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

ABSTRACT

In the past several decades, noble metal nanoclusters (NMNCs) have been developed as an emerging class of luminescent materials due to their superior photo-stability and biocompatibility, but their luminous quantum yield is relatively low and the physical origin of the bright photoluminescence (PL) of NMNCs remain elusive, which limited their practical application. As the well-defined structure and composition of NMNCs have been determined, in this mini-review, the effect of each component (metal core, ligand shell and interfacial water) on their PL properties and corresponded working mechanism were comprehensively introduced, and a model that structural water molecules dominated p band intermediate state was proposed to give a unified understanding on the PL mechanism of NMNCs and a further perspective to the future developments of NMNCs by revisiting the development of our studies on the PL mechanism of NMNCs in the past decade.

GRAPHICAL ABSTRACT

1. Introduction

‘Atoms on a small scale behave like nothing on a large scale, for they satisfy the laws of quantum mechanics’. As early as 1959, physicist Richard Phillip Feynman proposed the idea of arranging atoms on a microscopic scale and predicted the unique properties of small-size materials compared to macroscopic ones [1] With the continuous improvement of synthesis methodology and characterization methods, the synthesis of nanomaterials has achieved rapid development in the past two decades. A prominent case is that nano-sized noble metal (nanoparticles or nanoclusters) with magic atom numbers, also called the atomically precise NCs, can be synthesized at the molecular level [2–7], and as expected, these tiny particles or clusters exhibited unique electronic and optical properties, such as molecule-like energy gaps [8–13], photoluminescence (PL) [6,14–27], catalytic properties [28–39] and chirality [40–47]. In particular, the PL phenomenon is one of the most intriguing characteristics of NMNCs compared to bulk metals, which fascinated scientists in many areas of research and applications [48,49], but the luminous quantum yield of NMNCs is still relatively low [21]. These prominent optical properties of NMNCs were initially attributed to the quantum size confinement effect [14,50], whereas, the efficiency and wavelength of the luminescence of NMNCs was not only dictated by the size, composition and valence state of metal core [51–53], but also by the type, architecture of surface ligands [17,54–61], etc. Although the atomic structure of NMNCs can be determined [2,62], the elucidation on the origin of the optoelectronic properties of metal NCs is still diverse and even contradictory [21,63]. Thus it is highly important to describe the chemical and physical origin, which can guide a rational synthesis of highly luminescent NMNCs. This review will therefore mainly focus on the summary of the PL mechanism of NMNCs and give a further perspective to the future developments of NMNCs based on the literature survey and our long-term studies in the past decade, and the content of this review is organized as follows: in the first section, we briefly introduce the composition and structure of NMNCs and classify the NMNCs into two categories: organic ligand protected NMNCs and inorganic scaffold confined NMNCs. The second section describes the effect of each component (metal core, ligand shell and interfacial water) of NMNCs on the PL properties and corresponded working mechanism (quantum confinement effect (QCE), ligand to metal charge transfer (LMCT) or ligand to metal-metal charge transfer (LMMCT) and metallophilic interaction beyond aggregation-induced emission (AIE) effect, etc.). In the last section, the historical development of our studies on the PL mechanism of NMNCs is introduced, and a model that structural water molecules (SWs) dominated PL of NMNCs is addressed. We hope that this reviews will provide new insights to understanding not only the PL mechanism but also other unique properties (such as catalysis, chirality, etc.) of NMNCs and other low-dimensional nanomaterials (carbon dots, AIEgens, semiconductor quantum dots, etc.).

2. The composition and structure of NMNCs

The bulk metal has a continuous energy level structure and exhibits good electrical conductivity and metallic luster. When the size of metals is reduced to the nanometer level (2–100 nm), the density of states (DOS) in the band decreases [64], metal nanoparticles (NPs) exhibit obvious surface plasma resonance (SPR) absorption, which is caused by the collective oscillation of the conduction band electrons under optical radiation [65], at which time the noble metal NPs still have a quasi-continuous energy level structure that the gap between the highest occupied and the lowest unoccupied state (called the Kubo gap δ) is still less than thermal energy (kT), showing the properties of a metallic state [64]. When the size of noble metal NPs further decrease to less than 2 nm, the DOS in the band further decreases and the Kubo gap is larger than thermal energy (kT). Noble metal NPs at this scale are generally called noble metal nanoclusters (nanoclusters, NCs), which bridge the gap between organometallics and nanocrystals and thus show discrete energy level structure and molecule-like absorption [3–5,16,21,66–70]. Figure 1(a) is a schematic diagram of evolution of the energy level structure of metals as a function of size. And specifically, the critical number of gold atoms for a metallic state NCs has been determined recently, in which atomically precise gold nanoclusters Au₂₄₆(SR)₈₀ is nonmetallic but Au₂₇₉(SR)₈₄ (M_nL_m, n and m represent the number of metal atoms and ligands, respectively. SR: thiolate) is metallic (Figure 1(b)) [71,72].

Figure 1.

(a) Schematic illustration of the energy level of metal with decreased size [64]. Reproduced with permission from ref. [64]. Copyright 2006 the royal society of chemistry. (b) Schematic diagram of the grand transition from nonmetallic state to metallic state in Au NCs [71]. Reproduced with permission from ref. [7]. Copyright 2021, American chemical society.

Bare NMNCs are not stable in solution due to their high surface energy, thus NMNCs are generally protected by organic or inorganic scaffold. According to the difference in the properties of the protective scaffold on the surface of noble metal NCs, herein, noble metal NCs can be mainly divided into two categories: one is organic ligand protected NMNCs [2,73]; the other is inorganic scaffold confined NMNCs [6,74]. Both type of NMNCs have well-defined structure and compositions, as shown in Figure 2, organic ligand protected NMNCs mainly comprised by zero-valent metal core and metal(I)-ligand complex shell on the surface (Figure 2(a)): The crystal structure and the number of gold atoms and ligands can be accurately analyzed at the atomic level, and the NMNCs protected by atomically accurate organic ligands can be synthesized with molecular-level purity [3,4]. The inorganic scaffold confined NMNCs mainly consist of metal cores, adsorbed water molecules, and confined pores of inorganic templates (Figure 2(b)): No matter organic ligand protected NMNCs or inorganic scaffold confined NMNCs, each component of NMNCs shows a significant impact on the optical properties of NMNCs, which complicated the investigation on the PL mechanism of NMNCs. Besides, there are many different photophysical process occurring in NMNCs, mainly including radiative relaxation pathways (e.g. near-infrared fluorescence (NIRF)) and non-radiative pathways (e.g. heat, photoacoustic (PA) and reactive oxygen species (ROS)) as shown in Figure 3(a) (take ligand protected gold nanoclusters as an example), only radiative relaxation pathways (occurring from surface, core and triplet states) can lead to a rich and color-tunable PL of NMNCs [49]. For instance, the visible and near-infrared emissions in typical [Au₂₅(SR)₁₈]⁻ nanoclusters have been recently attributed to the core-shell charge transfer state and the electron decay of the Au₁₃ core excited state, respectively (Figure 3(b)) [75]. The authors conclude that these low-energy decay dynamics of Au NCs follows the multi-photon process, which could be found the interesting applications in the field of oncology [49]. Although a lot of types of NMNCs differed in the metal and/or ligand have been reported in the literature, the luminescence mechanism of them could follow the same principle, in this review, we will mainly focus on the systems of gold and silver-based nanoclusters, in some cases, the metal(I)-thiolates complexes are also introduced. In the following section 2 and section 3, we will separately introduce the effect of each component (metal core, ligand shell and interfacial water) of NMNCs on the PL properties but will not strictly separate these nanosystems to explain the origin of the PL and try to give a unified understanding of the luminescence mechanism of NMNCs.

Figure 2.

