Solvent-Shift Effects on Electronic Spectra and Excited-State Dipole Moments and Polarizabilities

Abstract

Publisher Summary The spectrum of a substance in a solution is changed compared to its spectrum in the vapor state. Solvent-shift effect is caused by a weak interaction between the solute and surrounding solvent molecules. The change in energy of the system because of this interaction will differ depending on if the solute molecule is in its ground or excited state and consequently there will be the change in excitation energy observed experimentally. The solvent–solute interaction, being weak, is best treated using quantum–mechanical perturbation theory that allows it to be divided into electrostatic type interactions and dispersion interactions. The electrostatic type interactions relate the solvent shift to the change in dipole moment and polarizability of the solute molecule on excitation from its ground state to its excited state and thus, can give experimental values-albeit once removed-of excited-state dipole moments and polarizabilities. The order-of-magnitude values for excited-state dipole moments can be obtained from solvent shift data. All solvent-shift theories are based on the assumption that solvent and solute molecules are sufficiently well separated that overlap of electronic distribution can be neglected. The chapter proves with the help of calculations that Abe's theory and the reaction-field method differ by very little in the final analysis. The solvent-shift data can also be used to obtain the order of magnitude estimates of excited-state dipole moments.