1,3,5-Tricyano-2,4,6-tris(vinyl)benzene derivatives with large second-order nonlinear optical properties.

Application of the nonlinear optical materials to the optical and optoelectronic devices requires large second-order nonlinear optical property and thermal stability.1,2 Much effort has been focused to optimize the physical properties of the donor-acceptor dipoles in order to meet these criteria. While there has been a great progress in improving these properties, it has become apparent that the dipolar molecules have certain limitations. One such problem is the difficulty associated with aligning the dipoles noncentrosymmetrically in the solid sate to achieve maximum bulk effect [ø(2)]. Because the dipoles favor antiparallel pairing in the solid state to nullify bulk nonlinearity, various approaches including crystallization, poled polymer, Langmuir-Blodgett film, and self-assembly technique have been employed to overcome the attractive forces between the dipoles.3 Recently, Dalton and Steier reported that dipole-based polymers can be fabricated to an electrooptic modulator with huge bandwidth acceptance and low modulation voltage, which is the major breakthrough in this area.1 An alternative approach to overcoming this difficulty would be to use the octupolar compounds with three-fold symmetry.4 These compounds are of particular interest not only because they exhibit moderate to very large â, but because they are less prone to relaxation due to the lack of the ground-state dipole moment, once they are assembled to exhibit large bulk nonlinearity. The challenge is the macroscopic organization of the two-dimensional octupoles in noncentrosymmetric assemblies. Because the electric poling method is not applicable to these octupolar molecules with zero permanent dipole moment, optical poling has been explored to tackle this problem.5 A straightforward solution to this problem would be the spontaneous arrangement of the NLO chromophores in the solid state. Planar octupoles with three-fold symmetry appear particularly attractive for this purpose. We recently suggested that, by using a simple three-state model, the first hyperpolarizability of the octupolar molecules increases as the extent of charge transfer from the peripheral donor to central acceptor is increased.6,7 This prediction was confirmed by carrying out both semiempirical and ab initio quantum chemistry calculations of crystal violet derivatives and other types of octupolar molecules. These observations provide a firm basis for the design and synthesis of the planar octupoles with large â values. Here we show that octupolar compounds containing three donoracceptor dipoles within a molecule not only show large first hyperpolarizability and high thermal stability, but also exhibit significant second harmonic generation (SHG) in the powder state. The synthesis of the octupolar compounds is shown in Scheme 1. Compounds 1b, 1c, 1, 2a, 2c, and 2e were prepared by refluxing tricyanomesitylene with N-formylamine dimethylacetal or substituted benzaldehyde. Compounds 2f and 3d-f were synthesized in 61-86% yields by the Wittig reaction of 1,3,5-tricyano-2,4,6tris[(diethoxyphosphoryl)methyl]benzene with p-diphenylaminobenzaldehyde or 4-(p-dialkylaminostyryl)benzaldehyde. The â values of the octupoles were measured at 1560 nm by Hyper-Rayleigh Scattering (HRS) method.8 To avoid complications due to the multiphoton excitation, the excitation wavelength was shifted to 1560 nm with the OPO laser (Continuum Surelite OPO, 5 ns pulses), which was pumped by 355 nm third harmonic of the Nd:YAG laser (Continuum SL-II-10, Q-switched,10 Hz).9 The possibility of the multiphoton fluorescence was examined by irradiating the solutions of 2a, 2f, 3d, and 3e whose λcut-off values are larger than others, at 1560 nm under the same condition. For all compounds, the spectra showed one sharp HRS signal at 780 nm without fluorescence background (Figures S1-4 in the Supporting Information). This result strongly negates such possibility. Furthermore, the observed signal showed strictly quadratic dependence on laser intensity, demonstrating that it is the HRS signal that we are observing (Figure S5 in the Supporting Information). It is well established that the dominant â tensor component of the dipolar compounds is âzzz. For the octupolar compounds, the nonzero tensor elements are âyyy, âyxx, âxyx, and âxxy. Here, the molecular y-axis is assumed to be one of the three C2-axis. We have measured the sum of orientationally averaged hyperpolarizabilities, 〈âZZZ〉 and 〈âXZZ〉, by using the HRS method, where the incident beam travels in the X-direction and the scattered light is measured in the Y-direction. More specifically, 〈âZZZ〉 ) (24/105)âyyy and 〈âXZZ〉 ) (16/105)âyyy. The HRS signal of the CHCl3 solution of the compounds was collected and the â was calculated by using the internal reference method.8 In this method the solvent is the reference and the â value of CHCl3 is -0.49 ×10-30 esu.10 The results of UV-vis absorption and second-order nonlinear optical measurements are summarized in Table 1. For all (1) (a) Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science, 2000, 288, 119. (b) Dalton, L. R.; Steier, W. H.; Robinson, B. H.; Zhang, C.; Ren, A.; Garner, S.; Chen, A.; Londergan, T.; Irwin, L.; Carlson, B.; Fifield, L.; Phelan, G.; Kincaid, C.; Amend, J.; Jen, A. J. Mater. Chem. 1999, 9, 1905. (2) Zyss, J.; Ledoux, I. Chem. ReV. 94, 77-105 1994. (3) (a) Marder, S. R.; Perry, J. W. Science, 1994, 263, 1706. (b) Saadeh, H.; Wang, L.; Yu, L. J. Am. Chem. Soc. 2000, 122, 546. (4) (a) Zyss, J. Nonlinear Opt. 1991, 1, 3. (b) Joffre, M.; Yaron, D.; Silbey, R. J.; Zyss, J. J. Chem. Phys. 1992, 97, 5607. (c) Zyss, J. J. Chem. Phys. 1993, 98, 6583. (d) Blanchard-Desce, M.; Baudin, J.-B.; Jullien, L.; Lorne, R.; Ruel, O.; Brasselet, S.; Zyss, J. Opt. Mater. 1999, 12, 333. (f) Thalladi, V. R.; Brasselet, S.; Weiss, H.-C.; Bläser, D.; Katz, A. K.; Carrell, H. L.; Boese, R.; Zyss, J.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 2563. (5) Brasselet, S.; Zyss, J. Opt. Lett. 1997, 22, 1464. (6) Lee, Y.-K.; Jeon, S.-J.; Cho, M. J. Am. Chem. Soc. 1998, 121, 10921. (7) Lee, H.; An, S.-Y.; Cho, M. J. Phys. Chem. B 1999, 103, 4992. (8) Hendrickx, E.; Clay, K.; Persoons, A. Acc. Chem. Res. 1998, 31, 675. (9) Stadler, S.; Dietrich, R.; Bourhill, G.; Brauchle, Ch. Opt. Lett. 1996, 21, 251. Scheme 1 6421 J. Am. Chem. Soc. 2001, 123, 6421-6422