The Effect of Dye Density on the Efficiency of Photosensitization of TiO₂ Films: Light-Harvesting by Phenothiazine-Labelled Dendritic Ruthenium Complexes

Abstract: A family of dendritic tris-bipyridyl ruthenium coordination complexes incorporating two or four carboxylate groups for binding to a TiO₂ surface site and another dendritic linker between the metal complex and highly absorptive dyes were formulated as thin films on TiO₂ coated glass. The family included phenothiazine-substituted dendrons of increasing structural complexity and higher optical density. The dye-loaded films were characterized by steady-state emission and absorption measurements and by kinetic studies of luminescence and transient absorption. Upon photoexcitation of the bound dyes, rapid electron injection into the metal oxide film was the dominant observed process, producing oxidized dye that persisted for hundreds of milliseconds. Complex decay profiles for emission, transient absorption, and optical bleaching of the dendritic dyes point to highly heterogeneous behavior for the films, with observed persistence lifetimes related directly to structurally enhance electronic coupling between the metal oxide support and the dendritic dyes.