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Enhanced Optical Nonlinearity and Improved Transparency of Inorganic-Organic Hybrid Materials Containing Benzimidazole Chromophores Jiancan Yu, Yuanjing Cui,* Junkuo Gao, Zhiyu Wang, and Guodong Qian* Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed: May 25, 2009; ReVised Manuscript ReceiVed: September 28, 2009
A series of novel inorganic-organic hybrid materials with benzimidazole chromophores covalently bonded into the inorganic silica network as pendants has been designed and synthesized via the wet chemistry routes. The measurement results of solvatochromic method and UV-vis spectra indicate that these benzimidazole chromophores exhibit higher microscopic optical nonlinearity compared with their analogues with benzene ring and without undesirable red-shift of the absorption band. After corona poling, the macroscopic nonlinear optical activity and poling alignment stability of the hybrid films were investigated by in situ second harmonic generation measurement. These hybrid films exhibit enhanced nonlinear optical coefficient from 25 to 54 pm/V with chromophore concentration varied, improved optical transparency (λmax < 470 nm), as well as excellent polar orientation stability at 160 °C. Introduction The researches of second-order nonlinear optical (NLO) materials have been greatly stimulated by their extensive applications in high-speed electro-optic (EO) devices.1-6 In the past decades, a series of highly efficient NLO chromphores with high first hyperpolarizabilities (β) and thermal stability have been successfully synthesized and effectively optimized.7-10 Recent developments, such as the employment of multicomponent Diels-Alder cross-linkable dendrimers5,11-13 and siteisolation moieties,14-16 have obtained many polymer systems with ultralarge EO coefficients, showing attractive prospects in applications. However, for practical application, NLO materials should simultaneously satisfy the requirements of large NLO coefficients, high stability, low optical loss, and reproducibility. Among them, the problem of “nonlinearity-stability-transparency trade-off” is a predominant challenge and postpones their application in devices. Chromophores with heteroaromatic ring as conjugated chargetransfer (CT) chains are of particular interest for their high optical nonlinearity and good thermal stability in comparison with their phenyl or polyenyl analogues.17-19 The incorporation of heterocycles in chromophores significantly enhances nonlinearity but generally leads to reduced transparency.20,21 As a result, the bathochromicly shifted absorption of chromophores associated with π-π* electronic excitation narrows the operating wavelength range for frequency doubling or EO devices.22 Fortunately, some heterocycle-based chromophores, which for example contain imidazole in the π-conjugation, typically exhibit good transparency.23-25 A class of heterocycle-based NLO chromophores containing 6(5)-nitrobenzimdazole moieties (Scheme 1) have been widely introduced in poled polymer systems for their high thermal stability and the convenience of synthesis.23,26,27 Although the chromophores have good thermal stability, it is still challenging to acquire polymeric materials with high poling orientation stability at the high temperatures. * To whom correspondence should be addressed. Phone: +86-57187952334. Fax: +86-571-87951234. E-mail:
[email protected] (Y.C.);
[email protected] (G.Q.).
