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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back of the book. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6" x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. A Bell & Howell Information Company 300 North Z ee b Road. Ann Arbor. M l 48106-1346 USA 313/761-4700 800/521-0600 DESIGN, SYNTHESES, AND CHARACTERIZATION OF ORGANIC MATERIALS FOR NONLINEAR OPTICAL APPLICATIONS by Joyce G. Laquindanum A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Chemistry) December 1995 Copyright 1995 Joyce G. Laquindanum UMI Number: 9617107 UMI Microform 9617107 Copyright 1996, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90007 This dissertation, written by . £ .» . . t a g u t a G s w w n . ...................................................... under the direction of h&x. Dissertation Committee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillm ent of re quirements for the degree of D O CTO R OF PH ILOSOPHY c . /S L i^s Dean o f Graduate Studies Date ..Se^tember^ 14, 1995 DISSERTATION COMMITTEE To Danny and Papa and Mama Mommy Jona, Jella, and Jetty ii Acknowledgements To the Lord God, for watching over me and giving me strength to continue with what I have started; to Danny and our families, for their constant support, encouragement, and love; to Prof. Larry R. Dalton, for being a very insightful adviser and for supporting me throughout most of my graduate studies; to Prof. Charley W. Spangler, my "surrogate" adviser, for being very supportive in more ways than one and for making me realize that lab work can also be fun. to Sean Garner, Nansheng Tang, Antao Chen, Srinath Kalluri, and Profs. W. Steier and Bob Hellwarth, for those very productive collaborations; to the members of the Dalton group both past and present, most especially, Chuck Xu, Linda Sapochak, Bo Wu, Andrea Hubbell, and Younsoo Ra for the stimulating discussions and the company; to the Spangler family, Max, and Abby, for welcoming me into their home and for all those wonderful dinners; to the members of the Spangler group at NIU for making my brief stay in DeKalb most enjoyable despite the weather; to the Chemistry staff at NIU for their invaluable help; to Michele Dea and the staff at the Chem office for all those cheerful conversations; to Jim Merritt, for masterfully creating some of the glasswares that I needed; to Luz Plopino, for the occasional lunches; to my friends at the convent, especially Puring and Glecy; and to Rosie Richards, my writing partner, who has the unenviable position of pushing me to start and continue writing this dissertation; To all of you, my heartfelt thanks and unending gratitude!!! Table of contents Page Dedication ii Acknowledgements iii List of Figures viii List of Tables xiv Abstract xv Chapter 1. Background of Nonlinear Optics and Review of Non linear Optical Materials 1 1.1 Introduction 2 1.2 Basis of Nonlinear Optics 4 1.3 Second-Order Nonlinear Optics 7 a. Second-order effects 7 i. Second-harmonic generation 7 ii. Electrooptic effect 9 b. Materials for second-order nonlinear optics 14 i. Chromophore design and synthesis 14 ii. Stabilization of poling-induced order 18 a. Guest-host systems 21 b. Covalent attachment of chromophores 24 Side-chain incorporation 24 Main-chain incorporation 25 c. Crosslinked systems 29 iv c. Crosslinked systems 29 Thermosets 29 Photochemically crosslinked polymers 32 Double-ended crosslinkable systems 35 Sol-gel systems 35 1.4 Third-Order Nonlinear Optics 37 a. Third-order effects 40 i. Third-harmonic generation 40 ii. Degenerate four-wave mixing 42 b. Materials for third-order nonlinear optics 44 i. ^-conjugated polymers 44 Polydiacetylenes 44 Polyacetylenes 45 Polyarylenes and Polyarylene vinylene 46 ii. a-conjugated polymers 47 iii. Ladder polymers 48 iv. Macrocycles 49 v. Fullerenes 50 1.5 Scope of this dissertation 51 1.6 References 53 Chapter 2 Third-order Nolinear Optical Studies of Bisthienyl Polyenes 64 2.1 Introduction 65 2.2 Experimental 69 v 2.3 Results and Discussion 72 a. Nonlinearities of -H and -SBu substituted bisthienyl polyenes 72 b. Studies on -SBu bisthienyl polyenes at 532 nm 80 c. Studies on-SBu bisthienyl polyenes at 1064 nm 87 2.4 Conclusion 91 2.5 References 94 Chapter 3 Novel Materials for Second-Order Nonlinear Optical Applications 97 3.1 Introduction 98 3.2 Experimental 103 a. General materials and methods 103 b. Synthesis of model compound 105 c. Synthesis of monomers 111 d. Synthesis of polymers 127 e. Film processing and electric field poling 133 f. NLO characterization 136 i. Second-harmonic generation 136 ii. Electrooptic coefficient measurement 139 g. Thermal stability measurements 141 3.3 Results and Discussion 143 a. Model compound 143 b. Double-ended crosslinkable system 153 c. Thermosetting polyurethane system 164 vi d. Polymers incorporating tricyano-functionalized chromophores 180 3.4 Conclusion 188 3.5 References 192 Chapter 4 Synthetic Studies of Functionalized Fused-Ring Chromophores for Use in Rigid-Rod Main-Chain Polymer System 194 4.1 Introduction 195 4.2 Experimental 201 4.3 Results and Discussion 208 4.4 Conclusion 225 4.5 References 226 vii List of Figures Chapter 1 Fig. 1.1 Schematic representation of a) frequency conversion, in general and b) second- and c) thid harmonic generation. 1.2 A Mach-Zehnder interferometer (from ref. 1). 1.3 A typical corona-poling set-up (from ref. 40a) 1.4 Chromophore arrangements in a main-chain polymer backbone. 1.5 a) A schematic representation of an accordion main-chain polymer (ref. 92) and b) an accordion main-chain polymer incorporating an NLO chromophore (from ref. 93). 1.6 Examples of epoxy systems containing NLO chromophores (from ref. 99 and ref. 100). 1.7 Scheme for the formation of thermally stable NLO- thermoset polyurethane (from ref. 101). 1.8 An NLO-polyimide glass composite (from ref. 116). 1.9 Schematic representation of the sol-gel approach utilizing atri-functionalized NLO chromophore (from ref. 117). 1.10 DFWM mixing experimental set-up using a) forward beam geometry and b) backward beam geometry (from ref. 1). Chapter 2 Fig. 2.1 MO diagram for the formation of the polaron and bipolaron states (from ref.35). 2.2 Scheme for the synthesis of -H and -SBu substituted bisthienyl polyenes. 2.3 DFWM experimental set-up (from ref. 35). 8 13 20. 26 28 31 33 38 39 43 68 70 71 viii 2.4 Absoprtion spectrum of doped -H substitued bisthienyl polyene (n=8) taken every five hours after doping. 73 2.5 Schematic representation of the formation of the bipolaronic bisthienyl polyenes. 74 2.6 DFWM signal of neutral -H substituted bisthienyl polyene (n=5). 77 2.7 DFWM signal of doped -H substituted bisthienyl polyene (n=5). 78 2.8 DFWM-interferometric set-up (from ref. 1). 81 2.9 Representative DFWM interference pattern using -SBu substituted bisthienyl polyene (n=5) (from ref. 35). 83 2.10 Plot of Y 1111 vs. log n to determine the chain length dependence of the third-order hyperpolarizability. 86 2.11 Plot of log yi 111 experimental (o) and log yi 111 theoretical (♦) vs. log n to determine the chain length dependence of the third-order hyperpolarizability. 88 Chapter 3 Fig. 3.1 Double-ended crosslinkable chromophore approach. 101 3.2 Thermosetting-polyurethane approach utilizing a tri hydroxy functionalized NLO chromophore. 104 3.3 Scheme for the synthesis of thiophene intermediates for Wittig and Horner-Emmons reactions. 106 3.4 Scheme for the synthesis of model compound (compound 6). 107 3.5 Scheme for the synthesis of cyanovinyl-substituted DEC-chromophore (compound 12). 112 3.6 Scheme for the synthesis of tri-hydroxy functionalized chromophore (compound 2 0 ) for incorporation into a thermosetting polyurethane system. 113 XX 3.7 Scheme for the synthesis of tricyanovinyl-functionalized chromophore (compound 25) for condensation polymerization. 114 3.8 Scheme for the synthesis of tricyanovinyl-functionalized chromophore (compound 26) for radical polymerization. 115 3.9 Scheme for the synthesis of polymer P1. 128 3.10 Scheme for the synthesis of thermosetting polyurethanes P2-XL1 and P2-XL2. 129 3.11 Scheme for the synthesis of Polymer P3. 130 3.12 Scheme for the synthesis of Polymer P4. 131 3.13 Electric-field poling set-up (from ref. 16). 135 3.14 Experimental set-up for second-harmonic generation measurements (from ref. 16). 137 3.15 Experimental set-up for the ATR method to measure electrooptic coefficients (from ref. 17). 140 3.16 Experimental set-up for the measurement of real time dynamic thermal stability of NLO activity (from ref. 16). 142 3.17 1H NMR (in CDCI3) spectrum of compound 6 . 146 3.18 13C NMR (in CDCI3) spectrum of compound 6 . 147 3.19 Absorption spectrum of compound 6 in dioxane. 149 3.20 Absorption spectra of compound 6 in dioxane (A.m a x = 580 nm) and in methanol (Xm ax = 600 nm). 150 3.21 TGA curve of compound 6 (heating rate = 10° C/ min, Ar atmosphere). 154 3.22 1H NMR (in CDCI3) spectrum of compound 12. 156 3.23 13C NMR (in DMSO-de) spectrum of compound 12. 157 3.24 1H NMR (in DMSO-de) spectrum of polymer P1. 159 X 3.25 TGA curve of polymer P1 (heating rate = 20 °C/ min, Ar atmosphere). 160 3.26 FTIR spectra of polymer P1 before and after heating/crosslinking at 120 °C for one hour. 161 3.27 Absorption spectra of polymer P1 before and after electric field poling and crosslinking. 163 3.28 Plot of SHG coefficients of p olymer P1 as a function of film temperature (heating rate = 10°C, in air). 165 3.29 Plot of normalized d coefficients of crosslinked polymer P1 as a function of time. Polymer film was continuously heated at 100 °C in an oven. 166 3.30 1H NMR (in DMSO-de) spectrum of compound 20. 168 3.31 13C NMR (in DMSO-de) spectrum of compound 20. 169 3.32 FTIR spectra of polymer P2-XL2 before and after heating/crosslinking at 120 °C for one hour. 171 3.33 Absorption spectra of polymer P2-XL1 before and after electric field poling and crosslinking. 172 3.34 Absorption spectra of polymer P2-XL2 before and after electric field poling and crosslinking. 173 3.35 Plot of SHG coefficients of polymer P2-XL1 as a function of film temperature (heating rate = 10 °C, in air). 176 3.36 Plot of SHG coefficients of polymer P2-XL2 as a function of film temperature (heating rate = 10 °C, in air). 177 3.37 FTIR spectra of crosslinked polymer P2-XL2 before and after electric field poling. 179 3.38 Plot of electrooptic coefficient as a function of chromophore concentration (compound 19) in PMMA host. 181 3.39 1H NMR (in DMSO-de) spectrum of compound 25. 182 3.40 13C NMR (in DMSO-de) spectrum of compound 25. 183 X I 3.41 1H NMR (in CDCI3) spectrum of compound 26. 184 3.42 13C NMR (in DMSO-ds) spectrum of compound 26. 185 3.43 TGA curve of compound 25. 186 3.44 TGA curve of compound 26. 187 3.45 Absorption spectra of polymer P3 before and after heating. 189 3.46 Absorption spectra of polymer P4 before and after heating. 190 Chapter 4 Fig. 4.1 a. General structure of rigid-rod polymers substituted with long alkyl chains, and b. with some of the alkyl chains replaced with NLO chromophores. 197 4.2 Rigid-rod polymers substituted with NLO chromophores in the side-chains, x:y = 1 a. m = 2, n = 15; b. m = 5, n = 7; c. m = 2, n = 5 (from ref. 30). 199 4.3 Rigid-rod polymers substituted with NLO chromophores in the main-chain (ref. 30). 200 4.4 Structure of the proposed rigid-rod polymer with an NLO- chromophore in the main-chain. 209 4.5 Scheme for the alternative synthesis of compound 4. 211 4.6 1H NMR spectrum of compound 1. 213 4.7 1H NMR spectrum of compound 2. 214 4.8 1H NMR spectrum of compound 3. 215 4.9 1H NMR spectrum of compound 4. 216 4.10 Scheme for the synthesis of functionalized 4. 217 4.11 1H NMR spectrum of compound 5. 219 4.12 1H NMR spectrum of compound 6 . 220 xii 4.13 Scheme for the incorporation and protection of diol groups into compound 6. 4.14 Scheme for the attempted preparations of donor-acceptor substituted fused-ring aromatic backbone. 4.15 Proposed scheme for the synthesis of asymmetrially functionalized donor-acceptor chromophores for ncoporation into a rigid-rod polymer. 221 223 224 xiii List of Tables Chapter 1 Table 1.1 Chapter 2 Table 2.1 2.2 2.3 2.4 Chapter 3 Table 3.1 3.2 3.3 pp of some repreentative stilbene-type donor-acceptor chromophores. List of maximum absorbances of neutral and doped species of -H and -SBu substituted bisthienyl polyenes. List of experimental x3 values of neutral and doped -H and -SBu substituted bisthienyl polyenes. Experimental Y 1111 of neutral -SBu substituted bisthienyl polyenes. Y i 111exp and Y m icalc of doped -SBu substituted bisthienyl polyenes. Y nncalc was determined using a two-level calculation based on absorption data and eqn. 2.5. Experimental conditions for poling the NLO polymers. Examples of second-order chromophores and their pp values. Second-order nonlinear optical properties of Polymers P2-XL1 and P2-XL2. 17 75 79 85 92 134 152 175 xiv Abstract Chapter 2 of this dissertation presents third-order nonlinear optical studies of bisthienyl polyenes (BTPs). Neutral forms of -SBu substituted BTPs (3 < n < 9) were studied by DFWM at 532 nm and it was found that, within experimental error, /x3n n / is proportional to nb where b ~ 5.5 The magnitude and phase of the x3i m response was measured using a DFWM-interferometric set-up. Bipolaronic forms of -SBu substituted BTPs (6 < n < 9) were also studied by DFWM at 1064 nm. It was found that experimental x3i in is also proportional to nb with b ~ 14. A good agreement between Y m iexp and Y n n calc was achieved provided Y im calc was derived using a two-level model treatment and assuming an adiabatic-following condition. Chapter 3 presents studies on the design, synthesis and incorporation of high pP chromophores into thermally stable lattices. A model chromophore (shown below) was synthesized and exhibited pP = 1700 x 10‘48 esu at 1.907 C(0)0Et X V pm. Variations of this model chromophore were designed and synthesized for incorpotation into a double-ended crosslinkable (DEC) and thermosetting polyurethane systems. NLO studies of both DEC and thermoset polymers showed second-order signals lower than expected from model compound studies. The low second-order response can be attributed to chromophore degradation under electric field poling conditions. In Chapter 4, a new chromophore backbone consisting of fused aromatic rings was proposed for use in rigid-rod main-chain polymer systems. A more efficient way of synthesizing this fused-ring backbone was presented. The backbone was functionalized with -SBu and -CHO groups. The -SBu substituted molecule was further functionalized by converting the methyl groups of the center ring to hydroxy groups thus making it accessible for incorporation into polymers. Nitration and tricyanovinylation reactions on the -SBu-substituted molecule, however, were not successful. xvi CHAPTER 1 BACKGROUND OF NONLINEAR OPTICS AND REVIEW OF ORGANIC NONLINEAR OPTICAL MATERIALS 1 1.1 Introduction As we enter the 21st century, a new technology based on photon interactions is expected to play an important role in the advancem ent of information and processing technologies. Photonics is the technology that uses photons instead of electrons in the acquisition, storage, transmittal and processing of information. Photonics is envisioned to replace electronics as it has many distinct advantages over the latter. One advantage is the gain in speed because photons travel faster than electrons. Another is the absence of magnetic and electrical interference resulting into reduced crosstalk between channels. Photonics also results to an increase in information density and is compatible with existing fiber optic networks. Furthermore, it has become apparent that the present electronic technology will no longer be compatible with processing rates greater than 50 GHz and transmission distances greater than one meter and thus the need for optical technologies. For practical applications, most photonic systems require that a material exhibit large nonlinear optical (NLO) effects. Nonlinear optical processes arise when an intense light pulse, usually from a laser, impinges on a medium. The strong oscillating field of the laser creates a nonlinear polarization in the medium yielding a nonlinear response. 2 Theoretically, all materials are capable of being nonlinearly polarized. However, with the currently available laser resources, only a few materials exhibit sizable NLO response. In addition, certain material requirement needs to be satisfied for practical use. Aside from large nonlinearities, candidate materials should also exhibit extraordinary stability to ambient conditions and under intense light sources. Furthermore, they must be resilient enough to withstand processing conditions and integration with other materials. These provide an impetus for the search for materials that are capable of exhibiting large optical nonlinearities and compatibility with processing conditions which which can eventually be used for device applications. NLO materials can be classified into two broad categories: crystals (including inorganic, organomettalic and organic) and polymers. Presently, inorganic crystals such as gallium arsenide (GaAs), lithium niobate (LiNbOa), potassium dihydrogen phosphate (KTP) are being utilized for device fabrication into electrooptic modulators or frequency doublers. However, because of the inherent difficulties in processing these materials, the cost of the finished product has been too enormous. Recently, much attention has been focused on the use of organic polymers for NLO applications. Organic polymers possess the necessary material requirements previously discussed for practical applications such as ease and low cost of device fabrication, high laser damage threshhold and low dielectric constant. Furthermore, the versatility of synthetic chemistry can always be used to modify the molecular structure to enhance the nonlinear optical response and other properties. 3 As proof of the emerging importance of organic polymers in the field of NLO, numerous papers and books have been published as well as conferences and symposia have been held. The following sections will attempt to briefly highlight the research activities done in both the field of second and third order nonlinear optics.1'22 1.2 Basis of Nonlinear Optics13 An organic molecule or polymer can be treated as a dielectric medium, which upon application of a low electric field of field strength E is linearly polarized according to the equation P = % (1 ) E (1.1) with P being the polarization vector and the proportionality constant %(1) being the linear susceptibility vector. If the electric field is due to an optical field, the response can now be described by the refractive index n, which is related to the x(1) coefficient by n2(co) = e = 1 + 4jix(1)(o)) (1 .2 ) At higher field strengths, the linear relationship between the field strength and polarization vectors is no longer valid. With the assumption that the size of the induced polarization of the medium is a lot weaker than the binding forces 4 between the electrons and the nuclei, the polarization can be expressed as a power series of the field strength E p = aE + PEE + yEEE + . .. . (1.3) where a is the polarizability term which accounts for the linear absorption and refraction behavior of the medium. The coefficients (3 and y, describe the microscopic nonlinear interactions. The first-order hyperpolarizability |3 is a third-rank tensor and constitutes the molecular origin of second-order nonlinearities, y, the second-order hyperpolarizability is a fourth rank tensor and contains the molecular basis of third order nonlinear effects. In bulk, the polarization equation (1.3) can be expressed as P = x(1) E + x(2) E E + jc (3)EEE+ . . . (1.4) Analogously, %(1), % (2), and x<3) are the linear, second- and third-order nonlinear susceptibilities, respectively. Symmetry considerations require that a medium be noncentrosymmetric in order for it to exhibit second order effects. This requirement however does not hold true for third order effects. Higher order terms originating from higher order processes can exist however, they are too small to be observed. Substituting the sinusoidal field equation, E(z,f) = Eq c o s (cof- kz) to the polarization equation gives 5 P= x(1)E0cos (cof - kz) + x<2)E02 cos2 (tot - kz) + % (3)E03 cos3 ( • w t - kz) (1.5) = %(1)E0 cos (tot - kz) + 1/2 % (2)E02 [ 1 + cos (2(01- 2kz)\ + % (3)E03 [3/4 cos (tot - kz) + 1/4 cos (3cot- 3kz)] (1.6) where E0 is the amplitude of the field, co is the frequency of light, k is the propagation constant and kz, the phase of the wave. The above equation clearly shows the different frequency components arising from the nonlinear polarization. The second-order term gives a frequency-independent contribution known as optical rectification and one at 2co which gives rise to second harmonic generation (SHG). Similarly, the third- order term shows a frequency response at to and 3co called the third-harmonic generation (THG). The SHG and THG phenomena can be used to convert long wavelength light to short wavelength light and can be used for high density optical data storage and many other applications. Another important nonlinear optical effect is the nonlinear index of refraction. More importantly, the different nonlinear optical processes previously discussed can be thought of as arising from changes in the refractive index of the medium. That is, an electric or optical field modulated the refractive index of the medium, n, which then alters the propagation characteristics of light (e.g. frequency, phase velocity). 6 1.3 Second-Order Nonlinear Optics a. Second-order effects i. Second-Harmonic Generation.1-22 23 27 Equation 1.6 clearly illustrates the basis of second harmonic generation. Second harmonic generation can be viewed as a three-wave mixing process where waves with frequencies coi and 002, respectively, interacts through a medium to produce a new wave of frequency 0 3 . (Fig. 1.1) Thus, in the case of SHG processes, the second-order susceptibility expression x 2 ijk(—0 0 3 :0 0 1 ,0 0 2 ) can be rewritten as X2ijk(-2 co;o)i,co2), where i, j, k are Cartesian indices. Experimentally, a SHG coefficient d is often used to describe the intensity of the second order effects. This coefficient is umabiguously related to % 2 by the equation X 2 ijk ( - c o ;c o i ,co2> = 2 d i jk ( 1.7 ) The SHG coefficient d is frequently written in contracted form using the convention (11)->1, (22)->2, (33)->3, (23,32)->4, (13,31)->5, (12,21 )->6 . These contractions are collectively known as Voigt notations. Contractions can be performed because the value of djjk is independent of the order in which the electric fields appear in the equation is a consequence of the fact that the frequencies of the two fields are equal. Consequently, the susceptibility terms can also be contracted in a similar manner such as x 233- 7 a. (0 noa b. c. (0 2(0 (0 3co Fig. 1.1 Schematic representation of a) frequency conversion, in general and b) second- and c) third-harmonic generation. 