RESONANCE-ENHANCED OPTICAL NONLINEARITIES IN AZOTHIOPHENES

The hyperpolarizabilities of a series of donor-acceptor azothiophene d yes have been assessed experimentally for the linear electrooptic effe ct (LEO) using a solvatochromic technique and theoretically using both a sum-over-states approach at the CNDO/S level and a time-dependent c oupled Hartree-Fock approach at the AM1 level. Reasonable correlations have been found between the magnitude of the experimental beta(xxx) v alues from solvatochromic measurements and those calculated, All the a zothiophenes examined possess large values which are substantially gre ater than those obtained for related azobenzenes. There is substantial resonance enhancement of the hyperpolarizabilities of the phenylazoth iophenes when the phenyl ring is replaced by a naphthalene ring becaus e of a lowering of the transition energy between the ground and first excited state. The naphthylazothiophenes have very large hyperpolariza bilities coupled with large dipole moments, which make them ideal cand idates for applications in poled polymer films as LEO modulators.