Ti as Mediator in the Photoinduced Electron Transfer of Mixed-Metal NH2–UiO-66(Zr/Ti): Transient Absorption Spectroscopy Study and Application in Photovoltaic Cell

A series of mixed-metal NH2–UiO-66(Zr/Ti) with different percentages of exchanged Ti have been prepared and studied by transient absorption spectroscopy (TAS). The photogenerated transients from mixed NH2–UiO-66(Zr/Ti) exhibit at short time scales two defined absorption bands, evolving to a continuous absorption band expanding from 300 to 700 nm at longer time scales. The observed spectral changes are compatible with an initial formation of Ti3+–O–Zr4+ and its further transformation to Ti4+–O–Zr3+ via metal–metal electron exchange, thus providing support to the role of substituted Ti as mediator to facilitate electron transfer from excited ligand to the (Zr/Ti)6O4(OH)4 nodes in mixed NH2–UiO-66(Zr/Ti). The slow recombination of photogenerated electrons and holes in the mixed NH2–UiO-66(Zr/Ti) has been advantageously used for the construction of a photovoltaic cell fabricated with the mixed NH2–UiO-66(Zr/Ti), reaching a higher photon-to-current efficiency than NH2–UiO-66(Zr).