8 A very important practical application of second harmonic generation is frequency conversion. Frequency conversion can be used for high density optical storage by converting light with wavelength of 820 nm to 410 nm. In order to be useful for frequency doubling applications, the active nonlinear optical material must have a sufficiently large second-order susceptibility. This would provide high conversion efficiency at a short interaction length. Another requirement is for the material to be transparent at both the fundamental and the second harm onic w avelengths. Unfortunately, there is a rough relationship betw een the chrom ophore's hyperpolarizabilty ( 3 and the w avelength of its lowest absorption peak .28 This m eans that the higher the (3 value is, the redder the absorption. To date, no system has been identified to p o sse ss a sizable second order nonlinearity and simultaneously have negligible absorption in the blue region. Finally, the second harmonic signals should be phase-m atched to permit maximum signal of the output light. Several approaches have been suggested and investigated to answ er the phase-m atching problem. Examples of th e se include quasi-phase matching29 and anom alous dispersion phase matching.30 Another way of achieving phase m atched condition is to launch the fundamental and second harmonic signals into different sizable m odes such that their indices of refraction are m atched.31-32 ii. Etectrooptic Effect.The optical properties of a seco n d -o rd er nonlinear optical material can be altered by the presence of a dc electric field. This effect results in a change in the refractive index of the medium that is linearly proportional to the electric field. This is clearly defined in Equations 1.5 and 1.6. This phenom enon is known as the electrooptic or Pockels effect.1 It 9 can be viewed a s a three-w ave mixing process where one of the field is a dc field. Thus, the second order susceptibility can be ex p ressed a s % 2iik (— to; 0,CO).1- 33,34 The change in refractive index due to the linear electrooptic can be defined as A(1/n2) = rjj «E (1.8) where H j is referred to a s the electrooptic (EO) coefficient. The EO coefficient is frequently contracted in a m anner similar to the SHG d coefficient. The contraction can be done in com plete generality since the output and input frequencies involved in the electrooptic effect are equal, i.e. co=g>i. The most often used elem ents are r33 and r3i and are related to each other in an isotropic model by the equality r33 = 3 ri3. The EO coefficient can be determ ined by m easuring the change of the dielectric constant or index of refraction when a field is applied acro ss the sam ple of interest. A num ber of techniques have been developed to do this. T h e se tech n iq u es include the Fabry-Perot interferom etric te c h n iq u e ,35 ellipsometry36 and attenuated total reflection (ATR) technique.37 The value of r can also be calculated from the SHG cofficient d. However, there is no simple relationship betw een r and d, since the second-order susceptibilities involved depend on different combinations of electric field frequencies. N onetheless, it has been shown that if the relationship 10 X2= N p f(coi) f(a>2) <cos3 0> (1.9) where N is the concentration of the second-order active material, p is the second order hyperpolarizability, co s3 0 is the order param eter, and f are local field factors, is used to relate the m acroscopic to m icroscopic properties, then an approxim ate relationship betw een the two quantities can be derived using the "two- lev el" model. This relationship can be written as: - 4 dK,u H fM jj f V (3co02 - co) (top2 - to'2 ) (cop 2 - 4 to'2 ) rUK" n,2 (< o ) n 2 (co) f2“ K K f® ’ ,, r 'j j 3o)o2 («o2 - co 2)2 (1.10) w here (o0 is the resonance frequency of the medium, to' is the frequency of the fundam ental field used in the SHG m easurem ent, co is the frequency used for the electrooptic m easurem ent and f(co) are local field factors.38 T h e "tw o-level" m odel a s s u m e s th a t th e s e c o n d -o rd e r hyperpolarizabilities can be described solely in term s of a m olecular ground state and a single excited state. This model has been shown to be adequate in describing effects observed in substituted benzene m olecules. However, this model fails for more complex system s. Since there are uncertainties in the reliability of the two-level model and approximations were m ade to calculate the local field factors, equation 1.10 can only be best used a s a rough estim ate of the EO coefficient given a m easured SHG data. 11 The linear optical effect has been used to m ake a variety of devices which include w aveguide switches, light modulators, filters, and polarization- transform ing devices. O ne such practical device is a M ach-Z ehnder intereform eter shown in Fig. 1.2. This device consists of a w aveguide of a nonlinear material split into channels that reunite. A voltage is applied to one of the channels thereby altering its index of refraction. This variation of the index results into a phase shift which can then be used to modulate light exiting the device.1-39 The phase shift caused by the EO effect is expressed as jm3rEL 0 = ----------- I (1 .11 ) w here E is the applied field, X is the wavelength of the light to be modulated and L is the propagation length of the nonlinear medium. From equation 1.11, it is clearly se e n that the p h ase shift is linearly proportional to n3 r. Efficient modulation is also aided by a small low-frequency dielectric constant. T hese two criteria result in a figure of merit (FOM) given by 22 electric field input beam \ — ». / output beam Fig. 1.2 A Mach-Zehnder interferometer (from ref. 1). 13 FOMs are very useful num bers when comparing probable material candidates for second order applications.22 b. Materials for second-order nonlinear optics From the previous section, it has becom e apparent that for a material to be even considered for device applications, sev eral things have to be ad d ressed . O ne of th ese is to com e up with a chrom ophore that has large molecular hyperpolarizability. Another is to be able to translate the m olecular nonlinear resp o n se into a bulk response which will be com patible with the ensuing material processing and operating conditions. The m olecular hyperpolarizability can be translated to a bulk second order reponse by equation 1.9 (ignoring the tensorial nature of the second order nonlinear optical susceptibility). Thus, to optimize bulk nonlinearity, synthetic chem ists can optimize three variables in this equation 1.9, namely, num ber density N, <cos3 0> and p, the magnitude of molecular hyperpolarizability. The following sections will attem pt to provide a brief overview of the major activities in the NLO community to address these points. i. Chromophore design and synthesis N um erous stu d ies have show n th at asym m etrically su b stitu ted conjugated m olecules of the type shown in I often p o s se ss large second 14 order m olecular hyperpolarizabilities ((3). T rends in the structure/function relationships have been successfully predicted by different sem iem pirical quantum mechanical calculations of the first hyperpolarizabilities. D O N O R — Jt-conjugating bridge — ACCEPTOR R ecent efforts to synthesize chrom phores with improved p values have com e up with som e interesting design features which is worthy of discussion. Prototypical devices have used the well known chrom ophore 4-dialkylamino-4'- nitrostilbene (DANS) as the second-order active material. DANS has a pp value of 580 x 10'48 esu and has been the reference currently used in the community to a s s e s s the optical nonlinearity of a new m aterial.43 Som e of the more interesting chrom ophores including their pP values are listed in Table 1.1. An inspection of the second-order nonlinear optical values in the table would show that pP values depend upon increasing the length of the 7 t-electron conjugating bridge (chrom ophores j and k; chrom ophore g) and increasing donor and acceptor strengths. From Table 1.1, an increase in second-order hypepolarizabilities w as observed a s the donor strength w as increased from methyl (a) to methoxy (b) to dimethylamino (c). Similarly, an increase w as also observed a s the acceptors w ere varied from nitro (e) to dicyanovinyl (g) to thiobarbituric acid (i) to tricyanovinyl (j). However, recent studies by M arder and cow orkers have indicated that there is an optimum combination of donor and acceptor strengths 15 to maximize the pP v alu e .41 Thus, it is not really n ecessary to have a combination of very strong acceptor and donors since a decrease in nonlinearity is expected past the optimum combination. M arder and Gorman analyzed the dependence of the nonresonant first hyperpolarizability to the change in dipole moment, oscillator strength and the transition energy of the charge transfer process using a tw o-state model. They show ed that the hyperpolarizability could be correlated to bond length alternation which in turn is related to the relative contributions of the ground and excited states 42 Chrom ophores which have arom atic ground states but non- arom atic quinoidal excited states tend to have d ecreased resonance energies upon electronic polarization. This can be attributed to the extra energy needed to lose aromaticity upon electronic polarization. On the other hand, m olecules like chrom ophore i that has a conjugating bridge that loses aromaticity upon polarization with an accom panying gain in aromaticity of the acceptor exhibit sizeable nonlinearities. This is due to the fact that both the neutral and charged state forms have com parable resonance energies and do not have significant loss or gain in aromaticity betw een the two forms upon charge transfer. An ap parent increase in nonlinearity is also observed w hen the the lone pair electrons on an am ine donor is locked in conjugation with the rest of the chromophore. This w as done by replacing alkylamine with donor groups such as julolidene (Chromophore d ) 28 A recent trend in chrom ophore design h as focused on two design features which were geared towards significant enhancem ent in second-order 1 6 Table 1.1 jj.p of some representative stilbene-typo donor-acceptor chromophores. Chromophore pP, x 10'48 esu (ref) (1.9 pm) Chromophore pP, x 10'4a esu (ref) (1.9 pm) a M e -Q ^-O -N 0 * 70.5 (28) CN , >-©{^cKC N n = 1 1300 (44a) 2 2300 (44a) 3 3800 (44a) b 0 M e - O - ^ ^ N° 2 126 (28) CN 2600 (43) c 580 (43) i Wv-d-vV 2400 (44b) d n^ - ^ 0 " n ° 2 672 (28) CN 6200 (43) 600 (43) CN 9100 (43) , C n 1040 (43) nonlineairty. The first of which w as described in detail in the previous paragraph, i.e., use of stronger acceptors and acceptors which gains aromaticity upon charge transfer. Another design feature involves the replacem ent of b en zen e rings with heteroarom atic five-m em bered rings such a s thiophene (chrom phores e-k).43-44 T hese heteroaromatic ring structures have much less arom atic stabilization energy which w as considered to be essential to achieving sizeable nonlinearity. While it is n e ce ssa ry for chrom ophores to p o s s e s s large optical nonlinearity, this condition is not sufficient for a material to be used for device applications. Another important are a to be ad d ressed is the ability of the chrom ophore to withstand processing conditions i.e., p o s se ss s therm al and chem ical stabilities. Garito and cow orkers have developed chrom ophores containing fused ring system s which displayed excellent thermal stabilities.45- 46 The sam e phenom ena were observed by Jen and coworkers.47 W orkers at IBM observed that therm al stability of am ine donor groups can be substantially increased by switching from alkyl am ines to aryl am ines.48- 49 More recently, they published a paper showing that an enhancem ent in nonlinearity w as also achieved by substituting alkyl with aryl am ines.50 ii. Stabilization of poling-induced order For chrom ophores to be utilized for nonlinear optical applications, they have to be assem bled into noncentrosymm etric lattices. In general, there are three ap p ro ach es currently used: a) crystals and inclusion com plexes, b) 1 8 sequential synthesis m ethods, and c) use of external poling fields. Since this dissertation deals mainly with polymeric materials, the discussion in this section will mainly focus on the use of electric poling fields. Before going any further, it is worthwhile to discuss the concept of electric field poling. As w as stated earlier, for any material to display second order effects, the system should p o sse ss asymmetry. For polymeric m aterials, this requirement translates to a polymer system consisting of chrom ophores oriented in such a way that the total system does not p o sse ss a center of symmetry. Experimentally, this w as achieved by application of a dc electric field to the polym er at a tem perature w here the chrom ophore's dipoles can be easily oriented. Several approaches were undertaken to induce polar order in the polymer. Interdigitated and coplanar electrodes have been used to orient the chrom phores' dipoles in the plane of the polymer film.51'53 High field strengths can usually be achieved using th ese techniques provided careful attention be given to material purity, film processing and electrode design. The utility of th ese approaches, however, is limited by charge injection especially at regions of high electric field near the electrode ed g e .53-54 The m ost commonly used electric field poling technique is corona poling.55'57 A typical corona set-up is shown in Fig. 1.3. A corona needle is charged with several kilovolts until electric breakdown of the surrounding atm osphere occurs. Electric fields obtained using corona poling are usually higher than those achieved with electrode poling. Corona needle can deposit either positive or negative ions on the surface of the film depending on its 19 Corona needle © + Voltage Supply © © Electrode © © © © © © © © © Polymer film III I III I III III! I l l l l l l l l l l l I l l l 11111111 l l l l l l l l I l l l l l l l l l l l III Heater Fig. 1.3 A typical corona-poling set-up (from ref. 40a). 20 polarity. It has been found that negative coronas tend to be less stable and are dependent on the chemical composition of the atm osphere in which discharge occurs.58-59 Equation 1.9 clearly illustrates that the quantity of % 2 d ep en d s on the precise orientation of chrom ophores in the polymer. The value of <cos3 0> can range from zero in the case of a completely disordered system and one in the c ase of a system in which all the dipoles are aligned along the sam e direction. The degree of polar order can be determined by m easurem ent of changes in the linear optical properties, absorption and index of refraction, of the system . For corona-poled system s, a typical value of <cos3 0> of 0.2 -0.3 have been achieved.56- 60 For parallel plate poling, th ese values have been smaller, <cos3 0> is on the order of 0.08.6 1 Optimization of <cos3 0> can be related to optimization of another variable in equation 1.9, namely N, the num ber density. It turns out that optimization of th ese two param eters is opposed by the repulsion betw een the high dipole moment chromophores. a. Guest-Host systems The earliest polymeric system studied has been the guest-host system . This is due to the e a se in its preparation. Typical preparation m ethod includes dissolving both the chrom ophore and polymer in appropriate solvents followed by spin casting the solutions into thin films. The earliest guest-host system 21 investigated consisted of a 2% by weight solution of 4-dim ethylam ino-4'- nitrostilbene (DANS) in a thermotropic nematic liquid crystal.51 The m easured susceptibilty w as 1 pm/V at 1.06 pm which decay ed quite rapidly. This illustrates two main concerns with polymeric system s of the guest-host type, nam ely 1) rapid relaxation of chrom ophore orientation leading to loss of nonlinearity and 2 ) low loading density due to finite solubility of the chrom ophores in the polymeric matrix. A variety of other guest-host system s have been also been studied incorporating different chrom ophores into different polymeric hosts such a s polymethyl m ethacrylates (PMMA), and bisphenol polycarbonate A.62'64 Several authors have investigated the effects of therm al history, dopant size, specific polymer relaxation m echanism s, and guest-host interactions on the poling order of guest-host system s.54- 65> 66 An important finding is, that for a relatively large chrom ophore doped in a polymer, the primary factor which influences the rate of relaxation is the Tg. Thus, a new class of guest-host system s using a high Tg polymer such a s polyimide w as paid close attention. Polyimides are generally insoluble and intractible hence processing is usually done at the polyamic acid precursor stag e .67 The curing of the polyamic acid precursor involves two distinct steps: 1) cyclocondensation stag e to form the polyimide followed by 2 ) a curing stage which provides a densification effect which usually happens at elevated tem peratures. It has been shown by Wu and cow orkers that densification of the cured polyimide Pyralin 2611 doped with erythrosin at 360°C dramatically increases its thermal stability.68 O ne exam ple 22 of a polyimide system is a fully imidized but still soluble isotropic poly(ether imide) Ultem (General Electric) doped with Lophine 1 (L1) and Lophine 2 (L2). L1 L2 T hese system s have outstanding therm al stabilities; they do not decom pose below 300°C .54-65- 69 O ther polyimide system s that have been investigated include a polyamic acid LQ-2200 (Hitachi) doped with DR-1 and Ultradel 4212 or 3 1 1 2 (A m oco) d o p e d with 4 -d ic y a n o m e th y le n e -2 -m e th y l-6 -p - (dimethylamino)styryl-4H-pyran (4-DCMP).70 Although all th e se polyimide system s p o ssess excellent thermal stability, the chrom ophores were not able to survive the high tem perature n ecessary for com plete curing. Wu et al. suggested that low-temperature chemical imidization will give equivalent results a s a high-temperature thermal imidization.71 Another approach used to slow the decay of poling induced order w as by the use of chromophore w hose charge transfer can occur in two directions a s in the c a se of N.N'-bis (p-nitrophenyl)methanediamine (p-NMDA).72 Although the nonlinearity of this molecule will inherently be lower than conventional charge- 23 transfer molecules, its polar order is expected not to decay rapidly hence leading to better long term stability. p-NMDA Overall, the use of guest-host system s for device applications h as been limited by a lot of factors. These include limited solubility of the chrom ophore in the polym er matrix, p h ase separation, aggregation of chrom ophores, and sublimation of chrom ophores at elevated tem peratures to nam e a few. This provided an incentive to find other ways of incorporating chrom ophores into polymeric system . b. Covalent attachment of chromophores Side-Chain Incorporation. It w as realized that higher nonlinearity and improved stability could be achieved if the chromophore w as chemically bonded to the polymer. The first such system synthesized w as a chrom ophore attached a s a side-chain to the polymer backbone.62- 73> 74 This sch em e h as the advantage of avoiding phase separation and prevents the crystallization of the chrom ophore. Furtherm ore, becau se of the attachm ent, the motion of the chrom ophore is limited thus retarding the relaxation of the poled order. It w as 24 also found out that the Tg of a side-chain polymer is substantially higher than a guest-host system containing the sam e amount of chromophores. A num ber of chrom ophores such a s DANS and 4-(dicyanovinyl)- 4'(dialkylamino)azobenzene (DCV). have been attached a s pendants to different polym ers.63-75 Exam ples of th e se polymeric b ack b o n es include PMMA, polystyrene, polyhydroxystyrene, polyphosphazene am ong o t h e r s . 4 0 b -73,77-82 Just like in guest-host system s, it is very desirable to achieve thermal stability of the poling-induced order. Hence, chrom ophores have also been attached a s pendant groups to high-Tg polymers such a s polyimides and polyphenylene oxides.80' 81 ■ 83 Main-Chain Incorporation. A chromophore is described a s incorporated in a m ain-chain fashion if it is attached to two points in the polymer or the chrom ophore's axis is orthogonal to the axis of the polymer backbone. The main difference betw een a main-chain attachm ent from a side-chain attachm ent is that there is the presence of a large segm ental motion during poling and relaxation of the chrom ophore in the former case. While it is expected that poling a main-chain system would be more difficult, subsequent relaxation of dipoles, however, would be hindered. The chrom ophores can be oriented in different fashion along the polymer chain: "head-to-head", "head-to-tail",and "random" (Fig. 1.4). Of th ese three, the earliest studied system w as the "head-to-tail" main-chain polymers b ecau se it w as originally s u g g e ste d th at this a rran g em en t would result in an 25 a. "head-to-head" b. "head-to-tail" random Fig. 1.4 ro 05 j \ j W : A / A / j\r Chromophore arrangem ents in a main-chain polymer backbone. an enhancem ent of the second-order nonlinear optical properties.85-86 It w as expected that in such arrangem ent, the chrom ophores would act a s a single dipole with dipole moment, ptotai = Np w here N is the chrom ophore density. Experimentally, it w as found that the enhancem ent in nonlinearity is limited if the polymer is not strictly linear or if, due to chain entanglem ent, the polymer chain h as not reached an optimum arrangem ent under the poling field. Xu and coworkers have synthesized sam ple polymers which fall into each of the class previously stated, i.e. "head-to-head", "random", and "head-to-tail" and they o b serv ed that provided th ere are long flexible seg m e n ts betw een the chrom ophore and the backbone, main-chain polymers can be effectively poled and gives rise to nonlinearities comparable to side-chain polymers.87’90 Main-chain polymer system s have also been synthesized wherein the dipoles were perpendicular to the polymeric backbone (Fig. 1.5).91 In this case, the polym ers w ere easier to pole than when the dipole axis lies along the polymer backbone. This w as only expected since only a small segm ent of the backbone w as needed to be moved to achieve chromophore alignment. A unique approach w s undertaken by Lindsay and coworkers when they synthesized "accordion" polymers. This polymer has the chrom ophore attached with U -shaped connecting bridges to achieved a roughly parallel arrangem ent. (Fig. 1.5a).92 An example of the polymer is shown in Fig. 1.5b93 which show s no nonlinearity in its unpoled state but a x 2 = 7 pm/V when poled at 215°C. The polymer also has a high Tg of 208°C. 27 a. B V H/ Hi V H v H / , t Bi H Fig. 1.5 a) A schem atic representation of an accordion main-chain polymer (from ref. 92) and b) an accordion main-chain polymer incorporating an NLO-chromophore (from ref.93 ). 28 As with the other polymer system s previously discussed, the problem of dipole relaxation is still present in main-chain system s. Thus, additional steps have to be introduced into the processing step s to en su re d e crea se in the chrom ophore and polymer mobility after poling. c. Cross-linked systems The rate of relaxation of poled polymer dictates the lifetime of devices m ade using th ese materials. Thus, one of the main focus of research in the field of polymeric second-order NLO materials is to maintain poling-induced order for extended periods of time. This can be done in two ways: u se a polymer with a high Tg such a s polyimides and/or use of crosslinked system s. The use of high Tg polym ers has already been ad d ressed in the previous sections. Crosslinking provides increased interaction betw een polymer chains thus decreasing polymer mobility. The discussion of crosslinked system s will be divided into four parts: therm osets, photochem cical crosslinked, double- ended crosslinkable and sol-gel system s. Thermosets. T herm oset or therm ally crosslinked m aterials are processed by heating such that it no longer p o ssess the properties of the initial polymer. The most widely used therm oset system s have been the epoxide resins thus it is not surprising that this w as also the first system tested for use in second-order NLO experim ents.94'98 Marks and coworkers used a guest-host system of either DANS or an azo benzene chrom ophore in a com m ercial therm osetting epoxy, EPO-TEK 301-2, a diglycidyl ehter of bisphenol A and a 29 polyfunctional am ine hardening ag en t.94-97 Pre-curing w as done by heating at 80°C before poling followed by final curing of the epoxide resin at elevated tem peratures. The poled sam ples exhibited d33 values ranging from 0.04-0.4 pm/V at 1.06 pm and at the sam e time displaying improved thermal stability than an uncrosslinked guest-host system . Although this early experim ent show ed that improved thermal stability can be achieved by crosslinking reactions, it w as still plagued by problems such as low chromophore densities, phase separation and plasticizing effects on the resulting polymer. Thus, they extended their studies wherein the chromophore w as covalently bound to the polymer.95 In this schem e, the chromophore w as not directly involved in the crosslinking reaction, rather, crosslinking w as done at different sites of the polymer. As expected, the tem poral stability w as enhanced relative to guest-host system s and even if the chrom ophore w as not directly crosslinked. Related studies were done by Marks and coworkers wherein they studied the effects of different crosslinking sites on the temporal stability of the covalently bound chrom ophores.98 The problem of low chrom ophore density w as ad d ressed by Eich and cow orkers w hen they designed an epoxy resin w herein am ine-containing chrom ophores w here used to initiate ring opening reactions and thus becom e part of the polymer backbone (Fig. 1.6a).99 A variation of this approach w as undertaken by Jungbauer et. al. when they used a bis-epoxide containing a bonded NLO chrom ophore (Fig1.6b).100 In both c a se s, prepolym ers were prepared by heating the reagents to form a viscous solution followed by final curing at elevated tem perature (usually 130-140 °C). Thermal stabilities of up 30 b. Fig. 1.6. nh2 A _ ch, o V ♦ 2 / S / ° - 0 - ? ^ 0 - ° v / A no? v 3 poling E OH OH _ CH3 _ “ I - ch2— c h - ch2 ch2 chch2 0 - < > c ^ > 0 -------- \ / OH CH3 -I -ch2ch ch2 N I 1 h J - J L ^..