ligand charge transfer [3]. However, ruthenium polypyridyl complexes contain a heavy metal, which is undesirable from point of view of the environmental aspects [4]. Moreover, the process to synthesize the complexes is complicated and costly. Alternatively, natural dyes can be used for the same purpose with an acceptable efficiency [2–9]. The advantages of natural dyes include their availability and low cost [3].
The sensitization of wide bandgap semiconductors using natural pigments is usually ascribed to anthocyanins [2–9]. The anthocyanins belong to the group of natural dyes responsible for several colors in the red–blue range, found in fruits, flowers and leaves of plants. Carbonyl and hydroxyl groups present in the anthocyanin molecule can be bound to the surface of a porous TiO2 film. This makes electron transfer from the anthocyanin molecule to