Ultrafast electron injection from metal polypyridyl complexes to metal-oxide nanocrystalline thin films

Interfacial electron transfer (ET) between molecular adsorbates and semiconductor nanoparticles has been a subject of intense recent interest. In this paper, we review our recent work in understanding ultrafast photoinduced electron injection from ruthenium and rhenium polypyridyl complexes to metal oxide nanoparticles. Electron injection rates were measured using femtosecond IR spectroscopy, which provided a direct probe of adsorbate vibrational spectra and IR absorption of injected electrons in semiconductor. The consequence of competition between ultrafast electron injection and intramolecular relaxation in these transition metal complexes was carefully examined. A two-state injection model was proposed to account for biphasic kinetics, allowing a comparison of injection rate in different systems. The components of the chromophore(donor)-bridge-nanoparticle (acceptor) complexes were systematically varied to examine their effect on ET rate. The observed trends were discussed by considering the change in the strength of electronic coupling and density of accepting states in semiconductor and compared with Marcus theory of interfacial electron transfer. (C) 2004 Elsevier B.V. All rights reserved.