Solvent Effects on Electronic Spectra

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Abstract
Electronic absorption and emission spectra of a molecule dissolved in a solvent medium in general are shifted in energy relative to the spectra of the isolated molecule. In some cases, these solvent shifts amount to more than 30% of the energy of the isolated-molecule transition, although shifts of the order of 102 to 103 cm-1 are more common. These effects have been the subjects of many investigations directed toward defining the molecular bases of the shifts [1–14]. Two broad categories of shift phenomena have been defined: (1) Many, especially the larger, shifts are attributed to specific chemical effects of the solvent on one or both electronic states of the chromaphore. Some important specific effects are: hydrogen-bond formation; proton or charge transfer between solvent and solute; and solvent-dependent aggregation, ionization, and isomerization equilibria. Such specific effects are outside the scope of this discussion. (2) The second broad category of shifts are attributed to physical interactions between the solute and solvent molecules. These interactions must be considered even when the chromaphore is not involved in a solvent-dependent reaction. Theories of these general effects assume that the chemical states of the isolated (at low pressure in the vapor phase) and solvated chromaphore are the same as a zeroth-order approximation and treat the solvent effects as a physical perturbation of the relevant molecular states of the chromaphore. Frequently, only perturbations due to attractive intermolecular interactions are included in the analysis.