Fluorescence band splitting of L-histidine functionalised copper nanoclusters in micro-heterogeneous aqueous ethanol mixtures.

The solvent effects on the fluorescence spectra of copper nanoclusters (CuNCs) are significant, influenced by solvent polarity, hydrogen bonding networks, conformational changes, charge transfer, and inter-nanocluster interactions. Herein, L-histidine-functionalised copper nanoclusters (L-His-CuNCs) with ∼3 nm size exhibit broad symmetric fluorescence at 470 nm that varies with weak asymmetric hydrogen bonding in aqueous ethanol mixtures. This study identifies aggregation-induced emission enhancement and fluorescence band splitting, leading to two blue-shifted peaks at 430 nm and 460 nm as the ethanol concentration rises from 0 to 95 vol%. This shift is attributed to destabilised excited states and inter-nanocluster interactions inside aggregates. The band splitting likely originates from micro-heterogeneous mixed domains of ethanol and water, yielding distinct emissive subpopulations, modulating primarily surface states while the core states have minimal effect. DLS and zeta potential reveal a correlation between the splitting band intensities at ethanol compositions with solvent-mediated dehydration of the CuNC surface and charge re-distribution of L-His imidazole ligands. Fluorescence enhancement and faster excited-state decay dynamics indicate restricted intramolecular motion in the aggregates. These findings highlight the role of the solvent hydrogen bonding network in altering the photophysics of CuNCs, offering physical insights for developing polarity-responsive fluorescent probes and improving the decay dynamics of the CuNCs in bioimaging and photonics applications.