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 function s 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'-bipyri dine; 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 (d mb = 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) phot oinduced electron transfer. These species adsorb firmly on nanoporous TiO2 films, via the dcb ligands of the Rh(III) units. The behavior of the adsorb ed species has been studied by means of nanosecond time-resolved emission a nd absorption measurements, as well as by photocurrent measurements. Photoc urrent action spectra demonstrate that light absorption by the Ru(II) chrom ophore leads to electron injection into the semiconductor. A detailed analy sis 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 beca use 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 p rocesses, i.e., intramolecular electron transfer, TiO2-Rh(III)-*Ru(II) --> TiO2-Rh(II)-Ru(III), followed by charge separation by electron injection,Ti O2-Rh(II)-Ru(III) - TiO2(II)-Rh(II)-Ru(III). The first process has comparab le rates and efficiencies as for the free dyads in solution. The second ste p is 40% efficient, because of competing primary recombination, TiOz-Rh(II) -Ru(III) - TiO2-Rh(III)-Ru(II), When the final recombination between inject ed 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 mononuc lear 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".