NONLINEAR EFFECTS IN DIPOLE SOLVATION .2. OPTICAL-SPECTRA AND ELECTRON-TRANSFER ACTIVATION

We present a theoretical analysis of the effect of nonlinear dipole so lvation on steady-state optical spectra and intramolecular electron tr ansfer (ET) reactions. The solvation nonlinearity is attributed to sat uration of a dipolar liquid produced by the solute dipole. The treatme nt explores the perturbation expansion over the solute-solvent dipolar interaction truncated in the form of a Pade approximant. The optical line shape and the free energies along the ET reaction coordinate are related to the chemical potential of solvation of a fictitious solute with a complex-valued dipole moment. Due to solvent dipolar saturation the spectrum of dipolar fluctuations is confined by a band of the wid th 2E(lim). Solvation nonlinearity was found to manifest itself for op tical transitions with high dipole moments in the initial state, most often encountered for emission lines. In this case, the spectral line approaches the saturation boundary E-lim bringing about ''line squeezi ng'' and decrease of the line shift compared to the linear response pr ediction. In the nonlinear region, the line shift dependence on the so lute dipole variation Delta m switches from the quadratic linear respo nse form proportional to Delta m(2) to a linear trend proportional to \Delta(m)\. The bandwidth may pass through a maximum as a function of \Delta m\ in the saturation region. Nonlinear solvation results thus i n a narrowing of spectral lines. For a transition with Solute dipole e nhancement, the bandwidth in emission Delta(e) is therefore lower that in absorption Delta(a):Delta(e)<Delta(a). As a result, the plot of be ta Delta(a,e)(2), beta = 1/k(B)T against the Stokes shift (H) over bar Delta(st) demonstrates the upward deviation of beta Delta(a)(2) and d ownward deviation of beta Delta(e)(2) from the linear response equalit y beta Delta(a,e)(2) = (H) over bar omega(st). We also explored the no nlinearity effect on charge separation/charge recombination activation thermodynamics. The solvent reorganization energy was found to be hig her for charge separation (lambda(1)) than for charge recombination (l ambda(2)). Both are smaller than the linear response result. For the r eorganization energies, the discrepancy between lambda(1) and lambda(2 ) is relatively small, whereas their temperature derivatives deviate s ignificantly from each other. The theory predictions are tested on spe ctroscopic computer simulations and experiment. Generally good quantit ative agreement is achieved. (C) 1997 American Institute of Physics.