OPTICAL AND RADIATIONLESS INTRAMOLECULAR ELECTRON TRANSITIONS IN NONPOLAR FLUIDS - RELATIVE EFFECTS OF INDUCTION AND DISPERSION INTERACTIONS

A microscopic theory of intramolecular optical and radiationless elect ron transitions in nonpolar fluids is developed. The solute is modeled by a polarizable dipolar hard sphere, and the solvent by polarizable hard spheres. The effect of the induction and dispersion interactions to the spectral line shift and width are calculated as a perturbation expansion in the solute-solvent attractions. The relative contribution s of both these effects depend significantly on the solute size. Only for large solutes the dispersions are found to dominate the first orde r energy shift, while inductions become important if the solute size i s comparable to that of the solvent molecules. If the solute dipole mo ment increases with excitation the dispersion and induction components of the first order spectral shift add up leading to a redshift. In th e converse case (dipole moment decreasing) the two components have opp osite signs, and the shift may switch from red to blue. Furthermore, b oth components cause the solvent reorganization energy to decrease sha rply with the solute size. However, dispersions are of minor importanc e relative to inductions, for the parameter values used in this study. The linear correlation of the first order line shift with the solvent dielectric function (epsilon infinity - 1)/(epsilon infinity + 2) of the dielectric constant epsilon infinity is traced back to a compensat ing effect of dispersions and inductions. The continuum theory is show n to overestimate the solvent response,substantially. Both the solvent reorganization energy and the Stokes shift (the difference between ab sorption and fluorescence energies) are predicted to vary inversely wi th temperature. If not masked by intramolecular reorganization, this d ependence can cause a maximum in the Arrhenius coordinates for electro n transfer rates in the near-to-activationless region. (C) 1995 Americ an Institute of Physics.