Visible to NIR‐II Photoluminescence of Atomically Precise Gold Nanoclusters

Atomically precise gold nanoclusters (NCs) have emerged as a new class of precision materials and attracted wide interest in recent years. One of the unique properties of such nanoclusters pertains to their photoluminescence (PL), for it can widely span visible to near‐infrared–I and –II wavelengths (NIR‐I/II), and even beyond 1700 nm by manipulating the size, structure, and composition. The current research efforts focus on the structure–PL correlation and the development of strategies for raising the PL quantum yields, which is nontrivial when moving from the visible to the near‐infrared wavelengths, especially in the NIR–II regions. This review summarizes the recent progress in the field, including i) the types of PL observed in gold NCs such as fluorescence, phosphorescence, and thermally activated delayed fluorescence, as well as dual emission; ii) some effective strategies that are devised to improve the PL quantum yield (QY) of gold NCs, such as heterometal doping, surface rigidification, and core phonon engineering, with double‐digit QYs for the NIR PL on the horizons; and iii) the applications of luminescent gold NCs in bioimaging, photosensitization, and optoelectronics. Finally, the remaining challenges and opportunities for future research are highlighted.


Introduction
3][4][5][6][7][8] Similar to metal-organic complexes, gold NCs are stabilized by organic ligands, but what distinguishes them from metalorganic complexes is the presence of free valence electrons in NCs.[36][37][38][39] Such a broad range of emission wavelengths of gold NCs is unparalleled by other luminescent materials such as dyes and metal complexes.When coupled with exceptional stability and biocompatibility, [40][41][42][43][44] gold NCs hold great potential for applications in bio-imaging, [45,46] light harvesting, [47][48][49] and optoelectronic devices. [50,51]Moreover, the atomically precise structures of these NCs provide a great opportunity to bridge up the structure and PL properties for a deep understanding of the PL mechanism.Nevertheless, despite more than a decade of development, the research on the PL properties of atomically precise gold NCs still confronts two primary challenges: the fundamental mechanism of PL and the relatively low quantum yields (QYs).Thus, understanding the details of the PL mechanism and developing strategies for raising QYs constitute the major efforts in the current research.
Currently, a rich library of gold NCs with determined structures has been discovered and constructed, but only a few of them have been thoroughly investigated regarding their PL mechanism.The challenge of elucidating the PL mechanism of gold NCs stems from two key aspects.First, gold NCs typically adopt a core-shell structure, [2] where Au(0) atoms constitute the inner metal core which is shielded by a gold-incorporated ligand shell (e.g., Au(I)-SR, where SR denotes thiolates), as opposed to the plain ligand shell.Consequently, Au(0) or Au(I) atoms can serve as the luminescent center, [52,53] and in certain circumstances, even both types of metal centers can simultaneously luminesce with mutual interactions. [54]This intricate interplay results in complicated electron dynamics [55,56] and makes it a significant endeavor to unravel the PL mechanism in gold NCs.Additionally, the presence of many heavy gold atoms in the NCs results in significant spin-orbit coupling (SOC), [57] enabling efficient intersystem crossing (ISC). [24,58]Therefore, when studying the PL mechanism of gold NCs, the triplet state should be taken into consideration.Earlier studies often regarded the metal NCs as being analogous to semiconductor quantum dots (QD) since both systems involve quantum confinement effects, but recent work suggests that the NCs are molecular-like, [59] especially in terms Scheme 1. Selected luminescent homogold NCs and their PL properties (QY and peak position).The ligands on the NCs are omitted for clarity.NIR = near-infrared, I: 700-1000 nm, II: 1000-1700 nm.
of the involvement of the triplet state; [20,60] the latter is common in organic dyes and metal complexes [61] but was overlooked in early research on the PL of NCs.Extra efforts are thus needed to ensure coherence between future studies and early results.
Compared to organic dyes and conventional quantum dots, the PL quantum yields of most gold NCs are often low (PLQY < a few %). [62,63]Therefore, in order to promote the applications of gold NCs, it is crucial to explore effective strategies for engineering the NCs to enhance their PLQY.Synthesizing novel structured NCs is always the most versatile method to manipulate the optical properties of NCs.However, the synthesis of new NCs requires much effort, and their optical properties may be uncertain, that is, a process of trial and error.Thus, direct synthesis of novel NCs with high PLQY at specific wavelengths (e.g., in NIR-II) remains a challenging task.On the other hand, tailoring the PL properties of the known gold NCs is an easier and more practical method. [9]Generally, ligand engineering [64][65][66] and heteroatom doping [22,23] are two effective strategies that have recently been developed for manipulating the PL properties of gold NCs toward the desired wavelength and PL lifetime for specific applications.These strategies typically allow for the improvement of PLQY and photostability without significant changes in the emission peak position and width of the parent nanocluster.69] Herein, we summarize the PL research progress of gold NCs.An overview of the present understanding of the PL mechanisms in gold NCs (Scheme 2) is first discussed.Generally, the emission mechanism can be characterized by four primary types: fluorescence, phosphorescence, thermally activated delayed fluorescence (TADF), and dual emission.For each type, we shall discuss some typical examples and provide essential factors or criteria Scheme 2. Overview of the main PL mechanisms, PL enhancement strategies, and applications of luminescent gold NCs. that can ascertain each individual PL mechanism.Then, we further discuss the reported strategies to improve PLQY of gold NCs from two aspects: the energy gap control and surface motif rigidification.[72] Finally, we highlight the applications of luminescent gold NCs and address the remaining challenges and opportunities for future research endeavors.5][76][77][78][79][80][81][82][83][84][85][86]

Emission Mechanism of Gold NCs
The photoluminescence in gold NCs can be classified into three primary processes (Figure 1A-C): fluorescence, phosphorescence, and TADF.While these three processes are commonly observed in organic dyes, the presence of heavy atoms in gold NCs adds complexity to the electron dynamics [87] and accordingly the emission. [63]Conventional methods used to determine the PL mechanism require adjustments before they can be applied to gold NCs.In addition to the three primary emission processes, several types of dual emission have also been observed in gold NCs.In this section, we will discuss some typical PL mechanisms with illustrative examples and provide guidance on how to differentiate the PL mechanism in gold NCs.

Fluorescence
Fluorescence is defined as the spontaneous emission of radiation from an excited molecular entity by maintaining the same spin multiplicity 2S+1=1 (singlet). [88]In later research, this concept is further extended to describe the PL in nanocrystalline semiconductors and metal nanoparticles where spin multiplicity holds less significance. [89,90]In these cases, fluorescence refers to an emission from an excited state that can be achieved by direct photoexcitation.][93][94][95][96] But recent research found that the electronic and optical properties of luminescent gold NCs are more akin to organic molecules and metal-organic complexes. [54,97,98]Therefore, it is more appropriate to describe the PL of gold NCs from a molecular perspective, as opposed to the use of QD language.It is important to note that all the cases of fluorescent gold NCs discussed here adhere to its definition depicted in Figure 1A, that is, the direct emission from the first singlet excited state (S 1 ) without the involvement of the first triplet state (T 1 ).