Schematic representation of (a) monolayer-protected Au NCs, Au(I)-thiolate complexes (Au(i)-SRs) [73] and (b) a hydrated Ag₄ cluster inside a sodalite zeolite cage [74]. Reproduced with permission from ref. [73]. Copyright 2007 American association for the advancement of science and ref. [74]. Copyright 2018 American association for the advancement of science.

Figure 3.

(a) Proposed mechanism for relaxation pathways in protected gold nanoclusters following (multi) photon absorption. NIRF: NIR fluorescence, PA: Photoacoustic, ROS: Reactive oxygen species, ISC: Inter system crossing [49]. (b) Mechanism of emissions in [Au₂₅(SR)₁₈]⁻ nanoclusters [75]. Reproduced with permission from ref. [75]. Copyright 2021 American chemical society.

2. Luminescence mechanism of NMNCs

2.1. The effect of metal core

In 1969, Mooradian firstly observed the photoluminescence of gold and copper films with the emission energy centered at 2.2 and 2.0 eV respectively (Figure 4(a)) [76], but the quantum yields were extremely low (~10⁻¹⁰), as bulk metal has a continuous energy level structure within the band, the emission was therefore attributed to the interband transition between electrons in conduction-band states below the Fermi level and holes in the d bands (Figure 4(b)). The PL intensity of bulk gold films can be increased by about 8 times through improving the roughness of the metal surface [78], but when the size of gold was further reduced to gold nanorods with a diameter of 20 nm, its fluorescence quantum yield was increased about one million times [79]. The size of metal obviously showed a more prominent effect on its PL properties, whereas, the luminescence properties of bulk metal and large metal NPs did not attract much more attention from photochemists and photophysicists due to their low quantum yields (<10⁻⁴) at this stage. With the development of synthesis methodology and characterization methods, smaller or even atomically precise metal NCs can be facilely synthesized, and these metal NCs exhibited more unique optical properties [3–5,16,21,66–70]. In 1998, Wilcoxon et al. investigated the size effect on the luminescence properties of nanosize gold clusters through liquid chromatographic analysis [24], they demonstrated that the large gold clusters with the size of 15 nm are non-luminescent, while the smaller gold clusters with the size of 5 nm or 2.5 nm can exhibit relatively intense visible photoluminescence with quantum yield of 10⁻⁴ ~ 10⁻⁵, notably, the emission is also dependent on the nature of the metal NCs surface (capping ligand), and the large, nonemitting gold nanoparticles can also be made luminescent by partial dissolution using potassium cyanide. In 2001, Huang and Murray prepared water-soluble Au NCs with different sizes (1.8 nm, 2.2 nm, 3.1 nm and 3.9 nm) by etching [26], only Au NCs with the size of 1.8 nm showed intense fluorescence, and its quantum yield reached 0.3%, while Au NCs with other sizes were not fluorescent; similarly, the efficiency and wavelength of the luminescence was also dependent on the surface ligand. At this stage, the luminescence mechanism of Au NCs is still considered to be consistent with the luminescence mechanism of bulk metals, that is the band gap transitions between the fully filled 5d¹⁰ band of a metal such as gold and the underfilled 6sp¹ band (Figure 4(b)), but the surface ligand was highlighted to influence the efficiencies and energies of the 6sp emitter states, the size effect on the luminescence properties of nano-size gold NCs is still not well-elucidated. Whetten et al. isolated a series of Au NCs in solution by fractional crystallization or column chromatography [50], these Au NCs are distinguished by a crystalline (or quasicrystalline) core of densely packed Au atoms, ranging in size from~1.1 nm (~40 atoms) to ~1.9 nm (~200 atoms), surrounded by surface ligand based on the characterization of mass spectrometry and high-resolution transmission electron microscopy (TEM), unlike bulk gold and large-sized Au NPs or Au NCs, Au NCs of corresponding size exhibit a size-dependent energy level structure in the UV-Vis absorption spectrum, indicative of transitions to the discrete lowest unoccupied levels of the conduction band due to the quantum confinement effect, that is, the transitions between discrete energy levels within the 6sp energy band (Figure 4(d)). Afterwards, Whetten et al. further investigated the luminescence properties of these molecular-like Au NCs [8,25,77,80]. The Au NCs composed of a 28-atom core and a glutathione (GSH) adsorbate layer consisting of 16 molecules exhibited two emission bands at 1.5 and 1.15 eV (Figure 4(c)), which were assigned to the recombination of the excited electron from higher excited states in the sp band with the hole in the lower-lying d-band (interband transition) and the relaxed radiative recombination across the gap between the highest occupied and lowest unoccupied molecular orbitals at 1.3 eV within the sp-conduction band (intraband transition), respectively. Thus the quantum confinement effect (QCE) was applied to account for the size effect on the luminescence properties of NMNCs. According to the mechanism of QCE, the emission wavelength of Au NCs can be tuned by changing the size of noble metal NCs. In 2002, Dickson et al. synthesized Au NCs with different sizes (Au₅→Au₃₁) using polyamide as a template [14], as-obtained Au NCs exhibit size-dependent fluorescence emission from ultraviolet to near-infrared light (Figure 5(a)), where the relationship between the emission wavelength (λ_em) of Au NCs and the number of gold atoms (N) approximately conforms to the relationship λ_em = E_Fermi/N_1/3, where E_Fermi represents the Fermi level of Au (Figure 5(b)). Besides, similar size-dependent optical properties was also observed in other NMNCs. For instance, Tsukuda et al. prepared glutathione (GSH) protected gold NCs by reductive decomposition of Au(I)-SG polymers [10,81], as-obtained Au NCs solution was separated into a number of fractions by polyacrylamide gel electrophoresis (PAGE), the fractionated nanoclusters are assigned to Au₁₀(SG)₁₀, Au₁₅(SG)₁₃, Au₁₈(SG)₁₄, Au₂₂(SG)₁₆, Au₂₂(SG)₁₇, Au₂₅(SG)₁₈, Au₂₉(SG)₂₀, Au₃₃(SG)₂₂, and Au₃₉(SG)₂₄ by electrospray ionization (ESI) mass spectrometry. As shown in Figure 5(c), these gold NCs aqueous solution shows size-dependent absorption in the UV-visible region and thus exhibited different colors in the optical images. These results are in good agreement with the quantum confinement mechanism. However, actually, most of NMNCs exhibited weak or undetectable luminescence [21], the QCE mechanism is difficult to reasonably explain the difference in the fluorescence quantum efficiency of noble metal NCs. In addition, the fluorescence emission wavelength of noble metal NCs is not always size-dependent. For example, in 2012, Xie et al. synthesized a series of atomically precise glutathione (GSH) ligand-protected noble metal NCs (Au₂₉SG₂₇, Au₃₀SG₂₈, Au₃₆SG₃₂, Au₃₉SG₃₅ and Au₄₃SG₃₇) [55], as the number of gold atoms increases, the fluorescence emission wavelength of Au NCs blue-shifts from 620 nm to 605 nm, the size effect of Au NCs is not obvious, and the variation of emission wavelength with size does not satisfy the QCE mechanism. Zheng et al synthesized Au NCs with the same size (~2.5 nm), but exhibited fluorescence emission at 600 nm and 810 nm, respectively, due to the different bonding modes of ligands on the surface [53]. These abnormal or conflicting experimental results indicated that the QCE mechanisms have significant limitations in explaining the luminescence properties of NMMCs, the surface ligand also shows a significant impact on the optical properties of NMNCs [63].

Figure 4.

Photoemission spectra of (a) bulk gold and copper films and (c) gold nanoclusters Au₂₈(SG)₁₆ [76,77]. Schematic illustration of (b) interband and (d) intraband excitation and emission [76,77]. Reproduced with permission from ref. [76]. Copyright 1969 American physical society and ref. [77]. Copyright 2002, American chemical society.

Figure 5.