To defeat the “stability-nonlinearity trade off”, thermoset polymers have been widely adopted as a host for chromophores.28,29 However, these acid-derived polymers typically exhibit poor reproducibility of the optical quality and large propagation loss in waveguide structure.30 Sol-gel silica, which features the merits of small free volume, excellent optical quality, inert chemical activity, and good mechanical integrity, is another thermoset matrix for stable NLO materials with high optical transparency.31,32 In nearly two decades, inorganic-organic hybrid NLO materials have achieved great achievements.33-37 However, only those chromophores with relative small size, such as disperse red (DR)33,38 and diethylamino nitrostibenes (DANS),39,40 have been employed in hybrid systems. Noting the fact that sol-gel hybrid NLO materials are achieved through a simultaneous poling and curing process, chromophores of large sizes and hyperpolarizabilities are able to rotate in free during poling at the high temperatures and then are gradually trapped in the cured matrix and consequently achieve highly noncentrosymmetric order. Moreover, the poled and cured materials bonded with bulky chromophores are proposed to increase the high alignment stability due to their difficult in rotation.41,42 Therefore, bonding long or bulky chromophores to inorganic network is a rational way to achieve nonlinear stability optimized NLO materials. To achieve stability nonlinearity transparency properties optimized chromophores and materials, we designed and synthesized two benzimidazole-based chromophores with relative long and tunable π-conjugated chains, as shown in Scheme 1. Hybrid films were prepared by sol-gel methods after chromophores functionalized with alkoxysilane, and their transparency and NLO performances were studied. Experimental Section Materials. 2-Amino-4-nitroaniline (97%, Alfa Aesar), paminocinnamic acid hydrochloride (97%, Fluka), N-ethyl-Nhydroxyethyl aniline (97%, TCI), (3-isocyanatopropyl)triethoxysilane (ICPTES, 95%, TCI), p-aminobenzoic acid, N,N′dimethylformamide (DMF), acetic acid, potassium carbonate (K2CO3), and concentrated hydrochloric acid (37%) were
10.1021/jp9048549 CCC: $40.75 2009 American Chemical Society Published on Web 10/20/2009
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SCHEME 1: Synthetic Routes to Chromophores and Alkoxysilane Dyes (ASDs)
obtained from commercial sources and used as received. Tetrahydrofuran (THF) and pyridine were dried over molecular sieves (3A). Characterization. 1H NMR spectra were recorded on a Bruker Advance DMX500 spectrometer (500 MHz) with tetramethylsilane as a reference. Fourier-transform infrared (FTIR) analysis was taken in the range of 400-4000 cm-1 for the synthesized chromophores, ASDs, and films in KBr disks. Elemental analysis was made on a Carlo Erba EA1110 elemental analyzer. UV-vis absorption spectroscopic study was performed with a Hitachi spectrometer U-4100. The thermal properties of synthesized compounds were studied on TA SDT Q600 simultaneous thermal analysis system and Netzsch 200 F3 differential scanning calorimeter (DSC) at a heating rate of 10 °C/min. The thickness of films was determined on a Tencor alpha-step 200 surface profiler (Lencoi). Synthesis of 2-(4-Aminophenyl)-6(5)-nitrobenzimidazole (3a). 4-Aminobenzoic acid 1a (7.54 g, 55 mmol) and 2-amino4-nitroaniline 2 (7.66 g, 50 mmol) were mixed with polyphosphoric acid as little as possible to give a stirrable paste. As the temperature was raised slowly to 160 °C, a dark solution formed. The solution was allowed to stir at 160 °C for 30 min, cooled to 100 °C, and then poured into cold water (500 mL) carefully. The mixture was neutralized with K2CO3, and the resulting precipitate was filtered. The filtered cake was dissolved in boiling ethanol, and the insoluble precipitate was removed by filtration and the filtrate was poured into water. The resultant precipitates were collected and dried in vacuo at 80 °C overnight to give 6.89 g (54%) of orange crude product 3a. No further purification was taken for the following steps. The recrystallized sample from DMF was characterized by 1H NMR. 1H NMR (DMSO-d6) δ (ppm): 13.09 (s, 1H, NH), 8.24-8.39 (d, 1H, ArH), 8.05 (d, 1H, ArH), 7.88 (d, 2H, ArH), 7.57-7.68 (d, 1H, ArH), 6.68 (d, 2H, ArH), 5.82 (d, 2H, NH2). Synthesis of (E)-2-(4-Aminostyryl)-6(5)-nitrobenzimidazole (3b). This synthesis was performed similarly to that for 3a except that the temperature of the reaction was elevated to 165 °C and the reaction time was extended to 1 h. Yield: 82%.
