Fused, Three-Ring Donor-Acceptor Molecules as Potential Materials for Efficient Second Harmonic Generation
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FUSED, THREE-RING DONOR-ACCEPTOR MOLECULES AS POTENTIAL MATERIALS FOR EFFICIENT SECOND HARMONIC GENERATION MALCOLM R. McLEAN, MAMOUN BADR and LARRY R. DALTON* Department of Chemistry ROBERT L. S. DEVINE and WILLIAM H. STEIER Department of Electrical Engineering, University of Southern California Los Angeles, CA 90089-1062 ABSTRACT A series of fused three-ring compounds have been synthesized and characterized for their possible use as organic crystals for second-harmonic generation. Polymer films doped with these compounds were aligned via corona poling and their second order non-linear optical properties compared with layers doped with stilbene like molecules. Values of !iJo have been obtained and the effects of different heteroatoms and substituents on X.max and on I±1o were examined. An X-ray diffraction crystal structure on one member of the series, 1 ,2,4-Trichloro-8-nitro-phenoxazin-3-one (Compound 1), revealed a centrosymmetric crystal habit in a P2i/n space group, with a --9.829(3), b = 8.300(2) and c = 15.542(4) A. Final R(F) -- 0.0939 for 754 reflections where I > 3o(I). INTRODUCTION Recent activity in the study of nonlinear optical (NLO) organic molecules has concentrated on either polyene- or azo-type dyes. Very large values of the second order hyperpolarizability, 13, have been obtained, with the 2 ) values for either electro-optic or secondcorresponding expectation of high 7x( harmonic generation (SHG) applications. The successful exploitation of these high 13values has, however, encountered several difficulties. In the case of thinfilm electro-optic applications using doped polymers, problems such as relaxation of the molecular orientation are well known. For SHG applications, utilization of bulk organic crystals is the most common approach. There are two fundamental problems which must be overcome here, however, before a useful crystal can be obtained. The first is to obtain a non-centrosymmetric crystalline structure, which is a necessary condition for the crystal to exhibit second-order NLO properties. Unfortunately, approximately 85% of known second-order NLO molecules crystallize in a centrosymmetric structure (in fact, this value can be extended to all known crystal structures of organic materials). Even if this condition is satisfied, however, the crystal must be phase-matchable if useful intensities of the second harmonic are to be obtained. [1] DISCUSSION In the search for useful materials for such applications we are currently studying a series of fused, three-ring aromatic compounds. Rationale for interest in such systems stems from the fact that some dyes, such as
Mat. Res. Soc. Syrup. Proc. Vol. 173. @1990 Materials Research Society
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merocyanine and disperse red 1 (DR1) have high 13values. Unfortunately, merocyanine, which possesses the highest 13value to date, is unstable and decomposes on exposure to the atmosphere, while DR1 is prone to thermal decomposition. This instability would appear to be due to the presence of a stilbene type linkage (merocyanines) and an azo linkage (DR1
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