ABSTRACT
Covalently linked photosensitizer-polyoxometalate (PS-POM) dyads are promising molecular systems for light-induced energy conversion processes, such as "solar" hydrogen generation. To date, very little is known of their fundamental photophysical properties which affect the catalytic reactivity and stability of the systems. PS-POM dyads often feature short-lived photoinduced charge-separated states, and the lifetimes of these states are considered crucial for the function of PS-POM dyads in molecular photocatalysis. Hence, strategies have been developed to extend the lifetimes of the photoinduced charge-separated states, either by tuning the PS photophysics or by tuning the POM redox properties. Recently, some of us reported PS-POM dyads based on cyclometalated IrIII complexes covalently linked to Anderson-type polyoxometalate. Distinct hydrogen evolution reactivity (HER) of the dyads was observed, which was tuned by varying the central metal ion M of the POMM (M=Mn3+ , Co3+ , Fe3+ ). In this manuscript, the photoinduced electron-transfer processes in the three Ir-POMM dyads are investigated to rationalize the underlying reasons for the differences in HER activity observed. We report that upon excitation of the IrIII complex, ultrafast (sub-ps) charge separation occurs, leading to different amounts of the charge-separated states (Ir.+ -POMM .- ) generated in the different dyads. However, in all dyads studied, the resulting Ir.+ -POMM .- species are short-lived (sub-ns) when compared to reference electron acceptors (e.g. porphyrins or fullerenes) reported in the literature. The reductive quenching of Ir.+ -POMM .- by a sacrificial donor, triethyl amine (1 m), to generate the intermediate Ir-POMM .- is estimated to be very efficient (70-80 %) for all dyads studied. Based on this analyses, we conclude that the yield instead of the lifetime of the Ir.+ -POMM .- charge-separated state determines the catalytic capacity of the dyads investigated. This new feature in the PS-POM photophysics could lead to new design criteria for the development of novel PS-POM dyads.
ABSTRACT
The visible-light-driven hydrogen evolution reaction (HER) by covalent photosensitizer-catalyst dyads is one of the most elegant concepts in supramolecular homogeneous solar energy conversion. The intricacies of catalyst reactivity and photosensitizer-catalyst interactions require a detailed fundamental understanding of the system to rationalize the observed reactivities. Here, we report three dyads based on the covalent imine-bond linkage of an iridium photosensitizer and an organo-functionalized Anderson polyoxometalate anion [MMo6 O18 {(OCH2 )3 CNH2 }2 ]3- (M=Mn3+ , Fe3+ , Co3+ ). Modification of the central metal ion M is used to modulate the HER activity. Detailed theoretical and experimental studies examine the role of the central metal ion M and provide critical understanding of the redox activity and light-driven HER activity of the novel dyads. Thus, the study enables a knowledge-based optimization of HER dyads by chemical modification of the reactive metal oxide components.
ABSTRACT
A general concept for the covalent linkage of coordination compounds to bipyridine-functionalized polyoxometalates is presented. The new route is used to link an iridium photosensitizer to an Anderson-type hydrogen-evolution catalyst. This covalent dyad catalyzes the visible-light-driven hydrogen evolution reaction (HER) and shows superior HER activity compared with the non-covalent reference. Hydrogen evolution is observed over periods >1â week. Spectroscopic, photophysical, and electrochemical analyses give initial insight into the stability, electronic structure, and reactivity of the dyad. The results demonstrate that the proposed linkage concept allows synergistic covalent interactions between functional coordination compounds and reactive molecular metal oxides.