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Article

Theory of the Salt Effect on Solvatochromic Shifts And Its Potential Application to the Determination of Ground-State and Excited-State Dipole Moments

Willem H. Mulder*

Department of Chemistry, University of the West Indies, Mona Campus, Kingston 7, Jamaica

Cyril Párkányi

Department of Chemistry and Biochemistry, Florida Atlantic University, 777 Glades Road, P.O. Box 3091, Boca Raton, Florida 33431-0991

J. Phys. Chem. A, 2002, 106 (49), pp 11932–11937

DOI: 10.1021/jp026505o

Publication Date (Web): November 13, 2002

Corresponding author. E-mail: wmulder@uwimona.edu.jm. Fax: +1-876-977-1835.

Abstract

The theory of solvatochromism based on a dielectric continuum description of the solvent and the classical Onsager cavity 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 the pure solvent effect, and the origin of the discrepancy is clarified. Finally, it is shown how the new 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 state of a solute molecule from a simple analysis of absorption and fluorescence data only, except in cases where the dipole moments are noncollinear. Completion of the analysis then requires an independent measurement of the ground-state dipole moment.

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