Electron injection dynamics from photoexcited porphyrin dyes into SnO ₂ and TiO₂ nanoparticles

The photoexcited electron injection dynamics of free-base and metallo-derivatives of tris(pentafluorophenyl)porphyrins bound to TiO ₂ and SnO₂ nanoparticle surfaces have been investigated using time-resolved terahertz spectroscopy (TRTS). The metallo-derivatives include Zn(II), Cu(II), Ni(II), and Pd(II). For the TiO₂-porphyrin assemblies, electron injection from the photoexcited dye to the semiconductor occurs only when using the zinc derivative as the sensitizer because it is the only dye studied in this report with long-lived excited states higher in energy than the TiO₂ conduction band edge. All of the dyes, however, have excited-state energies above the SnO₂ conduction band edge, and the electron injection rates vary widely from 0.4 to 200 ps depending on the sensitizer. For the SnO₂-porphyrin assemblies, electron injection is strongly influenced by competition with alternate deactivation routes that are accessible following Soret band excitation. These results offer thermodynamic and kinetic considerations for designing improved high-potential porphyrin photoanodes with applications to solar-powered water oxidation.