Push-Pull Quinonoid Compounds: Enhanced Powder SHG Utilizing the Effect of Chiral Centers on the Dipole Alignment

The molecular and crystal structure of a chiral diamino-substituted dicyanoquinodimethane and the observation of strong solid-state optical second harmonic generation (SHG) in this material are reported. The enhanced SHG was obtained by exploiting the strong influence of the rigidity of the stereogenic center environment on the alignment of the molecular dipole vectors. The intervector angles in this class of materials are found to correlate well with the powder SHG. Since a strong molecular twist (of the diaminomethylene unit with respect to the quinonoid ring) of ∼50° is found in this class of compounds, the dependence of the molecular hyperpolarizability (β) on the twist was examined. The interesting observation was that the computed β attained the maximum value at the twist angle corresponding to the optimized molecular structure, which in turn closely resembled the structure obtained from single-crystal X-ray analysis. This unusual behavior of β is explained on the basis of the two-state model.