^CrkCHCrta ------- r V \ ? V \ "1 CH2CHCH2 O C - 4 ^ - 0 — NOp OH CH3 j , C C 0 6 NO„ T poling NO, <> r < — — CH2 CHCH/ Y l L i H ^ N 0 2 OH ^ x ch2chch2 n - ch2c h ch J - CH2CHCH2 OH I OH Examples of epoxy system s containing NLO chrom ophores (from ref. 99 and ref. 100). 31 to 80 °C can be achieved using both a p p ro a ch e s and seco n d -o rd er nonlinearities of up to 50 pm/V (at 1.06 pm) can be achieved using this type of epoxy resins. Another polymer system which has been exploited a s NLO-therm oset system are polyurethanes. One very good example of the use of this system w as presented by Dalton and coworkers in Figure 1.7.101 In this schem e, a bifunctional chrom ophore w as copolymerized with diisocyanate to produce a polyurethane prepolymer and a trifunctional amine w as used a s a crosslinking reagent. The resulting material has a d33 value of 120 pm/V (1.06 pm) and r33 13 pm/V (1.06 pm) with a 30% chrom ophore density. The poled order w as stable for up to 3000 hours at room tem perature and retained 70% of the signal at 90°C . O ther types of therm osetting polyurethanes will be discu ssed in Chapter 3 of this dissertation. Aside form epoxy resins and polyurethanes, other polymeric system s which were used as NLO-thermosets include polyamides and polyimides.102' 104 Photochemically crosslinked polymers. The formation of therm oset system s usually require extended heating and poling at elevated tem peratures which could eventually lead to chromophore degaradation. To avoid such harsh consitions, one could use photochemical crosslinking. Robello and coworkers have u sed polyfunctional acrylates and m ethacrylates in photochem ical crosslinking studies.105 As with other NLO-crosslinking studies, the first material studied w ere g u est-h o st sy stem s com posed of NLO chrom ophores in 32 ho^ n^ oh n o 2 O C N H NLO segmeni|- N C 0 ,OCH3 OCN-kJ-^J-NCO 0 OCH{ Nj. r _ y -c 0 C H 3 - 1 — O y * |^| ^✓ O gC H N ~ ^~ ^~ ^~]y~NHCO I OCH3 N N O , > ^ -► H NLO s e g m e ^ - N H C O g ^ M ^ Q p C H N - H N LO segm ent| OH POlm9 } Q2CHN-r N L0 se9m e^ * — Fig.1.7 Schem e for the formation of a thermally stable NLO-thermoset polyurethane (from ref. 101). polyfunctional acrylates. The polymerization w as done by using a variety of photochem ical electron transfer sensitizer-activaror com binations known to be used for polymerization of acrylates. Just like other guest-host system s, the problem of low chrom ophore density as well a s the photochem ical reactivity of the chrom ophores limits the use of this system. NLO chrom ophores which w ere difunctionalized with acrylate or m a th a c ry la te g ro u p s w ere found to p ro d u c e h o m o p o ly m ers by photopolymerization.106 Second order nonlinearities of up to 1.4 pm/V were m easured however, results were irreproducible. However, th e se studies have indicated that sim ultaneous photochem ical crosslinking and poling can be potentially used to achieve poled and hardened lattice. Aside from acrylates and m ethacrylates, other sy stem s w ere used in photochem ical crosslinking studies such a s cin n am ates. W orkers like M atsuda107 and Tripathy108 have separately studied guest-host system s using poly(vinyl cinnam tes) a s the polymeric host. In Tripathy's work, the NLO chrom phores used w ere functionalized with photoreactive groups to ensure significant coupling to the densified polymer lattice. The resulting poled material displayed excellent thermal stability of up to 85 °C. P hotocrosslinkable sy ste m s w ere also sy n th esiz ed w herein the chrom ophore with photoreactive functionalities were covalently attached to the polymer backbone. An exam ple of th ese type of polymeric system is a linear polymer containing photoreactive functionalities on the main chain containing 34 NLO chrom ophores synthesized by Dalton and cow orkers.109 Tripathy and cow orkers syntheiszed both linear and branched nonlinear optical epoxies containing pendant cinnamoyl groups.110- 1 1 1 Their results indicated increased nonlinearity of the branched system s which w as due to th e in creased chrom ophore concentration. However, no stability studies w ere reported at elevated tem peratures. Double-ended crosslinkable systems. D ouble-ended crosslinkable (DEC) system is a unique way of stabilizing chromophore orientation developed by Dalton an d cow orkers.109 In this ap p ro ach , th e ch rom ophore is asymmetrically functionalized ensuring stepw ise processing using asym m etric reactions. The chrom ophore could be polymerized by addition reaction using a vinyl group and then undergo a condensation reaction with a polyfunctional crosslinker such as diisocyanates, or vice v ersa (i.e. udergo a condensation polymerization using the hydroxyl groups and then crosslink using the vinyl group). Another variation of this approach is to u se functionalities which undergo the sam e class of reaction but with significantly different reactivities. An exam ple is the use of a chromophore functionalized with am ine on end and hydroxy group on the other presented by Francis and cow orkers.112 The bifunctional chrom ophore is reacted with a polyfunctional isocyanate forming a hardened polyurethane. An r33 value of 3-4 pmA/ (1.3 pm) w as m easured with significant chromophore degradation. Soi-gel systems. Another example of a hardened lattice which could be u sed for NLO system s are sol-gel g lasses. Sol-gel g la sse s are inorganic 35 materials, com posed of silicon, aluminum or titanium which are characterized by excellent optical quality.113 Low-temperature polymerization of m onom ers is achieved by a sol gel process: SiOR + HOH ^ SiOH + ROH hydrolysis SiOH +SiOH SiOSi + HOH condensation SiOH + SiOH SiOSi + ROH condensation to form a clear and rigid glass matrix. Several NLO chrom ophores have been incorporated a s guests in th ese g lasses via the sol-gel technique. The sol-gel m aterials u sed for NLO studies can be categorized as guest/host system s and as covalently-linked system s. Zyss and coworkers used the NPP chromophore as a guest in a silica/zirconia g lass.114 A%2 value of 0.16 pm/V (at 1.064 pm) w as m easured with a chrom ophore concentration of 4%. The low loading density w as attributed to phase separation present at higher concentration. P rasad incorporated the sam e chrom ophore of SiC>2-Ti02 and studied its NLO behavior.115 Using a 15% loading density, x 2 values of 11 pm/V (at 1.064 pm) and 2 pm/V (at 633 nm) were m easured. However, the NLO signals decayed only after a few hours at ambient tem peratures. Several other research ers incorporated a variety of other NLO chrom ophores into different inorganic hosts. These guest-host system s often suffer from low chrom ophore- loading densities and phase separation. 36 Sol-gel system s w here the chrom ophores are covalently bound w ere also synthesized. An exam ple of this approach w as prepared by Tripathy and cow orkers who prepared clear films of NLO-polyimide-glas com posites (Fig. 1.8 ).116 A d33 value of 13.7 pmA/ w as m easured with a 27% reduction observed after 168 hours at 120°C . No signal decay w as o b serv ed at am bient tem peratures over the sam e period of time. Another m ethod for covalently incorporating NLO chrom ophores into organic g la sse s w as presented by Yang and coworkers (Fig. 1.9).117 In their work, a trihydroxy functionalized NLO chromophore w as used a s monomer. The hydroxy groups were converted to organosilane groups which can undergo hydrolysis and partial condensation to form a viscous medium. The viscous solution w as cast into a film and poled. Final curing of coated films at elevated tem peratures leads to a hardened matrix restraining the mobility of the poled chromophore. 1.4 Third-Order Nonlinear Optics Unlike the study of second-order NLO materials, the field of third order nonlinear optics can be considered in the infancy stage. Most of the accurate theoretical calculations which could provide quantitative estim ates of third-order polarizabilities can only be performed on atom s or very small m olecules. For organic com pounds of medium to large sizes, theoretical form alism s for predicting absolute hyperpolarizabilites are presently not available.118 Thus, design and synthesis of third-order com pounds rely on trends on series of 37 - O S } } . OH I NH - CH2CHCH20(CH2)3Si(0CH3)3 N N02 1. Acetone solution 2. Spin cast 3. A f ' ° | (CH2)3 j m o c h 2 chch2- nh N N NO, Fig. 1.8 An NLO-polyimide glass composite (from Ref. 116). 05 00 H O - A - ■ d C ,OH OH (OH)3S i-A — — D -Si(OH)3 -Si(OH)3 Prepolym er 1. film casting 2. poling/curing Si— O S i - 0 Fig. 1.9 Schem atic representation of the sol-gel approach utilizing a tri-functionalized NLO chromophore (from ref. 117). 39 chemically related- related com pounds. However, a material d o es not have to p o sse ss asymmetry to display third-order effects. a. Third-order effects For purposes of this dissertation, the discussion of third order effects will only be limited to third harmonic generation and degenerate four-wave mixing processes. i. Third-Harmonic Generation.1-118121 Equation 1.5 clearly describes the origin of third-harmonic generation (THG). THG is a frequency mixing process wherein three input beam s of different frequencies interact to create a nonlinear polarization at a new frequency and the redistribution of power am ong the beam s. The resulting nonlinear polarization can be expressed by: Pj(3co) = 5 C 3ijkl(-3co;co,co,co) Ej(co) Er(c o ) E|(c o ) (1.13) In harmonic generation, the phase of the resulting output wave at a new frequency has a definite relationship with the phase of the input wave. Thus, in this context, harmonic generation is considered a coherent process. It occurs alm ost instantaneously and the resulting coherent nonlinearities are derived only from electronic interactions and are independent of excited state populations. Dynamic nonlinearites, which depend on population density of excitation in the medium, are not probed by THG. The THG signals, y ( - 3 c o ;c o ,c o ,( o ) and % 3 (- 40 3co;co,co,co) are resonantly enhanced if the fundamental and/or the third-harmonic frequencies are near one-photon or multi-photon absorption bands. The pow er conversion efficiency of the third harm onic signal can be expressed a s q = — ------ I % 3(-3co;co,co,co) I 2 I W ^ k l j 2 j n(3co)n3(co)A 2c2 ' (Akl/2) (1.14) w here n(3co) and n(co) are the refractive indices at w avelenghts 3co and co respectively; I is the interaction length and Ak is the wave-vector mismatch: 67i(n3 (0 - nm ) Ak = -------- --------- A (1.15) Eqn. 1.14 predicts the dependence of the THG signal to wave-vector mismatch for medium of constant interaction length I. The maximum intensity signal can be achieved when Ak = 0, i.e. the w aves at frequencies to and 3to are phase- m atched. However, this condition is very difficult to achieve since dispersion in refractive indices at both frequencies, co and 3to, yield different velocities. However, one can still optimize THG signal by adusting the interaction length I at fixed value of Ak. An important concept in this case is121 I = — c Ak (1.16) 41 w here lc is the coherence length. The interaction length can be changed either by rotating the sam ple or translation of a wedged sam ple. W hen the interaction length is changed, the behavior of I30) can be described by a sinusoidal pattern or a fringe curve. For w edge-shaped sam ples, the interaction difference betw een two consecutive maxima in 1 30, is 2IC . Therefore, the coherence length can be experimentally determined by generating a fringe curve. ii. Degenerate Four-Wave Mixing. In degenerate four-wave mixing process (DFWM), three beam s, all having the sam e frequencies, interact to g en erate a fourth beam also of the sam e frequency.3-121-122 j h e resulting nonlinear polarization can be written as Pi(co) = 3 /4 % 3j jki (-to;(o,£o,-to)Ej(co)Ek(o))E*i(£o) (1 .1 7 ) DFWM process derives its contribution from both the real and imaginary parts of X3. An im portant requirem ent in DFWM p ro c e sse s, just like harm onic generation, is phase matching, i.e., ki + kj + kk + k| = 0 (1-18) Two common beam geom etries have been used for DFWM studies. The first u ses a forward beam arrangem ent (Fig. 1.10a)1 wherein all the incident beam s are propagating in the sam e direction. The other has a backward beam 42 NLO Fig. 1.10 DFWM mixing experimental set-up using a) forward beam geometry and b) backward beam geom etry (from ref. 1). 43 geom etry (Fig. 1.10b).1 In this case, a forward beam , Ef and a backward beam , Eb are counterpropagating and an incident beam , Ep is incident at a small angle 0 relative to Ef. In this geometry, the signal is generated counterpropagating to the probe beam automatically satisfying the phase matching condition, ks = - kp. b. Materials for third-order nonlinear optics1’118 i. 7t-Conjugated polymers. Extensive electron delocalization is n ecessary for enhanced third order effects. A polymeric backbone provides a framework for extensive ji-electron backbone. Polydiacetylenes. O ne of the most widely studied class of ^-conjugated polym ers are the polydiacetylenes. This class of polym ers h as the general structure \ / Rl c - c = c - n r / ^ The properties of the polymers can be varied by the nature of the substituents Ri and R2. Polydiacetylenes also behave differently depending on w hether they are formed in solid state or when they are recast from solution. Thus, th ese provide unique system s in studying conformational, substituent, and conjugation effects. 44 Interest in the study of polydiacetylenes has grown since W egner and coworkers were able to grow defect-free crystals of the polym ers.123- 124 Aside from crystals, x3 values of th ese polymers have already been m easured as solutions, Langmuir-Blodgett films, and films cast from solution.125 Studies using polydiacetylenes have com e out with som e correlation betw een % 3 and structure which include: a) the magnitude of x3 is larger along the direction parallel to the chain rather than perpendicular;126 b) enhancem ent in x3 is achieved as one-photon resonance is approahced;127 and c) a large increase in x3 is achieved as rc-conjugation length is increased.128 Polyacetylenes. Polyacetylene (-CH=CH-) is an o th er c la ss of conjugated polymers which have been studied with regards to its third-order response. Another interesting feature of this polymer is that it is intrinsically a sem iconducting material (bandgap ~ 1.8 eV) and with doping, can be highly conducting.129 H eeger and coworkers have com pared the nonlinearites of all-trans vs. an all-cis polyacetylene by THG m easurem ents.130- 1 3 1 They found out that an order of magnitude increase in nonlinearity can be achieved with the use of the all-trans polymer. A similar study w as done by Sinclair etal. who reported a x3= 4 x 1 0 '1° esu at 1.06 pm for an all-trans polyacetylene.132 They also found that the x3 value of the cis-polyacetylene is one-tenth that of the trans isomer. 45 O ne problem encountered in using polyacetylenes is that it is insoluble and extrem ely air-sensitive. This presents a big synthetic challenge since a significant % 3 enhancem ent can be obtained in structurally ordered and strongly oriented polyacetylynic system s. Polyarylenes and Polyarylene vinylenes. P o ly ary len es (polythiophenes (PT), in particular) and polyarylene vinylene (polyphenylene vinyles (PPV) and its thienylene analog, polythienylene vinylene (PTV)) are the most commonly studied polymers in this class. i r f i PT R = long alkyl chain n PPV X=Y= R, OR, H PTV R= long alkyl chain Unlike polyacetylenes, polythiophene is not air-sensitive and substituting alkyl chains on the 3-position m akes it soluble in various organic solvents. Polythiophenes have been extensively studied in several form s such a s solutions and a s Langmuir-Blodgett films. Electrochem ically polym erized polythiophenes have also been characterized by Prasad et al. using a DFWM 46 experim ent.133 They reported a resonance-enhanced x3 value of ~4 x 10_1° esu at 602 nm wih 350 fs pulse. Other experimental studies have com e from the groups of H eeger,134- 138 Vardeny,135 P rasad,136 Yang,137 W urland138 am ong others. PPVs and PTVs have attracted a great deal of attention b ecau se of the e a s e in their sy n th eses and obtaining optical quality films. PPV h as been particularly interesting since it can be used in polymeric light-emitting diodes (LEDs).139 A num ber of different laboratories have reported x3 values for PPV ranging from 1 0 '11 and 10'9 e su .140' 142 Kaino et al. recently reported higher values of x3 for PTV than PPV.1 ii. o-conjugated Polymers. Polysilanes and their germ anium analogs belong to a class of polymers that do not contain u-electrons in their backbones. They display interesting electrical and optical properties which can be attributed to the delocalization of o electrons along the polym er backbone. T hese polymers are transparent throughout the visible region. The properties of th e se polym ers can be tailored by varying the substituents Ri and R2 thus, studies like chain length, substituent and chain conformation d ependence are easily achievable. Typical off-resonsnce x3 values m easured by THG are on the order of 1 0 '12 to 10-11 esu .143-146 In one particular study by Kajzar et al. on X = Si, Ge 47 polysilanes with Ri= -CH3 and R2 = phenyl, (% 3 = 1.5 x 1CH2 esu at 1.064 pm), they su g g este d that the value of % 3 w as en h an ced by a three-photon resonance.143 iii. Ladder polymers. Ladder polymers are another group of conjugated polym ers com posed of rigid-rod segm ents. They are characterized by high m echanical strength and stability . Typical exam ples of th ese polymers are the p o ly (p -p h en y le n e b e n z o b isth ia z o le ), PBZT a n d p o ly (p -p h e n y le n e benzobisisoxazole), PBZO. values of ~ 10'11 esu at both 585 and 605 nm with subpicosecond response.147-148 PBZT — i n PBZO Another reason for the interest on ladder system s is the ab sen ce of ring torsion thereby maintaining electronic delocalization. An exam ple of such system s are polymers com posed of fused rings of the general formula X D O C K X = 0 ,S , NH 48 The fully fused polymer is very insoluble. To overcome this processing problem, Dalton and coworkers synthesized block copolymers of the ladder group and flexible alkyl segm ents as shown below149. The design has the advantage of controlling the nonlinearity, by varying the electroactive seg m en t and the solubility, by changing the flexible spacers. Preliminary % 3 values of ~ 1 0 8 esu (532 nm), ~4x 10'9 esu (585 nm) and 10’10-1 0 ‘1 1 at 1.064 pm w ere reported. iv. Macrocycles. Macrocycles are large conjugated m olecules which can be thought of as two-dimensional jc-electron system . The most widely used macrocycle for NLO studies are phthalocyanines. M = Fe, Ni, Co, Cu Zn, Pt, H2 49 Phthalocyanines are characterized by exceptional therm al and chemical stability. Materials for NLO studies can be prepared by multiple sublimation, by dissolving in appropriate solvents or by using phthalocyanines in polymeric form .1 Polymeric phthalocyanines are connected in a "shish-kabob" type arran g em en t through a bridging atom . A side from nonlinear optics, phthalocyanine have been used in energy conversion, liquid crystals, optical data storage, electrophotography, etc.150'153 A num ber of groups have studied the % 3 resp o n se of m acrocycles, especially phthalocyanines.154' 158 Special attention w as given to another characteristic of phthalocyanines, i.e., a strong electronic transition in the visible betw een 600 and 800 nm. This region is term ed Q -band and is a 71- 7 1 :* excitation with absorption coefficients of greater than 104 cm '1. Phthalocyanines can easily form com plexes with m etals such as Co, Ni, Cu, Zn, Pd or Pt. Metal substitution studies have shown that highest y values as well a s x 3 of 2 x 1 0 '1° with Pt substitution. In general, the presence of a metal enhances the value of y by factors of 5 to 50.1 v. Fullerenes. Carbon-cage molecules have aroused a lot of interest in different research a re a s including NLO.160- 161 For NLO applications, this system is particularly interesting since the re sp o n se will be devoid of contributions from vibrational harmonics associated with C-H bonds. 50 Initial studies on C6o in benzene and C70 in toluene show ed that off- resonance values of yto be on the order of 10 30 esu which translates to a % 3 ~ 10 '8 e s u .162- 163 However, recent experim ental studies by C h en g 164 and Kakafi165 reported y and x 3 three to four orders of m agnitude sm aller than previously reported. T hese results are in good agreem ent with a theoretical calculations perform ed on the basis of AM-1 optimized geom etries. Up to the present time, the NLO properties of fullerenes still continue to be of interest and subject of continuing research in several laboratories.167'168 1.5 Scope of this dissertation This dissertation will present studies on both third- and second-order organic nonlinear optical materials. C hapter 2 will discuss studies on neutral and doped bisthienyl polyenes on varying conjugation lengths. The dependence of the third order response of both sets of thienyl polyenes to the conjugation length will be a sse sse d . Furthermore, using an interferometric technique, the real and imaginary part of the neutral bisthienyl polyenes w ere determ ined. Lastly, the response of doped bisthienyl polyenes w ere correlated to a two-level model under the assum ption of an adiabatic-following condition. C hapters 3 and 4 of this dissertation will deal mainly with the study of materials for second-order applications. 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This is of importance in order to evaluate the best compromise betw een packing density and nonlinear optical efficiency. Several theoretical studies have addressed the question of conjugation length dependence of both (3 and y, and whether the nonlinearity levels out after som e specified conjugation length. The results consistently indicate that saturation s e ts in after about 50-60 carbon a to m s .2'7 More recently, a theoretical study using oligothiophenes showed that the y^to-conjugation length correlation can be described by: i) a rapid increase in the y response with the extension of chain length followed by ii) a regime w here the y value stays constant and iii) a decrease in y indicating a saturation regime. Regime ii occurs at about 3-4 and 6 thiophene rings and iii after about 7-8 thiophene rings.8 T hese results are in excellent agreem ent with the findings of P rasa d 9 and Meijer.10 The num ber of experimental work, however, is quite limited due to the insolubilities of longer polyene chains. Spangler and coworkers have reported several studies in which the main goal w as the synthesis of a series of stable 65 polyenes w hose third order nonlinear optical properties could be evaluated.1113 S everal polyenes w ere synthesized including a,to -bis thienyl p olyenes substituted with -SR and -OR groups in the 5 and 5' positions. However, the solubility of the resulting m aterials precluded further studies. A new set of thienyl polyenes w ere synthesized and w ere studied in their neutral and charged-state forms in this chapter. C onjugated polymers and oligomers p o sse ss a strong electron-phonon coupling. This m eans that there is a very close relationship betw een the electronic structure and geometric structure. Thus, formation of charged states i.e., alteration of electronic structure, results in a relaxation of m olecular geom etry giving rise to nonlinear excitations such a s polarons and bipolarons. As a consequence of changes in molecular geometry and electronic structure, a m ajor shift in oscillator strengths that can induce a highly nonlinear optical response can occur.1416 C harged states can be introduced into a medium by either of two ways: chemical doping or photonic excitation. Photonic excitation creates charges by photoexciting the originally neutral molecule with a strong laser p ulse.17 In chemical doping, a chemical is added to induce charge transfer in the medium. This results in the creation of either a positive or negative charge in the molecule. There are two w ays of introducing a positive radical into the system : protonic doping and oxidative doping. 