Large quadratic hyperpolarizabilities with donor-acceptor polyenes exhibiting optimum bond length alternation: Correlation between structure and hyperpolarizability

Donor-acceptor polyenes of various lengths, and that combine aromatic electron-donating moieties with powerful heterocyclic electron-withdrawing terminal groups, have been synthesized and characterized as efficient nonlinear optical (NLO) chromophores. Their linear and nonlinear optical properties have been investigated, and variations in these properties have been related to ground-state polarization (dipole mu) and structure. In particular, unprecedented quadratic hyperpolarizabilities (beta) have been achieved (up to beta(0) = 1500 x 10(-30) esu) by reduction of the bond-length alternation (BLA) in the polyenic chain. In each series of homologous compounds, increasing the number n of conjugated double bonds in the polyenic chain results in a marked bathochromic shift, more pronounced BLA, and exponential increases in mu beta(0) Values. As a result, polyenic chromophores displaying excellent optical quadratic nonlinearities (with mu beta values as large as 100 times that of the quadratic NLO benchmark 4-dimethylamino-4'-nitrostilbene (DANS)), as well as satisfactory solubility, have been obtained.