Electronic and molecular surface structure of Ru(tcterpy)(NCS)3 and Ru(dcbpy)2(NCS)2 adsorbed from solution onto nanostructured TiO2:: A photoelectron spectroscopy study

The element specificity of photoelectron spectroscopy (PES) has been used to compare the electronic and molecular structure of the dyes Ru(tcterpy)(NCS)(3) (BD) and Ru(dcbpY)(2)(NCS)(2) adsorbed from solution onto nanostructured TiO2. Ru(dcbpy)(2)(NCS)(2) was investigated in its acid (N3) and in its 2-fold deprotonated form (N719) having tetrabutylammonium (TBA(+)) as counterions. A comparison of the O1s spectra for the dyes indicates that the interactions through the carboxylate groups with the TiO2 surface are very similar for the dyes. However, we observe that some of the dye molecules also interact through the NCS groups when adsorbed at the TiO2 surface. Comparing the N719 and the N3 molecule, the fraction of NCS groups interacting through the sulfur atoms is smaller for N719 than for N3. We also note that the counterion TBA(+) is coadsorbed with the N719 and BD molecules although the amount was smaller than expected from the molecular formulas. Comparing the valence levels for the dyes adsorbed on TiO2, the position of the highest occupied electronic energy level is similar for N3 and N719, while that for BD is lower by 0.25 eV relative to that of the other complexes.