Triphenylamine-Cored Bifunctional Organic Molecules for Two-Photon

Jan 16, 2004 - Multiarmed nonlinear optical (NLO) molecules containing triphenylamine as a core and 4-(2-ethylhexylsulfonyl)benzene-(1E)-2-vinyl group...
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Chem. Mater. 2004, 16, 456-465

Triphenylamine-Cored Bifunctional Organic Molecules for Two-Photon Absorption and Photorefraction Hye Jeong Lee, Jiwon Sohn, Jaehoon Hwang, and Soo Young Park* School of Materials Science and Engineering, Seoul National University, San 56-1, Shillim-dong, Kwanak-gu, Seoul 151-744, Korea

Haeyoung Choi and Myoungsik Cha Department of Physics and Research Center for Dielectric & Advanced Matter Physics, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea Received May 19, 2003. Revised Manuscript Received November 16, 2003

Multiarmed nonlinear optical (NLO) molecules containing triphenylamine as a core and 4-(2-ethylhexylsulfonyl)benzene-(1E)-2-vinyl group or 4-{2-[4-(2-ethylhexylsulfonyl)phenyl](1E)-vinyl}benzene-(1E)-2-vinyl group as arms were synthesized (STEH series and SSEH series, respectively). Because triphenylamine linked to the sulfonylated stilbenic arms provided effective push-pull NLO structure and strong 2-D charge transfer, they were capable of both two-photon absorption (TPA) and photorefraction. Effective TPA cross sections of these molecules were as high as 0.94 × 10-46 cm4‚s and significantly enhanced as the number of arms and conjugation length increased. One- and two-armed STEH molecules (STEH1 and STEH2) showed moderate two-beam coupling gain coefficients (13.4 cm-1 at 45 V/µm and 17.4 cm-1 at 55 V/µm, respectively) and distinct diffraction efficiency (0.45% at 40 V/µm and 0.55% at 55 V/µm). On the other hand, the centrosymmetric three-armed STEH molecule exhibited no photorefraction at all. It is importantly claimed that the multiarmed bifunctional molecules of this work are easily fabricated into optically clear amorphous films by themselves, which is a key advantage toward potential applications such as solid-state optical limiting devices and two-photon excited photorefractive materials.

Introduction Organic nonlinear optical (NLO) materials have been extensively studied due to their broad applications in the area of electronics and photonics.1 Among many specific phenomena derived from the optical nonlinearity of organic molecules, two-photon absorption (TPA) and photorefraction are gaining increasing interest for their promising applications. TPA refers to the imaginary part of the third-order NLO response through which a molecule absorbs two photons simultaneously in the presence of an intense laser beam. This process holds a significant application potential for optical power limiting, two-photon upconverted lasing, confocal imaging, photodynamic therapy, and three-dimensional optical data storage.2-4 To date, various nonlinear optical molecules with large TPA cross section have been synthesized5-7 and their structure-property relation* To whom correspondence should be addressed. E-mail: parksy@ plaza.snu.ac.kr. Tel: +82-2-880-8327. Fax: +82-2-886-8331. (1) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1994, 94, 195. (2) Albota, M.; Beljonne, D.; Bre´das, J.-L.; Ehrlich, J.; Fu, J.-Y.; Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-Maughon, D.; Perry, J. W.; Ro¨ckel, H.; Rumi, M.; Subramanian, G.; Webb, W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653. (3) Kawata, S.; Sun, H.-B.; Tanaka, T.; Takada, K. Nature 2001, 412, 697. (4) Zhou, W.; Kueber, S. M.; Braun, K. L.; Yu, T.; Cammack, J. K.; Ober, C. K.; Perry, J. W.; Marder, S. R. Science 2002, 296, 1106. (5) Ventelon, L.; Moreaux, L.; Mertz, J.; Blanchard-Desce, M. Chem. Commun. 1999, 2055.

ship and molecular design rule have been investigated systematically.8,9 On the other hand, reversible and nonlocal modulation of the refractive index in the bulk of the material has been observed by the illumination of the light interference pattern when the second-order optical nonlinearity is combined with photoconductivity. This particular process, called “photorefraction”, is expected to provide novel and effective means for highdensity optical data storage, phase conjugation, and optical image processing.10-12 One inherent problem of this photorefractive optical recording has been image volatility during the reading process. Very recently, Kippelen et al.13 also proposed and demonstrated that (6) Cho, B. R.; Son, K. H.; Lee, S. H.; Song, Y.-S.; Lee Y.-K.; Jeon, S.-J.; Choi, J. H.; Lee, H.; Cho, M. J. Am. Chem. Soc. 2001, 123, 10039. (7) Kim, O.-K.; Lee, K.-S.; Woo, H. Y.; Kim, K.-S.; He, G. S.; Swiatkiewicz, J.; Prasad, P. N. Chem. Mater. 2000, 12, 284. (8) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt, J. C.; Kannan, R.; Yuan, L.; Guang, S. H.; Prasad, P. N. Chem. Mater. 1998, 10, 1863. (9) Rumi, M.; Ehrich, J. E.; Heikal, A. A.; Perry, J. W.; Barlow, S.; Hu, Z.; McCord-Maughon, D.; Parker, T. C.; Ro¨ckel, H.; Thayumananvan, S.; Marder, S. R.; Beljonne, D.; Bre´das, J.-L. J. Am. Chem. Soc. 2000, 122, 9500. (10) Zilker, S. J. Chemphyschem 2000, 1, 72. (11) Zhang, Y.; Burzynski, R.; Ghosal, S.; Casstevens, M. K. Adv. Mater. 1996, 8 (2), 111. (12) Wang, Q.; Wang, L.; Yu, L. Macromol. Rapid. Commun. 2000, 21, 723. (13) Kippelen, B.; Blanche, P.-A.; Schu¨lzgen, A.; Fuentes-Hernandez, C.; Ramos-Ortiz, G.; Wang, J.-F.; Peyghambarian, N.; Marder, S. R.; Leclercq, A.; Beljonne, D.; Bre´das, J.-L. Adv. Funct. Mater. 2002, 12, 615.