Gold NCs are typically comprised of tens of heavy atoms or more, resulting in strong spin-orbit coupling and rapid intersystem crossing (ISC) owing to the heavy atom effect.Consequently, in gold NCs, the rate of ISC could be comparable to or even surpass the rate of fluorescence, resulting in almost complete population transfer from the S 1 to the T 1 state.Therefore, fluorescence is less commonly observed in gold NCs compared to phosphorescence, and even if fluorescence is observed, it often coexists with phosphorescence. [99]Currently, the reported fluorescent gold NCs (as defined in Figure 1A) are generally of small sizes (i.e., the number of gold atoms is less than 20), [100][101][102] where the spin-orbit coupling is relatively weaker compared to larger-sized gold NCs (e.g., 20-50 atoms).The lifetime of photoluminescence plays a critical role in determining the underlying mechanism.In conventional luminescent materials such as organic dyes, fluorescence typically occurs within a range of tens of picoseconds (ps) to tens of nanoseconds (ns).Therefore, the emission processes with lifetimes shorter than a hundred nanoseconds are commonly assigned as fluorescence, while those with lifetimes longer than one microsecond (s) are attributed to phosphorescence.Nevertheless, this criterion is not strict in gold NCs.Due to the fast ISC and non-radiative relaxation, phosphorescence in gold NCs may also happen within 100 ns at room temperature.Therefore, in addition to the PL lifetime, it is necessary to assess the emission's sensitivity to triplet state quencher (e.g., 3 O 2 , perylene, etc.) when categorizing the observed emission. [103] typical example of fluorescent gold NCs is [Au 8 (dppp) 4 L 2 ]X 2 (dppp = bis(diphenylphosphino)propane, L = Cl, X = PF 6 ).[100] It features a bi-tetrahedral Au 6 core with two additional gold atoms protected by four dppp ligands and two chlorides (Figure 2A).[100,104] In a dichloromethane solution, Au 8 (ligands omitted) displays a strong absorption peak at 510 nm, corresponding to Reproduced with permission.[100] Copyright 2017, American Chemical Society.D) Schematic illustration of the triple-ligands layer-by-layer self-assembly on the Au 10 NCs.E) UV-vis absorbance and luminescence spectra of triple layer protected Au 10 NCs. F) Thexcited-state electron dynamics of triple-layer protected Au 10 NCs.Reproduced with permission.[101] Copyright 2023, Springer Nature. G) PL spetra of Au 13 in acetonitrile under Ar atmosphere (solid lines) and O 2 atmosphere (dotted lines).Reproduced with permission. [58] opyright 2022, Wiley-VCH.H) PL spectra of Au 13 (14.9μM) in deaerated N, N-dimethylformamide (DMF) with increasing concentration of perylene.I) PL spectrum obtained for the deaerated DMF solution of Au 13 (14.9μM) and perylene (1.0 mM) with 640 nm CW laser excitation.Reproduced with permission.[103] Copyright 2022, American Chemical Society.
the HOMO-LUMO transition. [105]Upon excitation at 510 nm, a broad emission band ranging from 500 to 750 nm is observed (Figure 2B upper panel).The excitation spectrum is found to be consistent with the absorption spectrum (Figure 2B lower panel), suggesting that the emission is from the first excited state (i.e., following Kasha's rule). [106]Time-dependent density function theory (TD-DFT) simulations showed that the HOMO-LUMO transition is mainly core-based. [107]Therefore, the emission was assigned to occur in the bi-tetrahedral Au 6 core. [102]Timeresolved PL measurements showed a single-exponential decay with an ultrashort lifetime of 55 ps (Figure 2C), indicating that the emission can be ascribed to fluorescence.The PL quantum yield of Au 8 is only 0.1%, which is too low for applications.Recently, Zhong et al. synthesized a 6-Aza-2-thiothymine (ATT) protected Au 10 NC with PL centered at 530 nm and dramatically increased its QY (≈90%) by rigidifying the Au 10 cluster's surface with L-arginine (ARG), and tetraoctylammonium (TOA) (Figure 2D). [101]The observed emission is ascribed to metal core-dictated fluorescence due to its narrow FWHM (≈30 nm), small Stokes shift, and the ns-level decay time (Figure 2E,F).
The extraordinary PLQY of Au 10 is owing to the trilayer ligand assembly strategy.
Triplet oxygen ( 3 O 2 , ground state) is the most widely used triplet state quencher to distinguish between fluorescence and phosphorescence in gold NCs. [24,58,103,108]Typically, 3 O 2 quenches the phosphorescence from the emitter, leading to singlet oxygen ( 1 O 2 , an excited state of oxygen) with emission at ≈1260-1275 nm.Therefore, the sensitivity of the NC emission to the presence of triplet oxygen allows one to infer the involvement of the NC triplet state.It is worth noting that the lack of emission quenching by triplet oxygen does not necessarily imply that the type of emission is fluorescence.For example, the emission from [Au 13 (dppe) 5 Cl 2 ] 3+ (dppe = 1,2-bis(diphenylphosphino)ethane, lifetime  = 2.6 μs) was attributed mainly to fluorescence because it presented almost no quenching under the O 2 atmosphere (Figure 2G). [58,109]However, another study found that the emission from Au 13 can be significantly quenched by perylene (Figure 2H) and the latter gives rise to an up-conversion PL peak at 472 nm (Figure 2I). [103]Therefore, Au 13 is an efficient triplet photosensitizer for perylene (albeit not for 3 O 2 ), suggesting that the emission of Au 13 is actually phosphorescence, in agreement with its long lifetime (2.6 μs).
Overall, although fluorescence constitutes one of the fundamental emission processes and is widely observed in luminescent molecules, distinguishing it in gold NCs is not a straightforward task.It requires a comprehensive assessment of factors such as the lifetime of PL, its sensitivity to triplet quenchers, and even the temperature-dependent trends of the emission before reaching a reliable conclusion.Due to the rapid ISC (e.g., ps), fluorescence is less prevalent in gold NCs compared to other emission processes (vide infra).

Phosphorescence
Phosphorescence refers to the luminescence involving a change in spin multiplicity, typically from triplet to singlet, that is, between the first triplet excited-state (T 1 ) and the singlet ground state (S 0 ). [110]As depicted in Figure 1B, the lowest-lying triplet state (T 1 ) is populated through an intersystem crossing from the S 1 state, and further relaxation gives rise to T 1 -S 0 phosphorescence radiation.In contrast to the extensive investigations of phosphorescence in organic dyes and metal complexes, the significance of the triplet state in the luminescent process of gold NCs has only gained attention recently.[113][114] In semiconductor quantum dots or nanocrystals, it is believed that, due to the strong spin-orbit coupling, the spin-sublevels responsible for PL may not be simply classified as triplet or singlet states. [88]Therefore, it is important to keep the concept of mixed S and T states in mind since it may emerge in larger-sized gold NCs (i.e., the number of gold atoms larger than 60).
