoporphyrins there is significant metal d  to
porphyrin *orbital interaction (metal to ligand * backbonding). This increases the
energy of the porphyrin to * transition, leading to a blue (hypsochromic) shift of the
spectrum. This effect increases with increasing atomic number of the transition metal,
e.g. in the series Ni (II), Pd (II) and Pt (II).

1.1.4 Excited state processes
Upon absorption of a photon with energy h , an electron in the ground state S
0

gets promoted to an excited state S
n
, which then decay through several possible
processes.
11
Promotion of electrons to different vibrational levels of the excited state
gives the vibronic structure in the absorption spectra. Rapid thermalization of the excess
vibrational energy follows the promotion of electrons, resulting in population of electrons
at the lowest vibrational state of that excited state, which can then undergo a non-
radiative process called internal conversion to a lower excited state while retaining the
spin multiplicity. Internal conversion may also occur when electrons are excited to a
higher excited state, such as S
2
, where it can non-radiatively relax down to S
1
or S
0
.
7

7

Figure 1.6 Illustration of the excited electronic and vibrational states of a
molecule.
12

Fluorescence is a radiative transition for electrons at the lowest excited state to
relax back to the ground state. Relaxation of electrons from the lowest excited state to
higher vibrational levels of the ground state gives the vibronic structure in the emission
spectra.

Figure 1.7 Absorption (blue) and emission (red) bands of a molecule
following the Franck-Condon progression.
12

1
23
4
56
R
Energy
S
0
S
1
T
1
V=3
V=2
V=1
V=0
S
0
V=3
V=2
V=1
V=0
S
1
(0,0)
Energy
Intensity
0→3
0→2
0→1
0→0
0→3
0→2
0→1
8

8
Electrons may also reach a lower excited state through intersystem crossing, a
non-radiative process that requires a flip of the electronic spin state and thus a change in
spin multiplicity from singlet to triplet. The rate of intersystem crossing is enhanced by
overlap of the vibrational levels of the singlet and triplet states, or can be increase
through the presence of heavy atoms, which enhances the spin-orbit coupling between the
spin and orbital angular momentum of the electron.
At the triplet excited state, electrons undergo similar processes as compared to the
singlet state, including vibrational relaxation to the lowest vibrational level in the triplet
excited state, back-intersystem crossing to the singlet excited state, and internal
conversion or non-radiative decay- phosphorescence, to the singlet ground state.
Energy gap law for radiationless transitions set forth by Englman and Jortner
13

quantitatively assess the rates of nonradiative deactivation k
nr
of emitting electronic states
by equation:

where ΔE is the energy gap between the emitting level and the next-lower state and ωmax
is the energy of a high frequency oscillator that mediates nonradiative deactivation.

1.2 Porphyrin as near-infrared absorber in organic photovoltaics
Organic photovoltaics (OPVs) represent a promising new technology due to their
low manufacturing costs, high absorptivities and ability to be fabricated on flexible
substrates.
14
The active layers of these devices typically consist of a mixture of electron-
donating (D) and electron-accepting (A) materials, which are sandwiched between two
k
nr
∝e
‐[constant ( E/ ћ 
max
)]
9

9
electrodes, a transparent anode and a metallic cathode on top. Light absorption leads to
the formation of an exciton that must diffuse to the D/A interface to undergo charge
separation, forming a hole and electron in the D and A layers, respectively. After the
hole and electron are generated, they are conducted through the D and A materials and
extracted by the electrodes and transferred to the outer circuit.
Under dark conditions the device shows a diode behavior with a characteristic
rectification ratio. When light is shined to the device the current is raised at reverse bias,
this is called photocurrent. The most important parameters of the device are the open
circuit voltage V
oc
; short circuit current density J
sc
; fill factor FF that measures the quality
of the cell and the maximum power P
max
that can be obtained from the device.
a) b)
Figure 1.8 Typical device structure for lamellar OPVs; Current-voltage curve
of OPV devices under dark and illuminated conditions.

The current power conversion efficiencies of OPVs are still below that of
convention crystalline Si cells, with a key limitation of the inability to utilize the near
infrared portion of the solar spectrum, which comprises nearly half of the energy in
sunlight that could be used to make electricity.
15
And thus the design of new
P
max
V
oc
J
sc
Voltage
J
current
density
Dark
Illuminated
J
ph
FF =
10

10
chromophores for application in OPVs has been an active area of research with important
implications in energy science.
16
One of the critically criteria for promising candidates is
the high absorptivities over a range of wavelengths to harvest the greatest possible
fraction of available light. This is especially important for devices employing organic
thin films, for which exciton diffusion and carrier conduction can impose strict limits on
the ideal thickness of active layers.
17

Porphyrin stands out as donor materials and light harvesting systems for OPVs
owing to the high extinction coefficients of their absorption in the visible region, rich
redox chemistry and remarkable p-type semiconducting behavior. Moreover, both the
high thermal stability of these molecules and the quality of the crystalline films prepared
by sublimation have also boosted their use in small molecule OPVs.
18-19
Strategies to
develop porphyrin-based NIR absorbing materials such as extension of  conjugations
through bridging two porphyrin moieties by fusion to a perylene center, and changing the
porphyrin core arrangement through replacing a pyrrole by pyrazole, will be discussed in
detail in Chapter 2 and Chapter 3 respectively.
11

11

Figure 1.9 Structure of perylene-bridged porphyrins before and after
extension; absorption coefficient of the corresponding porphyrins and solar photon flux
derived from ASTM G173-03 AM 1.5G spectral irradiance.

1.3 Porphyrin-single walled carbon nanotube hybrids
Carbon nanotubes (CNTs) have been the focus of intense investigation for more
than two decades since their discovery because of their outstanding properties and
versatility.
20
Single-walled carbon nanotube (SWNTs), a one-dimentional nanostructure
with aspect ratio up to 10
6
, exhibits characteristics including ballistic charge transport
along their axis, tunable bandgaps by employing different radii and chiralities, high
tensile strength and resiliency, high surface area (~1600 m
2
g
-1
) and electron-accepting
abilities, etc.
N N
N N
Zn
N N
N N
Zn
Por 1
Por 2
Por 3
N N
N N
Zn
N N
N N
Zn
N N
N N
Zn
N N
N N
Zn
400 600 800 1000
-3
-2
-1
0
1
2
3
4
5
0.0
0.5
1.0
1.5
4.0
4.5
5.0
Photon Flux (10
14
cm
-2
nm
-1
)
Wavelength (nm)
Absorption coefficient (10
5
cm
-1
)
Por 1
Por 2
Por 3
12

12
a) b)
Figure 1.10 a) SWNT: wrapping of a graphene sheet into a cylinder; b)
hexagonal lattice with vectors a
1
, a
2
, C
h
and T, and chiral angle .

Structurally SWNTs is often described as the result of seamless wrapping of a
graphene sheet into a cylinder. Consequently, SWNTs can be classified according to
how they are mapped into a single graphite layer. Considering the hexagonal lattice
shown in Figure 1.9 b), the unit vectors of the lattice are defined by a
1
and a
2
with the C-
C bond length being 1.42 Å. A SWNT is constructed by rolling the graphite layer along
a certain direction C
h
= n a
1
+ m a
2
making OB and AB' coincide. Perpendicular to C
h
,
the vector T points to the long axis of the SWNT. C
h
and T are referred as chiral vector
and translational vector, respectively. Together they define the unit cell of a SWNT as
the rectangle OAB'B. The indices (n, m) fully define the SWNT radius and chirality and
determine univocally its electronic structure. If |m – n| = 3k (k is integer), the SWNT is
metallic, where as for |m – n| ≠ 3k , the SWNT is semiconducting; about one-third of all
the SWNTs are metallic. The nanotube diameter d can be determined by equation:
d = 0.0783 (n
2
+ m
2
+ nm)
1/2
.

a
1
a
2
C
h
T
O
A
B
B
’
13

13
Optical properties of SWNTs are derived from electronic transitions within one-
dimensional density of states (DOS). As a consequence of the size-dependent
quantization of electronic wave functions around the circumference of the SWNT, the
DOS displays typical singularities feature, called van Hove singularities. Depending on
the structure of SWNTs, the energy between the van Hove singularities varies; the
bandgap of semiconducting SWNs in the DOS is inversely proportional to the diameter,
and thus the optoelectronic properties can be tuned by changing the structure of SWNTs.
The presence of the van Hove singularities dominates the spectral and redox features of
SWNTs.
Considering the tunable bandgaps, strong optical absorptivity in the near-infrared,
together with factors such as fast exciton and charge transport, chemical and thermal
stability, SWNT is a very promising candidate for use in a wide range of photonic and
electronic applications.
21

a) b)
Figure 1.11 a) Absorption spectrum of SWNTs and b) corresponding
transitions in the DOS diagram showing van Hove singularities and the bands involved.
21

Extensive work has been done in the literature on incorporating SWNTs into
polymer and hybrid photovoltaic devices as materials for electrodes
22
or for improved
14

14
charge collection or transport.
23
However, exploiting the optical absorptivity of SWNTs
in photovoltaic devices has been more challenging, as the use of electronically type-
sorted, semiconducting nanotubes is required to avoid exciton quenching by metallic
nanotube species.
24
Moreover, the demonstration of practical photovoltaic devices that
take advantage of the broadband absorption of SWNTs have been limited to difficulties
in the processability of these materials due to their intrinsic poor solubility in organic and
aqueous solvents.
In Chapter 4 of this dissertation, the supramolecular chemistry in a porphyrin-
SWNTs hybrid is discussed in details, presenting a full investigation which provides
deeper understanding of the properties of such novel hybrid material and it potential for
OPV applications.

Chapter 1 reference:
1. Kuster, W., Information on bilirubine and haemine. H-S Z Physiol Chem 1912, 82
(6), 463-483.
2. Fischer, H.; Walach, B., Synthesis of octamethyl porphin, of methyl analogues of
aetioporphyrin. Liebigs Ann Chem 1926, 450, 164-181.
3. Milgrom, L. R., The Colours of Life. Oxford University Press: 1997.
4. Vogel, E., The Porphyrins from the Annulene Chemists Perspective. Pure Appl
Chem 1993, 65 (1), 143-152.
5. Broring, M., How Should Aromaticity Be Described in Porphyrinoids? Angew
Chem Int Edit 2011, 50 (11), 2436-2438.
6. Wu, J. I.; Fernandez, I.; Schleyer, P. V., Description of Aromaticity in
Porphyrinoids. J Am Chem Soc 2013, 135 (1), 315-321.
7. Kadish, K. S., K; Guilard, R. , The Handbook of Porphyrin Science. World
Scientific: Hackensack, NJ, 2010.
8. Kadish, M.; Van Caemelbecke, E., Electrochemistry of porphyrins and related
macrocycles. J Solid State Electr 2003, 7 (5), 254-258.
9. Harriman, A., Luminescence of Porphyrins and Metalloporphyrins .1. Zinc(Ii),
Nickel(Ii) and Manganese(Ii) Porphyrins. J Chem Soc Farad T 1 1980, 76, 1978-1985.
10. Gouterman, M., Spectra of Porphyrins. J Mol Spectrosc 1961, 6 (1), 138-&.
15

15
11. Turro, N. J.; Ramamurthy, V.; Scaiano, J. C., Principles of Molecular
Photochemistry: An Introduction. University Science Books: 2009.
12. McQuarrie, D. A.; Simon, J. D., Physical Chemistry: A Molecular Approach.
University Science Books: Sausalito, CA, 1997.
13. Englman, R.; Jortner, J., The energy gap law for radiationless transitions in large
molecules Molecular Physics
1970, 18 (2), 145-164.
14. Sariciftci, N. S.; Sun, S. S., Organic Photovoltaics: Mechanisms, Materials, and
Devices. CRC: 2005.
15. Nelson, J., The Physics of Solar Cells. Imperial College Press: 2002; pp 17-39.
16. Hains, A. W.; Liang, Z. Q.; Woodhouse, M. A.; Gregg, B. A., Molecular
Semiconductors in Organic Photovoltaic Cells. Chem Rev 2010, 110 (11), 6689-6735.
17. Mishra, A.; Bauerle, P., Small Molecule Organic Semiconductors on the Move:
Promises for Future Solar Energy Technology. Angew Chem Int Edit 2012, 51 (9), 2020-
2067.
18. Martinez-Diaz, M. V.; de la Torrea, G.; Torres, T., Lighting porphyrins and
phthalocyanines for molecular photovoltaics. Chem Commun 2010, 46 (38), 7090-7108.
19. Walter, M. G.; Rudine, A. B.; Wamser, C. C., Porphyrins and phthalocyanines in
solar photovoltaic cells. J Porphyr Phthalocya 2010, 14 (9), 759-792.
20. Guldi, D. M.; Martin, N., Carbon Nanotubes and Related Structures: Synthesis,
Characterization, Functionalization, and Applications. John Wiley & Sons: 2010.
21. Cataldo, S.; Salice, P.; Menna, E.; Pignataro, B., Carbon nanotubes and organic
solar cells. Energ Environ Sci 2012, 5 (3), 5919-5940.
22. Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H. J.; Dennler, G.;
Sariciftci, N. S.; Hu, L. B.; Gruner, G., Organic solar cells with carbon nanotube network
electrodes. Appl Phys Lett 2006, 88 (23).
23. Li, Z. R.; Kunets, V. P.; Saini, V.; Xu, Y.; Dervishi, E.; Salamo, G. J.; Biris, A.
R.; Biris, A. S., Light-Harvesting Using High Density p-type Single Wall Carbon
Nanotube/n-type Silicon Heterojunctions. Acs Nano 2009, 3 (6), 1407-1414.
24. Shea, M. J.; Arnold, M. S., 1% solar cells derived from ultrathin carbon nanotube
photoabsorbing films. Appl Phys Lett 2013, 102 (24).

16

Chapter 2 -extended perylenyl porphyrins

2.1 -extended porphyrins
2.1.1 Structures of -extended porphyrins
Large, -conjugated materials with near-infrared (NIR) absorption/emission have
attracted considerable attention due to their increasing applications in the fields of solar
cells,
1-2
bioimaging
3
and nonlinear optics.
4-5
The -system of porphyrins can be extended
by singly, doubly or triply bonded connections at the meso or  positions.
6
Examples of
singly bonded extension at the meso position include construction of porphyrin
oligomers coupled through unsaturated alkyne
7
or phenylene bridges,
8
and meso-linked
porphyrin oligomers.
9
While these materials display red-shifted absorptions to some
extent, the effect is not pronounced due to the overall non-coplanar structure.
8-9
Aryl
substituents at porphyrin meso positions cause only a small perturbation to the electronic
structure, because there is minimal overlap between the aryl ring and the porphyrin due
to the large aryl-porphyrin dihedral angles, which result from steric interactions with the
hydrogens.
10
Because of the non-planarity caused by the meso aryl twist,
meso-phenylene-linked porphyrin oligomers or direct meso-meso bonded porphyrin
oligomers do not exhibit significant conjugation, and their absorption spectra are only
slightly red-shifted as compared to the corresponding monomers. On the other hand,
peripheral fusion of porphyrin, where the porphyrin moiety is doubly or triply bonded to
other aromatic units, is an effective means to expand π-conjugated electronic network of

17

porphyrins, as covalent connections provide forced overall coplanar conformations,
leading to more red-shifted absorptions with wavelengths beyond 2000 nm.
11

N N
N N
M
meso

b) meso, 
a)  , 
c)  ,meso, 

Figure 2.1 Fusion types of porphyrin.