66 Protonic doping is done by adding a strong acid to a neutral molecule. This results into the unpairing of two non-bonded electrons and thus, the formation of a m etastable structure. The optical properties in the protonic doped species have been less studied. In oxidative doping, an oxidizing agent such as antimony pentachloride is added to the solution which rem oves one or more electrons from the system .12' 18' 19 This process is illustrated in Fig. 2.1. The electrons are taken away separately going through a singly charged state (polaron) to a doubly charged state (bipolaron). The bipolaron state can be achieved upon adding an ex cess of the doping agent. This will be the doping method of choice in this chapter. Num erous theoretical investigations were m ade on non-resonant polaron and bipolaron states in conjugated polymer system s and a significant enhancem ent in th e third order nonlinearity w as predicted .20'22 Experim ental studies, however, w ere quite limited due to the difficulty in synthesizing longer polyene chains with appreciable solubility and stability. In this chapter, a new set of thienyl polyenes that are completely soluble in a variety of organic solvents were studied. The chain length dependence of third-order response of a series of -SR substituted thienyl polyenes are also d iscu ssed . Furtherm ore, using a d eg en erate four-w ave mixing (DFWM) interferometric set-up, the real and imaginary part of the third order response w ere determ ined. Finally, a correlation of the third-order nonlinearity of the doped thienyl polyenes to the two level model in the adiabatic-following regime is a sse ssed . 67 LUMO LUMO SOMO HOMO LUMO HOMO neutral polaron bipolaron Fig. 2.1 MO diagram for the formation of the polaron and bipolaron states (from ref.35). 68 2.2 Experimental The bis-thienyl polyenes (BTPs) used for this investigation w ere provided by Prof. C harles W. S pangler of Northern Illinois University and w ere synthesized according to the schem e on Fig. 2.2.23 The sam p les for the doping studies w ere p rep ared by dissolving pow dered BTPs into spectrophotom etric grade m ethylene cloride (Fisher Scientific, Inc.) with concentrations ranging from 10'2 to 10‘4 M. The doping studies w ere done by adding 0.1 M antimony pentachloride, SbCIs, into the neutral sam ples. The colors of the dissolved polyenes ranged from yellow to red and turned dark blue to brown upon doping. Third-order m easurem ents were performed by using a DFWM mixing set up (Fig. 2.3) utilizing a mode-locked Nd:YAG laser pulse of -3 0 psec at 1.06 pm or 532 nm split into three beam s: F, B, and S .24- 35 In Fig. 2.3, F, B, P, and S are the forward, backward, probe, and signal beam s, respectively. T hese polarizations of th ese beam s were controlled by the polarizers PF, PB, PP, and PS. The angle betw een F and P is approximately 17°. The set-up h as six mirrors, M1 to M6 , thrre delay lines (DF, DP, and DB), three beam splitters (BS1 to BS3), and two photodiodes (PDE and PDS). The input and signal pulse energies were m easured by the photodiodes and fed into the A/D converter port of a PC com puter.35 69 Br Br t i BuMgBr CI2 Ni(dppp) Bu Bu 8 Bu Bu BuLi/TMEDA_ S; Bui " BuLi/TMEDA DM F Bu Bu or Bu Bu H- SBu BuLi/TMEDA DMF Bu Bu O H O - H - S B U s C V c H 2P ‘ Bu3,B r DMF 10% h3 o + , t h f X— y — (CH=CH)n-CHO s Bu3PCH2CH=CHCH2PBu3 + , 2CI NaOEt, EtOH or DMF Bu3PCH2(CH=CH)2CH2PBu3+ , 2Br' NaOEt, EtOH or DMF Bu Bu Bu Bu Bu Bu Bu Bu X = H, BuS Fig. 2.2 Schem e for the synthesis of -H and -SBu substituted bisthienyl polyenes. 70 1.06 |im M 1 Mode-locked N d:Y A G laser M 2 D F B S1 M 3 BS2 D P PDE PP PS BS3 M 4 PDS Sample D B M 5 Fig. 2.3 DFWM experimental set-up (from ref. 35). 71 2.3 Results and Discussion a. Nonlinearities of -H vs -SBu substituted bisthienyl polyenes The materials used in this investigation w ere com posed of two sets of a,at thienyl polyenes, H -substituted bisthienyl polyenes (H-BTPs) an d SBu - substituted bisthienyl polyenes ( -SBu BTPs). T hese polyenes w ere com posed of long alkyl chain substituents in the (J and p' positions of each thiophene unit. Alkyl substitutions have long been shown to greatly enhance the solubility of thienyl or thienylene m oieties.25'28 T hese substituents w ere also utilized by Tour and coworkers to solubilize well-defined polythiophene oligomers up to the octam er level.29'32 The sam e behavior w as see n in the m aterials under investigation. The alkyl substituents provided solubility enhancem ent of the BTPs which enabled DFWM studies to be done both in its neutral and charged states. Bipolaron formation by oxidative doping w as done by adding a known am ount of SbCIs in methylene chloride. An ex cess of dopant w as used in all c ases. The resulting bipolaron is described in Fig. 2.4 and a typical absoption spectrum is shown in Fig. 2.5 . A list of maximum absorbances is listed in Table 2 . 1. A very striking trend in the optical spectrum of th ese materials is the shift of the oscillator strength upon doping. From Table 2.1, it can be see n that all neutral sam ples show an absorption maxima ranging from 420 nm to 500 nm. 72 Absorbanco 0.8 - - 0.6 - - 0.2 - - - 0.2 - Wavelength (nm) Fig. 2.4 Absoprtion spectra of doped -H substituted bisthienyl polyene (n=8) taken every five hours after doping. 73 B u Bu Bu B u X ^ - ( C H = C H ) - y - X SbCI5i -1 e B u Bu Bu Bu X - / 3 — CH—(CH=CH)— CH S n-1 S polaron SbCl5 > -1 e ' Bu B u B u B u + V - / C - H — CH—(CH=CH)— CH— n-1 S bipolaron n = 1,2,3,... Fig. 2.5 Schem atic representation of the formation of the bipolaronic bisthienyl polyenes. 74 Table 2.1 List of maximum absorbances of neutral and doped species of -H and -SBu substituted bisthienyl polyenes. Compound Xmax neutral, nm ^max doped, nm A . -H substitued, n= 3 399 655 4 418 661 5 435 715 6 450 719 7 464 849 8 477 914 9 489 971 B. -SBu substitued, n= 3 424 643 4 439 657 5 451 705 6 466 754 7 476 801 8 488 849 9 501 867 75 In th e d o ped sta te s, th e se a b so rb an c es w ere b leach ed out with an accom panying appearance of new absorption maxima betw een 610 to 870 nm. The red shift in the absorbances upon doping w as studied in detail by Logdlund et a /.33 and Dannetun et a /.34 using system s which w ere capped by phenyl substituents. They attributed the accompanying red shift to the creation of the bipolaron state. The values of the third order nonlinearity, x3 were calculated b ased on m easured signals in a DFWM experiment using the following equation: w here Is is the signal in the DFWM experiment and is m easured at the three- beam overlap position; Ic3 is the cubic of the incoming b eam s in a DFWM experim ent; T is transm ission; nSam and nret are the refractive indices of the sam ple and reference, respectively and Ic is the effective thickness. In the experim ent, C S2 w as u sed a s the reference and h as a x3 coefficient of 6 .8 x 10’13 esu at 532 nm and 4.7 x 10' 13 esu at 1064 nm. A typical DFWM signal is shown in Figs. 2.6 and 2.7. A list of x3 for selected BTPs can be found in Table2.2. 3 X X ref X sam n 2 ref (2 .1 ) 76 signal (a.u.) 3 2.5 2 1.5 1 0.5 0 -80 -60 -40 -20 0 20 40 60 80 time (ps) Fig. 2.6 DFWM signal of neutral -H substituted bisthienyl polyene (n=5). o Q o n 77 3 - 2 .5 - 2 - 1 .5 - 1 - 0 .5 - 0 - -0 .5 - -80 -60 -40 -20 0 20 40 60 80 time (ps) DFWM signal of doped -H substituted bisthienyl polyene (n=5). 78 Table 2.2 List of experimental % 3 values of neutral and doped -H and -SBu substituted bisthienyl polyenes. Compound ^-max (nm) X 3 neutral (x 10'13 esu) X 3 doped ( x 10-" * 3 esu) A . -H substituted, n= 5 532 2.7 7.8 7 532 43.0 11.0 8 532 258 14 5 1064 0.54 1.4 7 1064 0.85 3.2 8 1064 0.66 2.7 B. -SBu substitued, n= 5 532 48 b 7 532 300 b 8 532 1000 b 5 1064 b c 7 1064 b 30.7 8 1064 b 258 h indistinguishable from solvent c unstable 79 A com parison of the DFWM data betw een the H-BTP and SBu-BTP polyenes indicates a higher value for the latter series. This can be attributed to the highly polarizable -SR substituents. T hese substituents inject more electron density into the system providing m esom eric stabilization. B ecau se of the enhanced nonlinearities of the -SBu substituted polyenes over the -H substituted ones, the former series will be investigated more closely and are the materials which will be referred to in the succeeding paragraphs. b. Studies on SBu thienyl polyenes at 532 nm Third-order nonlinear optical processes are characterized by a fourth rank tensor and are com posed of both real and imaginary com ponents. In order to determ ine the phase of the third order signal, a DFWM interferometry set-up at 532 nm w as set-up by Tang according to the schem e in Fig.2.8.35 With this new technique, both the real and imaginary parts of the third order signal were m easured at an arbitrary time delay. This w as accom plished by interfering one of the transm itted counter-propagating beam s with the degenerate four wave mixing signal beam. A more thorough description of the set-up can be found in reference 35. Both neutral and doped sam ples of the SB u-substituted polyenes in m ethylene chloride were exam ined using the interferometric techniques at 532 nm. The doped sam ples do not show significant differences from the solvent 80 M 1 PDA M O BS2 Sample £ \P 2 D2 BS4 PD M 2 Fig. 2.8 DFWM-interferometric set-up (from ref. 1). 81 signal. However, the neutral sam ples showed a strong instantaneous response and small tails except for n= 3 and 4 SBu-BTPs, w here the DFWM signals decay in about 2 0 0 ps. A typical DFWM interference pattern using one of the sam ples n=5 SBu- BTP is shown in Fig. 2.9. It can be seen that the reference runs are quite reproducible. This is important since this shows the stability of the system and the ev en n ess of the cell surface. Any unnevenness in the surface will cau se a change in the beam direction and will alter the phase significantly. The m agnitude, y, of the third-order response w as calculated from the three beam overlap position and com pared to the C S2 reference a s previously described. Using the relationship - j L Y NL4 (2.2) w here N is the num ber density in m olecules per cm 3 and L is the local field factror which is related to the refractive index by equation: nre2 + 2 L = — -------- 3 (2.3) Since the solution w as very dilute, nre , the refractive index of the sam ple w as a ssu m ed to be the sam e a s that of the solvent, m ethylene chloride. The 8 2 4 C S 2 before 0 0.01 0.02 oscilloscope scan (s) Fig. 2.9 Representative DFWM interference pattern using -SBu substituted bisthienyl polyene (n=5) (from ref. 35). 83 resulting magnitude and phase of the hyperpolarizability is sum m arized in Table 2.3 Next, the chain-length dependence of the molecular hyperpolarizability y, of the sam ples at 532 nm were determined . Hermann et al36’37 and Rustagi 38 et al previously show ed the correlation of the chain length to the m olecular hyperpolarizability in a one dimensional system. They showed that y is related to n, the num ber of intervening double bonds, by a power law dependence Y=anb (2.4) with b = 4. This number w as found to be in good agreem ent in both theoretical and experim ental studies. Electron-electron correlation has recently been included in recent calculations.39’41 The obtained power law dependence is in the range of b = 4.6 to 5.4 for neutral molecules and b=6 for the bipolaron S tate.21- 22. 39-41 The dependence of the magnitude of the molecular hyperpolarizability, y, to the num ber of conjugated double bonds, n is shown in Table 2.3. A least- square fitting of y gives a power law dependence of b = 5.5± 0.07 and is shown in Fig. 2.10 . This value of b is slightly higher than theoretical predictions but is still within a 90% confidence range in b. The value of y obtained is resonance en h an ced since the laser operating wavlength sits within the characteristic absorption maximum of the polyenes under investigation. 84 Table 2.3 Experimental y n n of neutral -SBu substituted bisthienyl polyenes. n 3 4 5 6 7 8 9 71111 exp (x 10‘31 esu) 0.66 4.9 24.5 87 150 510 235 c p 3 151 ° 141 ° 172° 164° 186° 172 0 158 0 71111 real (x 10*3'I esu) -0.58 -3.8 -24.2 -83.6 -149.2 -505 -217.9 71111 imag. (x 10*31 esu) 0.32 3.1 3.4 24 -0.16 0.71 0.88 85 l o g Tim - 2 8 -29 -30 -31 -32 0.7 0.8 0.9 1 0.5 0.6 0.4 log n Fig. 2.10 Plot of Y 1111 vs. log n to determine the chain length dependence of the third-order hyperpolarizability. 86 c. Studies on -SBu thienyl polyenes at 1064 nm The neutral polyenes were also studied at the laser source wavelength of 1064 nm. At this wavelength, the sam ples do not show appreciable difference in the signal response from the solvent. However, the doped sam ples behave otherwise, i.e., they showed appreciable signal at 1064 nm and indistinguishable from the solvent at 532 nm. The doped sam ples were found to be stable for several days a s proven by the small changes in the optical absorption scans. However, this is only true for longer, n=6-9 polyene chains. For n=3-4, there w as a decomposition of the bipolaron state even after a few minutes after doping. This can be attributed to the significant overlap of the wavefunctions in the shorter chains. This results to an increased total energy of the molecule thus also increasing its intstability. The power law dependence of y to n for the doped sam ples is shown in Fig.2.11 A least square fit of the values from experimentally m easured third order response resulted to a b= 14.0 ± 2.6 , a num ber that is a lot bigger than any theoretical predictions. However, it should be noted that the predicted values w ere determ ined in the non-resonant regime. This can be used to explain the disparity in the observed and predicted values. Theory predicts that for m olecules w hose linear absorption and % 3 are cau sed by a near-resonant transition from the ground level (1) to an excited 87 lo g Yim -27 -2 7 .9 - -2 8 .8 - -2 9 .7 - -3 0 .6 - -31.5 0.9 0.95 1 0.85 0.75 0.8 log n Fig. 2 .1 1 Plot of log y n n experimental (0 ) and log y n n theoretical (♦) vs. log n to determ ine the chain length dependence of the third-order hyperpolarizability. 88 level (2 ), the bipolaron state can be approxim ated by a two-level model in the adiabatic following regim e.42 The third order susceptibilty per molecule, y n n at frequency v shifted above resonance can be expressed as y Y 1 1 1 1 = 3 (hA) (2.5) assum ing an "adiabatic-following" condition. An adiabatic-following state requires that the frequency spectrum of the pulse do not overlap the absorption line by a significant degree. In eq. 2.6, P12 is the magnitude of the transition dipole m oment betw een level 1 and 2 . A term A21, the spontaneous emission decay rate from excited level 2 to ground level 1, is also defined as 43 4p2 co 3 A - 1 2 21 “ “ T T 3 he (2.6) A frequency v dependent line shape function (Hz) is also used to define the relationship betw een A21 to the absorption spectrum . The line sh ap e function g2i(v) is normalized by Jg 2i(v)dv =1 (2 .7 ) This integral g o es over the range 600 to 1200 nm for the sam p les under investigation, i.e. n=6-9 SBu-BTPs. It w as also assum ed that the contribution of 89 the integrals from other regions insignificant. The absorption cross section o satisfies the well-known Fuchbaeur formula assum ing equal level degeneracies: *2 02i(v) . “ I — 5— A 21 87Cn2 re (2.8) w here X is the wavelength in vacuum and nre is the refractive index of the sam ple. From th e absorption spectrum , the line sh ap e function g 2i(v) and absorption cro ss section o can be determ ined. The value for g2i (v) can be calcu lated by converting th e w avelength to frequency th u s having an absorbance versus frequency plot. Integrating the absorbances over the entire frequency range gives a normalization number. Dividing the absorbance at a certain frequency by the normalization number gives the value of g2i(v). The absorption cro ss section of the sam ple is calculated from the absorption spectrum by using the relationship w here A is absorbance, I is the path length in cm and N is the num ber density in m olecules per cm3. Using the values for g2i(v), a and equation 2.6, A21 can be calculated. Plugging in the calculated value of A21 into equation 2.6, we find the dipole A = N al (2.9) 90 m om ent, p i 2- Finally, y n n can be derived by using equation 2.5. The calculated values of y are summarized in Table 2.4. The power law dependence of the magnitude of y is plotted in Fig. 2.11 An exponent of b= 14.0 ± 2.6 w as calculated from the least square fit using the experimental points (circles) and 14.5 ± 3.1 for the calculated values (diamond). It can be seen that the calculated values are close to the experimental values to within experimental error, considering uncertainties in the absoprtion data. 2.4 Conclusion Two se ts of bisthienyl polyenes, -H and -SBu substituted, w ere studied using DFWM techniques. A com parison of m easured x3 v alues show ed enhanced third-order nonlinearities with the -SBu substitued bisthienyl polyenes over the -H substituted ones. The enhancem ent can be attributed to the m esom eric effect of the -SBu substituents. The -SBu bisthienyl polyenes were further investigated at both 532 and 1064 nm laser source wavelengths. Results of experim ents done at 532 nm showed that for neutral sam ples with 3<n<9, x3iin * s proportional to nb where b ~ 5.5 . Furthermore, using a DFWM interferometry technique, the real and imaginary parts of the y response were m easured. The doped sam ples do not show appreciable signal at this wavelength. 91 Table 2.4 y ^ n ® * p and y im calc of doped -SBu substituted bisthienyl polyenes. y n n calc w as determined using a two-level calculation b ased on absorption data and eqn. 2.5. n 6 7 8 9 Y l 1 1 1 exp (x 10'30esu) Y l 1 1 1 calc (x 10'3oesu) 0.42 0.25 3.8 3.3 31.9 11.9 114 143 92 The correlation of the % z response to the num ber of double bonds w as also exam ined at 1064 nm. It was found out that for doped sam ples with 6<n<9, % 3 i n i is also proportional to nb with b~14. In th ese cases, the laser source wavelength is close to an absorption band. Finally, good agreem ent w as found betw een experimental and calculated % 3 values, assum ing a two-level system . 93 2.5 References 1. Parts of this chapter were published in N. Tang, J.P. Partanen, R.W. Hellwarth, J. Laquindanum, and L.R. Dalton, Proc. S P /E 2285,186-195, 1994. 2. J.L. Bredas, C. Adant, P. Tackx, and A. Persoons, Chem. Rev., 94, 243, 1994. 3. D.N. Beratan, J.N. Onuchic, and J.W. Perry, J. Phys. Chem. 91, 2696, 1987. 4. A.F. Garito, J.R. Heflin, K.Y. Wong, and Zamini-Khamiri, O, Nonlinear Optical Properties of Polymers. Mat.Res. Soc. Svmp. Proc.. A.J. Heeger, J. Orenstein, D.R. Ulrich, Eds., Materials ResearchSociety: Pittsburgh, 109,]19, 1988. 5. Z. Shuai and J.L. Bredas, Phys. Rev. B, 46, 4395,1992. 6 . Z. Shuai and J.L. Bredas, Phys. Rev. B, 45, 8264,1992. 7. G.J.B. Hurst, M. Dupuis, and E. Clementi, J. Chem. Phys, 89, 385, 1989. 8 . D. Beljonne, Z. Shuai, and J.L. Bredas, J. Chem. Phys., 98, 8819, 1993. 9. M.T. Zhao, B.P. Singh, and P.N. Prasad, J. Chem. Phys., 8 9 ,5 5 3 5 ,1 9 8 8 . 10. H. Thienpoint, G.L. Rikken, and E.W. Meijer, Phys. Rev. Lett., 65, 2141, 1990. 11. C.W. Spangler, L.S. Sapochak, and B.D. G ates, Organic Materials for Nonlinear O ptics. R. Hahn and D. Bloor, Eds., Royal Society of Chemistry, Publishers, London, 57-62,1989. 12. C.W. Spangler and K.O. Havelka, Polymer Preprints 31, 396,1991. 13. C.W. Spangler, P.K. Liu, A.A. Dembek, and K.O. Havelka, J. Chem. Soc. Perkin Trans. 1, 799,1991. 14. W.P. Su, J.R. Schrieffer, and A.J. Heeger, Phys. Rev. Lett., 42,1698,1979. 15. W.P. Su and J.R. Schrieffer, Proc. Natl. Acad. Sci. U.S.A., 7 7 ,5 6 2 6 ,1 9 8 0 . 16. J.L. Bredas and G.B. Street, Acc. Chem. Res., 18, 309,1985. 94 17. X.F. Cao, J.P. Jiang, R.W. Hellwarth, L.P. Yu, M. Chen, and L.R. Dalton, SPIE Proc. 1337,114,1990. 18. C.W. Spangler, P.K. Liu, and K.O. Havelka, J. Chem. Soc. Perkin Trans 2 , 1207, 1992. 19. C.W. Spangler and P.K. Liu, J. Chem. Soc. Perkin Trans 2,1959,1992. 20. J. R. Tallent, R.R. Birge, C.W. Spangler, and K.O. Havelka, Molecular Electronics-Science and Technology. A. Aviran, Ed. American Institute of Physics, New York, 191-203,1992. 21. C.P. de Melo and R. Silbey, Chem. Phys. Lett., 140, 537,1987. 2 2 . C.P. de Melo and R. Silbey, J. Chem. Phys., 8 8 , 2567,1988. 23. C.W. Spangler and M. He, submitted for publication. 24. R.W. Hellwarth, J. Opt. Soc. Am., 6 7 ,1 ,1 9 7 7 . 25. A.O. Patel, A.J. Heeger, and F. Wudl, Chem. Rev., 8 8 ,1 8 3 ,1 9 8 8 . 26. M. Sato, S. Tanaka, and K. Kuriyama, Synth. Met., 18, 229, 1987. 27. G. Heffner and D.S. Pearson, Synth. Met., 44, 341,1991. 28. A. Bolognesi, M. Catellani, W. Porzin, F. Sperani, R. Galarini, A. Musco, and R. Pontellini, Polymer, 34, 4150,1993. 29. J.M. Tour, R. Wu, and J.S. Schumm, J. Am. Chem. Soc., 112, 5662, 1990. 30. J.M. Tour, R. Wu, and J.S. Schumm, J. Am. Chem. Soc., 113, 7064,1991. 31. J. Guay, A. Diaz, R. Wu, J.M. Tour, and L.H. Dao, Chem. Mater.,4, 254, 1992. 32. J.M. Tour and R. Wu, Macromol., 25,1901,1992. 33. M. Logdlund, P. Dannetun, S. Stafstrom, W. Salaneck, M.G. Ram sey, C.W. Spangler, C. Fredriksson, and J.L. Bredas, Phys. Rev. Lett., 70, 970,1993. 34. P. Dannetun, M. Logdlund, C.W. Spangler, J.L. Bredas, and W.R. Salaneck, J. Phys. Chem., 98, 2853,1994. 95 35. N. Tang, Ph.D. Dissertation, University of Southern California, 1994. 36. J.P. Hermann, D. Ricardi, and J. Ducuing, Appl. Phys. Lett., 2 3 ,1 7 8 , 1993. 37. J.P. Hermann and J. Ducuing, J. Appl. Phys., 45, 5100, 1974. 38. K.C. Rustagi and J. Ducuing, Opt. Commun., 10, 258,1974. 39. S. Mukamel and H.X. W ang, Phys. Rev. Lett., 69, 65,1992. 40. J.R. Heflin, K.Y. Wong, O. Zamani-Khamiri, and A.F. Garito, Phys. Rev. B, 38, 1573, 1988. 41. J.R. Heflin, K.Y. Wong, O. Zamani-Khamiri, and A.F. Garito, Mol. Cryst. Liq. Cryst., 160, 37, 1988. 42. P.N. Butcher and D. Cotter, The Elements of Nonlinear O ptics. Cambridge University Press, New York, 1990. 43. H.A. Bethe and E.E. Salpeter, Quantum M echanics of O ne and Two- Electron System s. Handbuch der Phvsik Band XXXV. Atom I. Springer- Verlag, Berlin, 1957 96 CHAPTER 3 NOVEL MATERIALS FOR SECOND-ORDER NONLINEAR OPTICAL APPLICATIONS 97 3.1 Introduction The design and synthesis of materials for second-order nonlinear optical applications have been the major focus of continuing research. Major research efforts have been directed towards a) finding or developing a chrom ophore with a sizable m olecular second order activity and b) translating this m olecular activity into a stable bulk response. Most of the prototypical donor-acceptor poled polymer system s in the literature, such a s dimethylaminonitro stilbene (DANS), utilized a stilbene conjugating bridge containing two phenyl rings. Several studies have shown that replacem ent of the phenyl rings with heteroarom atic rings such a s thiophene led to an enhancem ent of nonlinearity.1-5 This can be attributed to the lower resonance stabilization energy of heteroaromatic rings over benzenoid rings (arom atic delocalization energy: benzene=36 kcal/mol, thiophene=29 kcal/mol, furan=16 kcal/mol, pyrrole=21 kcal/mol). The d e c re a se in the resonance stabilization energy m eans a more easily delocalized system which tran slates into a more effective electronic coupling betw een the donor and acceptor substituents. Since, the magnitude of nonlinear optical effects of push- pull polyenes depend upon the degree of coupling betw een the substituents through a conjugated bridge, a more perturbable conjugating system seem s to be more attractive for second-order applications. A side from altering the conjugating unit, an o th er way to in crease hyperpolarizability w as to find stronger acceptor groups. Jen and cow orkers have shown that replacem ent of nitro groups with a stronger tricyanovinyl group 98 led to a tenfold increase in nonlinearity.4-6 The use of cyanovinyl a s acceptor groups w ere also previously addressed by Katz et al.7 A model chrom ophore w as therefore designed using th ese features: a) a thiophene conjugating unit and b) cyanovinyl acceptor. Furthermore, the model chrom ophore w as also d esig n ed to contain an additional functionality which could be u sed for subsequent crosslinking reactions. The model chrom ophore has the following structure: C(0)0Et In order for this model chrom ophore to be useful for nonlinear optical applications, it has to be incorporated into noncentrosym m etric lattices and poling-induced order m ust be m aintained for extended periods of tim e at elevated tem peratures. In this chapter, two m ethods w ere used to achieve lattice hardening to stabilize poling-induced order. The first schem e m ade use of asym m etric reactions based on a double-ended crosslinkable (DEC) version of the model chrom ophore. In this schem e, each processing step (i.e., polymerization and crosslinking) is distinguishable and thus, lattice hardening can be controlled. The other sch em e involved the sy n th esis of a tri- functionalized variation of the m odel chrom ophore. In this c a s e , the 99 chrom ophore w as reacted under conditions to form a viscous solution and processed (i.e., spin casted, poled and crosslinked). Since the functionalities w ere all the sam e, there w as no distinct separation betw een the processing steps. As m entioned in the previous paragraph, the first approach that w as investigated to maintain the polar order w as by the u se of a double-ended crosslinkable (DEC) approach as shown in Fig. 3.1.8 In this schem e, the model chrom ophore w as tailor-m ade to have two functionalities on opposite ends. T hese functionalities were chosen such that they will have different reactivities under a given experimental condition. In this chapter, the model chronophore w as redesigned to contain a vinyl group on one end and a hydroxyl group on the other. In this case, the processing can be done stepw ise, i.e., a radical polymerization using the vinyl group and a condensation reaction using the hydroxyl group to lock-in the dipole alignment after electric-field poling. This approach w as shown to provide excellent long term therm al stability by Xu et al.8 C urrently, a m aterial d eveloped using this ap p ro a ch is being commercialized by AdTech Co.9 The DEC sch em e provides great flexibility in the sy n th esis and processing of the polymeric materials. Its flexibility allows for incorporation of different types of chromophore hence optimizing the second-order nonlinearity of a material. It also allows the use of different com onom ers to be able to vary the materials' properties ( e.g., glass transition tem perature, solubility, and 100 A— N LO chromophore .OH — D DEC Monomers OH Addition polymerization A i N L O D ho' OH XL crosslinking groups NLO polymer Spin Casting Optical-quality polymer film 1) Electric poling 2) Cross-linking I I I I I D D D D D s/V \^Jr^\A /V >A A A A />!A A A A ivV \/\A 3^A /V r' Dipole locked-in 3D network Fig. 3.1 Double-ended crosslinkable chromophore approach. 