Mp: 262 °C. 1H NMR (DMSO-d6) δ (ppm): 13.05 (s, 1H, NH), 8.35 (s, 1H, ArH), 8.07 (d, 1H, ArH), 7.63 (m, 2H, -ArCH)CHand ArH), 7.39 (d, 2H, ArH), 6.89 (d, J ) 16 Hz, 1H, sArCHdCHs), 6.60 (d, 2H, ArH), 5.68 (s, 2H, NH2). Synthesis of 2-(4-(4-(N-Ethyl-N-(2-hydroxyethyl)amino)phenylazo)phenyl)-6(5)-nitro-benzimidazole (BIZI). To a suspension of amine 3a (0.43 g, 1.7 mmol) in the dilute hydrochloric acid (20 mL, 4M) in an ice-water bath, the solution of NaNO2 (0.12 g, 1.7 mmol) dissolved in water (2 mL) was added dropwise to the resulting suspension while vigorously stirring. The reaction mixture was stirred at a low temperature for 30 min before N-ethyl-N-hydroxyethyl aniline 4 (0.30 g, 1.8 mmol) in ethanol was added to the above diazonium salt. The solution with a little suspension turns red at once, and the indissoluble solid was filtered off. When the solution of azo dye was neutralized with potassium carbonate, a lot of precipitates of azo compound formed. The compound was collected by filtration and recrystallized from DMF/H2O to give dark red microcrystalline solids (0.43 g, 59%). 1H NMR (DMSO-d6) δ (ppm): 13.75 (s, 1H, NH), 8.50 (s, 1H, ArH), 8.36 (d, 2H, ArH), 8.15 (d, 1H, ArH), 7.95 (d, 2H, ArH), 7.81 (d, 3H, ArH), 6.86 (d, 2H, ArH), 4.87 (s, 1H, OH), 3.61 (t, 2H, 3.51 (m, 4H, -NCH2CH3 and -NCH2CH2OH), -NCH2CH2OH), 1.16 (t, 3H, -NCH2CH3). Anal calcd. for C23H22N6O3 (430.46): C, 64.17; H, 5.15; N, 19.52. Found: C, 64.18; H, 5.09; N, 19.51. Synthesis of BIZII. The synthetic procedure of chromophore BIZII was similar to that of BIZI. 1H NMR (DMSO-d6) δ (ppm): 13.28 (s, 1H, NH in imidazole), 8.42 (s, 1H, ArH), 8.08 (d, 1H, ArH), 7.48-7.99 (m, 8H, 7H in ArH and sArCHdCHs), 7.33 (d, J ) 16.5 Hz, 1H, sArCH)CH-), 6.81 (d, 2H, ArH), 4.84 (s, 1H, OH), 3.60 (s, 2H, sNCH2CH2OH), 3.48 (t, 4H, sNCH2CH3 and sNCH2CH2OH), 1.13 (t, 3H, sCH3). Synthesis of BIZIASD. To chromophore BIZI (2.68 g, 6 mmol) under a dry nitrogen atmosphere were added dry pyridine (20 mL) and triethylamine (0.2 mL) as a catalyst. When the solution was heated to 50 °C, ICPTES (2.22 g, 9 mmol) was added dropwise in 5 min. The resulting solution was kept at
Hybrid Materials Containing Benzimidazole Chromophores 100 °C for 2 days and cooled to room temperature. The cooled solution was added dropwise to petroleum ether (300 mL), and a lot of precipitates occurred. The precipitation was collected by filtration and purified by flash column chromatography to give 2.05 g of orange red solid. Yield: 50%. 1H NMR (DMSOd6) δ (ppm): 13.62 (s, 1H, NH in imidazole), 8.42(s, 1H, ArH), 8.28 (d, 2H, ArH), 8.07(d, 1H, ArH), 7.88 (d, 2H, ArH), 7.66-7.80 (m, 3H, ArH), 7.21 (t, 1H, sO(CdO)NHs), 6.82 (d, 2H, ArH), 4.13 (t, 2H, sNCH2CH2-), 3.70 (q, 6H, sSi(OCH2CH3)3), 3.60 (t, 2H, sNCH2CH2s), 3.47 (d, 2H, sNCH2CH3), 2.95 (q, 2H, sSiCH2CH2CH2NHs), 1.44 (m, 2H, sSiCH2CH2CH2s), 1.13 (m, 12H, sSi(OCH2CH3)3 and sNCH2CH3), 0.51 (t, 2H, sSiCH2CH2CH2s). Anal. calcd. for C33H43N7O7Si (677.82): C, 58.47; H, 6.39; N, 14.46. Found: C, 58.35; H, 6.33; N, 14.35. Synthesis of BIZIIASD. The synthetic procedure of BIZIIASD was similar to that of BIZIASD. 1H NMR (DMSO-d6) δ (ppm): 13.31 (s, 1H, NH in imidazole), 8.45 (s, 1H, ArH), 8.12 (d, 1H, ArH), 7.72-7.85(m, 8H, 7H in ArH and sArCHdCHs), 7.36 (d, J ) 16.5 HZ, 1H, sArCHdCHs), 7.19 (t, 1H, sO(CdO)NHs), 6.88 (d, 2H, ArH), 4.14 (t, 2H, sNCH2CH2s), 3.73 (q, 6H, sSi(OCH2CH3)3), 3.63 (t, 2H, 3.51 (d, 2H), 2.95 (q, 2H, sNCH2CH2s), sSiCH2CH2CH2NHs), 1.44 (m, 2H, sSiCH2CH2CH2s), 1.14 (m, 12H, sSi(OCH2CH3)3), 0.51 (t, 2H, sSiCH2CH2CH2s). Anal. calcd. for C35H45N7O7Si (703.86): C, 59.72; H, 6.44; N, 13.93. Found: C, 59.03; H, 5.85; N, 14.19. Preparation of Hybrid Films. Hybrid NLO films were prepared through sol-gel methods. To avoid immediately precipitating, tetraethyl orthosilicate (TEOS) and dilute hydrochloric acid (pH ) 1) were diluted in THF before being added dropwise to the THF solution of BIZIASD and BIZIIASD while vigorously stirring. The molar ratio of water to TEOS was controlled to be 4:1. The solution was stirred at room temperature for 12 h and aged for another 4 days to increase the viscosity. The resulting sols were filtered through 0.22 µm syringe filters and spin-coated on the indium tin oxide (ITO) glass substrates to give orange red inorganic-organic hybrid films. Prior to poling process, these films were dried at 