10.1021/cm0343756 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/16/2004

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Scheme 1. Synthesis of Cores and Arm of STEH1, STEH2, and STEH3

combining the above two properties can provide a unique and efficient way for nondestructive readout in photorefractive polymeric media, which is called TPAsensitized photorefraction. In addition, Day et al. had demonstrated the use of continuous wave illumination for three-dimensional (3-D) bit optical data storage under two-photon excitation in photorefractive polymers. These achievements make it possible to develop a cheap, compact, sub-Tbyte rewritable optical data storage system to further extend the capabilities of compact disk and digital versatile disk technology.14 Although these approaches have demonstrated a proofof-concept breakthrough, materials development is very critical to the realization of practical application. In this work, we aimed to synthesize a bifunctional molecule which is capable of both TPA and photorefraction for their respective applications or the TPA-sensitized photorefraction. Our motivation for designing a bifunctional molecule was to eliminate the phase separation and mutual dilution problem inherent to the multicomponent mixtures. From the practical application viewpoint, we also intended this molecule to form the optically clear amorphous glass by itself. As a starting point, we wanted to adapt the known molecular design rules for efficient TPA, one of which states that the TPA could be significantly enhanced by incorporating a multibranched structure due to cooperative enhancement originating from arm-to-arm interaction and the enhanced 2-D charge transfer from the core to its periphery (or vice versa).15-17 On the basis of all of these considerations in this work, we have designed and synthesized a specific class of bifunctional multiarmed nonlinear optical molecules, (14) (a) Day, D.; Gu, M.; Smallridge, A. Opt. Lett. 1999, 24, 948. (b) Day, D.; Gu, M.; Smallridge, A. Adv. Mater. 2001, 13, 1005. (15) Chung, S.-J.; Kim, K.-S.; Lin, T.-C.; He, G. S.; Swiatkiewicz, J.; Prasad, P. N. J. Phys. Chem. 1999, 103, 10741.

which comprises triphenylamine as a core and sulfonesubstituted stilbene group as conjugated arms (see structure 19 in Scheme 4 as an example). It is expected that the electronic push-pull structures in the arm and their cooperative effect not only help the extended charge transfer for TPA but also afford the photorefractivity by electrooptic effect. Triphenylamine plays dual roles of an electron donor and a photoconductor for photorefractivity and an electron accepting sulfone group contributes to the formation of a nonlinear optical push-pull structure. Additional merit of using a sulfone group was the expectation of strong one-photon and twophoton pumped fluorescence.18,19 An ethylhexyl moiety was attached to the end of each arm to induce the amorphousness for optical transparency as well as the good solubility to various organic solvents. We tried to change the stilbenic conjugation length and the number of arms to investigate the effect of them on the nonlinear absorption and photorefractivity. Experimental Section Synthetic routes to the multiarmed bifunctional molecules and their intermediates are depicted in Schemes 1-4. 4-Diphenylamino-benzaldehyde (1). Phosphorus oxychloride (3.8 mL, 0.04 mol) was added dropwise to a stirred 6.2 mL (0.08 mol) of N,N-dimethyl formamide (DMF) at 0 °C. The mixture was stirred at 0 °C for 1 h and additionally stirred at room temperature for 1 h. After the addition of 10 g (0.04 (16) (a) Chung, S.-J.; Lin, T.-C.; Kim, K.-S.; He, G. S.; Swiatkiewicz, J.; Prasad, P. N.; Baker, G. A.; Bright, F. V. Chem. Mater. 2001, 13, 4071. (b) Drobizhev, M.; Karotki, A.; Rebane, A.; Spangler. C. W. Opt. Lett. 2001, 26, 1081. (17) Beljonne, D.; Wenseleers, W.; Zojer, E.; Shuai, Z.; Vogel, H.; Pond, S. J. K.; Perry, J. W.; Marder, S. R.; Bre´das, J.-L. Adv. Funct. Mater. 2002, 12 (9), 631. (18) Jung, H. K.; Lee, C. L.; Lee, J. K.; Kim, J. K.; Park, S. Y.; Kim, J-J. Thin Solid Films 2001, 401, 111. (19) Bhawalkar, J. D.; He, G. S.; Park, C.-K.; Zhao, C. F.; Ruland, G.; Prasad, P. N. Opt. Commun. 1996, 124, 33.

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Scheme 2. Synthesis of STEH1, STEH2, and STEH3

Scheme 3. Synthesis of Cores and Arm of SSEH1, SSEH2, and SSEH3

mol) of triphenylamine dissolved in dichloroethane, the mixture was stirred at 90 °C for 2 h. After cooling, the solution was poured into cold water. The resulting mixture was neutralized to pH 7 with 2 N NaOH aqueous solution and extracted with dichloromethane. The extract was washed with plenty of brine and the solvent was removed at reduced pressure. The residue was chromatographed on a silica gel column (ethyl acetate/n-hexane ) 1/3) to produce 5.8 g of yellowish solid, 53% yield. 1H NMR (acetone-d6, 300 MHz): δ 10.12 (s, 1H), 7.72 (d, 2H), 7.43-7.37 (t, 4H), 7.24-7.17 (t, 6H), 6.98-6.94 (d, 2H).

4,4′-Diformyl triphenylamine (2). Phosphorus oxychloride (46.6 mL, 0.5 mol) was added dropwise to a stirred 77.4 mL (1 mol) of DMF at 0 °C. The mixture was stirred at 0 °C for 1 h and then stirred at room temperature for another 1 h. After the addition of 10 g (0.04 mol) of triphenylamine dissolved in dichloroethane, the mixture was stirred at 80 °C for 48 h. After cooling, the solution was poured into cold water. The resulting mixture was neutralized to pH 7 with 2 N NaOH aqueous solution and extracted with dichloromethane. The extract was washed with plenty of brine and the solvent was removed at reduced pressure. The residue was chromato-

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Scheme 4. Synthesis of SSEH1, SSEH2, and SSEH3

graphed on a silica gel column (ethyl acetate/n-hexane ) 1/3) to produce 8 g of yellowish solid, 66% yield. 1H NMR (acetoned6, 300 MHz): δ 9.19 (s, 2H), 7.84 (d, 4H), 7.49-7.43 (t, 2H), 7.33-7.28 (t, 1H), 7.25-7.19 (m, 6H). Tris-(4-formyl-phenyl)amine (3). Phosphorus oxychloride (46.6 mL, 0.5 mol) was added dropwise to a stirred 77.4 mL (1 mol) of DMF at 0 °C. The mixture was stirred at 0 °C for 1 h and additionally stirred at room temperature for 1 h. After the addition of 6 g (0.02 mol) of 2 in dichloroethane, the mixture was stirred at 80 °C for 48 h. After cooling, the solution was poured into water. The resulting mixture was neutralized to pH 7 with 2 N NaOH aqueous solution and extracted with dichloromethane. The extract was washed with plenty of brine and the solvent was removed at reduced pressure. The residue was chromatographed on a silica gel column (ethyl acetate/n-hexane ) 1/2) to produce 1.35 g of yellowish solid, 20% yield. 1H NMR (CDCl3, 300 MHz): δ 9.92 (s, 3H), 7.84 (d, 6H), 7.24 (d, 6H). 4-(2-Ethylhexylthio)-1-methylbenzene (4). A solution of 10 g (0.08 mol) of 4-methyl-benzenethiol and 4.83 g (0.12 mol) of NaOH in 25 mL of ethanol was stirred at 65 °C for 1 h and then the solution of 17.2 mL (0.097 mol) of 1-bromo-2ethylhexane in ethanol was added. After the mixture was refluxed at 65 °C for 12 h, the resulting solution was poured into water and neutralized to pH 7 with 1 N HCl and extracted with ethyl acetate. The solvent was removed at reduced pressure and the residue was chromatographed on a silica gel column (ethyl acetate/n-hexane ) 1/7) to produce 18 g of transparent viscous liquid, 88% yield. 1H NMR (CDCl3, 300 MHz): δ 7.23 (d, 2H), 7.08 (d, 2H), 2.86 (d, 2H), 1.58-1.44 (m, 1H), 1.44-1.34 (m, 5H), 1.31-1.22 (m, 5H), 0.90-0.84 (m, 7H). 4-[(2-Ethylhexyl)sulfonyl]-1-methylbenzene (5). A solution of 8.5 g (0.036 mol) of 4 and 4.1 mL (0.072 mol) of acetic acid was stirred at 0 °C and treated with 11.8 mL (0.14 mol) of H2O2 (30% in water) and heated to 100 °C. After 5 h, the solution was cooled and poured into water and extracted with ethyl acetate. The extract was washed with plenty of water and the solvent was removed at reduced pressure. The residue was chromatographed on a silica gel column (ethyl acetate/nhexane ) 1/4) to produce 8.72 g of transparent viscous liquid,

