Photoinduced electron transfer across oligo-p-phenylene bridges. Distance and conformational effects in Ru(II)-Rh(III) dyads

A series of rodlike ruthenium(II)-rhodium(III) polypyridine dyads based on modular oligo-p-phenylene bridges, of the general formula [(Me(2)phen)(2)Ru-bpy-(ph)(n)-bpy-Rh(Me(2)bpy)](5+) (Me(2)phen = 4,7-dimethyl-1,10-phenanthroline; bpy = 2,2'-bipyridine; ph = 1,4-phenylene; n = 1-3), have been synthesized and their photophysical properties investigated. The dyad [(Me(2)bpy)(2)Ru-bpy-(ph)(3)'-bpy-Rh(Me(2)bpy)](5+) with the central phenylene unit bearing two hexyl chains has also been studied. The metal-to-metal distance reaches 24 A for the longest (n = 3) spacer in the series. For all of the dyads in a room-temperature CH3CN solution, quenching of the typical metal-to-ligand charge-transfer luminescence of the Ru-based chromophoric unit is observed, indicating that an efficient intramolecular photoinduced electron transfer from the excited Ru moiety to the Rh-based unit takes place. The rate constants for the electron-transfer process have been determined by time-resolved emission and absorption spectroscopy in the nanosecond and picosecond time scale. An exponential dependence of experimental transfer rates on the bridge length is observed, consistent with a superexchange mechanism. An attenuation factor beta of 0.65 A(-1) is determined, in line with the behavior of other systems containing oligo-p-phenylene spacers. Interestingly, for n = 3, the presence/absence of hexyl substituents in the central p-phenylene ring causes a 10-fold difference in the rates between otherwise identical dyads. This comparison highlights the importance of the twist angle between adjacent spacers on the overall through-bond donor-acceptor coupling.