Coordination compounds of the general type Ru(dmb)2(LL)(PF6)2, where dmb is 4,4‘-(CH3)2-2,2‘-bipyridine and LL is 4-(CH3)-4‘-(COOH)-2,2‘-bipyridine, or 4-(CH3)-4‘-((CH2)3COOH)-2,2‘-bipyridine, or 4-(CH3)-4‘-((CH2)3COCH2COOC2H5)-2,2‘-bipyridine were prepared for the attachment to semiconductor metal oxide surfaces. The optical and redox properties of these compounds in dichloromethane solution are reported. Binding to porous nanostructured TiO2 films was analyzed with the Langmuir adsorption isotherm model. Photoelectrochemical measurements of the modified TiO2 electrodes in regenerative solar cells are reported. The results indicate that intimate electronic coupling between the surface link and the chromophoric ligand is not a strict requirement in the design of sensitizers for photovoltaic applications. Interfacial kinetics for recombination of the electron in the solid with the oxidized form of the sensitizer were quantitated by excited state absorption spectroscopy.

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Time-resolved infrared measurements indicate ultrafast, < 350 fs electron injection from (4,4'dcb) 2 Ru(NCS) 2 (1) and (5,5'dcb) 2 Ru(NCS) 2 (2) to nanostructured TiO 2 electrodes. {(4,4'dcb) = (4,4'-COOH-2,2'-bipyridine)} Although rapid, the injection from 2 apparently occurs with a lower quantum yield, explaining a lower overall photon-to-current efficiency for 2/TiO 2 solar cells. Transient visible spectroscopy reveals similar rates of both halide oxidation and injected electron-oxidized dye recombination for the two sensitizers. Substituting SnO 2 for TiO 2 increases the electron injection yield from 2 in the case of transparent metal oxide films and improves the photon to current efficiency. Results indicate competition between electron injection and vibrational relaxation of the sensitizer excited state.

Time-resolved infrared (IR) spectroscopy in the 6 µm region was employed to study the excited-state properties of [Ru(4,4′-(COOCH 2 CH 3 ) 2 -2,2′-bipyridine)(2,2′-bipyridine) 2 ] +2 and [Ru(4,4′-(COOCH 2 CH 3 ) 2 -2,2′-bipyridine)-(4,4′-(CH 3 ) 2 -2,2′-bipyridine) 2 ] +2 in solution and anchored to nanostructured thin films of TiO 2 and ZrO 2 . Excited-state spectra reveal a shift in the ν(CdO) of the ester groups for the free molecules in solution as well as attached to insulating ZrO 2 substrates. For these molecules attached to TiO 2 semiconductor films, a transient absorption appears that is attributed to electrons injected into TiO 2 . This absorption appears within the instrumental time resolution (ca. 30 ps) yielding an approximate 20 ps upper limit time constant for electron transfer from the sensitizer excited state to TiO 2 .