How does the metal doping in mixed metal MOFs influence their photodynamics? A direct evidence for improved photocatalysts

Mixed-metal organic frameworks (MMOFs) have arisen as one of the most promising photocatalysts for a wide number of reactions. In general, the introduction of a second or even a third metal atom in the MOFs metal cluster, having a better overlap with the π∗ orbital of the organic linker, enhances the photocatalytic activity of the MOFs. This can be a consequence of a more favorable ligand-to-cluster charge transfer (LCCT) process in the MMOFs, which in fact generates a higher population of long-lived charge-separated states (CSSs), the latter being the driving force of a photocatalytic reaction. However, proving a direct relation between these photo processes and the improved photocatalytic performance in MMOFs frequently remains elusive. Hence, herein we perform an exhaustive and systematic spectroscopic and photodynamical characterization of three UiO-66-NH2-type MOFs, where the metal cluster composition is varied from 100% Zr, a mixture of Zr/Ce (80/20%), and a mixture of Zr/Ce/Ti (70/7/23%). In comparison with the linker photodynamics, we observe the opening of new deactivation channels in the MOFs, associated with the LCCT phenomenon with time constants of 5–7 ps and 70–90 ps. Upon the successive incorporation of Ce and Ce/Ti in the MOFs, the LCCT becomes more efficient, which is reflected in the quenching of the photoluminescence quantum yield and in the generation of a higher population of CSSs, demonstrated by the flash photolysis experiments. These observations are in agreement with the photodegradation of a pollutant dye, Nile Blue, where the trimetallic MOF shows higher activity than the monometallic one. Hence, our results provide conclusive experimental evidence for the relationship between the photodynamics of the MMOFs and their photocatalytic activities.