Fused porphyrins can be classified into the following fusion types from a
structural point of view: 1) - fusion type, 2) meso- fusion type, and 3)
-meso- fusion type (Figure 2.1).
6
Depending on the units the porphyrin -system is
extended through, fused porphyrins can also be classified into fused porphyrin oligomers
and porphyrins fused with aromatic hydrocarbons and heterocycles. Despite markedly
red-shifted absorptions, syntheses of large fused porphyrin oligomers have been very
challenging, as they typically require multi-step synthesis with low overall yields.
Moreover, these oligomers show pronounced aggregation in solution and poor
processability due to π- π stacking. Facile oxidative degradation due to raised HOMO
levels and very short S
1
excited state lifetimes
12-13
also limit the application of these
porphyrin oligomers in a number of areas. On the other hand, π-extended porphyrins
fused with aromatic hydrocarbons and heterocycles have advantages of higher solubility,
better air stability and larger scale preparation over porphyrin oligomers. A schematic

18

illustration of the direct fusion reaction studied is shown in Figure 2.2, along with the
PAHs such as naphthalene, pyrene, perylene and coronene.
1

N N
N N
M
N N
N N
M
H
H
,meso
fusion
H H
H
H
H

Figure 2.2 Direct meso- fusion with PAHs.
1

2.1.2 Photophysical properties of -extended porphyrins
Photophysical properties of π-extended porphyrins have been studied to provide
understanding of the fundamental electronic properties in these conjugated systems. As
aforementioned, porphyrins display large absorption in the visible region, including an
intense transition to the second excited state (S
0
to S
2
) at around 400 nm (the Soret or B
band) and a weak transition to the first excited state (S
0
to S
1
) at about 550 nm (the Q
band). Upon fusion, absorption spectra of -extended porphyrins often include broadly
split Soret bands and unusually intense and broadened Q bands. In the case of
-meso- triply fused porphyrin oligomers, it is shown that the splitting of the Soret
bands is due to exciton coupling between the adjacent porphyrin units with a large
coupling energy.
13
The Q band corresponding to an excitation to the lowest excited state
is very weak before fusion due to the accidental cancellation of the transition dipole
moments. However, efficient -conjugation of the fused porphyrins lifts the accidental

19

cancellation of the transition dipole moments, leading to the broadened and intensified Q
band. Delocalization of electrons in the porphyrin chromophore gives rise to the
significant red shift of both the Soret and Q bands.
13

Figure 2.3 UV-vis-NIR absorption spectra of -meso- fused porphyrin
tapes.
11

Besides absorption spectra, the relaxations of excited-state energy in fused
porphyrin are of great interest. One of the earliest examples studied by Osuka et. al. is
the meso-meso directly linked porphyrin dimers.
13
It is shown that before fusion, the
internal conversion process occurs very fast with a time constant of about 300 fs and
decay slowly with a time constant of about 1.9 nm from the Q state to the ground state.
After fusion, however, the internal conversion time constant from the Soret to Q state was
estimated to be about 500 fs in the transient anisotropy decay dynamics and then the
internal conversion process occurs with a time constant of about 4.5 ps to the ground
state. In a more recent work by Osuka et. al., femtosecond transient absorption
measurements in the IR region are carried out to explore the singlet electronic excited

20

state dynamics of the triply connected porphyrin tapes up to 8 units, showing excited
state life time gradually reduces from 4.7 ps for dimer to 0.59 ps for octamer, mainly due
to the acceleration of nonradiative relaxation processes cause by reduced HOMO-LUMO
energy gap (Figure 2.4).
12

Figure 2.4 Schematic molecular structures of triply linked Zn(II) porphyrin
arrays and schematic diagram of energy relaxation dynamics of TBn after
photoexcitation to the S
1
state.
12

While the elongated -conjugation pathway is shown to be a crucial factor to
determine the lowest excited state lifetime for the fused porphyrin array systems,
increased fluorescent quantum efficiencies are observed in the pyrenyl porphyrins: bis-
pyrenyl-porphyrin (BPP), monofused and doublyfused-bis-pyrenyl-porphyrin (MFBPP

21

and DFBPP) exhibit room temperature emission that undergoes red-shift with extending
conjugation from 596 nm for BPP to 721, 829 and 839 nm for MFBPP, syn- DFBPP and
anti- DFBPP, respectively. The fluorescence quantum efficiencies ( 
f
) increase from
3.3% for the starting non-fused BPP, to 8%, 8% and 13% for MFBPP, anti-DFBPP and
syn-DFBPP, respectively. This behavior is in contrast to what is expected from the
energy gap law,
14-15
which states that the nonradiative deactivation rate increases for
related molecules as the S
0
–S
1
energy gap decreases. Such unexpected behavior may be
due to the hindrance of vibrational deactivation modes by ring fusion. Many twisted
acenes are known to exhibit strong fluorescence,
16
suggesting that the distortion in fused
porphyrins may also contribute to the increased fluorescence efficiency. The fluorescence
efficiencies are comparable to those of the most efficient NIR-fluorescent porphyrin
oligomers.
17
In contrast, closely related porphyrin tapes or directly linked , meso-
porphyrins have very low fluorescent efficiencies in the NIR ( 
f
= 10
−5
) due to very short
S
1
excited state lifetimes.
12, 18

N N
N N
Zn
Ar
Ar
Ar = 3,5-di-tert-butyl-phenyl
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
BPP MFBPP anti-DFBPP
syn-DFBPP

Figure 2.5 Bis-pyrenyl-porphyrins

22

2.1.3 Synthesis of -extended porphyrins via intramolecular oxidative coupling
Synthetic routes for fusion of PAHs to porphyrin macrocycles can be divided into
intermolecular and intramolecular categories.
19
In the case of PAHs substituted
porphyrins, intramolecular fusion has been shown previously to occur under the
palladium-catalyzed coupling,
20
which involves attack of on an exocyclic double bond by
aryl halides attached to the porphyrin at an adjacent meso-position, and oxidative
aromatic coupling reaction conditions often using high-valent metal reagents such as
DDQ/Sc(OTf)
3
and Fe(III) salts.
19
Although intramolecular oxidative coupling has been
applied to well-known aromatic hydrocarbons and even more complex heterocyclic
fragments, such approach requires activation of the porphyrin by metalation with
nickel(II) and/or activation of the singly connected aromatic rings with bulky electron
donating alkoxy, aryloxy or amino groups to increase electron density in the porphyrin
and/or PAHs moieties.
21-22
Unfortunately, metalation of porphyrins with nickel (II)
causes fast deactivation of the porphyrin excited states through low-lying metal-based d-d
states limiting their application in a range of optoelectronic and photovoltaic
applications.
23
In addition, demetalation of nickel porphyrins with extended conjugation
often leads to extensive decomposition,
22, 24
or low yields of the demetalated free-base
product. For these reasons we have sought synthetic approaches to , meso fused
porphyrin systems that require neither nickel metalation nor donor groups on the aromatic
moiety to activate the system for fusion.
Thermal cyclodehydrogenation, an entirely new approach towards the synthesis of
meso- fused porphyrins, has been developed in our laboratory.
1
Thermal activation of
C-H bonds in aromatic systems for direct C-C bond formation is known to proceed under

23

flash vacuum pyrolysis conditions,
25
but it had never been used before in the context of
porphyrins. Porphyrins are thermally stable and relatively inert compounds. For
example, bis-pyreneyl zinc porphyrin can be sublimed without decomposition at 430 °C
in vacuum (10
-5
Torr). However, when sublimation is suppressed under an atmosphere of
nitrogen, bis-pyreneyl porphyrin undergoes thermal ring closure at elevated temperatures
(500-530 °C) to afford singly or doubly fused porphyrins. Moreover, fusion of
unactivated pyrene as well as other unactivated PAHs, such as naphthalene and coronene,
which cannot proceed under a wide range of oxidative reaction conditions is made
possible by our thermal fusion approach.
1
Overall, thermal cyclodehydrogenation
conditions, formation of an additional bond between the porphyrin skeleton and the
meso-substituded aromatic unit without any addition of oxidizing agents, does not require
nickel metallation or donor groups on the aromatic moiety to activate the system for
fusion and can be applied to a variety of PAHs.

In this chapter, the fusion of unactivated perylene to one or two porphyrin
moieties under oxidative aromatic coupling and thermal cyclodehydrogenation is
discussed. A series of perylenyl monporphyrins and perylene bridged porphyrin dimers
are studied and compared to provide insights on the scope and limitations of different
fusion approaches based on different porphyrin and PAHs building blocks.

24

2.2 Results and discussion
2.2.1 Synthesis of perylenyl porphyrins
The starting building block for our fused PAH porphyrin synthesis is a 5,15-
unsubstituted porphyrin with two meso positions open for PAH functionalization. This
building block can be obtained using methods well developed by Lindsey et al., starting
from dipyrromethane
26
and 3,5-di-tert-butyl-benzaldehyde,
27
as shown in Scheme 2.3.
Functionalization of meso-H positions of the porphyrins was achieved by bromination
with 1 equiv of N-bromosuccinimide (NBS), follow by borylation of
monobromoporphyrin with pinacolboronane, which is a necessary step for isolation of the
monosubstitued porphyrin pinacol boronate ester from the unsubstituted and 5,15-
disubstituted side products. Suzuki coupling of the porphyrin pinacol boronate ester and
monobromoperylene with equal equivalance using Pd(PPh
3
)
4
/Cs
2
CO
3
in toluene afford
desired product 1, while Suzuki coupling using a mixture of 3,9- and 3-10-Br
2
-perylene
and 2 equiv of porphyrin pinacol boronate ester afford product 2 as a mixture of
isomers anti-2 and syn-2. (Figure 2.6) Yields of the perylenyl derivatives 1 and 2 are all
30-40% after column chromatography purification followed by crystallization from
dichloromethane and methanol. Derivative 1 has good solubility in organic solvents
(e.g., toluene, dichloromethane), whereas the solubility of the derivative 2 is more
limited.

25

N N
N N
Zn
Ar
Ar
NH
NH
HN
HN
Ar
CHO
Ar
CHO
N N
N N
Zn
Ar
Ar
Br
i
ii iii
N N
N N
Zn
Ar
Ar
B(pin)
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
1
syn-2 anti-2
Br
Br
Br
Br Br
+
3,9-Br
2
-perylene 3,10-Br
2
-perylene

Figure 2.6 Ar = 3,5-di-tert-butyl-phenyl; i a) TFA, CH2Cl2, followed by
DDQ; b) Zn(OAc)2, CH2Cl2, MeOH, Yield 50%; ii NBS, CH2Cl2-pyridine, -10 °C,
yield = 80-95%; iii Pd(PPh3)4, Cs2CO3, pyridine, toluene, 110 °C, 10 - 20h

Separation of the anti- and syn-2 isomers is very challenging, as both isomers
have very similar solubility in organic solvents, almost identical polarity and size.
Attempts for separation of the porphyrin products through crystallization and column
chromatography have failed. An example of 3,9- and 3,10-dibromo perylene isomers
separation is found in a German paper published in 1925.
28
Even though both isomers
have very limited solubility in low boiling point solvents, even in boiling toluene
solubility is not high, they display good solubility in nitrobenzene and analine. The
3,9-isomer can be separated by consecutive recrystalization from nitrobenzene, analine,
xylene, and analine:xylene 1:1 mixture until long, needle like, dark orange crystals are
achieved. However, the 3,9-Br
2
-perylene precursor has very limited solubility, and

26

separation has yield of only 5-10% for the 3,9-Br
2
-perylene. Nevertheless, separation of
the 3,9-Br
2
-perylene precursor followed by Suzuki coupling as aforementioned leads to a
single isomer perylenyl porphyrin product anti-2; decreased solubility is also observed in
anti- 2 as compared to the isomer mixture 2.
All perylenyl derivatives 1 and 2, including isomeric mixture as well as single
isomer anti-2, are treated with Scholl-type oxidative conditions (FeCl
3
/CH
2
Cl
2
).
Porphyrin 1 undergoes fast -fusion (within 5min upon addition of FeCl
3
) to give the
perylene-fused porphyrin 1m. MALDI-TOF indicates the presence of the corresponding
free-base fused porphyrin, which can be treated with zinc acetate to afford desired
product. As electron-rich porphyrin moieties can be chlorinated in the presence of FeCl
3
,

prolonged reaction time leads to chlorinated side products, which are difficult to separate
from desired product by column chromatography due to the insignificant difference in
polarity.
In the case of 2, two porphyrin moieties are bridged by a perylene unit, also
providing two sites for potential -fusion. To our great surprise, with addition of
20 equivalence of FeCl
3
, only one of the porphyrin moieties in 2 undergoes -fusion,
giving the mono-fused perylenyl porphyrin 2m with decent yield of 60-65%, whereas the
second porphyrin moiety remains unfused, even under more forcing conditions
(Sc(OTf)
3
/DDQ/110 °C). However, decreasing the amount FeCl
3
from 20 to 10
equivalence gives both mono-fused and doubly-fused in very low yields (<5%); the
majority of porphyrin 2 stays unfused on either side, or demetallated to the free-base non-
fused porphyrin, as indicated by MALTI-TOF. UV-vis spectroscopy monitoring reaction
mixture absorptions show no spectral evolution from mono-fused 2m to doubly-fused 2d,

27

suggesting under oxidative aromatic coupling conditions 2m may not be a necessary
intermediate for the formation of 2d. Nonetheless, separation yield of 2d from Scholl-
type oxidative conditions (FeCl
3
/CH
2
Cl
2
) is too low for potential applications of the
material.
Thermal cyclodehydrogenation conditions are also applied to porphyrins 1 and 2.
In the case of 1, thermal fusion of 1 to 1m proceeds at 530 ºC. Unfortunately, the high
temperature of this process leads to elimination of some of the aryl groups from 1, giving
a mixture of 1m and related fused porphyrins where either one or both of the 5,15-aryl
groups are replaced by a proton (Figure). Likewise, thermal cyclodehydrogenation of
non-fused porphyrin isomer mixture 2 undergo at 530-550 ºC with the formation of
inseparable mixture of the doubly fused porphyrin tapes 2d together with side products
with the loss of di-tert-butyl-phenyl groups. Interestingly, in the case of single isomer
anti-2, thermal fusion at 530 ºC for 5 minutes gives a mixture of monofused anti-2m and
doubly fused syn-2d in much lower yield as compared to the isomer mixture sample, with
the majority of starting anti-2 remain unfused. Increasing thermal fusion time does not
change the yield of either anti-2m or anti-2d, but more decomposed products are
observed. Increasing temperature from 530 ºC to 550 ºC leads to only anti-2d, however
the yield is so low that product can only be detected by UV-vis spectroscopy while the
rest of the sample turns to insoluble decomposed product.
In contrast, use of the monofused precursors 2m (isomer mixture) or anti-2m for
thermal cyclodehydrogenation gives the perylene-fused porphyrin dimer 2d with good
yields (>80%) at lower temperature 480 ºC than that needed for preparation of 1m or 2d
using 2 as precursor, and occurs without loss of the 5,15-aryl groups (Figure, only one

28

isomer of 2d is shown). MALDI-TOF spectrum of the crude reaction mixture after
thermal cyclodehydrogenation in the range 100-5000 Da contains only a single peak of
the molecular ion of 2d. This result indicates that thermal fusion is favored in porphyrins
where at least one of the meso,positions is annulated with PAHs.
We performed thermal gravimetric analysis (TGA) on 1, 2 and 2m, and found that
all three samples lose about 30% of total weight, which corresponds to the lost of di-tert-
butyl groups, with a temperature onset of 500ºC. This result explains the observed
fragmentation of singly connected perylenyl-substituted porphyrins 1 and 2 during
thermal cyclodehydrogenation process with temperatures of 530ºC and 550ºC, while no
fragmentation is observed for 2m at thermal fusion temperature 480ºC. In contrast, for
monoporphyrins with two open meso positions, no fragmentation is observed with
temperatures up to 600ºC. We therefore suspect fragmentation of di-tert-butyl groups is
associated with melting of the materials. Indeed, we observe that 1 and 2 start melting
before thermal fusion reaction, but not in the case of 2m. Differential scanning
calorimetry (DSC) measurements of 1, 2 and 2m give major peak at 550ºC, most likely
corresponding to the fragmentation of di-tert-butyl groups in all samples.