90% yield. 1H NMR (CDCl3, 300 MHz): δ 7.8 (d, 2H), 7.3 (d, 2H), 3.0 (d, 2H), 2.44 (s, 3H), 1.94-1.86 (m, 1H), 1.46-1.33 (m, 4H), 1.25-1.44 (m, 4H), 0.86-0.78 (m, 6H). Diethoxy({4-[(2-ethylhexyl)sulfonyl]phenyl}methyl)phosphino-1-one (6). Bromination of compound 5 was carried out using N-bromosuccinimide (NBS) as the bromine source. The solution of 8 g (0.03 mol) of 5 and 8 g (0.045 mol) of NBS in 20 mL of benzene was refluxed at 80 °C for 48 h. The resulting solution was poured into water and then was extracted with ethyl acetate. After the extract was washed with water several times, the solvent was removed at reduced pressure. And then phosphonation of the brominated compound was carried out without further purification. The mixture of 18.5 mL (0.11 mol) of triethyl phosphite and the dried crude material was stirred at 130 °C for 12 h. Unreacted triethyl phosphite was removed by the distillation; the residue was chromatographed on a silica gel column (ethyl acetate/ methanol ) 30/1) to produce 2.83 g of viscous liquid, 26% yield. 1H NMR (CDCl , 300 MHz): δ 7.85 (d, 2H), 7.50 (d, 2H), 4.23 4.02 (m, 10H), 3.27-3.20 (d, 2H), 3.02 (d, 2H), 1.97-1.91 (m, 1H), 1.45-1.10 (m, 8H), 0.88-0.78 (m, 6H). 1-{(1E)-2-[4-(Diphenylamino)phenyl]vinyl}-4-[(2-ethylhexyl)sulfonyl]benzene (7, STEH1). 1 (0.7 g, 2.56 mmol) was added to a mixture of 1.04 g (2.56 mmol) of 6 and 0.431 g (3.84 mmol) of potassium tert-butoxide in 20 mL of tetrahydrofuran (THF). After refluxing at 80 °C for 3 h, THF was removed at reduced pressure. The mixture was poured into water and neutralized to pH 7 with 1 N HCl and then extracted with dichloromethane. The solvent was removed at reduced pressure and the residue was chromatographed on a silica gel column (ethyl acetate/n-hexane ) 1/10) to produce 0.35 g, 89% yield. Elem. Anal. Calcd (%) for C34H37NO2S: C, 77.97; H, 7.12; N, 2.67; S, 6.12. Found: C, 78.15; H, 7.07; N, 2.64; S, 5.66. 1H NMR (CDCl3, 300 MHz): δ 7.85 (d, 2H), 7.63 (d, 2H), 7.39 (d, 2H), 7.3-7.25 (t, 5H), 7.19 (d, 1H, J ) 16.29 Hz), 7.13-7.04 (m, 10H), 7.09 (d, 1H, J ) 16.29 Hz), 3.02 (d, 2H), 1.98-1.89 (m, 1H), 1.49-1.35 (m, 4H), 1.25-1.16 (m, 4H), 0.84 (q, 6H). IR (KBr pellet, cm-1): 1580 and 1500 (vinyl and aromatic CdC), 1310 and 1150 (SO2). 1-[(1E)-2-(4-{[4-((1E)-2-{4-[(2-Ethylhexyl)sulfonyl]phenyl}vinyl)phenyl]phenylamino}phenyl)vinyl]-4-[(2-ethyl-

