Spectroelectrochemical studies of hole percolation on functionalised nanocrystalline TiO2 films: a comparison of two different ruthenium complexes

Abstract

We report the application of spectroelectrochemical techniques to compare the hole percolation dynamics of molecular networks of two ruthenium bipyridyl complexes adsorbed onto mesoporous, nanocrystalline TiO₂ films. The percolation dynamics of the ruthenium complex cis-di(thiocyanato)(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(2,2′-bipyridyl-4,4′-tridecyl) ruthenium(II), N621, is compared with those observed for an analogous dye with an additional tri-phenyl amine (TPA) donor moiety, cis-di(thiocyanato)(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(2,2′-bipyridyl-4,4′-bis(vinyltriphenylamine)) ruthenium(II), HW456. The in situoxidation of these ruthenium complexes adsorbed to the TiO₂ films is monitored by cyclic voltammetry and voltabsorptometry, whilst the dynamics of hole (cation) percolation between adsorbed ruthenium complexes is monitored by potentiometric spectroelectrochemistry and chronoabsorptometry. The hole diffusion coefficient, D_eff, is shown to be dependent on the dye loading on the nanocrystalline TiO₂ film, with a threshold observed at ∼60% monolayer surface coverage for both dyes. The hole diffusion coefficient of HW456 is estimated to be 2.6 × 10⁻⁸ cm²/s, 20-fold higher than that obtained for the control N621, attributed to stronger electronic coupling between the TPA moieties of HW456 accelerating the hole percolation dynamics. The presence of mercuric ions, previously shown to bind to the thiocyanates of analogous ruthenium complexes, resulted in a quenching of the hole percolation for N621/TiO₂ films and an enhancement for HW456/TiO₂ films. These results strongly suggest that the hole percolation pathway is along the overlapped neighbouring -NCS groups for the N621 molecules, whereas in HW456 molecules cation percolation proceeds between intermolecular TPA ligands. These results are discussed in the context of their relevance to the process of dye regeneration in dye sensitised solar cells, and to the molecular wiring of wide bandgap inorganic materials for battery and sensing applications.