Energy and electron transfer processes in polymethine dyes

Abstract

Polymethine dyes and its derivatives are attractive for their interesting optical and photo‐electric properties. They are used as very efficient spectral sensitizers and laser dyes. Due to the high rate constant of deactivation channels of such dyes the primary processes of bimolecular processes as energy or electron transfer proceed within not more than some picoseconds or even shorter. In the case of a polymethine which does not isomerize we were able to show by means of time‐resolved absorption spectroscopy that the singlet state photoelectron transfer to methyl‐ and benzylviologen had an efficiency of 0.15 with rate constants of 6.7⋅109 and 4.6⋅109 l/mole⋅s, respectively, yielding the polymethine dication radical. The photoreduction with tetraphenylborate and potassium rhodanide is also very efficient with an efficiency of about 0.10 with rate constants of 2.4⋅1010 and 1.6⋅1010 l/mole⋅s, respectively, yielding the polymethine neutral radical. The spectral differences of the observed radical spectra are small. The investigation of the temperature dependence of the photo induced electron transfer of the investigated polymethine to methylviologen results in an activation energy ΔG*=24 kJ/mole and a value of the frequency factor of A=4.7⋅1014 l/mole⋅s. Strong deviation from a linear Arrhenius plot was observed at low temperatures which can be explained by solvent‐solute interaction decreasing the electron transfer rate constant at lower temperatures. The calculated electron transfer rate constants agree with the assumption of the investigated process as a diffusion‐controlled one. Energy transfer occurs as a efficient competitive deactivation channel from photo excited polymethine dyes to other chromophore systems with a strong overlapping of the fluorescence and the absorption bands of the donor and the acceptor, respectively. We have investigated the time and spectral evolution of the energy transfer process from a polymethine dye to different energy acceptor dyes in solution. The general question within this respect was the involvement of an intermediate electron transfer as competitive process in the energy transfer process. Whereas the Förster energy transfer radius calculated from the time‐resolved data exceeds the value received from the overlap integral by 15%, indicating deviation from a normal Förster decay type the semilogarithmic plot of the ground state recovery kinetics vs. square root of time results in an ideal straight line dependence. No intermediate spectra as well as intermediate time behaviour was found in these complexes.