Nonlinear optical response in acceptor-substituted carotenoids:: A theoretical study

Correlated semi-empirical quantum-chemical calculations are performed to rationalize the origin of the large first- (alpha), second- (beta), and third- (gamma) order molecular polarizabilities in carotenoidlike molecules capped at a single end by an acceptor group. The degree of ground-state polarization of the chromophores is made to vary via the application of an external homogeneous electric field in the range 10(7)-10(8) V/cm. The most interesting feature is a stepwise evolution of the longitudinal component of the dipole moment with field, caused by charge transfer toward the acceptor end of the molecule. Since the steps are abrupt, this results in very large values of the molecular polarizabilities, in accordance with the derivative relationships among the dipole moment and alpha, beta, and gamma. The longitudinal components of the molecular polarizabilities are analyzed as a function of both the external field and the bond-order alternation (BOA) parameter. The ability of simple models to describe the evolution of the molecular polarizabilities is assessed. (C) 2000 American Institute of Physics. [S0021-9606(00)31645-2].