(a) Photoemission spectrum of Au NCs with different gold atoms. (b) Relationship between the number of gold atoms and emission energy [14]. Reproduced with permission from ref. [14]. Copyright 2004 American physical society. (c) Digital photographs (top) and optical absorption spectra (down) of aqueous solutions of Au NCs with different gold atoms [10,81]. Reproduced with permission from ref. [10]. Copyright 2005 American chemical society and ref. [81]. Copyright 2007 Wiley-VCH.

2.2. The effect of ligand shell

Organic ligands [3,4,21,29,68], such as small organic molecules [82], polymers [51], proteins and DNA [83–87], are typically used to control the particle size of metal core and protect tiny metal clusters from irreversible aggregation in the synthesis of NMNCs, whereas, more and more experimental results demonstrate that the optical properties of NMNCs are highly dependent on the nature and aggregation or assembly state of surface ligand. In 2004, Murray et al. investigated the ligand effect on the luminescence properties of Au NCs through the methodology of ligand exchange [9]. The PL of Au₃₈ NCs was more intense after partially substituting the phenethylthiol (PhC₂SH) ligand of Au₃₈(PhC₂S)₂₄ by a thiol-substituted polyethylene glycol ligand (PEG₁₃₅SH) without changing the number of gold atoms, indicating that the luminescence properties of noble metal NCs are closely related to the nature of surface ligands. In 2010, Jin et al. studied in detail the role of ligand in the fluorescence of atomically monodisperse, 25-atom gold nanoclusters (Au₂₅SR₁₈, -SR = SC₂H₄Ph, SC₆H₁₃, SC₁₂H₂₅ and -SG (glutathione)) [54]. The fluorescence intensity of Au₂₅SR₁₈ NCs increased with the enhancement of the electron-donating ability of the ligand and the increase in the oxidation state of Au NCs (Figure 6(a,b)). The authors suggest that the surface ligands can influence the fluorescence in two different ways: (i) charge transfer from the ligands to the metal nanoparticle core (i.e., LMNCT) through the Au-S bonds, and (ii) direct donation of delocalized electrons of electron-rich atoms or groups of the ligands to the metal core (Figure 6(d)). Thus the fluorescence of Au NCs further enhanced by employing ligands (long-chain peptide nucleic acid, PNA) with electron-rich atoms and groups (Figure 6(c)), but the fluorescence efficiency of these noble metal NCs is still very low (QY < 10⁻³). Intriguingly, in 2012, Xie et al. discovered that non-luminescent Au(I)-thiolate (Au(I)-SRs) complexes can generate strong luminescence emission after dense aggregation by either a solvent-induced or cation-induced method (Figure 7(a-d)) [55], which is reminiscent of the fluorescence enhancement phenomenon of organic chromophores in the aggregated state, namely AIE, discovered by Tang et al. in 2001 (Figure 7(e-g)) [88]. The intensity and color of the luminescence were largely determined by the degree of aggregation, the emission from the Au(I)-SRs aggregates was directly attributed to the ligand-to-metal charge transfer (LMCT) or ligand-to-metal-metal charge transfer (LMMCT) from the sulfur atom in the thiolate ligands to the Au atoms, and subsequent radiative relaxation. The authors suggested that the stronger intra- and inter-complex aurophilic Au(I)···Au(I) interactions and more restrained molecular vibrations within these Au(I)-SRs aggregates were the possible two reasons for the luminescence enhancement of Au(I)-SRs complexes in the aggregated state. Besides, based on the mechanics of AIE, the authors prepared highly luminescent Au-thiolate NCs with a Au(0)@Au(I)-thiolate core-shell structure, in which the emission wavelength of the luminescence was identical to that of Au(I)-SRs complexes, and its quantum yield can reach about~15%. These results further emphasized the crucial role of ligand or metal(I) complexes in the PL of NMNCs and inspired the researchers to reconsider the contribution of metal core and ligand in the PL of NMNCs. Note that, the d¹⁰ coinage metal organic chalcogenolates (MOCs) with the formula [M(ER)]_n, where M = Cu(I), Ag(I), Au(I) and ER = -SR, -SeR, -TeR, is also an intriguing type of luminescent materials [89–92], they can exist as oligomeric species or coordination polymers in the solution or solid state, and the photoemission of MOCs was also highly dependent on their structure and the ligand, but the PL mechanism of MOCs also remains elusive. In the follow-up research work, Xie et al. further revealed the impact of surface Au(I)-thiolate motifs on the AIE properties of Au NCs by employing a series of atomically precise glutathione coordinated Au complexes and NCs as a model (Au₁₀SR₁₀, Au₁₅SR₁₃, Au₁₈SR₁₄, and Au₂₅SR₁₈) [56]. Spectroscopic investigations show that the emission energy and emission pathway of Au NCs was highly dependent on the length or size of the Au(I)-SRs motifs (Figure 8(a,b)), Au NCs with longer Au(I)-SRs motif showed a strong AIE effect and thus exhibited an AIE-type phosphorescence with longer lifetime and higher emission energy, whereas, Au NCs with shorter Au(I)-SRs motif only exhibited an Au(0)-core-dictated fluorescence with shorter lifetime and lower emission energy (Figure 8(c)). Indeed, AIE is an effective strategy to synthesize highly luminescent NMNCs [93–99], and there is no doubt that the restriction of intramolecular motion could result in the luminescence enhancement from AIE luminogens (AIEgens) in the aggregated state due to the decrease in the probability of non-radiative relaxation of the excited states [100,101]. Whereas, the origin of the emission of metal(I)-complexes [94] and other organic small molecules without conjugated chromophore [102] but showing AIE effect is still mysterious [103], the metallophilic interaction, a specific intra- and intermolecular bonding between closed-shell metal centers such as Au⁺[5d₁₀] [104], were generally involved in the accounting for the AIE effect of metal(I)-complexes, however, the physical origin of the metallophilic interaction and the relationship between metallophilic interaction and optical properties of metal(I)-complexes aggregates are still unclear.

Figure 6.

Photoemission spectra of Au₂₅ species with different ligand (a) or different charge states (b) or the addition of PNA (c); (d) Schematic illustration of possible interactions of amine and carboxyl groups of glutathione (−SG) ligands to the gold surface [54]. Reproduced with permission from ref. [54]. Copyright 2010 American chemical society.

Figure 7.

(a) Schematic illustration of solvent-induced AIE properties of oligomeric Au(I)-thiolate complexes. (b) Digital photos of Au(I)-thiolate complexes in mixed solvents of ethanol and water with different f_e under visible (top row) and UV (bottom row) light. (c) UV-vis absorption and (d) photoemission spectra of Au(I)-thiolate complexes in mixed solvents with different f_e. Inset: Relationship between the luminescence intensity and f_e [55]. Reproduced with permission from ref. [55]. Copyright 2012 American chemical society. (e) Molecular structure and conformational rotamers of 1; (f) PL spectra of 1 in water-ethanol mixture (90 + 10 by volume), absolute ethanol, and solid film; (g) Quantum yield of 1 vs. solvent composition of the water-ethanol mixture [88]. Reproduced with permission from ref. [88]. Copyright 2001 the royal society of chemistry.

Figure 8.

(a) Aggregation-induced emission of Au NCs with decreased length of Au(I)-SRs motifs (from top to bottom). (b) Summary of the corresponding emission lifetimes. (c) Schematic illustration of the excited-state relaxation dynamics of Au NCs [56]. Reproduced with permission from ref. [56]. Copyright 2020 Wiley-VCH.

