The deactivation of singlet excited all-trans-1,6-diphenylhexa-1,3,5-triene by charge transfer processes 2. Formation and dynamics of charge transfer (CT) intermediates

Abstract

The fluorescence quenching mechanism of single excited all-trans-1,6-diphenylhexa-1,3,5-triene ( 1DPH ∗) by the electron acceptor molecule p-dicyanobenzene (p-DCB) was investigated with time-resolved fluorescence and absorption measurements in 5 solvents of different polarity. In the polar solvents (dielectric constant ε ≳ 10) the interaction between 1DPH ∗ and p-DCB leads to formation of cationic DPH. In the less polar solvents ( ε ≲ 10) an additional emission in the fluorescence spectrum and a bi-exponential fluorescence decay of 1DPH ∗ in the presence of p-DCB is observed. Formation of triplet DPH occurs in every solvent employed. With the assumption that only one CT intermediate is dominant in a given solvent, rate constants of triplet DPH 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 CR rate constant of the CT intermediates exhibits the most pronounced variation upon changing the solvent. E.g. in acetonitrile solution, it is ca. 50 times larger than in toluene solution, whereas the standard free energy change for the CR processes — ΔG CR is larger in toluene than in acetonitrile solution. The experimental results for the CR rate constants are in good agreement with estimations of rate constants in the framework of non-adiabatic electron transfer theory using experimental values for reorganization energies and electronic matrix coupling elements.