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hexyl)sulfonyl]benzene (8, STEH2). 0.32 g (1.06 mmol) of 2 was added to mixture of 1.3 g (3.2 mmol) of 6 and 0.36 g (3.2 mmol) of potassium tert-butoxide in 20 mL of THF. Compound 8 was prepared by the same procedure as described for 7. Purification was carried out by chromatography on a silica gel column (ethyl acetate/n-hexane ) 1/10) to produce 0.22 g, 25% yield. Elem. Anal. Calcd (%) for C50H59NO4S2: C, 74.87; H, 7.41; N, 1.75; S, 8.00. Found: C, 74.45; H, 7.28; N, 1.59; S, 7.97. 1H NMR (CDCl3, 300 MHz): δ 7.87 (d, 4H), 7.64 (d, 4H), 7.43 (d, 4H), 7.31 (t, 3H), 7.21 (d, 2H, J ) 16.29 Hz), 7.15 (d, 2H), 7.11 (d, 4H) 7.03 (d, 2H, J ) 16.29 Hz), 3.03 (d, 4H), 1.981.90 (m, 2H), 1.49-1.35 (m, 8H), 1.25-1.16 (m, 8H), 0.84 (q, 12H). IR (KBr pellet, cm-1): 1580 and 1500 (vinyl and aromatic CdC), 1310 and 1150 (SO2). 1-[(1E)-2-(4-{bis[4-((1E)-2-{4-[(2-Ethylhexyl)sulfonyl]phenyl}vinyl)phenyl]amino}phenyl)vinyl]-4-[(2-ethylhexyl)sulfonyl]benzene (9, STEH3). 3 (0.23 g, 0.7 mmol) was added to a mixture of 1.41 g (3.5 mmol) of 6 and 0.4 g (3.5 mmol) of potassium tert-butoxide in 20 mL of THF. Compound 9 was prepared by the same procedure as described for 7. Purification was carried out by chromatography on a silica gel column (ethyl acetate/n-hexane ) 1/4) to produce 0.38 g, 50% yield. Elem. Anal. Calcd (%) for C66H81NO6S3: C, 73.36; H, 7.56; N, 1.30; S, 8.90. Found: C, 73.07; H, 7.55; N, 1.21; S, 8.81. 1H NMR (CDCl3, 300 MHz): δ 7.87 (d, 6H), 7.65 (d, 6H), 7.47 (d, 6H), 7.23 (d, 3H, J ) 16.29 Hz), 7.14 (d, 6H), 7.05 (d, 3H, J ) 16.29 Hz), 3.03 (d, 6H), 1.94 (m, 3H), 1.47-1.38 (m, 12H), 1.23 (m, 12H), 0.78 (q, 18H). IR (KBr pellet, cm-1): 1580 and 1500 (vinyl and aromatic CdC), 1310 and 1150 (SO2). (4-Bromophenyl)diphenylamine (10). Triphenylamine (1.5 g, 6.11 mmol) and 3 g of silica gel were added to the solution of 1.1 g (6.11 mmol) of NBS in 500 mL of dichloromethane and the round-bottom flask was wrapped with aluminum foil. After the solution was stirred for 12 h at 25 °C, silica gel was removed through the filtration. The solution was washed with water and brine several times. The solvent was removed at reduced pressure and the residue was chromatographed on a silica gel column (n-hexane) to produce 0.93 g, 44% yield. 1H NMR (CDCl3, 300 MHz): δ 7.31 (d, 2H), 7.15 (t, 4H), 7.04 (q, 6H), 6.94 (d, 2H). Bis(4-bromophenyl)phenylamine (11). Triphenylamine (0.5 g, 2.04 mmol) and 1 g of silica gel were added to the solution of 0.73 g (4.08 mmol) of NBS in 280 mL of dichloromethane. Compound 11 was prepared by the same procedure as described for 10. Purification was carried out by chromatography on a silica gel column (n-hexane) to produce 0.44 g, 53% yield. 1H NMR (CDCl3, 300 MHz): δ 7.34 (d, 4H), 7.26 (t, 2H), 7.05 (m, 3H), 6.93 (d, 4H). Tris(4-bromophenyl)amine (12). Triphenylamine (0.5 g, 2.1 mmol) and 2 g of silica gel were added to the solution of 1.2 g (6.3 mmol) of NBS in 400 mL of dichloromethane. Compound 12 was prepared by the same procedure as described for 10. Purification was carried out by chromatography on a silica gel column (n-hexane) to produce 0.8 g, 86% yield. 1H NMR (CDCl , 300 MHz): δ 7.36 (d, 6H), 6.93 (d, 6H). 3 (2-Ethylhexylsulfanyl)benzene (13). The mixture of 14 mL (0.136 mol) of benzenethiol and 11.5 g (0.203 mol) of KOH in 120 mL of ethanol was refluxed at 80 °C for 1 h and 29 mL (0.163 mol) of 1-bromo-2-ethylhexane was added to it. After the mixture was stirred for 12 h, ethanol was removed at reduced pressure. The mixture was poured into water and neutralized to pH 7 with 1 N HCl. After the solution was extracted with dichloromethane, the solvent was removed at reduced pressure. The residue was chromatographed on a silica gel column (n-hexane) to produce 30 g, 98% yield. 1H NMR (CDCl3, 300 MHz): δ 7.33-7.32 (m, 4H), 7.13 (t, 1H), 3.12 (d, 2H), 1.58 (m, 1H), 1.58 (m, 1H), 1.42 (m, 4H), 1.27 (m, 4H), 0.88 (m, 6H). 1-Bromo-4-(2-ethylhexylsulfanyl)benzene (14). 13 (17 g, 0.076 mol) and 10 g of silica gel were added to the solution of 13.6 g (0.076 mol) of NBS in 450 mL of dichloromethane and the round-bottom flask was wrapped with the aluminum foil. After the solution was stirred for 48 h at 25 °C, silica gel was removed through the filtration. The solution was washed with water and brine several times. The solvent was removed

Lee et al. at reduced pressure and the residue was chromatographed on a silica gel column (n-hexane) to produce 17 g, 74% yield. 1H NMR (CDCl3, 300 MHz): δ 7.38 (d, 2H), 7.18 (d, 2H), 2.87 (m, 2H), 1.35-1.5 (m, 4H), 1.27-1.18 (m, 5H), 0.91-0.85 (m, 6H). 1-Bromo-4-(2-ethylhexane-1-sulfonyl)benzene (15). H2O2 (30% in water) (18.5 mL, 0.225 mol) was added dropwise to a stirred solution of 17 g (0.056 mol) of 14 and 6.4 mL (0.112 mol) of acetic acid at 0 °C. And then the mixture was stirred at 100 °C for 8 h. The obtained product was poured into water and extracted with ethyl acetate. The extract was washed with brine several times and the solvent was removed at reduced pressure. The residue was chromatographed on a silica gel column (ethyl acetate/n-hexane ) 1/4) to produce 18 g, 93% yield. 1H NMR (CDCl3, 300 MHz): δ 7.78 (d, 2H), 7.71 (d, 2H), 3.01 (d, 2H), 1.98-1.86 (m, 1H), 1.53-1.35 (m, 4H), 1.28-1.10 (m, 4H), 0.88-0.79 (m, 6H). Divinylbenzene. Terephthaldicarboxyaldehyde (5 g, 0.037 mol), 33.3 g (0.093 mol) of methyltriphenylphosphonium bromide, and 13 g (0.093 mol) of potassium carbonate were dispersed in 120 mL of 1,4-dioxane and 1.8 mL of distilled water. The mixture was refluxed at 110 °C for 12 h under the N2 gas atmosphere. The obtained product was poured into water and neutralized to pH 7 with 1 N HCl. After the solution was extracted with dichloromethane, the solvent was removed at reduced pressure. The residue was chromatographed on a silica gel column (n-hexane) to produce 2.8 g, 57% yield. 1H NMR (CDCl3, 300 MHz): δ 7.37 (s, 4H), 6.75-6.65 (q, 2H), 5.74 (d, 2H), 5.23 (d, 2H). 1-{2-[4-(2-Ethylhexane-1-sulfonyl)phenyl]vinyl}-4vinylbenzene (16). A mixture of 5.12 g (0.015 mol) of 15, 2 g (0.015 mol) of divinylbenzene, 0.275 g (1.2 mmol) of palladium(II) diacetate, 0.75 g (2.4 mmol) of tri-o-tolylphosphine, and 2.4 mL of triethylamine in 4 mL of DMF was stirred at room temperature for 30 min with N2 bubbling. And then the mixture was stirred at 110 °C for 24 h in a N2 atmosphere. After cooling, it was poured into water and neutralized to pH 7 with 1 N HCl and then extracted with dichloromethane. After drying with MgSO4, the solvent was removed at reduced pressure. The residue was chromatographed on a silica gel column (ethyl acetate/n-hexane ) 1/7) to produce 1.5 g, 26% yield. 1H NMR (CDCl3, 300 MHz): δ 7.88 (d, 2H), 7.66 (d, 2H), 7.51 (d, 2H), 7.3 (d, 2H), 7.24 (d, 1H, J ) 16.29 Hz), 7.13 (d, 1H, J ) 16.29 Hz), 6.77-6.68 (q, 1H), 5.80 (d, 1H), 5.29 (d, 1H), 3.0 (d, 2H), 1.93 (m, 1H), 1.49-1.35 (m, 4H), 1.25-1.16 (m, 4H), 0.87-0.79 (m, 6H). 4-[2-(4-{2-[4-(2-Ethylhexane-1-sulfonyl)phenyl]vinyl}phenyl)vinyl]phenyl}diphenylamine (17, SSEH1). A mixture of 0.18 g (0.56 mmol) of 10, 0.19 g (0.5 mmol) of 16, 0.01 g (0.044 mmol) of palladium(II) diacetate, 0.026 g (0.09 mmol) of tri-o-tolylphosphine, and 2.4 mL of triethylamine in 4 mL of DMF was stirred at room temperature for 30 min with N2 bubbling. And then the mixture was stirred at 110 °C for 24 h in a N2 atmosphere. After cooling, it was poured into water and neutralized to pH 7 with 1 N HCl. After extracted with dichloromethane, the extract was dried with MgSO4. The solvent was removed at reduced pressure and the residue was chromatographed on a silica gel column (ethyl acetate/nhexane ) 1/10) to produce 0.31 g, 89% yield. Elem. Anal. Calcd (%) for C42H43NO2S: C, 80.60; H, 6.93; N, 2.24; S, 5.12. Found: C, 80.40; H, 7.12; N, 1.98; S, 4.86. 1H NMR (CDCl3, 300 MHz): δ 7.87 (d, 2H), 7.66 (d, 2H), 7.52 (s, 4H), 7.39 (d, 2H), 7.29-7.22 (m, 4H), 7.15-6.96 (m, 12H), 3.01 (d, 2H), 1.94 (m, 1H), 1.49-1.35 (m, 4H), 1.25-1.16 (m, 4H), 0.85-0.79 (q, 6H). IR (KBr pellet, cm-1): 1580 and 1500 (vinyl and aromatic CdC), 1310 and 1150 (SO2). Bis{4-[2-(4-{2-[4-(2-ethylhexane-1-sulfonyl)phenyl]vinyl}phenyl)vinyl]phenyl}phenylamine (18, SSEH2). Compound 18 was prepared by the same procedure as described for 17 using a mixture of 0.17 g (0.42 mmol) of 11, 0.34 g (0.88 mmol) of 16, 0.016 g (0.07 mmol) of palladium(II) diacetate, 0.043 g (0.14 mmol) of tri-o-tolylphosphine, and 3 mL of triethylamine in 5 mL of DMF. Purification was carried out by chromatography on a silica gel column (ethyl acetate/ n-hexane ) 1/7) to produce 0.2 g, 47% yield. Elem. Anal. Calcd (%) for C66H71NO4S2: C, 78.77; H, 7.11; N, 1.39; S, 6.37.

