A Brief General Background on Electronic Spectroscopy in the Condensed Phase

A Brief General Background on Electronic Spectroscopy in the Condensed Phase 

For simplicity we focus on transitions between two electronic states of a solute molecule a - , in a dilute solution (so solute-solute interactions and 

These expressions have been used as the starting point for the development of a number of approximate statistical mechanical and continuum model formulas for the solvent-induced spectral shift. Because a detailed discussion of this topic is outside our scope, we refer the reader to the literature for a few examples of 

However, it is useful for future reference to highlight some results. If the solute is modeled as a point dipole inside a spherical cavity of radius R 

It is well known experimentally that the ability of a solvent to interact differently with the ground and excited states typically involves much more than just its dielectric constant it may also depend on the details of the solvent-solute interaction and the solvent structure. The solvent polarity scale is an empirically based approach to express quantitatively the differential solvation of the ground and excited states of a solute.It uses the electronic spectral shift as a convenient one-parameter characterization of the ability of the solvent to interact with the solute. Several solvent polarity scales have been developed on the basis of the spectral shift of several different dye molecules. For example, one of the most widely used scales, called the Ej-(30) scale, is equal to the spectral shift in kcal/mol for the Jt n transition in pyridinium N-phenolate betaine dye. The Et(30) values, as well as other polarity scales, have been tabulated for many solvents.  

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