Binary Chromophore Systems in Nonlinear Optical ... - ACS Publications

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J. Phys. Chem. C 2008, 112, 8091–8098

8091

Binary Chromophore Systems in Nonlinear Optical Dendrimers and Polymers for Large Electrooptic Activities† Tae-Dong Kim,‡ Jingdong Luo,‡,| Yen-Ju Cheng,‡ Zhengwei Shi,‡ Steven Hau,‡ Sei-Hum Jang,‡,| Xing-Hua Zhou,‡ Yanqing Tian,‡,| Brent Polishak,‡ Su Huang,‡ Hong Ma,‡,| Larry R. Dalton,§,| and Alex K.-Y. Jen*,‡,| Department of Materials Science and Engineering, Department of Chemistry, and, Institute of AdVanced Materials and Technology, Box 352120, UniVersity of Washington, Seattle, Washington 98195 ReceiVed: December 22, 2007

Recent developments of molecular architectural control and solid-state engineering have led to exceptionally large electro-optic (EO) activities in organic and polymeric nonlinear optical (NLO) materials. A new generation of NLO dendrimers has been developed to generate well-defined nano-objects, minimize strong intermolecular electrostatic interactions, and improve poling efficiency and stability. A facile and reliable Diels-Alder “click” reaction was applied for lattice hardening to improve physical properties of cross-linkable EO polymers. The “click” chemistry also provides means to study the relationship between EO activity, chromophore shape, and number density of the chromophore, systematically. The NLO dendrimers or polymers were used as hosts for guest chromophores to increase chromophore concentration and improve poling efficiency. A variety of nanostructured organic and polymeric materials with ultrahigh r33 values (>350 pm/V at the wavelength of 1310 nm, more than 10 times that of LiNbO3) and excellent temporal alignment stability at 85 °C were achieved by the approaches. Introduction Organic and polymeric electro-optic (EO) materials have been intensively studied for several decades due to their potential applications for the information technologies, THz generation/ detection systems, integrated circuits, and multifunctional nanodevices.1–4 To obtain device-quality materials, several factors need to be addressed simultaneously: (1) the design and synthesis of chromophores with high-molecular nonlinearity (µβ) along with efficient translation of the nonlinearity to macroscopic EO response, (2) long-term stability of polar order, (3) minimal optical loss at operation wavelengths, and (4) good mechanical strength and processability for multilayer fabrication. Synthesis of highly polarizable dipolar chromophores with a large dipole moment and extended π-conjugation have been reported recently.5 However, achieving EO activity appropriate for the device by incorporating such molecules into a polymer matrix has proven to be difficult due to the strong dipole-dipole electrostatic interactions between chromophores. Theoretical understanding has shown to be critical for molecular structure/ property relationships, and diverse molecular-engineering protocols are necessary to optimize nonlinear optical (NLO) performance of the materials.6 Our major efforts have been focused on developing efficient shape-engineered chromophores and increasing polar order through careful control of the nanoscale architecture of macromolecules. Dendronized NLO chromophores and polymers have been demonstrated to improve poling efficiency by encapsulating chromophore with dendritic substituents that can electronically shield the π-electron core and form spherical † Part of the “Larry Dalton Festschrift”. * To whom correspondence should be addressed. E-mail:ajen@ u.washington.edu. ‡ Department of Materials Science & Engineering. § Department of Chemistry. | Institute of Advanced Materials and Technology.

molecular shapes.7,8 The unique nanoscale environment created by the shape, size, and location of dendritic moieties in the chromophores plays critical roles in maximizing the macroscopic properties of the EO polymers. In situ generation of side-chain NLO polymers with dendronized chromophores in the Diels-Alder (DA) “click chemistry” provided a material design strategy to increase the number density of chromophores further as binary composites without sacrificing material properties.9 This opens new avenues as an efficient combinatorial material design approach to build a guest/ host library of materials and develop efficient means to control the lattice-hardening process in polymeric NLO materials. Figure 1 illustrates our recent development of these high-performance NLO materials showing dramatically enhanced EO activity compared to the past two decades. In this report, we describe a new generation of NLO dendrimers and polymers with binary chromophores showing cooperatively enhanced r33 values up to 387 pm/V with excellent temporal alignment stability at 85 °C. Experimental Section General Method. Dichloromethane (CH2Cl2) and tetrahydrofuran (THF) were distilled over phosphorus pentoxide and sodium benzophenone ketyl, respectively, under nitrogen prior to use. The synthesis of the dipolar NLO chromophores, 1-8, 11, 15, and 17-19, has been previously reported by our group.8,10 9, 10, and 14, 4-(dimethylamino)pyridinium-4-toluenesulfonate (DPTS), 1,6-bismaleimido-hexane (BMI), and tris(2-maleimidoethyl)amine (TMI) were prepared according to the methods described in the literature.11 All other chemicals were purchased from Aldrich and were used as received. Reactions were carried out under an inert nitrogen atmosphere unless otherwise specified. 1H NMR spectra (300 MHz) were taken on a Bruker AV-301 spectrometer. UV/vis spectra were obtained on a