2.3. The effect of interfacial water

In addition to traditional organic ligand, NMNCs can also be protected by the inorganic scaffolds [105,106], such as glass [107,108], metal-organic frameworks (MOFs) [105] and zeolites [109]. These inorganic materials with uniform microporous structure can provide strong physical confinement effect on NMNCs after encapsulating NMNCs inside their void nanospaces. Typically, crystalline zeolite with molecularly sized cages are perfectly suited to accommodate NMNCs, but in fact only a very small part of zeolite encapsulated NMNCs can observe bright photoluminescence [19,20,109–111], and unexpectedly, their optical properties were highly dependent on the hydration status of NMNCs [112]. In 2009, Vosch et al. prepared highly luminescent silver nanoclusters (Ag NCs) through thermal treatment of Ag⁺-exchanged LTA- and FAU-type aluminosilicate zeolites (Figure 9(a)) [19], the luminescence properties of these Ag NCs within zeolite matrixes were dependent on the zeolite topology, the silver loading and the nature of counterions [19,113,114]. The dominant emission bands of Ag NCs in LTA-type zeolite were situated around 550 nm and 690 nm for the samples with, respectively, low and high silver content, whereas, Ag NCs in FAU-type zeolites only exhibited a broad emission band centered at around 550 nm. These emitters with characteristic luminescence colors were primarily ascribed to Ag_x⁺ clusters with different number of silver atoms within zeolites, typically Ag₃⁺ cluster for emitters at 550 nm and Ag₆⁺ cluster for emitters at 690 nm, based on the analysis of the fluorescence and electron spin resonance properties. Besides, the cations balanced the negative charge of aluminosilicate can also efficiently tune the emission color of these Ag NCs. For instance, Steele et al. observed that the gradual incorporation of Li⁺ into LTA zeolites (counterions: Na⁺) resulted a successive hypsochromic shift of the emission energy from 2.05 eV (605 nm) to 2.43 eV (510 nm) of these confined Ag NCs [113], but the excitation maxima and external quantum efficiency remain nearly constant (Figure 9(b)), the authors suggested that these changes arise through guest-host-guest interactions induced by Li⁺ exchange, which cause a reduction of the lattice parameter of zeolite and thus changed bond lengths of the Ag NCs (Figure 9(c)). Much efforts have been devoted to build the relationship between optical properties and structural parameters (such as the cluster size, geometry and charge) of these confined Ag NCs, which is similar to that of organic ligand protected NMNCs, but unexpectedly, the optical properties of these zeolite confined Ag NCs are closely related to their hydration state. In 2014, Lin et al. observed reversible emission evolution of Ag NCs in LTA zeolite upon dehydration/hydration in vacuum/water vapor [112], the yellow-green emission at 550 nm of these confined Ag NCs was quenched under vacuum conditions, accompanying with the enhancement of the red emission at 690 nm, and these changes were reversible when water vapor was added (Figure 10(a)). Based on these intriguing experiments, the authors suggested that origin of the emission from Ag activated zeolite may not be the quantum confinement effect of Ag NCs, but may be the charge transfer transition from the framework oxygen to Ag⁺-Ag⁰ with the participation of H₂O molecules. In the follow-up research work, the authors discovered that the emissions of the thermally-treated Ag⁺ exchanged SOD zeolites were insensitive to the environmental change [116], in contrast to the notable reversible green/red dominant emission evolution of thermally-treated Ag⁺ exchanged LTA zeolites. The difference is ascribed to the difficult H₂O permeation in SOD zeolites in comparison with LTA zeolites. Therefore the authors proposed the following emission mechanisms for thermally-treated Ag⁺ exchanged LTA and SOD zeolites: the yellow-green emission is due to the transition from ligand-to-metal (framework O₂⁻ → Ag⁺) charge transfer (LMCT) state to the ground state and the red emission is due to the transition from the metal-metal (Ag⁺-Ag⁺) charge transfer (MMCT) state to the ground state, the hydrated water only distributed a shielding effect on neighboring Ag⁺ ions, which could increase the distance between the neighboring Ag⁺-Ag⁺ ions and impede the formation of Ag_n^m+ clusters, and thus the yellow-green emission at 550 nm observed for Ag NCs at hydration state was dominated by the mechanism of LMCT, while the red emission at 690 nm for Ag NCs at dehydration state was dominated by the mechanism of MMCT. Grandjean et al. also observed the remarkable reversible on-off photoluminescence switching of Ag NCs confined in LTA zeolite upon dehydration and rehydration [115], but it was demonstrated that the fully dehydration of the samples, confirmed by the absence of the water and hydroxyl vibrational bands in FTIR spectra, can be only achieved after a treatment up to 450 ºC under vacuum (0.1 mbar pressure), and the fully dehydrated samples were almost non-luminescent but the green-yellowish emission at 560 nm was regenerated after rehydration of the dehydrated samples at room temperature (Figure 10(b)). The authors believed that water coordinated, diamagnetic, tetrahedral Ag clusters [Ag₄(H₂O)₄]²⁺ with Ag atoms positioned along the axis of the sodalite six-membered rings are at the origin of the green-yellowish luminescence in the hydrated sample. Upon dehydration, luminescent [Ag₄(H₂O)₄]²⁺ clusters are transformed into non-luminescent (dark), diamagnetic, octahedral Ag clusters [Ag₆(O_F)₁₄]²⁺ with Ag atoms interacting strongly with zeolite framework oxygen (O_F) of the sodalite four-membered rings. Besides, by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent-density functional theory calculations, and time-resolved spectroscopy, the authors further elucidated that the optical properties of the Ag₄(H₂O)_x²⁺ (x = 2 and x = 4) clusters (Figure 10(c-h)) originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster (Figure 10(i,j)) [6]. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states. In these cases, there is no doubt that hydrated water take a crucial role in tuning the luminescence properties of Ag NCs confined in zeolites, and the emission energy and pathways of these Ag NCs was highly dependent on their hydration degree. Whereas, the local environment of these confined Ag NCs was much more intricate due to the multiple host-guest interaction between zeolite and Ag NCs, by comparing with organic ligand-protected NMNCs, and the structure of the Ag NCs is also sensitive to the environment changes, the mechanism of the role of hydrated water needs more experimental and theoretical investigation. Besides, as water is ubiquitous in the solvent or air atmosphere, it is unavoidable that there is trace amount of water adsorbed on the organic ligand protected NMNCs. For instance, Perić et al. recently demonstrated that the water surrounding on the metal NCs core significantly influenced the PL properties of multi-shell zwitterion functionalized gold nanoclusters, which depended on the penetration depth of water molecules into NCs core [117]. The authors observed that more dense packing of surface protective ligands on the core produced the stronger PL emission by blocking the water diffusion on the metal core, and they pointed out that, the surrounding water molecules indeed played a quenching role for the PL emission of metal NCs. In fact, in our structure water model, the same quenching role of surrounding water on metal NCs PL emission was also observed, which is attributed to the destabilization effect of surrounding water on SWs due to hydrogen bonding interactions. Obviously, the surrounding water is hardly to influence the metal core state with defined compositions and geometry (Figure 3), suggesting the rationality of SWs as emitter center for PL emission. In the section 3, we will discuss this point in details.

Figure 9.

(a) Schematic of the LTA and FAU zeolite frameworks and their building blocks (top) [20], position of the most pronounced luminescence bands of the heat treated silver-exchanged zeolites (down) [19]. Reproduced with permission from ref. [19]. Copyright 2009 American Chemical Society and ref. [20]. Copyright 2016 springer nature. (b) 2D emission and excitation profiles of the different hydrated Ag₁-exchanged LTA zeolite samples, with a parent Li_xNa_12-x-LTA composition as depicted in the legend (top), maximal emission wavelength and EQEs of the Ag₁Li_xNa_11-x-LTA zeolites depending on the Li content of the exact composition of the luminescent zeolite (down). Inset shows the emission color of the different samples [113]. (c) Schematic representation of the changes observed for the ground and excited states of the tetrahedral Ag clusters confined inside the sodalite cages of the LTA zeolite as determined by Tr- (left) and XEOL-EXAFS (right), respectively (top), changes to lattice parameters of unit cell of Ag₁- and nonexchanged Li_xNa_12-x-LTA zeolites and Ag_c-Ag_c distances of the tetrahedral Ag clusters compared to Li-Na content of the Ag₁-exchanged Li_xNa_12-x-LTA zeolites (down) [113]. Reproduced with permission from ref. [113]. Copyright 2018 American chemical society.