Triphenylamine-Cored Bifunctional Organic Molecules Found: C, 78.92; H, 7.35; N, 1.32; S, 6.31. 1H NMR (CDCl3, 300 MHz): δ 7.87 (d, 4H), 7.66 (d, 4H), 7.52 (s, 8H), 7.42 (d, 4H), 7.29-7.22 (m, 2H), 7.15-6.98 (m, 7H), 3.01 (d, 4H), 1.93 (m, 2H), 1.46-1.37 (m, 8H), 1.23-1.17 (m, 8H), 0.73 (q, 12H). IR (KBr pellet, cm-1): 1580 and 1500 (vinyl and aromatic CdC), 1310 and 1150 (SO2). Tris{4-[2-(4-{2-[4-(2-ethylhexane-1-sulfonyl)phenyl]vinyl}phenyl)vinyl]phenyl}amine (19, SSEH3). Compound 19 was prepared by the same procedure as described for 17 using the mixture of 0.18 g (0.375 mmol) of 12, 0.48 g (1.23 mmol) of 16, 0.021 g (0.09 mmol) of palladium(II) diacetate, 0.06 g (0.19 mmol) of tri-o-tolylphosphine, and 2.7 mL of triethylamine in 4.5 mL of DMF. Purification was carried out by chromatography on a silica gel column (THF/ dichloromethane/n-hexane ) 1/1/4) to produce 0.3 g, 57% yield. Elem. Anal. Calcd (%) for C90H99NO6S3: C, 77.94; H, 7.19; N, 1.09; S, 6.94. Found: C, 78.31; H, 7.35; N, 0.99; S, 7.01. 1H NMR (CDCl3, 300 MHz): δ 7.88 (d, 6H), 7.67 (d, 6H), 7.53 (s, 12H), 7.44 (d, 6H), 7.25 (d, 3H), 7.15 (d, 6H), 7.11 (s, 6H), 3.0 (d, 6H), 1.94 (m, 3H), 1.94-1.37 (m, 12H), 1.25-1.20 (m, 12H), 0.87-0.79 (q, 18H). IR (KBr pellet, cm-1): 1580 and 1500 (vinyl and aromatic CdC), 1310 and 1150 (SO2). Instruments. 1H NMR spectra were measured on a JEOL JNM-LM300 (300 MHz) using CDCl3 and acetone-d6 as solvents. Infrared spectra were measured on a Bomem FT-IR spectrophotometer using a KBr pellet. Element analysis was carried out on EA1110 (CE Instrument, Italy). Thermal analysis was carried out under a nitrogen atmosphere at the heating rate of 10 °C/min on a Perkin-Elmer DSC 7. UVvisible spectra were obtained on HP 8452-A. MALDI-TOF mass spectra were measured on a Voyager-DE STR Biospectrometry Workstation. Photoluminescence spectra were obtained on a Shimadzu RF-500 spectrofluorophotometer. Device Preparation. Samples (STEH1, STEH2, STEH3) with 1 wt % 2,4,7-trinitrofluorenone (TNF) as a photosensitizer were dissolved in dichloromethane, filtered through a Teflon membrane filter (Millipore, 0.22 µm), and then cast on indium-tin-oxide (ITO)-coated glass. After drying at room temperature overnight, solvents were completely removed at 100 °C in a vacuum. Another ITO glass was pressed down to obtain the sandwiched devices. Polyimide films of 25-µm thickness were used as a spacer for the photorefractivity and electrooptic measurements. On the other hand, glass beads of 3-µm diameter were used for the photoconductivity measurement. Measurements. Nonlinear absorption phenomena were investigated by a direct nonlinear transmission (NLT) method.20,21 First, two-photon excitation spectra for photoluminescence were measured to locate the effective TPA resonance. Then, intensity-dependent transmission was measured at the resonance wavelength using the laser pulses with 6-ns duration. The laser beam passed through a 1-cm path quartz cuvette filled with the solution and the intensities of transmitted beam were measured as a function of incident intensity. The dark- and photoconductivity were determined by measuring a steady-state current using a Keithley 6517 electrometer and a 633-nm He-Ne laser as a light source. The birefringence (∆n) was determined using the transmission ellipsometric method22 with a 633-nm He-Ne laser. The sample was exposed to a light beam incident under an internal angle of 45° with respect to the sample normal. The beam polarization was set to +45° with respect to the p-plane and analyzed through a polarizer set at -45° placed after the sample. A compensator was placed between the sample and the second polarizer. Photorefractivity was evaluated by the conventional twobeam coupling and the degenerated four-wave mixing technique using a 633-nm He-Ne laser.23 In the two-beam coupling (20) He, G. S.; Weder, C.; Smith, P.; Prasad, P. N. IEEE J. Quantum Electron. 1998, 34 (12), 2279. (21) Lee, K. S.; Lee J. H.; Choi, H. Y.; Cha, M.; Chung, M. A.; Kim, Y. J.; Jung, S. D. Mol. Cryst. Liq. Cryst. 2001, 370, 155. (22) Bittner, R.; Daubler, T. K.; Neher D.; Meerholz, K. Adv. Mater. 1999, 11 (2), 123.

