Environmental Effects on Nonlinear Optical Chromophore Performance. Calculation of Molecular Quadratic Hyperpolarizabilities in Solvating Media

The molecular quadratic hyperpolarizabilities of a wide series of pi-conjugated, donor-acceptor chromophores in various solvation media are calculated using the INDO/S (ZINDO) sum over excited particle hole states formalism. The energy terms of the perturbation theory are corrected for solvent effects by means of a continuum Onsager cavity model, based on the reaction field model. Calculated hyperpolarizability values including solvent effects are in excellent agreement with experimental electric field induced second-harmonic generation data taken in solution. The calculations show that red shifts of the lowest energy electronic charge-transfer transition upon solvation are the most important feature altering hyperpolarizability values on passing from the gas phase to solution. A linear correlation found between the hyperpolarizability and the energy of the lowest charge-transfer transition demonstrates the general validity of the simple two-state model in predicting solvation trends in hyperpolarizability for donor-acceptor chromophores. The consistency of this model is probed by calculating chromophore hyperpolarizabilities in different solvents and for different fundamental laser frequencies.