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Design and synthesis of novel second order nonlinear optical materials
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Design and synthesis of novel second order nonlinear optical materials
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DESIGN AND SYNTHESIS OF NOVEL SECOND ORDER NONLINEAR OPTICAL MATERIALS by Rima Ghosn A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (CHEMISTRY) May 1996 Copyright 1999 Rima Ghosn UNIVERSITY OF SOUTHERN CALIFORNIA T H E G R A D U A TE S C H O O L U N IV E R S IT Y PARK LOS A N G E LE S , C A L IF O R N IA 0 0 0 0 7 This thesis, •written by Rim a G hosn ______________________ __ under the direction of h pm. Thesis Committee, and approved by a ll its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements fo r the degree of ----------- Iiasiec..Qf„Science.. j (L .. D taa Date _ _ THESIS COMMITTEE g y r,. . .j }d?. &^S&rrrT / Gbmfnittm Chsi r J r : — ACKNOWLEDGMENTS This research was completed under the guidance of my advisor, Professor Larry R. Dalton, and the rest of my defense committee, Professor Thieo E. Hogen-Esch and Professor William H. Steier. I am most grateful for the guidance and continued support of Professor Dalton. His patience and continued encouragement through difficult times is greatly appreciated. He introduced me to the field of NLO, which has proven to be the most fulfilling scientific research I have undertaken. His insight, not only into chemistry, but into life in general wiil be always useful. He is truly the best research advisor for whom I could ask. Thank you for your continued belief in all of your students. I would like to thank the Dalton group for their wonderful support and friendship. I will definitely miss all of you and will always remember the good times we shared, i especially want to extend my gratitude to Aaron Harper for his continued insightful discussions and bright ideas, and for proofreading this thesis. I want to also thank my friends Younsoo Ra Darin Files and Andrea Hubbell for their continued support. I would also thank pervious group members, Dr. Robert Montgomery, for getting me started in the lab and for synthesizing key starting materials. I thank Dr. Chuck Xu, and Dr. Bo Wu for their help and advice. I want to extend my gratitude to the chemistry faculty members, staff members, and graduate students for their support. The past years at USC have been filled with wonderful memories. Finally, I want to thank my family for their continued support and encouragement. They have been a driving force in my education. They are all that I can wish for and more. iii Table of Contents Acknowledgments.................................................................................ii List of Figures....................................................................................... v List of Tables....................................................................................... vii Abstract...............................................................................................viii CHAPTER 1: A Review of Organic Nonlinear Optics and Polymeric Nonlinear Optical Materials 1.1 Introduction.................................................................................. 1 1.2 Principles of Second-Order Nonlinearities..................................2 1.2.1 Polarization by a Light Field......................................... 3 1.2.2 Second Harmonic Generation......................................4 1.2.3 Electrooptic Phenomenon............................................ 7 1.3 Review of Second Order-Nonlinear Optical Polymers.............. 9 1.4 Review of Second Order-Nonlinear Optical Materials............ 15 1.4.1 Guest-Host Polymer Composites............................. 15 1.4.2 Incorporation of NLO Dyes into Polymer Matrix 16 1.4.3 Crosslinked Polymer Materials..................................20 1.5 Motivation and Research Objective......................................... 24 1.6 References................................................................................25 Chapter 2: Novel Functionalized Second Order Nonlinear Optical Chromophores 2.1 Introduction................................................................................ 32 2.2 Material Design..........................................................................34 2.3 Experimental Section................................................................38 2.3.1 General Materials and Methods................................38 2.3.2 Synthesis of NLO monomers.................................. 39 2.4 Results and Discussion.............................................................50 2.5 Conclusion.................................................................................. 52 2.6 Future Research.........................................................................53 2.7 References................................................................................. 54 V List of Figures Fig. 1.1 Second Harmonic generation phenomenon caused by the interaction of light with NLO media........................................... 5 Fig. 1.2 General three-wave mixing and second harmonic generation.................................................................................... 6 Fig. 1.3 A representative Mach-Zehnder interferometer for electrooptic light modulation................................................................8 Fig. 1.4 Electric field poling alignment and relaxation of chromophore dipoles.........................................................................12 Fig. 1.5 Examples of polymers with different chromophore configuration............................................................................. 19 Fig. 1.6 The reported thermoset NLO materials........................... 23 Fig. 2.1 Charge transfer process of DANS.................................... 33 Fig. 2.2 Schematic representation of the DEC monomers and the polymerization techniques.................................................................38 Fig. 2.3 Synthetic route of the donor-bridge of NLO chromophores.......................................................................... 40 Fig. 2.4 Preparation of N-ethyl, N’-( 2-hydroxyelhyl) thiourea.....43 Fig. 2.5Synthesis of functionalized bis(phenyl) thiourea 46 Fig. 2.6 Large b-value monomers synthesized. ......... 48 Fig. 2.7 Future polymerization schemes.......................................... 53 Fig. 2.8 Organic thermosetting polyurethanes.............................. 54 List of Tables Table 1.1 Nonlinear optical properties of some important chromophores........................................................................... 35 Table 2.1 Nonlinear optical properties of thiobarbituric acids.......50 Abstract This dissertation focuses on the design, synthesis and nonlinear optical properties of large second-order nonlinearity chromophores. A review of the fundamentals of nonlinear optics and polymeric materials is provided in the first chapter. The second chapter deals with the design and synthesis of chromophores derived from thiobarbituric acids. Conventional nonlinear optical organic materials employ chromophores possessing a large transition dipole moment between the ground and first excited states. This requirement is fulfilled using conjugated molecules containing strong electron donor and acceptor moieties. Barbituric acid is a strong electron acceptor that yields molecules with a large first hyperpolarizabilities. There are two classes of chromophores investigated, both capable of covalent polymer incorporation, which include the main chain and double-end crosslinkable chromophores. These chromophores that are both high in nonlinearity and thermal stability, are regarded as candidates for electrooptical devices. viii Chapter 1 A Review of Nonlinear Optics and Polymeric Nonlinear Optical Materials 1.1 Introduction The pressing need for better optical communication technology has exploded the field of nonlinear optics. Electronics, the process which uses electrons to acquire, store, transmit and process information, cannot keep up with current advances in the telecommunication industry, so that the field of photonics, the analog of electronics that uses photons instead of electrons, has widely emerged. The