101 processibility). Lastly, the locking-in of both ends of the chrom ophore's dipole limits its rotational freedom thus enhancing the poling-induced order. Thermally-induced crosslinking reactions have always been utilized a s a m eans of achieving hardened lattices. T hese reactions have even been used to stabilize poling induced order of chrom ophores. O ne widely used therm oset system are therm osetting p o ly urethanes.10 T herm osetting p o ly u reth an e system s have been attractive because of e a se in synthesizing it and its fast formation. A num ber of studies have been m ade incorporating nonlinear optically active materials into therm osetting polyurethanes. Dalton and coworkers have constructed a prototypical electrooptic device using diisocyante capped disperse red-19 (DR-19) which form ed a polyurethane network upon reaction with triethylamine.11 In this system , only one end of the chrom ophore w as anchored to the polymer chain. W orkers at 3M synthesized a new class of double-end functionalized NLO chrom ophores based on hydroxy and am ine groups and their corresponding crosslinked p o ly urethanes. 12 The nonlinearity of the resulting materials, however, w as lower than expected due to the degradation of the chromphore. The polar order on the other hand has been quite impressive. B oogers e t al. recently published the synthesis of a trihydroxy functionalized amino-sulfone benzene and its incorporation into a crosslinked polyurethane.13 In this case, both ends of the chromphore were anchored to the network. A 34% loss in nonlinearity at 70° C w as observed which w as attributed to incomplete curing. Wu recently presented novel tri-linked and tetralinked chrom ophores 102 which w ere incorporated into thermosetting polyurethane.14 A similar schem e w as presented in this chapter as illustrated in Fig. 3.2. A tri-hydroxy variation of the model com pound previously discussed w as synthesized. The tri-hydroxy chrom ophore w as reacted with diisocyanates to form a viscous solution that can be spin-casted and poled. Subsequent curing at elevated tem peratures would lead to a thermally crosslinked three-dimensional network. Finally, several paragraphs will be devoted to discuss several attem pts to incorporate a tricyanovinyl-substituted chrom ophores into polymeric system s and study its second-order nonlinearity. 3.2 Experimental 3.2.a. General materials and methods Conventional 1H and 13C NMR spectra w ere taken using a Bruker 250 and 200 MHz sp ectrom eters. The chem ical shifts w ere referen ced to tetram ethylsilane (TMS) a s an internal standard. FTIR spectra w ere obtained using a Perkin-Elm er 1760 FTIR spectrophotom eter. G lass transition tem p eratu re (Tg) and therm al decom position te m p e ratu res (Td) w ere determ ined using a Perkin-Elmer Differential Scanning Calorimeter (DSC)-7 and Thermogravimetric Analyzer (TGA)-7, respectively, employing a heating rate of 20 °C under an argon atm osphere. Elemental an alyses w ere perform ed by Atlantic Microlab, Inc. and P aulanne Rider of Northern Illinois University. Polym er m olecular weights w ere obtained by size-exclusion chrom atography 103 hon oh D N 1 L OCN-k/w/vNCO T~ A OH 1) prepolymer formation 2) film casting 3) poling and crosslinking » N-C-O. I H V _L ? , o-c- \ O O r ‘ 'U H I I J* N - C — O O - C - N 1 ' / i H D H N' L \ O -C ii o H I N . / ^ N m * * “ T vA N-C-O, O -C -N V JL I I O H ’O l iAAAAA/'N'C_ 0 \ Dipole locked -In 3D network Fig. 3.2 Thermosetting-polyurethane approach utilizing a tri-hydroxy functionalized NLO chromophore. 104 (SEC) using 1% triethylamine in tetrahydrofuran (THF) a s the solvent and polystyrene as standard. Dioxane, tetrahydrofuran and diethylether were dried by refluxing with sodium and benzophenone as indicator. Pyridine w as dried by refluxing over sodium hydroxide for one hour. All moisture sensitive solvents and reagents w ere stored in a drybox. All other starting materials were purchased from various com panies and used a s received unless otherwise noted. 3.2.b. Synthesis of model compound The synthesis of the model compound w as carried out according to the reaction schem es shown in Fig. 3.3 and 3.4 2-chloromethylthiophene (1) Pyridine (44.5 ml, 0.55 mol) w as added dropwise to a solution of 2- thiophenem ethanol (57.1 g, 0.5 mol) under a nitrogen atm osphere at 0 °C. After com plete addition, thionyl chloride, SOCI2 (40 ml, 0.55 mol) w as added dropwise while maintaining the tem perature at 0 °C. The resulting solution w as stirred for one hour at 0 °C and then for two hours at room tem perature. The solution w as w ashed with brine (3 x100 ml), then with w ater (3 x 50 ml) and dried overnight over calcium chloride,CaCl2 - The 105 CHoOH * 5 SOCl2 pyridine CHoCI • 6 2 triphenyl phosphine CH2PPh3+ Cl Fig. 3.3 Schem e for the synthesis of thiophene interm ediates for Wittig and Horner-Emmons reactions. 1 0 6 w ^ - P ( 0 ) ( 0 E t ) 2 K+ tBuO', THF CHO nBuLi DM F CNCH2C(0)0Et piperidine (cat) 1.NaCN 2. HOAc 3. Pb(OAc)4 N C ^ - h C(0)0Et N C ^ ' CN C(0)0Et Fig. 3.4 Scheme for the synthesis of model compound (compound 6). 107 dried liquid w as concentrated and the resulting brown oil w as distilled under reduced pressure (5 torr, 65 °C) to yield the desired chlorideas a tan colored oil ( 81.4% yield). 1H NMR (CDCI3 ) relative to TMS (in ppm): 4.81 (s, 2H, -CH2); 6.96 (d, 1H, thiophene 3-H); 7.07 (d, 1H, thiophene 5-H); 7.29 (m, 1H, thiophene 4-H). Ethyl 1-thiophene phosphonate ester (2) A solution of 2-chloromethylthiophene (27 g, 0.2 mol) and triethyl phosphite (34 g, 0.2 mol) w as heated under nitrogen atm osphere at 140 °C for 24 hours. The tem perature w as then lowered to 110 °C and the solution w as stirred at this tem perature for another 48 hours while m aintaining inert atm osphere. The solution w as o p en ed to air and nitrogen w as bubbled into the solution in order to get rid of ex cess triethyl phosphite. The resulting brown oil w as used in succeeding step s without further purification. Compound 3 A solution of 4-dimethylamino benzaldehyde (5 g, 0.03 mol) and 2 (9.03 g, 0.038 mol) in 20 ml of freshly distilled tetrahydrofuran, THF w as cooled to 0°C under an inert atm osphere. Potassium tert-butoxide (4.26 g, 0.038 mol) w as then added in three portions. The resulting solution w as stirred overnight in the absence of light. The mixture w as concentrated en vacuo and the residue w as redissolved in methylene chloride (20 ml). The 1 0 8 solution w as then w ashed with brine ( 3 x 1 0 ml), w ater (3 x 10ml) and dried over magnesium sulfate, MgS0 4 . The crude product w as purified by co lu m n c h ro m a to g ra p h y on a silic a g el co lu m n u sin g hexane/dichlorom ethane as eluent (84% yield). 1H NMR (CDCI3) relative to TMS: 3.02 (s, 6 H, -CH3); 6.72 (d, 2H, phenyl Hs); 6.91 (s, 1H, =CH); 7.08 (m, 3H, thiophene Hs); 7.27 (s, 1H, =CH); 7.38 (d, 2 H, phenyl Hs). Compound 4 A 50 ml round bottom flask fitted with an addition funnel and stirring bar w as charged with a solution of stillbene 3 (5 g, 0.02 mol) and N.N.N'.N'-tetramethylenediamine, TMEDA (2.56g, 0.022 mol) in 20 ml dry ether. The solution w as cooled to -78 °C and n-butyllithium, n-BuLi (13.75 ml, 1.6 M solution in hexanes) w as added dropwise and stirred for 40 m inutes. The tem perature w as raised to 0 °C and the mixture w as stirred for one hour. Ten ml of 3N hydrochloric acid, HCI w as added and stirred for an additional 30 minutes. After pouring into iced-w ater and sub seq u en t work-up, the crude product w as recovered a s a brown oil. Purification by colum n chrom atography using 2:1 ethyl a c e ta te / dichloromethane a s eluent yielded the title compound 4 a s a bright orange solid (4.4g, 63% yield). 1H NMR(CDCI3) relative to TMS: 3.01 (s, 6 H, -CH3); 6 .6 8 (d, 2H, phenyl Hs); 7.03 (s, 1H, =CH); 7.07 (d, 1H, thiophene H); 7.23 (s, 1H, =CH); 7.42 (d, 2H, phenyl Hs); 9.81 (s, 1H, -CHO). 109 Compound 5 Com pound 4 (5 g, 0 .0 2 mol) and m ethylcyanoacetic acid (2.18 g, 0.022 mol) w ere mixed together in ethanol. A catalytic am ount of piperidine w as added and the mixture w as stirred overnight. A color change from orange to deep red w as observed a s the reaction proceeded. The solution w as concentrated en vacuo and the solid redissolved in dichlorom ethane, CH2CI2. The organic layer w as w ashed with w ater (3 x 30 ml) and dried over magnesium sulfate, MgS0 4 . The crude product w as purified by column chromography on silica gel (dichloromethne) to afford a red solid (4.4 g, 63% yield). 1H NMR(CDCl3) relative to TMS: 1.46 (t, 3H, -CH3); 3.01 (s, 6 H, -CH3); 4.42 (q, 2 H, -CH2); 6.85 (d, 2 H, phenyl Hs); 7.01 (s, 1H, =CH); 7.16 (d, 1H, thiophene H); 7.21 (s, 1H, =CH); 7.5 (d,2H, phenyl Hs); 7.67 (d, 1H, thiophene H); 8.21 (s, 1H, =CH). From the NMR data, the methyl peak of methylcyanoactete disappeared accom panied by th e a p p e ra n c e of an ethyl peak. This w as attrib u ted to the transesterification reaction due to an ex cess am ount of alcohol in the reaction mixture. The product w as still used for the subsequent step. Compound 6 A stirred solution of 5 (4 g, 0.01 mol) in 50 ml of dimethylformamide, DMF w as cooled to 0 °C in ice bath. After stirring for 30 minutes, 30% ex cess 3N sodium cyanide (NaCN) w as ad d ed and stirring w as continued for 10 minutes. A change in color from red to pink 110 w as noted upon addition of the salt. Concentrated hydrochloric acid, HCI (ml) w as added immediately followed by lead tetraacetate, Pb(OAc)4 (4.43 g, 0.01 mol). Stirring w as continued for another 30 minutes and the green solution w as poured into ice water and w as stirred further for five minutes. The precipitate w as filtered and redissolved in methylene chloride. After s u b se q u e n t w ork-up, th e crude product w as purified by flash chrom atography (1 :1:1 hexane:dichlorom ethane:ethyl acetate) to afford the title com pound a s a fine black solid (2.2g, 58% yield). 1H NMR (CDCI3) relative to TMS (in ppm): 1.45 (t, 3H, -CH3); 3.01 (s, 6 H, -CH3): 4.51 (q, 2 H, -CH2); 6.82 (d, 2 H, phenyl Hs); 7.13 (s, 1H, =CH); 7.2 (d, 1H, thiophene H); 7.22 (s, 1H, =CH); 7.4 (d, 2H, phenyl Hs); 8.01 (d, 1H, thiophene H). 13C NMR (in ppm): 1 3 .7 5 ,4 2 .3 ,6 5 .1 ,1 1 2 .5 ,1 1 5 .1 ,1 1 6 .9 , 124.6, 127.5, 130.2, 133.75, 138.4, 142.5, 151.25, 158.75, 183.8. Calculated for C2iH 19N30 2 S: C, 66.28; H, 5.07; N, 11.13; Found: C, 66.73; H, 5. 32; N, 11.21. 3.2.c. Synthesis of monomers The synthesis of the m onom ers and their interm ediate com pounds are illustrated in Figs. 3.5 - 3.8 and are described as follows: 111 o CH, o ch • i I H t 3 \ ^ O H \ ^ 0-C-C=CH2 \ a .O-C-C=CH N V g CH3 N ^ N ^ c i-c -c = c h 2 X . pocia r^ k J L j J pyridine 1^1) DMF * L j l CHO CH2OP(OEt)2 ^ K+ t-BuO' 0 c h 3 I I I o c h3 \ "C * C= C H 2 POCI3 9 OH n c -c h 2- c - o ^ 11 ► piperidine \ ^ / \ ^ 0 *C-C=CH2 DMF CHO 10 O CH3 "0 • c — ch2 o=c— o \ ^ O H 12 Fig. 3.5 Scheme for the synthesis of cyanovinyl-substituted DEC chromophore (compound 12). H 0 ^ ^ ° H C H 3 I C H g O ^ i ^ O C H a p o c u ( f S —— ifS * n u o . ^ DMF DMSO CH2PPh3 +CI' c h 3o / M ^ o c h 3 S 'S < Z w S rfS N V > M i ^ NaOEt > W CHO 0C H 3 14 16 OCH3 nBuLi '_J-\S~-(5 'irO H O CNCH2C( ° ) ° CH2CH2 ° CH3 DMF ^ \ = / piperidine OCH3 17 NaCN OCH C (0 )0C H 2CH20C H 3 CN 18 HO Ac Pb(OAc)4 CN OCH CN OCH3 19 C (0)0C H 2CH20C H 3 BBr3 -78° C C (0 )0C H 2CH20 H Fig. 3.6 Schem e for the synthesis of tri-hydroxy functionalized chrom ophore (compound 2 0 ) for incorporation into a therm osetting polyurethane system. 113 IH O v ^ n a v OH AcOv a n ^ v OAc 1 (CH3CO)2Or 1 POCI3 pyridine D M F 21 CH2OP(OEt)2 AcO. ^ n" V ° A C 0 ^ 1 A c O ^ /^ O A c < > k2 co3 K+ t-BuO" CHO 22 MeOH / H20 23 H C V ' N'V OH HOv^ N-V OH NC CN NC CN DMF 24 n c - t ^ c n CN 25 Fig. 3.7 Schem e for the synthesis of tricyanovinyl-functionalized chromophore (compound 25) for condensation polymerization. 114 TCNE, DMF O CH3 s o -c -c = ch2 N V NC 26 Fig. 3.8 Scheme for the synthesis of tricyanovinyl-functionalized chromophore (compound 26) for radical polymerization. 115 Compound 7 A 500 ml round-bottom flask equipped with an addition funnel and magnetic stirrer w as charged with methyl phenyl ethanolam ine (20 g, 0.13 mol) and pyridine (14.4 g, 0.182 mol) in 75 ml dry tetrahydrofuran, THF. The solution w as cooled to 0 °C and methacryloyl chloride (19 g, 0.182 mol) w as added dropwise. The resulting mixture w as stirred overnight and poured into iced-water. After subsequent work-up and removal of solvent, the crude product w as purified by column chrom atography using 4:1 dichloromethane-.hexane a s eluent to yield a brown viscous oil ( 27.54 g, 96.5% yield). 1H NMR(CDCl3) relative to TMS: (in ppm) 1.89 (s, 3H, -CH3); 2.96 (S, 3H, -CH3); 3.6 (t, 2H, -CH2); 4.29 (t, 2H, -CH2); 5.5 (s, 1H, =CH); 6.04 (s, 1H, =CH); 6.7 (d, 2H, phenyl Hs); 7.21 (m, 3H, phenyl Hs). Compound 8 P hosphorus oxychloride, POCI3 (24 g, 0.156 mol) w as added dropw ise into a round-bottom ed flask c h arg ed with 2 0 0 ml of dimethylformamide, DMF at 0° C. After com plete addition, the solution w as stirred at 0 °C for one hour and then at room tem perature for an additional hour. Compound 7 (27.54 g, 0.13 mol) w as then added and the mixture heated at 90°C for six hours. The reaction mixture w as cooled and poured into iced-water and extracted several tim es (3 x 100 ml) with dichlorom ethane. The com bined organic extracts w as w ash ed with saturated sodium bicarbonate (3 x 50 ml), then with w ater with a small 116 am ount of ammonium chloride, NH 4 CI (3 x 50 ml) and dried over m agnesium sulfate, M gS04. The crude product w as purified by column chromatography on silica gel (dichloromethane) yielding the pure aldehyde in 87 % yield. 1H NMR(CDCl3) relative to TMS (in ppm): 1.89 (s, 3H, -C H 3); 3.11 (s, 3H, -CH3); 3.76 (t, 2 H, -CH 2); 4.35 (t, 2 H, -C H 2); 5.5 (s, 1 H, =CH ); 6.04 (s, 1H, =CH); 6.78 (d, 2 H, phenyl Hs); 7.75 (d, 2H, phenyl Hs); 9.74 (s, 1 H ,-CHO). Compound 9 Com pound 8 (19 g, 0.06 mol) w as reacted a s d escribed for com pound 3 using (8.08 g, 0.072mol) of potassium tert-butoxide and (16.87 g, 0.072 mol) com pound 2. The crude product w as purified by column chromatography (4:1 ether:hexane) to yield the desired stilbene as a yellow solid (g, % yield). 1H NMR(CDCI3) relative to TMS (in ppm): 1.92 (s, 3H, -CH3); 3.03 (s, 3H, -CH3); 3.68 (t, 2H, -CH2); 4.30 (t, 2 H, -CH2); 5.56 (s, 1H, =CH); 6.06 (s, 1H, =CH); 6.74 (d, 2H, phenyl Hs); 6 .8 8 (s, 1H, =CH); 6.99 (m, 3H, thiophene Hs); 7.23 (s, 1H, =CH); 7.33 (d, 2H, phenyl Hs). Compound 10 Formylation of com pound 9 (10 g, 0.03 mol) w as achieved by using the procedure used for product 8 using (5.6 g, 0.036 mol) of phosphorus oxychloride in 75 ml of dimethylform am ide, DMF. After 117 subsequent work-up, the crude product w as p assed through a silica gel column to afford compound 10 a s a bright orange solid (6 .8 g, 65% yield). 1H NMR(CDCl3) relative to TMS (in ppm): 1.91 (s, 3H, -CH3); 3.05 (s, 3H, -CH3); 3.69 (t, 2 H, -CH2); 4.33 (t, 2 H, -CH2): 5.56 (s, 1 H, =CH); 6.06 (s, 1 H, =CH); 6.75 (d, 2H, phenyl Hs); 6.97 (s, 1H, =CH); 7.05 (d, 1H, thiophene H); 7.26 (s, 1H, =CH); 7.37 (d, 2H, phenyl Hs); 7.63 (d, 1H, thiophene H); 9.82 (s, 1H, -CHO). Ethanol-2-cyanoacetic acid (11)1 5 Cyanoacetic acid (20 g, 0.24 mol) and ethylene glycol (58.35 g, 0.94 mol) in 150 ml toluene w as vigorously stirred and refluxed for 24 hours using a Dean-Stark trap. Toluene sulfonic acid m onohydrate (1g) w as used a s a catalyst. The solution w as poured into 200 mlwater and extracted several tim es with m ethylene chloride (5 x 50 ml). The com bined organic extracts w as w ashed with brine (3 x 50 ml), w ater (3 x 100 ml) and dried over m agnesium sulfate, MgS0 4 .overnight After removal of solvent, the transesterification product w as recovered and used without further purification (19 % yield). 1H NMR(CDCl3) relative to TMS (in ppm): 3.73 (s, 2 H, CN-CH2); 3.87 (t, 2 H, -CH2); 4.14 (t, 2 H, -CH2); 5.05 (br, 1H,-OH). 118 Compound 12 Product 11 (2.2 g, 0.012 mol) w as reacted with com pound 10 (5 g, 0.01 mol) a s described in the procedure for synthesizing com pound 1 2 . The crude product w as purified by colum n chrom atography using chloroform a s eluent to afford 12 as a red solid (1.2 g, 25.75% yield). 1H NMR (DMSO) relative to TMS(in ppm): 1.84 (s, 3H, -C H 3 ); 3.08 (s, 3H, -C H 3); 3.63 (t, 2 H, -C H 2); 3.86 (t, 2 H, -C H 2); 4.27 (t, 2H, -C H 2); 4.35 (t, 2H, -C H 2); 5.49 (s, 1H, =CH ); 5.99 (s, 1H, =C H ); 6.65 (d, 2 H, phenyl Hs);6.91 (s, 1H, =CH); 6.99 (d, 1H, thiophene H); 7.19 (s, 1H, =C H ); 7.35 (d, 2H, phenyl Hs); 7.55 (d, 1H, thiophene H); 8.19 (s, 1H, =C H ); 13C NMR (in ppm): 19.89, 26.3, 27.4, 52.1, 63.4, 65.3, 78.8, 82.1, 100.5, 112.4, 111.5, 117.3, 124.9, 128.6, 129.2, 132.8, 137.5, 140.0, 144.4, 153.7, 168.9, 172.1, 178.6. Elem ental analysis: C alculated for C24H26N2O 5S: C, 63.42; H, 5.77; N, 6.16 Found: C, 63.74; H, 5.34; N, 6 .1 1 . Compound 13 Pow dered potassium hydroxide, KOH (74,32 g, 1.32 mol) w as added to 500 ml dimethylsulfoxide in a 1000 ml round-bottom flask. After stirring for 15 m inutes, N-phenyldiethalnolamine (30 g, 0.17 mol) w as ad d ed , followed immediately by methyl iodide (94g, 0.68 mol). The reaction mixture w as left to stir overnight. After which, the mixture w as 119 poured into w ater and extracted several tim es with dichlorom ethane, CH2CI2 (3 x 2 0 0 ml). The combined organic extracts w as w ashed with w ater (5 x 100 ml) and dried over m agnesium sulfate, MgSC>4 . The product w as used for the subsequent steps without further purification. 1H NMR (CDCI3) relative to TMS( in ppm): 3.5 (s, 6 H, -CH3); 3.59 (t, 8 H, -CH2); 6.7 (multiplet, 3H, aromatic Hs); 7.22 (d,2 H, aromatic Hs). Compound 14 C om pound 14 w as synthesized following the procedure for com pound 8 using the following materials: 13 (15 g, 0.08 mol), POCI3 (13.8 g, 0.09 mol) in 100 ml DMF. Colum n chro m ato g rap h y (dichlorom ethane/hexanes) afforded the desired product in 89.6% yield. 1H NMR (CDCI3) relative to TMS (in ppm): 3.4 (s, 6 H, -CH3): 3.6 (t, 4H, -CH2); 3.66 (t, 4H, -CH2); 6.7 (d, 2 H, aromatic Hs); 7.69 (d, 2 H, aromatic Hs); 9.72 (s, 1H,-CHO). 2-chloromethylthiophenene triphenylphosphonium salt (15) A 500-ml round bottom flask fitted with a reflux co n d en ser and stirring bar w as charged with 3 (20g, 0.15 mol) and triphenyl phosphine (g, 0.15mol) in toluene (100 ml). The reaction vessel w as subm erged into an oil bath that had been preheated to 90 °C and the mixture stirred for 72 hours. After cooling, the resulting product w as filtered and several tim es with anhydrous diethyl ether (2 x 100 ml). No further purification w as 120 necessary and the reagent w as used directly for the Wittig condensation step. Compound 16 Compound 16 w as synthesized using typical Wittig condensation procedure a s follows. Sodium ethoxide (33 ml, 2.0 M in ethanol) w as added dropwise to a stirred solution of the aldehyde 8 (15 g, 0.06 mol) and triphenylphosphonium salt 15 in 50 ml dry tetrahydrofuran, THF. The reaction mixture w as heated overnight at approximately 50-60 °C. W ater (500 ml) w as added and the solution stored in the freezer overnight. The yellow solid w as filtered and redissolved in m ethylene chloride (50 ml). The organic layer w as w ashed with brine (2 x 50 ml), w ater (2 x 50 ml) and dried over m agnesium sulfate, MgSC>4 . Purification by silica gel chrom atography (4:1 CH2Cl2:hexanes) afforded the d esired stilbene (85.7% yield). 1H NMR (CDCI3) relative to TMS (in ppm): 3.35 (s, 6 H, -CH3); 3.58 (t, 8 H, -CH2); 6.7 (d, 2H, phenyl Hs), 6.9 (s, 1H, =CH); 6.97 (multiplet, 3H, thiophene Hs); 7.26 (s, 1H, =CH); 7.31 (d, 2H, phenyl Hs). Compound 17 Com pound 17 w as synthesized according to the procedure employed in making compound 4 using the following starting materials: 16 (5 g, 0.016 mol), nBuLi, 1.6 M in hexanes (11 ml, 0.0176 mol), TMEDA (2.05 g, 0.0176 mol), 3N HCI ( 20 ml) in 20 ml of dry ether. The aldehyde 121 product w as recovered by purification of the crude material on a silica gel column (4:1 hexanes: ethyl acetate) in 79.6% yield. 1H NMR (CDCI3) relative to TMS (in ppm): 3.39 (s, 6 H, -CH3); 3.5 (t, 8 H, -CH2); 6.68 (d, 2 H, phenyl Hs); 6.94 (s,1H, =CH); 7.05 (d, 1H, thiophene H); 7.26 (s, 1H, =CH); 7.34 (d, 2H, phenyl Hs); 7.61 (d, 1H, thiophene H); 9.8 (s, 1H, -CHO). Compound 18 This material w as synthesized according to the procedure for making com pound 5. However, in order to prevent transesterification, tetrahydrofuran, THF w as used a s solvent. The following starting materials and their correspondng am ounts were used: 17 (4 g, 0.011 mol), 2 -m ethoxyethylcyanoacetic acid (1.82 g, 0.013 mol), piperidine (0.2 ml). The crude product w as purified by flash chrom atography (2:1 ethyl acetate: hexane) to yield a red solid (77% yield). 1H NMR (CDCI3) relative to TMS (in ppm): 3.36 (s, 6 H, -CH3); 3.43 (s, 3H, -CH3); 3.55 (t, 8 H, -CH2); 3.72 (t, 2H, -CH2); 4.43 (t, 2H, -CH2); 6.71 (d, 2 H, phenyl Hs); 6.94 (s, 1H, =CH); 7.15 (d, 1H, thiophene H); 7.26 (s, 1H, =CH); 7.39 (d, 2H, phenyl Hs); 7.61 (d, 1H, thiophene H); 8.25 (s, 1H, CN=CH). Compound 19 Com pound 19 w as synthesized according to the procedure for com pound using the following materials: 18 (3.5 g, 7.4 mmol), 4N NaCN 122 (2.4 ml), cone HOAc (15 ml), Pb(OAc)4 (3.28g, 7.4 mmol). The crude product w as purified by flash chrom atography (1 :1:1 ethyl a cetate: m ethylene chloride: hexanes) to afford the title com pound in 76% yield. 1H NMR(CDCI3) relative to TMS(in ppm):3.33 (s, 6 H, -CH3); 3.45 (s, 3H, -CH3); 3.57 (t, 8 H, -CH2); 3.71 (t, 2H, -CH2); 4.43 (t, 2 H, -CH2); 6.78 (d, 2H, phenyl Hs); 7.35 (d, 1H, thiophene H); 7.38 (s, 1H, =CH); 7.40 (s, 1H, =CH) 7.51 (d, 2H, phenyl Hs); 8.09 (d, 1H, thiophene H). Compound 20 A solution of 19 (3 g, 6.0 mmol) in 15 ml of dichlorom ethane in a 250-ml round bottom flask w as cooled to -78° C. Boron tribromide, 1.0 M in dichlorom ethane (36 ml, 36 mmol) w as added dropwise and stirred at -78 °C for 30 minutes. The tem perature w as raised to -10 °C and the mixture w as stirred for an additional four hours. The reaction w as quenched by adding 3 ml of diethyl ether. W ater w as then added and the resulting solution w as stirred for 30 minutes. The solution w as extracted with ethyl acetate and following subsequent work-up yielded a green solid. Flash chrom atography using acetone as eluent afforded the desired triol (1 g, 24% yield). 1H NMR(DMSO-de) relative to TMS (in ppm): 3.66 (t, 4H, -CH2); 3.82 (t, 4H, -CH2); 4.3 (br, 3H, -OH), 4.83 (t, 2H, -CH2); 5.01 (t, 2H, -CH2); 6.78 (d, 2H, phenyl Hs); 7.35 (d, 1H, thiophene H); 7.38 (s, 1H, =CH); 7.56 (m, arom atic Hs); 8.05 (d, 1H, thiophene H); 13 c NMR (in ppm): 53.1, 57.9, 58.1, 100.8, 110.9, 111.6, 113.7, 114.8, 116.1, 122.6, 125.9, 126.4, 129.5, 131.7, 136.4, 140.7, 149.5, 160.5, 162.1, 189.6. 