Geminate Recombination versus Cage Escape in the Reductive Quenching of a Re(I) Carbonyl Complex on Mesoporous ZrO2

In the wider context of artificial photosynthesis, this work aims to explore the functionality and characteristics of rhenium tricarbonyl complexes as photosensitizers in heterogeneous water reduction systems. To that end, the reductive quenching of an excited (ReCl)-Cl-I(2,2'-bipyridine-4,4'-bisphosphonic acid)(CO)(3) adsorbed on a redox-neutral scaffold by phenothiazine was observed by transient IR spectroscopy. From the spectroscopic and time response, the full reaction cycle could be elucidated and the intrinsic lifetime of the excited Re complex, together with rates for reductive quenching, cage escape, as well as geminate recombination and secondary back electron transfer could be determined. Three different scenarios have been explored, which decrease the mobility of the reactants in the system in a stepwise manner: Starting from a reference system with all compounds in solution, first the Re complex was immobilized on the surface, and in a second step also the quencher. The overall reaction cycle for all reactants in solution is preserved as long as the quencher is in solution, with relatively minor changes of the rates of the individual reaction steps. The overall cage escape yield was found to be larger on the surface. As soon as the quencher is coadsorbed alongside the Re complex, however, the reaction cycle changes completely. Electron transfer occurred only from quencher molecules that sit next to an excited Re complex, and there is no possibility of cage escape. Varying the ratio of quencher molecules and Re complexes, it is concluded that molecules do not cluster on the surface and that excitation energy migration is not a very efficient process.