Photoinduced electron transfer in ruthenium(II)-rhodium(III) terpyridine dyads

A series of Ru(II)-Rh(III) dyads of general formula (ttpy)Ru-tpy-(ph)(n)-tpy-Rh(ttpy)(5+) with n = 0, 1, 2 [ttpy = 4'-p-tolyl-2,2':6,2 ''-terpyridine; tpy-(ph)(n)-tpy = bridging ligand where two 2,2':6',2 ''-terpyridine units are connected at the 4'-position through a variable number of p-phenylene spacers] have been studied, in acetonitrile at room temperature, by picosecond and nanosecond time-resolved emission spectroscopy. When n = 1, excitation of the Ru(II)-based molecular component is followed by efficient intramolecular quenching by electron transfer to the Rh(III) center. The rate constant, k greater than or equal to 3 x 10(9) s(-1), is high despite the relatively small driving force of the process (ca. 0.1 eV). When n = 2, with the same driving force as above, no intramolecular electron transfer quenching is observed (upper limit for the rate constant of the electron transfer process: k < 5 x 10(8) s(-1)). The decrease in electron transfer rate obtained in going from n = 1 to n = 2 is in line with the behavior of other systems containing poly-p-phenylene spacers. The dyad with rt = 0 is definitely not homogeneous with the other two: the intercomponent electronic coupling is much stronger, the Ru(II)-based excited state is lower in energy, and the electron transfer has a smaller driving force (Delta G approximate to 0). The lifetime of the Ru(lI)-based emission is 17 ns. The lack of an obvious model compound makes it difficult to draw firm conclusions for this system.