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

Abstract

Photoinduced electron transfer reactions between singlet excited all- trans-1,6-diphenylhexa-1,3,5-triene ( IDPH ∗) and p-dicyanobenzene ( p-DCB) were investigated by electrochemical methods, stationary and time-resolved fluorescence and absorption measurements and charge transfer (CT) emission spectral fitting in five solvents of different polarity. Rate constants of triplet DPH ( 3DPH ∗) 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 processes were determined. The S 1-S n absorption spectra of the CT intermediates in the less polar solvents provided solid evidence that the CT intermediates can be approximately described by pure CT states with negligible contributions of locally excited states. A bandshape analysis of the CT emission spectra (CT emission spectral fitting) was employed to determine the Franck-Condon factors which depend, among other parameters, on the vibrational and solvent reorganization energies ( λ i, λ 0), on the electronic matrix coupling element ( V CR) and on the free energy change ( ΔG CR G). With the experimental values for λ i, λ 0, δG CR G, and V CR, rate constants of the charge recombination reaction of the CT intermediates were calculated in the framework of non-adiabatic electron transfer theory, and the results were found to be in good agreement with experimental rate constants.