Sensitization of TiO2 by the MLCT Excited State of CoI Coordination Compounds

Co I coordination compounds, based on porphyrin or glyoxime ligands, anchored to TiO 2 nanocrystallites interconnected in a mesoporous thin film were found to efficiently inject electrons into TiO 2 when excited with visible or near-infrared light. When a cobalt metalloporphyrin sensitizer was employed, electron injection was rapid and led to a long-lived (millisecond time scale) charge-separated state whose recombination kinetics were nonexponential but were first order in the concentration of TiO 2 electrons. The MLCT excited state was found to inject electrons more efficiently than the π → π* excited state of the porphyrin, even though unfilled d orbitals were present. These findings provide an earth-abundant metal to use in coordination compounds for sensitization of wide-band-gap semiconductor nanoparticles, especially those whose conduction band edges are generally closer to the vacuum level.