10.1021/jp712037j CCC: $40.75  2008 American Chemical Society Published on Web 04/29/2008

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Figure 1. Recent development of organic NLO chromophores and polymers for EO devices.

Perkin-Elmer Lambda-9 spectrophotometer. Glass transition temperatures (Tg) were measured by differential scanning calorimetry (DSC) using a DSC-2010 in TA instruments with a heating rate of 10 °C/min. The molecular weights and polydispersities (relative to polystyrene standards) were determined using a Waters 1515 gel permeation chromatograph with a refractive index detector at room temperature (THF as the eluent). 12. To a solution of 11 (1.00 g, 0.94 mmol), 9 (0.20 g, 0.28 mmol), and DPTS (0.02 g, 0.07 mmol) in 10 mL of THF was added 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide (0.20 g, 1.29 mmol). The reaction mixture was allowed to stir at room temperature overnight. After filtration of the byproduct urea, the solvent was evaporated under reduced pressure. The crude product was purified by column chromatography using ethyl acetate as an eluent to afford 12 as blue-greenish solid (0.75 g, 70%). 1H NMR (CDCl3, TMS, ppm): δ 8.10 (d, J ) 15.3 Hz, 1H), 7.54 (s, 1H), 7.45 (d, J ) 8.7 Hz, 2H), 7.35-7.23 (m, 10H), 7.22 (d, J ) 15.6 Hz, 1H), 7.11 (d, J ) 15.6 Hz, 1H), 6.80 (s, 1H), 6.79 (d, J ) 8.7 Hz, 2H), 6.51 (d, J ) 15.3 Hz, 1H), 5.40 (s, 2H), 5.11 (s, 4H), 5.02 (s, 2H), 4.46 (t, J ) 6.3 Hz, 2H), 3.70 (t, J ) 6.9 Hz, 2H), 3.55 (q, J ) 7.0 Hz, 2H), 2.05 (s, 3H), 1.98 (s, 3H), 1.28 (t, J ) 7.2 Hz, 3H). MALDITOF: Exact mass calcd for C194H123F39N12O24S3 [M + Na]+: 3863.72. Found [M + Na]+: 3863.94. UV/vis spectrum of thin film: λmax ) 714 nm. Tg ) 116 °C. 13. It was synthesized and purified with a similar procedure as described for 12 using 0.23 g (0.21 mmol) of 10 to afford 13 (0.71 g, 65%). 1H NMR (CDCl3, TMS, ppm): δ 8.11 (d, J ) 15.3 Hz, 1H), 7.55 (s, 1H), 7.43 (d, J ) 8.7 Hz, 2H), 7.38-7.24 (m, 10H), 7.22 (d, J ) 15.6 Hz, 1H), 7.11 (d, J ) 15.6 Hz, 1H), 6.80 (s, 1H), 6.79 (d, J ) 8.7 Hz, 2H), 6.52 (d, J ) 15.3 Hz, 1H), 5.41 (s, 2H), 5.13 (s, 4H), 5.05 (s, 2H), 4.98 (s, 1H), 4.45 (t, J ) 6.3 Hz, 2H), 3.73 (t, J ) 6.9 Hz, 2H), 3.57 (q, J ) 7.0 Hz, 2H), 1.99 (s, 3H), 1.27 (t, J ) 7.2 Hz, 3H). MALDI-TOF: Exact mass calcd for C261H162F58N16O34S4 [M + Na]+: 5315.93. Found [M+Na]+: 5316.24. UV/vis spectrum of thin film: λmax ) 715 nm. Tg ) 123 °C. 16. A mixture of 0.08 g (0.07 mmol) of 14 and 0.25 g (0.21 mmol) of 15 was completely dissolved in 2.95 g of 1,1,2trichloroethane. After filtration through a 0.2 µm syringe filter, it was spin-coated onto a glass substrate. The films were baked in a vacuum oven at 65 °C overnight to ensure the removal of the residual solvent. The prepared films were further annealed