29

N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
1 1m
A: FeCl
3
/ CH
2
Cl
2
B: 530- 550
o
C
N N
N N
Zn
Ar
N N
N N
Zn 1m

Figure 2.7 Annulation of a perylene ring to a porphyrin core

N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
anti-2 anti-2m
anti-2d
A: FeCl
3
, no reaction
B: 450- 480
o
C
Ar = 3,5-di-tert-butylphenyl
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
A
B: 530-550
o
C
+ fragmentation of (t-Bu)
2
Ph
-

Figure 2.8 Annulation of two porphyrin rings to a perylene core.

2.2.2 Photophysical and electrochemical characterizations
The absorption spectra of 1m, 2m, and 2d in CH
2
Cl
2
(Figure 2.9) show
significant bathochromic shifts with respect to those of their singly connected
perylenyl-porphyrin precursors 1 and 2, and significant increase of the Q band intensity.
Nonfused precursor 1 displays absorption peaks at 417, 453 and 544 nm with molar
absorptivity of 3.0, 0.9 and 0. 

M
-1
cm
-1
), respectively. Upon fusion, 1m displays

30

broadened and red shifted absorption peaks at 424, 541, 717 and 779 nm with 0.4, 0.6,
0.2 and 0.4 

M
-1
cm
-1
) molar absorptivity. In the case of the perylene-bridged
porphyrin dimer, nonfused precursor 2 displays absorption peaks at 414, 468 and 544 nm
with molar absorptivity of 4.7, 1.13, 0.58 ( 

M
-1
cm
-1
). Upon the first fusion on one side
of the perylene bridge, 2m displays absorption peaks at 415, 548, 717 and 785 nm with
molar absorptivity of 1.8, 0.5, 0.2 and 0.4 ( 

M
-1
cm
-1
); while fusion on both sides of the
perylene bridge leads to absorption peaks at 412, 556,, 848 and 942 nm with molar
absorptivity of 1.0, 0.5, 0.5 and 0.8 ( 

M
-1
cm
-1
).

400 600 800 1000
0
1
2
3
4
5
600 800 1000
0
1
 (10
5
M
-1
cm
-1
)
 (10
5
M
-1
cm
-1
)
Wavelength (nm)
1 2
1m 2m
2d

Figure 2.9 UV-vis absorption spectra of 1, 1m, 2, 2m and 2d in CH
2
Cl
2
.

Compounds 1m, 2m and 2d all display NIR photoluminescence ( 
max
= 811 ,
825 and 978 nm respectively). Although photoluminescence quantum yields are quite
low, 2% for 1m and 2d and less than 0.5% for 2d (Figure 2.10), both the absorption and
emission profiles of our fused products are consistent with those of the previous reported
porphyrins with similar structures by Wu et al.

31

500 600 700 800 900 1000 1100
0.0
0.2
0.4
0.6
0.8
1.0
Emission intensity (a.u.)
Wavelength (nm)

Figure 2.10 Emission spectra of 1, 1m, 2, 2m and 2d in CH
2
Cl
2
.

The electrochemical properties of compounds 1, 1m, 2, 2m and 2d were
investigated by cyclic voltammetry. The fused porphyrins have lower oxidation
potentials E
ox
than the nonfused precursors. Compounds 1 and 1m have E
ox
values of
0.38 and 0.14 V, respectively, while compound 2, 2m and 2d give E
ox
values of 0.29,
0.08 and -0.22 V, respectively. The fused porphyrins also show a significant decrease in
the separation between E
ox
and E
red
potentials ( ΔE
ox-red
). Compounds 1 and 1m give
ΔE
ox-red
value of 2.22 and 1.44 V, respectively, while compound 2, 2m and 2d give ΔE
ox-
red
value of 2.30, 1.25 and 1.00 V, respectively, which is consistent with the significant
bathochromic shifts observed in the absorption and emission profiles of the fused
porphyrins. All electrochemical data is summaries in Table 2.1.

32

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
1m
1
in 0.1 M Bu
2
NPF
6
/CH
2
Cl
2
Current (A)
*
**
Voltage vs Fc/Fc
+
(V)
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5
**
**
*
2d
2m
Current (A)
2
Voltage vs Fc/Fc
+
(V)
in 0.1 M Bu
2
NPF
6
/CH
2
Cl
2

Figure 2.11 Electochemical characterization of perylenyl porphyrins by cyclic
voltammetry. * peaks from internal standard ferrocene. Fc
+
/Fc = 0 V; ** peaks from
internal standard decamethylferrocene. dcFc
+
/dcFc = -0.54 V vs. Fc
+
/Fc.

Table 2.1 Electrochemical potentials of compounds 1, 1m, 2, 2m and 2d
Complex E
1/2
ox
/E
pa
ox
(V) E
1/2
red
/E
pc
red
(V) ΔE (V)
1 0.38 -1.84 2.22
1m 0.14 -1.30 1.44
2 0.29 -2.01 2.30
2m 0.08 -1.17
b
1.25
2d -0.22
a
-1.22
b
1.00
Measured by cyclic voltammetry in 0.1 M TBAPF, referenced to an internal
ferrocenium/ferrocene potential. (a) Irreversible oxidation. (b) Irreversible reduction.

33

2.2.3 Structural characterizations of perylenyl porphyrins
Characterization of large -extended porphyrins is known to be a common
problem due to strong aggregations. Common strategies to improve characterization
involve extensive use of solubilizing substituent in the structures, which becomes a huge
trade-off in the products for photovoltaics applications, as additional solubilizing groups
largely decrease charge transport in the thin films.
29-30
To make useful materials for
photovoltaics, we need to minimize the use of solubilizing groups and acquire complete
characterization.
We perform concentration, temperature and solvent dependent
1
H NMR on the
fused-porphyrins to get well-resolved spectra. Even though
1
H NMR spectra of fused-
porphyrins show a varying degree of line broadening and shifting of peaks in the
aromatic region under different conditions, we are able to perform reasonable
assignments based on chemical shift, multiplicity, coupling constant, integration from
1
H
NMR and through bond cross coupling information from 2D gCOSY; all assignments are
consistent with the structures we proposed before and after fusion.
Achieving the single isomer anti-3 by using single isomer 3,9-Br
2
-perylene is of
great importance for assigning NMR peaks and determining structures of its fused
derivatives anti-3m and anti-3d, as it sets apart isomer peaks from additional aggregation
peaks, even though all single isomer derivatives are less soluble as compared to their
isomer mixture counterparts.

34

Figure 2.12 a)
1
H NMR of a mixture of 3,9 and 3,10-Br
2
-perylene in CDCl
3
at
room temperature; b)
1
H NMR of single isomer 3,9-Br
2
-perylene in DMSO at 90 °C.

Due to the anisotropic effect from the porphyrin ring current, the NMR signals for
the deshielded meso and protons show up at low field, generally observed at  from 9.7
to 11.2 ppm for the meso-protons and at  from 8.5 to 9.9 ppm for the -protons. For
anti-2, we assign the singlet at 10.2 ppm for the meso-protons, the 4 doublets from
8.8 ppm to 9.4 ppm for the -protons (Figure 2.13). Based on the difference in coupling
constant, we can also tell the -protons (J~8) and the protons on the perylene ring (J~4)
apart. 2D NMR gCOSY shows through bond cross coupling between the doublets at 8.6
and 8.3 ppm, doublets at 8.4 ppm and triplets at 7.1 ppm, triplets at 7.1 ppm and doublets
at 6.9 ppm, and thus we assign these doublets and triplets as d
1
to d
4
and t as the perylene
protons. However, we are not able to tell d
1
and d
2
, or d
3
and d
4
apart.

35

N N
N N
Zn
Ar
Ar
N N
N N
Zn
Ar
Ar
d
4
d
1
d
2 t
d
3
meso

d

d

d

d
H
o
H
p
H
p
Ar =

36

Figure 2.13 a)
1
H NMR and b) gCOSY spectra of anti-2 in chloroform-
d/pyridine-d
5
at 58 °C

In order to assign NMR peaks for anti-2m, we need assignment information on
similar structure 1m. Upon fusion with perylene, meso proton of the -extended
porphyrin moiety shifts up field from 10.2 to 9.75 ppm; all of the -protons become
nonequivalent, leading to 1 singlet and 6 doublets from 8.8 to 9.4 ppm. The emergence
of 
s
at 9.4 ppm is important to confirm fusion of the perylene unit to the porphyrin
moiety. For protons on perylene, 8 doublets and 2 triplets are expected. Based on
gCOSY, we find through bond cross coupling between the doublets at 8.7 and peaks at
8.9 ppm, doublets at 8.54 and peaks at 7.7 ppm, doublets at 8.5 and peaks at 7.6 ppm,
peaks at 7.7 and 7.8 ppm. Since the triplets have cross coupling with two sets of
doublets, we assign 2 triplets from 7.6 to 7.7 ppm and 2 doublets from 7.8 to 7.9 ppm.

37

More detailed assignment is shown in Figure 2.14, but we are not able to tell d
1
and d
2
, or
d
3
and d
4
apart.

38

Figure 2.14 a)
1
H NMR and b) gCOSY spectra of 1m in chloroform-
d/pyridine-d
5
at 58 °C.
N N
N N
Zn
Ar
Ar
d
4
d
1
d
2
t
d
3
meso

d

d

d

s
H
o
H
p
H
p
Ar =
d
1
d
2
d
3
t
d
4

d

d

d

39

NMR spectra of anti-2m is expected to be a combination of anti-2 and 1, as the
perylene fused moiety and nonfused moiety are structurally identical to that of anti-2 and
1. The emergence of 
s
at 9.5 ppm is important to confirm the perylene fused porphyrin
moiety. Unfortunately,
1
H NMR only display one distinct meso proton of the nonfused
porphyrin unit, while the other meso proton of the fused moiety appears to be a broad
peak around 10 ppm, possibly due to aggregations in solution. A comparison of the
NMR spectra of 2m isomer mixture and anti-2m provides information on the set of peaks
contributed from syn-2m; however aggregation seems to be more pronounced in the
single isomer sample, as broad features are shown in the regions centered around 10 ppm,
8.9 ppm and 8.1 ppm.

40

Figure 2.15 a)
1
H NMR spectra of anti-2m in chloroform-d/pyridine-d
5
at
58 °C; b) comparison of 2m isomer mixture and anti-2m.

H
o
H
p
H
p
Ar =
N N
N N
Zn
Ar
Ar
d
d
d
t
d
meso

d

d

d

s
d
d
d d

d

d

d
N N
N N
Zn
Ar
Ar

41

NMR spectra of anti-2d is expected to be very similar to that of 1m. Deshielded
meso protons of anti-2d shift further up field to 9.56 ppm as compared to 9.75 ppm for
1m as the -conjugation of porphyrin moiety extends through fusion on both sides of the
perylene bridge, consistent with the pervious observation for the NMR spectra of 1m.
The emergence of 
s
at 9.4 ppm confirms fusion of the perylene unit to the porphyrin
moieties. Moreover, integration of peaks from di-tert-butyl protons from 1.0 to 1.4 ppm
further confirms that there is no loss of 5,15-aryl groups under the reported thermal
cyclodehydrogenation conditions.

Figure 2.16
1
H NMR spectra of anti-2d in chloroform-d/pyridine-d
5
at 58 °C.
H
o
H
p
H
p
Ar =
N N
N N
Zn
Ar
Ar
d
meso

d

d

d

s
d
d d

d

d

d
N N
N N
Zn
Ar
Ar

42

We also perform HRMS and MALDI-TOF spectroscopy to characterize our -
extended porphyrins. While HRMS provides calculated mass and molecular formula
consistent with our structures, MALDI-TOF further proves the purity of our final doubly-
fused product 2d.
31
Porphyrins as neat films are very stable towards laser irradiation and,
in most cases, give only a single peak of the molecular ion without any fragmentation.
The presence of any porphyrin-related impurities, i.e. fragmentation peaks from di-tert-
butyl groups in a sample containing a mixture of porphyrins, is visible in the MALDI-
TOF measurements; therefore the absence of any fragmentation peaks in our MALDI-
TOF spectra shows purity of 2d from any other porphyrin based impurities.

2.3 Conclusion
In summary, perylene-bridged diporphyrin has been synthesized to study
subsequent meso,  coupling reaction as a model towards alternative porphyrin tapes.
Unlike porphyrins and porphyrin tapes, partially fused perylenyl bridged diporphyrins are
not active towards further oxidative coupling in the Scholl reaction. However, the second
porphyrin ring of the mono-fused porphyrin can be annulated by thermal fusion reaction
that proceeds at significantly lower temperatures than thermal cyclodehydrogenation of
the previously reported PAH-porphyrins. Lowering the temperature of thermal fusion
avoids previous problems with fragmentation of meso aryl groups for the first time. These
results demonstrate that reactivity of -extended system of partially fused porphyrins is
inversed in the Scholl reaction vs. thermal cyclodehydrogentation. Our data suggests that

43

further lowering the temperature of thermal cyclodehydrogenation should be possible by
the appropriate choice of porphyrin and PAH precursors.

