Magnetic field and temperature dependencies shed light on the recombination kinetics of a transition metal donor/acceptor system.

The radical pair recombination of an intramolecular electron-transfer system containing a transition metal moiety has been addressed by femtosecond spectroscopy. The radical pair is formed by ultrafast electron transfer (90 fs) from a ferrocene residue to a photoexcited Nile blue moiety. Its recombination proceeds on the picosecond time scale in a multiexponential fashion. The kinetic pattern is a manifestation of spin processes competing with electron transfer. Magnetic field effects on these kinetics allow one to disentangle the two contributions. Their temperature dependencies yield the activation parameters of the two processes. The discussion focuses on the mechanism of electron spin relaxation. Strong evidence for the Orbach/Kivelson mechanism will be given.