Effect of charge and spin multiplicity on hyperpolarizabilities of donor-acceptor substituted polyenes

Theoretical studies of the ground state structure and nonlinear optical (NLO) properties of a number of donor-acceptor substituted singlet and triplet (neutral) and doublet (mono-positive and mono-negative) polyenes have been carried out. The variation of NLO property of the investigated molecules has been explained by using previously derived relationships between the different order polarizabilities and ground state dipole moment obtained in the framework of the standard sum-over-state expressions of Orr-Ward-Bishop and the generalized Thomas-Kuhn sum rule. The effect of charge and spin multiplicity on the molecular hyperpolarizabilities have been discussed in terms of relative changes of polarizability and ground state dipole moment. Among the investigated polyenes, the isotropic polarizability and the second-hyperpolarizability are predicted to be larger for the doublet anions in which the NH 2 group is pyramidal. The position of nitrogen atom in the p-conjugative path strongly modulates the magnitudes of both the first- and second-hyperpolarizabilities of the investigated polyenes.