impact of photonics and electronics on our lives can be seen all around us. Their uses range from integrate governmental telecommunication devices to everyday appliances, such as the television, and telephone networks. The field of photonics has been greatly studied in nonlinear optics (NLO). Nonlinear optics, the discipline which takes into account the effect the nonlinear polarization of a material in response to a strong oscillating electric field is at the frontiers of photonic research and application. Using NLO phenomena, propagation 1 properties of light can be uttered by a strong electric field, which disrupts the internal electric field of matter. NLO application include light modulation, frequency conversion, optical switching, memory and sensor protection. Novel optical materials has been the focus of much of the research of nonlinear optics. Inorganic materials, such as lithium niobate and potassiun dihydrogen phosphate (KDP), have dominated the electronic field until now, but the need for materials exhibiting superior properties has opened up the field for organic materials. Organics exhibit unique properties which might prove essential for the advancement of this field of optics. These properties include fast response time, large optical nonlinearity, low dielectric constant, good processibility, increased availability, low cost, and ease of synthesis without inherent defects as those encountered in crystal growth. The main problems associated with inorganic crystals include high dielectic constant, difficulty in growing crystals, and their high damage susceptibility. The focus of this dissertation will be the synthesis of chromophores exhibiting large nonlinear optical properties, high thermal stability, and possessing the capability incorporation into polymer systems. 2 1.2 Principles of second-order non linearities'1 -6 The principles of nonlinear optics can be divided into three categories. These are the polarization by light, second harmonic generation, and electrooptic response of the NLO material. (A) Polarization by a light field Polarization is achieved when a nonconducting or nonmagnetic media is subjected to an external electric field. There are two cases of polarization, linear and nonlinear. The first case arises when a relatively low field strength is applied causing a linear polarization. This can be given by the microscopic polarization equation : p = a E (1.1) where (p) is the polarization and (E) is the field strength. The other case arises when the field is strong enough to cause a more pronounced interaction of the molecule with the electric field. The polarization response will then be nonlinear. The microscopic induced polarization (p) of the molecule can be expressed by the following power series of the electric field. This 3 is only applicable when (p) is small, and the dipolar approximation holds. p=aE+pEE+y EEE + (1.2) where a is the polarizability, and p and y are referred to as the first and second hyperpolarizability tensors respectively. The coefficient p is third rank tensors and describes the second-order nonlinear response of the molecule, which will prove very important in this research project. The linear polarizability is a special case of the nonlinear polarization equation at low electric field intensities ( where the higher order terms are negligible). The macroscopic relationships between the polarization of a material and the electric field can be expressed in the following equations: This expresses the linear polarization of the bulk material, where the linear susceptibility is independent of the field strength. In strong applied fields, the macroscopic polarization can be expressed in the following power series expansion: P = %0) E (1.3) P = X < 1) E + X < 2) EE + x(3) EEE + (1.4) 4 where % (1), % (2) and %(3) are referred as to the first-order, second-order and third-order susceptibility tensors respectively. (B) Second harmonic generation (SHG) Equation 1.4 can be expressed in term of the sinusoidal equation for the oscillating electric field of light by first noting that E (z ,t) = E0cos(cot-kz) (1-5) where Eo is the amplitude, w is frequency, k is the proportionality constant, and kz describes the relative phase of the light. Simple manipulation of equations 1.4 and 1.5 gives equation 1.6 with new frequency terms imposed by nonlinear polarization. P = % (1 )E0cos(cot - k z )+ %(2)eJcos2 (cot - kz) + x(3 )EqCos3 (cot - kz) = x(t)E0cos(oX -k z) + ^ x (2)E j[l+ c o s(2 c o t-2k z)] +z(3 ,E 2 3 1 I -cos(cot-kz)+~cos(3cot-3kz)J (1*6) These components, at 2co and 3to, are resultants of the phenomena known as second harmonic generation (SHG) and third harmonic generation (THG). In the SHG process, as illustrated in Figure 1.1, two photons at frequency w combine 5 through interactions with the medium to generate a new photon at 2 c o . 2co ► SHG Figure 1.1. Second harmonic generation phenomenon caused by the interaction of light with NLO media. General three-way mixing and the second harmonic generation can also be illustrated by figure 1.2 . c o -| =0)2=0) and 0)3 = 2c o Figure1.2. General three-wave mixing and second harmonic generation. 6 SHG reflects the second-order NLO effects of materials. The % (2) coefficient of a material is determined by the measured intensity of the second harmonic light. Traditionally, the SHG coefficient d is often used to describe the experimental measurement results of the second-order NLO effects. These coefficients are related by the following equation: %ijk~2djjk (1-7) where subscripts i, j, k are Cartesian indices. SHG applications are numerous and include the conversion of light from long to short wavelengths which can be used for the storage of optical data at much higher density. However, the realization of many of the devices depend on the synthesis of new NLO materials with large second-order nonlinearity for high conversion efficiency. This is limited by optical transparency which is decreased with the increase of nonlinearity. A balance between transparency, at the fundamental and the second harmonic wavelength in order to avoid optical loss, and SHG efficiency is needed for device application. This has been a hindrance that has so far limited much of polymeric materials from being practicle for SHG application. 7 (C) Electrooptics Nonlinear optics can be viewed in terms of a field-induced modulation of the refractive index (n). A fast alteration of the index of refraction in the presence of a low frequency electric field is needed for any application. The Pockels effect or the electrooptic (EO) effect is the result of a linear change in the dielectric constant with respect to the electric field. This involves a change in the NLO material’s refractive index due to the polarization of the material.. where rjj is referred to as the EO coefficient of a material. The most used electrooptic coefficients are the tensor components ri 3 and r3 3 , where r i 3= 3r33 . Experimental measurement include interferometry, ellipsometry and wave guiding, while estimated values can be obtained from SHG measurement of the %(2) susceptibility. The EO effect is used to modulate light by an external electric field. The EO modulator is a useful device for fast optical signal processing and optical switching for telecommunications by transducing electric signals into optical signals. (1.8) 8 Output beam Input beam Electric field Fig 1.3 A representative Mach-Zehnder interferometer for electrooptic light modulation. A schematic diagram of the Mach-Zehnder interferometer is represented in figure 1.2 in which an input light beam is diverged into two channels, which then travel through two nonoptically active waveguide channels. One beam is then phase shifted by an external electric field. The particular phase shift is dependent on the field strength used. The interference of the two beams as they recombine possesses a new intensity as dictated by the phase shift incurred. This output beam is now encoded with the information carried by the applied field’s electric signal. The phase shift of the beam induced by the EO effect can be expressed as 2nAnL Tin*rEL 9 where E is the applied electric field, X is the wavelength of light to be modified, and L is the propagation length in the nonlinear medium. For a given E and L, the phase shift (A < J > ) is proportional to n3r, which is traditionally referred to as the figure of merit (FOM) of second-order NLO materials for EO modulators. For most applications, phase shift of p is needed for modulation. NLO material must then be optimized to give the largest FOM to be used in devices, which includes the r coefficient. These are dependent on the values of E, X and L. The larger the % (2), the lower the external voltage and the lower the light propagation path is needed. Therefore, one of the targets of NLO research is to optimize the optical nonlinearity of polymeric materials. 