Evidence for triplet-state-dominated luminescence in biicosahedral superatomic molecular Au₂₅ clusters

In photoluminescence (PL) quenching and triplet fusion upconversion experiments with fluorescent organic-molecule quenchers, it was revealed that a rod-shaped, phosphine- and thiolate-protected biicosahedral Au₂₅ cluster (a representative di-superatomic molecule) exhibits only phosphorescence, not fluorescence, at room temperature with an intersystem crossing quantum yield of almost 100%. By virtue of these photophysical properties, this cluster can be used as a triplet sensitizer that undergoes direct singlet–triplet transitions in the near-infrared (NIR) region (730–900 nm), inducing photon upconversion from NIR to visible light.