Figure 10.

(a) PL evolution of Ag_4.4, Na_7.6-A from air to vacuum then to water vapor [112]. Reproduced with permission from ref. [112]. Copyright 2014 AIP Publishing LLC. American chemical society. (b) PL intensity of the sample at different excitation (Ex.) and emission (Em.) maxima in the hydrated (H), dehydrated (D) and rehydrated (R) state [115]. Reproduced with permission from ref. [115]. Copyright 2018 the Royal Society of Chemistry. (c to h) Structures of (c) Ag₄(H₂O)₄ and (f) Ag₄(H₂O)₂, including [(d) and (g)] Ag_R cations and [(e) and (h)] embedded in the sodalite cage (~0.66 nm free diameter). (i) Frontier orbitals of [Ag₄(H₂O)₄(Si₂₄H₂₄O₃₆)]²⁺ and (j) energy level diagram of Ag₄(H₂O)₂²⁺ and Ag₄(H₂O)₄²⁺ clusters in Ag₃K₉-LTA [6]. Reproduced with permission from ref. [6]. Copyright 2018 American association for the advancement of science.

3. PBIS model

As discussed above, both types of NMNCs have well-defined structure and composition [2,6,73,74], which mainly comprised by the metal core and ligand or ligand-metal complexes shell, and each components showed a significant effect on the PL properties of NMNCs. The size effect of metal core (quantum confined effect), the coordination interaction between ligand and metal (ligand-to-metal charge transfer transitions, LMCT and/or LMMCT) and the interaction between metal cations within ligand-metal complexes are all involved to understand the luminescence mechanism of NMNCs, whereas a unified understanding of luminescent NMNCs have not been achieved, the elucidation on the origin of the optoelectronic properties of NMNCs is still diverse and even contradictory [63]. Intriguingly, the interaction between ligand (including ubiquitous water molecules) bound or confined on the metal core at nano-interface or -space have been largely overlooked. Recently, in contrast to conventional metal-centered emission mechanism, we firstly attributed the unique optical properties of NMNCs to the strong electronic interaction between ligand bound on the confined metal core based on our continuous and long-term investigation on the nature of the PL of NMNCs [22,63,103,118–124]. A completely new p band intermediate state (PBIS) model, stemmed from the overlapping of p orbitals of the paired or more adjacent heteroatoms (O and S) of interfacial ligand through space interaction, was proposed to understand the nature of the bright PL of NMNCs. The interaction model within PBIS can be approximate described by the molecular orbital theory, in which both the spatial distance and orientation between adjacent heteroatoms of ligand showed a significant impact on the strength of PBIS. The PBIS model can well rationalize the abnormal optical phenomena reported in the literature. In this section, we describe the history of the PBIS model.

To fully understand the photoemission mechanism of NMNCs, the relationship between structure and optical properties of NMNCs must be firstly elucidated. In our earlier investigation, we separately investigated the contribution of the ligand shell and the metal core to understand the nature of photoluminescence of Ag NCs using carboxylate-protected silver nanoclusters (Ag-carboxylate NCs) as a model [22]. Several crucial structural components were identified for achieving highly luminescent Ag NCs: (i) Highly fluorescent Ag NCs were synthesized using one-step aqueous photoreduction in the presence of multidentate ligands made of polyelectrolyte poly(methacrylic acid) (PMAA) anchoring groups. The absorption and PL intensity was positively related to the radiation time, suggesting the formation of metal core is conductive to construct the emission center for highly luminescent Ag NCs (Figure 11(a,b)). (ii) Diagnostic experiments, in which dosing Ag⁺ and Cl⁻ resulted the increase and decrease in PL intensity respectively, suggested the indispensable role of Ag⁺ in the construction of the emission center (Figure 11(c,d)). (iii) When the poly(methyl vinyl ether-alt-maleic acid) (PMVEM) with the same anchoring ligand as PMAA was used as the surface anchoring ligand, the wavelength of both absorption and luminescence was blue shifted (Figure 11(e,f)). Notably, when the surface anchoring ligands were replaced by poly-(sodium-4-styrenesulfonate) free of carboxylate groups, as-synthesized Ag NCs with similar particle size showed undetectable PL intensity. These results emphasized the vital role of ligands to regulate the PL properties of Ag NCs. Based on these experiment results, a Ag(0)NCs@Ag(I)-carboxylate complex core-shell structural model was proposed to understand the nature of photoluminescence of Ag NCs (Figure 11(g)). We concluded that the origin of photoluminescence from Ag NCs cannot be simply attributed to the quantum confined effect (discrete Fermi energy levels) in the Ag NCs but highly dependent on the ligand-to-metal-metal charge transfer (LMMCT) from Ag(I)-carboxylate complexes to the Ag atoms. However, how the surface ligand regulated the optical properties of NMNCs is still unclear at this stage.

Figure 11.

(a) UV-vis absorption and (b) photoemission spectra of the freshly prepared Ag-carboxylate NCs using PMAA as scaffold at various radiation times (a, 0 min; b, 10 min; c, 20 min; d, 30 min; and e, 40 min.). Inset images show the corresponding photographs of Ag-carboxylate NCs under room and UV light exposure at λ = 365 nm. Luminescence enhancement and quenching by addition of (c) Ag⁺ and (d) Cl⁻ into the aqueous Ag-carboxylate NC solutions. Inset in (c) displays the relationship between PL intensity and added AgNO₃. (e) UV-vis absorption and (f) photoemission spectra of Ag NCs prepared with PMVEM scaffold under UV photoreduction. (g) Schematic illustration of the structures of our luminescent Ag-carboxylate with Ag⁺-carboxylate complexes shell [22]. Reproduced with permission from ref. [22]. Copyright 2014 American chemical society.

Followed the above research, in the aqueous solution, both Ag NCs synthesized using PMVEM and PMAA as templates (labeled as PMVEM-Ag NCs and PMAA-Ag NCs respectively) exhibited a fluorescence with low quantum yield (QY ≈ 1.0%) and short lifetime (τ ≈ 1.0 ns). Whereas, in the dimethyl sulfoxide (DMSO) solution, these water-soluble Ag NCs suffered aggregation due to the hydrogen-bond breaking capacity of DMSO, and the optical properties of these aggregated Ag NCs was highly dependent on the nature of surface ligand [118], the PMVEM-Ag NCs exhibited the fluorescence-phosphorescence dual solvoluminescence including a short-wavelength emission at ~460 nm with a short lifetime of 1.6 ns, which is same as the one in water, and a new generated long-wavelength emission at ~530 nm with a remarkably long lifetime of 97.1 μs and high QY of 40% (Figure 12(a-d)). Whereas, no phosphorescence was observed for PMAA-Ag NCs in the DMSO solution. Although both the polymer templates bear abundant carboxylate groups on the flexible carbon chain, the difference in the conformation of carbonyl groups resulted the difference in optical properties of Ag NCs at aggregation state. The phosphorescence of PMVEM-Ag NCs was attributed to the aggregation of surface carboxylate ligands on the metal core, that is, the cluster formation of carbonyl groups, the efficient delocalization of electrons in overlapped C=O double bonds between neighboring carbonyl groups triggered by strong n → π* interactions in the polymer cluster formed an exact chromophore of metal NCs for AIE mechanics. However, the conjugation or delocalization of electrons between carbonyl groups cannot occur in the PMAA-Ag NCs due to the steric effect of methyl groups connected to the proximal carbonyl group of the polymer chain (Figure 12(e)). The mechanism highlighted the strong interaction between surface ligand rather than the metallophilic interaction on the metal NC center within AIE mechanics.

Figure 12.

