Dipole Solvation in Nondipolar Solvents:  Experimental Studies of Reorganization Energies and Solvation Dynamics

Abstract

Steady-state and time-resolved emission measurements of the solvatochromic probe coumarin 153 are used to study solvation of a dipolar solute in nondipolar solvents such as benzene and 1,4-dioxane. Contrary to the predictions of dielectric continuum theories, the Stokes shifts (or nuclear reorganization energies) that accompany electronic excitation of this solute are substantial in such solvents (∼1000 cm-1). The magnitudes of the shifts observed in both nondipolar and dipolar solvents can be consistently understood in terms of the relative strength of the interactions between the permanent charge distributions of the solute and solvent molecules. (Information concerning these charge distributions is derived from extensive ab initio calculations on the solute and 31 common solvents.) The dynamics of solvation in nondipolar solvents, as reflected in the time dependence of the Stokes shifts, is qualitatively like that observed in polar solvents. But, whereas the dynamics in polar solvents can be rather simply modeled using the solvents dielectric response as empirical input, no simple theories of this sort are currently capable of predicting the solvation dynamics in nondipolar solvents.

Femtosecond dynamics of rubredoxin: Tryptophan solvation and resonance energy transfer in the protein

“…Third, the initial Trp dynamics also show different temporal behaviors in different solvents. In p-dioxane it occurs in several picoseconds (21) whereas in water it is on the femtosecond time scale, as expected for water solvation (22,23). Finally, all transients gated at various emission wavelengths are nearly independent of excitation wavelength (265 nm and 288 nm), ruling out a large contribution from vibrational cooling.…”

Solvent Reorganization Entropy of Electron Transfer in Polar Solvents

“…The latter is normally obtained from spectroscopic measurements in nonpolar solvents. 22 Most of what we know theoretically about the thermodynamics of solvent reorganization comes from dielectric continuum models of solvation. 2,[23][24][25] These models predict that all the information required to describe the variation of λ s with changing external parameters is concentrated in the Pekar factor used for ion solvation and long-distance ET * Author to whom correspondence should be addressed.…”

Equilibrium solvation in quadrupolar solvents

“…1,18 The remaining contribution from dispersion interactions, normally associated with mechanical solvation, 19,20 does not typically exceed 100−200 cm −1 , which is small compared to usual values of the Stokes shift arising from dipolar and quadrupolar solvation. 2 The electrostatic component of solvation free energy in non-dipolar solvents arising from (partial) solute charges interacting with solvent quadrupoles is the focus of the present equilibrium solvation theory. * E-mail:dmitrym@asu.edu.…”

“…The electrostatic component of non-dipolar solvation, in particular quadrupolar solvation, has received little attention 2,3,4,6,21,22,23,24 compared to the very extensive literature on solvation in dipolar solvents. 6,25,26 In contrast to dipolar solvation, where dielectric measurements provide the basis for modeling thermodynamics and dynamics of solvation, 25,27,28,29,30,31 there is no obvious method to extract the dynamic and thermodynamic response functions in non-dipolar solvents from existing data.…”