The emission lifetime stands as the most straightforward criterion to determine the phosphorescence nature of gold NCs.Normally, an emission process with a lifetime longer than one microsecond is considered phosphorescence.However, it has been reported that the lifetime of phosphorescence in metal complexes can be as short as 100 ns. [87]The same situation can be possible in gold NCs, therefore, for a rigorous identification of phosphorescence (which strictly follows Figure 1B), one needs to combine the lifetime value with several more criteria as follows: 1) The decay curve from time-resolved PL measurements should be well fitted by a mono-exponential function; 2) The emission intensity can be quenched by triplet state quenchers like 3 O 2 , perylene, rubrene, etc.; 3) The PL lifetime should be consistent with the result from transient absorption spectroscopy (TAS); and 4) The excitation spectrum should be consistent with the absorption spectrum, otherwise it may indicate that the emission is from a meta-stable state or impurities.Of note, the emission process shown in Figure 1B only considers radiative relaxation from the T 1 state that typically requires an efficient ISC (i.e., ISC efficiency ≈100%).Co-existence of fluorescence (S 1 ) and phosphorescence (T 1 ) will be discussed in Section 3.
The rod-shaped [Au 25 (PPh 3 ) 10 (SC 2 H 4 Ph) 5 Cl 2 ] 2+ NC (abbrev.Au 25 rod) is a typical example of phosphorescent gold NCs.The investigation on the PL of Au 25 rod spanned over a decade but only recently have the full emission spectrum and PL mecha-nism been elucidated. [20,60,115,116]Single crystal X-ray diffraction revealed that the core of Au 25 rod is composed of two Au 13 icosahedral units by sharing one vertex, and ligands of PPh 3 , SC 2 H 4 Ph, and Cl wrap over the gold core (Figure 3A). [117]The optical absorption spectrum of the Au 25 rod exhibits a strong peak at 680 nm with a weak tail that extends to 900 nm (Figure 3A blue line).Upon 640 nm excitation, a strong emission peak (PLQY = 8%) can be observed at 900 nm under deaerated conditions (Figure 3A red line). [20,60]Under aerated conditions, the PL intensity was found to be reduced by 10% (Figure 3A black dash line), suggesting the involvement of a triplet state in the emission process.The time-resolved PL measurement found that the lifetime of the emission peak is ≈3 μs, which is in good agreement with the longest lifetime component of the Au 25 rod determined by TAS. [20,60,118]Furthermore, the emission from the Au 25 rod was found to be significantly quenched by rubrene via a triplet energy transfer mechanism (Figure 3B). [60]Considering all the evidence, it can be firmly concluded that the NIR emission from the Au 25 rod is phosphorescence.
Recently, phosphorescence has been observed in many gold NCs.For instance, the Tsukuda group synthesized a series of [MAu 12 (dppe) 5 Cl 2 ] NCs (Figure 3C,D, where M = Au, Pd, Pt, Rh, or Ir).They assigned the Pt, Rh, and Ir doped cases to phosphorescence because their emission peaks can be completely quenched by oxygen (Figure 3C dash lines).As we mentioned in the last section, although the emission from the original Au 13 NC cannot be completely quenched by oxygen, it should still be assigned to phosphorescence.Similarly, PdAu 12 is expected to exhibit the same emission process as Au 13 in this regard.Moreover, Li et al. found that the Au 38 S 2 (S-Adm) 20 (HS-Adm = adamantanethiol) NC exhibits bright phosphorescence at 900 nm with QY up to 15%. [114,120]Regarding the polarization property of phosphorescence, Kong et al. determined an intriguing circularly polarized phosphorescence in the R/S-[S 2-@Au 12 @S 8 @Ag 32 (PS) 24 ] 2+ NC (where, PS = binaphthyldithiophosphoric). [121]n addition to the core-based phosphorescence, the surface Au(I)-SR staple motif was reported to be a source of phosphorescence in Au 22 (SG) 18 , Au 18 (SG) 14 , and Au 15 (SG) 13 (where, SG = glutathione). [122,123]Typically, the excitation spectrum for motifbased emission does not align consistently with the optical absorption spectrum of the corresponding NC [123,124] , which results in challenges in identifying the true origin of the emission.A rigorous study of this kind of mechanism is currently still lacking; thus, further efforts are required in future studies.To establish a reliable understanding, it is recommended that one should collect the time-dependent UV-vis absorption and PL emission spectra of the sample, ensuring that the emission does not stem from any decomposition of the NC or from any impurity.If the emission is from the decomposed product(s) of the original NC, the PL intensity will progressively increase over time, while the optical absorption features will diminish concurrently.
It is worth noting that the above cases all show a microsecond scale lifetime at room temperature that is in consensus with phosphorescence.However, the phosphorescence in gold NCs may also happen in tens or hundreds of nanoseconds.Liu et al. demonstrated that the NIR PL from Au 25 (PET) 18 − (PET = phenylethanethiolate) should be ascribed to phosphorescence, [125] albeit the PL lifetime is only 155 ns at The inset shows the corresponding PL spectra.Reproduced with permission. [60]Copyright 2022, Royal Society of Chemistry.C) PL spectra of MAu 12 in acetonitrile under Ar atmosphere (solid lines) and O 2 atmosphere (dotted lines).Inset shows the plot of the PLQYs of MAu 12 under Ar atmosphere.D) Time-resolved PL decays of MAu 12 under the Ar atmosphere.Black lines represent the fitting results with single exponential functions.Inset shows the plot of the PL lifetimes of MAu 12 under the Ar atmosphere.Reproduced with permission. [58]Copyright 2022, Wiley-VCH.E) PL processes for [RuAu 12 (dppm) 6 ] 2+ at temperatures of more than 140 K (left) and less than 130 K (right).Reproduced with permission. [24]Copyright 2021, American Chemical Society.F) Energy level diagrams showing the photophysical processes of Ag 29 -TBA (left) and Ag 29 + Ag NCs (right).Reproduced with permission. [119]Copyright 2023, American Chemical Society.room temperature.Moreover, when a Hg atom is doped into Au 25 (PET) 18 − , the lifetime of NIR phosphorescence is shortened to 53 ns.Therefore, in the research on metal NCs, the lifetime cannot be used as the sole criterion to distinguish fluorescence and phosphorescence.Rather, one needs to combine multiple pieces of evidence, for example, from the triplet sensitizer quenching test, TAS analysis, and lowtemperature PL measurements in order to reach a solid conclusion.
