The availability of a range of excited states has enriched zero-, one- and two- dimensional quantum nanomaterials with interesting luminescence properties, in particular for noble metal nanoclusters (NCs) as typical examples. But, the elucidation and origin of optoelectronic properties remains elusive. In this report, using widely used Au(I)-alkanethiolate complex (Au(I)-SRs, R = -(CH2)12H) with AIE characteristics as a model system, by judiciously manipulating the delicate surface ligand interactions at the nanoscale interface, together with a careful spectral investigations and an isotope diagnostic experiment of heavy water (D2O), we evidenced that the structural water molecules (SWs) confined in the nanoscale interface or space are real emitter centers for photoluminescence (PL) of metal NCs and the aggregate of Au(I)-SRs complexes, instead of well-organized metal core dominated by quantum confinement mechanics. Interestingly, the aggregation of Au(I)-SRs generated dual fluorescence-phosphorescence emission and the photoluminescence intensity was independent on the degree of aggregation but showed strong dependency on the content and state of structural water molecules (SWs) confined in the aggregates. SWs are different from traditional hydrogen bonded water molecules, wherein, due to interfacial adsorption or spatial confinement, the p orbitals of two O atoms in SWs can form a weak electron interaction through spatial overlapping, which concomitantly constructs a group of interfacial states with π bond characteristics, consequently providing some alternative channels (or pathways) to the radiation and/or non-radiation relaxation of electrons. Our results provide completely new insights to understand the fascinating properties (including photoluminescence, catalysis and chirality, etc.) of other low-dimension quantum dots and even for aggregation-induced emission luminophores (AIEgens). This also answers the century old debate on whether and how water molecules emit bright color.
We first provided the key evidences that the distribution of surface protecting ligands on the metal core played a paramount role to tune the optoelectronic properties of noble metal NCs by series of series systematic investigations ( J. Am. Chem. Soc. 2014, 136, 5, 1686; J. Phys. Chem. Lett. 2017, 8, 17, 3980; Communications Chemistry, 2019, 2 132; Nanomaterials 2020, 10(2), 261; ; Nanoscale, 2021, 13, 15058; Front. Chem. 9:756993; ACS Phys. Chem Au 2021, https://doi.org/10.1021/acsphyschemau.1c00020) in the past decade, but the definitive assignment on the real emitter center of metal NCs are not clearly addressed by the experiment. . Our results answers the century old debate on whether and how water molecules emit bright color (Ewles, J. Nature 1930, 125, 706; Przibram, K. Nature 1958, 182, 520).