1.3 Review of Second Order Nonlinear Optical Polymeric Materials In theory, all materials can be noniinearly polarized. However, with the limited laser power sources available, nonlinear effects can not be achieved for most materials. As discussed earlier, one of the requirements for second order nonlinear optical materials is the lack of a center of symmetry at the molecular level. This is imposed by the odd-ordered tensor property of % (2). At the molecular level, it has been observed that a molecule containing an electron donor and an electron acceptor connected 10 via a delocalized p system exhibits a large p value. These second- order NLO chromophores have been studied extensively and many have been d e v e lo p e d .23~30 By varying the electron donation and withdrawal strengths, researchers have been working to maximize the beta value. The macroscopic nonlinearity % (2) is dictated, among other things by the combined microscopic molecular nonlinearity p of the medium. The mathematical relationship between %(2) and p is approximated by an oriented gas model of the polymeric material. General three wave mixing and second harmonic generation proportionality substitution from figure 1.2 gives rise to the SHG expression: X{2) = iV /2(0) ) / ( 2 G>)p{cos3e ) (1.9) where N is the number density describing the NLO chromophore concentration in the bulk materials, f(co) and f(2c a ) are the local field factors at frequency c o and 2ca respectively, and <cos3 0> is the alignment factor describing an average orientation of all the dipolar chromophores, where 0 is the angle of the chromophore dipole axis with respect to the external field. This relationship emphasizes the need of polymers possessing a higher number density of large p chromophores in high macroscopic noncentrosymmetric order. 11 Electric field poling is used for the alignment of the chromophore dipoles at a temperature where the dipoles can be easily oriented. Figure 1.4 illustrates the alignment of chromophore dipoles upon electric field poling. The most common poling technique is corona poling.31-33 Higher fields across the polymer films are realized as compared with other methods such as that of electrode poling. In order to pole the polymer material effectively, the polymeric film is heated near or at its glass transition temperature, Tg, where it is believed that the chromophore, thus the polymer unit, is in the mobile phase. While the polymer is being heated, an external electric poling field is applied to align the dipole moments of the chromophores. The optimization of the poling efficiency is achieved by a tedious balancing process of the poling temperature, applied external field strength and the poling duration. Theoretically, the higher external field strength used, the lower the poling temperature is needed to shift the equilibrium in favor of the ordered state. But, there are many other factors that play a role in the poling process. Poling is done at the lowest needed temperature to eliminate any possibility of dielectric breakdown of the polymer or thermal decomposition of the chromophore. However, if the poling temperature is too low, the polymer chain segments will be too restrained to allow for dipole 12 iww>wrownw«niiH'iiUiMir I 1) Heated to above Tg 2) Poled by electric field Electric field off relaxes to random state Isotropic state J C (2 ) = 0 Ep = 0 f polar axis z Anistropic state %{2) * 0 E p ^ 0 Electric field off Ordered lattice O ' a Q B m f ^ B n C J * The poled order state can go back to the isotropic state or stay at anistropic state Figure 1.4 Electric field poling alignment and relaxation chromophore dipoles (represented by arrows). 13 reorientation. In addition to poling conditions, the larger the dipole moment of the polymer, the larger the poling efficiency will be. After the dipole moments are aligned, the electric field is turned off and the polymeric material will either reiaxe to the random state or will remain oriented in an ordered lattice state. This will be dependent on the chromophore type and the polymer system. In general, poled polymer systems tends to lose the NLO activity (especially at elevated temperatures) as the chromophores relax to the thermodynamically stable isotropic state. This process is related to the mobility of the chromophore in a polymer matrix and can be measured by monitoring the recovery of the absorption intensity or by following the decay of the SHG and EO coefficients. 12-22, 35-36 The poling methods are assessed by the degree of poling, also termed the poling efficiency, of the material which is related to the alignment factor <cos30>. The alignment factor <cos30> can be calculated from the order parameter values of O.34 The order parameter can be obtained by measuring the absorbance of an unpoled system Ao and of a poled system Aa:3^-34 < I> = 1 - Aa / A0 (1.10) There are competing forces which play part in determining poling efficiency. These are the inherent thermal energy of the 14 unpoled polymer due to the more thermodynamically stable random dipole orientation, and the dipole alignment energy which pushes to align the dipole moment in an ordered direction to counteract the applied electric field . NLO polymers must be thermally stable at temperature up to 100 °C over long periods of time, and be stable to short term fabrication temperatures of 280 oc,37 it is therefore critical to stabilize the NLO activity of the polymeric materials in order to meet these prerequisites. 1.4 A review of second-order NLO polymeric materials Polymeric systems’ properties make them very promising materials for photonic devices. There are three types of polymeric materials that have been studied for device applications. They are (1) guest/host polymer composite materials, (2) polymers with covalently bonded NLO chromophores, and (3) cross-linked NLO polymers. The advantages and the disadvantages will be discussed in this section. (A) Guest-host polymer composites Guest-host materials were the earlier materials studied due to their ease of syntheses. The NLO chromophores are doped into 15 polymer matrices, where there is no covalent attachment of the polymer and the chromophore. Polymethyl methacrylate (PMMA) and polystyrene (PS) were commonly used as polymer host due to their good optical qualities. In general, the doped systems that have been thoroughly studied have achieved a maximum loading density of 15% by weight. One of the most studied guest-host polymer systems is PMMA doped with the azo dye Disperse Red 1 (DR1).31i 38-40 Measured resonantly-enhanced values of r33=2.5 pm/V at 633 nm and of d33=6.7 pmA/ at 1.58 mm were realized. Another system of relatively high-p chromophore, 4-dicynovinyl-4,-(dialkylamino) azobenzene into PMMA measured a high d33 value of 74 pmA/ at 1.58 mm. However, due to rapid relaxation of the dipole moment, this value decayed to 19 pmN within several days even at room temperature.41 The intramolecular degree of freedom of the host polymer played a significant role in the orientational relaxation of the guest molecules.42 The randomization of the dipole order is a serious and limiting problem in NLO. The usefulness of guest-host systems for NLO materials is also limited by the low loading density of the chromophore. When the NLO chromophore is doped in higher loading density, the chromophores tend to aggregate and even crystallize, resulting in phase separation. Therefore, although 16 such systems are simple to formulate, much of the research has recently been focused on covalent incorporation of the chromophores, which will be discussed in the next section. (B) Covalent incorporation of NLO chromophores into polymer matrices. The