J. Am. Chem. Soc. 2001, 123, 6421-6422
6421
Scheme 1
1,3,5-Tricyano-2,4,6-tris(vinyl)benzene Derivatives
with Large Second-Order Nonlinear Optical
Properties
Bong Rae Cho,* Soon Bong Park, Seung Jae Lee,
Kyung Hwa Son, Sang Hae Lee, Myung-Ja Lee, Jun Yoo,
Yun Kyong Lee, Geon Joon Lee, Tae Im Kang,
Minhaeng Cho, and Seung-Joon Jeon*
Molecular Opto-Electronics Laboratory
Department of Chemistry and Center for
Electro- and Photo-ResponsiVe Molecules
Korea UniVersity, 1-Anamdong, Seoul 136-701, Korea
ReceiVed July 13, 2000
ReVised Manuscript ReceiVed January 2, 2001
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 calcula-
tions 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 donor-
acceptor 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 sub-
stituted benzaldehyde. Compounds 2f and 3d-f were synthesized
in 61-86% yields by the Wittig reaction of 1,3,5-tricyano-2,4,6-
tris[(diethoxyphosphoryl)methyl]benzene with p-diphenylamino-
benzaldehyde 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 complica-
tions 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 pos-
sibility. 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.3 For the octupolar
compounds, the nonzero tensor elements are âyyy, âyxx, âxyx, and
4,6
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, â2
and â2 , by using the
ZZZ
XZZ
HRS method, where the incident beam travels in the X-direction
and the scattered light is measured in the Y-direction. More
(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.
8
specifically, â2
) (24/105)â2yyy and â2
) (16/105)â2
.
ZZZ
XZZ
yyy
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
(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.
The results of UV-vis absorption and second-order nonlinear
optical measurements are summarized in Table 1. For all
(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.; Bla¨ser, D.; Katz, A. K.; Carrell, H. L.;
Boese, R.; Zyss, J.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120,
2563.
(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.
(5) Brasselet, S.; Zyss, J. Opt. Lett. 1997, 22, 1464.
10.1021/ja0025595 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/09/2001