Recent temperature-dependent PL measurements have revealed that the room temperature phosphorescence in NCs may transform into a different emission process at low temperatures.Takano et al. reported the phosphorescence from [RuAu 12 (dppm) 6 ] 2+ (dppm = (Ph 2 )PCH 2 P(Ph 2 )) transforms into fluorescence when the temperature is lower than 130 K (Figure 3E). [24]They proposed that there is an energy barrier between S 1 and T 1 states and lower temperatures result in in-sufficient thermal energy, thereby suppressing the intersystem crossing. [24]Another interesting temperature-dependent transformation is observed in a silver-based Ag 29 + Ag NC, [119] which can offer valuable insights and guidance in the investigation of gold NCs.Specifically, after adding Ag(PPh 3 ) 4 NO 3 into [Ag 29 (BDT) 12 (PPh 3 ) 4 ] 3-NC (BDT = 1.3-benzenedithiol), four Ag(I) complexes were found to bind at the periphery of Ag 29 and the PL emission peak shifts from 680 to 770 nm (Figure 3F).However, at low temperatures, the 770 nm emission in the Ag 29 + Ag composite shifts back to 690 nm, which is close to that of the original Ag 29 NC.Therefore, the 770 nm emission is ascribed to a T 1 ′ state centered at the Ag complexes and an energy barrier is proposed to exist between the T 1 state (mainly in Ag 29 ) and the T 1 ′ state on the complex (Figure 3F).Copyright 2018, Springer Nature.D) PL decay curves of (Carbene)M(Cz) (M = Cu, Ag and Au) complexes.E) Ultrafast timeresolved emission decay traces for (Carbene)Cu(Cz) in toluene solution at room temperature.F)Temperature-dependent lifetimes of (Carbene)M(Cz) in polystyrene.Reproduced with permission. [135]Copyright 2019, American Chemical Society.

Thermally Activated Delayed Fluorescence
Photoexcitation in gapped systems leads to the formation of excitons (i.e., electron-hole pairs).In molecular systems, the lowest excited state comprises the singlet (S 1 ) and the triplet (T 1 ).When the energy difference between the S 1 state and the T 1 state (ΔE S-T ) becomes sufficiently small (e.g., <0.2 eV), the triplet excitons can be efficiently converted into emissive singlet excitons through a thermally assisted reverse intersystem crossing (RISC) process.[128] To differentiate between these two emission processes from the same S 1 state, the standard fluorescence is referred to as prompt fluorescence (PF), while the slow fluorescence is termed thermally activated delayed fluorescence (TADF).TADF is now widely observed in many designed organic molecules and metal complexes and has driven the development of novel optoelectronic devices. [129,130]Metal NCs share some similarities with metal complexes in terms of their PL properties, which makes the NCs a promising candidate for novel TADF materials, but the current understanding of TADF in gold NCs is still quite limited.Therefore, further investigation into the TADF mechanism and improving the TADF efficiency of gold NCs will hold significant importance in future efforts.Below we first discuss TADF in organic dyes, which would benefit the understanding of subsequent discussions on the TADF in gold NCs.
The kinetics of a TADF process is much more complicated than fluorescence and phosphorescence because it may coexist with PF and phosphorescence (Figure 1C).When a RISC process is involved, the full photophysical processes can be described by a three-state model in Figure 4A.The closedform solutions to the two differential equations are too complicated and hard to correlate with physical parameters. [131,132]evertheless, by introducing appropriate assumptions (e.g., Φ phos = 0, which is justifiable for organic systems), the solutions can be simplified.This, in conjunction with the experimentally measured lifetimes and PLQY, enables the estimation of key parameters (i.e., k ISC , k r S , etc.) for the photophysical processes. [133]xperimentally, TADF materials can be categorized into two types, depending on the rate of ISC. [136]In many organic TADF molecules, the ISC rate is typically within the range of 10 5 − 10 8 s −1 , allowing for a clear distinction between PF and TADF in the micro-second-scale PL decay measurement.For example, CzDBA is a diboron-compound-based TADF material (Figure 4B,C) that presents a small ΔE S-T (≈20 meV). [134]As shown in Figure 4C, the decay curve of CzDBA follows a typical bi-exponential function.The shorter component represents the lifetime of PF while the longer component represents the TADF lifetime.On the other hand, when the ISC rate increases to the range of 10 10 − 10 11 s −1 , a pre-equilibrium between the S 1 and T 1 state is established before any emission process. [126,136] complexes where only a mono-component decay curve is observed in the microsecond-scale PL measurement (Figure 4D). [135]Nevertheless, an ultrafast component (≈160 ps) can be observed in ultrafast time-correlated single photon counting measurements (Figure 4E).Since the rate constant k ISC >> k r S or k nr S , the observed 160 ps lifetime should be assigned to the lifetime of ISC.
Like phosphorescence, TADF can also be quenched by triplet state quenchers such as 3 O 2 and perylene.Therefore, the determination and analysis of TADF heavily rely on temperaturedependent steady-state and time-resolved PL measurements.In general, the intensities of fluorescence and phosphorescence increase with decreasing temperature owing to the inhibition of non-radiative relaxations.In TADF materials, the temperature dependence of reverse intersystem crossing (k RISC ) follows an Arrhenius formula: k RISC (T) = A exp(− ΔE S−T k B T ). [133]Consequently, as the temperature decreases, the rate of RISC is suppressed since it is an up-hill process and requires thermal energy input; thus, lower temperatures lead to a reduction of TADF intensity.This reduction stands as the clearest sign of TADF existence in temperature-dependent steady-state PL measurements.Regarding the temperature-dependent time-resolved PL measurements, the short lifetime component usually varied a little, whereas the long lifetime component increases significantly (Figure 4C,F).When the RISC is completely suppressed, the long lifetime component becomes the lifetime of phosphorescence.The equation given in Figure 4F is widely applied to extract the k fl , k ph , and ΔE S-T from the temperature-dependent lifetime, where fl = fluorescence and ph = phosphorescence.
In gold NCs, TADF typically coexists with phosphorescence (as in metal complexes) and the small ΔE S−T makes the two emission processes overlap or merge into one apparent peak, which causes extra difficulties in identifying TADF. [37,102]Generally, the decrease of k RISC with the temperature drop will result in the following three phenomena that can serve as the criteria for identifying the existence of TADF.First of all, the most important phenomenon is the decline in the overall radiative rate with decreasing temperature (Scheme 3A).In examples of metal complexes, at low temperatures, the overall k r (PLQY = k r  ave ) can be suppressed to a quarter or even less when compared to the room temperature case. [137]Second, the PL intensity will experience a zig-zag trend as the temperature decreases (Scheme 3B); note: when there is no interference from TADF, the PL intensity of materials should be a monotonic increase due to the suppression of non-radiative relaxation as the temperature decreases.When TADF is involved, the suppression of TADF at low temperatures typically leads to a decrease in the overall PL intensity in a certain temperature range.However, it should be noted that the increase of phosphorescence with the temperature decrease may offset the decrease of TADF, which makes the zig-zag trend vary in each case.Third, the PL peak position will also experience a zig-zag trend (Scheme 3C) as the temperature decreases.Theoretically, the PL peak position should present a monotonic blue shift as the temperature decreases because of the suppression of electron-vibration coupling, but the suppression of TADF at low temperatures will decrease the PL intensity at shorter wavelengths and make the PL peak from phosphorescence (at longer wavelengths) become dominant.