at 92 °C for 30 min to form NLO dendrimer 16. UV/vis spectrum of thin film: λmax ) 709 nm. Tg ) 95 °C. PMMA-AMA10. A solution of 1.0 g (10.0 mmol) of methylmethacrylate, 0.3 g (1.09 mmol) of 9-anthrylmethacrylate, and 0.01 g (0.06 mmol) of AIBN in 4 mL of THF was degassed using the freeze-pump-thaw procedure three times and filled with nitrogen. The solution was first heated at 55 °C for 2 h and then at 62 °C for 12 h. The viscous solution was poured into 100 mL methanol to precipitate the polymer. The polymer was redissolved in THF and reprecipitated twice into methanol to yield 1.1 g (85%) of the polymer. 1H NMR (CDCl3, TMS, ppm): δ 8.60-8.10 (br m), 8.10-7.80 (br m), 7.65-7.20 (br m), 6.20-5.80 (br s), 3.70-3.05 (br m), 2.15-1.50 (br m), 1.15-0.50 (br m). Tg ) 135 °C. Molecular weight: Mn ) 35 000 with a polydispersity of 2.09. PMMA-AMA20. It was synthesized and purified with a similar procedure as described for PMMA-AMA10 using 0.83 g (8.3 mmol) of methylmethacrylate and 0.57 g (2.08 mmol) of 9-anthrylmethacrylate to afford PMMA-AMA20 (1.0 g, 71%). 1H NMR (CDCl , TMS, ppm): δ 8.60-8.10 (br m), 8.10-7.80 3 (br m), 7.65-7.20 (br m), 6.20-5.80 (br s), 3.70-3.05 (br m), 2.15-1.50 (br m), 1.15-0.50 (br m). Tg ) 143 °C. Molecular weight: Mn ) 40 000 with a polydispersity of 2.22. 24. To a solution of 0.80 g (5.92 mmol) of PMMA-AMA20 in 5 mL of 1,1,2-trichloroethane was added 0.20 g (0.16 mmol) of 15 and dissolved at room temperature for 30 min. It was refluxed at 120 °C for 3 h and concentrated under vacuum. The polymer was isolated through precipitation of the filtered solution to hot methanol. Further purification was conducted by redissolving the polymer in CH2Cl2, filtering, and reprecipitating to afford the NLO polymer 24. 1H NMR (CDCl3, TMS, ppm): δ 8.60-8.10 (br m), 8.10-7.80 (br m), 7.60-7.20 (br m), 7.20-7.10 (br m), 6.80-6.49 (br m), 6.10-5.80 (br s), 5.45 (br s), 5.05 (br s), 4.81 (br s), 4.25 (br s), 3.80-3.10 (br m), 2.12-1.77 (br m), 1.37-1.21 (br m), 1.12-0.87 (br s), 0.85-0.59 (br s). UV/vis spectrum of thin film: λmax ) 691 nm. Tg ) 145 °C. Molecular weight: Mn ) 48 000 with a polydispersity of 2.30. Results and Discussion Highly Efficient NLO Chromophores through Shape Modification. One of the major challenges in the development of polymeric EO materials based on NLO chromophores is to

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Figure 2. The molecular structures of representative dipolar chromophores.

TABLE 1: Physical and Optical Properties of Guest/Host EO Polymers contentsa

EO polymer 1/PMMA 2/PMMA 3/PMMA 4/PMMA-AMA10 5/PMMA-AMA10c 6/PMMA 7/PMMAc 8/PMMA-AMA10c

dye (wt %) 20 24 22 32 20 22 27 21

b

applied voltage (V/µm)

r33 (pm/V)

100 100 100 100 100 100 80 90

53 60 69 130 156 145 262 208

a Core chromophore moiety (formula of 1-4: C28H21F3N4OS, molecular weight 518.6. Formula of 5-8: C33H28F3N4O, molecular weight 553.6) counted by total loading weight. b EO coefficient measured at 1310 nm by simple reflection method.12 c 5/PMMA-AMA10: ref 10a. 7/PMMA and 8/PMMA-AMA10: ref 10b.