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13. Cho, H. S.; Jeong, D. H.; Cho, S.; Kim, D.; Matsuzaki, Y.; Tanaka, K.; Tsuda, A.;
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44

15. Berezin, M. Y.; Achilefu, S., Fluorescence Lifetime Measurements and Biological
Imaging. Chem Rev 2010, 110 (5), 2641-2684.
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Near-IR Dyes. Chem-Eur J 2011, 17 (24), 6610-6614.
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23. Jiao, C. J.; Zu, N. N.; Huang, K. W.; Wang, P.; Wu, J. S., Perylene Anhydride
Fused Porphyrins as Near-Infrared Sensitizers for Dye-Sensitized Solar Cells. Org Lett
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45

Chapter 3 Pyrazoloporphyrins

3.1 Carbaporphyrins
3.1.1 Structures of carbaporphyrins
N-confused porphyrin (NCP), or 2-aza-21-carbaporphyrins (1), is an isomer of
porphyrin with an inverted pyrrole ring, containing three nitrogens in the porphyrin core
and one at the porphyrin peripheral (Figure 3.1). The structure and property of NCP were
predicted by Linus Pauling and Melvin Calvin in 1943-1944.
1
However, not until 1994,
the synthesis and characterization of NCPs were reported independently by Furuta's
2
and
Latos-Grazynski's
3
groups. A large number of new NCP related organic and inorganic
compounds have been reported since the discovery of NCPs, which drastically extends
members of the porphyrin family and opens up new areas of research in diverse fields.
Contemporaneous with the studies on NCPs, other examples of carbaporphyrinoid
structures have been reported, including benziporphyrins (2),
4
oxybenziporphyrins (3),
5

azuliporphyrins (4),
6
tropiporphyrins (5)
7
and carbaporphyrins (6 and 7) as shown in
Figure 3.1.
8
Carbaporphyrinoids, porphyrin analogs with one or more carbon atoms
within the macrocyclic cavity, may be fully aromatic, non-aromatic, or somewhere in
between depending on the specific structures, providing valuable insights into the nature
of aromaticity in porphyrins and related structures. Moreover, these carbaporphyrins
have been shown to exhibit unique reactivity, including the ability to generate
organonmetallic derivatives under mild conditions.
9
For instance, azuliporphyrins have
been shown to readily form stable organometallic derivatives with nickel(II),

46

palladium(II) or platinum(II) salts,
10
whereas copper(II) salts give rise to a regioselective
oxidation at the internal carbon.
11
Benzocarbaporphyrins, on the other hand, act as
trianionic organometallic ligands, stabilizing unusual oxidation states such as silver(III)
and gold(III);
12
while oxybenziporphyrin acts as both a dianionic or a trianionic ligand
generating palladium(II), platinum(II), copper(III), silver(III) and gold(III) complexes.
13

N NH
N
HN
Ar
Ar
Ar Ar
N
H
N
N
HN
Ar Ar
Ar
Ar
N
N
HN N NH
HN
N
N
HN
N NH
HN
O
N NH
HN
N NH
HN
1a (N-Confused porphyrin) 1b
2 3
4
7
6
5
Benziporphyrins Oxybenziporphyrins Azuliporphyrins
Tropiporphyrins Carbaporphyrins Benzocarbaporphyrins

Figure 3.1 Examples of carbaporphyrinoid systems with highlighted
delocalization pathway.

3.1.2 Photophysical properties of carbaporphyrins
Besides the exciting reactivity, carbaporphyrinoids also display intriguing
spectroscopic properties as compared to regular porphyrins. Absorption spectra for 1a

47

and 1b (inner-3H and inner-2H form, respectively) display striking differences compared
to regular tetraphenyl free-base porphyrin (H
2
TPP) and are highly solvent dependent.
14

In a polar solvent such as dimethylformamide (DMF), the inner-2H form is more stable,
presumably because of either hydrogen-bonding or dipole-dipole interactions of the
exocyclic N-H bond with solvent, whereas a less polar solvent such as CHCl
3
tends to
favor the inner-3H form. As compared to the Soret band of H
2
TPP (419nm), the Soret
band of 1b and 1a is 25 and 19 nm red-shifted, respectively. Similarly, the Q bands are
∼37-50 nm lower in energy than the corresponding absorptions in H
2
TPP. Unlike
H
2
TPP, however, the intensities of the Q bands increase slightly with decreasing energy
(Figure 3.2).

Figure 3.2 Color and absorption spectra of NCP in CH
2
Cl
2
(inner-3H form,
left) and in DMF (inner 2H-form, right).
14

DFT calculations on carbaporphyrins have been carried out by different research
groups to provide better understanding of the electronic structures and spectra of these
porphyrins.
15
Feng et. al. have found that by substituting the nitrogen atoms with carbon

48

atoms in the porphyrin core, HOMO LUMO gaps of the carbaporphyrins are reduced
increasingly as compared to the regular porphyrin, in the order of monocarbaporphyrin,
dicarbaporphyrin, tricarbaporphyrin and tetracarbaporphyrin, leading to more red-shifted
absorption and emission of these carbaporphyrins as more nitrogen atoms are substituted
by carbon atoms.
15b
Moreover, the HOMO and HOMO-1 orbitals of carbapophryins are
contiguous with other occupied orbitals, which make the Gouterman four-orbital model
not hold for them, as the basis of the Gouterman model is that the HOMO, HOMO-1 and
the almost degenerated LUMOs are well separated from other MOs. Unlike regular
porphyrins, the oscillator strength for transitions in longer wavelengths can be improved
for carbaporphyrins, which explains the more intense, red-shifted and broadened Q
bands.
Despite that a number of carbaporphyrinoids have been made, displaying novel
structures, unusual reactivity and intriguing absorption properties, applications of these
materials remain largely unexplored. Only a few examples regarding applications of
carbaporphyrinoids can be found in the literature, including the use of carbaporphyrin
ketals as potential agents for photodynamic therapy,
16
and cobalt carbaporphyrin as
catalyst for cyclopropanation,
17
but no application has been reported so far in the field of
organic photovoltaic cells (OPVs) to the best of our knowledge.
Absorption properties of porphyrins and analogs are very attractive for artificial
photosynthesis and solar energy conversion. Porphyrins have played an important role in
understanding and development of organic photovoltaics, especially in small molecule
OPVs containing porphyrins as dyes.
18
One of our focuses is to develop new porphyrin

49

based NIR absorbing materials to capture and convert photons in the NIR part of the solar
spectrum for improved OPV efficiencies. In this regard, carbaporphyrinoids could be a
promising candidate considering their broadened and red-shifted absorptions as compared
to conventional porphyrin systems even without further modification of the porphyrin
framework, e.g. extension of conjugations.
In this chapter, the synthesis, characterizations and OPV applications of a Pd (II)
pyrazole-containing carbaporphyrinoid is discussed, providing information on the solid
state properties of carbaporphyrinoids and exploring their potentials as NIR absorbing
dyes in OPVs.

3.1.3 Brief summary of porphyrin and carbaporphyrinoid synthesis
Porphyrin synthesis has a long history, which dates back to 1926 when Fischer
published the first paper on porphyrin synthesis, where pyrromethene intermediates were
utilized.
19
Since then, a variety of approaches have been developed and employed in
porphyrin synthesis. In 1935, almost a decade after the discovery, Rothemund reported a
stepwise condensation of monopyrroles with aliphatic or aromatic aldehydes,
20
which
was reinvestigated by Lindsey in late 1980s and greatly improved the yields to 50-60%,
21

allowing this approach to be one of the most popular methods for constructing porphyrin
macrocycles. Another method is "2+2" type synthesis based on dipyrromethane or
dipyrromethene condensation initially suggested by MacDonald
22
and Woodward
23
in
1960. Johnson also made use of tripyrrolic intermediated known as tripyrranes and
introduced the concept of carrying out the synthesis of porphyrinoids by a "3+1"

50

variation on the MacDonald condensation, incorporating one or two furan or thiophene
subunits.
24
Cyclization of linear tetrapyrrolein the presence of a transition metal was
reported in 1976, where the tetrapyrrole was obtained by multi-step condensation of
pyrroles, allowing the synthesis of totally asymmetrical porphyrin systems. However,
this method tends to involve a significantly larger number of steps, which leads to lower
overall yields. Common routes for porphyrin synthesis is summarized in Figure 3.3.

N
HN N
NH
R
R
R R
HN
HN
OHC
NH
CHO
NH
CHO
CHO
R
R
R R
HN
X
NH
X
HN
CHO
NH
OHC
X = H or COOH
N
NH N
HN N
NH N
HN
R
R
HN NH
NH HN
CHO R OHC R
N
X N
Y
X = O or S
Y = NH, O or S
N
X
N
Y
HOOC
HOOC
OHC
CHO
N
NH N
HN
N
H
N
H
N
H
N
H
N
H
N
H
N
H
N
H
or

Figure 3.3 Common routes for porphyrin synthesis.

Although the "3+1" approach was initially reported in the early 1970s, it wasn't
further pursued until a direct route was developed to obtain the tripyrrane intermediate by
Sessler in 1987,
25
where two equivalents of an acetoxymethylpyrrole are condensed with
a 3,4-diethylpyrrole under acidic condition (p-toluenesulfonic acid) in refluxing ethanol,
followed by debenzylation by catalytic hydrogenation, producing the
tripyrranedicarboxylic acid. It was only in 1994 the first synthesis of a tetrapyrrolic
porphyrin structure by the "3+1" approach was reported by Boudif and Momenteau.
26
In
1996, the first synthesis of carbaporphyrin, benzocarbaporphyrin, was accomplished by

51

the "3+1" methodology, in which a tripyrrane was condensed with a dialdehyde in the
presence of an acid catalyst followed by an oxidation step to generate the porphyrinoid
system.
27
This methodology has been proved to be particularly effective in synthesizing
novel porphyrinoid structures; carbachlorins, azuliporphyrins, tropiporphyrins,
benziporphyrins and oxybenziporphyrins were synthesized by using the "3+1" version of
the MacDonald condensation.

3.2 Results and discussion
3.2.1 Synthesis of pyrazoloporphyrin
The pyrazoloporphyrin (pz-por) was synthesized by the "3+1" approach,
28

condensing equal equivalence of diformylpyrazole (pz)
29
with tripyrrane (tpy)
25
as shown
in Figure 3.4.
N
N
CHO
OHC
H
3
C
pz
NH
HOOC
HN
HOOC
HN
Me
Me
Et Et
Et
Et
tpy
[O]
N
N
N
CH N
HN
CH
3
Me
Et
Et
Et Et
Me
NH
N
N
CH HN
N
CH
3
Me
Et
Et
Et Et
Me
TFA
pz-por

Figure 3.4 Acid catalyzed condensation followed by oxidation for free-base
pyrazoloporphyrin pz-por.
1-methylpyrazole-3,5-dicarboxaldehyde (pz) was prepared from 1-H-pyrazole-
3,5-dicarboxylic acid (pz-1) shown in Figure 3.5. The pyrazole-1,3-dicarboxylate (pz-2)
was obtained by acid-catalyzed esterification of the dicarboxylic acid. Alkylation with

52

methyl iodide then affords the N-methyl-pyrazole (pz-3). Reduction of the diester with
lithium aluminum hydride gave the related dicarbinols (pz-4), and subsequent oxidation
with activated manganese dioxide in refluxing dioxane then gave the required
diformylpyrazole (pz) in good yields.
N
N
H
HOOC
COOH
N
N
H
H
3
COOC
COOCH
3
N
N
H
3
COOC
COOCH
3
H
3
C
N
N
HOH
2
C
CH
2
OH
H
3
C
N
N
OHC
CHO
H
3
C
CH
3
I
K
2
CO
3

CH
3
OH
H
2
SO
4
LiAlH
4
MnO
2
dioxane

pz-1 pz-2
pz-3
pz-4
pz

Figure 3.5 Synthesis of the diformylpyrazole (pz) precursor.

The acetoxymethylpyrrole (py-1) was prepared by a Knorr synthesis (Figure
3.6).
30
Oxime solution was first prepared by nitrosation of benzyl acetoacetate overnight,
which was then reduced by zinc dust generating the relevant amine in situ to react with
2,4-pentanedione in glacial acetic acid under external cooling. The resulting pyrrole was
reduced with diborane, followed by acetoxylation with lead tetraacetate to afford
py-1.

53

O
O
O
NaNO
2
CH
3
COOH, < 5
o
C
O
O
O
N
OH
Oxime from nitrosation
O O
Knorr pyrrole synthesis
CH
3
COOH
Zn,
NaBH
4
, BF
3
.Et2O
THF, < 5
o
C
N
H
BnOOC
N
H
BnOOC
O
Pb(OAc)
4
5% HCl to pH=4
CH
3
COOH
N
H
BnOOC
O
O
N
H
N
H
N
H
BnOOC
COOBn
H
2
, 10% Pd/C
THF N
H
N
H
N
H
HOOC COOH
N
H
BnOOC
O
EtOH, p-TSA
N
H
Et
Et
py-1
tpy
py-2

Figure 3.6 Knorr pyrrole synthesis and tripyrrane (tpy) precursor synthesis.

The key precursor tripyrrane (tpy) is synthesized in three steps from monomeric
pyrroles (py-1 and py-2) shown in Scheme 3.4. Condensation of two equivalents of
pyrroles py-1 and one equiv py-2 under acid catalyzed condition (p-toluenesulfonic acid)
in refluxing ethanol affords the tripyrrane diester. Debenzylation by Pd/C catalytic
hydrogenation produces the tripyrrane diacid (tpy) in quantitative yield while the
catalyst is removed by vacuum filtration with Celite under nitrogen atmosphere. The
tripyrrane is rather unstable in air thus it is not characterized but used directly for
condensation with diformylpyrazole (pz).
Methyl-pyrazole-dialdehyde (pz) was reacted with tripyrrane (tpy) in the
presence of TFA in dichloromethane for 18 hours, affording a phlorin intermediate that
must then be oxidized to afford the fully conjugated pyrazoloporphyrin (pz-por).
However, attempts to oxidize the crude product with DDQ led to complete

54

decomposition. Instead, dilute aqueous solutions of ferric chloride (FeCl
3
) was used as a
mild oxidant. By vigorously shaking the reaction solution in a separatory funnel with
0.1% aqueous ferric chloride solution, phlorin intermediate was oxidized to the fully
conjugated product. Higher concentration of FeCl
3
aqueous solution, ca. 0.5% or
prolonged shaking (ca. 10 min) also led to complete decomposition, while shorter
exposure times failed to oxidize the intermediate.
Metalation studies were performed on the free-base pz-por. When reacted with
palladium (II) acetate in refluxing acetonitrile, fully conjugated palladium (II)
pyrazoloporphyrin (Pdpz) was obtained in good yield (70%), displaying bright purple-
brown color in solution and dark purple color in neat solid. However, attempts for
metalation with zinc (II) acetate have failed, as no respected mass of the expected zinc
pyrazolopophyrin was observed from MALDI analysis of the crude reaction mixture.
Metalation with platinum (II) chloride in refluxing benzonitrile for 6 hours also failed,
leading to complete decomposition of the starting free-base porphyrin. Decomposition is
possible due to prolonged reaction at high temperature (200 °C), although no expected
metalation product was observed either by monitoring reaction mixture with UV-vis
absorption spectra and MALDI in shorter reaction time. Platinum (II) acetate prepared
through ligand exchange of PtCl
2
and silver (I) acetate, was also used to react with the
free-base porphyrin in refluxing benzonitrile. Expected mass of the platinum porphyrin
was observed from MALDI analysis of the crude reaction mixture, however, we were not
able to obtained the isolated product through column chromatography on basic alumina
or recrystalization from chloroform and methanol.

55

N
N
N
CH N
HN
CH
3
Me
Et
Et
Et Et
Me
pz-por
Pd(OAc)
2
acetonitrile, reflux
N
N
N
C N
N
CH
3
Me
Et
Et
Et Et
Me
Pd
Pdpz

N
N
N
CH N
HN
CH
3
Me
Et
Et
Et Et
Me
Zn(OAc)
2
CH
3
OH/CH
2
Cl
2
N
N
N
C N
N
CH
3
Me
Et
Et
Et Et
Me
Zn
PtCl
2
bezonitrile, reflux
N
N
N
C N
N
CH
3
Me
Et
Et
Et Et
Me
Pt
Pt(OAc)
2
bezonitrile, reflux

Figure 3.7 Metalation of free-base pz-por.

Thermal stability of Pdpz is characterized by thermal gravimetric analysis, which
indicates thermal decomposition of Pdpz with onset temperature at 320 ⁰C.

Figure 3.8 Thermal gravimetric analysis of Pdpz.

