Transient absorption spectroscopy of ruthenium and osmium polypyridyl complexes adsorbed onto nanocrystalline TiO2 photoelectrodes

Transient absorption spectroscopy has been used to probe the electron injection dynamics of transition metal polypyridyl complexes adsorbed onto nanocrystalline TiO2 photoelectrodes. Experiments were performed on photoelectrodes coated with Ru(H2L')(2)(CN)(2), Os(H2L')(2)(CN)(2), Ru(H2L')(2)(NCS)(2), or Os(H2L')(2)(NCS)(2), where H2L' is 4,4'-dicarboxylic acid-2,2'-bipyridine, to study how the excited-state energetics and the nature of the metal center affect the injection kinetics. All of these complexes exhibited electron injection dynamics on both the femtosecond and picosecond time scales. The femtosecond components were instrument-limited (<200 fs), whereas the picosecond components ranged from 3.3 +/- 0.3 ps to 14 +/- 4 ps (electron injection rate constants k(2)' = (7.1-30) x 10(10) s(-1)). The picosecond decay component became more rapid as the formal excited-state reduction potential of the complex became more negative. Variable excitation wavelength studies suggest that femtosecond injection is characteristic of the nonthermalized singlet metal-to-ligand charge-transfer ((MLCT)-M-1) excited state, whereas picosecond injection originates from the lowest-energy (MLCT)-M-3 excited state. On the basis of these assignments, the smaller relative amplitude of the picosecond component for the Ru sensitizers suggests that electron injection from nonthermalized excited states competes more effectively with (MLCT)-M-1 --> (MLCT)-M-3 conversion for the Ru sensitizers than for the Os sensitizers.