need for an increase in loading density of the NLO unit, and increased stability with lower relaxation rate has been achieved with covalent attachment of the NLO unit. In this case, aggregation and phase separation is minimized, and the chromophores are not as mobile for rapid relaxation as in the doped system. There are two major ways to covalently incorporate NLO chromophores into polymer units. Main-chain polymers, where the chromophore is part of the polymer backbone, can be subdivided into the head to tail, head to head, and random configurations of the NLO unit relative to the backbone. The other method is typified by side-chain polymers, where the NLO unit is an attached pendant from the polymer backbone. The covalently attached chromophores exhibit better temporal stability than the guest host systems. In a study conducted by Singer,38 side-chain, covalently-attached dicyanovinyl azobenzene dyes to PMMA decayed 10% from the 17 original poled polymer value, (see Figure 1.5). Similar guest-host system decayed 75% under the same conditions within 35 days. Covalent incorporation of NLO dyes into polyimides were also studied to overcome the problems associated with composite systems. The literature is filled with many examples where covalently incorporated NLO units exhibited a higher retention percentage of the NLO activity as opposed to guest host systems. Recently, Dalton, e t a f f l have successfully synthesized a side- chain DR-19 polyimide and achieved a large nonlinearity (d33 = 200x10-9 esu) and improved NLO activity retention rates. Synthesis of head-to-tail NLO polymers46-50 was persued for the belief that since the dipole moment of the chromophores are lined up in the same direction, the nonlinearity would be significantly large. However, poling difficulty of the polymer in the solid state and film fabrication has been a major hindrance in testing these systems. Early work of several head-to-head NLO polymers5'1 “53 were synthesized, but their nonlinearities were never reported. Recently, results reported by Lindsay and coworkers suggest that, provided that there is a flexible spacer between the head-to-head connected dipoles, their dipole moments were not canceled canceled (see figure 1.5).55 Dalton, e t a l 20,56 have resently demonstrated that random NLO polymers (see Figure 1.5) can be 18 Side chain polymer: H - £ h2c - ' H 3 ^ t r ~ H c h 3 h2c - c - I c=o I V N c=o I OCH3 CN CN Head-to-tail polymer: CN n- G - f COOR H CN — S H CO Head-to-head polymer: CO— N H N H — CO C N -i. l 7 CN " ^ i _ A N v n Random polymer: _ N_ f V N" so^- < ch*>°- ° - f 1 CH2CH2- O - O C - NH(CH2)eNH— CO— Figure 1.5 Examples of polymers with different chromophore configurations. poled successfully and have exhibited sizable nonlinearities. In summary, covalent incorporation of chromophores onto the polymers increased their nonlinearity and their thermal stability. Much work is still needed to address the structure- function relationship that will be most effective in developing NLO polymers with larger nonlinearities. (C) Crosslinked Polymeric Materials There has been many attempts at extending the lifetime of poled polymers which depends mainly on the thermal and orientational stability of a NLO material. Consequently, there has been much work done to stabilize the poled order of polymers dipoles. The explored methods for decreasing the relaxation rate of the poled chromophore units is by increasing the glass transition temperature (Tg) of a polymer system, or by crosslinking which results in the partial immobilization of the chromophores and/or segments of the polymer chain. Polyimides have taken center stage in the development of high Tg polymers. They have been used in a variety of systems, including guest-host and covalently incorporated systems. Very recently, Dalton and coworkers incorporated DR19 into a 20 polyimide backbone with improved thermal stability and large nonlinearities (d33 ~200 x 10-9 esu). Another method for realizing stable nonlinearity is by crosslinking of the chromophore to restrict the rotation of the aligned NLO units. Crosslinking can be either photochemically or thermally induced. Although photo-induced crosslinking has been exploited17,58-60 thermally-induced crosslinking methods have been more extensively studied. There are four main approaches to thermally crosslink NLO systems. Side-chain polymers are synthesized and subsequently form crosslinking between the polymer chains. In the first approach crosslinking can be achieved by functionalization of the chromophores or the polymer backbone. In a recent effort, Marks and c o -w o r k e r s 8 1 prepared hydroxyi-functionalized polystyrene with side chain NLO chromophore. Epoxy resin was used for crosslinking, which allowed the polymer to maintain -90% of initial SHG signal after 350 h at room temperature. In this crosslinked system, the chromophore units were not involved in the crosslinking process, and therefore were able to rotate freely. Dalton, e t a/.18* 19 realizing the need to tie down the chromophore from both ends have researched a side-chain NLO polymer with the crosslinking sites (hydroxyl group) on the free end of the chromophore. After poling and thermal curing with 21 diisocyanate, there was no detectable decay in NLO signal at room temperature. Even at 90 OC, there was only a small initial decay of NLO activity with the polymer material retaining the majority of its activity after several days. The second crosslinking method is to synthesize the main chain NLO polymer, and then crosslink it while poling.20. 56> 57 In this case, both ends of the chromophore are covalently bonded to the polymer chain which also contains crosslinking sites. This process allows for the alignment of the NLO dye during the poling process, and the units are tied down while the electric field is still applied. This process will require a flexible spacer between the rigid dye, which will put a limit on the loading density of the chromophore. The third approach involves thermosetting of the NLO monomer or oligomers into a crosslinked lattice. This approach have been favored because high loading density is achieved. The increase in the loading density improves the nonlinearity, but at the same time decreases the processibility of the material. Eich and coworkers at IBM 62»63 pioneered this effort by using a multifunctional nitroaniline-epoxy reaction (see Figure 1.6 a). A nonlinearity of d33=13.5 pmA/ at 1.06 mm was reported with no observed signal decay at 80 °C for 40 minutes. An increase in the Tg of the system was also reported. Dalton and co-workers'! 2> 1 3 22 ( a ) NH2 ► Thermoset epoxy n o 2 ( b ) OCN‘ 1 NCO H C ^O H OH Thermoset polyurethane (c) ,, Thermoset H2n o h + o c n ~ n c o polyurethane H C k x N /\y O H N (d) c h 3 V so2 NCO (CH2)2 1 OH NCO ^ Thermoset polyurethane Figure 1.6 The reported thermoset NLO materials 23 have demonstrated that large nonlinearity (d33=120 pmA/ at 1.06 mm, r33=13 pm/V at 633 nm) can be realized for the thermosetting polyurethane system (see Figure 1.6 b), with 70% of the signal still remained at 90 °C for 3000 hours. Since then, there had been many systems reported. But the need for improved poling efficiency and curing conditions remains to be of great concern. The fourth approach involves crosslinking through the sol- gel process. This approach is desirable for the rigid inorganic/organic composite network it creates while maintaining excellent optical clarity. Originally, only NLO sol-gel systems with low nonlinearities have been reported (d33 values ranging from 0.04 pm/V to 13.7 pm/V at 1.06 m m )J° However, a polyimide/sol- gel interpenetrating polymer network (IPN) system with relatively larger-p chromophores and improved poling efficiency yielded a large d33 value (33 pm/V at 1.06 mm). No measurable decay of SHG intensity after being heated at 100°C for 168 hours was observed.66 1.5 Motivation and objective of research Keeping in mind that macroscopic nonlinearity is the direct result of the average contribution of the individual chromophore units, thus, a large p chromophore is essential for increased 24 macroscopic nonlinearity. This has opened the door for hundreds of new chromophores to be synthesized and their structure- property relationships tested. However, there has not been any NLO system that has met all application requirements mainly, due to either or both small nonlinearity or poor stability of the polymer material. Although many NLO polymers have been developed and some prototype devices fabricated, the need for improved nonlinearity and orientational stability at elevated temperatures is the basis of this research effort. The objective of this thesis therefore, was to develop NLO chromophores of high nonlinearity and processibility, along with increased thermal stability. The design and syntheses of a variety of single and double-end crosslinkable chromophores are reported. The design and syntheses of novel multilinkable chromophoric monomers are also illustrated. In order to study the structure-function relationship, different donor and acceptor groups are explored. 