120 °C in vacuo for 6 h to remove the residual solvents. The film samples with 20 mol % BIZI were denoted as FBIZI and BIZII as FBIZII. The thickness of films generally increased with the concentration of organic units. The films with 10 and 20 mol % of ASDs are around 400 nm in thickness, while those with 40 mol % of ASDs reached 850-1000 nm. The second-order NLO properties of hybrid films were determined by in situ second harmonic generation (SHG) measurements. The details of measurements were given elsewhere.43 In this experiment, a poling voltage 5.0-5.5 kV was applied across the ITO layer and the coronal needle, and the distance between needle and the films was 1 cm. The poling temperature was determined by the TGA results and the NLO responses of films when they were poled. Results and Discussion Synthesis and Characterization. The synthetic routes to chromophores and alkoxysilane dyes are sketched in Scheme 1. The benzimidazole-based chromophores were synthesized in two steps. Condensations of 1 (1a and 1b) and o-phenylenediamine 2 in the presence of polyphosphoric acid at a relative high temperature afforded benzimidazole derivatives amine 3a, 3b in moderate yields. The chromophores were easily obtained by a diazo coupling reaction from the corresponding benzimidazole-based amine and aniline 4. For the synthesis of 3b, a
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Figure 1. FT-IR spectra of BIZII, BIZIIASD, and film FBIZII.
lower temperature of 165 °C was chosen as the reaction temperature than that reported in a similar reaction (200 °C),27 because p-aminocinnamic acid, which is the intermediate after the evolution of hydrochloride, decomposes at 173-175 °C.44 ASDs were subsequently obtained from the reactions between respective chromophores and ICPTES. In consideration of the poor solubility of chromophores and the purification in the following steps, pyridine is chosen as solvent for the coupling reactions due to its good solvency and moderate boiling point. The formations of chromophores and ASDs were verified by 1 H NMR and FT-IR spectroscopies. 1H NMR data of chromophores are generally in agreement with the previous reports.27 The IR spectra of BIZII and BIZIIASD are illustrated in Figure 1. For BIZII, the broad absorption band above 3000 cm-1, which is much broader than normal hydroxyl stretch absorption, arises from the overlapping of the hydroxyl group and the secondary amino group in imidazole heterocycle. The variation of absorption frequencies (about 30 cm-1) for chromophore and ASD between 3200-3700 cm-1 due to overlapped NH or OH stretch absorptions could be observed, suggesting that BIZIIASD was obtained from the coupling reaction between hydroxyl groups in chromophores and isocyanate groups in ICPTES. Additionally, the new carbonyl absorption band at 1702 cm-1 identifies the formation of carbamate (RO(CdO)NHR′).45 In 1H NMR spectrum of BIZIIASD, the emerging triplet proton peaks around δ ) 7.19 ppm, which is assigned to the proton in carbamate, further identified the formation of BIZIIASD.46 Unlike poorly soluble chromophores, the consequent ASDs are much more readily soluble in common spin-coating solvents such as THF and DMF, which allow to heavily loading of NLO active components in hybrid materials. Because of the great enhancement in solubility of ASDs, the following sol-gel process acted well, and uniform films with high chromophore concentrations were obtained as expected. The IR spectrum of film FBIZII is illustrated in Figure 1. For hybrid films, compared with ASDs, the C-H stretch in the O-CH2CH3 groups at 2800-3000 cm-1 has significantly decreased. This is due to the hydrolysis of ASDs and TEOS and then co-condensation in sol-gel and curing process. In addition, the broad absorption band around 1080 cm-1 suggests the formation of Si-O-Si network.45 UV-vis absorption spectral study reveals that chromophores BIZI and BIZII have good transparency in DMF, with a relative short absorption edge wavelength of approximately 550 nm, as shown in Figure 2. Both chromophores have a strong broad UV-vis absorption peak with a wavelength of maximal absorption (λmax) value around 480 nm in DMF, about 20 nm
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Figure 4. TGA curves of BIZI, BIZII, and DR1. Figure 2. UV-vis spectra for chromophores BIZI, BIZII, and DR1 in DMF.
