Excited-State Pathways in Near-Infrared Emitting DNA-Stabilized Silver Nanoclusters with Different Geometries

DNA-stabilized silver nanoclusters (DNA-AgNCs) have emerged as a promising class of biocompatible fluorophores with tunable emission wavelengths and lifetimes governed by the scaffolding DNA sequence. Unlike conventional organic ligand-protected metal clusters, the structure-photophysical correlation of DNA-AgNCs has not been fully elucidated due to the paucity of crystal structures. To address this, we employ ultrafast transient absorption spectroscopy to unravel the excited-state relaxation processes in three DNA-AgNCs with distinct rod-like and spherical geometries. For the rod-like DNA-AgNC, we observe a nanosecond fluorescent state forming from the Franck-Condon state, followed by the formation of a microsecond-lived state from the nanosecond excited state. For the two DNA-AgNCs with suggested spherical geometries, a microsecond-lived luminescent state is directly formed from the Franck-Condon state on a subpicosecond time scale. These new insights into the relationship between shape and luminescence response will contribute to the photophysical understanding and structural engineering of DNA-AgNCs for specific imaging applications.