A series of mixed-metal NH 2-UiO-66(Zr/Ti) with different percentages of exchanged Ti have been prepared and studied by transient absorption spectroscopy (TAS). The photo-generated transients from mixed NH 2-UiO-66(Zr/Ti) exhibit at short time scales two defined absorption bands, evolving to a continuous absorption band expanding from 300 nm to 700 nm at longer time scales. The observed spectral changes are compatible with an initial formation of Ti 3+-O-Zr 4+ and its further transformation to Ti 4+-O-Zr 3+ 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) 6 O 4 (OH) 4 nodes in mixed NH 2-UiO-66(Zr/Ti). The slow recombination of photo-generated electrons and holes in the mixed NH 2-UiO-66(Zr/Ti) has been advantageously used for the construction of a photovoltaic cell fabricated with the mixed NH 2-UiO-66(Zr/Ti), reaching higher photon-to-current efficiency than NH 2-UiO-66(Zr).