0 100 200 300 400 500
20
40
60
80
100
120
TG weight %
Temperature (
o
C)

56

3.2.2 Photophysical characterization
The absorption property of Pdpz is characterized by UV-vis absorption spectra in
both solution and in thin film. In CH
2
Cl
2
, Pdpz solution shows a Soret-like band at
320 nm to 417 nm, Q band-like absorptions at 530 nm and 570 nm, as well as a series of
weaker bands at 670 nm, 735 nm and 813 nm. Pdpz thin film shows slight broadening
and 10 nm red-shift of the Soret band, and very similar absorption peaks in the region of
540 nm to 830 nm while each peak red-shifted 15 nm to 20 nm as compared to that in the
absorption spectrum of solution . No emission is observed in 2-methyltetrahydrofuran
solution of Pdpz at 77 K or room temperature from 800 nm to 1500 nm.
400 600 800 1000
0.0
0.2
0.4
0.6
0.8
1.0
normalized absorbance
wavelength (nm)
Pdpz solution in CH
2
Cl
2
vapor deposit film

Figure 3.9 Absorption spectra of Pdpz in CH
2
Cl
2
solution and in a vapor
deposit thin film on glass.

In order to obtain information of the excited state kinetics that follow the
photoexcitation of Pdpz, transient absorption experiments are performed. Optical
excitation of the porphyrin is achieved through an excitation pulse centered at 520 nm.
Photobleaches corresponding to Pdpz absorption at 530 nm and 570 nm appear

57

immediately upon photoexcitation, indicating a depletion of the porphyrin’s ground state.
Induced absorption features are observed in 450-500 nm, 600-700 nm and 850-950 nm
that decay over the course of the first 10 ps. As the time delay between pump and probe
is increased, induced absorption peaks and photobleaches do not evolve spectrally with
the time window of measurements (1 ns).

a)
b)
Figure 3.10 Transient spectra for Pdpz in THF following photoexcitation at
520 nm.

The entire spectral and temporal datasets were fit simultaneously using a global
analysis procedure, and the resulting kinetics can be best described by Figure 3.11. The
global fitting data, taken together with the fact the Pd-porphyrins generally have high
intersystem crossing yields due to heavy metal effect, suggest that the 10-15 ps time scale
represents the intersystem crossing pathways that promotes Pdpz from its first singlet

58

excited state S
1
to its first triplet excited state T
1
, which returns Pdpz to its ground state in
a time scale much longer than our 1 ns measurement window and cannot be accurately
determined. The absorption spectra of S
1
and T
1
resulting from global fitting are shown
in Figure 3.12 a) The time profile of the normalized population of the two excited suggest
evolution from the initially populated S
1
state of Pdpz to a new excited state, presumably
T
1
(Figure 3.12 b)). No emission of Pdpz is observed in the measurement wavelength
range up to 950 nm.

Figure 3.11 Excited state kinetics following photoexcitation of Pdpz.

a) b)
Figure 3.12 a) Absorption spectra of S
1
and T
1
and b) time profile of
normalized population from global fitting.
S
0
S
1
T
1
10‐15 ps
8‐10 ns
h 

59

3.2.3 Electrochemical and theoretical characterization
Electrochemical properties of Pdpz are characterized by cyclic voltammetry (CV)
and differential pulse voltammetry (DPV), using decamethylferocene as internal
reference; while CV provides information on reversibility of peaks, DPV provides more
accurate reading of the peak positions. The first oxidation peak of Pdpz at 0.39 ev is
reversible with a small shoulder at 0.28 ev. The first reduction peak at -1.36 ev is also
reversible while the second reduction peak at -2.04 ev is quasireversible.
-2 -1 0 1
*
*
in 0.1 M Bu
2
NPF
6
/CH
2
Cl
2
Current (A)
Voltage (V)

b)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
Reduction scan
*
*
Oxidation scan
Differenciate Current (A)
Voltage (V)

Figure 3.13 Electrochemical characterization of Pdpz by CV (a) and DPV (b).
* peaks from internal standard decamethylferrocene dcFc
+
/dcFc = -0.54 V vs. Fc
+
/Fc.

60

Theoretical calculations (B3LYP/LACVP**) are performed on the model
palladium pyrazole carbaporphyrin without methyl and ethyl groups on positions of
pyrroles, as a comparison to a regular palladium porphyrin (Figure 3.14). In the regular
Pd porphyrin, the calculated HOMO and HOMO-1, LUMO and LUMO+1 are
degenerated and well separately from HOMO-2 and LUMO+2, respectively. On the
other hand, the asymmetric structure of Pd pyrazol carbaporphyrin lifted the degeneracy
of HOMOs and LUMOs, and thus the Gouterman four orbital model cannot be applied,
which is consistent with previous theoretical calculations in other carbapophyrinoid
systems. Although no emission of Pdpz can be observed, calculation predicts the first
triplet excited state is 0.99 ev above the ground state; spin density of the Pdpz is shown
in Figure 3.15. 

61

Figure 3.14 Theoretical calculations on regular Pd porphyrin and Pdpz.

Figure 3.15 Spin density of Pdpz triplet.

62

3.2.4 Device fabrication
Porphyrins have been shown to act as donor and hole transporting materials in
heterojunction OPVs. The HOMO and LUMO relative to vacuum for Pdpz estimated
from electrochemical redox potentials are 3.2 ev and 4.9 ev, respectively. The energy
offsets between Pdpz and fullerene C
60
E
HOMO
= 6.2 eV, E
LUMO
= 4.0 eV should provide
sufficient driving force for exciton separation for Pdpz to act as donor material in an
OPV using a C
60
acceptor layer. However, the low-lying triplet state of Pdpz at 3.9 ev as
indicated by theoretical calculation may potentially act as a trap state in the device.
Due to low thermal sublimation yield of Pdpz resulting from similar sublimation
temperature (310-315 ⁰C) to decomposition temperature (320 ⁰C), Pdpz thin films are
achieved by spin-casting from solution for OPV devices. Solution-process conditions are
evaluated by a screening of solutions with different Pdpz concentration (2, 3 and
5mg/mL) in different solvents (chloroform, toluene and chlorobenzene) under different
spin-casting speed (500, 1500, 2000 and 3000 rpm). Optimum condition is determined
by comparison of thin film surface roughness through root-mean-square (rms) value
under atomic force microscopy (AFM). Use of 3mg/mL chlorobenzene Pdpz solution
for spin-casting at 1500 rpm for 40 s gives the smoothest film with rms of 1.7 nm on ITO
(Figure 3.16). Thickness of the film is determined by ellipometry to be 190 Å. Solar
cells incorporating Pdpz are fabricated using a structure: ITO/Pdpz(190 Å)/C
60
(400 Å)/BCP(100 Å)/Al(1000 Å). However, such structure gives only shorting devices
without any rectifying current-voltage characteristics, possibly due to direct contact
between the anode and cathode caused by the crystalline porphyrin layer. Same linear
current-voltage curves are observed for devices with thermal annealing at 95 ⁰C for 3 min

63

before or after C
60
deposition. Morphologies of spun-cast porphyrin thin films on ITO
with and without annealing are shown in Figure 3.16 a) and b), indicating annealing
results in more crystalline Pdpz surface. Films annealed before C
60
deposition also
display more crystalline surface with a rms value of 2.7 nm, as compared to a rms value
of 1.5 nm from films annealed after C
60
deposition with (Figure 3.16 d) and e)).

Figure 3.16 a) ITO/Pdpor(190 Å) before annealing and b) after annealing; c)
ITO/MoO
3
(150 Å)/Pdpor(190 Å); d) and e) ITO/Pdpor(190 Å)/C
60
(400Å) annealed
before and after C
60
deposition, respectively; f) ITO/MoO
3
(150 Å)/Pdpor(190 Å)/
C
60
(400 Å), non-annealed.

In order to prevent shorting of devices from direct anode-cathode contact, 150 Å
of MoO
3
which is used in OPVs to promote hole injection, is deposited on ITO prior to

64

spin-casting Pdpz solution. Devices with structures of
ITO/MoO
3
(150 Å)/Pdpz(190 Å)/ C
60
(400 Å)/BCP(100 Å)/Al(1000 Å) show rectifying
current-voltage curve for both under dark and illumination conditions. The dark current
shown in Figure 3.11 displays significant current leakage, which can be explained by
electrons tunneling through the MoO
3
layer to the ITO anode, as spin casting donor layer
on MoO
3
leads to the most crystalline film with rms roughness of 3.2 nm (Figure 3.16 c)).
External quantum efficiency (EQE) overlaid with normalized film absorption of Pdpz
donor and C
60
acceptor, shows that the majority of the device photoresponse comes from
C
60
. Interestingly, small peaks from the EQE at around 700 nm, 780 nm and 840 nm
coincide with the Pdpz thin film absorption peaks in the same region, suggesting
possible photoresponse from Pdpz in the NIR. However, further device optimization is
needed to investigate the properties of Pdpz in OPV applications.

Figure 3.17 Current-voltage curve of device ITO/Pdpz(190 Å)/C
60
(400
Å)/BCP (100 Å)/Al(1000 Å), with J
sc
= 0.12 mA/cm
2
, V
oc
= 0.159 V, FF= 0.29 and
PCE= 0.005; EQE and normalized porphyrin Pdpz thin film, C
60
thin film, and device
absorption.
-0.5 0.0 0.5
-0.4
0.0
0.4
0.8

Current Density (mA/cm
2
)
Voltage (V)
Dark
Light
MoO
3
Pdpz
C
60
BCP
2.3
5.2 5.0
3.2
4.0
6.2
6.4
1.7
4.3
Al
4.8
ITO
400 500 600 700 800 900 1000
Normalized Abs
EQE %
Wavelength (nm)
EQE
Pd_por
C
60
device aborption
0
1
2
3
4
0.0
0.5
1.0
1.5
2.0
2.5

65

3.3 Conclusion
A Pd(II) pyrazole containing carbaporphyrin was synthesized successfully by
"3+1" acid catalyzed condensation of tripyrrane and diformylpyrazole precursors.
Absorption properties of the pyrazoloporphyrin solution and thin film are studied,
together with DFT frontier orbital calculations show the asymmetric carbaporphyrin does
not follow the Gouterman four orbital model. Excited state dynamics are studied by
femosecond transient absorption, indicating intersystem crossing follows photoexcitation
of the pyrazoloporphyrin in 10-15 ps timescale, resulting in a relatively long lived (ns
time scale) triplet state. Photovoltaic devices are fabricated using carbaporphyrins as
donor materials for the first time, with structures of ITO/MoO
3
(150 Å)/Pdpz(190 Å)/
C
60
(400 Å)/BCP(100 Å)/Al(1000 Å), where the pyrazoloporphyrin thin film is achieved
by spin-casting from solution. Crystalline porphyrin thin film leads to leakage in the dark
current, resulting in a relatively low device V
oc
, while low J
sc
is most likely due to lack of
photoresponse from the donor layer resulting from insufficient energy offset with C
60
.
However, further optimization of device fabrication including improving thin film
morphology as well as using an acceptor material with lower LUMO as compared to C
60
will provide more insight for the evaluation of such pyrazoloporphyrin in OPV
applications.

Chapter 3 references:
1. Senge, M. O., Extroverted Confusion-Linus Pauling, Melvin Calvin, and
Porphyrin Isomers. Angewandte Chemie-International Edition 2011, 50 (19), 4272-4277.
2. Furuta, H.; Asano, T.; Ogawa, T., N-Confused Porphyrin - a New Isomer of
Tetraphenylporphyrin. Journal of the American Chemical Society 1994, 116 (2), 767-768.

66

3. Chmielewski, P. J.; Latosgrazynski, L.; Rachlewicz, K.; Glowiak, T., Tetra-P-
Tolylporphyrin with an Inverted Pyrrole Ring - a Novel Isomer of Porphyrin.
Angewandte Chemie-International Edition in English 1994, 33 (7), 779-781.
4. Stepien, M.; Latos-Grazynski, L., Tetraphenyl-p-benziporphyrin: A
carbaporphyrinoid with two linked carbon atoms in the coordination core. Journal of the
American Chemical Society 2002, 124 (15), 3838-3839.
5. Lash, T. D., Oxybenziporphyrin, a Fully Aromatic Semiquinone Porphyrin
Analog with Pathways for 18-Pi-Electron Delocalization. Angewandte Chemie-
International Edition in English 1995, 34 (22), 2533-2535.
6. Lash, T. D.; Chaney, S. T., Azuliporphyrin: A case of borderline porphyrinoid
aromaticity. Angewandte Chemie-International Edition in English 1997, 36 (8), 839-840.
7. Bergman, K. M.; Ferrence, G. M.; Lash, T. D., Tropiporphyrins, cycloheptatrienyl
analogues of the porphyrins: Synthesis, spectroscopy, chemistry, and structural
characterization of a silver(III) derivative. Journal of Organic Chemistry 2004, 69 (23),
7888-7897.
8. (a) Berlin, K., Carbaporphyrins. Angewandte Chemie-International Edition in
English 1996, 35 (16), 1820-1822; (b) Lash, T. D.; Hayes, M. J., Carbaporphyrins.
Angewandte Chemie-International Edition in English 1997, 36 (8), 840-842.
9. Lash, T. D., Metal Complexes of Carbaporphyrinoid Systems. Chemistry-an
Asian Journal 2014, 9 (3), 682-705.
10. Graham, S. R.; Ferrence, G. M.; Lash, T. D., Organometallic chemistry of
carbaporphyrinoids: synthesis and characterization of nickel(II) and palladium(II)
azuliporphyrins. Chem Commun 2002, (8), 894-895.
11. Colby, D. A.; Ferrence, G. M.; Lash, T. D., Oxidative metalation of
azuliporphyrins with copper(II) salts: Formation of a porphyrin analogue system with a
unique fully conjugated nonaromatic azulene subunit. Angewandte Chemie-International
Edition 2004, 43 (11), 1346-1349.
12. Lash, T. D.; Colby, D. A.; Szczepura, L. F., New riches in carbaporphyrin
chemistry: Silver and gold organometallic complexes of benzocarbaporphyrins.
Inorganic Chemistry 2004, 43 (17), 5258-5267.
13. (a) Stepien, M.; Latos-Grazynski, L.; Lash, T. D.; Szterenberg, L., Palladium(II)
complexes of oxybenziporphyrin. Inorganic Chemistry 2001, 40 (27), 6892-6900; (b) El-
Beck, J. A.; Lash, T. D., Tetraphenyloxybenziporphyrin, a new organometallic ligand for
silver(III) and gold(III). Org Lett 2006, 8 (23), 5263-5266.
14. Toganoh, M.; Furuta, H., Blooming of confused porphyrinoids-fusion, expansion,
contraction, and more confusion. Chem Commun 2012, 48 (7), 937-954.
15. (a) AbuSalim, D. I.; Lash, T. D., Relative Stability and Diatropic Character of
Carbaporphyrin, Dicarbaporphyrin, Tricarbaporphyrin, and Quatyrin Tautomers. Journal
of Organic Chemistry 2013, 78 (22), 11535-11548; (b) Liu, X. J.; Pan, Q. H.; Meng, J.;
Feng, J. K., Substitution effect of nitrogen atoms with carbon atoms in porphyrin on the
electronic structure and excited states properties. Journal of Molecular Structure-
Theochem 2006, 765 (1-3), 61-69.
16. Morgenthaler, J. B.; Peters, S. J.; Cedeno, D. L.; Constantino, M. H.; Edwards, K.
A.; Kamowski, E. M.; Passini, J. C.; Butkus, B. E.; Young, A. M.; Lash, T. D.; Jones, M.
A., Carbaporphyrin ketals as potential agents for a new photodynamic therapy treatment
of leishmaniasis. Bioorganic & Medicinal Chemistry 2008, 16 (14), 7033-7038.

