Mulder, Willem H.; Párkányi, Cyril
Author Affiliation, Ana.
Department of Chemistry
Theory of the salt effect on solvatochromic shifts and its potential application to the determination of ground-state and excited-state dipole moments
Journal of Physical Chemistry A
Date of Publication
Electronic edition: DOI: 10.1021/pj026505o
The theory of solvatochromism based on dielectric continuum description of the solvent and the classical Onsager activity model is revisited and extended to include the effect of an added 1:1 salt. An expression is derived for the reaction field inside the solute cavity, which is applicable in the limit of low salt concentrations. Using this result, expressions are obtained for the shifts in the (0-0) absorption and fluorescence maxima on the basis of Marcus' approach to the calculation of the medium reorganization free energy in the ground state and excited (Franck-Condon) state of the solute molecule. The lifetime of the excited state is assumed to exceed the longest relaxation time characteristic of the medium. For the salt-free case, our equations differ markedly from several others reported in the literature dealing with pure solvent effects, and the origin of the discrepancy is clarified. Finally, it is shown how the equations can be used, in principle, to obtain estimates for the Onsager radius, polarizability, and dipole moments in the ground state and the lowest excited of a solute molecule from a simptle analysis of absorption and fluorescence data only, except in cases where the dipole momemts are noncollinear. Completion of the analysis then requires and independent measurement of the ground-state dipole moment. (AU)....