Figure 3. UV-vis spectra of films FBIZI and FBIZII.
hypsochromic shifted in comparison with the widely researched dye disperse red 1 (DR1), which features similar structure to these benzimidazole-based chromophores. This hyposochromic effects can be explained as the noncoplanar arrangement of phenyl rings and benzimidazole, which hampers the transfer of electrons. The distortions from coplanarity have been convinced by previous researches in the similar molecules.27 Interestingly, BIZII, whose structure has a more vinyl group in the π-conjugation of BIZI, shows almost competitive transparency with BIZI. These results are desirable for transparent requirement in NLO materials. The films derived from these chromophores exhibit similar transparency with their organic components from UV-vis spectra (Figure 3). Compared with the solution of chromophores, the λmax values of FBIZI and FBIZII shift hypsochromicly to 466 and 456 nm, respectively, due to the difference in media polarity. The improved transparency of films allows them to be applied in a broader wavelength range than that derived from DR1 and other less transparent chromophores. In addition, the shapes of absorption band are similar to their chromophores, suggesting the chromophores have been successfully incorporated in the silica networks, without deterioration. The thermal stability of the chromophores and alkoxysilane dyes was evaluated by simultaneous thermogravimetric analysis (TGA)-DSC. The decomposition temperatures (Td) of chromophores BIZI and BIZII are 274 and 244 °C, respectively, determined by the TGA (Figure 4). Although have a longer conjugated chain than DR1, chromophore BIZI even has a higher decomposition temperature than DR1, by nearly 30 °C. BIZII, which has an additional vinyl and benzimidazole moiety in the π-conjugation with respective to DR1, still has a
comparable Td value to DR1, although it exhibits lower stability than BIZI. The introduction of vinyl group typically leads to reduced thermal stability;47 nevertheless, similar desirable results were frequently obtained in previous studies that good thermal stability of chromophores retains by incorporation heterocyles such as thiophenyl moiety into π-conjugation.18 Therefore, heterocycles are typically promising candidates for π-conjugation of chromophores to increase NLO activities without sacrificing thermal stability. NLO Activity. To investigate the microscopic NLO property, the solvatochromic effects of these chromophores were studied to estimate their first hyperpolarizabilities. Table 1 illustrates the λmax values of chromophores in DMF and chloroform (CHCl3), which are of great difference in polarity. For comparison, the solvatochromic data of DR1, collected in the same condition is listed aside. The βCTµg values (products of the hyperpolarizability along the CT direction and the permanent dipole moment of the ground state) of chromophores were calculated by the following simplified equation48
βCTµg ) 4.612 × 10-5 × f(λ) )
f(λ)ε∆V1/2∆Va3 ∆f(D)
λ3λ40
(λ20 - 4λ2)(λ20 - λ2) 2(D - 1) f(D) ) 2D + 1
(1)
Where λ is the maximum absorption wavelength of molecule due to π-π* transition in dipolar solvent; ε, ∆V1/2, ∆V, λ0, and a are the maximum of absorption coefficient in dipolar solvent, the full frequency width at half-maximum of absorption band, frequency shift of the maximum absorption in different solvents, wavelength of fundamental frequency, and the radius of chromophores, respectively. The CGS unit system is adopted, the unit of ε is mol-1 L cm-1 and D is Debye. For BIZI and BIZII, their βCT · µg values are much larger than that of DR1, by about 3-4 times. Compared with DR1 in structure, these chromophores have additional benzimidazole moieties of low resonant energy in the extended π-conjugated chains for CT, thereby representing much larger µgβCT values. In spite of noncoplanar structures of chromophores, these chromophores exhibit expected large optical nonlinearity. The possible reason is that the longer CT chains and lower delocalization energy of imidazole compensate the influence of undesirable structures on NLO activity. These results demonstrate that introduction
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TABLE 1: Hyperpolarizabilities of Chromophores Estimated by the Solvatochromic Method chromophore DR1 BIZI BIZII a
λmaxa(nm)
ε
503 476 480
(104mol-1 · L · cm-1)
∆ν1/2(cm-1)
2.87 1.92 2.09
3819 3629 3859
λmaxb(nm) 480 454 454
βCT(1907nm)µg
(10-30esu · D)
814 2306 3129
Measured in DMF. b Measured in CHCl3.
