Mechanism of Photoluminescence in Ag Nanoclusters: Metal-Centered Emission versus Synergistic Effect in Ligand-Centered Emission

It remains unclear whether the emission center of ligand-encapsulated metal nanoclusters (MNCs) is the surface ligands or the metal core. In this paper, we simultaneously observed metal-centered and ligand-centered emissions in Ag nanoclusters. The contributions of the surface ligands and the metal core were individually investigated to understand the nature of AgNC photoemission. A new ligand synergistic emission effect was observed. The amino correlated nπ∗ state provides a pivot to bridge the carboxyl correlated ππ∗ and nπ∗ states to enhance the charge transfer efficiency between different surface electronic states. Consequently, the photoluminescence quantum yields were significantly improved (∼1 to ∼10%). Transient absorption studies revealed that decreasing the pH could expand the potential energy curve and generate a conical intersection. This would facilitate the charge transfer and relaxation of excited electrons via a radiative pathway, thereby enhancing the emission intensity. These new insights into the photoemission mechanisms of MNCs should stimulate additional experimental and theoretical studies and could benefit the molecular-level design of luminescent MNCs for optoelectronics and other applications.