Enhancing the efficiency of two-photon absorption by metal coordination

Abstract

The intensity of the two-photon absorption (TPA) spectrum of a terpyridine ligand acting as a D–π–A chromophore (D = donor and A = acceptor) is enhanced by a factor of about 2 upon coordination to ZnCl₂. Based on an analysis of linear absorption and fluorescence spectra of both the ligand and its Zn(II) complex, we have defined essential-state models for the two species. Linear and TPA spectra of the ligand are well reproduced in terms of a two-state model accounting for the D–π–A ↔ D⁺–π–A⁻ charge resonance. However, the enhancement of the TPA response of its Zn(II) complex can only be understood by extending the model to account for the active role of the “ZnCl₂” moiety acting as a virtual A_v acceptor group of a D–π–AA_v structure. The virtual D⁺AA_v⁻ state of the relevant three-state model has negligible weight in the ground state but contributes to the first excited state. The resulting increase of the excited-state dipole moment is responsible for the enhancement of the TPA cross section, and also explains the increase of the second order nonlinear optical response upon coordination.