1.6 References 1. Dalton, L. R.; Sapochak, L. S.; Chen, M.; Yu, L. P. In M o le c u la r E le ctro n ics a n d M o le c u la r E le c tro n ic Devices', Sienicki, M., Ed.; CRC Press: Boca Raton, FL, 1993. 25 2. Prasad, P. N.; Williams, D, J. In tro d u ctio n to N o n lin e a r O p tic a l E ffe cts in M o le cu le s a n d P olym ers, Wiley: New York, 1991. 3. Williams, D. J. A ngew . C hem . Intl. Ed. Engl. 1984, 23 690. 4. Boyd, R. W. N o n lin e a r O ptics; Academic Press: New York, 1992. 5. Shen, Y. R. The P ricip ie s o f N o n lin e a r O ptics; Wiley: New york, 1984. 6. Lytel, R.; Lipscomb, G. F.; Stiller, M.; Thackara, J. I.; Ticknor, E. J. P roc. S P IE, 1989, 971, 277. 7. Bjorklund, G. C.; Boyd, R. W.; Garter, G.; Garito, A. F.; Lytel, R. S.; Meredith, G. R.; Prasad, P.; Stamatoff, J.; Thakur, M. A p p l. O p t 1987, 26, 227. 8. Eaton, D. F. C H E M T E C H 1992, 308. 9. Chemla, D.; Zyss, J. N o n lin e a r O p tic a l P ro p e rtie s o f O rg a n ic M a te ria ls a n d C rystals; Academic Press: Orlando, FL., 1987, 1, 23. 10. Burland, D. M.; Miller, R. D.; Walsh, C. A. C hem . R e v ie w 1994, 94, 31 and the references therein. 11. Dalton, L. R.; Yu, L. P.; Chen, M.; Sapochak, L. S.; Xu, C. S ynth. M et. 1993, 5 4 ,155. 12. Mai, C.; Yu, L.; Dalton, L. R.; Shi, Y.; Steir, W. H. M acro m o l. 1991,25,4032. 13. Shi, Y.; Steir, W. H.; Mai, C.; Yu, L.; Dalton, L. R.; A p p l. P hys. Lett. 1992, 60, 2577. 26 14. Shi, Y.; Ranon, P. M.; Steir, W. H.; Xu, C.; Wu, B.; Dalton, L. R.; Wang, W.; Chen, D.; Fetterman, H. P roc. S P IE 1993, 2 0 25 , 535. 15. Wu, J. W.; Vally, J. F.; Ermer, S.; Binkley, E. S.; Kenney, j. T.; Lytel, R. A ppl. P hys. Lett. 1991, 59, 2214. 16. Lon, J. T.; Hubbard, M. A.; Marks, T. J.; Lin, W.; Wong, G. K. C hem . M ater. 1992, 4, 1148. 17. Chen, M.; Yu, L.; Dalton, L. R.; Shi, Y.; Steir, W. H. M a cro m o l. 1991,2 4 , 5421. 18. Xu, C.; Wu, B.; Todorova, O.; Dalton, L. R.; Shi, Y.; Ranon, P. M.; Steir, W. H. M acrom ol. 1993, 26, 5303. 19. Shi, Y.; Ranon, P. M.; Steir, W. H.; Xu, C.; Wu, B.; Dalton, L. 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C om m un. 1992, 672. 28. Rao, V. P.; Jen, A. K.; Wong, K. Y.; Drost, K.; Mininni, R. M. In N o n lin e a r O p tica l P ro p e rtie s o f O rg a n ic M a te ria ls V; Williams, D. J., Ed.; Proc. SPIE, 1992; 1775, 32. 29. Jen, A. K.; Wong, K. Y.; Rao, V. P.; Drost, K.; Mininni, R. M. Presented in O rg a n ic M a te ria ls fo r N o n lin e a r O p tica l A p p lica tio n s; MRS Spring Meeting; San Fransisco, April, 1993. 30. Ulman, A.; Willand, C. S.; Kohler, W.; Robello, D. R.; Williams, D. J.; Handley, L. J. A m . C hem . Soc. 1990, 112, 7083. 31. Mortazavi, M.; Knoesen, A.; Kowel, S,; Higgins, B.; Dienes, A. J. O pt. S oc. A m . 1989, B6, 733. 32. Hampsch, H.; Torkelson, J.; Bethke, S.; Grudd, S. J. A ppl. P hys. 1990, 6 7 ,1037. 33. Knoesen, A.; Molau, N.; Yankelevich, D.; Mortazavi, M.; Dienes, A. int. J. N o n lin e a r Opt. P hys. 1992, 1, 73. 28 34. Page, R. H.; Jurich, M. C.; Beck, B.; Sen, A.; Twieg, R. J.; Swalen, J. D.; Bjorklund, G. C.; Wilson, C. G.; J. O pt. Soc. A m . B 1990, 7, 1239. 35. Jones, P.; Jones, W.; Williams, G. J. C hem . S oc. Fraad. Trans. 1990, 8 6 ,1013. 36. Cicerone, M.; Ediger, M. J. C hem . P hys. 1992, 97, 2156. 37. Lytel, R,; Lipscomb, G. Mat. R es. Soc. S ym p. Proc. 1992, 247, 17. 38. Singer, K. D.; Sohn, J. E.; Lalama, S. J. A ppl. P hys. L e tt 1986, 49, 248. 39. Singer, K. D.; Kuzyk, M.; Sohn, J. E.; n. J. O pt. S oc. A m . 1987, B4, 968. 40. Singer, K. D.; Kuzyk, M.; Holland, W. R.; Sohn, J. E.; Lalama, S. J.; Comizzoli, R. B.; Katz, H.; Schilling, M. A ppl. P hys. Lett. 1988, 5 3 ,1800. 41. Katz, H.; Singer, K. D.; Sohn, J. E.; Dirk, C.; King, L.; Gordon, H. J . A m . C hem . S oc.. 1987, 109, 6561. 42. Hampsch, H. L.; Yang, J.; Torkelson, J. M. M a cro m o l. 1988, 21, 526. 43. Ermer, S.; Valley, J. F.; Lytel, R.; Lipscomb, G. F.; VanEck, T. E.; Girton, D. G. A ppl. P hys. Lett. 1992, 2272. 44. Valley, J. F.; Wu, J. W.; Ermer, S.; Stiller, M.; Binkley, E. S.; Kenney, J. T.; Lipscomb, G. F.; Lytel, R. A p p l. P hys. Lett. 1992, 6 0 ,160. 45. Lin, J. T.; Hubbard, M. A.; Marks, T. J.; Lin, W.; Wong, G. K. C hem . M ater. 1992, 4,1148. 29 46. Stenger-Smith, J. D.; Fischer, J. W.; Henry, R. A. M akro m o l. C h e m . , R a p id C om m un., 1984, 1 1 ,141. 47. Kohler, W.; Robello, D. R.; Dao, P. T.; Willand, C. S.; Williams, D. J. J . C hem . P hys. 1990, 92, 9157. 48. Mitchell, M. A.; Tomida, M.; Padias, A. B.; Hail, Jr. H. K. C hem . M ater. 1993, 5 , 1044. 49. Fuso, F.; Padias, A. B.; Hall, Jr. H. K. M acrom ol. 1991, 24, 1710. 50. Mitchell, M. A.; Hall, Jr. H. K.; Mulvanney, J. E.; Willand, C.; Williams, D. J.; Hampsch, H. P olym . P repr. 1992, 3 3 ,1060. 51. Wang, C. H.; Guan, H. W. Proc. S P iE 1992, 1775, 318. 52. Wright, M. E.; Mullick, S. M acrom ol. 1992, 25, 6045. 53. Wright, M. E.; Sigman, M. S. M acrom ol. 1992, 25, 6055. 54. Lindsay, G. A.; Stenger-Smith, J. D.; Henry, R. A.; Hoover, J. M.; Nissan, R. A.; Wynne, K. J. Macromol. 1992, 25, 6075. 55. Unsay, G. A.; Stenger-Smith, J. D.; Henry, R. A.; Nissan, R. A.; Merwin, L. H.; Chafin, A. P.; Yee, R. Y. "High-Temperature Sierrulate Nonlinear Optical Polymers" in O rg a n ic Thin F ilm s fo r P h o to n ic A p p lica tio n s T e ch n ica l D igest, Optical Society of America: Washington, D. C.,1993, 1 7 ,14. 56. Ranon, P. M.; Shi, Y.; Steir, W. H.; Xu, C.; Wu, B.; Dalton, L. R. A p p l. P hys. Lett. 1993, 62, 2605. 57. Xu, C.; Wu, B.; Dalton, L. R. Ranon, P. M.; Shi, Y.; Steir, W. H.; M acrom ol. 1992, 25, 6716. 30 58. Mandal, B. K.; Lee, J. Y.; Zhu, X. F.; Chen, Y. M.; Prakienavincha, E.; Kumar, J.; Tripathy, S. K. S yn. M et. 1991, 41-43, 3143. 59. Mandal, B. K.; Kumar, J.; Huang, J. C.; Tripathy, S. K. M akrom of. C hem ., R a p id C om m un. 1991, 12, 63. 60. Mandal, B. K.; Chen, Y. M.; Lee, J. Y.; Kumar, J.; Tripathy, S. K. A pp l. P hys. L e tt. 1991, 58, 2459. 61. Park, J.; Marks, T. J.; Yang, J.; Wong, G. K. C hem . M ater. 1990, 2, 229. 62. Eich, M.; Reck, B.; Yoon, D. Y.; Willson, C. G.; Bjorklund, G. C. J. A ppl. P hys. 1989, 66, 3241. 63. Jungbauer, D.; Reck, B.; Tweig, R.; Yoon, D. Y.; Willson, C. G.; Swalen, J. D. A pp l. P hys. Lett. 1990, 56, 2610. 64. Francis, C. V.; White, K. M.; Boyd, G. T.; Moshrefzadeh, R. S.; Mohapatra, S. K.; Radcliffe, M.D.; Trend, J. E.; Williams,R. C. C hem . M ater. 1993, 5, 506. 65. Boogers, J. A. F.; Klaase, P. Th. A.; Vlieger, J. J. de.; Tinnemans, A. H. A. M acrom ol. 1994, 27, 205. 66. Chen, J. I.; Marturunkakul, S.; Li, L.; Jeng, R. J.; Kumar, J.; Tripathy, S. K. M acrom ol. 1993, 2 6 ,7379. 31 CHAPTER 2 Novel Functionalized Second-Order Nonlinear Optical Chromophores 2.1 INTRODUCTION In the recent years, many studies have been focused on the development of second-order nonlinear optical (NLO) chromophores for polymer incorporation. Keeping in mind that the macroscopic nonlinearity is sum of the averaged individual contribution of each chromophore, the need to synthesis a large p-chromophore is essential for the realization of a large macroscopic nonlinearity. There are many issues which need addressing when designing a chromophore. The chromophore need to exhibit large nonlinearity and thermal stability. A typical chromophore is characterized by a rc-electron donor and acceptor connected by a rc-electron segment. An example of one such chromophore is given in figure 2.1. When a laser irradiates an NLO chromophore, the electron density migrates from the donor to the acceptor end. This electron polarization interaction with the electric field gives rise to the electrooptic activity. 