Similarity and difference of exciplexes with complete and partial electron transfer

I.V. Soboleva

XX ICP International Conference on Photochemistry, Moscow, Russia, July 30 - August 4, 2001, Book of Abstracts, p. 515.

ABSTRACT. Feature of exciplexes with complete and partial electron transfer formed are analyzed. Exciplexes of both types are formed as a result of mixing of wave functions of charge-transfer (CT) and locally excited (LE) states. Variation of contribution of each state into wave function of exciplex results in changing of degree of charge transfer, dipole moment, contribution of various kinds of interaction into the bond energy of exciplex causing change of physicochemical properties of exciplexes. As an example of exciplexes with partial charge transfer spectral and thermodynamic properties of exciplexes of 9-cyanophenanthrene, 9-cyanoantharacene, pyrene and 1,12-benzoperylene with methoxybenzenes and naphtalenes are investigated. The dependencies of these properties on solvent polarity and electron transfer driving force are compared with that for exciplexes with complete charge transfer. For exciplexes with complete charge transfer (the degree of charge transfer z>0.8 and the energy gap between CT- and LE-states in vacuum (driving force of electron transfer) (H220 - H110) < -0.3 eV), the main contribution to the exciplex energy gives the energy of electron transfer. In this case exciplex energy -UEx ~= H220 - mf(e) (m = (mu0)^2/r^3, mu0 is dipole moment of contact radical-ion pair, r is radius of exciplex solvent shell). When (H220 - H110) > -0.3 eV (z < 0.8) the contribution of H12 (electronic matrix coupling element the CT- and LE-states) in exciplex stabilization increases simultaneously the role of solvent polarity and intermolecular repulsion decreases. Exciplex energy is substantially lower than that of LE-state even when (H220 - H110) > 0. Increase of H12 contribution demonstrates why just exciplexes with partial electron transfer are stable in polar solvents in contrast to exciplexes with complete electron transfer. The latter easily dissociate to radical ions in polar solvents. Kinetic properties of exciplexes (rate constants of fluorescence, intersystem crossing, dissociation to radical-ions and parent molecules) also change according to variations of z. Thus the general model of correlated polarization of exciplex and medium considering electronic coupling, solvation change and molecular repulsion energy describes properties of both exciplexes with partial electron transfer and exciplexes with complete electron transfer. This work is supported by grant of Russian Foundation for Basic Research (99-03-32337).

Laboratory of Photochemistry