Theoretical studies of third-order nonlinear optical responses for fused benzenoid polycyclic hydrocarbons having from two to seven rings

The frequency - dependent third - order nonlinear optical polarizabilities with different optical processes have been calculated for fused benzenoid p olycyclic hydrocarbons having from two to seven rings, in terms of the INDO - SDCI quantum chemical method combining with the sum - over - states meth od. The calculated dispersions of the third - harmonic generation (THG), el ectric - field - induced second - harmonic generation (EFISHG) and degenera tion four- wave mixing (DFWM) have been discussed, and influences of the pi - electron delocalizations upon the third - order NLO responses have been described for these hydrocarbons. Additionally, the explicit expression of the calculating DFWM polarizability has been presented in this work. It is found from the obtained results that the third - order NLO polarizability g ets larger as the numbers of benzene rings increase in a linear arrangement , and pi - pi* electronic charge transfers make significant contributions t o the third - order polarizability in these hydrocarbons.