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 ZnCl(2). 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 "ZnCl(2)" 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.

Dimers of quadrupolar chromophores in solution: electrostatic interactions and optical spectra.

Two dimers of a heteroaromatic quadrupolar (acceptor-donor-acceptor) chromophore have been synthesized with different interchromophoric distances. Optical spectra of dimers in solution show a red shift of the linear absorption band upon decreasing the interchromophore distance, while fluorescence and two-photon absorption spectra are only marginally affected by the interactions. A bottom up approach is adopted to describe the spectra: via a detailed spectroscopic analysis of the monomeric species in solution, we define an essential-state model for the isolated chromophore and use this information to set up a model for the dimers also accounting for interchromophore electrostatic interactions. To discriminate between static screening governed by the static dielectric constant and dynamical screening at optical frequencies, we first solve the problem in the mean-field approximation and then define the excitonic Hamiltonian on the resulting best excitonic basis. Along this line, the evolution of spectral properties with the interchromophore distance is properly rationalized.