Figure 5. DSC-TGA curves of films FBIZI and FBIZII.
Figure 6. Temperature-dependent decays of NLO coefficients for FBIZI and FBIZII.
of benzimidazole moieties to π-conjunction is an efficient way to solve the problem of “nonlinearity-transparency trade off”. To determine appropriate poling and curing temperatures, TGA-DSC studies of films that previously baked at 120 °C for 6 h were carried out, as shown in Figure 5. The onset temperatures of weight loss for hybrid films FBIZI and FBIZII are about 170 °C, at which condition no apparent exothermal reaction are observed from the simultaneous DSC curves, indicating that the weight loss mainly attributes to the condensation of silanols. From the DSC curves of films, the exothermic reactions occur above 280 °C, suggesting the decomposition of the film. This temperature is high enough for poling and curing processes. By consideration that the highly randomization of chromophores orientation at the high temperature above 180 °C, the hybrid films were poled and cured at 180 °C. After properly poled for 0.5 h, these films, with various molar concentration of chromophores from 10-40%, have large NLO coefficients d33 values ranging from 25 to 54 pm/V calculated by the following equations49
d33,s ) d11,q
Is Lqc F Iq Ls
′ ) d33,sexp(R2ωLs/4)(R2ωLs/4)/sinh(R2ωLs/4) d33,s
(2)
(3)
where d11,q ) 0.45 pm/V is the d11 component of NLO coefficient of quartz crystal, and the coherence length of quartz Lqc ) 20.6 µm. F is correction factor of the experimental setup, and if the thickness of the poled film is less than Lqc, F ) 1.2. Equation 3 gives the corrected value d33,s′ when the absorption effects of films are considered, and R2ω is the absorption coefficient of samples at the doubling frequency. Among these films, the film with 20 mol % of BIZII, which has extended π-conjugated chain, has the largest d33 value (54 pm/V). With respect to these previously reported polymers containing similar chromophores,23 the great improvement in d33 values for hybrid materials was achieved mainly due to the high concentrations of chromophores and the high poling efficiency by virtue of their rigid silica matrix derived from poling and curing process.
Compared with the hybrid films containing DR1 (d33 )36 pm/ V),46 the benzimidazole-base films exhibit enhanced NLO activities. Stability of NLO Activity. The stability of NLO performance is one of the most challenging issues in poled NLO materials. Two approaches were used to evaluate their stability in this study. The first one is temperature-dependent decay of d33 values at a constant heating rate in the absent of poling electric field. Figure 6 illustrates the decay of two samples FBIZI and FBIZII. The results indicate that for both films, there are no obvious decays of d33 values below 120 °C. FBIZI begins to decay at a relative lower temperature than FBIZII but decays more slowly than FBIZII at a high temperature above 150 °C. Although NLO property of FBIZII decays rapidly above 160 °C, it retains its original NLO activity until around 140 °C. For practical applications, hybrid films with BIZII are more promising for their stable feature. The second way to evaluate NLO stability of films was performed on the other two FBIZII samples prepared at the same time to monitor the isothermal decay process of d33 values. We chose two samples rather than one to eliminate the influence of thermal history on films. Before isothermal decay process was detected, the films that had been poled at 180 °C continued to be poled at the test temperature (160 or 140 °C) for several minutes to stabilize the temperature and SHG signals. SHG signals were collected once the poling electric field was removed. The isothermal decay curves of d33 values are shown in Figure 7. Decays of NLO coefficients for these samples at different temperature involve two different stages: the fast decay and the slow decay processes. After a rapid decay (lasts about 1 min), the latter process is extremely slow at 140 °C such that less than 5% of d33 decay was observed in the following 15 min. Even at 160 °C, only less than 15% decay of d33 was detected in the next 15 min. These films exhibit much better stable NLO properties than hybrid films containing DR1 in our previous studies, in which a film containing 17 mol % of DR1, less than 40% of original d33 remains after keeping at 160 °C for 10 min.46 The fast decay process, which lasts for only several minutes, probably ascribes to the alignment adjustments of individual
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Yu et al. and careful design of a matrix for NLO materials is urgently required even for inorganic-organic hybrid matrix. Conclusion
Figure 7. Isothermal decay of d33 values for film FBIZII at 140 and 160 °C.
