Enhancing the efficiency of two-photon absorption by metal coordination

The intensity of the two-photon absorption (TPA) spectrum of a terpyridine ligand acting as a D-pi-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-pi-A <-> D+-pi-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-pi-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.