| Summary: | Abstract The effects of halide ligands on the photoluminescence (PL) and photoredox properties were examined on [IrAu12(dppe)5X2]+ (1X; X=Cl, Br, I; dppe=1,2‐bis(diphenylphosphino)ethane). The PL quantum yield was enhanced in the order of 1Cl (67 %)<1Br (75 %)<1I (77 %). The lifetime of the photoexcited state was shortened in the order of 1Cl>1Br>1I due to the acceleration of radiative relaxation by the heavy atom effect in 1I. Despite the shortest PL lifetime, cluster 1I exhibited the highest conversion in the photocatalytic [2+2] cycloaddition of bisenone. The PL from 1Cl and 1Br was quenched by adding Lewis acid Li+, required for activation of bisenone, whereas that from 1I remained unaffected. Theoretical calculations showed that the Au−Cl and Au−Br bonds of 1Cl and 1Br were weakened by interaction with Li+, while the Au−I bond of 1I was not. Clusters 1Cl and 1Br underwent halide exchange with isocyanide in the presence of Li+, whereas 1I did not. These results suggested that the photoredox activity of 1Cl and 1Br was suppressed by the Li+‐induced nonradiative quenching of the photoexcited state. This study highlights the unexpected importance of halide ligands in the design of efficient cluster photocatalysts.
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