Enhancement of third-order nonlinear response in excited state of metallophthalocyanines (PcM; M = H-2, Mg, Ni)

In this paper we calculate the dynamic third-order nonlinear optical polari zabilities of three optical processes, third-harmonic generation, electric- field-induced second-harmonic generation, and degenerate four-wave mixing, for both ground and first excited singlet states in metal phthalocyanines i n terms of the sum-over states formula. The transition moment elements and the transition energies in this formula are obtained using the INDO-SDCI me thod. The calculated second-order hypolarizabilities show a dispersion beha vior with the difference from ground state and an enhancement of third-orde r nonlinear response in the first excited state. Among the PcM (M = H-2, Mg , Ni), there is a large enhancement of the first excited singlet state when there is a small third-order optical polarizability of the ground state. T he calculated value of PcNi is compared with the observed one of the substi tuted PcNi in the order of magnitude. We also gain an understanding of enha ncement mechanism for nonlinear processes in the excited state.