Efficient acceptor groups for NLO chromophores:: competing inductive and resonance contributions in heterocyclic acceptors derived from 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran

The 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) group has been identified as a particularly strong acceptor suitable for the realization of highly efficient second-order NLO chromophores. To provide guidelines for a further improvement of heterocyclic acceptors, we present Sum-Over-States (SOS) calculations of second-order polarizabilities (beta) for a series of donor-pi-bridge-acceptor (D-pi-A) chromophores. In particular, we investigate the effect of replacing the oxygen atom of the furan ring in TCF by a variety of other groups, X (SiH2, CH2, C=CH2, NH, C=O, S, C=CHNO2, SO, and SO2). Overall beta is found to increase with the inductive electron- withdrawing character of X, with beta being further increased or decreased when X has pi-accepting or pi-donating character. Within the framework of the two- level model, the effects associated with the pi-donating and accepting character can be understood in terms of destabilizing and stabilizing effects on the lowest unoccupied molecular orbitals of the dipolar chromophores. When using the stronger 3-methyl-4-cyano-5-dicyanomethylidene-2-oxo-3-pyrroline (TCP) acceptor and replacing the pyrroline nitrogen atom with the same range of X groups, the inductive electron-withdrawing and pi-accepting or pi-donating characters of X have the same impact on beta as in the TCF motif.