FORMATION AND DYNAMICS OF INTERMOLECULAR CHARGE-TRANSFER (CT) INTERMEDIATES - A STUDY WITH PHOTO-EXCITED ALL-TRANS-1,6-DIPHENYLHEXA-1,3,5-TRIENE AND P-DICYANOBENZENE

Photoinduced electron transfer reactions between singlet excited all-t rans-1,6-diphenylhexa-1,3,5-triene ((DPH)-D-1) and p-dicyanobenzene ( p-DCB) were investigated by electrochemical methods, stationary and ti me-resolved fluorescence and absorption measurements and charge transf er (CT) emission spectral fitting in five solvents of different polari ty. Rate constants of triplet DPH ((DPH)-D-3) formation, fluorescence (CT emission), and radiationless deactivation (charge recombination, CR) of the respective CT intermediates (solvent-separated ion pairs or exciplexes) and the free energy changes of the corresponding processe s were determined. The S-1-S-n absorption spectra of the CT intermedia tes in the less polar solvents provided solid evidence that the CT int ermediates can be approximately described by pure CT states with negli gible contributions of locally excited states. A bandshape analysis of the CT emission spectra (CT emission spectral fitting) was employed t o determine the Franck-Condon factors which depend, among other parame ters, on the vibrational and solvent reorganization energies (lambda(1 ), lambda(0)), on the electronic matrix coupling element (V-CR) and on the free energy change (Delta G(CR)(G)). With the experimental values for lambda(1), lambda(0), Delta G(CR)(G), and V-CR, rate constants of the charge recombination reaction of the CT Intermediates were calcul ated in the framework of nonadiabatic electron transfer theory, and th e results were found to be in good agreement with experimental rate co nstants. (C) 1997 Elsevier Science S.A.