Photophysical properties of an amphiphilic cationic hemicyanine dye in solution and adsorbed on a TiO2 mesoporous film

The photophysical behavior of the amphiphilic cationic hemicyanine dye N-n-heptyl-4-(((dimethylamino)phenyl)ethenyl)pyridinium bromide (H7HC) has been examined in various environments with the intention to assess its state of adsorption on a TiO2 mesoporous film. In some solvents, such as short-chain alcohols or chloroform, the dye is dissolved as a monomer. In other solvents, like water and cyclohexane, it is found in concentration-dependent aggregate forms. H7HC monomer absorption and fluorescence spectra show a symmetric solvatochromic effect previously found for other amphiphilic hemicyanines (Fromherz, P. J. Phys. Chem., 1995, 99, 7188). Aggregates are of the H-type; i.e., they are formed by repulsive interaction and display a hypsochromic shift in their absorption spectra. When adsorbed on nanostructured mesoporous TiO2 films, H7HC is found in aggregate form, which evolves in the course of time by displaying a hypsochromic shift in the absorption maximum. Photoelectrochemical studies with an ITO-TiO2-H7HC electrode revealed that aggregates are responsible for the ability of the dye to photosensitize TiO2.