Tuning the degree of bond-length alternation in organic nonlinear optical materials is a powerful paradigm for the design of organic materials with large molecular hyperpolarizabilities ( beta ). Several research groups have employed this paradigm in the deisgn and synthesis of NLO materials incorporating donor-acceptor polyenes. Increased bond-length alternation in polyenes leads to decreased barriers to rotation about C=C bonds and, hence, increased conformational flexibility. Since the degree of bond-length alternation is solvent dependent, so is the degree of conformational flexibility. In an effort to probe the influence of conformational flexibility on NLO response, we synthesized as series of simple donor-acceptor p0olyenes that are either conformationally flexible (1a, 2a, 3a) or rigid (1b, 2b, 3b). For each pair of molecules ZINDO sum-over-states calculations predict a larger value of beta _( mu ) for the conformationally flexible isomer, but EFISH measurements (CHCl_3) display mixed results. Various explanations for this behavior will be considered.
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