engineer molecular materials to minimize the formation of strong dipole-dipole electrostatic interactions, which produce centrosymmetric aggregates of chromophores in solid state. Recent attempts to improve µβ of chromophores has been accelerated with the invention of conjugated thiophene-bridge (1, 2, 3, and 4) and extended polyene-bridge (5, 6, 7, and 8) systems with strong multicyano-containing heterocyclic electron acceptors. The molecular structures of these chromophores are shown in Figure 2, and the optical and NLO properties are summarized in Table 1. To prevent close packing of such extended chromophore structures and improve their solubility, tertbutyldimethylsilyl (TBDMS) or perfluorinated-dendron groups were used and functionalized on either the donor-end or the center-bridge site of the chromophores. Poly(methyl methacrylate) (PMMA) and PMMA-AMA10 were used as a host polymer. Various concentrations of chromophores were considered to study substitution effects on the dipolar NLO chromophore for EO activity. The r33 values were measured using simple reflection technique at the wavelength 1310 nm.12

Chromophore 1, known as AJL8, has been widely used as a standardized reference material for many device applications, which shows consistent EO activity (∼50- 60 pm/V) in a single layer poled film and EO device structure.13 As shown in Table 1, it is clear that bulky substituents on chromophore precursors impact poling efficiency significantly by decreasing electrostatic interactions among the chromophore building block. Among conjugated thiophene-bridge-based chromophores, the chromophore 4 shows its highest r33 of 130 pm/V with 32 wt % chromophore loading in PMMA-AMA10. Compared to other systems, 4 can be loaded up to 42 wt % with fairly proportional EO activities to the chromophore concentration.9 On the contrary, chromophore loading levels beyond 25% for 1, 2, and 3 in the polymer gave poor film quality and could not be poled. It is noted that 3 attached with a TBDMS group on its corecenter shows a slightly better EO activity than 1 and 2 with similar poling conditions. This result suggests that the bulky substituents in the middle of the chromophore can effectively reduce intermolecular interaction. The perfluorophenyl dendritic chromophores 7 and 8 offer notable enhancements for EO activities compared to those of the extended polyene-bridge chromophores, 5 and 6. The poled films showed very high EO coefficients up to 262 pm/V for 7/PMMA and 208 pm/V for 8/PMMA-AMA10, respectively. This value is exceptionally large compared to those reported for guest-host EO polymers containing polyene-type chromophores.10b Such a great increase in EO activity is attributed to structural features of the chromophores spatially isolated by both the isophorone ring and the center-anchored perfluoroaromatic dendron. This spatial arrangement also increases the solubility of 7 and 8 resulting in a higher number density of chromophores that can be incorporated into the polymer matrix. The dendron also provides effective site isolation to decrease the strong electrostatic interactions among chromophores. In addition, their all-trans conformations could provide a better

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Figure 3. Synthesis of 12 and 13 for multiarm NLO dendrimers.

Figure 4. The structure and EO activity for 12 and 13.

conjugation and significantly increase the µβ value. The fluorinated dendrons can not only provide improved macroscopic optical nonlinearity, but also help to minimize absorption loss from the C-H vibrational overtones by substituting with C-F groups. Multiarm NLO Dendrimers. In general, the EO activity increases with increasing concentration of the chromophore and shows saturation behavior as a result of strong dipole-dipole electrostatic interactions. EO coefficients of guest-host polymers can be significantly improved by encapsulating chromophore with substituents that can electronically shield the core and form spherical molecular shapes. To create a structurally more well-defined and stable NLO material, we have constructed dendrimers with multiple dendritic chromophores branched out from a passive core unit. The most important advantage of NLO dendrimers is that the active volume fraction of chromophore can be maximized without phase separation and aggregation.7b It occurs easily in the guest/host polymeric systems when the chromophore is highly loaded. The dendrimers are also expected to have reproducible physical and optical properties with welldefined molecular structures. Three-arm and four-arm NLO dendrimers (12 and 13) were synthesized by postesterification reactions between the carboxyl group on the three or four branches of the desirable core molecules (9 or 10) and the hydroxy-containing chromophore