32 NLO phenomena occurs when either or both of two situations are present. The first is the change in dipole moment accompaning the n to n * transition of the NLO chromophore when subjected to an irradiation of light at a frequency characteristic of that chromophore. The second is the change of the dipole moment caused by a high intensity optical field where rc-electrons migrate within the same quantum level, also known as hyperpolarization. Both cases cause a shift in ^-electron balance of the molecule. 4-dimethylamino-4’-nitrostilbene (DANS) has been extensively studied for NLO application. Figure 2.1 gives the neutral and the charge-separated forms. The nonlinearity is associated with the difference in energies between the ground and excited states. In this case, there is a loss of aromaticity when going from the benzenoidal neutral state to the quinoidal charge separated state. neutral form charge-separated form Fig. 2.1 Charge transfer process of DANS 33 Marder and coworkers have reported that there needs to be a balance between the two forms for optimal nonlinearity. Therefore, there needs to be new criteria to design chromophores. Conventional chromophores are more characteristic of the neutral form and poses not enough of the charge-separated form. 2.2 Material Design When designing a chromophore, there are many factors to be taken to consideration. The choice of electron donor and acceptor, as well as the electron re-bridge are of crucial importance. Secondary requirements include optical transparency, thermal stability, and ease of synthesis. The electron donating and accepting strengths, as well as the bridging re-bonds are important in determining the nonlinearity strength as well as the thermal stability of a given chromophore. It has been thought that the larger the electron donation and withdrawal strengths of the donor and exceptor, the larger the nonlinearity is expected. However, recently it has been shown that for a given connective segment, there is an optimal contribution of donor and acceptor strengths. Table 2.1 gives a list of some important chromophores. These will be used to give molecular structure and susceptibility trends. In general, there is a correlation of larger p with the 34 increase of electron donating and withdrawing strength of a given chromophore, as weil as with an increase in the rc-bridge length. When the donor strength is increased Table 2.1 Nonlinear optical properties of some important chromophores # 1 2 3 4 5 6 Molecule h3c o — Q — n o 2 ^n— no2 t j - n o 2 ,N— N 02 (CH3)2N— Ns V H( 3 ( 1 0 '48 P o ( 10 " 30 esu)________esu)______ 275 (1.9) 330 (1.9) 71 (1.9)* 140(1.3) 12 358 (1.3) 25 106 \ / / n o 2 580 (1.9) 55 35 8 9 10 11 12 13 14 15 16 17 \ / N / N— N'v /= \ w n- Q - n o 2 800(1.58) 51 1470(1.58) 101 CN CN 4 1 0 ° ( 1 -5 8 ) 1 5 4 CN no2 ,N— ^°2 560 (1.9) 700 (1.9) r n \ j ^ u r N O = 1040 (1.9) JT N 0 2 8 u Bu _ CN > - 0 - n -n_ , s ^ o N N Cl CN ^ - O V t y ^ c N n—^ /)""V CN ■ ^r<fkcN 2190(1.58) 104 5320(1.58) 147 1300 (1.9) 6200 (1.9) CN 36 18 9100< 1-9> CN from a methoxy, to a dimethylamino to dithiolydinemethyl group, a significant increase in the hyperpolarizability is observed. The same can be seen for the withdrawing strengths. By increasing the rc-bridge, either by a longer aromatic or nonaromatic system, the p value is increased. Increase in p is also observed when the chromophore has a planar 7 r-bridge, which allows for better tc- orbital overlap. Better electron overlaps favors the charge transfer process, donor-acceptor interaction, which gives rise to a higher nonlinearity. This is apparent in #5 where the phenyl rings are planar to each other by the fluorene structure. The phenyl rings, however, in #4 are twisted due to steric hindrance which decreases the nonlinearity. In order to obtain a stable second-order material, the chromophores need not only be large in hyperpolarizability, but also be capable of incorporation into a polymeric system. Therefore the need to synthesis a double-end crosslinkable (DEC) chromophore monomer is essential. Figure 2.3 gives the schematic representation of the DEC monomers and the polymerization techniques. Although there are alcohols on both 37 sides of the donor and the acceptor, each has a different reactivity. This will aid in the polymerization on one side using one set of conditions, and then crosslinking on the other end by varying the reaction conditions. ^ A— * I NLo rc-conjugation ,OH HO' Condonsation polymorizalion DEC Monomers OH Addition polymn. — D“ I A HO OH X L J---- f crosslinking group “ I 1 XL I— A f _|_ Crosslinking groups {OH) A HO OH Slde*chaln polymers with crosslinkable dye ends I 0 @ @ @ @ i D D D D Q D W ^A/VW NAAA/V\AAAA/W VVVVW \AAA/VVfc 3D network with dipole alignment locked In Fig 2.2 Schematic representation of the DEC monomers and the polymerization techniques. 2.3 EXPERIMENTAL SECTION 38 (A) General materials and methods All reagents and solvents were of analytical grade quality, purchased commercially and used without further purification unless otherwise noted. The general methods for characterizing the structures and properties of the materials synthesized were by NMR, Bruker-250 spectrometer operating at 250 MHZ, with TMS as an internal standard. Thin layer chromatography and melting points were used when applicable. Solvents were dried and stored in the glove box for moisture sensitive reactions. Tetrahydrofuran (THF), and dioxane were refluxed under argon with sodium and the indicator benzophenone. All other anhydrous solvents were purchased as such and stored in sure sealed bottles. (B) Synthesis of the monomers The intermediate materials and monomer 4 are synthesized according to the scheme in Figure 2.3. and 2.4. There were four monomers synthesized, two capable of side chain incorporation into a polymer, and the other two are applicable toward DEC polymer schemes. 39 N,N-Bis[(2-methoxyeth)2-methoxyethyl)]aniline (1). A three necked flask, equipped with a stir bar under argon, containing N,N-bis(2hydroxyethyl)-aniline {16.367g, 90mmol) was dissolved in dry THF and cooled down to 0 °C. Sodium hydride (5.42g, 2.5 eq) was added carefully and stirred until all the bubbling has stopped. MEM chloride (27.0g, 217mmol) was added dropwise at 0°C and then stirred for 1 hr. The reaction mixture was H i ^ - ch2 oh HO NaH, CIChfcOCH 2CH2 0CH3 OoC 1. conc. HBr, Ote 2. P(OEt)3 ^ - C H 2P(0)(0E I)2 MEMO 1. tBuOK, THF 2. DMF/POCb 3. 2nBr2, CH2CI2 H i ’ r Figure 2.3 synthetic rout of the donor-bridge of NLO chromophores 40 then quenched with water and extracted with ether (3x100ml). The combined organic layer extract was dried over anhydrous magnesium sulfate, then concentrated. The residue was then purified by column chromatography (silica gel; hexanerether, 2 :1) to afford the yellow oil at 92% yield. 1H NMR (CDCI3), 2.08(s,3H); 3.55(t, 2 H); 4.20(t, 2H); 6.75(m, 3H); 7.23(t, 2H). 4-N,N“Bis(2methoxyethoxy)-methoxyethyI)amino benzaldehyde(2). In a three necked flask, dropwise funnel, condenser under argon dry DMF was cooled to 0 °C and POCI3 (14.97g, 97 mmol) was added dropwise and stirred for 1 hr. The phenyl amine (35g, 97mmol) was added dropwise with stirring while maintaining the 0 °C temperature. The reaction mixture was then refluxed for 2 hrs at 90 °C. When the reaction was cooled down, the mixture was poured over ice, and then neutralized with sodium acetate. Then, the mixture was extracted with dichloromethane and dried over magnesium sulfate, concentrated and purified through column chromatography (2 dichlomethane: 1 hexane) to yield a yellow oil. (82% yield). NMR (CDCI3): 2.08(s, 3H); 3.75(t, 2H); 4.30(t, 2H); 6.60(d, j=8.8 ; 2H), 7.65(d, j=8.8; 2H),9.65(s, 1H). 2-Chloromethylthiophene (3). In an ice cooled solution of 2-thiophene methanol (25g, 0.219mol) in dry dichloromethane, under argon, triethylamine (25.3g, .025mols) was added dropwise 41 for over 15 mins. Thionyl chloride (29.75g, 025mols) was then added dropwise over a 1 hr period, at then was allowed to stir for 2 hours at room temperature. The mixture was washed with water, neutralized, dried and concentrated. The crude product was purified by column chromatography (silica gel; 2:1 chloroform: hexane) to afford a clear oil, 80% yield. 