The effects of anchoring groups on electron injection from adsorbate to nanocrystalline thin films were investigated by comparing injection kinetics through carboxylate versus phosphonate groups to TiO 2 and SnO 2 . In the first pair of molecules, Re(L A )(CO) 3 Cl (ReC1A) and Re(Lp)(CO)3Cl (ReC1P), [L A ) 2,2′-bipyridine-4,4′-bis-CH 2 -COOH, Lp) 2,2′-bipyridine-4,4′-bis-CH 2 -PO 3 H 2 ], the anchoring groups were insulated from the bipyridine ligand by a CH 2 group. In the second pair of molecules, Ru(dcbpyH 2 ) 2 (NCS) 2 (RuN3) and Ru(bpbpyH 2 ) 2 (NCS) 2 (RuN3P), [dcbpy ) 2,2′-bipyridine-4,4′-biscarboxylic acid, bpbpy ) 2,2′-bipyridine-4,4′-bisphosphonic acid], the anchoring groups were directly connected to the bipyridine ligands. The injection kinetics, as measured by subpicosecond IR absorption spectroscopy, showed that electron injection rates from ReC1P to both TiO 2 and SnO 2 were faster than those from ReC1A. The injection rates from RuN3 and RuN3P to SnO 2 films were similar. On TiO 2 , the injection kinetics from RuN3 and RuN3P were biphasic: carboxylate group enhances the rate of the <100 fs component, but reduces the rate of the slower components. To provide insight into the effect of the anchoring groups, the electronic structures of Re-bipyridyl-Ti model clusters containing carboxylate and phosphonate anchoring groups and with and without a CH 2 spacer were computed using density functional theory. With the CH 2 spacer, the phosphonate group led to a stronger electronic coupling between bpy and Ti center than the carboxylate group, which accounted for the faster injection from ReC1P than ReC1A. When the anchoring groups were directly connected to the bpy ligand without the CH 2 spacer, such as in RuN3 and RuN3P, their effects were 2-fold: the carboxylate group enhanced the electronic coupling of bpy π* with TiO 2 and lowered the energy of the bpy orbital. How these competing factors led to different effects on TiO 2 and SnO 2 and on different components of the biphasic injection kinetics were discussed.

No abstract

The preparation and the properties of several ruthenium complexes of the general formula cis-RuL(2)X(2) with L = 2,2'-bipyridine-4,4'-bisphosphonic acid, L' = 2,2'-bipyridine-5,5'-bisphosphonic acid, and X = Cl, CN, or NCS are reported. The synthesis of these complexes relies on the preparation of the key intermediates cis-Ru(bipyridinebis(diethyl ester phosphonate))Cl(2). The ground-state second pK(a) values of the thiocyanato complexes were determined and are 6.0 and 6.1 for cis-RuL(2)(NCS)(2) and for cis-RuL'(2)(NCS)(2), respectively. For these species, (13)C NMR and IR demonstrate that the thiocyanato ligands are bound to Ru via the N atom. The new complexes exhibit a blue-shifted electronic absorption spectrum with respect to the analogous complexes containing carboxylic acid groups. Density functional theory molecular orbital calculations show that the LUMO of the bipyridine phosphonated ligands is at higher energy than the corresponding dicarboxylate complexes and that the thiocyanato ligands are not simple spectator ligands, whose role is to enrich electron density on the ruthenium, but are also involved in transitions from PiRu-NCS to Pibpy that extend the absorbance of the dye in the low energy part of the absorption spectrum. The photoaction spectra recorded in a sandwich regenerative photovoltaic cell indicate that the cyano and thiocyanato complexes containing the bipyridine substituted in 4,4' positions exhibit a 90-95% photoconversion efficiency on the MLCT band, whereas those containing the bipyridine substituted in 5,5' positions display lower efficiency (60-65%). The most efficient complex in the series is cis-RuL(2)(NCS)(2); however, its overall efficiency is about 30% lower than the analogue cis-Ru(H(2)dcb)(2)(NCS)(2) (H(2)dcb = 2,2'-bipyridine-4,4'-dicarboxylic acid) due to a lower absorbance in the red part of the visible spectrum.

This paper describes the synthesis of a new series of molecules composed of a ruthenium cation liganded by a chloro or a thiocyanato, a 4,4'-(diethoxycarbonyl)-2,2'-bipyridine, and a 2,2':6',2' '-terpyridine substituted in its 4' position by a difluoroborazaindacene or a zinc phthalocyanine. A set of conditions are reported to conveniently synthesize these dyads by a Stille cross-coupling reaction between the trimethyltin derivative of the organic chromophore and the corresponding ruthenium complex with 4'-bromo-2,2':6',2' '-terpyridine and 4,4'-(diethoxycarbonyl)-2,2'-bipyridine. The dyads were studied by UV-visible absorption spectroscopy, steady-state fluorescence, and electrochemistry. The results of these studies indicate strong electronic coupling between the zinc phthalocyanine unit and the ruthenium complex but weakly electronically coupled systems in the case of dyads containing a difluoroborazaindacene unit. The new bichromophoric systems display strong absorbance in the visible spectrum. An efficient quenching of the fluorescence of the organic chromophore by the nearby ruthenium complex was also observed in all of the dyads. In dyads connected to the borazaindacene, excitation spectra indicate efficient photoinduced energy transfer from the borazaindacene to the ruthenium complex.

No abstract