(a) UV-vis absorption and (b) photoluminescence spectra of PMVEM-Ag NCs in the varied volume fraction f_d of DMSO in the mixed solvent (f_d = V_DMSO/V_DMSO+water). (c) Correlations of the emission intensities of the two peaks centered at ∼460 and ∼530 nm versus f_d. (Inset) Photographs of PMVEM-Ag NCs at different f_d under visible (top row) and UV (bottom row) light. (d) Time-resolved luminescence decay profiles of PMVEM-Ag NCs in DMSO measured at 460 and 530 nm (inset), respectively. (e) Schematic illustration of the solvent-induced clustering process of Ag NCs (top) depending on the unique molecular structure of the polymer used as a template for the synthesis of Ag NCs (bottom-right) and the energy-level structure of Ag NCs in water solution and DMSO solution (bottom-left) [118]. Reproduced with permission from ref. [118]. Copyright 2017 American chemical society.

Based on the above mechanism, the optical properties of NMNCs should be strongly dependent on the binding group and assembly model of surface ligand on metal core. In the following research, by manipulating the delicate surface ligand interactions at the nanoscale interface of a single metal nanocluster, the superlattice, and mesoporous materials, we isolated the role of the local patterning or spatial arrangement of surface ligands in the PL emissions of metal NCs, the photoemission of Au NCs was more sensitive to the type and the assembly of surface ligand rather than the size of metal core and the type of metal [120], as shown in Figure 13, water-soluble Au NCs protected by glutathione (Au NCs@GSH) bearing both carbonyl and thiol group exhibited a broad emission peak at ~560 nm with a long lifetime of 14.6 μs in the ethanol solution, showing a strong solvent-enhanced PL emission behavior (Figure 13(a,d)). The broad emission can be decomposed into two peaks located at 550 and 620 nm using Lorentzian functions, whereas, when the hydrophilic GSH ligand of Au NCs was replaced by 1-dodecanethiol (Au NCs@DT) with a single thiol group through a simple surface ligand exchange reaction, as-obtained oil-soluble Au NCs exhibited a sharp emission peak at ~620 nm with a long lifetime of 3.7 μs in the aqueous solution (Figure 13(c,d)), as Ag NCs protected by PMVEM with single carbonyl group exhibited a similar phosphorescence with the emission peak at 530 nm [118], we hypothesized that the two emission peaks at 550 and 620 nm of water soluble Au NCs@GSH was the consequence of the clustering of carbonyl and thiol group respectively due to the solvent-induced aggregation. Furthermore, except for Au NCs@DT, Cu NCs protected by DT (Cu NCs@DT) ligand can assembly different superlattices (nanoribbon and nanosheet), which exhibited different emission color, Whereas, when breaking the superlattices into isolated Cu NCs by ultrasonic treatment, the PL of superlattices was quenched, but when these isolated and hydrophobic Cu NCs redispersed in the water, the Cu NCs@DT exhibited a red emission at 620 nm (Figure 13(e)), which is identical to that of Au NCs@DT. These results further supported the hypothesis, that means the clustering of the function group (-COOH/-SH) bearing on the targeting ligands could generate an intermediate state to activate the radiative transition of triplet state, we defined the intermediate state as PBIS, resulting from the strong overlapping of p orbitals of the heteroatoms (O, N, and S) bearing on the targeting ligands though space interactions (Figure 13(f)). Differed in the metal-centered free-electron model, the PBIS model give a totally new insight to understand the nature of the PL of NMNCs and other low other quantum dot systems. More importantly, based on the concept of PBIS model, we successfully construct the chromophore by the assembly of ligand in the confined mesopores of mesoporous silica nanoparticles (MSNs) through functionalized by non-luminescent organosilanes with amino and carbonyl groups free of any metals (Figure 14), the amino-functionalized and carbonyl-functionalized MSNs exhibited a blue emission at 430 nm and a red emission at 615 nm, respectively (Figure 14(c,d)). Both the nano-sized metal core of NMNCs and mesopores of MSNs imposed a strong space interaction on the adjacent p orbitals of the heteroatoms (O, N, and S) bearing on the ligands, which is conductive to the formation of PBIS. The concept of PBIS can rationalize these abnormal PL phenomena, however, the physical-chemical nature of PBIS is still unclear. Further investigation on the optical properties of inorganic scaffold confined NMNCs give us some new inspiration on the nature of PBIS [123]. Luminescent Ag NCs confined in the LTA zeolite (Ag NCs@LTA) was readily prepared through a cation-exchange and a thermal reduction process. The Ag NCs confined in the zeolite was highly hydrated, in which the water acts the role of ligand, thus it is rational that the optical properties of Ag NCs was strongly dependent on the hydration state based on the PBIS model. The hydration degree of Ag NCs@LTA can be tuned through thermal treatment or dispersing in different solvents (Figure 15(a-d)), fully hydrated Ag NCs@LTA at room temperature or dispersed in water exhibited a broad emission peak at 525 nm with a short lifetime of 3 ns, whereas, partially hydrated Ag NCs@LTA after thermal treatment at 250 ºC or dispersed in DMSO exhibited a red emission peak at 605 nm with a long lifetime of 231.1 us (Figure 15(a,d)), and fully dehydrated Ag NCs@LTA was non-luminescent as introduced in the section 2. Besides, the PL intensity of hydrated Ag NCs@LTA was gradually decreased with increasing time of vacuumizing but increased when replacing solvent water (H₂O) by heavy water (D₂O) accompanying with a sight red-shift in emission wavelength (Figure 15(e,f)). Notably, without any noble metal, NH₄⁺ exchanged zeolite Y also exhibited a blue emission at 430 nm after thermal treatment at 200 ºC (Figure 15(g)), and the blue photoemission was ultra-stable at liquid nitrogen temperature with an ultra-long lifetime up to second scale (Figure 15(h)). The PL of Ag NCs@LTA was therefore attributed to the formation of hydrated hydroxide complex OH⁻·H₂O (defined as SWs) in the confined nanocavity beyond PBIS mechanism, but a critical question is that what is the physical nature or chemical principle for the formation of PBIS. For instance, hydrogen-bonded water is a colorless liquid, it is confusing that the interfacial water bound to the NMNCs or confined in the nanopores can emit bright color. Therefore, it is important to elucidate the difference in chemical nature between hydrogen bonded water and interfacial water with photoemission. Free water molecules firstly formed an adsorption bond between oxygen atom and interfacial sites X (X could be Ag⁺ ions, alkali metal ions, and protons), subsequently, the nano-sized metal NCs or micropores of zeolite imposed strong space interaction on these adjacent adsorption bond, which could induce the re-distribution of the localized chemical bonds on the molecule level (Figure 16). The formation of PBIS could be described as a result of a linear combination of extended sp hybrid orbitals between O p orbitals and X s orbitals based on the principle of molecular orbital (MO) theory. The derived PBIS contained a π* state with π bond character due to the overlapping of p orbitals of oxygen atoms, which can account for the physical origin of the PL of SWs, and a σ* state with a σ bond character due to the overlapping of s orbitals of X, which could give a new fundamental understanding on the origin of metallophilic interactions between the closed-shell metal centers. Besides, combined with time-resolved steady state and ultra-fast femtosecond (fs) transient optical spectroscopy, a scenario of deactivation involving the energy delocalized excitation producing an ensemble of states (Q_n and Q’_n) with mutually different polarizations that decay via multiple channels was represented in Figure 16(c). The observed multiple sets of quantum transient states (Q_n and Q’_n) are concurrently produced, and the main optical absorption and emission are extremely susceptible to delicate change of microenvironments surrounding the emitter center. Therefore, the effect of metal core (size, composition, valence state, etc.), ligand shell (type, architecture, etc.) and external stimulus (pH, solvent, temperature, ions, etc.) on the optical properties of NMNCs can be rationalized by the chemical nature of PBIS which followed the bonding principle of molecular orbital theory. For instance, the negative influence of the PMMA on the PL emission efficiency of Ag NCs in our earlier investigation can be easily understood since the adsorbed content and the acting mode of structural water on the Ag NCs core are changed owing to the hydrophobicity of methyl groups bearing on the PMMA [118]. Besides, the fundamentals of PBIS mechanisms could give a unified interpret for the luminescence properties in the community of ligand protected metal NCs and also answer the century debate about whether and how adsorbed water can emit bright colors [125,126].

