Origin of the Bright Photoluminescence of Thiolate-protected Gold Nanoclusters: Confined Structural Water Molecules as Real Emitters

Abstract

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.

Structural water molecules dominated p band intermediate states as a unified model for the origin on the photoluminescence emission of noble metal nanoclusters: from monolayer protected clusters to cage confined nanoclusters

“…For instance, strong Cotton effects (circular dichroism, CD) of NMNCs protected by chiral ligands have been reported a long time ago [ 40 , 43 ], e.g . the in-situ-formed Ag(I)-thiolate polymers upon mixing AgNO 3 and chiral ligand (cysteine) exhibited intense absorption and CD signals in the range 200–400 nm, which affords signal amplification for cysteine sensing [ 45 ], whereas, the absorption characteristics of the chiral Ag(I)-thiolate polymers were almost identical to that of Au(I)-thiolate polymer using achiral dodecanethiol as ligand reported in our recent research work [ 121 , 122 ], which have been, herein, ascribed to the formation of PBIS within SWs on the NMNCs, that means the new-formed CD signals of these NMNCs can be also well rationalized by the SWs dominated PBIS model.…”

Green carbon science: fundamental aspects

“…Based on the molecular orbital theory (MO), the essence of the PBIS can be simply understood as that, due to the spatial overlap of the orbital of the adsorbed atoms (or molecules), two pairs of local electrons in surface chemical bonds can be highly delocalized into four interfacial bonding regions (Fig. 8c ), like ‘electron pool’ at the nanoscale interface ( here, localized electrons must invariably be transformed to a collective-electron description, or called ‘interface conjugation or delocalization’ ) [ 31 ]. The physical nature of PBIS is exactly the same as the transition state (TS) of the reaction intermediate, where the old bond is not completely broken and the new bond is not completely formed.…”

“…Using the PBIS model (Fig. 8 ) [ 31–33 ], we proposed that, the structural water molecules (SWs) adsorbed on the cubane cluster play a main role in mediating synergetic electron and proton transfer. When two centers of oxidation and reduction are far away, for example, the long-range electron and proton transfer between PSII and PSI could occur by water bridge or water line in the enzyme pocket, where two more water molecules are linked together by p orbital overlaps [ 40 ].…”