Chem. Mater., Vol. 16, No. 3, 2004 461 experiment, p-polarized beams with the identical intensities of 60.7 mW/cm2 were intersected in the samples. For the fourwave mixing technique, two s-polarized beams of 60.7 mW/ cm2 were used as writing beams, and a p-polarized reading beam of 1.27 mW/cm2 counterpropagated to one of the writing beams. The normal of the sample surface was tilted 60° with respect to the symmetric axis of the two intersected beams, and the external interbeam angle was 11° in this work.

Results and Discussion Synthesis. We have successfully synthesized six kinds of bifunctional nonlinear optical molecules, which contain triphenylamine as an electron donor, sulfone as an electron acceptor, and stilbene as a π-conjugation bridge between them. 4-(2-Ethylhexylsulfonyl)benzene(1E)-2-vinyl group or 4-{2-[4-(2-ethylhexylsulfonyl)phenyl]-(1E)-vinyl}benzene-(1E)-2-vinyl group was singly, doubly, or triply substituted to the positions para to the nitrogen in triphenylamine and they are denoted as STEH1, STEH2, STEH3, SSEH1, SSEH2, and SSEH3, respectively. It should be noted that the conjugation length of the SSEH series is longer than the STEH series because the SSEH series consists of two stilbene moieties per each arm, although the STEH series contains one stilbene moiety. The synthetic routes to these bifunctional molecules are described in Schemes 1-4. The STEH series was synthesized by Wadsworth-Emmons reaction of formylated triphenylamine core with sulfone derivative, diethoxy({4-[(2-ethylhexyl)sulfonyl]phenyl}methyl)phosphino-1-one (6). Compound 6 was prepared by alkylation, oxidation, bromination, and phosphonation of 4-methyl benzenethiol. The SSEH series was obtained through Heck reaction of brominated triphenylamine core with 1-{2-[4-(2-ethylhexane-1-sulfonyl)phenyl]vinyl}-4-vinylbenzene (16), which was prepared by alkylation, bromination, and oxidation of benzenethiol and subsequent Heck reaction of compound 15 and divinyl benzene. The structural characterization of each compound was conducted by 1H NMR, FT-IR, elemental analysis, and MALDI-TOF mass measurements. The chemical shift of trans-stilbene protons with the coupling constant of 16.29 Hz at 7.05-7.23 ppm could be assigned in the NMR spectra, and the stretching bands of the sulfone group were identified at 1150 and 1310 cm-1 in the FT-IR spectra. In addition, molecular weights of 523.7 (STEH1, calcd 523.73), 802.1 (STEH2, calcd 802.14), 1080.5 (STEH3, calcd 1080.55), 625.9 (SSEH1, calcd 625.86), 1006.4 (SSEH2, calcd 1006.41), and 1386.9 (SSEH3, calcd 1386.95) were confirmed by a MALDI-TOF mass spectrometer. Thermal and optical properties of bifunctional NLO molecules of this work are summarized in Table 1. Due to the presence of ethylhexyl moiety in their structures, all these molecules formed transparent amorphous glasses by themselves. From differential scanning calorimetry, STEH1, STEH2, and STEH3 showed their glass transitions around room temperature (22, 33, and 40 °C, respectively). On the other hand, SSEH1, SSEH2, and SSEH3 containing the extended stilbene unit showed the higher glass transition temperatures (38, 68, and 103 °C, respectively). Therefore, in the case of (23) Sohn, J.; Hwang, J.; Park, S. Y.; Lee, G. J. Jpn. J. Appl. Phys. 2001, 40, 3301.

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Figure 1. (a) and (b) UV-vis absorption spectra (line) and two-photon-induced fluorescence excitation spectra (line and symbol); (c) and (d) nonlinear transmission of (c) STEH1, STEH2, and STEH3 and (d) SSEH1, SSEH2, and SSEH3 in solution. The solid lines in (c) and (d) are the fitting results with eq 1. Table 1. Thermal and Optical Properties of Materialsa

STEH1 STEH2 STEH3 SSEH1 SSEH2 SSEH3

Tg (°C)

λmax abs. (nm)

λmax PL (nm)

Q

22 33 40 38 68 103

390 412 412 410 426 428

459 463 468 482 486 491

0.49 0.72 0.29 0.42 0.36 0.20

a Q: relative fluorescence quantum yield: Q ) Q (I/I )(OD / R R R OD). QR: quantum yield of reference (9,10-diphenyl anthracene, 0.84024). I: integration of peak area in photoluminescence spectrum. OD: optical density in UV-abs. spectrum. Solvent: benzene, concentration 10-5 mol/L.

the STEH series, optically clear thick films could be obtained, but with the SSEH2 and -3, unfortunately, thick devices could not be fabricated due to their brittleness. As shown in Table 1, λmax of absorption and λmax of photoluminescence were red-shifted as the number of arms and stilbenic conjugation increased, due to the strong interarm coupling and extended π-electron conjugation. Particularly, it is noteworthy that these molecules are strongly fluorescent in terms of the relative fluorescence quantum yield which is as high as 0.72 determined with 9,10-diphenyl anthracene as reference in benzene.24 Such a strong fluorescence in bifunctional NLO molecules is very important in many potential applications such as two-photon excited fluorescence microscopy and two-photon upconverted lasing.25 (Stronger fluorescence was observed as the number of arms (24) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229.

increased, but the optical density of materials also increased, and therefore, the relative fluorescence quantum yield was not increased with the number of arms.) Two-Photon Absorption. We obtained the effective TPA molecular cross section (σ2) of the STEH and SSEH molecules with a concentration of 0.005 mol/L in chloroform. Two-photon-induced fluorescence excitation spectra were measured to locate the effective TPA resonance shown in Figure 1 (a), and (b) along with one-photon absorption spectra. Parts (c) and (d) of Figure 1 show the optical power limiting measured by the NLT method at the maximum wavelength of two-photon-induced fluorescence excitation. It is seen that the intensity of the transmitted laser beam (I′) increased nonlinearly as that of the incident laser beam (Io) increased. The nonlinear absorption coefficient was obtained by fitting the experimental data with eq 1,8 which is included as solid lines in Figure 1.

