Stepwise charge separation in heterotriads. Binuclear Ru(II)-Rh(III) complexes on nanocrystalline titanium dioxide

Two novel Ru(II)-Rh(III) polypyridine dyads, containing carboxylic functions at the Rh(III) unit, Rh-III(dcb)(2)-(BL)-Ru-II(dmp)(2) and Rh-III(dcb)(2)-(BL)-Ru-II(bpy)(2) (bpy = 2,2'-bipyridine; dcb = 4,4'-dicarboxy2,2'-bipyridine; dmp = 4,7-dimethyl-1,10-phenanthroline; BL = 1,2-bis[4-(4'-methyl-2,2'-bipyridyl]ethane) have been synthesized. Their photophysical behavior in solution, compared with that of the mononuclear Ru-II(dcb)(2)(dmb) model (dmb = 4,4'-dimethyl-2,2'-bipyridine), indicates the occurrence of fast (10(8)-10(9) s(-1)) and efficient (>95%) Rh(III)-*Ru(II) --> Rh(II)-Ru(III) photoinduced electron transfer. These species adsorb firmly on nanoporous TiO2 films, via the dcb ligands of the Rh(III) units. The behavior of the adsorbed species has been studied by means of nanosecond time-resolved emission and absorption measurements, as well as by photocurrent measurements. Photocurrent action spectra demonstrate that light absorption by the Ru(II) chromophore leads to electron injection into the semiconductor. A detailed analysis of the transient behavior of the TiO2-Rh-III(dcb)(2)-(BL)-Ru-II(bpy)(2) system indicates that about one-third of the adsorbed dyads (probably because of different orientation at the surface or accidental contacts in small cavities) undergo direct electron injection from the excited state of the Ru(II) chromophore. The remaining dyads display stepwise charge injection processes, i.e., intramolecular electron transfer, TiO2-Rh(III)-*Ru(II) --> TiO2-Rh(II)-Ru(III), followed by charge separation by electron injection,TiO2-Rh(II)-Ru(III) - TiO2(II)-Rh(II)-Ru(III). The first process has comparable rates and efficiencies as for the free dyads in solution. The second step is 40% efficient, because of competing primary recombination, TiOz-Rh(II)-Ru(III) - TiO2-Rh(III)-Ru(II), When the final recombination between injected electrons and oxidized Ru(III) centers is studied, a remarkable slowing down is obtained for the supramolecular systems, e.g., TiO2-Rh-III(dcb)(2)-(BL)-Ru-II(bpy)(2), relative to analogous systems containing simple mononuclear sensitizers, e.g., TiO2-Ru-II(dcb)(2)(dmb). Stepwise charge separation and slow recombination between remote sites are distinctive features that suggest the labeling of these systems as "heterotriads".