][140] In 2019, Han et al. synthesized an alkynyl-protected Au 22 ( t BuC≡C) 18 NC (Figure 5A, denoted as Au 22 ) and found TADF in this NC. [37]The UV-vis absorption spectrum of Au 22 exhibits a broad absorption peak between 400 and 550 nm, corresponding to the HOMO-LUMO transition.At room temperature, Au 22 shows a single emission peak between 550 and 850 nm.As the temperature decreases, the single peak splits into two peaks and the high energy peak completely disappears when the temperature is lower than 150 K (Figure 5B).The high energy peak was assigned to TADF, whereas the low energy one was ascribed to phosphorescence.More recently, Luo et al. reported the existence of TADF in Au 38 (PET) 26 (denoted as Au 38 ).Unlike Au 22 , the PL intensity of Au 38 was found to monotonically increase as the temperature decreases (Figure 5C).It is because the RISC in Au 38 is less efficient so the increase of phosphorescence (due to the suppression of non-radiative decay) completely offsets the decrease of TADF.The temperature-dependent PL decay curves of Au 38 clearly show the disappearance of the TADF component as the temperature decreases (Figure 5D).Unfortunately, the PLQY of both Au 22 and Au 38 are low (PLQY < 2% in solution), which hampers their application in optoelectronic devices.Very recently, the Zang group prepared several Au(I)-Cu(I) clusters that give rise to efficient TADF in the visible range (PLQY = 30−95%). [139,140]These NCs with excitation at 530 nm.Reproduced with permission. [37]Copyright 2019, Wiley-VCH.C) Temperature-dependent emission spectra of Au 38 with excitation at 400 nm.D) Decay profiles of Au 38 at different temperatures.Reproduced with permission. [138]Copyright 2023, Springer Nature.clusters show great potential for applications as scintillators and high-performance OLED (organic light-emitting diodes).Overall, the design and synthesis of NIR TADF gold nanoclusters remain an intriguing and challenging task, which is worth further effort in future research.

Dual Emission
In addition to the usual single emission peak, several types of intriguing dual emission processes have been reported in gold NCs. [99,141,142]These dual-emissive gold NCs are able to emit light at two different peaks, which makes such gold NCs a promising candidate for application in multi-color imaging, ratiometric sensing, and novel optoelectronic devices.Of note, all the dualemissive cases discussed in this section have a distinction between the two emission peaks, as opposed to the superimposition of emission peaks into one.
The first type of dual emission is simultaneous fluorescence and phosphorescence (Figure 6A).This phenomenon indicates that k ISC is comparable to k r S (i.e., the radiative rate of the S 1 state), and no RISC exists.A good example is the Au 42 (PET) 32 rod (denoted as Au 42 ). [99,143]Under 380 nm excitation, Au 42 presents a high-energy PL peak at 875 nm and a low-energy PL peak at 1040 nm (Figure 6B).The excitation spectra of both emission peaks are found to be consistent with the optical absorption spectrum of Au 42 , suggesting that both emission processes follow Kasha's rule. [99,106]Time-resolved PL measurements found that the lifetime of the 875 nm peak is 0.72 ns, whereas the lifetime of the 1040 nm peak is 2400 ns (Figure 6B).Furthermore, the 1040 nm peak can be quenched by 3 O 2 while the 875 nm peak is barely affected.Therefore, the dual emission of Au 42 is revealed to be fluorescence (for the 875 nm emission) and phosphorescence (for the 1040 nm emission).Beyond Au 42 , there is no other report about the coexistence of fluorescence and phosphorescence in gold NCs.One of the reasons is that the ΔE S-T is usually ultrasmall (e.g., ≈50 meV) in gold NCs, [24,37,138] which makes fluorescence overlapping with phosphorescence without peak separation.There are some reports that have found the coexistence of a short lifetime and a long lifetime for a single emission peak. [62,144]Such a scenario could imply that the emission peak comprises both fluorescence and phosphorescence, but further investigation into the emission mechanism of these cases is still required before drawing a solid conclusion.
The second type of dual emission is dual phosphorescence (Figure 6D).In recent work, Si et al. [130] synthesized an alkynyl-protected [Au 20 (CZ─PrA) 16 ] 2− [denoted as Au 20 ; CZ─PrAH = 9-(prop-2-yn-1-yl)-9H-carbazole].Upon excitation with 365 nm light, Au 20 exhibits two emission peaks at 820 and 940 nm (Figure 6E).Interestingly, both emission peaks possess microsecond-scale lifetimes (Figure 6F), and their intensity was suppressed in an oxygen atmosphere, suggesting the phosphorescence nature of the two emission processes.However, the excitation spectra of both emission peaks were found to be inconsistent with the UV-vis spectrum of Au 20 . [141]Combining the observations with TD-DFT calculations, the two emission processes  [99] Copyright 2022, American Chemical Society.D) Emission pathway of dual phosphorescence.E) UV-vis absorption spectrum (green) and PL spectrum (orange) of Au 20 .F) Decay profiles of PL I and PL II.Reproduced with permission. [141]Copyright 2023, The American Association for the Advancement of Science.G) Emission pathway of dual fluorescence.H) UV-vis absorption spectrum (black line) and PL spectrum (grey line) of Au 24 .I) Decay profiles of PL I and PL II.Reproduced with permission. [142]Copyright 2020, Springer Nature.
were ascribed to ligand-to-metal charge transfer state ( 3 LMCT) and metal-to-metal charge transfer state ( 3 MMCT). [141]The discovered dual phosphorescence mechanism in Au 20 is close to the situation of the Ag 29 + Ag case that we discussed above and can inspire future studies of the PL mechanism in gold NCs.
The third type of dual emission observed in gold NCs relies on an excited-state transformation (Figure 6G), akin to the widely recognized twisted intramolecular charge transfer in organic molecules. [106]In this scenario, upon photoexcitation, the excited electron first relaxes into an electron-redistribution (locally excited) state, which gives rise to PL I (high energy).Sub-sequently, after hundreds of picoseconds or several ns, the electron further relaxes into a distorted electron-redistribution, leading to PL II. [142,145]Notably, the distorted excited state is much more polar than the local excited-state, making it more stabilized in polar environments.This type of mechanism was first observed by Li et al. in a series of bi-tetrahedral gold NCs such as Au 24 (TBBM) 20 [denoted as Au 24 , TBBM = 4-(tert-butyl)benzyl mercaptan)].The emission spectrum of Au 24 features dual emission at 670 nm (PL I) and 1050 nm (PL II) as shown in Figure 6H.The excitation spectra of both emission peaks were found to be consistent with the absorption spectrum of Au 24 .