precursor (11) that is surrounded by perfluorinated-phenyl dendrons as the exterior moieties (Figure 3). The resulting N-acrylurea and anhydride byproduct can be removed by repetitive precipitation after the condensation reaction. Figure 4 shows the structure of 12 and 13. The weight % of active chromophore content is 40% for 12 and 39% for 13. The glass transition temperatures of the dendrimers, determined by DSC, were 116 °C for 12 and 124 °C for 13. The NLO dendrimers can be directly spin-coated to form a monolithic molecular glass without any prepolymerization process. The resulting dendrimer films exhibit appreciably higher EO activities (up to 83 pm/V) compared to the nondendronized guest/ host polymeric systems based on conjugated thiophene-bridge chromophores (1, 2, and 3 in PMMA). This suggests that the screening effect, provided by the peripheral groups of dendrimer, allows the chromophores to be spatially isolated, and the large void-containing structure of dendrimers provides the needed space for efficient reorientation of the chromophores. Furthermore, the globular geometry of dendrimers is ideally suited for the spherical shape modification of chromophores. In terms of chromophore alignment stability, the dendrimers also showed very promising results retaining higher than 90% of their original r33 values after several hundred hours at 85 °C. Amazingly, the NLO dendrimers can be used as a host matrix for structurally compatible guest chromophores to additionally

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Figure 5. Synthesis of NLO dendrimer 16 by in situ Diels-Alder click reaction in solid films.

enhance poling efficiency by increased net chromophore concentration. The highly efficient NLO chromophore 5 has been doped as a guest chromophore in NLO dendrimers. It also showed excellent film-forming quality with no indication of phase separation and aggregation. Although the optimum poling field was limited to less than 60 V/µm due to the high conductivity and low dielectric strength of the NLO dendrimer, very large r33 values (193 and 198 pm/V) were achieved for 12 and 13, respectively. The postesterification condensation method for the synthesis of 12 and 13 can generate byproduct and a trace amount of residual ionic impurities, which significantly attenuate the effective poling electrical field and possibly cause the DC bias drift during device operation. To alleviate this problem, the DA click chemistry was selected as an alternative for generating a new high-performance NLO dendrimers. The DA reaction involves a ring-forming coupling between a dienophile and a conjugated diene that can be described by a symmetry-allowed concerted mechanism without forming biradical or zwitterionic intermediates. Recently, DA reactions have been successfully adopted for side-chain and cross-linkable EO polymers for postfunctionalization.9,10,11b To conduct DA click reactions for the NLO dendrimer, a maleimide-containing NLO chromophore (15) and an anthryl-containing diene core (14) were prepared. One of the attractive features of this methodology, which makes the NLO dendrimer in the bulk state, is that it requires no solvent or catalyst, provides quantitative yields, and regio-specifically forms adducts without any generation of byproduct. As a

consequence, it is possible to precisely control the chromophoreloading concentration by adjusting the ratio of chromophores in the feed. As shown in Figure 5, the dendrimer 16 was constructed by DA click reaction during the poling in solid films. The absorption bands at 320, 370, and 380 nm, which correspond to the anthracene moiety, have completely disappeared after annealing by the DA reaction. Through an in situ postfunctionalization process during the electric field poling, a very large EO coefficient (r33 ) 109 pm/V) was achieved for 16. This is significantly higher than the r33 values from 12 and 13, which were prepared by catalyzed postesterification condensation. This is probably due to easier alignment of dendronized chromophores before forming DA adducts covalently bonded with anthracenyl core units. A sequential process for the chromophore alignment and functionalization can also provide improved dielectric strength that the electric field up to 110 V/µm applied to the NLO dendrimer film. Unfortunately we could not achieve any appreciable r33 values from 16 with additional guest polyenic-type chromophores due to chemical sensitivity of the polyenic chromophore toward the DA reaction. Very recent spectroscopic studies reveal that highly polarizable polyenic-type chromophores tend to react with dienophiles such as maleimides through cycloaddition during the heating process, which lead to decomposition of chromophores.14 However, because a polar substitution such as a methoxy group in the middle of the polyenic chromophore has provided significantly reduced dienic reactivity, we expect that improved poling efficiency and chemical stability are highly

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Figure 6. In situ poling and Diels-Alder cross-linking of guest/host cross-linkable EO polymers containing binary chromophores.

