Confined Structural Water Molecules as Alternative Potential Emitters for Bright Photoluminescence of Thiolate-Gold Complexes

The debate about water as an emitter has spanned nearly a century, but how it emits bright colors remains elusive. In this report, using the widely used Au(I)-alkanethiolate complex (Au(I)-SRs, R=−(CH₂)₁₂H) with AIE properties as a model system, by carefully manipulating the delicate surface-ligand interactions at the nanoscale interface, together with a careful spectral investigation and an isotopic diagnostic experiment of heavy water (D₂O), we demonstrated that the structural water molecules (SWs) trapped in the nanoscale interface or space are the true emission centers of metal nanoclusters (NCs) and the aggregates of Au(I)-SRs complexes, instead of a well-organized metal core dominated by quantum confinement mechanics. Unlike conventional hydrogen-bonded water molecules, due to interfacial adsorption or spatial confinement, the p-orbitals of two O atoms in SWs can form strong electronic interactions through spatial overlap, thus constructing a set of interfacial states, one of which is characterized by π-bonding, thus providing alternative channels (or paths) for the relaxation decay of the excited electrons. Using the one-dimensional free-electron gas model, the energy levels calculated by the Schrödinger equation are in perfect agreement with the experimental observations, further validating the SWs model.