Ultrafast electron transfer, recombination and spin dynamics

The quenching of excited singlet methylene blue ((MB+)-M-1*) by N,N-dimethylaminomethylferrocene (FcN) in acetonitrile at room temperature has been studied using femtosecond pump-probe absorption spectroscopy. At high FcN concentration static quenching via an intermolecular electron transfer mechanism constitutes the predominating decay channel for (MB+)-M-1* The time constants of the large amplitude components for the forward electron transfer from FcN donor to (MB+)-M-1* and the subsequent recombination process recovering the ground states have been determined to be 390 fs and 1 ps, respectively. Thus, the majority of radical pairs has recombined before paramagnetic relaxation induces spin mixing giving rise to triplet-phased radical pairs. These can escape geminate recombination as the system does not offer low-lying local triplet states to be occupied in the recombination process. (C) 1997 Elsevier Science B.V.