achievable in the NLO dendrimer systems afforded by DA click reaction in the future.15 Cross-Linked EO Polymers Containing Binary Chromophores. As described, the multiarm NLO dendrimers can be used as a host matrix for structurally compatible, chemically stable guest chromophores to achieve enhanced EO activity. The long-term temporal stability and mechanical strength are major concerns in dendrimers due to relatively low glass transition temperatures and molecular weights. Many studies of polymeric EO materials have shown that lattice hardening approaches can significantly improve longterm alignment stability. However, a reduction of ∼20-40% in EO activity is usually accompanied with such approaches, because typical poling of conventional thermoset EO polymers is achieved through sequential lattice-hardening and poling process, resulting in severely limited chromophore reorientation. The DA cycloaddition reaction for the lattice-hardening method can tune temperature windows for cross-linking and poling. This DA reaction has been applied successfully in lattice-hardening processes of EO polymers with both high nonlinearity and thermal stability. We have extended the concept of binary systems into crosslinkable EO polymers through DA click reactions to further incorporate highly polarizable NLO chromophores. This combined effort demonstrates that binary mixtures of chromophores can be loaded into side-chain EO polymers and efficiently poled to give EO activities higher than the summed value of two added chromophores. These systems can also be mildly cured to ensure a thermally stable EO response. The cross-linkable EO polymers containing binary chromophores exist as a three-component guest-host system (Figure 6): PMMA-AMA10 as a host polymer, guest chromophore 5, and secondary chromophore 17, 18, or 19. The chromophores are functionalized with maleimido moieties, which can act as an active cross-linker to react with the anthracenyl side-chains

on PMMA-AMA10. After solid-state DA click reactions, they form chromophore-embedded networks. BMI, 1,6-bismaleimidohexane, was also included as a passive cross-linker for a parallel comparison. After curing PMMA-AMA10/BMI/5, the main absorption bands (two major absorption subpeaks, centered at the wavelengths of 799 and 951 nm) of 5 remained unchanged when the same amount of anthracenyl and maleimide groups were equivalent. However, it underwent a significant decrease in intensity (25-40%) if an excess amount of maleimide was used. This suggests that the anthracenyl group has a higher reactivity toward maleimide and can serve as a scavenger to prevent the polyenic chromophore from reacting with maleimide. In all cases, the chromophore absorption bands remained almost unchanged throughout the thermal curing and poling, indicating that good chromophore stability was achieved under a mild curing condition and carefully adjusted diene/dienophile ratios. All poling processes were performed at temperatures of around 110 °C with a poling field ranging from 75 to 125 V/µm. Both poling fields and currents were monitored in situ to optimize the entire process. All of the binary systems exhibited very large r33 values (up to 237-263 pm/V). However, it is hard to achieve more than 200 pm/V in a singular chromophore system even though at higher chromophore loading levels. These results suggest structural features for the chromophores and processing control should be desirable in the binary systems. The shape of a guest chromophore, 5, is a roughly prolate ellipsoid while chromophoric cross-linkers (17, 18, 19) are Λ-shaped. During poling and annealing processing, such a combination of chromophores can minimize the formation of antiparallel or close head-to-tail centrosymmetric stacking between chromophores. Furthermore, chromophoric crosslinkers can provide further modification of the polymer hosts, through the in situ DA cross-linking, leading to better homogeneity and stability to additional polyenic chromophore dopants. In this process-induced morphological confinement,

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Figure 7. A cross-linked EO polymer, 24, in situ generated by Diels-Alder reaction in a side-chain EO polymer with binary chromophores.

Figure 8. Poling profile and monitored current flow for the crosslinked polymer 24.

both guest chromophores and in situ generated active polymer networks could respond cooperatively to the poling field. Side-chain EO polymers can also cross-link with DA reactions in the form of binary chromophoric systems. Figure 7 shows a schematic illustration for chromophore-aligning/lattice-hardening processes from a side-chain EO polymer 24. Side-chain chromophore contents were adjusted to 8 wt % with an anthracenyl group in PMMA-AMA20 to further cross-link the binary systems. A guest chromophore 6 and passive cross-linker TMI were added with different concentrations to maximize poling efficiency and alignment stability. Direct spin-coating of the dissolved mixture in 1,1,2trichloroethane (∼8 wt %) gave high optical quality thin films, which were then subjected to the electric field to form a poled cross-linked polymer, 24. An optimal poling condition for 24 is shown in Figure 8. After films were mounted on the poling stage at 40 °C, the initial voltage was set to 20 V/µm. The temperature was then ramped up to 110 °C with 10 °C/min.