molecules those are in extremely unfavorable sites or in the vicinity of locally soft sites. The decays of SHG signals after the removal of electric field are always observed even at room temperature in our previous tests, but only last for a few minutes. Therefore, to a certain extent, it is inevitable but not detrimental to preparation of stable devices. The slow decay is a result of structural relaxations of molecules in thermodynamically unfavorable polar order. The relaxation process is largely dependent on the rigidity of matrix and the rotational mobility of chromophores. As Boyd et al. reported that rotational freedom generally decreased with increasing the length of chromophores in matrix,41 the excellent temporal stability of these hybrid films is proposed to derive from the rigidity of films and large lengths of chromophores. Besides, it is worth identifying whether imidazole amines in chromophores serve as a basic catalyst for condensation of silanols, hence promoting the integrity of silica network. The role of amine in alkoxysilane precursor serving as a catalyst for inorganic polymerization has been investigated by Innocenzi et al. in sol-gel hybrid materials.50 However, introduction of amine-terminal component as an efficient catalyst that leads to reduce the gelation time to a large extent is not desirable for NLO materials, because it is difficult to accomplish homogeneous hybrid copolymer films for optical devices without a delicately balanced hydrolysis-condensation process. Instead of strong base, introduction of a weak basic moiety to chromophores as a mild catalyst for smooth condensation is an alternative route to achieve uniform films with high optical quality and alignment stability. If we put the temperature-dependent and isothermal decay curves of FBIZII together, it is found that there seems to be an unexpected contradiction between them. Only about 12% of d33 decay occurs during 2-16 min at 160 °C from isothermal decay curves, while from temperature-dependent decay curve nearly 23% of d33 value is lost when the temperature is raised to 160 °C (in 12 min). As a matter of fact, it should be noted that the isothermal decay curves were obtained after the structural equilibrium including the alignment of molecules and matrix under poling electric field at this test temperature. In the isothermal decay process, the decay of NLO properties is mainly derived from the randomization of chromophores alignments, while the temperature-dependent decays are the synergetic results of relaxation of chromophores and matrix at Various temperatures. It suggests the temperature-driven structural relaxation of matrix, for instance, thermal expansion, plays a significant role contributing to the decays of optical nonlinearity,
A series of benzimidazole-based NLO chromophores and ASDs of optimized stability-transparency-nonlinearity were synthesized and characterized. The introduction of benzimidazole to π-conjunction efficiently improves the transparency of chromophores, compensates the negative influence on nonlinearity due to undesirable noncoplanar structure, and also supplies a mild basic catalyst for materials process. The merits of chromophores successfully endow the resulting hybrid films simultaneously with good stability, transparency, and nonlinearity. An optimal hybrid film FBIZII exhibits excellent NLO alignment stability at 140 and 160 °C, with