Synthesis, Thermal Behavior, and Second-Order Nonlinear Optical Properties of Conjugated, Acceptor/Donor Compounds
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SYNTHESIS, THERMAL BEHAVIOR, AND SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF CONJUGATED, ACCEPTOR/DONOR COMPOUNDS JAMES F. WOLFE, SUSAN P. ERMER, STEVEN M. LOVEJOY, DORIS S. LEUNG* KENNETH P. ARONI GLENN A. HANSENr AND STEVEN P. BITLER** *Lockheed Missiles & Space Co., Inc., 3251 Hanover St., Palo Alto, CA 94304 **SRI International,
333 Ravenswood Ave,
Menlo Park, CA 94025
ABSTRACT New noncentrosymmetric compounds having both electron-accepting and electron-donating substituents on a conjugated, aromatic heterocyclic structure were synthesized and their spectroscopic, thermal, and solubility properties were determined. D.C. electric field-induced second harmonic generation (EFISH) experiments were performed to provide values of the second-order nonlinear optical susceptibility/dipole moment product P1. Long alkyl substituents were required in heterocycle-containing compounds for sufficient solubility to conduct EFISH analysis. 2,4-Dinitro-substitution on these imine-linked materials gave the highest fi in the series.
INTRODUCTION Our overall objective of this work has been to develop the capability of synthesizing and evaluating materials for applicability in second-order nonlinear optical devices. The material of choice, when processed into a device, must have an optimized combination of both nonlinear optical activity and stability of performance. In other words, the nonlinear optical moiety must have not only a high microscopic nonlinear optical susceptibility, but it must also be processible and thermally stable structurally, be capable of high degrees of orientation, and maintain its orientation with time. Our approach has been to study the effect of the nature and position of substituents on the second-order nonlinear optical susceptibility in a homologous series of compounds that allow the adjustment of these secondary, but just as necessary, characteristics. We have given the name ALHD compounds to this series because of the four components comnmon to each, namely, electron-Acceptor, Linking group, Heterocycle, and electron-Donor. The compounds synthesized in this study have the following general structure:
AC
1
No
23 R NR
2
A A 3 where Al, A2, and A are either H or electron-accepting groups such as nitro, 1 2 cyano, or dicyanoethenyl. R is either H or methyl, and R and R3 are either H, methyl, n-hexyl, n-octadecyl, or phenyl. The heterocyclic moiety for this study was a benzoxazole unit (n-i), although ALD compounds, i.e., n-0, were prepared for comparison. The linking group is an imine moiety in all cases.
RESULTS AND DISCUSSION Synthesis The specific compounds synthesized are shown in Table I. The general method used to prepare compounds 1-12 is shown in Scheme 1. Compound 13 was prepared in an analogous fashion by condensing p-nitrobenzaldehyde with 5amino-2-[4-(dimethylamino)phenyl]benzoxazole. Compounds 14-20 were prepared in toluene by Schiff base condensation of the appropriate aldehyde or ketone
Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
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Table I.
Comp
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