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17. Fields, K. B.; Engle, J. T.; Sripothongnak, S.; Kim, C.; Zhang, X. P.; Ziegler, C.
J., Cobalt carbaporphyrin-catalyzed cyclopropanation. Chem Commun 2011, 47 (2), 749-
751.
18. Martinez-Diaz, M. V.; de la Torrea, G.; Torres, T., Lighting porphyrins and
phthalocyanines for molecular photovoltaics. Chem Commun 2010, 46 (38), 7090-7108.
19. Fischer, H.; Walach, B., Synthesis of octamethyl porphin, of methyl analogues of
aetioporphyrin. Justus Liebigs Annalen Der Chemie 1926, 450, 164-181.
20. Rothemund, P., Formation of porphyrins from pyrrole and aldehydes. Journal of
the American Chemical Society 1935, 57, 2010-2011.
21. Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz, A. M.,
Rothemund and Adler-Longo Reactions Revisited - Synthesis of Tetraphenylporphyrins
under Equilibrium Conditions. Journal of Organic Chemistry 1987, 52 (5), 827-836.
22. Arsenault, G. P.; Bullock, E.; Macdonald, S. F., Pyrromethanes and Porphyrins
Therefrom. Journal of the American Chemical Society 1960, 82 (16), 4384-4389.
23. Woodward, R. B.; Ayer, W. A.; Beaton, J. M.; Bickelhaupt, F.; Bonnett, R.;
Buchschacher, P.; Closs, G. L.; Dutler, H.; Hannah, J.; Hauck, F. P.; Ito, S.; Langemann,
A.; Legoff, E.; Leimgruber, W.; Lwowski, W.; Sauer, J.; Valenta, Z.; Volz, H., The Total
Synthesis of Chlorophyll. Journal of the American Chemical Society 1960, 82 (14), 3800-
3802.
24. Broadhur.Mj; Johnson, A. W.; Grigg, R., Synthesis of Porphin Analogues
Containing Furan and/or Thiophen Rings. Journal of the Chemical Society C-Organic
1971, (21), 3681-&.
25. Sessler, J. L.; Johnson, M. R.; Lynch, V., Synthesis and Crystal-Structure of a
Novel Tripyrrane-Containing Porphyrinogen-Like Macrocycle. Journal of Organic
Chemistry 1987, 52 (19), 4394-4397.
26. Boudif, A.; Momenteau, M., A new convergent method for porphyrin synthesis
based on a '3+1' condensation. Journal of the Chemical Society-Perkin Transactions 1
1996, (11), 1235-1242.
27. Berlin, K.; Steinbeck, C.; Breitmaier, E., Synthesis of carba-porphyrinoids from
tripyrranes and unsaturated dialdehydes. Synthesis-Stuttgart 1996, (3), 336.
28. (a) Lash, T. D.; Young, A. M.; Von Ruden, A. L.; Ferrence, G. M., Adding to the
confusion! Synthesis and metalation of pyrazole analogues of the porphyrins. Chem
Commun 2008, (47), 6309-6311; (b) Young, A. M.; Von Ruden, A. L.; Lash, T. D.,
Pyrazole analogues of porphyrins and oxophlorins. Org Biomol Chem 2011, 9 (18), 6293-
6305.
29. Kumar, M.; Bhalla, V. S. N.; Kumar, V.; Singh, M.; Singh, G., Synthesis of new
macrocyclic polyaza compounds containing 1-methylpyrazole. J Incl Phenom Macro
2001, 39 (3-4), 241-245.
30. Awruch, J.; Frydman, B., The Total Synthesis of Biliverdins of Biological
Interest. Tetrahedron 1986, 42 (15), 4137-4146.

68

Chapter 4 Porphyrin modified SWNT hybrids

4.1 Introduction
Single-walled carbon nanotubes (SWNTs) have unique optical,
1-4
mechanical
5-7

and electronic
8-9
properties that have been utilized in a wide range of applications in
fields such as biology, molecular electronics, and solar energy conversion.
10-15
However,
the polydispersity and insolubility of SWNTs in common organic solvents and water
hamper the extension of SWNTs to further applications.
16
Among efforts to increase the
processability of SWNTs, the noncovalent binding of small molecules or polymers to
SWNT surfaces is a powerful strategy to introduce chemical functionalities to SWNTs
without significantly affecting the structure and electronic properties of the nanotubes.
17-
18
In this context, polycyclic aromatic molecules and conjugated polymers have been
used to modify and disperse SWNTs by forming π- π interactions with nanotube
sidewalls.
19-20
The attachment of functional units to SWNTs through the formation of
solubilizing π- π interactions offers great opportunity to develop SWNT-based hybrids for
various applications. In particular, SWNT-based photosensitizing donor-acceptor multi-
chromophore nanohybrids
21-22
are promising materials for photovoltaic applications due
to their light-harvesting characteristics and the excellent charge transport properties of
SWNTs.
23-24

Chromophores have been tethered to polycyclic aromatic compounds, such as
pyrene, which noncovalently bind to SWNT surfaces through π- π interactions,
25-26
but the
spatial distance between the chromophores and the SWNT surface can hinder desirable
electronic coupling in such photosensitized donor-acceptor hybrids. Heterocyclic

69

polyaromatic molecules such as porphyrins
27-28
can bind more directly to SWNTs
through the  system of the chromophores, resulting in porphyrin-SWNT hybrids with
the potential for improved electronic coupling between the SWNT and bound
chromophore. However, these hybrids can be easily dissociated, indicating a weak
affinity for the SWNT surface. Due to larger size of their -system, triply connected
porphyrin tapes
29-30
and polymers
31
were found to display a larger binding affinity for
SWNTs compared to porphyrin monomers. However, these conjugated porphyrins are
not only difficult to synthesize, but also display limited solubility and processability.
Moreover, making a stable suspension using triply connected porphyrins is possible only
in trifluoroacetic acid containing solvents or by adding multiple bulky alkyl groups to the
porphyrin tape.
30
The further rational design of photoactive SWNT donor-acceptor
hybrids requires a fundamental understanding of the noncovalent chemistry, the nature of
- interactions between light-harvesting molecules and SWNTs, and its effect on
photosensitized energy and electron transfer processes.
Large -extended porphyrins obtained by the fusion of porphyrin to meso-
connected polycyclic aromatic rings have attracted significant attention recently.
32-39

Among them porphyrins fused at meso, -positions with unsubstituted polycyclic
aromatic rings represent unique types of large -systems that are not sterically blocked,
and thus available to form - stacking interactions with SWNT surfaces.
38-39
At the
same time these porphyrins have improved solubility in organic solvents due to out-of-
plane distortions of the porphyrin core that prevent strong aggregation. Previously, we
demonstrated that such porphyrins can exhibit -  interactions with each other and also

70

with fullerenes, as evidenced by a significant bathochromic shift of the porphyrin Q-band
transition in thin film absorption spectra.
38-39

N N
N N
Zn
Ar
Ar
1
N N
N N
Zn
Ar
Ar
2
N N
N N
Zn
Ar
Ar
anti-3
N N
N N
Zn
Ar
Ar
syn-3
Figure 4.1 Structures of non-fused bis-pyrenyl porphyrin 1, mono-fused
pyrenyl porphyrin 2, and doubly-fusedpyrenyl porphyrin 3 including the isomers anti-3
and syn-3. Ar indicates 3,5-di-tert-butylphenyl groups.

In this study, we have probed the concept of using the unhindered -system of
fused porphyrins, minimizing the spatial distance between the porphyrin chromophore
and the SWNT surface, to enhance their affinity to bind to SWNT sidewalls and to
facilitate electronic coupling. We chose non-fused bis-pyrenyl porphyrin 1, mono-fused
pyrenyl porphyrin 2, and the doubly-fused pyrenyl porphyrins anti-3 and syn-3, shown in
Figure 4.1, to systematically study how extending the size and shape of the π-systems of
these pyrenyl-porphyrins modifies their interaction with SWNTs.
We demonstrate the first example of stable and easily processable nanohybrids
formed by strong noncovalent interactions between SWNTs and -extended porphyrins
in nonpolar organic solvents. These porphyrin-SWNT hybrids show very promising
light-harvesting properties, possessing absorption transitions that span from the visible to
the NIR due to complementary absorption features of the porphyrin chromophore and
SWNTs. The binding interaction between SWNTs and attached porphyrins displays not
only selectivity towards specific porphyrin isomers, but is also sufficiently strong to
prevent dissociation of the porphyrin from the SWNT when strongly coordinating ligands

71

are added, allowing further hierarchical modification by metal-ligand axial coordination
without disturbing the stability of the hybrids. Femtosecond transient absorption
experiments reveal the presence of ultrafast photoinduced electron transfer between
porphyrin molecules and the SWNT to which they are bound, but also show rapid
electron-hole recombination following electron transfer.

4.2 Results and discussion
4.2.1 Preparation of porphyrin modified SWNTs hybrids
The formation of noncovalent π- π interactions between nanotube sidewalls and
conjugated materials, including aromatic molecules and conjugated polymers, is a useful
and simple approach to modify and disperse SWNTs. The solubilizing ability of different
conjugated materials has been shown to depend on the size of the π-system of the
noncovalent modifier. For example, pyrene groups adsorb onto SWNTs and impart
solubility to the nanotubes, whereas phenanthrene binding groups show only limited
solubilizing ability and naphthalene binding groups have no solubilizing affect.
40
In
addition, the affinity of conjugated porphyrin oligomers to bind to SWNTs was
demonstrated to increase sharply with the length of the porphyrin’s π-system.
41

To determine the affinity with which pyrenyl porphyrins 1, 2 and 3bind to
SWNTs, porphyrin-SWNTs suspensions were prepared by sonicating SWNTs with
porphyrin solutions. 1, 2 and 3 were prepared as previously described.
38-39
Compound 3
is obtained as a 1:1 mixture of anti and syn isomers. Sonication of SWNTs with
solutions of 2 or 3 for approximately one hour results in a homogenous suspension of

72

SWNTs, whereas insoluble SWNTs aggregates could still be observed when 1 was
employed. Formation of a well-dispersed suspension was found to occur in a wide range
of solvents, including dimethylformamide (DMF), chlorobenzene, CH
2
Cl
2
, THF and
toluene. The suspensions of SWNTs with1, 2 and 3 were filtered through a 0.4 μm-pore
polytetrafluoroethylene (PTFE) membrane and washed repeatedly with the solvent used
for sonication until the filtrate was colorless, indicating the removal of any
excess/unbound porphyrin.
29-30
The prepared porphyrin-SWNT hybrids could be
quantitatively re-suspended after filtration and washing. 1-SWNT suspension began to
precipitate upon standing for an hour in the absence of excess porphyrin, while2-SWNT
did not show precipitation within 24 hours of preparation but started aggreagting after a
week. In contrast, 3-SWNT showed no sign of precipitation after a week of standing
without the presence of excess porphyrin.
Absorption spectroscopy was used to characterize suspensions of these porphyrin-
SWNT hybrids. Optical absorption spectra of the porphyrin-SWNT hybrids along with
spectra of the individual components are shown in Figure 4.2. Compounds 1, 2 and 3
display Soret and Q-band absorptions characteristic of porphyrins at below and above
550 nm, respectively, which red-shift and broaden as the -system of the porphyrin is
extended through consecutive pyrene fusion. The Soret and Q-band absorptions of 1
( 
max
= 429 and 560 nm, respectively) did not change upon suspending with SWNTs,
while those of 2 ( 
max
= 498 and 725 nm, respectively) broadened and red-shifted to 500
and 743 nm following the addition of SWNTs to solution, indicating interaction between
2 and the added SWNTs. The broad features attributed to SWNT absorption in the region
of = 1000–1400 nm are unaltered by the presence of 2, suggesting that many of the

73

SWNTs present in solution do not directly interact with 2, possibly due to the presence of
large SWNT bundles. Absorption bands of 3 are observed at 510, 530 and 834 nm, and
show a similar broadening and red-shift to 512, 534 and 867 nm once SWNTs are added
to solution. In the region of = 1000–1400 nm, SWNTs show distinct absorption
features that are narrower and better resolved with respect to the absorption of a pristine
SWNT suspension, suggesting substantial if not complete dissociation of the SWNT
bundles.

74

400 600 800 1000 1200 1400
0.0
0.1
0.2
0.3
X10
A
Absorbance
Wavelength (nm)
1-SWNT
1
400 600 800 1000 1200 1400
0.0
0.2
0.4
0.6
B
Absorbance
Wavelength (nm)
2-SWNT
SWNT
2

Figure 4.2 Absorption spectra of (A) non-fused bis-pyrenyl porphyrin 1 and
1-SWNT hybrids in DMF. Insert: porphyrin Q-band at 560 nm; (B) Mono-fused
porphyrin 2, CoMoCAT SWNTs and 2-SWNT hybrids in DMF; (C) Doubly-fused
porphyrin 3, CoMoCAT SWNTs and 3-SWNT hybrids in DMF.

The dissociation of SWNT bundles by 3 into small bundles or individual SWNTs
was further confirmed by transmission electron microscopy (TEM) characterization of
the hybrids. Before modification by 3, SWNTs are aggregated (Figure 4.3A), forming a

400 600 800 1000 1200 1400
0.0
0.1
0.2
0.3
0.4
0.5
C
Absorbance
Wavelength (nm)
SWNT
3-SWNT
3

75

dense network of bundles due to strong inter-nanotube van der Waals attractions. In
contrast, 3-SWNT shows substantial unwinding of the aggregates into small bundles or
individual tubes (Figure 4.3B), indicating that π- π interactions between 3 and the SWNT
surface are strong enough to overcome inter-nanotube van der Waals attraction and
prevent SWNTs from re-bundling after formation of the porphyrin-SWNT hybrids.

Figure 4.3 (A) TEM characterization of the SWNT sample before porphyrin
3 modification and (B) after modification.The black scale bar in both images indicates
200 nm.

To further investigate the interaction between 3 and SWNTs, we varied the ratio
between 3 and SWNTs in solution and instead of filtering off unbound porphyrin after an
hour of probe sonication, ultracentrifigation for 30 minutes at 42,000 g was used to
remove large SWNT bundles. Absorption spectra of the supernatant solution after
centrifugation are shown in Figure 4.4A. When an excessive amount of 3 was used (i.e.
2:1 weight ratio), absorption spectra of the supernatant solution show both free porphyrin

76

and 3-SWNT hybrid features. As the amount of 3 was reduced (1:1), no absorption from
free porphyrin was observed in the supernatant, suggesting an irreversible attachment of
3 to SWNTs. When the weight ratio between 3 to SWNT fell below 0.3, the supernatant
became colorless and displayed no absorption from either 3 or SWNTs, suggesting the
amount of 3 was not enough to break apart SWNT bundles so that both 3and SWNTs are
quantitatively removed by ultracentrifigation from solution. Therefore a threshold
porphyrin to SWNT ratio exists for efficient SWNT dispersion. When a mixture of 1, 2
and 3 is used, only the free porphyrin absorption of 1 and 2 is observed in the
supernatant, with 1 being enriched, while absorptions corresponding to either free
SWNTs or 3are not observed (Figure 4.4B), suggesting that 3 has the highest binding
affinity for SWNTs, followed by 2 and lastly 1. This study provides strong evidence that
the degree of interaction between monoporphyrins and SWNTs can be increased through
the extension of the π-system of monoporphyrins. Even though 1 is singly connected to
two pyrenyl groups, a functional group well known to bind to SWNTs,
42
steric hindrance
between porphyrin and pyrenyl rings in the structure of 1 prevents the binding of either
pyrene moiety or porphyrin core to the sidewalls of SWNTs. Upon pyrene-fusion, steric
hindrance was eliminated and the extended π-conjugation enhanced the binding
interactions between either 2 or 3 with SWNTs.