a NLO coefficient d33 value of 54 pm/V and a maximal absorption wavelength λmax at 456 nm. The benzimidazole-based inorganic-organic hybrid materials represent a kind of promising materials for the requirements of second-order NLO and EO applications. Acknowledgment. This work was financially supported by PCSIRT, the National Natural Science Foundation of China (Nos. 50532030, 50625206, 50802084, and 50972127), and the Natural Science Foundation of Zhejiang Province (No. Y4090067). References and Notes (1) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. ReV. 1994, 94 (1), 31–75. (2) Shi, Y. Q.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science 2000, 288 (5463), 119–122. (3) Enami, Y.; Derose, C. T.; Mathine, D.; Loychik, C.; Greenlee, C.; Norwood, R. A.; Kim, T. D.; Luo, J.; Tian, Y.; Jen, A. K.-Y.; Peyghambarian, N. Nat. Photon. 2007, 1 (3), 180–185. (4) Ohkoshi, S.-i.; Saito, S.; Matsuda, T.; Nuida, T.; Tokoro, H. J. Phys. Chem. C 2008, 112 (34), 13095–13098. (5) Shi, Z.; Hau, S.; Luo, J.; Kim, T.-D.; Tucker, N. M.; Ka, J.-W.; Sun, H.; Pyajt, A.; Dalton, L.; Chen, A.; Jen, A. K.-Y. AdV. Funct. Mater. 2007, 17 (14), 2557–2563. (6) Zhang, C.-Z.; Lu, C.; Zhu, J.; Wang, C.-Y.; Lu, G.-Y.; Wang, C.S.; Wu, D.-L.; Liu, F.; Cui, Y. Chem. Mater. 2008, 20 (14), 4628–4641. (7) Robinson, B. H.; Dalton, L. R.; Harper, A. W.; Ren, A.; Wang, F.; Zhang, C.; Todorova, G.; Lee, M.; Aniszfeld, R.; Garner, S.; Chen, A.; Steier, W. H.; Houbrecht, S.; Persoons, A.; Ledoux, I.; Zyss, J.; Jen, A. K. Y. Chem. Phys. 1999, 245 (1-3), 35–50. (8) Jang, S. H.; Luo, J.; Tucker, N. M.; Leclercq, A.; Zojer, E.; Haller, M. A.; Kim, T. D.; Kang, J. W.; Firestone, K.; Bale, D.; Lao, D.; Benedict, J. B.; Cohen, D.; Kaminsky, W.; Kahr, B.; Bredas, J. L.; Reid, P.; Dalton, L. R.; Jen, A. K. Y. Chem. Mater. 2006, 18 (13), 2982–2988. (9) Luo, J.; Hau, S.; Bale, D. H.; Kim, T.-D.; Shi, Z.; Lao, D. B.; Tucker, N. M.; Tian, Y.; Dalton, L. R.; Reid, P. J.; Jen, A. K.-Y. Chem. Mater. 2007, 19 (5), 1154–1163. (10) Gao, J.; Cui, Y.; Yu, J.; Wang, Z.; Wang, M.; Qiu, J.; Qian, G. Macromolecules 2009, 42 (6), 2198–2203. (11) Sullivan, P. A.; Olbricht, B. C.; Akelaitis, A. J. P.; Mistry, A. A.; Liao, Y.; Dalton, L. R. J. Mater. Chem. 2007, 17 (28), 2899–2903. (12) Kim, T. D.; Luo, J. D.; Cheng, Y. J.; Shi, Z. W.; Hau, S.; Jang, S. H.; Zhou, X. H.; Tian, Y.; Polishak, B.; Huang, S.; Ma, H.; Dalton, L. R.; Jen, A. K. Y. J. Phys. Chem. C 2008, 112 (21), 8091–8098. (13) Shi, Z.; Luo, J.; Huang, S.; Cheng, Y.-J.; Kim, T.-D; Polishak, B. M.; Zhou, X.-H.; Tian, Y.; Jang, S.-H.; Knorr, D. B., Jr.; Overney, R. M.; Younkin, T. R.; Jen, A. K.-Y. Macromolecules 2009, 42 (7), 2438–2445. (14) Shi, Z.; Luo, J.; Huang, S.; Zhou, X.-H.; Kim, T.-D.; Cheng, Y.J.; Polishak, B. M.; Younkin, T. R.; Block, B. A.; Jen, A. K.-Y. Chem. Mater. 2008, 20 (20), 6372–6377. (15) Hammond, S. R.; Clot, O.; Firestone, K. A.; Bale, D. H.; Lao, D.; Haller, M.; Phelan, G. D.; Carlson, B.; Jen, A. K. Y.; Reid, P. J.; Dalton, L. R. Chem. Mater. 2008, 20 (10), 3425–3434. (16) Adkins, C. T.; Harth, E. Macromolecules 2008, 41 (10), 3472– 3480. (17) Rao, V. P.; Wong, K. Y.; Jen, A. K. Y.; Drost, K. J. Chem. Mater. 1994, 6 (12), 2210–2212. (18) Cai, C.; Liakatas, I.; Wong, M. S.; Bosch, M.; Bosshard, C.; Gunter, P.; Concilio, S.; Tirelli, N.; Suter, U. W. Org. Lett. 1999, 1 (11), 1847– 1849.
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