Enhanced luminescence of Au₂₂(SG)₁₈ nanoclusters via rational surface engineering

We report design strategies for the preparation of highly luminescent Au₂₂(SG)₁₈ clusters, where SG is glutathione, by the functionalization of the cluster shell. In these strategies, the cluster shell was covalently modified with small aromatic molecules and pyrene chromophores that led to a 5-fold PL enhancement by rigidifying the shell-gold. Highly luminescent water-soluble gold clusters with a PL quantum yield of 30% were obtained at room temperature. To further enhance the luminescence, the pyrene chromophores in the functionalized Au₂₂ clusters were photoexcited at 350 nm to induce energy transfer from pyrene to the Au₂₂ cluster. Steady-state and time-resolved PL measurements have shown evidence of enhanced rigidity with increased PL lifetimes for the functionalized Au₂₂ clusters. However, the energy transfer efficiency was found to be only 14% because of the competing electron transfer deactivation pathway as evidenced by the formation of the pyrene anion radical revealed in the ultrafast transient absorption measurements. To suppress the electron transfer pathway, the pyrene functionalized Au₂₂ clusters were ion-paired with tetraoctylammonium (TOA) cations that could break the electron transfer pathway, leading to a dramatic 37-fold increase in PL brightness with the resonance energy transfer efficiency of ca. 80%. This work presents effective design strategies for the preparation of highly luminescent gold clusters by the combination of rigidifying effect and energy transfer sensitization.