During the poling, we monitored current to optimize poling efficiency and to control electrical breakdown of the films. The poling current started to increase rapidly from 85 to 93 °C and dropped suddenly at the temperature beyond 95 °C with 20 V/µm of constant voltage. This might be due to the changing complexity of charge injection and trapping behavior of materials in the polymers with high chromophore concentrations as the DA cross-linking reaction progress. The voltage was then slowly increased to 90-100 V/µm before the temperature was reached at 110 °C. Again, the poling current rapidly increased. This second current increase is possibly due to the chromophore alignment and rearrangement of the polymer, which is mostly achieved at the Tg of the polymer. We found there was a significantly reduced EO response when the currents were not increased because of lowered poling temperature than Tg for 24. The films were finally cooled to room temperature while the electric field was maintained. With these careful poling processes, very large r33 values (300-380 pm/V) were achieved. We have observed lowered poling efficiency at an excess amount of TMI due to polyenic chromophore decomposition by maleimide groups. The concentration of a side-chain chromophore, 15, can also affect poling efficiency in the binary cross-linked polymer 24. For an example, we have previously reported side-chain EO polymers, which were constructed with 20∼25% of 15.10c In these EO polymers, the additional polyenic guest chromophores have been doped with various concentrations. However, the maximum r33 values showed 180∼200 pm/V in 23% of guest chromophores. This implies that there is an optimal concentration between two chromophores to minimize their electrostatic interactions. Careful control of TMI content (1∼2 wt %) and side-chain chromophore concentration (8%) was taken into account to maximize EO activity and lattice hardening.

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Kim et al. under Agreement No. DMR-0120967), the Defense Advanced Research Projects Agency (DARPA) MORPH program, and the Air Force office of Scientific Research (AFOSR) under the MURI Center on Polymeric Smart Skins are acknowledged. A.K.-Y.J. thanks the Boeing-Johnson Foundation for its support. T.-D.K. thanks the Nanotechnology Center at the University of Washington for the Nanotechnology fellowship. References and Notes

Figure 9. Experimental EO activities as a function of active polyenic chromophores (core D-π-A skeleton) for binary NLO systems compared with singular guest/host NLO polymers.

A continuous increase of r33 values can be seen with the increase of guest chromophore 6 in 24. The highest r33 value of 387 pm/V was obtained for the film containing 30% of 6, which is significantly larger than that (198 pm/V) of singular guest chromophore 6 in PMMA-AMA20. This result again suggests that binary chromophores in different morphological confinement can cooperatively respond to the poling field. The binary systems may also provide a unique nanoenvironment for enhancing local field factors. However, further increase of the chromophore contents led to saturation or decrease in EO coefficients due to severe aggregation of chromophores. In addition, a poling voltage higher than 75 V/µm across the films resulted in catastrophic electrical breakdown. All cross-linked EO polymers showed good thermal alignment stability. After an initial fast decay, ca. 75% of these EO activities could be maintained at 85 °C for over 500 h. This demonstrates that the binary systems can be efficiently poled and cured to form thermally stable EO lattices. These results are again a great demonstration of the advantages offered by binary chromophoric systems combining with well-controlled lattice-hardening and poling methodology via DA click reactions. Conclusion Nanoscale architectures of chromophores, dendrimers, and polymers have been systematically exploited to extend the potential of organic NLO materials with exceptionally high r33 values. A facile and reliable DA click reaction has been used to improve thermal alignment stability via rationally controlled lattice-hardening process without compromising EO activities. We have also employed binary chromophores to enhance EO activities further at high chromophore concentration. Compared to the conventional NLO polymer based on a singular chromophore, binary NLO materials including dendrimers, sidechain polymers, and cross-linked polymers showed exceptional poling efficiency as shown in Figure 9. Furthermore, their r33 values were higher than summed values of individual chromophores in the binary NLO materials. The exceptional EO coefficients enable not only unprecedented performances in conventional device formats, but also the development of new devices such as hybrid modulators and EO polymers integrated with silicon photonics. Acknowledgment. Financial supports from the National Science Foundation (NSF-NIRT and the NSF-STC Program

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