I′ )

Io 1 + IoLβ

(1)

In eq 1, L is the thickness of the sample (inner space of the quartz cuvette) and β is the nonlinear absorption coefficient which is related to the effective TPA molec(25) (a) Ventelon, L.; Charier, S.; Moreaux, L.; Mertz, J.; BlanchardDesce, M. Angew. Chem., Int. Ed. 2001, 40 (11), 2098. (b) Abbotto, A.; Beverina, L.; Bozio, R.; Bradamante, S.; Ferrante, C.; Pagani, G. A.; Signorini, R. Adv. Mater. 2000, 1963. (c) He, G. S.; Bhawalkar, J. D.; Zhao, C. F.; Park, C.-K.; Prasad, P. N. Opt. Lett. 1995, 20, 2393. (d) He, G. S.; Lixiang Yuan; Cui, Y.; Li, M.; Prasad, P. N. J. Appl. Phys. 1996, 81, 2529. (e) He, G. S.; Yuan, L.; Cui, Y.; Li, M.; Prasad, P. N. J. Appl. Phys. 1997, 81, 2529. (f) Zhou, G.; Wang, D.; Yang, S.; Xu, X.; Ren, Y.; Shao, Z.; Jiang, M.; Tian, Y.; Hao, F.; Li, S. Appl. Opt. 2002, 41, 6371.

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Table 2. Two-Photon Absorption Properties in Solution

STEH 1 STEH 2 STEH 3 SSEH 1 SSEH 2 SSEH 3 AF-50

λ measurement (nm)

β (cm/GW)

σ2 (×10-46 cm4‚s)

810 830 860 860 820 860 740

0.14 ( 0.64 × 10-3 0.24 ( 1.4 × 10-3 0.60 ( 2.8 × 10-3 0.35( 3.6 × 10-3 0.70 ( 7.4 × 10-3 1.23 ( 9.8 × 10-3 0.83 ( 6.9 × 10-3

0.11 ( 0.5 × 10-3 0.19 ( 1.1 × 10-3 0.46 ( 2.2 × 10-3 0.27 ( 2.8 × 10-3 0.56 ( 5.9 × 10-3 0.94 ( 7.5 × 10-3 0.74 ( 6.1 × 10-3

Figure 3. Linear absorption spectra obtained from 100-µm films of the STEH series. Inset shows the DSC thermograms obtained from second heating traces of the STEH series.

Figure 2. Effective TPA cross sections (σ2) in solution. Inset shows the effective TPA cross section as a function of the total number of stilbene moieties in a multibranched molecule. Solid line is the best linear fit and the dashed line is the thermodynamic limit of a collection of stilbene moieties.

ular cross section as follows:8

σ2 ) hνβ/N0

(2)

Here, N0 is the molecular density in the ground state, hν is the energy of the incident photon, and σ2 is in units of cm4 s. These effective TPA properties of bifunctional NLO molecules are summarized in Table 2. Although the cooperative effect is not dramatic seemingly,15,16 it is readily concluded that the effective TPA molecular cross sections of these molecules are actually enhanced according to the increased number of arms and conjugation length as shown in Figure 2. The inset of Figure 2 shows the effective TPA cross sections as a function of the total number of stilbene units. As one can see, the dependence is very close to the linear one but the obtained values of the effective TPA cross section were higher than the thermodynamic limit of a collection of noninteracting stilbene moiety. This cooperative enhancement of the effective TPA cross section in these multibranched structures is in good agreement with the reported structure-property relationships that the increase of arms and conjugation length leads to the enhancement in two-photon absorption.15,16 We also measured effective σ2 of AF-50, a well-known TPA dye,26 in the same experimental condition as a reference sample and included its valve in Table 2. It should be noted that the effective σ2 values of our bifunctional NLO molecules are almost comparable to that of AF50. (26) He, G. S.; Yuan, L.; Cheng, N.; Bhawalkar, D.; Prasad, P. N.; Brott, L. L.; Clarson, S. J.; Reinhardt, B. A. J.Opt. Soc. Am. B 1997, 14 (5), 1079.

In addition to this attractive molecular capability, it should be emphasized that we were able to fabricate optically clear thick films of the STEH series which are potentially useful for the solid-state optical limiting device.27,28 Figure 3 shows the absorption spectra of 100-µm films of the STEH series. In the region of near-IR, we could not observe any evidence for the nonhomogeneity of films such as scattering. In addition, although STEH1 exhibited the crystalline melting peak at 117 °C in the first heating trace of the DSC curve, if cooled quickly from the melt, it formed a stable glass with a Tg ∼ 22 °C without further crystalline melting. In the cases of STEH2 and -3, they exhibited the glass transitions at 33 and 40 °C, respectively, even in first heating traces without the crystalline melting (inset of Figure 3). These results indicated that the STEH series have excellent capability to form stable amorphous films for the various optical applications. It should also be pointed out that the inherent assumption in the NLT measurement is that TPA is the predominant process causing the observed intensitydependent nonlinear absorption. However, as many researchers have indicated, the strong TPA process considerably increases molecular population in the excited state, which, in turn, creates an additional contribution to the observed nonlinear absorption of the input laser beam. The TPA cross section is, therefore, commonly used as a parameter to compare the relative magnitude of TPA-dominated nonlinear absorptivity or TPA/excited-state absorption combined nonlinear absorptivity among various nonlinear media. Because it was not possible to separate pure TPA from ESA contribution through the ns NLT measurement, we came to use the term “effective TPA cross section” to described the σ2 values for the materials in the paper.29 (27) He, G. S.; Bhawalkar, J. D.; Zhao, C. F.; Prasad, P. N. Appl. Phys. Lett. 1995, 67, 2433. (28) He, G. S.; Gvishi, R.; Prasad, P. N.; Reinhardt, B. A. Opt. Commun. 1995, 117, 133. (29) Kannan, R.; He, G. H.; Yuan, L.; Xu, F.; Prasad, P. N.; Dombroskie, A. G.; Reinhardt, B. A.; Baur, J. W.; Vaia, R. V.; Tan, L.-S. Chem. Mater. 2001, 13, 1896.

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Figure 5. Total birefringence of STEH1, STEH2, and STEH3. The solid lines are guides to the eyes. Figure 4. Photoconductivity of STEH1, STEH2, and STEH3.