77

400 600 800 1000 1200 1400
0.0
0.2
0.4
0.6
0.8
A
Absorbance
Wavelength (nm)
3 to SWNT 2 :1
3 to SWNT 1.5 : 1
3 to SWNT 1:1
400 600 800 1000 1200 1400
0.0
0.2
0.4
400 500 600
B
3
1
2
Wavelength (nm)
Absorbance
supernatant
1, 2 and 3 mixture

Figure 4.4 Absorption spectra of the supernatant. (A) Samples with different
3 to SWNT weight ratios. Black arrows indicate the disappearance of absorption features
from free porphyrin 3. (B) Samples with a mixture of porphyrins 1, 2 and 3 (red: initial
porphyrin mixture absorption; blue: supernatant after sonication with SWNTs and
centrifugation). Insert: supernatant absorption from 400–600 nm plotted on an expanded
scale.

Possible detachment of 3 from SWNTs was investigated by repeating the
washing, filtering and re-suspension cycle of the 3-SWNT hybrids. The relative
absorbance of the porphyrin compared to that of SWNTs was unchanged after three
cycles (Figure 4.5A) indicating no measurable detachment of 3 from SWNTs. Moreover,
no detachment of 3 from SWNTs was observed following the addition of coordination
additives, such as pyridine and dodecylamine, to 3-SWNT suspensions. While the added

78

amines did not lead to release of 3, the porphyrin Q-band absorption of the nanohybrids
displayed a red-shift (Figure 4.5B) due to the coordination of the additives to 3. Similar
red-shifted Q-band absorption was observed when the same additives were added to the
porphyrin solution. This coordination reaction allows for further modification of
3-SWNT hybrids by metal-ligand axial coordination without disturbing the stability of
the hybrids.

600 800 1000 1200 1400
0.4
0.8
1.2
800 850 900 950
B
Absorbance
Wavelength (nm)
3-SWNT
with py
with dda

Figure 4.5 Absorption spectra of 3-SWNT hybrids (A) after one and three
cycles of washing, filtering and re-dispersing, and (B) with pyridine (py) and
dodecylamine (dda) as coordination ligands.

400 600 800 1000 1200 1400
0.04
0.06
0.08
0.10
0.12
0.14
A
Absorbance
Wavelength (nm)
3 time redispersion
1 time redispersion

79

4.2.2 Modification ratio estimation:
The fraction of the SWNT surface covered by 3 in 3-SWNT hybrids was estimated
by two methods, absorption spectroscopy and elemental analysis. In the first method, the
amount of bound 3 in the nanohybrids was estimated by using the molar absorptivity of
the Q-band for 3 in CH
2
Cl
2
at 867 nm (1.1 x 10
5
M
-1
cm
-1
), assuming this value remains
unchanged upon binding to SWNTs. The nanohybrids were prepared using a measured
amount of SWNTs and an excess of 3. After filtration of 3-SWNT and repeated washing
to eliminate excess 3, the 3-SWNT sample was suspended in a fixed volume of DMF and
the absorption spectrum measured to estimate the amount of 3 in the 3-SWNT sample.
The measured modification ratio was 177 SWNT carbon atoms per molecule of 3. To
estimate the percentage of the SWNT surface covered by 3, we consider the fact that we
used a CoMoCAT SWNT mixture, which contains a majority (>50%) of (7,6)
semiconducting SWNTs. (7,6) SWNTs have a diameter of 0.88 nm with 508 carbon
atoms per unit cell. Given the measured modification ratio, we estimate that 2.9
molecules of 3 exist per unit cell of (7,6) SWNT.
The modification ratio was also evaluated by elemental analysis of the unbound 3
in the filtrate from the absorbance measurement. Unbound 3 was first demetallated,
followed by EDTA titration to determine the Zn concentration. Given that each
porphyrin molecule coordinates a single Zn atom, the titrated amount of Zn in the filtrate
is complementary to that in the porphyrin-SWNT hybrids. The modification ratio
determined by this method was 157 SWNT carbon atoms per molecule of 3, which is
nearly double the modification ratio of 280 carbon atoms to 1 molecule of previously

80

reported pyrene derivatives.
43
Based on a (7,6) SWNT, the modification ratio was
estimated to be 3.2 molecules of 3 per unit cell of (7,6) SWNT, reaching a 64% surface
coverage of SWNT. The estimated modification ratios from both absorption
spectroscopy and elemental analysis are in good agreement given that our modification
ratio calculated from the molar absorptivity of 3 does not account for the broadening and
red-shifting of the porphyrin Q-band upon binding to SWNTs. If the oscillator strength
of the porphyrin Q-band is preserved upon SWNT binding, our methodology will lead to
an underestimation of the amount of porphyrin bound to the SWNT surface.

Figure 4.6 Top: NMR spectrum of 3 before modification with SWNTs;
Bottom: NMR spectrum of the filtrate following the addition of SWNTs.

The molecular morphology of the pyrene-fused porphyrins was also investigated.
As aforementioned, the as-synthesized 3 was a 1:1 mixture of anti and syn isomers. In
both isomers, the length of the cavity between the two di-tert-butylphenyl groups is 0.89
nm, matching well the diameter of (7,6) SWNTs. The binding of the two isomers to
SWNTs was probed with NMR. The NMR spectrum (Figure 4.6) of the initial solution
of 3 displays a 1:1 ratio of the two isomers, based on the integration of H
α
peaks. After

anti syn
H
α
9.90 9.85 9.80 9.75 9.70
Chemical shift (ppm)

81

treatment with SWNTs, the NMR spectrum of the filtrate shows a 1:9 ratio of the anti
and syn isomers, indicating that anti-3 is bound to SWNTs in preference to syn-3. While
anti-3 and syn-3 are similar chemically, they have very different shapes. The geometry-
optimized structuresof the two compounds are shown in Figure 4.7. The length of the
molecule along the SWNT π- π interaction axis is 2.3 nm for the linear anti-3 isomer
(Figure 4.7A) and 1.5 nm for the bent syn-3 isomer (Figure 4.7B), which suggests that
anti-3 has a larger surface area available to form π- π interactions with a neighboring
SWNT. In both cases, the modification ratio of 3.2 molecules per (7,6) unit cell is very
compact, as the length of a (7,6) unit cell is 4.8 nm, twice and three times the length of
anti-3 and syn-3, respectively. The side view of the isomers also shows a difference in
the surface curvature between anti-3 and syn-3. In the case of anti-3, the two fused-
pyrene groups are slightly twisted in opposite directions (Figure 4.7C), which matches
well with the surface curvature of (7,6) SWNTs. In the case of syn-3, the two fused-
pyrene groups are curved in the same direction, distancing the π-system of syn-3 from the
SWNT surface (Figure 4.7D). Therefore we hypothesize that 3-anti interacts more
strongly with SWNTs compared to syn-3 due to the larger π- π interaction area and
favorable surface curvature of anti-3.

82

Figure 4.7 (A)& (B) Comparison of the -system size and shape of anti-3 and
syn-3 with that of the SWNT along the SWNT’s long axis. (C) & (D) optimized
structures of model porphyrins anti-3 and syn-3 without the 3,5-di-tert-butylphenyl
groups, calculated at the B3LYP/6-31G*/LANL2DZ level. Reprinted with permission
from V. V. Diev, C. W. Schlenker, K. Hanson, Q. Zhong, J. D. Zimmerman, S. R. Forrest
and M. E. Thompson, J. Org. Chem., 2012, 77, 143. Copyright (2012) American
Chemical Society.

4.2.3 Photophysical characterizations
The photophysical properties of 3-SWNTs were first studied by fluorescence
experiments. Emission spectra were obtained by exciting the peak of the porphyrin Soret
band. Before modification with SWNTs, strong NIR emission was observed for 3 at
830 nm in DMF. However upon the formation of 3-SWNTs, over 80% of the porphyrin

83

emission was found to be quenched. This quenching may result from a variety of
processes, including energy transfer from 3 to the bound SWNT or photoinduced charge
separation.

Figure 4.8 Emission spectra of 3 and 3-SWNT hybrids in DMF.

To distinguish the excited state kinetics that follow the photoexcitation of 3,
femtosecond transient absorption experiments were performed. Optical excitation of the
porphyrin Q-band was achieved through an excitation pulse centered at 800 nm (Figure
4.9). Photobleaches corresponding to both the Soret (550 nm) and Q bands (860 nm) of 3
appear immediately upon photoexcitation indicating a depletion of the porphyrin’s
ground state. Interestingly, a photobleach at 660 nm that matches a feature in the
absorption spectra of the unmodified SWNTs (Figure 4.2) also arises within the
experiment’s time resolution (100 fs). The concurrent growth of this feature along with
photobleaches due to the porphyrin suggests that the electronic ground state of 3 partially
extends onto the SWNT backbone. This observation is consistent with the large red-shift
that occurs in the absorption spectrum of 3 upon binding to a SWNT, suggesting an
extension of the π-system of 3.

700 750 800 850
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (a.u.)
Wavelength (nm)
3 in DMF
3-SWNT

84

500 600 700 800 900
-8
-6
-4
-2
0
2
4
A
Abs (10
-3
)
Wavelength (nm)
50 fs 210 fs 1 ps
130 fs 510 fs
500 600 700 800 900
-2
-1
0
1
2
B
Abs (10
-3
)
Wavelength (nm)
1 ps 12 ps 525 ps
3 ps 125 ps
024 68 10
-0.8
-0.4
0.0
0.4
0.8 C
Abs (Norm.)
t (ps)
870 nm
920-935 nm
fitting

Figure 4.9 (A)&(B) Transient spectra measured for 3-SWNT hybrids in DMF
following photoexcitation at 800 nm. (C) Temporal slices that correspond to the
relaxation of the photobleach of the Q-band of 3 (black squares) and the induced
absorption attributed to the cation of 3 (red circles). The dash dot blue lines correspond
to biexponential fits with time constants of 260 fs and 6 ps.

As the time delay between pump and probe is increased, the photobleaches
corresponding to both the SWNT and Soret and Q bands of 3 rapidly decay over the

85

course of 1 ps. This result is qualitatively different from that observed in measurements
of 3 dissolved in DMF. In these experiments, photoexcitation gives rise to Soret and Q-
band photobleaches as well as a broad induced absorption band between these features.
However, all of these features are long-lived compared to those seen following
photoexcitation of 3-SWNTs. Excitation of 3 in DMF leads to a photobleach and
induced absorption features that decay exponentially with a rate constant of 1.8 ns and do
not evolve spectrally with time. These observations taken together with the fact that Zn-
porphyrins generally have low intersystem crossing yields, suggests that the 1.8 ns time
scale represents the sum of the rates for radiative and internal conversion pathways that
directly return 3 to its ground state. Returning to the transient spectra of 3-SWNT
(Figure 4.9A), as the relaxation of the Soret and Q-band photobleaches occurs, a new
induced absorption band appears from 900–970 nm suggesting evolution from the
initially populated S
1
state of 3 to a new excited state. This new absorption feature peaks
at a time delay of 1 ps, decays over the course of a few tens of ps, and has largely
vanished by 100 ps. Likewise, beyond 1 ps, the decay of the Q-band photobleach slows
and tracks the relaxation of the induced absorption feature between 900-970 nm (Figure
4.9B). An isosbestic point at 890 nm can be observed between the Q-band bleach and the
induced absorption band, indicating that the relaxation of this feature leads to a
repopulation of the porphyrin’s ground state. Both the growth and decay of the induced
absorption band as well as the decay of the Q-band photobleach can be described by a
biexponential function with time scales of 260 fs and 6 ps (Figure 4.9C).
The rapid photobleach loss and growth of a new absorption feature following the
photoexcitation of 3 is unlikely to result from energy transfer from 3 to the SWNT

86

backbone. Such a process would result in the return of 3 to its ground state, causing the
photobleach of its Soret and Q bands to fully recover as the new absorption feature
appears. The fact that these bands show an additional slow decay that tracks the loss of
the new absorption feature argues against this scenario. A more likely explanation for the
observed spectral evolution is that photoexcitation of 3 drives an electron transfer
reaction between it and the bound SWNT. In this case, the appearance of new absorption
features corresponding to both the cation of 3 and the SWNT anion are expected to
appear. Furthermore, if these features overlap spectrally with the Soret and Q bands of 3,
a decrease in the photobleach of these bands will be observed as the 3cation and SWNT
anion absorption features grow as the charge transfer reaction takes place. Once charge
transfer has occurred, if charge recombination takes place that results in a nonradiative
return of 3-SWNT to its ground state, then both the photobleach of 3 and any
cation/anion induced absorption features will decay with the same time scale. Prior
studies that have utilized pyrene-based linkers to tether electron rich groups to SWNTs
have observed photoinduced charge transfer between the tethered electron donor and
SWNT,
44-47
but generally on longer time scales (a few ps to tens of ns) than the sub-ps
decay observed here. However, the 3-SWNT system described here differs from the
materials investigated in these studies in that the doubly bridged pyrene groups of 3 cause
it to directly sit on the sidewall of the SWNT, leading to a smaller separation between the
π-systems of 3 and the SWNT than in systems tethered to SWNTs via pyrene linkers.
To test if the spectral evolution we observe in the transient spectra can indeed by
attributed to photoinduced electron transfer, we measured the cation absorption spectra of
3 by electrochemically oxidizing it in a cyclic voltammetry cell (Figure 4.10). The

87

measured cation absorption spectrum shows a strong reduction in the intensity of both the
Soret and Q bands, but gains appreciable intensity to the red of both of these bands,
notably near 600 nm and from 870 to 1000 nm. This latter region is the same spectral
range over which the new absorption feature is observed in the transient spectra.
Likewise, even though the intensities of the Soret and Q bands are diminished in the
cation absorption spectrum, the cation still shows an appreciable absorption in the range
that these bands appear, lending support to the hypothesis that the sub-ps decay of the
Soret and Q bands arises from charge transfer. The rapidity with which charge transfer
occurs in 3-SWNT gives rise to the strong quenching observed in steady-state
photoluminescence experiments (Figure 4.8).

Figure 4.10 Absorption spectra of the porphyrin 3 cation measured through
electrochemical oxidation.

