Electronic Characterization of Nonlinear Optically Active 8,8-Dicyano-3-(4‘-N,N-dimethylamino)phenylheptafulvene

Abstract

Some new 8,8-dicyanoheptafulvene derivatives have been synthesized, among which the title compound (1a) is found to crystallize in the polar space group of P1 showing nonlinear optical characteristics. The molecule is composed of a strong acceptor of the dicyanomethylidene group and a strong donor of the dimethylamino group, both of which are combined with the π-conjugated heptafulvene skeleton. Compound 1a is therefore typical of an intramolecular charge transfer compound, for which a high second-order hyperpolarizability is expected. For this reason, the electronic structure has been investigated in solution and in the solid state on the basis of the molecular and crystal structures together with molecular orbital calculations. The solid-state spectrum is found to be strikingly different from the solution spectrum because of the difference in effective conjugation between the seven-membered ring and phenyl ring. There are two electronic transitions A (about 450 nm) and B (580−650 nm) in the solid. Band A is due mainly to the 8,8-dicyanoheptafulvene skeleton, while band B is of charge-transfer character due to the dicyanomethylidene and dimethylamino groups.