1H NMR: 4.8(s, 2H); 6.8 (d, 1H); 7.10{d. 1H). Ethyl 2-thiophene methyl phosphonate (4). A solution of 2-chloromethylthiophene (67g, .5 mol) in triethylphosphite was heated to 140 °C under argon for 24hrs. The solution was then cooled to 110°C and stirred for 48hrs. The crude product was used without purification. Trans-7-[4-(N,N,diethylmethoxyaminobenzene)]- ethenylthiophene (5). To a stirred solution of the aldehyde(3.28g, 8.5mmol), and the Ethyl 2-thiophene methyl phosphonate(2.0g, 8.94 mmol) in dry THF, potassium t-butoxide (0.98g, 8.9mmol) was slowly added at 5 °C. The mixture was then stirred in darkness at room temperature. The mixture was then concentrated, dissolved in dichloromethane, washed with water, and dried over magnesium sulfate. The reaction residue was concentrated on silica gel and purified by chromotography over silica gel (3:1; hex:CH2 Cl2). This afforded a fluorescent pale yellow solid. 1H NMR: 2.08(s, 3H); 3.75(t, 2H); 4.30(t, 2H); 6.67(d, 42 j=9; 2 H), 7.00(d, j=15.6; 1H); 7.13(d, j=4.5, 1H); 7.26(d, j=15.6, 2H) 7.42(d, j=8.8 ; 2H); 7.50(m, 2H). Trans-7-[4-(N,N,diethyImethoxyaminobenzene)]- ethenylthiophen-1-al (6). In a three necked flask, fitted with an addition funnel, a condenser under argon, dry DMF was cooled to 0°C and POCI3 (4.52, 29.5 mmol) was added dropwise and mixed N=C=S + H2N 0 H CHaCI,, 0°C N H ,OH H 1) EtONa, EtOH >7 h, 100 °C 2) HaO, HCI S Figure 2.4 Preperation of N-ethylN’-(2-hydroxyethyl) thiobarituric acid. 43 for one hour. The thiophene (1 .Og, 27mmol) was added dropwise with stirring while maintaining the 0 °C temperature. The reaction mixture was then refluxed for 2 hours at 90 °C. When the reaction was cooled down, the mixture was poured over ice, and then neutralized with sodium acetate. The mixture was extracted with dichloromethane and dried over magnesium sulfate, concentrated and purified by chromatography (3:2; ether: hexane) to yield a florescent yellow solid. 1H NMR: 2.08(s, 3H); 3.75(t, 2H); 4.30(t, 2H); 6.67(d, j=9; 2H), 7.00(d, j=15.6; 1H); 7.13(d, j=4.5, 1H); 7.26(d, j=15.6, 2 H) 7.42(d, j=8.8 ; 2H); 7.50(d, j=4.5, 1H); 9.70(s, 1H). Thiobarbituric acid acceptors. Thiobarbituric acids were prepared by the following procedure, using the appropriate substitutions. N-EthylN’-(2-hydroxyethyl )thiourea. In modification of literature synthesis, ethanolamine (16.91 ml, 0.26 mol) was dissolved in anhydrous chloroform and ethylisothiocyanate (22.2 g, 0.26 mmol) was added dropwise. The solution was heated at reflux for 3 h. Upon cooling, 30.1 g (79.6%) product precipitated as a white powder: NMR d (DMSO d6) 1.04 (t, J= 7.2 Hz, 3H), 3.34 (bs, 2H), 3.41 (bs, 2H),3.45(t, j=5.0 HZ, 2H), 4.71 (bs, 1H), 7.28 (bs, 1H), 7.44 (bs, 1H).; 13C NMR d (CD3OD) 14.63, 39.95, 44 40.08, 47.34, 61.68; IR 3246(OH), 2952(C-H), 1570(C=S), 1059(C-O) cm-1; EIMS , m/z 138 (M, 55), 130(M-H2O, 45), 71(52), 60(59), 44(100), 43(43); Anal. Calod. for C5 H12N2 OS: C, 40.52; H, 8.16; N, 18.90; 21.63. Found: C, 40.49; H, 8.20; N, 18.85; S, 21.70. N-EthylN-(2-hydroxyethyl )thiobarbituric Acid. In analogy to the literature procedure for N.N’-thiobarbituric acid derivatives, 5 250 ml absolute ethanol was stirred in a 1 liter flask and sodium (5.75 g, 0.25 mol) added in two portions. When the sodium had reacted, diethyl malonate (40 g, 0.25 m ol), was added followed by N-ethyIN’-(2-hydroxyethyl)thiourea (36.5 g, 0.25 mol) in ethanol (250 ml).The solution was heated at relux for 24 h. After cooling, 22.5 ml of concentrated HCI was added, and the resulting precipitate was filtered. The ethanol was removed to give a viscous orange crude oil: Upon standing for long periods, a side product crystallized. Purification was not achieved in this case. Many tedious silica columns were ran with no sucess. Recrystalization of the solid did not purify either. Bisphenol thiourea, into a one litter round bottom flask, fitted with a reflux condenser,containing carbon disulfide (15.8ml, .26 mols, r=1.26) and 25 ml of ethanol; was added two equivalents of 4-aminopheno! (83ml, 0.52mols, r=0.945) in 100 ml ethanol. The reaction mixture was gently refluxed for 5 hrs. The 45 reaction mixture was cooled, and the thiourea began to precipitate. The crystals were collected, and air dried overnight. Bisphenyl thiourea is synthesised in the same fashion to produce a yellow powder mp 151-152 °C. Protected bisphenol thiouea. Bisphenolthiourea (40g, 0.154 mol) was added to acetic anhydride (47.1 g, 0.461 mols), and sodium hydroxide (18.4g, 0.461 mol). The reaction mixture was then refluxed overnight. The mixture was cooled down, poured over ice, and extracted with dichloromethane. The combined organic layer extracts were concentrated to a yield a pale solid with m.p. 168-169 OC. Bis(acetoxyphenyl) thiobarbituric acid, bisphenol thiourea, (37.1 g, 0.108mol) was placed in a round bottom flask, equipped with a reflux condenser, malonic acid, ( 11.23g, 0.11 mol) was added with vigorous mixing. Acetyl chloride ( 15.3 ml, 0.216 mol, r=1.1) was added slowly, and the reaction mixture heated to reflux for 4hrs. The reaction solution was cooled down, extracted with aqueous sodium hydroxide solution. The solution was then filtered to remove any insoluble matter, and then slowly diluted HCI was added with rapid stirring to precipitate the yellow product. NMR and elemental analysis were satisfactory. 46 H H Ethanol, heat R 2-acetyl chloride >4h, 100 °C 11 Figure 2.5 Synthesis of funstionalized Bis(phenyl)thiobarbiturc acid Once the aldehyde is synthesized, it can be either reacted with nonfunctionalized thiobarbituric acid 1,3-(N,N- diphenei)thiobarbitur-5-ylidene, to afford a chromophore for side chain polymerization. Or reaction with functionalized thiobarbeturic acid 1,3-(N,N-diphenol)thiobarbitur-5-y!idene, to afford a DEC chromophore. Synthesis of monomer 1. 4-(N,N- dimethylamino)benzaldehyde was completely dissolved in warm j u 47 ethanol (100 mis). A solution of the functionalized thiobarbituric acid, 1,3-(N,N-diphenol)thiobarbitur-5-ylidene, (0.21 g, 6.8x10-4 mols) in 10 mis of ethanol was added causing a gradual darkening. A couple of drops of piperidine was then added and the reaction mixture was heated to about 35 °C for 4-6 hrs. The addition of the base caused the color to immediately darken. The mixture was then cooled and diluted with petroleum ether and the product was filtered and washed with ethanol/petroleum ether. The crude is then purified with column chromatography. Purification via silica gel column, (100% ethyl acetate), afforded the purple dye. Further purification was preformed by recrystallization with dichloromethane: pet ether; and then with dichloromethane: hexane 1H NMR d (CD2CI2) 8.45 (dd, J = 14.8, 12.6 Hz, 1H), 8.17 (d, J = 12.4 Hz, 1H), 7.62 (d, J = 8.9 Hz, 2H), 7.48 (d, J = 14.8 Hz, 1H), 6.71 (d, J =9.0 Hz, 2H), 4.54, 4.52 (each q, J = 7.1 Hz, 2 H), 3.11 (s, 6H),1.28,1.26(each t, J = 6.9 Hz, 3H). 13c NMR d 178.80, 161.13, 159.67, 158.35, 153.31, 132.66, 123.49, 121.09, 111.87, 110.46, 43.51, 42.97, 40.14, 12.47, 12.42; Anal. Calcd. for Ch 9H23N3O2S: C, 63.84; H, 6.49; N, 11.75. Found: C, 63.88; H, 6.52; N, 11.71. Imax (solvent, nanometers): cyclohexane, 530; toluene, 550; chloroform, 572; methylene chloride, 570; acetone, 564; methanol, 574. 48 HO HO OH HO Figure 2.3 NLO Monomers Synthesized Synthesis of monomer 2. 4-{N,N- dimethylamino)cinnamaldehyde was completely dissolved in warm ethanol and the same knoevenagel coupling reaction scheme was carried out. 1H NMR d (CD2CI2) 8.06 (m, 2 H), 7.45 (d, J = 8.9 Hz, 2 H), 7.37 (dd, J = 13.8, J = 11.0 Hz, 1H), 7.05 (m, 2H), 6.69 (d, J = 8.9 Hz, 2H), 4.51 (m, 4H), 3.06 (s, 6H), 1.27 (m, 6H). 