123 Calculated for C23H23N3O5S: C, 60.91; H, 5.11 ; N, 9.27. Found: C, 60.81; H, 5.15; N, 9.24. Compound 21 Protection of N-phenyldiethanolamine (10 g, 0.06 mol) with acetate groups w as done by refluxing with acetic anhydride (9.38 ml, 0.132) and pyridine (10.4 g, 0.132 mol). After cooling to room tem perature, the m ixture w as poured into w ater and ex tracted sev eral tim es with dichlorom ethane (3 x 50 ml). The combined organic extracts w as w ashed with brine (2 x 50 ml), w ater (2 x50 ml) and dried overnight over sodium sulfate, N a2 SC>4 . The crude product w as purified by colum n chrom atography using dichlorom ethane/ethyl acetate a s eluent (91% yield). 1H NMR(CDCl3) relative to TMS (in ppm): 2.05 (s, 6 H, -OCH3); 3.62 (t, 4H, -CH2); 4.24 (t, 4H, -CH2); 6.79 (m, 3H, phenyl Hs); 7.74 (d, 2 H, phenyl Hs). Compound 22 21 (11 g, 0.05mol) w as reacted with phosphorus oxychloride (9.2 g, 0.06 mol) a s described for compound 8 to afford the title com pound (87.8% yield). Purification w as done by column chrom atography using ethyl acetate/hexane a s eluent. 1H NMR(CDCl3) relative to TMS (in ppm): 2.02 (s, 6 H, -CH3); 3.7 (t,4H, -CH2); 4.25 (t, 4H, -CH2); 6.7 (d, 2H, phenyl Hs); 7.7 (d, 2H, phenyl Hs); 9.7 (s, 1H, -CHO). 124 Compound 23 Com pound 23 w as synthesized via a typical H orner-Em m ons reaction as described for the synthesis of com pound 3. The following materials were used: 22 (10 g, 0.04 mol), 2 (10.8 g, 0.046 mol), potassium tert-butoxide (5.16 g, 0.046 mol). The crude product w as purified by column chromatography (5:1 hexane:dichloromethane) to yield the desired stilbene (71% yield). 1H NMRfCDCh) relative to TMS(in ppm): 205 (s, 6 H, -CH3); 3.6 (t, 4H, -CH2); 4.25 (t, 4H, -CH2); 6.76 (d, 2H, phenyl Hs); 6.87 (d, 1H, =CH); 6.99 (m, 3H, thiophene Hs); 7.34 (d, 2H, phenyl Hs). Compound 24 Compound 23 (5 g, 15.8 mmol) w as dissolved in 100 ml of a 10:1 mixture of m ethanokw ater. The mixture w as treated with potassium carbonate (4.79 g, 34.66 mmol) and warmed to 40° C overnight. The resulting solution w as concentrated en vacuo and extracted several times with dichlorom ethane (3 x 50 ml). The com bined organic extracts w as w ashed with brine (2 x 30 ml) then with water (2x 30 ml) and dried over sodium sulfate, Na2S 0 4 . The crude product w as purified by column chrom atography (1 : ethyl acetate/hexane) to give the desired product a s a tan solid (3.1 g, 6 8 % yield). 1H NMR(CDCl3) relative to TMS (in ppm): 3.69 (t, 4H, -CH2); 4.27 (t, 4H, -CH2); 6.76 (d, 2H, phenyl Hs); 6.87 (s, 1H, =CH); 6.97 (m, 3H, thiophene Hs); 7.02 (s, 1H, =CH); 7.37 (d, 2H, phenyl Hs). 125 Compound 25 A round-bottom flask containing 25 ml dimethylformam ide w as cooled to 0 °C and charged with com pound 24 (3 g, 0.01 mol) and tetracyanoethylene, TCNE (1.54 g, 0.012 mol). The solution w as stirred for at least 72 hours and poured into crushed ice. The resulting precipitate w as filtered and redissolved in 20 ml of dichlorom ethane. The organic solution w as w ashed with brine ( 3 x 20 ml) , w ater (2 x 20 m l) and dried over calcium chloride. The crude product w as purified by chrom atography (3:1 ethyl acetate: hexane) to give a fine black solid (1.15 g, 29.5% yield). 1H NMR(DMSO-d6) relative to TMS (in ppm): 3.69 (t, 4H, -CH2); 4.27 (t, 4H, -CH2); 6.80 (d, 2 H, phenyl Hs); 7.08 (s, 1H, =CH); 7.19 (d, 1 H, thiophene H); 7.26 (s, 1H, =CH); 7.46 (d, 2H, phenyl Hs); 7.95 (d, 1H, thiophene H); 13C NMR(DMSO-de): 2.5, 54.3, 58.7, 76.3, 113.7, 116.5, 118.2, 124.1 128.1, 130.5, 132, 140, 145.3, 152.1, 161.8.Elem ental analysis: Calculated for C21H18N4O2S: C, 64.6; H, 4.65; N, 14.3. Found: C, 65.02; H, 4.33; N, 14.78. Compound 26 The sam e procedure w as used a s in com pound 25 using the following starting materials: 10 (5 g, 0.015 mol), TCNE (2.35 g, 0.018 mol) in 20 ml DMF. The crude product w as purified by flash chrom atoraphy using 1:2:1 ethyl acetate: hexane: dichlomethane to give a green solid (5 g, 78% yield). 1H NMR (CDCI3) relative to TMS (in ppm): 1.91 (s, 3H, 1 2 6 -CH3); 3.1 (s, 3H, -C H 3); 3.743 (t, 2 H, -CH2); 4.35 (t, 2 H, -CH2); 5.56 (s, 1H, =CH); 6.09 (s, 1H, =CH); 6.77 (d, 2H, phenyl Hs); 7.07 (s, 1H, =CH); 7.26 (d, 1H, thiophene H); 7.46 (d, 2H, phenyl Hs); 7.95 (d, 1H, thiophene Hs); 13C NMR (DMSO-d6): 17.5, 24.7, 26.3, 51.2, 63.2, 66.3, 77.5, 82.3, 113, 115.5, 116.2, 128.3, 129.4, 130, 135.8, 139.3, 142.8, 151.3, 158.3, 162.1, 167.9. Calculated for C24H20N4O2S: C, 67.27; H, 4.7; N, 13.1. Found: C, 67.53; H, 4.52; N, 13.5. 3.2.d Synthesis of polymers The polymerization reactions w ere done according to the sch em es illustrated in Figs. 3.9-3.12 and are described a s follows: i) Polymer P1 A polymerization tube w as charged with com pound 12 (0.1 g, 0.2 mmol), methyl methacrylate, MMA (0.07 g, 0.7 mmol) and 1 ml dry DMF. The radical initiator, AIBN, w as added. The tube w as cooled in an acetone-dry ice bath, d eg assed for at least half an hour and sealed under vaccuum . The polymerization solution w as stirred at 65 °C for 48 hours, cooled to room tem perature and poured into 100 ml m ethanol. The methanol solution w as vigorously stirred and the resulting precipitate w as filtered. The solid w as redissolved in a minimum am ount of DMF and reprecipitated in methanol to obtain 0.12 g a red solid. 1H NMR(DMSO- d6) relative to TMS (in ppm): 0.4-1.2 (m, vb); 1.7-2.1 (m, vb); 3.2 (m, br); 127 o I I V V 0_C“ ? =CH* CH NC I I O CH3 c = ch2 o=c I o ch3 1) AIBN (initiator) in dry DMF 2) seal under vacuum 3) stir at 70 °C for 48 hours H2C -C ch2ch2 ^ C ( 0 ) 0 C H 2CH20 H CN P1 Fig. 3.9 Schem e for the synthesis of polymer P1. 1 2 8 NC ^ -C (0 )0 C H 2CH20 H OCN -at NCO stir at 75°C for 30 minutes P R E P O LY M E R N \ C = 0 cast into thin film heat, pole, crosslink C = 0 O I N U II I I s /F -C (0 )C H 2CH20 - C — -N — ] |^W V ' i H P2 STABILIZED 3-D NETW ORK OCH, O C N -| |-N C O = O C N - ^ - C H g O C N - ^ - ^ - N C O CH30 OCN XL1 XL 2 Fig. 3.10 Scheme for the synthesis of thermosetting polyurethanes P2-XL1 and P2-XL2. 129 o tl ' . , ^ 0 -c "C=CH2 c h 3 c = c h 2 o= c + I o ch3 1) AIBN (initiator) in dry DMF 2) seal under vacuum 1 1 3) stir at 70 °C for 48 hours C=CH2 i o = c , ? H*n f ^ N f h 2c - c — 4 — ( - h 2c - c ------ t f h2c c ---------- i — " C ,J S A = n 7 3rT C=0 3n OCH3 ch2 ch2 OH P3 Fig. 3.11 Schem e for the synthesis of Polymer P3. 130 H C V m^ 0H NC-t^CN CN o o I I /= \ I I n Cl—C -^ _ ^ -C -C I + c h 3 n HOCHo-C-CHoOH i c h3 pyridine DMF Fig. 3.12 Schem e for the synthesis of Polymer P4. 131 3.5-3.65 (m, vb); 4.25 (d, 2H); 4.9 ( d , 2H); 6.8 (b, 2H); 7.1-7.3 (m, vb, aromatic Hs); 7.4 (b, 2H); 7.94 (b, 2H); 8.5 (b, 1H). ii. Polymer P2-XL1 Compound 20 { 0.1 g, 0.2 mmol) was dissolved in a 10:1 mixture of dioxane and dimethylformamide. Toluene diisocyante (TDI) (0.02 g, 0.12 mmol) w as added and the solution was stirred at 70° C for 30 min. The resulting solution will be the prepolym er used for su b seq u en t film processing and NLO m easurem ent. iii) Polymer P2-XL2 The sam e procedure a s in P2-XL1 w as em ployed. Dianisidine diisocyante (0.032 g, 0.12 mmol) w as used instead of TDI. iv) Polymer P3 Com pound 26 (0.1 g, 0.21 mmol) w as radically polym erized according to the procedure for polymer P1. The following com onom ers w ere used: MMA (0.04 g, 0.426 mmol) and hydroxyethyl m ethacrylate (HEMA) (0.028 g, 0.21 mmol). AIBN (0.003 g) w as u sed a s radical initiator. The final product w as a blue solid (0.83 g) which w as reprecipitated twice in methanol. 1H NMR(CDCl3) relative to TMS (in 132 ppm): 0.7-1.19 (m, vb); 1.82-2.1 (m, vb); 3.53 (m, vb); 4.27 (b, 2H);4.80 (b, 2H); 6.78 (b, 2H); 7.3-7.8 (m, vb, aromatic Hs); 8.09 (b, 2H). v) Polymer P4 Recrystallized terephthaloyl chloride (0.53 g, 3 mmol) and 2 ,2 - dimethyl propanediol (0.16 g, 1.5 mmol) in 10 ml DMF w as added dropwise into a 20 ml stirred solution of com pound 25 (0.5 g, 1.5 mmol) and pyridine in DMF. The solution was refluxed overnight, colled to room tem perature and precipitated in 200 ml of 1/1 m ethanol/water mixture. A blue powder w as recovered (0.32 g). 1H NMR (CDCI3) relative to TMS (in ppm): 0.9-1.2 (m, vb); 3.5 (m,vb); 4.1 (m, vb); 4.2-4.4 (b, 2H); 4.5 (m, vb); 6.7 (br, 2H); 7.4-7.7 (m, vb, aromatic Hs); 8.02 (b, 2H). 3.2.e Film processing and electric field poling The polymers w ere dissolved in a 10:1 mixture of dry dioxane:DM F to form a 7 -1 0 % solution. The solutions were filtered through a 0.2 p. syringe filter and spin coated onto indium-tin oxide (ITO) slides. The films were dried at 40° C under vacuum . The thickness of the films were m easured using a Dektak II m easurem ent system . Electric field poling w as done using the voltage listed in Table 3.1 The electric field poling set-up used to pole the films is illustrated in Fig. 3.13.16 A needle electrode, with a tip to plane distance of 1.5-2.0 cm, w as 133 Table 3.1 Experimental conditions for poling the NLO polymers. Polymer Temperature, °C Time, min Poling voltage, kV P1 a. 130 a. 60 7 -9 b. 170 b. 60 P2-XL1 a. 100 a. 60 9 b. 120 b. 60 P2-XL2 100 60 9 P3 83 60 4 P4 100 90 3 Note: Two entries denote stepwise heating and/or poling (i.e. condition a followed by condition b). 134 Needle Electrode " © © © + High Voltage Supply Si wafer _ A TE Coupler Heater Multimeter Fig. 3.13 Electric-field poling set-up (from ref. 16). 135 connected to a high voltage supply and w as used to discharge charges to pole the chrom ophores. The maximum voltage can be a s high a s 25 kV. The tem perature of the films can be adjusted from room tem perature to 150 °C. The films were poled in several step s at program m ed tem peratures to ensure high poling efficiciency and crosslinking density while mainitaining the optical quality of the films. The listed poling voltages in Table 3.1 w as the maxim um volatge used. Likewise, the tem perature w as th e final polng tem perature used. The time listed w as the poling duration at the poling tem perature for each film. The listed poling conditions w ere the optimum conditions found from a num ber of conditions tested and the the results in this chapter w ere obtained using these poling schedules. 3.2.f NLO Characterization i. Second-Harmonic Generation The second-harm onic generation m easurem ent (SHG) set-up is shown in Fig. 3.14. The fundam ental laser beam source used w as a Spectro-Physics DCR-11 mode-locked Q-switched Nd:YAG laser (A, = 1.064 pm ) with a pulse width of 10 ns and repetition rate of 10 Hz. This filtered infrared laser beam was split into a signal beam and a reference beam . A Y-cut quartz crystal, positioned at approximately 6 ° angle to the incident laser beam and the sam ple filter w as at 45° angle to the incident beam w as used a s a reference. The 136 s 1.064 5 3 2 0.532 ND BS Sample Nd: YAG laser X=1.064 pm 1.064 ^ RS 3 2 R1 0 6 4 Ref IRF 1.064 • 1064 Q-switch control ,1.064 ND 0.532 D2 Boxcar Integrator Trigger Fig. 3.14 Experimental set-up for second-harmonic generation m easurem ents (from ref. 16) 137 neutral density filter (ND) w as used to control the second-harm onic signal intensity to obtain a large signal range. The effective SHG coefficient of a NLO polymer, d ef f> w as calculated from the com parison of the second harmonic signals between the reference and the sam ple using the following equation:16 w here d n is the second harmonic coefficient of the reference, I is the SHG intensity, n2a>the refractive index at second harmonic (SH) wavelength, Lc is the coherence lengtn, T2a>is the transm ittance at the SH w avelength, tw the am plitude transission coefficient at fundam ental w avelength, I the absorption coefficient of the polymer film at 2w, L the thickness of the polymer film, and the subscripts p and r denote polymer sam ple and reference, respectively. The refractive indices w ere m easured at three w avelengths using the four-zone averaging m ethod with an ellipsom eter. The coherence length (lc) can be calculated from the refractive indices (no and n2o a) and the transm itted angles (0to and 02(o) by the equation:16 (3.1) c 4 (n2(0 cos 02 to - nw cos 0ffl) (3.2) 138 For the p-polarized incident fundam ental beam that w as used in the m easu rem en t, d33 of the polymer sam ple can be obtained after geom etry correction by the equation: deff = f t C O S ' em + sin' 0m I sin 02 o ) + _2 3 cos0(oSin0(oCOS2( C O d 33 (3-3) ii. Electro-optic coefficient measurement The electrooptic coefficient r w as m easured using the attenuated total reflection (ATR) method. The experimental set-up w as based on the design by Herm inghaus et al. and is shown in Fig. 3.15.17 In the ATR technique, optical quality films of the polymers were spin-coated onto a glass-ITO sustrate and the films dried in the vacuum oven overnight. The polymer films w ere poled and overlayed on a prism containing a 100 A layer of chrom e. The multi-layer structure w as then subjected to a laser beam (X = 1064 nm). The reflectivity of the laser probe beam with the transverse electric (TE) and transverse magnetic (TM) polarizations w ere recorded for a given range of incidence angles. At certain angles, the reflectivity corresponding to the TE and TM m odes will exhibit dips which occurs when a waveguiding resonance condtion is met. Fitting th ese experim ental shifts to a se t of theoretial reflectivity curves, field-induced chang esp o led . The poled polymer film w as then overlayed on a prism containing a 100 of njE and njM can be obtained and so are the electro-optic coefficients r13 and r33 by using the following relationships: 139 glass ITO active material laser polarizer detector lock-in amplifier oscillator Fig. 3.15 Experimental set-up for the ATR method to m easure electrooptic coefficients (from ref. 17). 140 where E is the electric field and njE and njM are refractive indices. 3.2.g. Thermal stability measurements The real-time NLO stability of the polymer films w as studied a s a function of tem perature by an in-situ SHG m easurem ent w hose configuration is shown in Fig. 3.16.16 This set-up allows for the determ ination of maximum device processing tem perature and is a quick evaluation of tem poral and therm al stability of the materials. The tem perature of the film w as controlled by the current flow of an external power supply through the ITO layer on the back side of the glass substrate to avoid possible charge injection effects. A therm ocouple anchored to the polymer film w as used to monitor the film tem perature. The tem perature ramping rate w as controlled by an O m ega CN6081 ram p-soak tem perature controller, which has an experimental error of ± 4 °C. The SHG signal and tem perature readings were recorded in a chart recorder. The tem poral stability of the crosslinked polym er films at elevated tem peratures w as studied by monitoring the SHG signals as a function of time. After the poling electric field w as removed, the films were continuously annealed Digital Ramp/Soak Tem perature Controller Process T emp I l |7 |4 |*C Set Point Temp f 11 7 15 1 - C Heater Output - o o ITO Glass Substrate' Heat Sink Compound Therm o couple /-=532 nm /-=1064 nm X= 1064 nm (a). Direct Heating Method. To To Digital Tem peraure To + H eater Controller - Heater O utput ^ | Output Thermo couple 7^=1064 nm 7-=532 nm 7»=1064 nm ITO Heater (b). Indirect Heating M ethod. Polymer Glass Substrate Fig. 3.16 Experimental set-up for the m easurem ent of real time dynamic thermal stability of NLO activity (from ref. 16). 142 in the oven at 100 °C, respectively under air. At various time intervals, th ese films were taken out of the oven and their SHG coefficients w ere m easured. 3.3 Results and Discussion 3.3.a. Model Compound The model com pound 8 w as synthesized through a series of synthetic tech n iq u es starting from 4-dim ethylam ino b en zald eh y d e (4-DMAB). A n ecessary intermediate, com pound 2 , w as achieved from a tw o-step reaction starting from commercially available 2 -thiophenem ethanol. The first step involved the chlorination of 2 -thiophenemethanol using thionyl chloride to yield 2-chloromethylthiophene 1. Reaction of compound 1 with an equimolar am ount of triethyl phosphite under the conditions of a Michaelis-Arbuzov reaction yielded 2 -thiophene methylenediethyl phosphonate 2 . The formation of the stilbene bridge, compound 5 w as accom plished by a typical Horner-Emmons reaction of 4-DMAB with compound 2 in the presence of potassium tert-butoxide in dry tetrahydrofuran. The Horner-Emmons reaction or W adsworth-Emmon reaction is a type of Wittig reaction wherein the ylide used w as prepared from phosphonates. The reaction is a s follows: 143 R R i base I — C = 0 (RO)2P-CH-R (RO)2P-C-R' ii ii o o — C=C-R" + (R 0)2P 0 3- R’ O ne advantage of this reaction over the Wittig method is that the phosphorus product is a phosphate ester and hence soluble in water. This m akes the separation of the olefin product a lot easier.18 The succeeding formylation step using BuLi and dimethylform am ide resulted in the formation of com pound 4. Knovenagel condensation of 4 with m ethylcyanoactetate in ethanol with catalytic am ount of piperidine afforded com pound 5. The Knoevenagel reaction is a condensation reaction betw een aldehydes or ketones usually not containing an a-hydrogen with com pounds of the form Z- CH2-Z or Z-CHR-Z. O thers limit the use of this nam e to only som e of the active hydrogen com pounds that give the reaction. In this case Z and Z' are -CN and -C(0 )0 CH3 , respectively.18 An NMR analysis of the product indicated the presen ce of a triplet at 1.46 ppm an d a q u artet at 4.42 ppm. The methyl p eak from the m ethylcyanoactetate w as surprisingly absent. The new peaks correspond to the 144 -CH2 CH3 group which can be attributed to a transesterification reaction betw een ethanol and the methylcyanoacetate. The general procedure for addition of another nitrile group to com pound 5 w as to add hydrogen cyanide to the double bond.19 This w as accom plished by addition of a q u eo u s sodium cyanide to a dim thylform am ide solution of com pound 5. Acidification by addition of hydrogen chloride forms the adduct Y which w as accom panied by a color change from red to pink. The adduct w as oxidized to yield the final product by adding an equim olar am ount of lead tetraacetate. Extra care w as m ade in ensuring only an equivalent am ount of lead tetraacetate w as used to avoid oxidation of the final product. The proton and carbon NMR spectra of com pound 5 are shown in Figs. 3.17 and 3.18, respectively. NC H ,s ,H "CN C (0 )0E t Adduct Y 145 r™.......|M M H Fig. 3.17 n y 11 i ii i i i 111 .............i i | m i m i i | n i m m | i n m i i i | M i m m | H Tv. iiM|- e.O 6 .0 4 .0 2 .0 PPM 1H NMR (in CDCI3) spectrum of com pound 6 . 1 5 0 . Fig. 3.18 0 1 0 0 . 0 5 0 . 0 PPM 13C NMR (in CDCI3) spectrum of compound 6 . 147 The absorption spectrum (Fig. 3.19) consists of two absorption bands, an intense band at around 580 nm and a w eaker absorption band at lower regions. The latter band is assigned to an intermolecular charge transfer (ICT) resulting from the excitation from the donor to acceptor group. The model com pound also displays solvatochrom ic effect as seen from the absorption spectra in different solvents (Fig. 3.20). This is a very good indication of a large ch ange in the dipole moment between the ground and excited states. Solvatochrom ic m easurem ent is a very useful synthetic guideline and screening m ethod for m aterials which have potential u ses for second order applications. This method w as used by de Martino et al.20to calculate the dipole moment difference Ap betw een ground and excited states. In this m ethod, the absorption spectra of a compound w as taken in solvents of varying polarity. The resulting spectral shifts can then be used to calculate p for certain classes of m olecules a s long a s certain guidelines are followed for its applicability. As an approximation tool however, it is sufficient to say that a com pound displaying solvatochrom ic behavior also exhibits a big difference in its dipole m om ent betw een its ground and excited states. Nonlinear optical characterization w as accom plished by a DC-electric field-induced second-harm onic generation (EFISH) method. The main idea of this m ethod is to achieve a noncentrosymmetric alignment of m olecules using a dc-electric field.21 The experim ent m easures the quantity pP, which com bines both the second-harm onic generation hyperpolarizability,p, and the ground state perm anent dipole moment. The model compound w as dissolved in a non-polar 148 Absorbance M i-s r o e i < 500 700 se e 9 3 8 430 603 Wavelength (nm) Fig. 3.19 Absorption spectrum of compound 6 in dioxane. 149 A bsorbance dioxane methanol 260 W avelength (nm) Fig. 3.20 Absorption spectra of compound 6 in dioxane (A .max = 580 nm) and in methanol (Xmax = 600 nm). 150 solvent, dioxane. This w as done in order to employ a Lorentz-Lorentz correction for the treatm ent of the local field effect.2 The m easurem ent w as done at a fundam ental wavelength of 1.907 pm. Since the maximum wavelength of the model com pound (580 nm) is far away from the wavelength at which the second harmonic signal is m easured (0.95 pm), the value m easured w as free from resonance enhancem ent. A com parison of the m easured pP value of the model com pound with similar system s can be found in Table 3.2. All values in this table w ere taken at 1.907 pm and using dioxane a s solvent. The model compound has a pP value of 1700 x 10-48 esu. This value is alm ost three tim es that of 4-dialkylamino-4'- nitrostilbene (DANS) which has been used a s prototype for poled polymer applications. Replacem ent of one phenyl ring with a thiophene ring only gives a marginal increase on the second-order nonlinearity. However, literature findings indicate a dram atic enhancem ent of nonlinearity when both benzene rings were replaced with thiophene rings. B ecause of synthetic e a se , only one thiophene ring w as replaced in this study. C om pared to a dicyanovinyl, the pP of the model com pound w as a bit higher because of an added electron-accepting strength d u e to the p resen ce of the carbonyl group. Katz h a s previously discussed that multiple groups can enhance the total electron-accepting ability of a su b stitu ted .7 Although the value is lower than a tricyanovinyl group, it is high enough to be considered for further functionalization and incorporation into a thermally stable polymeric system. 151 Table 3.2 Examples of second-order chromophores and their pP values. Compound Xmax (nm) In dioxane pp ( x 1< r48 esu) at X = 1.907 pm 424 580 CN ' \ J r a 468 1100 CN 513 1300 CN ___. , __^ S v /^ C O O E t * ■ ■ . / \ ■y \\ // i / N 'L-y CN 580 1700 CN / = \ ,__C N ',n_ C~^_ / - ^ - ^ cn a 630 6200 a from ref. 4 152 The model compound w as also characterized with regards to its stability at elevated tem peratures. The decomposition tem perature w as found to be 252.01 (Fig. 3.21). Furthermore, the optical absorbances were taken before and after rep eated heatings of a guest-host film of the m odel com pound in polymethyl m ethacrylate (PMMA). Optical scan s indicated no ch an g es in the absoprtion spectra of the films even after heating in the oven at 150 °C. T hese results indicate that the model com pound d oes not d ecom pose under the conditions of elevated tem perature alone. Combined with a sizable second-order nonlinearity and therm al stability, this model compound w as a very good candidate for incorporation into thermally stable polymeric system and will be topic of discussion in the succeeding paragraphs. 3.3.b. Double-Ended Crosslinkable system One way to incorporate the model compound into a hardened lattice is to utilize the double-ended crosslinkable approach. The model com pound needed to be asymmetrically functionalized with a vinyl group on one end and a hydroxyl group on the other end of the chromophore, a s shown below. CN C(0)CH2CH2OH 153 W EIGHT 100 From. 2 2 9 .0 0 C To. 2 7 5 .0 0 C Onsot a t. 252.01 75 50 25 0 d o . oo daToa d o . oo d o . oo o k o o d o .o o ife o o T em perature(C ) 1.00 Fig. 3.21 TGA curve of compound 6 (heating rate = 10° C/ min, Ar atm osphere). 