Figure 13.

Photoemission and excitation spectra of water-soluble Au NCs@GSH (a) and oil-soluble Au NCs@DT (c) in mixed solvents with different volume fraction R (Inset, relationship between the luminescence intensity and R (R_EtOH = Vol_EtOH/Vol_EtOH+H2O, R_H2O = Vol_H2O/Vol_EtOH+H2O). (b) UV-vis absorption spectra of Au NCs@GSH in mixed solvents with different R_EtOH. Inset shows the digital photos of water-soluble Au NCs@GSH and oil-soluble Au NCs@DT in mixed solvents of ethanol and water with varied volume fraction R_EtOH and R_H2O under UV light. (d) Time-resolved luminescence decay profiles of solvent-induced luminescent Au NCs@GSH and Au NCs@DT. (e) Excitation (dash) and emission spectra of Cu NCs SL with nanoribbon structure (blue), nanosheet structure (green), and individual Cu NCs in water (red). The insert shows the corresponding digital photos under UV light. AIE stands for the assembling-induced emission of the SL, and SIE is solvent-induced emission for individual metal NCs in a poor solvent. (f) Schematic illustration of the ligand exchange process and solvent-induced emission (SIE) properties of Au NCs (inset: the energy-level structure of Au NCs in water and ethanol mixed solution). The p-band formed by the overlapping of p-orbitals of electron-rich sulfur and oxygen heteroatoms of well-organized surface ligands is used as an intermediate state or dark state to tune the optoelectronic properties [120].

Figure 14.

Scanning electron microscopy (SEM) (a) and TEM (b) images of as-synthesized fluorescent mesoporous silica nanoparticles. The inset shows the assembly of amino- and carbonyl- groups in the confined nanopores. (The scale bar in a and b is 200 and 100 nm, respectively.) (c) Excitation and emission spectra of aminopropyl-functionalized MSNs. (d) Absorption and emission spectra of propylsuccinic-functionalized MSNs [120].

Figure 15.

(a) Photoemission and (b) UV-vis absorption spectra of Ag@LTA-450°C in the varied volume fraction of DMSO in the mixed solvent (f_d = V_DMSO/V_DMSO+water). (c) Contour representation of the excitation and emission data with varied f_d for Ag@LTA-450°C. (d) Time-resolved luminescence decay profiles of Ag@LTA-450°C in water (green) and DMSO (red) solution. (e) PL decrement of hydrated Ag@LTA-450°C with increasing time of vacuumizing. The inset shows the corresponding PL intensity attenuation (at 525 nm) during the dehydration process. (f) Comparison of emission properties of Ag@LTA-450°C in H₂O and D₂O. (g) Excitation (dashed line) and emission spectra (solid line) of NH₄⁺-Y-type zeolite annealed at 200°C. (h) Time-resolved luminescence decay profiles of NH₄⁺-Y-type zeolite at room temperature (RT) and liquid nitrogen temperature (LNT). Inset shows the naked eye visible long afterglow emission at LNT [123]. Reproduced with permission from ref. [123]. Copyright 2021 the Royal Society of Chemistry.

Figure 16.

Schematic illustration of the interfacial orbital redistribution through space interactions induced bright and wavelength adjustable photoluminescence of Ag@LTA and corresponding energy level structure derived from molecular orbital theory [123]. Reproduced with permission from ref. [123]. Copyright 2021 the royal society of chemistry.

The luminescence of SWs in the system Ag NCs@LTA inspired us to revisit the nature of the PL of ligand protected NMNCs as water is ubiquitous in the natural world, and the formation of PBIS between SWs is more available than that between organic ligands which could show a large steric effect on the overlapping of p orbitals of the heteroatoms (O, N, and S) bearing on the ligands. In the following research, the silver cations were introduced into the oil-soluble Au NCs@DT, the obtained Ag-Au bimetallic NCs exhibited an interesting silver cation-mediated dual-emission behavior, in which the blue emission at 440 nm with a lifetime of 5.4 ns is arouse by the doping of silver and the red emission at 620 nm with a lifetime of 2.0 us is the same as the PL of monometallic Au NCs@DT (Figure 17(a-d)) [122]. Furthermore, the dual emission of Ag-NCs was highly pH-dependent in the ethanol solution, in which the red emission dramatically reduced and almost quenched while the blue emission increased two times after dosing NaOH in the solution (Figure 17(e,f)), the variation in pH of the solution can hardly affect the assembly of the nonionic alkylthiol (DT) ligands but show a significant effect on the interfacial water, therefore, it is more rational to attribute the PL of Ag-Au bimetallic NCs to the formation of PBIS between unconspicuous interfacial water molecules adsorbed on the metal sites (Figure 17(g-j)). This again answers that, why the PL properties of monolayer protected clusters and metal-thiolates complexes are extremely sensitive to the composition of metal NCs, the type of surface protective ligand and even the surface ligand packing mode or assembly [22,118–120,127]. Very recently, some of us demonstrated that [127], just by manipulating noncovalent interactions of the ligand-ligand and ligand-metal interactions, the silver-glutathione (Ag(I)-GSH) supramolecular assemblies could be readily synthesized with blue and orange color, and interestingly that their color can be reversibly tuned, and exhibited a 4 orders of magnitude variation in fluorescence lifetime and dramatically different excited state dynamics. In addition, using PBIS dominated structural water model, the surface-ligand coverage dependent and non-size-dependent PL emission of GSH protected Au NCs can be easily understood: at high (or dense) ligand coverage, due to the limited number of adsorbed of water, the short-range conjugation of p orbitals of O atoms leads to the high energy emission at 600 nm, while at low coverage of surface protective ligands of GSH, more water molecules are adsorbed on the metal core surface, the long-range conjugation of p orbitals of O atoms results in the low energy PL emission at 810 nm [53].

Figure 17.

(a) Luminescent photographs of Ag_xAu@DT NCs under UV 365 nm (top) and 260 nm (bottom) irradiations. (b) Photoemission spectra and (c) absorption spectra of Ag_xAu@DT NCs. (d) Time-resolved luminescence decay profiles of Ag₁Au@DT NCs measured at 440 nm (left) and 630 nm (right), respectively. (e) Photoemission spectra of as-synthesized Ag₁Au@DT NCs after adding increasing amounts of 50 mM NaOH aqueous solution. The inset displays the relationship between PL intensity and the added mole amount of NaOH. (f) PL intensity of Ag1Au@DT NCs upon cyclic switching of the pH by adding 50 uL 50 mM NaOH and 50 uL 50 mM HCl aqueous solution. (g) Schematic illustration of structural water molecules (SWs) confined at nanoscale interface of Au-Ag bimetal NCs packed by protective ligands emit bright dual-photoluminescence, (h) the red emissive SWs strongly binding on the single metal Au NCs, (j) the blue emissive SWs weakly binding on the fully core-shell structured Au-Ag bimetal Au NCs though water bridge which strongly coordinated with Ag⁺ and (i) energy diagram of dual emissions of Au-Ag bimetal NCs [122].