13c NMR d 178.80, 160.97, 159.93, 158.43, 158.32, 151.98, 147.09, 49 130.33, 127.42, 123.94 , 123.82, 112.00, 111.57, 43.53, 43.01, 40.09, 12.44, 12.42; Anal. Calcd. for C2 1H2 5 N3O2S: C, 65.77; H, 6.57; N, 10.96. Found; C, 65.86; H, 6.57; N, 10.97. A-max (solvent, nanometers): cyclohexane, 544; toluene, 570; chloroform, 604; methylene chloride, 600; acetone, 586; methanol, 598. Synthesis of monomer 3. Trans-7-[4- (N,N,diethylmethoxyamoinobenzene)]- ethenylthiophene-1-al (6) (0.25g, 6.8x10"4 mols) was completely dissolved in warm ethanol (100 mis). A solution of the thiobarbituric acid (0.21 g, 6.8x10-4 mols) in 10mls of ethanol was added causing a gradual darkening. A couple of drops of piperidine was then added and the reaction mixture was heated to about 35 °C for 4-6 hrs. The addition of the base caused the color to immediately darken. The mixture was then cooled and diluted with petroleum ether and the product was filtered and washed with ethanol/petroleum ether. The crude is then purified by column chromatography. A silica gel column, (100% ethyl acetate), afforded the purple dye. Further purification by recrystallization with dichloromethane: pet ether; and then with dichloromethane: hexane afforded a blue dye. The dye exhibited solvatochromic behavior, which caused the dye to absorb at different wavelengths with the change in solvent polarity. NMR: 1H NMR: 2.1 (s, 3H); 3.75(t, 2H); 4.30(t, 2H); 6.67(d, j=9; 2H), 6.85(m, 4H), 7.00(d, j=15.6; 1H); 7.05(m,4H) 7.13(d, j=4.5, 1H); 7.26(d, 1=15.6, 2H) 7.42(d, j=8 .8; 2H); 7.50(d, j=4.5, 1H); 8.39(s, 50 1H), 8.60(s, 2H). The dye is purple in diethyl ether, but blue in dichloromethane, ethanol, THF, DMF and chloroform. 2.4 Results and discussion (a) Monomer Design and Synthesis Table 2.2 Nonlinear optical properties of Barbituric Acids # Molecule H P ( 1 0 -48 P o ( 10'30 esu)________esu)______ 2400(1.9) 9830(1.9) N 770 Thiobarbituric acids exhibit large first molecular hyperpolarizabilities as depicted in Table 2.2 . It is well known that molecules containing strong electron accepting and donating 51 groups exhibit large NLO effects, therefore by using thiobarbituric acid as an electron acceptor, enhanced nonlinearity is expected. Thiobarituric acids are also useful for their ease in functionalization. This allows for a DEC scheme which is more thermally stable, by restricting the dye from relaxing to the more thermodynamically favored isotropic dipole moment distribution state, (due to the steric bulk of the acceptor). When synthesizing the chromophores, the alcohols on both the donating (N,N,-bis(2- hydroxyl-ethylanaline}, and the accepting sides (barbituric acid} were protected and later deprotected following literature methods. The choice of protecting group was not simple. The protecting group needed to be a stable to the harsh acidic conditions of the Vilsmeier formylation reaction, as well as to harsh basic conditions of the Wittig coupling reaction. The protecting groups used were MEM (CH3OCH2CH2OCH2), methoxy ether (CH3O), and acetoxy (Ac). But all of the protecting groups tried were deprotected partially at different points of the synthesis. All of the intermediate compound and monomers synthesized gave satisfactory NMR analysis. Elemental analyses was obtained for monomers 1 and 2, however, for monomer 3 high resolution mass spectrometry was used. When synthesizing the barbituric acids, there was a lot of difficulty encountered in the purification process of N-ethyl,N’-(2- 52 hydroxyethyl) thiobarbituric acid. The decision to abandon this route and to start synthesizing dyes with the phenyl-based barbituric acid was made for thermal stability reasons. Phenyl groups are significantly more stable than ethyl groups, but this is counterbalanced by the fact that flexible ethyl groups solublize the chromophore, where the bulky rigid phenyl groups decrease the solubility of the dye. The difunctionalized monomer 3 was synthesized in hope that the different reactivity of the alcohols, one being a primary alcohol and the other a phenol, can be useful in controlling polymerization rates. By polymerizing at one end only and then crosslinking while poling, it is expected to lock the chromophores in place. This is useful because, if both ends were to polymerize at the same time, the crosslinked polymer will be unprosessible. 2.5 Conclusion Novel research has been undertaken to synthesize new second-order nonlineariy optically active chromophores with large molecular hyperpolarizabilities. These functionalized chromophores are suitable for incorporation into a variety of polymer systems, which include side chain and DEC polymers. 53 Thiobarbituric acids are promising acceptors for ultimately achieving stable polymers with large second order nonlinearities. OH Double-End Crossllnkable Scheme OH (EtO)3Si(C^)„NHCOO (ElOJjSitCH^NHCOO OOCNH(CHj)^S!{OEt)3 Sol-Gel Scheme OOCNH(CHj)^l(OEI)4 Fig 2.6 Future Polymerization schemes. 2.6 Future Research The demand for thermally stable chromophores with large second-order nonlinearity is crucial for device application. Two of the chromophores that are under investigation are depicted in figure 2 .6 . 54 The double bond allows for a variety of radical polymerizations, which is simple to carry out. The monomer need not be as pure for radical polymerization as for condensation polymerization. After the monomer is polymerized radically, it can be crosslinked with diisocyante crosslinkers to lock in the chromophores. This polymerization can be also carried out in reverse, condensation polymerization followed by radical polymerization. Sol-gels are also of great interest, this tetrafunctionalized monomer is readily synthesized from monomer 3. Organic "Soi-Gei" Analog (Thermosetting Polyurethanes) 2. Electric field poling 3. Heat to effect crosslinking 1. Spin cast onto substrate ► 3D stabilized NLO matrix Fig 2.8 Organic Thermosetting polyurethanes 2.7 References: 55 1. S.R. Marder, D. Beratan, L-T- Cheng, S cie n ce 1991, 252,103. 2. B.G. Tiemann, S.R. Marder, L-T-Cheng, J. C hem . Soc. Chem. Commun.1992, 735 3. S.R. Marder, C.B. Gorman, L-T. Cheng, B.G. Tiemann, P roc. S P IE 1992, 1775, 19. 4. S.R. Marder, C.B. Gorman, P ro c N atl. A ca d . Sci. U S A 1993, 90 11297. 5. S.R. Marder, L.-T. Cheng, B.g. Tiemann, A.c. Friedli, M.BIancharddesce, 6. 6. J.W. Perry, J. Skindhj, S c ie n c e 1994, 263, 5117. C.W Dirk, H.E. Katz, M.L. Schilling, L.a. King, Chem.Mater. 1990, 2 , 700. 8 . A.k.-Y. Jen, V.p. Rao, K.Y. Wong, K. J. Drost, J. C hem . S oc., C hem . C om m un. 1993, 90. 9. V.P Rao, A.K.-Y. Jen, K.Y. Wong, K.J. Drost, R.M. Minnini, P roc. S P IE 1992, 1775,32 10. V.P Rao, A.K.-Y. Jen, K.Y. Wong, K.J. Drost, T e tra h e d ro n Lett. 1993, 1747. 13. G.mignani, F. Leiesing, R. Meyrueix, H.Samson, T e tra h e d ro n Lett. 1990, 31, 4743. 14. Rao, V. P., Jen, A.K, and Drost, K, J. C hem . Soc, C hem . C om m un. 1993, 1118. 15. F. Wurthner, F. Leising, R. Wortman, P. Kramer, C hem . P hys. 1993, 173, 305. 16. L.-T. Cheng, W. Tam, S.H. Stevenson, G.R. Merrdith, G. Rikken, S.R. Marder, J. P hys. C hem . 1991, 95,10631. 56 17. Corel 1. Marder. S.R , perry, J et al. A d v a n c e d M ate ria ls, 1994, #6, 494. 18. A. K.-Y. Jen, K.Y. Wong, V.P Rao, K.J. Drost, R.M. Minnini, M ater. R e s. S ym p. Proc. 1992, 247, 59. 19. C. Xu, B. Wu, L.R. Dalton, Y. Shi, P.M. Ranon, W.H. Steier,M4cromo/ect//es1992, 25, 6714. 2 0. Archer, W.L.and Campaigne, E, O rg a n ic S yn th e se s, V, 1963, 331. 21. C. Xu, B. Wu, L.R. Dalton, P.M. anon, Y. Shi, W.EIER< M a c rm o ie c u ie s 1992, 25, 6716 22. Ulman et al, J A C S Vol112, No 2 0 ,1990, 7085. 23. Gogte, V. N, and Tilak, B.D, Tetrahedron, 2 3 ,1967, 2443. 24. K.D. Singer, S.L. Laalama, J.E. Sohn, R.D. Small, In Nonlinear Optical Properties of Organic Molecules and Crystals I. (Eds: D.S. Chemla, J. ZYss), Academic Press, San diego, Ca 1987, p.437. 25. C.C. Teng, H.T. Man, A ppi. P hys. Lett. 1989, 56, 1734. 26. J.S. Schildkraudt, A p p l. O p tics 1990, 29, 2839. 27. J.L. Oudar, H.E Le Person, O p t C om m un. 1977, 6 6, 2664. 28. Becher, J. Angew. C hem . int. Ed. Engi. 1980, 8 , 589-612. 29 Nikolajewski, H.E.; Dahne, S.; Hirsch, B. C hem . Ber. 1967, 100, 2616-2619. 30 Katayama, H.; Ohkoshi, M.; Kaneko, K. Chem. Pharm Bull., 1984 32(5), 1770-1779. 57 31 Corey, E.J.; Venkateswarlu, J. A m . C hem . Soc. 1972, 23, 6190. 32 Prasad, P.N, Williams,D Introduction to Nonlinear Optical Effects___________ in Molecules and Polymers. John Wiley & Sons: N Y 1991. 33 Dickey, J. V.; Gray, A. R.; O rg. S ynth. Col. Vol. II. 1943, Blatt, A. H ., ed., 60 58 INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. U M I 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. 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Design and synthesis of novel second order nonlinear optical materials
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