154 The schem e to synthesize the bifunctionalized com pound is illustrated in Fig. 3.12. This w as accom plished by using N-methyl,N ethanol aniline a s starting material which w as reacted with methacryloyl chloride using the conditions of a Schotten-Baum ann reaction to yield compound 7. The succeeding step s up to the formation of the stilbene linkage w as similar to th at u sed for the model com pound. Formylation of com pound 9 w as done by using a Vilsmeier formylation reaction. This w as done to prevent hydrolysis of the m ethacrylate group, which would occur if the formylation w as done using n-butyllithium. The acceptor, ethanol-2-cyanoacetic acid (11) w as synthesized by a transesterification reaction betw een cyanoacetic acid and ethylene glycol. A ttachm ent of the accep to r group w as accom plished via K novenagel condensation reaction with compound 10. N um erous attem pts w ere m ade to attach an o th er nitrile group to com pound 12. However, all attem pts w ere not successful. Thus, the final m onom er has only one nitrile group and appeared as a bright red solid. The m onom er gave excellent results when characterized by different analytical techniques. The proton and carbon NMR spectra of compound 12 are shown in Fig. 3.22 and 3.23, respectively. The m onom er w as polymerized using radical polymerization conditions with AIBN a s initiator and dry DMF as solvent. The structure of the polymer Pi is shown in Fig. 3.9 The formation of the polymer w as confirmed by NMR and size exclusion chrom atography (SEC). The vinyl protons of the m ethacrylate 155 Fig. 3.22 1H NMR (in CDCI3) spectrum of compound 12. 156 1 5 0 .0 100.0 PPM 5 0 .0 Fig. 3.23 13C NMR (in DMSO-de) spectrum of com pound 12. 157 group (1H: 6.07, 5.54 an d 1.93 ppm) d isap p eared after polym erization accom panied by the broadening of the other p eak s (Fig. 3.24). SEC m easurem ents indicated an Mn of 65,000 and Mw of 98,000. The thermal properties of polymer P1 were studied using DSC and TGA. The polymer exhibits a relatively high Tg of 130 °C which can can be attributed to the p re sen c e of hydrogen bonding groups (-OH). A decom position tem perature, Td of 276.58 °C w as m easured by TGA (Fig. 3.25). Films of polymer P1 were spin-cast onto ITO slides from a 10% solution of P1 in a 10/1 dioxane/DM F solvent system . The films w ere dried undervacuum at 40 °C to evaporate the solvents. Films of thickness ranging from 0.3 to 0.5 micron were used for NLO characterization studies. The crosslinking reaction w as monitored by the d ecrease of the peak at -2250 c m '1 after heating in a vacuum oven for at least an hour at 120 °C. A complete disappearance of this peak w as not expected since nitrile groups also absorb around that region (Fig. 3.26). The fully-cured films w ere insoluble in various organic solvents such a s DMF, chloroform, and DMSO which further indicated the formation of cross-linked network. The thin film of the polmer Pi exhibited an absorption maximum at 500 nm. No other absorption maxima w as observed betw een 800-2500 nm. The absorption maximum was attributed to a charge-transfer excitation. 158 J i 0 4 .3 i 0 3 .0 Fig. 3.24 1H NMR (in DMSO-d6) spectrum of polymer P 1. 159 <:»> 100 From sea. 57 C Toi 347. 14 C 07 5 ., Onoot o ti 270.58 U J : « 62.5 • iao o iJora dam Tnmporoturo(C) aJaca ito o u i f e w ;.co Fig. 3.25 TGA curve of polymer P1 (heating rate = 20° C/ min, Ar atm osphere). 160 4000 3000 3000 2000 2000 1000 ca -1 Fig. 3.26 FTIR spectra of polymer P1 before and after heating/crosslinking at 120° C for one hour. 161 Optical scan s of the polymer film before and after electric-field poling show ed a d ecrease in absorbance of the absorption maximum. This d ecrease w as attribute to the alignm ent of the chrom ophore dipoles and w as used to calculate the alignment factor <cos3 0> and order param eter 0 . The value of <cos3 6> w as caculated using a well-used electrochromic technique. The order param eter w as calculated using w here A0 and Ap are the peak absorbances before and after poling, respectively. From the data shown in Fig. 3.27, the following param eters w ere calculated: <cos3 0> = 0.3 and O = 0.15 The second-order NLO resp o n se of polymer P1 w as m easu red by second-harm onic generation (SHG). Using the second-harm onic output, refractive indices, film thickness and ab so rb an ces at the second-harm onic w avelength, the SHG coeficient d33 = 112 p n W w as calculated using eq. 3.3. Using this m easured value of d, a more useful param eter in term s of device standpoint, electro-optic coefficient r w as also estim ated. By using the "two- level" model approximation, r33 values of 6.3 pmA/ (at 1.3 pm); 7.2 pmA/ (1.06 pm), and 12.2 pmA/ (800 nm) were calculated. To a s s e s s the dynamic therm al stability of the polymer film, the NLO response w as monitored a s a function of the film tem perature. The tem perature of the film w as increased in 10° increment and the NLO signal w as m easured 162 Absorbance 1 .4 - 1.2 - 1.0 - Initial Poled (120°C, 1 hr) Depoled (Ramped to 120°C) 0 . 8 - 0 .6 - 0 .4 - 0 .2 - 0 .0 - 1200 800 1000 400 600 Wavelength (nm) Fig. 3.27 Absorption spectra of polymer P1 before and after electric field poiing and crosslinking. 163 sim ultaneously (Fig. 3.28). The NLO response started to decay at about 120 °C. The long term stability of the NLO response of polymer Pi w as studied by heating the poled film at 100 °C and the SHG d coefficients w ere m easured at various tim e intervals. Figure 3.29 show s that the second-order nonlinear optical signal started to decay to about 70% of the original signal after 200 hours at 100 °C. This tem perature w as a little bit lower than th o se of other DEC- system s. This decrease can be attributed to the am ount of crosslinking density and the extent of crosslinking present in the polymer system . It is expected that long-term stability will be enhanced if i) the num ber of crosslinking sites will be increased and ii) the polymer will be completely cured. 3.3.C. Thermosetting Polyurethane System A num ber of polymeric system s have been designed to stabilize the poling order of the NLO chrom ophores. One of the more interesting system is the therm osetting polyurethane approach. This system h as b een quite attractive for its synthetic e a s e and the fast formation of the polyurethane network. In this section, the model compound w as tailor-m ade to contain a tri hydroxy group which w as subsequently reacted with diisocyanates to form the urethane linkage. The synthesis of the tri-hydroxy monomer w as outlined in Fig. 3.6. In the sy n th etic sch e m e , th e hydroxyl groups of the startin g m aterial, N- 164 Normalized d 0 .8 - 0 . 6 - 0.4- 0 . 2 - Fig. 3.28 " " * 1 *■ "t • • •■ » i a ■ M • ■ 11 1 ■1 11 'i'1 ' 1 11 11 1 ‘ l 11 i ■ | ' i i ‘ i ' 1 1' 1 1 111 11 1 11 j i 11 i-11 11 i [ i 11 11 1 1 1 11 f1 4 0 60 80 100 120 140 160 T em perature (°C) Plot of SHG coefficients of polymer P1 a s a function of film tem perature (heating rate = 10° C, in air). 165 Normalized d 1.0— 1 I 0 . 8 - 0 . 6 - 0.4- 0 . 2 ' Long Term Stability (100°C) ® | I I I I | 1 I I I | I I I I | I I I I | I I I l '"| ~ I T I I | I I I I | I I I I | I I I I ' l ' T I I I | I I I I 'I 'V* 0 200 400 600 800 1000 Time (hour) Fig. 3.29 Plot of normalized d coefficients of crosslinked polymer P1 as a function of time. Polymer film w as continuously heated at 100° C in an oven. 1 6 6 phenyldiethanolam ine, w as protected by a methylation reaction using methyl iodide under basic conditions. The choice of an ether rather an ester protecting group w as critical since the monom er also contains an ester group which could easily be hydrolyzed during deprotection reactions. The formation of the stilbene brige w as accom plished by standard Wittig c o n d en sa tio n reactio n s using a phosphonium salt derived from the chloromthylthiophene. The Wittig reaction proceeds in the following m anner: x- + n-BuLi PPh3 + X-CH-R -------► PPhg-CH-R --------------- ► Ph3P+-C'-R 3 I R' Ph3P=C-R 3 I R’ —C=C-R + Ph3PO l 3 R' Another key step in the synthesis of the tri-hydroxy m onom er w as the deprotection of the m ethoxy-protecting groups. Boron tribrom ide, BBr3 in m ethylene chloride at -78 °C w as u sed without hydrolyzing the e ste r functionalities or decom posing the whole chrom ophore altogether.20 The yield after column chrom atography of the final tri-hydroxy m onom er w as low which w as attributed to the adhesion of the monomer to the column. The proton and carbon NMR spectra of the trihydroxy monomer is shown in Figs. 3.30 and 3.31, respectively. 167 j d U _ i J L p I n inii'imi n iw | 1 Ml I im p n MTU r 1 11 1 1 n »11 p i n n m p m i 11 « t y r i.C E.0 6.0 4.0 PPM a.o 3.30 1H NMR (in DMSO-d6) spectrum of compound 20. Fig. 3.31 13C NMR (in DMSO-de) spectrum of com pound 20. Two sep arate therm osetting system s w ere prepared. Both system s consisted of two com ponents consisting of the trihydroxy monomer and toluene- diisocyanate in one system and dianisidine diisocyanate in the other system . The reaction schem e in illustrated in Fig. 3.10. An isocyanate-hydroxy group ratio of 1.15:1.0 w as used since isocyanates were more reactive than -OHs and could easily react with any residual water left in the solution. In both case s, a 10:1 mixture of dioxane:DMF w as the optimum solvent combination found for use to obtain optical quality films. The polymerization and crosslinking reactions were monitored by FT-IR m easurem entsbefore and after heating in the vacuum oven for an hour at 120 °C. FTIR spectrum also show ed a large d ecrease of the peak at -2200 cm-1 indicative of the disappearance of the the isocyanate groups (Fig. 3.32). This w as accom panied by the broadening of the other peaks in the spectrum . Furthermore, the fully cured film w as insoluble in different organic solvents such a s DMF, CHCI3, and DMSO. Optical absorption scan s of the polymer films before and after in-situ poling show ed a d e crea se in the absorption maxima at 616 nm for both polym ers P2-XL1 (Fig. 3.33) and P2-XL2 (Fig. 3.34), respectively. This w as attributed to the alignment of the chrom ophore dipoles. The d ecrease in the abso rb an ces were used to calculate the alignment factor. A <cos3 0> value of 0.207 for polymer P2-XL1 and 0.285 for P2-XL2 were calculated. 170 a) before heating b) after heating <900 2000 cm -1 Fig. 3.32 FTIR spectra of polymer P2-XL2 before and after heating/crosslinking at 120° C for one hour. 171 Absorbance 3.0 n 2 .5 - 2 .0 - 1 .5 - Initial Poled (120°C) Depoled (Ramped to 110°C) 1.0 - 0 .5 - 0 .0 - 400 600 800 1000 1200 1400 1600 1800 2000 Wavelength (nm) Fig. 3.33 Absorption spectra of polymer P2-XL1 before and after electric field poling and crosslinking. 172 Absorbance 0 .5 - 0 .4 - — Initial - After Poling (100°C, 9 kV ) 0.3- 0.2 0.1 - 0.0 400 600 800 1000 1200 Wavelength (nm) Fig. 3.34 Absorption spectra of polymer P2-XL2 before and after electric field poling and crosslinking. 173 The second-order NLO responses of the polym ers w ere determ ined by SHG m easurem ents using the set-up in Fig. 3.12 and are sum m arized in Table 3.3. d33 coefficients of 39 and 67 pmA/ were calculated for Polym ers P2-XL1 and P2-XL2, respectively. The higher value obtained from the latter can be ex p lain ed in th e context of diple-dipole repulsion b etw een a d jac e n t chrom ophores. Polymer P2-XL1 has a chrom ophore loading density of 48% com pared to 36% for P2-XL2. The dynam ic therm al stability of the m aterials w ere studied by a fast- ram ping m ethodology. The SH signals of th e films w ere m easu red instantaneously a s the tem perature of the films w ere increased in a stepw ise manner. Fig. 3.35 and 3.36 illustrates the behavior of the films a s a function of tem perature. For device applications, the electro-optic coefficient r is more useful in assessin g the nonlinearity of a material. Since the A ,m ax of the polym ers lie in- betw een the fundam ental (1064 nm) and second harm onic frequencies (532 nm), a simple approximation of the r value from the m easured d coefficient using a two-level model w as not applicable. Instead, the r coefficient w as m easured by an ATR technique and a value of r ~ 6 pm/V w as m easured for P2-XL2. The m easured r coefficient w as surprisingly lower than expected. Since the acceptor group in this c ase (dicyano) is stronger than the acceptor group used in the DEC-case (compound 12) of the previous section, it w as expected that the r value of the former would be higher. One plausible explanation w as 174 Table 3.3 Second-order nonlinear optical properties of Polymers P2-XL1 and P2-XL2. Crosslinker x P i f thickness (nm) Tstab(°C) cos3 9 d33(pm/V) CH3“ ^ ^ ~ nco NCO 616 1.09 95 0.207 39 OCH3 0CN~ \ / ~ ~ 4 i y ~ NC0 OCHf 616 0.19 100 0.285 67 1.0-n 0 .8 - 0 .6 - 0 . 4 - 0 .2 - 40 80 60 1 00 T em perature (°C) Fig. 3.35 Plot of SHG coefficients of polymer P2-XL1 as a function of film tem perature (heating rate = 10° C, in air). 176 Normalized d 1.0-1 0 .8 - 0 . 6 - 0 .4 - 0 .2 - 0 . 0 i i i r - j - i - i i r | i i i i | i i i - i - | i i i i j " i i i ' 1' T T i i i | m i | i i t i | T i i i | i 40 60 80 100 120 T em perature (°C) Fig. 3.36 Plot of SHG coefficients of polymer P2-XL2 a s a function of film tem perature (heating rate = 10° C, in air). 177 the dicyano-chrom ophore (compound 2 0 ) used for the therm osetting study w as not stable under the conditions of electric field poling. One experimental support for this assum ption w as the appearance of a new peak at ~ 1700 cm -1 after subjecting the crosslinked film to the electric field (Fig. 3.37). To investigate the possibility that the com pound 2 0 w as not able to withstand electric-field poling under high tem peratures, guest-host system s of the protected m onomer (i.e., compound 19) were studied. The guest-host film contained the sam e loading density a s the therm oset film. The m easured r coefficient for the guest host film w as 10-12 pmA/. The r of therm oset film is about half of the value for the guest-host system. This proves that the integrity of th e chrom ophore w as not retained after electric-poling at elev ated tem p eratu res. Two things might have happened to the chrom ophore: a) oxidation of the chromophore or b) the hydroxyl group could have reacted with the nitrile group of a neighboring chromophore or the nitrile group of the sam e chrom ophore in the following fashion: In a sep arate set of electrooptic coefficient m easurem ents, protected m onom ers w ere u sed to prepare g u est-h o st sy stem s of varying host OH 178 a) before poling b) after poling 4000 9600 9000 2600 2000 1900 1000 493 ca -1 Fig. 3.37 FTIR spectra of crosslinked polymer P2-XL2 before and after electric field poling. 179 concentration, i.e., 10-60% guest chrom ophore in PMMA host. The resulting plot of chrom ophore concentration versus r is shown in Fig. 3.38. Theoretically, it is expected that the response would increase with increasing chrom ophore co n cen tratio n up to a point w here dipole-dipole repulsion b etw een chrom ophores will result to an over-all d ecrease in the nonlinear response. However, an almost linear response w as observed which could be indicative of chromophore degradation. This, however, is a necessary but not sufficient proof for the presence of chromophore degradation. Several other factors which could contribute to this type of response (e.g., optimizing the poling conditions, film thickness). 3.3.d. Polymers incorporating tricyano-functionalized chromophores Tricyano functionalized m onom ers w ere also synthesized a s shown in Figs. 3.7 and 3.8. Spectroscopic and analytical test results indicate that the m onom ers w ere synthesized and obtained in pure forms. Proton and carbon NMR spectra of the tri-cyano functionalized chrom ophores are shown in Figs. 3.39-3.42 Therm al analysis data show ed a decom position tem perature of 232.28 °C and 233.73 °C for com pounds 25 and 26, respectively (Fig. 3.43 and 3.44). Com pound 26 w as polymerized by radical polymerization to produce polym er P3 w hile com pound 25 w as polym erized by co n d en satio n polymerization to produce polymer P4. The structure of the polymers are shown in Figs. 3.11 and 3.12. The ability of the polym ers to w ithstand e x p o su re to elev ated tem p eratu res w as a ss e ss e d by taking absorbance sc a n s before and after 1 8 0 Measured r. 1 1 1 8 - 6 - 4 - 2 - 0 - 40 45 25 30 35 50 20 Doping Level (%) Fig. 3.38 Plot of electrooptic coefficient as a function of chromophore concentration (compound 19) in PMMA host. 1 8 1 l J I | I I I I | TIT l | 1111 I I I I I 9. 0 [ 111111111[ 111111111| 111111111] 11111 n 11| 1111 j 11 n | r n 111111| i m j 1111| n i t 5. 0 7 . 0 6.0 5 . 0 PPM 4 . 0 3. 0 2.0 Fig. 3.39 1H NMR (in DMSO-de) spectrum of compound 25. 182 ^ i ' 1 5 0 .0 1 0 0 . 0 5 0 . 0 PPM 0.0 Fig. 3.40 13C NMR (in DMSO-d6) spectrum of com pound 25. 183 a X j L J . [ i t t t j n “r r j n i t p T T T ] * . ? » * . o y.u 7 . 1 Fig. 3.41 1H NMR (in CDCI3) spectrum of compound 26. 184 j i j j , flH N f J I J l— a 150.0 I 100.0 PPM 50.0 Fig. 3.42 13C NMR (in DMSO-d6) spectrum of com pound 26. 185 W EIGHT (3) 100 Krom. 21 5 .4 4 C To* 249. 10 C 0 7 ^ 5 .. Onoot Gt» 23 2 .2 0 H z l l L O O 2&1.00 2^0.00 3 it). 00 siaGO ife o o Tem poraturo<C) t a o o fe o o 1 jo. 00 Fig. 3.43 TGA curve of compound 25. 186 W EIGHT (2) 100 Fromi 1 SB. 43 C Toi 2 5 5 .9 2 C Onoot a ti 2 3 3 .7 3 ite o o 2 ^ 0 .0 0 24 d o 3 k 0 0 3 k 0 0 i/0 .0 0 T em p eratu re C C ) Fig. 3.44 TGA curve of compound 26. 187 heating. Results showed that thin films of tricyanovinyl polymers bleached upon exposure to heat even in the ab sen ce of electric field poling. This can be illustrated by the decrease in their respective absorption maxima a s illustrated in Figs. 3.45 and 3.46. In a typical poled-polymer system with no chrom ophore degradation and the conditions are optimum, a d ecrease in the absorbance is only observed when the chromophore is subjected to electric-field poling. In this case, since the tricyano-polymer films were not subjectd to an electric field, the absorption drop can be attributed to chromophore degradation. These results do not correlate with the findings of Jen and coworkers. In their work, radical polymers containing tricyanovinyl-substituted chrom ophores w ere prepared and exhibited an r33 valueof 23 pm/V.5 The main difference betw een Jen 's schem e and the schem es presented in this chapter w as that in the former case, the tricyanovinylation reaction w as done in the last step, i.e. TCNE w as added in the polymer stage. This prevents the participation of a tricyanovinyl group in any chemical reaction thus preserving its integrity. It is believed th at the apparently high reactivity of the tricyanovinyl group neccesitates that it be incorporated in the last step of any reaction. 3.4 Conclusion A model chrom ophore w as synthesized b ased on DANS using the following variations: a) replacem ent of one phenyl ring with a thiophene ring as part of th e chrom ophore backbone, b) dicyanovinyl accep to r, an d c) incorporation of a functionality which can be used for crosslinking. The model 188 A bsorbance Before and After Heating (no poling) 0 . 8 - 0 . 6 - Initial After 115°C Heating After 2nd 115°C Heating 0 .4 - 0 .2 - 1000 1200 1400 1600 1800 600 800 400 W avelength (nm) Fig. 3.45 Absorption spectra of polymer P3 before and after heating. 189 Absorbance 0 . 7 -1 0 . 6 - 0 .5 - - Initial After 90°C Heating - - 10 Hours After 90°C Heating - After 115°C Heating ITO_Absorbance 0 .4 - 0 .3 - 0 . 2 - 0.0 — j 400 600 800 1000 1200 1400 1600 1800 Wavelength (nm) Fig. 3.46 Absorption spectra of polymer P4 before and after heating. chromophore exhibited sizable second-order nonlinearity (pP = 1700 x 10-48 esu at 1.9 pm, 3-4 times DANS) and high thermal stability. Variations of the model chromophore were synthesized enabling it to be incorporated into thermally stable polymeric system s. Two app ro ach es were u sed to achieve this goal — DEC m onom er approach an d the u se of therm osetting polyurethane system . In both c a se s, the second-order NLO behavior of the resulting m aterials w as limited by th e stablity of the chrom ophores under electric field poling conditions. Tricyanovinyl functionalized m onom ers w ere also sy n th esized and polymerized using both radical and condensation polymerization conditions. The resulting polymers were unstable even at elevated tem peratures alone (i.e., without electric field poling). These results were in contrast to the results of Jen and coworkers which led us to believe that for tricyanovinyl polymers to be used for second-order applications, tricyanovinylation reactions should be the very last step in any reaction schem e. 191 3.5 References 1. V.P. Rao, A.K.Y. Jen, K.Y. Wong, and K.J. Drost, Tetrahedron Lett., 34, 1747,1993. 2. K.Y. Wong, A.K.Y. Jen, V.P. Rao, K. Drost, and R.M. Mininni, Proc. SPIE, 17 7 5 .7 4 .1 9 9 2 . 3. V.P. Rao, A.K.Y. Jen, K.Y. Wong, K. Drost, and R.M. Mininni, Proc. SPIE, 1775, 32,1992. 4. V.P. Rao, A.K.Y. Jen, K.Y. Wong, and K.J. Drost, J. Chem. Soc.,Chem. Commun., 1118,1993. 5. K.J. Drost, V. Rao, and A.K.Y. Jen, J. Chem. Soc., Chem. Commun., 369, 1994. 6 . A.K.Y. Jen, Y. Liu, Y. Cai, V. Rao, and L.R. Dalton, J. Chem. Soc., Chem. Commun., 2711, 1994. 7. H.E. Katz, K.D. Singer, J.E. Sohn, C.W. Dirk, L.A. King, and H.M. Gordon, J. Am. Chem. Soc., 109, 6561,1987. 8 . C. Xu, B. Wu, L.R. Dalton, Y. Shi, P.M. Ranon, and W. Steier, Macromolecules, 25, 6714,1992. 9. L.R. Dalton, R. Ghosn, A. Harper, J. Laquindanum, J. Liang, W. Steier, H. Fetterman, R.V. Mustacich, and A.K.Y. Jen, Angew. Chem. Intl. Ed., 1995, in press. 10. D. Burland, R.D. Miller, and C.A. Walsh, Chem. Rev., 9 4 ,1 9 5 ,1 9 9 4 . 11. M. Chen, L.R. Dalton, L. Yu, Y. Shi, and W. Steier, Macromolecules, 25, 4032.1992. 12. C.V. Francis, K.M. White, G.T. Boyd, R.S. M oshrefzadeh, S.K. M ohapatra, M.D. Radcliffe, J.E. Trend, and R.C. Williams, Chem. Mater., 5 ,5 0 6 , 1993. 13. J.A. Boogers, P. Th. A. Klaase, J.J. de Vlieger, and A.H.A Tinneman, Macromolecules, 27, 205,1994. 14. B.Wu, Ph.D. Dissertation, University of Southern California, 1994. 15. A. Strobel, Delmar, and S. Catino, U.S. Patent 3,644,466,1972. 192 16. Y. Shi, Ph.D. Dissertation, University of Southern California, 1992. 17. S. Herminghaus, B.A. Smith and J.D. Swalen, J. Opt. Soc. Am. B, 8 , 2311,1991. 18. J. March, Advanced Organic Chemistry. J. Wiley and Sons: New York, 1985. 19. B.C. McKusick, R.E. Heckert, T.L. Cairns, D.D. Coffman, and F.H. Mower, J. Am. Chem. Soc., 80, 2806,1958. 20. R.N. de Martino, E.W. Choe, G. Khanarian, D. Haas, T. Leslie, G. Nelson, J. Stamatoff, D. Stutz, C.C. Teng, and H. Yoon, In Nonlinear Optical and Electroactive Polymers. P.N. Pasad and D.R. Ulrich, Eds., Plenum: New York, 1988. 21. K.D. Singer and A.F. Garito, J. Chem. Phys., 75, 3572,1981. 22. T. G reene, Protective G roups in Organic Synthesis. Wiley: New York, 1981. 193 CHAPTER 4 SYNTHETIC STUDIES OF FUNCTIONALIZED FUSED-RING CHROMOPHORE BACKBONE FOR USE IN RIGID-ROD MAIN-CHAIN POLYMER SYSTEMS 194 4.1 Introduction One of the key challenges in the design of polymeric second-order NLO m aterials is the synthesis of m aterials with high degree of polar order and maintain this orientation for extended periods of time at a given tem perature. This subject has been addressed by several research groups and som e of their works w ere described in Chapter 1 of this dissertation. Side-chain incorporation of NLO chrom ophores into a polymer backbone provided significant enhancem ent in both nonlinearity and temporal stability over guest-host system s.1" 5 The stability of the poled order can be further enhanced even at higher tem peratures by incorporation into high Tg polymers or by the use of crosslinking reactipns.6-7 Crosslinking can be accom plished either by thermal or photochemical m eans.8-9 NLO chrom ophores w ere also incorporated a s part of m ain-chain polym ers. 10" 13 Higher loading densities were achieved this w as over side-chain counterparts, thereby translating into higher nonlinearities. However, the poling p ro cess w as not always easy and so, several variations w ere developed to a d d re ss this problem. An exam ple of this variation is the accordion-type polymers synthesized by Lindsay and coworkers. 14-15 Another class of polymers which has been investigated with regards to optical nonlinearities consists of rigid-rod polymers.16" 18 Rigid-rod polymers are characterized by increased thermal stability, tensile strength, and high modulus. 