Figure 4.11A plots transient spectrabut now measured in the NIR spectral range
after photoexcitation of the 3-SWNT hybrid at 850 nm. A series of strong photobleach
bands are found to arise within the instrument response, the most prominent of which
appears at 1165 nm and matches a feature in the ground state absorption spectrum of the
3-SWNT hybrid that is attributable to the bound SWNT. Similar to what was observed

400 600 800 1000
0.00
0.05
0.10
0.15
0.20
in 0.1M Bu
4
NPF
6
/CH
2
Cl
2
Absorbance
Wavelength (nm)
3
3 cation

88

for the Soret and Q bands of 3, after a delay of 1 ps, the decay of the SWNT photobleach
tracks the disappearance of the 3cation (Figure 4.11B). This indicates that the electronic
relaxation of the bound SWNT is linked to that of the porphyrin, as would be expected
for the recombination of photogenerated charge carriers. At long time delays (~ 1 ns),
the SWNT photobleach is found to relax to a 3% offset, indicating that a small amount of
charge carriers persist in the system following photoexcitation.
1100 1200 1300 1400
-30
-20
-10
0
A
Abs (10
-3
)
Wavelength (nm)
100 fs
760 fs
3.2 ps
22 ps
525 ps

Figure 4.11 (A) Spectral slices showing the evolution of transient absorption
spectra of 3-SWNT hybrids in the NIR spectral range following photoexcitation at 850

0.1 1 10 100 1000
-1.0
-0.5
0.0
0.5
1.0
1.5
B
Abs (Norm.)
t (ps)
920-935 nm
1165 nm flipped
fitting

1000 1100 1200 1300 1400
0.0
0.2
0.4
0.6
0.8
1.0
C
Absorbance (Norm.)
Wavelength (nm)
Linear Abs
- Abs( t=1 ps)

89

nm. (B) Temporal slices showing the relaxation of the photobleach of the SWNT (black
squares) and 3 cation induced absorption (red circles). The sign of the SWNT
photobleach has been inverted to enable better comparison with the 3 cation induced
absorption profile. After 1 ps, these features show similar relaxation profiles. (C)
Comparison of the absorption spectrum of 3-SWNT hybrids and the transient spectrum
measured at a time delay of 1 ps.

Given that we observe induced absorption features that correspond to the cation of
3 following photoexcitation, we also expect to observe the appearance of induced
absorption features that are characteristic of SWNT anions. Prior transient absorption of
similar SWNT hybrid systems that undergo photoinduced charge transfer have attributed
the appearance of induced absorption features between 1200-1600 nm to SWNT anions.
45
However, for the system studied here, no net induced absorption signal is observed in this
spectral range. Rather, a comparison of the absorption spectrum of 3-SWNT and the
transient spectrum obtained at a 1 ps time delay where charge transfer is expected to be
largely complete shows that the transient spectrum is not solely comprised of a
photobleach of ground state absorption features. This may in part arise from
contributions to the spectrum from weak SWNT anion induced absorption bands as well
as a shift in the location of the van Hove singularities as a result of SWNT anion
formation, as was observed in a tetrathiafulvalene sensitized SWNT system.
43

90

4.3 Conclusions
A systematic study of the binding between π-systems of fused-pyrenylporphyrins
and SWNTs has been reported. High solubility and unhindered π-system of fused-
pyrenyl porphyrins allow efficient dispersion of SWNTs in common organic solvents,
e.g.toluene, CH
2
Cl
2
, DMF. The binding of SWNTs to porphyrins has been found to be
dependent on the length and shape of the porphyrin’s π-system. While the non-fused
porphyrin1 shows only limited ability to solubilize SWNTs, the ability of porphyrins to
bind to the SWNT surface increases across the series: mono-fused 2, doubly-fused syn-3,
doubly-fused anti-3. Very high selectivity in binding (>9:1) between different isomeric
porphyrins with different -system shapes has been observed. The linear π-system of the
anti-3 isomer fits well to a linear-shaped SWNT, resulting in improved binding to the
surface of the SWNT compared to bent isomers with a similarly sized -system. The
formation of stable 3-SWNT hybrids, characterized by UV-vis and SEM, allows further
hierarchical modification by axial coordination of nitrogen containing ligands to the
porphyrin metal center to generate three-component nanohybrids. Transient absorption
experiments indicate that while rapid photoinduced charge transfer occurs in 3-SWNT
hybrids (1/k
CT
= 260 fs), the majority of the photoinduced charges recombine over the
course of a few picoseconds. This is the first example of strong electronic binding
between -extended porphyrins and SWNTs. Future modification of the nanohybrids
through the addition of coordination ligands that move the positive charge further from
the SWNT may be able to suppress the rapid charge recombination observed in the
hybrids studied here, leading to a system suitable for applications in solar energy
conversion.

91

Synthesis & Experimental Methods:
SWNTs used in our experiments were purification-grade HipCO SWNTs purchased
from Unidym and CoMoCAT (7,6) SWNTs purchased from Sigma Aldrich; materials
were used as purchased without any further purification. PTFE membranes with a pore
size of 0.4 μm were purchased from Millipore. Ultrasonication (sonicator 3000, Misonix)
was implemented by immersing an ultrasonic probe (standard 12.7 mm diameter tip) into
5-7 ml of the SWNT suspension. During sonication, the solution was immersed in an
ice-water bath to prevent heating. Ultracentrifugation was performed using a Beckman
XL-90. Absorption measurements were carried out using a Cary 14 UV-Visible
spectrometer while steady-state emission measurements were performed using a Photon
Technology International QuantaMaster Model C-60 fluorimeter. TEM was performed
on a JEOL JEM-2100 microscope at an operating voltage of 200 kV, equipped with a
Gatan Orius CCD camera, and samples were prepared by drop casting SWNT
suspensions onto 300 mesh Formvar-coated copper grids (Ted Pella,Inc.).
1
H NMR
spectra were recorded on a Varian 400-MR spectrometer and chemical shifts were
reported in ppm relative to the residual non-deuterated solvent CHCl
3
(  7.26 ppm).
Ultrafast transient absorption experiments were conducted with a Ti:Sapphire
regenerative amplifier operating at a 1kHz repetition rate (Coherent Legend, 3.5 mJ, 35
fs). Excitation pulses centered at 800 nm were derived from the direct output of the
amplifier while pulses centered at 850 nm were produced by pumping a visible OPA
(Spectra Physics OPA-800C) with ~10% of the amplifier output. White light super-
continuum probe pulses were created by focusing a small amount of the amplifier output

92

into either a 1 mm thick CaF
2
plate or a c-cut sapphire plate. CaF
2
was found to give a
stable continuum from 320–1080 nm while sapphire yielded a continuum that extended
further into the near-IR (450–1400 nm). To prevent photodamage, the CaF
2
plate was
rotated slowly during data collection. Following its generation, the white light continuum
was collimated and focused into the sample using a pair of off-axis aluminum parabolic
mirrors. The pump was focused to a point after the sample with a CaF
2
lens to give a
spot size of 300 m (FWHM) at the sample. After passing through the sample, the probe
was dispersed by a spectrograph (Oriel MS1271) onto a 256-element photodiode array.
A Si diode array was used to measure spectra from 400–970 nm, while a
thermoelectrically cooled InGaAs array (Hamamatsu G9213-256S) recorded transient
spectra in the range of 1000–1450 nm.
Samples for transient absorption experiments consisted of a solution of 3-SWNT
in DMF held in a 1 mm quartz cuvette. Samples were slowly translated perpendicular to
the path of the pump and probe by a linear stage to prevent sample photodamage. The
cross correlation of the pump and probe in a sample cuvette filled with neat DMF had an
average FWHM of 100 fs across the probe spectrum. Transient spectra were recorded for
a number of different pump energies between 60 and 900 nJ. Over this range, spectra
were found to scale linearly with the pump fluence, suggesting that contributions to the
spectra from bimolecular annihilation processes are minimal. For experiments that probe
the NIR spectral range (Figure 4.11A), the sample concentration was adjusted such that
the peak optical density of the Q-band of 3 was 0.3. A pump energy of 60 nJ was used,
and the polarization between the pump and the probe was set to the magic angle. Due to
the large scattering background in the visible region that arises from SWNTs, the sample

93

concentration was lowered by 3x to record the spectra in Figure 4.9 and the pump energy
was raised to 550 nJ. For this data set, the pump and probe were oriented perpendicular
to one another. This allowed the suppression of scattered pump light by passing the
probe through a polarizer after the sample.

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47. Bartelmess, J.; Ballesteros, B.; Torre, G. d. l.; Kiessling, D.; Campidelli, S.; Prato,
M.; Torres, T.; Guldi, D. M., Phthalocyanine-pyrene conjugates: A powerful approach
toward carbon nanotube solar cells. J Am Chem Soc 2010, 132, 16202-16211.

97

Future outlook
The design and detailed studies of NIR absorbing materials discussed in previous
chapters, including  extended perylene bridged porphyrins, pyrazole carbaporphyrins, as
well as porphyrin-modified SWNTs hybrids, have provided insights into applications of
these materials in organic photovoltaics.
The perylene-bridged porphyrins in Chapter 2 are synthesized by porphyrin
periphery fusion. They display pronouncedly red-shifted and intensified absorptions,
with Soret band (400-600 nm) in the visible region and the Q band (700-1020 nm) in the
NIR region. However, despite the promising absorption property of the perylene bridged
porphyrins, such large  extended porphyrin systems are not sublimable, which hampers
the incorporation of these materials in OPVs through thermal vapor deposition for thin
film fabrication. Moreover, the perylene bridged porphyrins display poor solubility in
common organic solvents due to significant aggregations from intramolecular -
stacking, which leads to poor solution processability for OPV applications.
Besides extension of porphyrin  systems through periphery fusion, we found that
replacement of the nitrogen atom in the porphyrin core by a carbon atom can also be a
promising approach for NIR absorptions. The pyrazole carbaporphyrin in Chapter 3
displays red-shifted absorptions up to 813 nm; they are soluble and sublimable and thus
can be applied to OPV devices by either solution process or thermal vapor deposition.
However, the intensity of the NIR absorption bands are relatively week, with molar
absorptivity an order of magnitude lower than that of the absorption band in the visible
region.

98

The incorporation of periphery fusion and core modification of porphyrins may
provide materials that have strong NIR absorptions as well as improved solution/thermal
vapor processability for OPV applications. For example, tetrabenzoporphyrins have an
extended  system through ,  fusion with benzene units with red-shifted and intensified
Q band absorption. Each additional benzene unit included in the macrocycle causes a
bathochromic shift of about 30 nm. Examples of extended carbaporphyrins through ,
 fusion mode are shown in Figure 5.1.

Figure 5.1 Examples of extended carbaporphyrins through ,  fusion mode.

The porphyrin-SWNTs hybrids in Chapter 4 display broad absorption, which
spans from 400 nm to 1100 nm, and good solubility in common organic solvents;
photoinduced charge transfer from the porphyrins to the SWNTs occurs in the hybrids
within 260 fs time scale, showing promising potentials for OPV applications. The major
drawback of this specific system, however, is the rapid charge recombination (6 ps) due
to the close proximity of the porphyrin unit with the SWNT surface.
Efficient charge separation has been observed in SWNT-fullerene nanohybrids
(Figure 5.2), where the SWNT behaves as an electron donor and the fullerene as an
electron acceptor. In this specific system, the rates of charge separation and charge
recombination were found to be 3.46 × 10
9
and 1.04 × 10
7
s
_1
, respectively. The

99

calculated lifetime of the radical ion-pair was found to be over 100 ns, suggesting charge
stabilization in the nanohybrids.

Figure 5.2 SWNT-C
60
nanohybrids using alkylammonium functionalized
pyrene and benzo-18-crown-6 functionalized fullerene. (Endohedral and exohedral
hybrids involving fullerenes and carbon nanotubes)

The design and investigation of novel porphyrin-SWNT-fullerene systems shown
in Figure 5.3 will be a promising approach to generate solution processable, broad band
absorbing nanohybrids with added advantages of rapid photoinduced charge separation
and charge stabilization. Specifically, to compete with the ps time scale charge
recombination between the porphyrins and SWNTs, the second charge transfer from the
SWNTs to the fullerenes should be well tuned through the type and length of the linkage
between the SWNT and fullerene.

100

Figure 5.3 Example of porphyrin-SWNT-fullerene nanohybrids.

e
-

N N
N N
Zn
Ar
Ar
anti-3
e
-

h
+

h
+

101

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Abstract (if available)

Abstract The conservation and transformation of energy is essential to the survival of mankind, and thus concerns every modern society. Solar energy, as an everlasting source of energy, holds one of the key solutions to some of the most urgent problems the world now faces, such as global warming and the oil crisis. Advances in technologies utilizing clean, abundant solar energy, could be the steering wheel of our societies. Solar cells, one of the major advances in converting solar energy into electricity, are now capturing people’s interest all over the globe. While solar cells have been commercially available for many years, the manufacturing of solar cells is quite expensive, limiting their broad based implementation. The cost of solar cell based electricity is 15-50 cents per kilowatt hour (¢/kwh), depending on the type of solar cell, compared to 0.7 ¢/kwh for fossil fuel based electricity. Clearly, decreasing the cost of electricity from solar cells is critical for their wide spread deployment. This will require a decrease in the cost of light absorbing materials and material processing used in fabricating the cells. ❧ Organic photovoltaics (OPVs) utilize organic materials such as polymers and small molecules. These devices have the advantage of being flexible and lower cost than conventional solar cells built from inorganic semiconductors (e.g. silicon). The low cost of OPVs is tied to lower materials and fabrication costs of organic cells. However, the current power conversion efficiencies of OPVs are still below 15%, while convention crystalline Si cells have efficiencies of 20-25%. A key limitation in OPVs today is their inability to utilize the near infrared (NIR) portion of the solar spectrum. This part of the spectrum comprises nearly half of the energy in sunlight that could be used to make electricity. ❧ The first and foremost step in conversion solar energy conversion is the absorption of light, which nature has provided us optimal model of, which is photosynthesis. Photosynthesis uses light from the sun to drive a series of chemical reactions. Most natural photosynthetic systems utilize chlorophylls to absorb light energy and carry out photochemical charge separation that stores energy in the form of chemical bonds. The sun produces a broad spectrum of light output that ranges from gamma rays to radio waves. The entire visible range of light (400-700 nm) and some wavelengths in the NIR (700-1000 nm), are highly active in driving photosynthesis. Although the most familiar chlorophyll‐containing organisms, such as plants, algae and cyanobacteria, cannot use light longer than 700 nm, anoxygenic bacterium containing bacteriochlorophylls can use the NIR part of the solar spectrum. No organism is known to utilize light of wavelength longer than about 1000 nm for photosynthesis. NIR light has a very low‐energy content in each photon, so that large numbers of these low‐energy photons would have to be used to drive the chemical reactions of photosynthesis. This is thermodynamically possible but would require a fundamentally different molecular mechanism that is more akin to a heat engine than to photochemistry. ❧ Early work on developing light absorbing materials for OPVs was inspired by photosynthesis in which light is absorbed by chlorophyll. Structurally related to chlorophyll is the porphyrin family, which has accordingly drawn much interest as the potential light absorbing component in OPV applications. ❧ In this dissertation, the design and detail studies of several porphyrin‐based NIR absorbing materials, including π‐extended perylenyl porphryins and pyrazole‐containing carbaporphyrins, as well as porphyrin modified single‐walled carbon nanotube hybrids, will be presented, dedicating efforts to develop novel and application‐oriented materials for efficient utilization of sustainable solar energy.

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Asset Metadata

Creator Zhong, Qiwen (author)

Core Title Porphyrin based near infrared‐absorbing materials for organic photovoltaics

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 07/14/2014

Defense Date 05/14/2014

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

Tag carbon nanotube,near infrared,near infrared‐absorbing materials,OAI-PMH Harvest,organic photovoltaics,porphyrin,solar cells,solar energy

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Malmstadt, Noah (committee member), Reisler, Hanna (committee member)

Creator Email qiwenzho@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-437037

Unique identifier UC11287630

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Legacy Identifier etd-ZhongQiwen-2667.pdf

Dmrecord 437037

Document Type Dissertation

Format application/pdf (imt)

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Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

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