Raman resonance effect in a monolayer of polypyridyl ruthenium(II) complex adsorbed on nanocrystalline TiO2 via phosphonated terpyridyl ligands

A high Raman intensity is observed when a dye containing Ru(II) and different polypyridinium ligands is adsorbed on nanocrystalline TiO2. This effect is attributed to a strong resonance obtained when the laser wavelength corresponds to a charge transfer between the central ruthenium atom and one of the ligands, With a judicious choice of the ligands, such modified titanium oxide electrodes used in photoelectrochemical cells can give high photocurrent efficiencies, The dye studied here contains two different ligands: L = (4,4'-Me-2-2,2'-bpy), bpy = bipyridine, and L' = (4'-PO3H-terpy), terpy = 2,2':6,2"-terpyridine; in this case, the strong adsorption on TiO2 is ensured by the PO3H group of the L' ligand; with the green laser line, the resonance arises also in this L' ligand. An enhancement of the bridging ligand Raman features-not very strong but however significant-is observed during the polarization in the photoelectrochemical medium. In the case of a complicated charge-transfer complex, one can therefore identify the nature of the bridging ligand as well as, on the opposite side, assign the spectral features to a particular ligand, Very promising perspectives for the understanding of the adsorption mechanisms and of the configuration of the surface complex formed by the dye and the semiconductor during the modification of TiO2 are now opened.