Interestingly, the intensity of PL I is significantly enhanced in non-polar solvents, whereas the intensity of PL II decreases.Moreover, high-viscosity or high-pressure environments were found to favor PL I, but not PL II. [142]All these results indicate that PL II is from a distorted charge transfer state.Time-resolved PL measurements showed that the lifetime of PL I is ≈70 ns, and the lifetime of PL II is 265 ns (Figure 6I).The 1.6 ns component (from TAS measurements) was assigned to the transformation from the local excited-state to the distorted charge-transfer state.

Strategies to Improve Photoluminescence Quantum Efficiency
After photoexcitation, the excited electron relaxes back to the ground state through radiative and non-radiative pathways.According to the definition of PLQY (PLQY = k r k r + k nr ), one can enhance the radiative rate or suppress the non-radiative rate to improve the PLQY.In gold NCs, the values of k r are usually ≈10 4 to 10 5 s −1 , whereas the k nr can vary from 10 5 to 10 7 s −1 . [20,24,58]pparently, non-radiative relaxation holds more potential in PL improvement and it is practically much easier to manipulate than the radiative relaxation. [144]Many early works, unfortunately, did not provide the value of k r for gold NCs, which made it difficult to understand the key factors that affected k r in those studies.It is well-known that the rate of spontaneous emission is proportional to the transition dipole moment and cube of the PL energy. [58,133]ut the understanding of this relationship in gold NCs is still poor.Therefore, in this review, we focus on the suppression of non-radiative decay of photoexcited states.To achieve this, it is crucial to minimize the coupling between the vibrational states and the electronic state.According to the energy gap law, [146] a larger HOMO-LUMO gap leads to slower non-radiative relaxation.Therefore, expanding the HOMO-LUMO gap is an important method to tune the PLQY.Nevertheless, for NIR luminescent NCs, the value of their energy gap must be small and is inevitable; therefore, approaches to tuning the electron-vibration interaction under the given HOMO-LUMO gap become essential.Here, we will discuss the methods such as reducing the density of vibrationally excited states by rigidifying surface Au-ligand motifs [147] and suppressing the electron-core-vibration coupling. [125]

HOMO-LUMO Gap Expansion
In accordance with the energy gap law, a decrease in the energy gap between the excited state and ground state is predicted to lead to an increase in the rate of non-radiative decay processes. [148]herefore, the expansion of the HOMO-LUMO gap (E g ) is an effective way to improve the PL performance of golf NCs. [149]he E g of gold NCs can be influenced by multiple factors, in which ligand modification and heteroatom doping are straightforward and intensely studied. [3]To elucidate the impact of these two variables, we use the icosahedral Au 13 (Au@Au 12 ) superatom with eight valence electrons, protected by various ligands such as Au 2 (SR) 3 oligomers, phosphines, Au 2 (C≡CR) 3 oligomers, and N-heterocyclic carbenes (NHCs), as models for illustration.152] The electronic and steric characteristics of ligands on gold NCs have been demonstrated to profoundly influence both the electronic/geometric structures of nanoclusters and their optical performance. [153,154]Differences in the electron-donating capability of ligands can substantially impact the LMCT process, therefore regulating the PL performance. [155]To clarify the influence of the ligand nature, the optical absorption spectra of multiple Au 13 superatoms protected by different ligands are depicted in Figure 7A for comparison. [150]The organic ligands used to protect these superatoms include electronegative ligands, for example, halides, thiolates, selenolates, and alkynyls, and electroneutral ligands such as phosphines, stibines, and NHC.The spectral profiles of the Au 13 superatoms are remarkably affected by the nature of the ligand layer, and the optical energy gaps estimated by the absorbance onsets vary with ligands, ranging from 0.9 to 2.2 eV.This finding underscores the susceptibility of electronic structures to subtle changes in geometric structures and electrostatic environment induced by the ligand layer, correspondingly displaying distinct emission properties.A prototypical example is the NHC-protected Au 13 which has a wide E g of 2 eV. [26]Multiple CH- and -- interactions between the benzyl substituents and both rings of the benzannulated NHC core are contained in the NHC-protected Au 13 NCs (Figure 7B).As a result, the improved structural rigidity restricts the vibrational and rotational motions of ligands (Figure 7C), limiting pathways for non-radiative decay processes, which leads to a QY (16%) for Au 13 superatoms (Figure 7D).Nonetheless, the implications of ligand type on structural aspects, such as geometric symmetry, rigidity, and electronic cloud density and distribution, present a complex scenario.Therefore, systematic investigations on the ligand effects on the PL properties are still needed in future explorations.
Doping of foreign elements (M) presents an effective approach to manipulating the electronic properties of gold NCs, owing to the differences in electronegativity between M and Au. [156,157]The disparity in electron densities also allows for clear identification of doping positions via X-ray crystallography analysis.A series of atomically precise syntheses have revealed that foreign elements can be regioselectively doped into either the Au kernel or the surface motifs. [23]Taking the icosahedral Au 13 (Au@Au 12 ) superatom as an example, elements from Group IX (Rh, Ir) and X (Ni, Pd, Pt) tend to occupy the central position to form M@Au 12 icosahedra, whereas Group XI (Ag, Cu) and XII (Cd, Hg) elements favor the surface position, forming Au@MAu 11 icosahedra.Moreover, the preferential sites of individual dopants are maintained in the co-doped superatoms such as M@Ag n Au 12-n (M = Pd, Pt). [158]The impact of doping on electronic structures can be explicated through a two-step spherical jellium model, wherein the host (Au) and dopant atoms contribute distinct yet uniform background potentials, as depicted in Figure 8A. [159]Group IX and X atoms, when doped as M − and M 0 , respectively, possess a lower valency and thus provide a shallower potential than that of the Au atom, which results in an upshift of 1P orbital of the M@Au 12 superatom.In contrast, group XII atoms are doped as M 2+ with a higher valency, and therefore offer a deeper potential, instigating a downshift of the superatomic orbitals. [150]igure 7. A) Optical absorption spectra of Au 13 superatoms protected by different ligands, with the right table showing the molecular formulas.Reproduced with permission. [150]Copyright 2021, American Chemical Society.B) Crystal structure of NHC-protected Au 13 NCs.C) Subset of inter-ligand CH- interactions in NHC-protected Au 13 NCs, showing the high degree of organization of the ligand shell.D) Emission and excitation spectra of NHC-protected Au 13 NCs.Reproduced with permission. [26]Copyright 2019, American Chemical Society.
Accordingly, the E g can be either widened or narrowed.Notably, the degree to which the E g expands is contingent upon the group and period of the doping element. [150]As illustrated in Figure 8B, the E g , as determined from the optical spectra, increases when doping a smaller group element from the same period at the center of Au 13 (Ir > Pt > Au > Hg).On the other hand, the E g diminishes when doping a smaller period element from the same group (Pt > Pd > Ni) at the center of Au 13 .
