Solvent Effects on Conformationally Induced Enhancement of the Two-Photon Absorption Cross Section of a Pyridinium-<i>N</i>-Phenolate Betaine Dye. A Quantum Chemical Study

Abstract

A study of the influence of the solvent on the electronic contributions to the two-photon absorption cross section (δ) of the simplest pyridinium-N-phenolate betaine dye molecule is presented. The calculations of δ, for the first excited singlet state (connected with the intramolecular charge transfer), were performed as a function of the interplanar angle between the pyridinium and the phenolate ring. It was found that in the gas phase the two-photon absorption cross section is enhanced several times near 80° as compared to the planar structure. The calculations show that in aqueous solution there is no distinct maximum value of δ. The computations of the first- (β) and second-order (γ) hyperpolarizabilities performed for the gas phase have shown that the angle at which δ has its maximum value is exactly the same for β and γ. The behavior of these two parameters in water resembles that of δ. The demeanor of all three nonlinear parameters in the gas phase as well as in aqueous solution has been successfully explained using the two-level model, accounting the lowest-lying charge-transfer state only. The solute/solvent interaction was included within Langevine dipoles/Monte Carlo formalism. All parameters were evaluated by applying the semiempirical GRINDOL method based on the NDO-like Hamiltonian.