I.V. Soboleva, E.V. Dolotova
XVIIth IUPAC Symposium on Photochemistry, Dresden, German, July 22-27, 2000, Book of Abstracts, p. 558.
ABSTRACT. Exciplexes with partial charge transfer are key transients in excited-state electron transfer reactions. To describe quantitatively the effects of electron transfer driving force DGet and medium polarity on their emission spectra and energy of formation we developed recently a model of correlated polarization of the exciplex and medium. This model considers a correlated polarization of the exciplex and medium as a coupling of locally excited (LE) and charge transfer (CT) states in the solvent shell corresponding to the equilibrium solvating of an exciplex with partial charge transfer and provides a quantitative description for main features of the solvent and DGet effects on exciplex electronic structure, emission spectra and enthalpy of formation. Several donor-acceptor systems with DGet > -0.2eV were studied in solvents of various polarities from heptane to acetonitrile. Most spectacular evidence for mutually correlated polarization of the exciplex and medium is observed in polar solvents with e > 10 for exciplexes with DGet > 0 as sigmoidal dependence of exciplex emission shift hDv vs. solvent polarity function f(e) ("sudden polarization"). The essentially non-linear dependence of hDv on f(e) is caused by the substantial growth of the extent of charge transfer in the narrow range of the solvent polarity. The parameters characterizing the exciplex electron structure, i.e. the electronic matrix element H12 coupling the LE and CT states, the energy gap (H22o-H11o) between these states in vacuum (driving force of electron transfer), and the energy of CT state solvation (m/2) in the medium of infinite dielectric permeability (concerned with the exciplex dipole moment and size) were estimated from the dependencies of hDv vs. f(e) as well as from temperature dependencies of relative exciplex emission quantum yields. Both dependencies give similar values of H12, (H22o-H11o), and m for each exciplex. Some difference was observed, however, in the parameters obtained experimentally from hDv - f(e) dependence and from the temperature dependence of quantum yields. Possible reasons for such deviations such as temperature dependence of dielectric constant and entropy of exciplex formation etc. are discussed.
The results are evidence for unambiguous interrelation between enthalpy of exciplex formation, DGet and solvent polarity. This interrelation allows to describe quantitatively the dependence of rate constants of electron transfer on DGet and solvent polarity using the proposed relatively simple and sufficiently general model.
This work is supported by grants of Russian Foundation for Basic Research (99-03-32337) and "Russian University - Basic Research".