The E g variation can effectively impede the electronic relaxation, thereby facilitating an increase in the PLQY. [24]For instance, when group IX (Rh, Ir) and X (Pd, Pt) elements are doped into the central position of the Au@Au 12 (dppe) 5 Cl 2 NCs (Figure 8C), [58] an expansion of E g is observed, which exhibit a trend of Pd < Au < Rh < Pt < Ir (Figure 8D).In terms of PL emission, the PL peaks exhibit a monotonic blueshift correlating to a decrease in the group and an increase in the period of the doping element (Figure 8E), which aligns with the changes noted in the HOMO-LUMO gap as mediated by doping.The PLQYs of PtAu 12 , RhAu 12 , and IrAu 12 , which are 60.6%, 46.4%, and 66.5%, respectively, significantly exceed that of the undoped Au 13 (11.7%)(Figure 8F).On the other hand, the PLQY of PdAu 12 (11.4%)was observed to be comparable to that of Au 13 .These improvements in the PLQY are attributable to the widening of the gap, a change that promotes the radiative process and simultaneously inhibits the non-radiative process.Additionally, the effectiveness of this doping-mediated enhancement of PLQY was validated through a comparative analysis with PLQYs of M@Au 12 superatoms protected by a different ligand, dppm. [58]he E g expansion-induced PL enhancement is also corroborated in other Au NCs.As shown in Figure 8G, when the Au 25 rod is doped with varying numbers of Ag atoms, the biicosahedral structure remains intact, but the electronic structure undergoes distinctive alterations, which leads to critical differences in PL performance. [116]In the Au 25 rod, the HOMO and LUMO orbitals are localized on the ten Au atoms at the waist and the central Au atom, respectively.As different numbers of Ag atoms are introduced, they first replace the two vertex Au atoms preferentially, followed by the ten Au atoms at the waist and the central Au atom.Consequently, when the 13 Au atoms are replaced by Ag, the LUMO, consisting of the Au 6s and 6p orbitals, will be largely altered, as the Ag 5s orbital is higher than the Au 6s.This gives rise to an increased HOMO-LUMO energy gap, boosting the QY dramatically (up to 40.1%) (Figure 8H).A recent revisit of the PL properties of the Au 25 rod and silver-doped Au 25-x Ag x rods discovered that both Ag 13 Au 12 and Ag 12 Au 13 are crucial for the strong PL intensity in Au x Ag 25-x rods. [111]The PLQYs of Au 25 , Ag 13 Au 12 , and Ag 12 Au 13 were determined to be 8%, 29%, and 56%, respectively.Similarly, Mitsui et al. attributed the PL enhancement to the larger S 0 -T 1 gap in Ag 13 Au 12 and Ag 12 Au 13 , which suppresses the non-radiative process. [111]Nevertheless, considering the small differences in HOMO-LUMO energy gap (≈0.1 eV) between the rod Au 25 and Au x Ag 25-x (x = 12 or 13), the significance increase of PLQY in Au x Ag 25-x may not be just because of the energy gap law, and other factors should be considered in future research.Furthermore, heteroatom doping also has the capacity to modulate the HOMO-LUMO gap of silver [125,160] and copper NCs [161][162][163] , consequently impacting both radiative and non-radiative processes.However, there is a deficiency in mechanistic insights regarding how heteroatoms promote the radiative process in gold, silver, and copper NCs, necessitating future research for clarification.

Surface Motif Rigidification
Gold NCs are typically characterized by a core-shell structure, comprised of an inner Au kernel and outer Au-ligand motifs. [2]he PL properties of gold NCs are intimately associated with the surface Au-ligand motifs.The restriction of intramolecular motions of these surface motifs has been identified as an effective way to reduce energy loss from photoexcited states via nonradiative relaxation, therefore enhancing the emission efficiency Figure 8. A) Two-step jellium potentials for doping lower (left) and higher (right) valent atoms into Au 13 .B) Optical gaps of M@Au 12 (dppe) 5 Cl 2 (M = Ir, Pt, Au) (red), M@Au 12 (Au 2 (PET) 3 ) 6 (M = Pt, Au, Hg) (blue), and M@Au 12 (PMe 3 ) 11 Cl (M = Ni, Pd, Pt) (green).Reproduced with permission. [150]opyright 2021, American Chemical Society.C) Superatomic cores of MAu 12 .D) UV-vis absorption spectra of MAu 12 in acetonitrile.Insets correspond to the expanded spectra in the onset region.The upper right is the plot of the HOMO-LUMO gaps of MAu 12 .E) Plot of the energy of the PL peak of MAu 12 .F) Plot of the PL QY of MAu 12 under Ar atmosphere.Reproduced with permission. [58]Copyright 2022, Wiley-VCH.G) X-ray structures of rodshaped Au 25 doped with different numbers of Ag atoms.H) Photoluminescence spectra of rod-shaped Au 25 , Ag x Au 25-x (x ≤ 12), and Ag x Au 25-x (x ≤ 13).Reproduced with permission. [116]Copyright 2014, Wiley-VCH. of gold NCs. [122]So far, various methods have been developed to impede the intramolecular motions of the surface Au-ligand motifs, thereby inducing rigidity. [9]These methods can be grouped into four primary strategies: 1) bulky group bonding, 2) hostguest interactions, 3) aggregation-induced emission, and 4) selflocking effect.Here we select some representative work to illustrate these strategies.
The first strategy is termed the bonding strategy, which involves bonding (e.g., covalent bonding and other interactions such as H-bonds, dipolar, van der Waals, or electrostatic interactions) between bulky groups, such as counter-ions and the sur-face Au-ligand shell of a given gold NC.The compact packing of the introduced bulky groups onto the nanocluster surface will rigidify the Au-ligand shell, reducing energy loss through intramolecular rotations and vibrations and thus increasing the PL intensity of the as-engineered gold NCs.A representative example is the bonding between Au 22 (SG) 18 (where SG = glutathione) and bulky tetraoctylammonium (TOA) cations, as demonstrated by Lee's group (Figure 9A). [122]The interaction between the alkyl chains of TOA cations and the carboxylate anions of glutathione on Au 22 makes the Au(I)-thiolate shell rigid.As a result, the PL efficiency of the rigidified Au 22 NCs is enhanced remarkably and  [122] Copyright 2015, American Chemical Society.B) Schematic diagram of intramolecular cross-linking of ligands by the formation of bis-Schiff linkages on the Au 22 (SG) 18 surface.C) PL spectra of Au 22 (SG) 18 and PDA-Au 22 (SG) 18 NCs.D) Effect of the content of surface-bound Au(I)-SG complexes on the PDA-induced PL enhancement of Au NCs.Reproduced with permission. [123]Copyright 2022, Springer Nature.E) Schematic diagram of host-guest assembly of CB (7) and CB (8) with FGGC-Au 22 and the corresponding PL photographs.Reproduced with permission. [165]Copyright 2020, Royal Society of Chemistry.F) Schematic diagram of AIE of non-luminescent oligomeric Au(I)-thiol complexes and Au(0)@Au(I)-thiol core-shell NCs.Reproduced with permission. [52]Copyright 2012, American Chemical Society.G) Schematic diagram of the self-locking effect in Au 38 S 2 (SR) 20 NCs.Reproduced with permission. [120]Copyright 2021, American Chemical Society.H) Schematic diagram of structural differences of Au 28i and Au 28ii NCs.I) PL spectra of Au 28i and Au 28ii NCs.Reproduced with permission. [166]Copyright 2020, American Chemical Society.
