<title>Energy and electron transfer processes in polymethine dyes</title>

Abstract

Polymethine dyes and its derivatives are attractive for their interesting optical and photo- electric properties. 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 (DOT) 10<SUP>9</SUP> and 4.6 (DOT) 10<SUP>9</SUP> l/mole(DOT)s, respectively, yielding the polymethine dication radical. The photo-reduction with tetraphenylborate and potassium rhodanide is also very efficient with an efficiency of about 0.10 with rate constants of 2.4 (DOT) 10<SUP>10</SUP> and 1.6 (DOT) 10<SUP>10</SUP> l/mole(DOT)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 (Delta) G* equals 24 kJ/mole and a value of the frequency factor of A equals 4.7 (DOT) 10<SUP>14</SUP> l/mole(DOT)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. We have investigated the time and spectral evolution of the energy transfer process from a polymethine dye to different energy acceptor dyes in solution as well as in Langmuir-Blodgett layers. The general question within this respect was the involvement of an intermediate electron transfer as competitive process in the energy transfer process.