Followed this mechanism, we also re-investigated the nature of the PL of luminescent MSNs functionalized by non-luminescent organosilanes (Figure 18(a-f)) [103]. The emission wavelength of amino-functionalized MSNs was blue-shift with the increase in the grafting amount of hydrophobic functional groups (trimethyl chlorosilane) within the mesopores (Figure 18(e)). More prominently, the PL of amino-functionalized MSNs was almost quenched after vacuum treatment (Figure 18(h)), that means the SWs was the true emitter for the luminescent MSNs and the amino group took a similar role of metal in the NMNCs for the adsorption of water. Similarly, the conceptual of confined SWs as a bright emitter is also possible to account for the AIE effect which is both observed for conjugated and non-conjugated organic molecules and metal-thiolate complexes. The nature of AIE is indeed a solvent driven molecular self-assembly process, the aggregation of AIE luminogens (AIEgens), such as typical AIEgens of diphenyl-1-pyrenylphosphine (DPPP, Figure 18(g,i)) and classical organic chromophores (1-bromopyrene, BP, Figure 18(h,j)) trigged by the introducing of bad solvents could also possibly construct a soft nanocavity for the formation of SWs emitters (Figure 18(k)), which is analogous to the rigid nanocavity (mesopores) in luminescent MSNs (Figure 18(b)). Therefore, in some of case, the AIE effect is not sensitive to the structure of AIEgens, the classical mechanism of restriction of intramolecular motions (RIM) was still working during the aggregation process, but the PL may not be originated from the AIEgens itself but the SWs confined in the soft nanocavity constructed by the AIEgens.

Figure 18.

(a) SEM images of as-synthesized fluorescent mesoporous silica nanoparticles (FMSNs). (b) the formation of SWs in an amino and carbonyl-group-functionalized enzyme-like nanocavity. (c) Excitation and emission spectra of amino-functionalized FMSNs. (d) Absorption and emission spectra of propylsuccinic-functionalized FMSNs. (e) Emission wavelength regulation of FMSNs by controlling the hydrophobicity of the nanopores. (f) Evolution of PL emission intensity of amino-functionalized FMSNs at ca. 450 nm by vacuum dehydration treatment. (g, h) Photographs and (i, j) fluorescence spectra of diphenyl-1-pyrenylphosphine (g, i) and 1-bromopyrene (h, j) in mixed solvents with different volume fractions R_w (R_w = Vol_w/Vol_e+w, the subscripts w and e represent water and ethanol, respectively). Inset shows the relationship between the luminescence intensity and R_w. (k) Proposed mechanism for water-induced AIE phenomenon. (l) Ultraviolet-visible (UV-vis) absorption spectra of 1-bromopyrene (BP) in mixed solvents with different R_w [103].

4. Summary and outlook

In this mini-review, we firstly summarized recent advances in the PL mechanism of NMNCs, several prevailing PL mechanisms, including quantum confinement effect (QCE), ligand to metal charge transfer (LMCT) or ligand to metal-metal charge transfer (LMMCT) and metallophilic interaction beyond AIE effect, etc., were involved in explaining the effect of structural components (metal core, organic ligand shell and interfacial water), local environments and external stimulus (pH, solvent, ions, etc.) of NMNCs on their PL properties, but there is still no basic consensus on the elucidation of the PL mechanism of NMNCs. Then, we revisited our studies on the PL mechanism of NMNCs, a completely new PBIS model, stemmed from the overlapping of p orbitals of the paired or more adjacent heteroatoms of interfacial ligand (especially emphasized the oxygen atom of SWs) through space interaction, was proposed to understand the nature of the bright PL of NMNCs. Note that water in the term of SWs does not distinguish between the three possible protonation states, water/hydroxo/oxo, which is extremely sensitive to the surrounding microenvironment of SWs at confined nanoscale interface. By combining the literature survey and our long-term studies in the past decade, we draw the conclusion that SWs dominated PBIS model can well rationalize the abnormal optical phenomena reported in the literature and give a unified understanding on the PL mechanism of NMNCs.

Based on the fundamentals of the SWs dominated PBIS model, not only the optical properties of NMNCs, we believe that it can be also extended to understand other unique properties (such as catalysis, chirality, etc.) of NMNCs. 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 AgNO₃ 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.

Furthermore, the principle for the formation of PBIS within SWs was also analogous to that for the formation of transition state between reactant molecules, NMNCs, which provided confined interface and/or space, can largely reduce the potential barrier for the formation of strong intermolecular interaction (interfacial state or transient state), which favored the formation of PBIS within SWs for bright PL, or favored the formation of metastable transient state between reactants for fast reaction kinetics [103,128–133]. Within this scenario, these unique properties of NMNCs can be well elucidated, and therefore the rational design of metal NCs with excellent optical and catalytic properties can be guided although much work remains to be done, even beyond the often used noble metal NCs (Au/Ag/Cu/Pt), such as transition metals and transition metal oxides [134–136]. Herein, in particular at basic conditions (pH > 7.0), we take hydrogen evolution reaction (HER) as a typical example to show the key role of SWs as reaction center to enhance reaction kinetics [132]. With Co-C-N as an prototype electrocatalyst, the reaction dynamics of HER does not follow the classical two-step process of Volmer step and Heyrovsky step (or Tafel step), while working the p band mediated electron transfer process for the direct activation of O-H bonds of water: the dissociated H⁺ from water don’t need to adsorb on the metal active sites, where SWs adsorbed on metal cations can directly interact with the water reactants, owing to the space interaction of p orbitals in SWs and water reactants, the energy level of the formed p band anti-bonding level (π*) is even higher than that of the σ anti-bonding (σ*) of O-H bonds in water, thus the electron can be directly fill with σ* energy level, resulting in the O-H bond activation and hydrogen evolution [132,137]. Therefore, the p-band model proposed by us not only provides a new insight into the origin of PL emission of metal NCs, but also reveals the physical essence of interface (or surface) states at nanoscale interface for heterogeneous catalysis. On the one hand, the p-band accelerates the surface electron transfer through surface delocalization (or conjugation), and on the other hand, stabilizes the transition state of the chemical reaction.

Finally, from the perspective of the common nature of nanomaterials surface usually covered by water molecules [138,139], this significant conceptual advance of SWs model as emitters can be readily extended to understand the optical and catalytic properties of other quantum dot systems, including carbon and graphene nanodots [140–144], semiconductor quantum nanostructures [145–152], and, also provides a new insight into the PL emission mechanism of green fluorescent protein (GFP) [153–163].

Biographies

Bo Peng, born in Yichun (Jiangxi Province, China), is currently a postdoctoral researcher at the College of Chemistry and Molecular Engineering of East China Normal University (ECNU, China) working with Prof. Kun Zhang and Prof. Ming-Yuan He. He received his B.S. (2017) from Donghua University (DHU, China) and Ph.D. (2022) from East China Normal University (ECNU, China). His research interests are mainly focused on the mechanistic understanding on the unique properties (photoluminescence, catalytic properties and chirality) of nanomaterials and therefore rational design of high efficient nanocatalysts towards the activation of small molecules (water and carbon dioxide) related to energy conversion。

Kun Zhang, born in Gaomi (Shandong Province, China), is currently a full Professor at the College of Chemistry and Molecular Engineering of East China Normal University (ECNU, China). In 2008, He received a Ph.D. in the Chemistry Laboratory at Ecole Normale Supérieure de Lyon (ENS-Lyon, France) working with Prof. Laurent Bonneviot and Prof. Ming-Yuan He. His research interests are mainly focused on the design of bio-inspired nanocatalysts towards the activation of small molecules related to energy conversion, such as water, methane (CH₄), carbon dioxide (CO₂), carbon monoxide (CO), oxygen (O₂), hydrogen (H₂) and nitrogen (N₂).

Funding Statement

This research was funded by the National Science Foundation of China [22172051, 21872053, and 21573074], the National Key R&D Program of China [2021YFA1501401], the Research Funds of Happiness Flower ECNU [2020ST2203], the Open Project Program of Academician and Expert Workstation, Shanghai Curui Low-Carbon Energy Technology Co., Ltd., and the JORISS program.

Disclosure statement

No potential conflict of interest was reported by the author(s).