More detailed analyses including pulse width dependence and TPA properties of STEH films are the topics of future research. Photorefractivity. It is expected that both STEH and SSEH series could exhibit photorefractivity due to the presence of photoconducting triphenylamine core30 and conjugated push-pull structures in the arms. Because the STEH series was easily fabricated to excellent optical films suitable for photorefractive measurement, we have limited our attention on STEH series in this work. First, photoconductivity and electrooptic property, which are elementary properties for photorefraction, were measured. Photoconductivity of STEH series (with 1 wt % TNF as a photosensitizer) was measured with 3-µm-thick film using 633-nm He-Ne laser. The current was monitored in a dark state or photo state and the conductivity could be calculated from

σ)

IL AV

(3)

where σ is the photo- or dark conductivity, I is the current, L is the thickness of the film, V is the voltage applied to the film, and A is the area. It is seen that the photoconductivity of STEH1, STEH2, and STEH3 increased with the number of arms and/or intensity of electric field as shown in Figure 4. In an organic amorphous glass, hole or electron transport occurs by the transfer of charges from states associated with the donor or acceptor molecules, respectively. This can be described as the one-electron oxidation-reduction or donor-acceptor process between molecules in their neutral and charged states, which is essentially known as “hopping”. Since the charge transport process by hopping is strongly dependent on the intermolecular distance and proximity of hopping sites, it is speculated that the increase of photoconductivity with the number of arms is related to the enhanced probability of the hopping process due to the increased density of chromophore units (arms).31,32 Densities of chromophore units in film states of STEH1, -2, and -3 were deter(30) (a) Wright, D.; Gubler, U.; Moerner, W. E.; DeClue, M. S.; Siegel, J. S. J. Phys. Chem. B 2003, 107, 4732. (b) Park, S. H.; Ogino, K.; Sato, H. Polym. Adv. Technol. 2000, 11, 349.

mined to be 2.18 × 10-3, 2.48 × 10-3, and 3.84 × 10-3 mol/cm3 by measuring the molecular densities (2.18 × 10-3, 1.24 × 10-3, and 1.28 × 10-3 mol/cm3), respectively. The electrooptic properties of STEH1, STEH2, and STEH3 were determined by the transmission ellipsometric method. The electric field-induced birefringence (∆n) was calculated from

(2πλl∆n)

T ) sin2

(4)

where λ is the wavelength of the laser, l is the length of the light path, and T is the transmittance. As shown in Figure 5, the ∆n of STEH1 and STEH2 were 0.8 × 10-4 and 1.0 × 10-4 at 40 V/µm, respectively. As expected, STEH3 did not show any detectable ∆n because of the lack of birefringence due to its centrosymmetric molecular structure. Photorefractivity of the STEH series was measured by the two-beam coupling and the degenerated fourwave mixing technique at the wavelength of a 633-nm He-Ne laser. The energy transfer between two incident beams with the same intensities is the unique characteristics of the photorefractive effect derived from the phase shift between the light intensity pattern and the refractive index grating.33 The gain coefficient was estimated as follows in the two-beam coupling experiment,

1 Γ ) [ln(γoβ) - ln(β + 1 - γ0)] L

(5)

where L is the beam path length, β is the ratio of beam intensities, and γ0 ) P/P0 is the beam coupling ratio (P0 is the signal beam intensity without the pump beam and P is the signal beam intensity with the pump beam). Figure 6 shows the gain coefficients of them. STEH1 and STEH2 showed the gain coefficient of 13.4 cm-1 at 45 V/µm and 17.4 cm-1 at 55 V/µm, respectively. In the four-wave mixing measurement, the diffraction efficiency was determined by the ratio of the intensity of the diffracted beam to the sum of intensities of the (31) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for Xerography; Marcel Dekker: New York, 1998; p 289. (32) Strohriegl, P.; Grazulevicius, J. V. Adv. Mater. 2002, 14, 1439.

Triphenylamine-Cored Bifunctional Organic Molecules

Figure 6. Gain coefficients of STEH1, STEH2, and STEH3.

Chem. Mater., Vol. 16, No. 3, 2004 465

where d is the sample thickness, ∆n is the refractive index modulation, λ is the wavelength, and θ1 and θ2 are the internal angle of incidence of the two writing beams, it is speculated that the diffraction efficiency of the STEH series will be enhanced further with an increase of film thickness. Although STEH1 and -2 show relatively good TPA properties, their photorefractivity was an order of magnitude smaller than that of the state-of-the-art photorefractive glasses at this stage. In this paper, however, we wanted to show the novel approach to organic glasses that possess TPA as well as photorefractivity in single chromophores, and we believe that this molecular class will be a good candidate for the twophoton photorefractive materials aiming at nondestructive readout in holographic data storage35 via the optimization of chromophore design and enhancement of each property. Conclusion

Figure 7. Diffraction efficiency of STEH1, STEH2, and STEH3.

diffracted beam and transmitted beam as shown in eq 6.

η)

Idiffracted

(6)

(Idiffracted + Itransmitted)

Figure 7 shows the diffraction efficiencies of STEH1, STEH2, and STEH3 in 25-µm-thickness films. STEH1 and STEH2 showed the maximum diffraction efficiency of 0.45% at 40 V/µm and 0.55% at 55 V/µm, respectively. These correspond to the index change of ∆n ) 3.47 × 10-4 and 4.01 × 10-4 for d ) 25 µm in eq 7. As expected, STEH3 did not show any detectable photorefractive response, which could be attributed to the absence of electrooptic activity in STEH3. Since the diffraction efficiency is dependent on the thickness of the sample as follows,34

η ) sin2

[

]

πd∆n cos(θ1 - θ2) λxcos θ1 cos θ2

(7)

We have designed and synthesized a novel class of bifunctional multiarmed NLO molecular glasses, which comprised the triphenylamine as a core and sulfonesubstituted stilbene group as conjugated arms (STEH and SSEH series according to the different conjugation lengths). These materials were bifunctional, exhibiting both TPA and photorefractivity originating from the conjugated push-pull structures in the arm and photoconducting tripheneylamine core. It is emphasized that our bifunctional NLO molecules were able to form optically clear films free from the phase separation and mutual dilution problem inherent to the multicomponent mixture, which will provide the distinct advantage for realizing the various TPA-based applications in the solid state. STEH and SSEH series of this work exhibited two-photon absorption properties comparable to that of AF50, which showed systematic enhancement with increasing the number of arms and conjugation length. It was also demonstrated with STEH1 and STEH2 that the bifunctional NLO molecules of this work show distinct photorefractivity. It is suggested that these molecules are good candidates for two-photon excited photorefractivity aiming at nondestructive readout in holographic data storage.35 Acknowledgment. This work was supported in part by CRM-KOSEF. CM0343756 (33) Moerner, W. E.; Grunnet-Jepsen, A.; Tompson, C. L. Annu. Rev. Mater. Sci. 1997, 27, 585. (34) Chun, H.; Moon, I. K.; Shin, D.-H.; Kim, N. Chem. Mater. 2001, 13, 2813. (35) Blanche, P. A.; Kippelen, B.; Schu¨lzgen, A.; Fuentes-Hernandez, C.; Ramos-Ortiz, G.; Wang, J. F.; Hendrickx, E.; Peyghambarian, N.; Marder, S. R. Opt. Lett. 2002, 27 (1) 19.