Molecules with quinoid ground state: a new class of large molecular optical nonlinearities?

The extent of p-electron delocalization in polyconjugated organic materials is the basic molecular structural property required for attaining large molecular hyperpolarizabilities (b, g) and large nonlinear optical (NLO) response (w(2), w(3)). When the electronic and the vibrational contributions to hyperpolarizabilities are considered we have shown that for low band-gap molecules at least the longitudinal be and ge are as large as bv and gv. Since molecular vibrations (frequencies and intensities in IR and Raman) can be derived both experimentally and theoretically by ab initio methods, the vibrational contributions to b and g can be obtained from either experimental or calculated IR and Raman spectra and the two values compared. New materials with improved NLO properties need to be devised for a successful breakthrough in technology. We have synthesized low band-gap molecules whose ground electronic state is forced to be quinoid (planarity and the consequent p-electron delocalization should yield large g values) and its first excited state is aromatic. Very little is known on NLO properties of quinoid systems. All our molecules have been characterized by UV-Vis, 1H-NMR, FT-IR, FT-Raman and their properties theoretically studied by DFT methods. With this work we try to identify the molecular features which optimize the NLO properties and we find that the vibrational contribution to molecular hyperpolarizability gv (measured from spectra and calculated by quantum chemical methods) and the w(3) value (experimentally determined by third-harmonic generation, THG) of the quinoid structures are extremely large.