195 They also display unusual optical properties and have been shown to form liquid-cystalline phases and highly ordered structures.19’22 The use of rigid-rod polymers for NLO applications h as mainly been limited to third-order applications.23-24 The u se for seco n d -o rd er NLO applications, on the other hand, has not been fully exploited. Jin e t at. reported the sy n thesis of polyphenylene vinylene substituted with d o n or-acceptor groups 25 Poling th ese polymers, to date, has not been succesful. The sam e w as true for fused five-ring ladder polym ers.26 R ecent activites in W egner's group proved very interesting and will be discu ssed in m ore detail in the succeeding paragraphs.27’29 W egner exploited the idea that enhanced second-order response can be achieved by using self-assembling rigid-rod polymers. It w as shown by Ballauf, W egner, and others that solution-casted films of rigid polymers substituted with long alkyl chains formed layered structureswith the layer plane parallel to the surface (Fig. 4.1).30’33 Addition of the alkyl chains enhance the solubility of the polym er and interaction betw een neighboring alkyl chains induces the self organization to form the layered structure. The idea w as by replacing som e of the alkyl chains with NLO chrom ophores, a parallel and upright arrangem ent of the chrom ophores will be attained. A system with an inherent degree of order is expected to be poled more readily than such c ase s wherein the chrom ophores are free to move around in different directions. Thus, a highly ordered and exceptionally thermally stable system could be expected. Two rigid-rod system s 196 a. limim m TI-------iiiimiihh^ — Im m im m l------lim iim in i I imiii i mill— In i mi i ii n I— in m u miii I------liim m m il 1111M1111M11 — liii^ m ^iil--------IiiiiiiiiiimI lim m im iJ SUBSTRATE b. I < J < 1 I ? ___L 1 ii iTI----111 i 1 1 1 1 1 1 1 i~p— ImmiiimTI-------liiiimiiiiil Ii im i i i i i i i ii I-------1............ ii i iTI imiiimiin limiiimiTl T T j \ i s<> ? j f IiiiiiiiiiimI IiiiiiiiiiimI--------1 iiin m im l—-— I m i him m l SUBSTRATE donor rigid-rod v 5 ® main chain ihmiiiiiiiii T T alkyl chain s " —^ acceptor Fig. 4.1 a. General structure of rigid-rod polymers substituted with long alkyl chains and b. with som e of the alkyl chains replaced with NLO chromophores (from ref. 30). 197 have b e en investigated - chrom ophores a tta ch e d a s sid e-ch ain s and chrom ophores in the main chain. Stilbene chrom ophores and flexible alkyl chains have been attached to the arom atic backbone of a rigid-rod polyester (Fig. 4.2). The polym ers have varying length which could also explain the following results that w ere obtained. Polymer a (Fig. 4.2a) has a very low y2 response of 0.06 pm/V at 632.8 nm .29 The low nonlinearity w as attributed to the cancellation of dipoles b ecau se of the large distance betw een layers of the main chain. On the other hand, for polym ers with shorter alkyl chains (Fig. 4.2b and 4.2c), relatively higher % 2 values were obtained (3.3 and 11.8 pm/V), respectively.29 It w as proposed that due to the short distance betwen the layers, the chrom ophores are forced to bend from the alkyl spacer. This results to the chrom ophores being sandw iched betw een layers with orientations parallel to the main-chain axis. Substituted rigid-rod type main-chain polymers of the type shown in Fig. 4.3 w ere also sy n th esiz ed , ch aracterized and th eir NLO re sp o n se investigated.29-33 X-ray diffractograms of these polymers indicate the formation of layered structures. % 2 values of up to 15 pm/V were m easured in th e se system s. The polymers exhibited thermal stability, with polymer 4.3c retaining the response at up to 920 hours at 100 °C .29 From the above discussion, it becam e ap p arent that highly stable system s can be achieved using substituted rigid-rod main chain polymers. This system could be further improved using chrom ophores with high second- 198 (CH^m Fig. 4.2 Rigi-rod polymers substitued with NLO chrom phores in the side-chains, x:y = 1 a. m = 2, n = 1 5 ;b . m = 5 ,n = 7;c. m = 2, n = 5 (from ref. 30). 199 o _ _ o / = (N(CHt _ O -c - \ y ^ - c - \ ^ c -N^ y j - c - ° ~ y j ~ { 0 ----------- H2n 1c n° N CnH2n+10 H2m+1 CmO Jx n = 6, m = 14 N(CH3)2 NC CN X s s - OCnH2n+ 1 : i CnH2n+iO . 4.3 Rigid-rod polymers substituted with NLO chrom ophores in the main-chain (from ref. 30). 200 order nonlinearity and thermal stability. In this line, this chapter presents studies tow ards the synthesis of an asymmetrically functionalized fused-ring arom atic m olecule as chrom ophore backbone. This backbone system provides longer conjugation length which will enh an ce the optical nonlinearity. It contains thiophene moieties which m akes it accessible to functionalization with donor and acceptor groups. 4.2 Experimental a. Materials and Methods The m ethods used for the characterization of synthetic products w ere the sam e a s to those discussed in C hapter 3. All chem icals w ere purchased from different chem ical com panies and were used a s received unless otherw ise indicated. Diethyl ether and tetrahydrofran w ere dried by refluxing with finely divided Na metal and using benzophenone as indicator. b. Synthetic Procedures i. Compound 133 Bromoacetaldehylde dimethyl acetal (131.45 g, 0.667 mol) and potassium iodide (5g) were added to a solution prepared from sodium sufide nonahydrate (120 g, 0.5 mol) and sulfur (24 g, 0.75 mol) in 500 ml of 95% ethyl alcohol. The resulting mixture w as vigorously stirred and heated at reflux for 16 hours. The 201 ethanol w as rotovapped off and w ater (500 ml) w as added. The solution w as extracted several tim es with ether. The com bined ether extracts w as w ashed with brine (2 x 100 ml) then with water (2 x 100 ml). Removal of the ether left 61.7 g light yellow oil which w as distilled (100-105 °C , 0 .6 mm) to give 32.9 g (86.3 % yield) of the disulfide. 1H NMR (CDCI3) relative to TMS (in ppm): 1.22 (t,12H, -CH3); 2.97 (d, 4H, -CH2); 3.65 (q, 8 H, -CH2): 4.71 (t, 2H, -CH). ii. Compound 2 2,5-dibrom o-p-xylene (50 g, 0.19 mol) w as dissolved in 300 ml dry tetrahydrofuran and t-BuLi, 1.7 M solution in hexane (234 ml, 0.38 mol) at -78oC. After com plete addition, the tem perature w as raised to -10 °C. Sulfur (6.28 g, 0.19 mol) w as added in one portion w ashed with tetrahydrofuran under inert a tm o s p h e re . W hen th e sulfur had co m p letely d isso lv e d , brom oacetaldehyde dimethyl acetal (34.3 ml, 0.228 mol) w as added. The mixture w as left to reflux overnight. The solvent w as rotavapped off and then redissolved in w ater (500 ml). The aqueous solution w as extracted with ether (5 x 100 ml) and the combined extracts w as w ashed with brine and then dried over m agnesium sulfate. The crude product w as vacuum-distilled at -1 6 0 °C at 1 mm pressure to give a yellow oil (45.6 g, 76.2% yield). 1H NMR (CDCI3) relative to TMS (in ppm): 1.21 (t, 6 H, -CH3); 2.33 (s, 6 H, -CH3); 3.08 (d, 2 H, -SCH2-); 3.67 (q, 4H, -OCH2-);4.64 (t, 1H, -CH-); 7.19 (s, 1H, phenyl H); 7.32 (s, 1H, phenyl H). 202 iii. Compound 3 A solution of compound 2 (36.7 g, 0.11 mol) in 300 ml dry tetrahydrofuran w as cooled tp -78 °C. n-BuLi, 1.6 M solution in hexane (75.65 ml, 0.121 mol) w as added dropwise. The solution w as stirred for 2 hours at -78 °C and then com pound 1 (38 g, 0.13 mol) w as added. The tem perature w as m aintained at -78° C for at least six hours. The solvent w as rotavapped off and the residue redissolved in water. The aqueous solution w as extracted several tim es with ether (5 x 100 ml), w ashed with brine (3 x 100 ml) and dried over m agnesium sulfate. The crude product w as w as vacuum distilled at -1 8 0 ° C at 1 mm pressure to yield a light brown oil (39 g, 88.1% yield). 1H NMR (CDCI3) relative to TMS (in ppm): 1.24 (t, 1 2 H, -CH3); 2.34 (s, 6 H, -CH3); 3.08 (d, 4H, -SCH2-); 3.63 (q, 8 H, -OCH2-); 4.62 (t, 4H, -CH-); 7.15 (s, 2 H, phenyl H). iv. Compound 4 A solution of com pound 3 (23.25 g, 0.06 mol) in 300 ml benzene w as added to 132.86 g polyphosphoric acid and the mixture w as refluxed for 24 hours. Afterwards, th e mixture w as cooled to room tem p eratu re and the benzene layer w as decanted. Another portion of benzene (250 ml) w as added and the mixture refluxed for another 12 hours. The procedure w as repeated twice. The combined benzene extracts w as w ashed with brine (5 x 2 0 0 ml) and dried over m agnesium sulfate. The solvent w as rotovapped off and the crude product w as purified by column chrom atography using ben zen e a s eluent to yield an orange solid (9 g, 71.4 % yield). 1H NMR (CDCI3) relative to TMS (in 203 ppm): 2.81 (s, 6 H, -CH3); 7.48 (s, 4H, aromatic Hs). Calculated for C12H10S 2: C, 66.02; H, 4.62; S, 29.37. Found: C, 66.13; H, 4.53; S, 29.30. v. Compound 5 A round-bottom ed flask w as charged with com pound 4 (2 g, 9 mmol), TMEDA (1.04 g, 9 mmol) and 20 ml dry tetrahydrofuran. The solution w as cooled to -78 °C and 6 ml of 1.6 M n-BuLi in hexanes w as added dropwise. The solution w as stirred for 30 m inutes after which, N-methylformanilide (0.8 ml, mmol) w as added dropwise. After complete addition, the solution w as stirred overnight, acidified with ml 3N HCI then poured into iced-water. The aqueous solution w as extracted with ether ( 3 x 10 ml) and then w ashed with brine, dried over m agnesium sulfate then concentrated. The crude product w as purified by column chrom atography using hexane as eluent to give a yellow solid (1.3 g, 70 % yield). 1H NMR (CDCI3) relative to TMS (in ppm): 2.73 (s, 3H, -CH3); 2.79 (s, 3H, -CH3); 7.43 (d, 1H, thiophene a-H); 7.61 (d, 1H, thiophene P-H); 8.14 (s, 1H, thiophene p'-H); 10.06 (s, 1H, -CHO). vi. Compound 6 A solution of 5 (2.0 g, 9 mmol) and TMEDA (1.15 g, 9.9 mmol) in 20 ml dry tetrahydrofuran w as cooled to -78 °C. 6 ml of 1.6 M n-BuLi in hexane w as added dropwise. The solution w as refluxed for 30 m inutes after which sulfur (0.32 g, 9.9 mmol) then 1-iodobutane (1.14 ml, 9.9 mmol) w as added and stirred at room tem perature overnight. The reaction w as quenched by the addition of 204 ice w ater and w as extracted with ether (3 x 30 ml), w ashed with brine (2 x 30 ml). Purification w as done using column chrom atography using hex an es a s eluent. Yield: 67 %. 1H NMR (CDCI3) relative to TMS (in ppm): 0.93 (t, 3H, -CH3); 1.26 (m, 2H, -CH2-); 1.69 (m, 2H, -CH2-); 2.75 (s, 3H, -CH3); 2.88 (s, 3H, -CH3); 2.99 (m, 2H, -CH2-); 7.43 (s, 1H, p'-H); 7.49 (m, 2H, thiophene Hs). vii. Compound 7 Com pound 6 (1.5 g, 5 mmol) w as dissolved in b en zen e under inert atm osphere. Benzoyl peroxide (2.91 g, 12 mmol) and N-brom osucinim ide (NBS) (2.14 g, 12 mmol) w as added to the solution. The resulting mixture w as left to stir overnight at room tem perature. The mixture w as poured into w ater and extracted several tim es with ether. The com bined organic extracts w as w ashed with brine and dried over magnesium sulfate. The product (2.0 g) w as used directly in the succeeding step. viii. Compound 8 A round bottom flask w as charged with com pound 5 (2 g, 4.3 mmol), concentrated NaOH (5 ml), 5 ml w ater and 20 ml methanol. The solution w as warm ed to 40 °C and w as left to stir at this tem perature overnight. The solution w as poured into w ater and extracted several tim es with ether. The com bined ether extracts w as w ashed with brine, dried over MgS0 4 , concentrated and the crude product w as used directly in the succeding step. 205 ix. Compound 9 Com pound 8 (1.5 g, 4 mmol) w as dissolved in 10 ml tetrahydrofuran and cooled to 0 °C. Acetyl chloride (0.3 ml, 4.2 mmol) w as added and the solution w as stirred at 0 °C and w as left to warm to room tem perature overnight. The solution w as poured in ice, extracted with dichloromethane (3x10 ml), w ashed with brine ( 2 x 1 0 ml) and dried over m agnesium sulfate. The product w as purified using 10% ethyl acetate in hexane to yield a yellow solid (1 g, 56% yield). 1H NMR (CDCI3) relative to TMS (in ppm): 0.97 (t, 3H, -CH3); 1.52 (m, 2 H, -CH2-); 1.77 (m, 2 H, -CH2-); 3.03 (s, 3H, -CH3); 3.11 (s, 3H, -CH3); 3.53 (m, 2H, -CH2-); 7.43 (s, 1H, p'-H); 7.49 (m, 2H, thiophene Hs). x. Attempted bromination of compound 4 Com pound 4 (1 g, 4 mmol) w as dissolved in 10 ml chloroform at room tem perature. Bromine (0.2 ml, 4.05 mmol) w as added and the solution w as refluxed overnight. The reaction w as poured into 20 ml w ater and extracted several times with ether. The combined organic extracts w as w ashed with brine and concentrated. The crude product contained a mixture of mono- and multi- brom inated com ponents. The sam e procedure w as repeated this time, addition of brom ine w as done 0 °C and the reaction w as left to warm to room tem perature overnight. The crude product still consisted of a mixture of brominated products. 206 xi. Attempted tricyanovinylation of compound 6 Tetracyanoethylene (0.23 g, 1.92 mmol) w as ad d ed to a solution of com pound 6 (0.5 g, 1.6 mmol) in 7 ml dimethylformamide. The solution turned green a s the reaction progressed. The solution w as left to stir for at least 72 hours and then poured into water. The aqueous solution w as w ashed several tim es with brine, dried over MgS0 4 , concentrated and purified, recovering the starting material 6 . The sam e procedure w as repeated this time using elevated tem peratures with similar results. xii. Attempted nitration of compound 6 Two procedures w ere used to introduce a nitro group to com pound 6 . The first procedure m ade u se of a solution containing 3.0 g (20 mmol) of trifluoromethane sulfonic acid dissolved in 50 ml of dichlorom ethane placed in a 200 ml round-bottm ed flask. A 0.63 g sam ple of anhydrous nitric acid w as added to this solution forming a white crystalline solid which sep arated out of solution. The tem perature w as lowered to -60 °C, com pound 6 (0.11 g, 0.35 mmol) w as added and the solution w as stirred at -60 °C for 1 hour and w as left to warm to room tem perature. After work-up (i.e., w ashed with water, dried over MgSC>4, and concentrated) and separation of reaction products, no nitro- substituted fraction w as recovered. Another nitration protocol w as carried out using nitric acid/acetic acid a s the nitrating agent. A solution of 6 (0.11 g, 0.35 mmol), 10 ml nitric acid/ acetic 207 acid in dichlorom ethane w as refluxed overnight. This reaction still did not give a nitrated product. 4.3 Results and Discussion The main objective of the work presented in this chapter w as to conduct synthetic stu d ies tow ards the form ation of a family of d o n o r-accep to r funtionalized rigid-rod system s. T hese rigid-rod system s are designed to self- assem ble in bulk and form ordered arrays. In order to ad d ress this objective, a rigid-rod polymer w as designed as illustrated in Fig. 4.4 This polymer system consists of a fused-ring arom atic backbone and a phenyl ring functionalized with long alkoxychains. Since the rigid-rod polymer will be potentially used for second-order applications, an asym m etric system is desired. The fused-ring arom atic system will be u sed a s achrom ophore backbone and will be functionalized with donor and acceptor groups. O ne advantage of the proposed chromophore backbone is the presence of thiophene rings, w hose 5 and 5' positions are highly reactive and therefore easily substituted. The backbone p o sse sse s a longer conjugation pathw ay betw een the donor and acceptor groups which is expected to lead to an enhancem ent in second-order nonlinearity. Furthermore, the backbone is very robust and is expected to display excellent thermal stability. Another com ponent of the rigid-rod polymer system is the phenyl ring with alkoxy groups attach ed to it. The purpose of incorporating th e alkoxy- fuctionalized phenyl moiety is to enhance the stiffness of the chains and at the 208 @ = donor, — Q = acceptor * a a / w ' = flexble group Structure of the proposed rigid-rod polymer with an NLO- chromophore in the main chain. 209 sam e time increase solubility. B ecause of the p resence of the long alkoxy ch ain s, it w as predicted th at the ch ain s will stack , producing an orderedstructure. The proposed design has also the flexibility of varying the chrom ophore loading density and solubility by incorporating flexible-chain as com onom ers. The critical step in the formation of the proposed rigid-rod polymer system w as the synthesis of the fused three-ring aromatic backbone. The synthesis of com pound 4 has already been reported by several groups.35- 36 In this chapter, another route w as presented to synthesize this compound. C om pared to the previously published protocols, the new route w as synthetically e asie r and provided higher over-all yields. The new synthetic route (Fig. 4.5) is similar to Pom erantz’ s schem e which u se s 2,5-dibrom o-p-xylene a s the starting m aterial.34 However, in the new schem e, the attachm ent of the thioacetaldehyde dimethyl acetal groups w as done in two steps. The step-w ise addition of the thioacetaldehyde dimethyl acetal groups led to an improvement in the over-all yield of the substituted product 4. The first debromination step required a much harsher condition (i.e. use of t-BuLi) to form the monoanion. The formation of the dianion can also be visually detected a s t-BuLi w as added, however, it w as unstable and easily reverted back to the more stable monoanionic form which upon reaction with sulfur and brom oacetaldehyde dimethyl acetal led to the formation of 2 . 210 Na2S + S --------► Na2S2 K l Na2S2 + BrCH2CH(OCH2CH3)2 -----► (OCH2CH3)2CHCH2SSCH2CH(OCH2CH 3)2 1 9 H3 c h 3 c h 3 2 - A T 6 ' t-BuLi r A - 1. S reflux X .S C H 2 CH(OCH2 CH3 )2 j t r * - -7 r V Br Y -78° C B r ^ ^ 2. BrCH2CH(OC2H5)2 24 hours B r' c h 3 c h 3 c h 3 2 CH3 SCH 2CH (OC2H5)2 nBuLi ^ y S C H 2CH (O CH 2CH 3)2 SC H 2CH(OC2H5)2 -78° C I -78° C, 2 hours CH3 2 hours 1 3 CH3 , h h ■ ? H3 A . S C H 2CH(OCH2CH3)2 polyphosphoric acid v t - A s v - S . (OC2H5)2CHCH2S V ■ " reflux, 72 hours S c h 3 c h 3 Fig. 4.5 Schem e for the alternative synthesis of compound 4. The substitution of a second thioacetaldehde dim ethylacetal group to form com pound 3 w as achieved by using disulfide 1. C om pound 1 w as synthesized using the procedure by Parham and cow orkers.34 The synthesis w as relatively straightforward, the critical step, being the separation of the d esired disulfide from mono-, tri-, and other polysulfide byproducts. The cyclization reaction to form the desired fused-ring arom atic backbone 4 w as done using polyphosphoric acid. Higher over-all yields were achieved when the reaction w as done under completely dry and inert conditions. NMR characterization of the reaction products described above are illustrated in Figs. 4.6-4.9. The stepwise substitution of each thioacetaldehylde dim ethyacetal group w as clearly shown by the disappearance of an arom atic singlet in the mono-substituted case (Fig. 4.7) to show a single arom atic peak at 7.15 ppm corresponding to the equivalent protons for the disubstituted c ase (Fig. 4.8). The cyclization product, 4, exhibited two p eak s at 2.81 (methyl protons) and 7.48 (arom atic protons) ppm respectively accom panied by the d isa p p e a ra n c e of all o th er p eak s corresponding to th e ace ta ld eh y d e dimethylacetal group (1.21, 3.08, 3.64, and 4.64 ppm) (Fig. 4.9). Several attem pts w ere m ade to attach different groups to com pound 4 (Fig. 4.10). The first functional group attached w as an aldehyde group. Functionalization with an aldehyde group is an important reaction since the product can be used as a precursor for incorporating different acceptors and donors containing active hydrogens (e.g., thiobarbituric acid, malonitrile, etc.). The reaction w as carried out using n-BuLi to form the anion. Two anionic forms 212 6.0 1 I I I I I 1 1 I I J_1 l_L I 1 t 1 I I I I 1 1 1 1.1 I t 1 1 1 4.0 3.0 2.0 1l 1 1 1 >i I ■ 11 i-l-i-i 5.0 Xi-iJ. 1.0 8 . C 7 .0 PPM Fig. 4.6 1H NMR spectrum of compound 1. 213 Fig. 4.7 1H NMR spectrum of compound 2. 214 0.0 6.0 — T ■ n " W| , , . T ~ | T - T - r - r - ^ - i . |l , ...i tT t j . .1 | . , I . | . 7 0 6.0 6.0 4.0 3 0 2.0 1 0 PPM Fig. 4.8 1H NMR spectrum of compound 3. 215 Fig. 4.9 1H NMR spectrum of compound 4. 216 h3c CH 1. nBuLi, -78° C 2. N-Methyl formanilide 3. H+ H,C CH SCH2CH2CH2CH3 1. nBuLi, -78° C 2. S, IBu CH CHO Fig. 4.10 Schem e for the synthesis of functionalized 4. 217 tof 4 can exist— mono- and di-anionic forms. Of the two, the monoanionic form w as found to be more stable and is colored blue in solution. Treatm ent of the m onoanion with N-methylformanilide followed by deprotection with an acid gavehe desired aldehyde product. The formation of the product can be clearly seen from the appearance of a signal at 10.06 ppm which corresponds to an aldehyde proton (Fig. 4.11). Several other synthetic reactions were tried to derivatize the fused-ring backbone 4 (Fig. 4.9). Bromination reactions were run at different tem peratures, however, the products obtained in each c ase were multi-brominated species which w as very difficult to separate by column chromatography. Another functionality that w as substituted into compound 4 w as a donor group, thiobutyl (-SBu). In order to attach this substituent, the anion of com pound 4 w as m ade similar to that described in the previous paragraph. This w as followed by addition of a slight excess of sulfur and iodobutane. An NMR analysis of the product show ed peaks at 0.93, 1.26, 1.69, and 2.99 ppm corresponding to butyl groups (Fig. 4.12). In order to introduce a functionality to the fused-ring backbone which could be used a s a point of attachm ent into a polymer, the methyl groups w ere reacted with N-bromosuccinimide using benzoyl peroxide a s radical initiator (Fig. 4.13). The halogentaed product w as then hydrolyzed to form a diol. The presence of the hydroxyl groups m akes the fused-ring backbone accessible for 218 pmimrpmT 10.0 8.0 6 .0 4 .0 2 .0 PPM Fig. 4.11 1H NMR spectrum of compound 5. 219 > r-fj-t-r-TTj-T-rrri t j — m ~.*j->- Fig. 4.12 1H NMR spectrum of compound 6. 220 H3 C“W - CH3 C ?s e SCH2CH2CH2CH3 NBS, benzoyl peroxide in d-feCfe NaOH in MeOH/ HaO 8 s c h 2c h 2c h 2c h 3 1. acetyl chloride 2. pyridine g SCH2CH2CH2CH3 Fig. 4.13 Schem e for the incorporation and protection of diol groups into compound 6. 221 condensation polymerization reactions. The hydroxy group w as protected by reacting with acetyl chloride using Schotten-Baumann reaction conditions. Several attem pts to attach acceptor groups to com pound 6 w ere also perform ed (Fig. 4.14). Tricyanovinylation with the use of tetracyanoethylene w as perform ed using different reaction conditions. However, the substitutions w ere not successful and the starting material w as recovered. Two different w ays of attaching nitro groups were also tried. In both c a s e s the starting material remain w as recovered and remained unreacted. The stability of the poled order of the proposed rigid-rod polymer can be en h an ced by the introduction of crosslinking groups onto the functionalized backbone itself. However, the crosslinking reaction should be carefully chosen such that the necessary conditions for crosslinking will not lead to the hydrolysis of the other functional group (e.g., ester linkages) present in the polymer chain. To a d d ress this point, a synthetic schem e is proposed in Fig. 4.15. In this schem e, the fused ring backbone is functionalized with vinyl groups which will be used to crosslink the poled-polymer system. The proposed crosslinking is to b e c a rrie d o ut u sin g a re a c tio n of th e vinyl groups with tetram ethylcyclotetrasiloxane. This therm oset reaction is catalyzed by Pt at elevated tem peratures.36 222 0 C to RT 72 hours TCNE reflux 24 hours CN ^i-C N H 3C-<\ /hCH3 HNOg/HOAc r _ / V p m reflux triflic acid/HN03, -60° C SBu Fig. 4.14 Schem e for the attem pted preparations of donor-acceptor substituted fused-ring aromatic backbone. 223 Fig. 4.15 nBuli D M F S p o c i HO, D M F 2. nBuLi CHO I D r D M SO H.O* 1. N B S.B PO CH-.CL A cO OA c N a 0 H M e 0 H /H 20 Br 2. A c: 0 . pyridine in T H F AcO O O I K; C O , M eO II H : 0 HO OH C F i 0^0 N H i - ? - O A o W n ^ cf o - N H O A cO piperidine catalyst O -S -O S H O C n lH n * COCI “\ V CO Cl O C nIN n+ i 2. heat 3. pole 4 crosslink with Pi catalvst STABLE NLO MATERIAL S i-O Me ?' Y '-h " S i , M e ' O - S i ' Proposed schem e for the synthesis of asymmetrially functionalized donor-acceptor chromophores for incoporation into a rigid-rod polymer. 224 4.4 Conclusion A new and more efficient way of synthesizing 4 w as presented. The new schem e m ade use of step-w ise attachm ent of the thioacetaldehyde dimethyl acetal groups which led to an over-all higher yield of com pound 4. This compound can be used as a chromophore backbone for synthesizing rigid-rod polymers for second-order nonlinear optical applications. In this line, synthetic studies to functionalize com pound 4 were undertaken. C om pound 4 w as functionalized with a donor group, -SB to form compound 6. Com pound 6 w as further functionalized with diol groups to form com pound 8 by converting the methyl groups at the center ring to hydroxy groups. The hydroxyl groups can be used for incorporating compound 8 into polymeric system s. Com pound 4 also form ylated resulting to an ald eh y d e-substituted molecule. This can be used as an intermediate for attaching other acceptor groups (e.g., malonitrile, thiobarbituric acid, phenyl isoxzolone). O ther functionalization reactions (i.e., tricyanovinylation and nitration), however, were not successful. 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