Femtosecond Transient Absorption Study of Supramolecularly Assembled Metal Tetrapyrrole-TiO2 Thin Films

Photoexcited electron injection and back electron transfer dynamics of metal tetrapyrrole bound to TiO2 nanoparticle surfaces via the metal ligand axial coordination approach have been investigated using femtosecond pump probe transient spectroscopic technique. The employed metal tetrapyrroles include zinc and magnesium metalated meso-tetraarylporphyrins having halogen substituents on the peripheral aryl groups, perfluorinated zinc phthalocyanine derivatives, and zinc naphthalocyanine. The employed metal tetrapyrroles covered absorption at different portions of the visible and near-IR region of the spectrum with excited-state reduction potentials ranging between -0.61 eV and -1.34 V, that is, having energy higher than the TiO2 conduction band edge (-0.57 V vs NHE). Two linkers, pyridine and phenylimidazole, have been employed to visualize electronic coupling between the dye and metal oxide surface for optimal electron injection and back electron transfer dynamics. In agreement with the previously reported photocurrent generation of dye-sensitized solar cells constructed using this self-assembly approach (J. Am. Chem. Soc. 2009, 131, 14646), spectral evidence for electron injection from the excited metal tetrapyrrole to the TiO2 nanoparticle in the form of a tetrapyrrole radical cation has been obtained. The time profile of the pi-cation radical of tetrapyrroles revealed the occurrence of ultrafast electron injection (time constant = 470-700 fs), while the back electron transfer processes were found to be complicated due to the intricate environment of the metallotetrapyrrole-TiO2 interface.