Sensitization of TiO2 by a symmetric anionic polymethine dye with three conjugated chromophores

Based on the analysis of spectral, electrochemical and energy parameters of the anionic polymethine dye 9-ezenil-2,4,5,7-tetranitro-[1-(2,4,5,7-tetranitro-9N-fluoren-9-ezenil) methyliden] triethylammonium 9-N-fluoren, a principal possibility for its utilization as an effective sensitizer of titanium dioxide has been shown. It was found that the bathochromic shift of the long-wave absorption band occurs at deposition of the dye on the surface of TiO2. The degree of this shift depends on the dye content in the heterostructure. In addition, the deposition also causes some redistribution in the intensity of the absorption bands: they become wider and expand over almost the entire visible and near IR spectral zones. This effect can be used to increase an efficiency of the heterostructures because of increased light absorption. The oxidation and reduction potentials of the polymethine dyes were determined by cyclic voltammetry and then their excited redox potentials were calculated using those values. It was shown that the dyes can perform sensitizing of TiO2 by transfering electrons into the conductivity band of titania. Photocatalytic activity of the heterostructure was determined using the model reaction of methylene blue reduction under different irradiation intensities and at various concentrations of the dye. The processes of energy transformation between TiO2 and the dye-sensitizer induced by the light absorption have been discussed. It was proven that the electronic processes required for both direct and photocatalytically-sensitized reduction of methylene blue are allowed thermodynamically in the systems containing the heterostructure described in this work.