EXPERIMENTAL INVESTIGATIONS OF ORGANIC MOLECULAR NONLINEAR OPTICAL POLARIZABILITIES .2. A STUDY OF CONJUGATION DEPENDENCES

In the present contribution, we broaden our investigations to include the conjugation dependences of molecular optical hyperpolarizabilities. In addition to the benzene and stilbene derivatives reported earlier, dc electric field induced second-harmonic generation (EFISH) and third-harmonic generation (THG) measurement results on the hyperpolarizabilities of a wide range of donor-acceptor-substituted pi-systems are presented. These include results for styrene, biphenyl, fluorene, diphenylacetylene, various phenylvinyl heterocyclics, oligomeric polyphenyl, alpha,omega-diphenylpolyyne, alpha-phenylpolyene, and alpha,omega-diphenylpolyene, as well as other extended phenylvinyl derivatives. Drawing from the experimental phenomenology illustrated by a large set of internally consistent data, issues concerning the efficacies of various conjugation units, conjugation planarity, aromaticity and bond alternation, and electron localization and delocalization effects, as well as the conjugation length and end-group dependences, are examined.