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 ≥ 3 × 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 × 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 n = 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 (Δ G ≈ 0). The lifetime of the Ru(II)-based emission is 17 ns. The lack of an obvious model compound makes it difficult to draw firm conclusions for this system.