Adjacent- versus Remote-Site Electron Injection in TiO2 Surfaces Modified with Binuclear Ruthenium Complexes

Nanocrystalline thin films of TiOg cast on an optically transparent indium tin oxide glass were sensitized with ruthenium homo- and heterobinuclear complexes, [LLʹRu(BL)RuLL {n = 2, 3), where L and Lʹ are 4,4ʹ-dicarboxy2,2ʹ-bipyridine (deb) and/or 2,2ʹ-bipyridine (bpy) and BL is a rigid and linear heteroaromatic entity (tetrapyrido[3,2a:2ʹ,3ʹ-c:3",2"-2ʹ",3-y]phenazine (tpphz) or 1,4-bis([1,10]phenanthroline[5,6-d]imidazol-2-yl)benzene (bfimbz)). . The photophysical behavior of the Ru(ll)-Ru(ll) diads in solution indicated the occurrence of intercomponent energy transfer from the upper-lying Ru — bpy charge-transfer (CT) excited state of the Ru(bpy)2 moiety to the lower-lying Ru — dcb CT excited state of the Ru(bpy)(dcb) (or Ru(dcb)2) subunit in the heterobinuclear complexes. These sensitizer diads adsorbed on nanostructured TiC)2 surfaces in a perpendicular or parallel attachment mode. Adsorption was through the dcb ligands on one or both chromophoric subunits. The behavior of the adsorbed species was studied by nanosecond time-resolved transient absorption and emission spectroscopy, as well as by photocurrent measurements. In the TiO2-adsorbed samples where BL was bfimbz, the electron injection kinetics was very fast and could not be resolved because an electron is promoted from the metal center to the deb ligand directly linked to the semiconductor. In the TiC)2-adsorbed samples where BL was tpphz, for which, in the excited state, a BL localization of the lowest-lying metal-to-ligand charge transfer (MLCT) is observed, slower injection rates (9.5 x 10 s in [(bpy)2Ru(tpphz)Ru(bpy)(dcb-)] TiO2 and 5.5 x W s in [(bpy)(dcb)Ru(tpphz)Ru(bpy)(dcb-)]Ti02) were obtained. Among the systems, the heterotriad assembly [(bpy)2Ru(bfimbz)Ru(bpy)(dcb)]/Ti02 gave the best photovoltaic performance. In the first case, this was attributed to a fast electron injection initiated from a dcblocalized MLCT; in the second case, this is attributed to improved molecular orientation on the surface, which was due to rigidity and, at the same time, linearity of the heterotriad system, resulting in a slower charge recombination between the injected electron and the hole.