the QY at room temperature increases from 8.0% to >60%. [122]urthermore, the PL intensity of the cation-paired Au 22 (SG) 18 could be finely tuned via control over the bulkiness of the introduced counterions by changing the chain length.Additionally, covalent bonding between the Au(I)-thiolate shell with aromatic molecules and pyrene chromophores can also achieve rigidification of the surface shell of Au 22 (SG) 18 , leading to a fivefold PL enhancement and a high QY of 30%. [164]Recently, the Chen and Xie groups illustrated the shell rigidification by forming the bis-Schiff base linkages between dialdehydes such as 2,6pyridinedicarboxaldehyde (PDA) and Au 22 (SG) 18 (Figure 9B). [123]he PDA can react with amino functionalities on the surface of Au 22 (SG) 18 to form imine (─CH═N─) bonds, inducing crosslinking of Au(I)-SG motifs.As a result, the flexibility of the Au(I)-SG motifs was significantly suppressed, leading to a remarkable decrease in the non-radiative decay rate and an obvious increase in the radiative decay rate, which boosted the PL inten-sity of Au 22 (SG) 18 over eleven-fold (QY of 48% at room temperature) (Figure 9C).Moreover, this dialdehyde-mediated intracluster cross-linking strategy is not limited to Au 22 (SG) 18 and can be extended to other gold NCs with a core-shell structure.A higher content of Au(I)-SG complexes on the cluster shell can further enhance the luminescent effect, showing the potential applicability and versatility of this approach (Figure 9D).
The second strategy involves host-guest self-assembly-a specific type of molecular interaction by which smaller molecules are encapsulated within a larger host structure.Such assembly has received extensive attention in molecular recognition and supramolecular chemistry. [167,168]This assembly process is typically driven by non-covalent forces, including ion-dipole interactions, hydrophobic effects, van der Waals forces, and H-bonding effects.The tailorable ligand functionalities of gold NCs allow for the creation of abundant recognition sites on the surface, which may facilitate the triggering of the host-guest self-assembly between gold NCs and other molecules such as macrocyclic cucurbit(n)urils (CB[n]) to rigidify the Au-thiolate shell.Recently, the Ma group developed a CB(n)-assisted (n = 7, 8) host-guest self-assembly to rigidify the surface of Au-FGGC NCs, for example, Au 22 (FGGC) 18 , using CB-FGGC (FGGC = N-terminal Phe-Gly-Gly-Cys peptide) recognition (Figure 9E). [165]The FGG fragment on the FGGC peptide served as a binding site for CB (7)  and CB (8) to form the compressed Au(I)-thiolate surface on gold NCs, which promotes the radiative decay and suppresses the nonradiative decay processes simultaneously.As a consequence, the QY of the rigidified Au 22 (FGGC) 18 was up to 51% for CB (7) and 39% for CB (8), while the plain Au 22 (FGGC) 18 , without the hostguest self-assembly, exhibited a QY of only 7.5% under the same conditions.
The 3 rd strategy is the widely reported aggregation-induced emission (AIE), which describes a photophysical phenomenon in which molecules or other materials, typically non-luminescent or only weakly luminescent, exhibit significantly enhanced luminescence upon aggregation in poor solvents or the solid state. [169]his concept was first proposed by the Tang group to clarify some organic fluorophores involved in photophysical variations upon aggregation.When it comes to gold NCs, the Xie group first highlighted the significance of AIE in the PL of gold NCs (Figure 9F). [52]It was found that non-luminescent oligomeric Au(I)-thiolate complexes become intensely luminescent upon aggregation, with the intensity and color of the PL being dependent on the degree of aggregation.Furthermore, Au(0)@Au(I)-thiolate core-shell NCs, formed by the induction of Au(I)-thiolate complex aggregation onto the Au(0) core, displayed strong PL that is generated by the AIE of surface Au(I)-thiolate complexes.The PL of AIE-type luminescent gold NCs is superior to the regular Au n (SR) m NCs with short Au(I)-SR motifs.This is because both intramolecular and intermolecular motions of the long staple motifs, for example, Au(I)-thiolate complex, are effectively restricted.As a result, the degree of photoexcitation energy dissipation by molecular motions is remarkably minimized.The AIE in gold NCs is fundamentally tied to the surface-emission, arising primarily from the LMCT and/or ligand-to-metal-metal charge transfer (LMMCT) state. [68]Therefore, the central strategy to enhance the performance of AIE-type luminescent gold NCs should involve the favorable manipulation of the LMCT and/or LMMCT processes.The AIE mechanism has received broad acceptance and continuous refinement in the research on luminescent gold NCs, but as Xie et al. pointed out, [68] there is no clear consensus yet regarding the specific underlying principles of AIE as it comes to the luminescent metal NCs.
The fourth strategy pertains to the self-locking effect, which is characterized by the deliberate construction of specific surface motifs into elongated or interlocked arrangements, such as ring-like metal(I)-ligand configurations.This design induces a 'locked' orientation in the surface motifs, limiting their mobility and restricting intramolecular motions within the NCs.Consequently, it leads to a reduction in non-radiative decay pathways, effectively increasing the QY of radiative decay.The Au 38 S 2 (SR) 20 NCs provide an illustrative example of the self-locking effect (Figure 9G).The Au 38 S 2 (SR) 20 NCs feature an Au 8 S 9 "lock ring" motif and surface "lock atoms" that bridge the fundamental Au kernel units (e.g., tetrahedra, icosahedra, etc.). [120]The surface "lock rings" and "lock atoms" facilitate the conformational locking of the nanocluster in the desired excited-state geometry, thereby increasing PL efficiency by minimizing the energy dissipation through non-radiative decay pathways.As a result, the Au 38 S 2 (SR) 20 NCs display a bright NIR PL at 900 -nm with QY up to 15% in common solvents (such as toluene) under ambient conditions.In addition to the ring-like motifs, longer motifs can also demonstrate the function of the self-locking effect in enhancing the PL of gold NCs.As illustrated in Figure 9, Wu's group reported a pair of isomers, in which the Au 28i NC comprises a four-tetrahedral Au 14 kernel capped by two Au 3 (SR) 4 trimers and four Au 2 (SR) 3 dimers, but the Au 14 kernel of the isomeric Au 28ii NC is covered by four Au 3 (SR) 4 trimers and two Au(SR) 2 dimers (Figure 9H). [166]The structural differences between these two isomeric Au 28 NCs lead to a substantial change in their PL properties.Specifically, the PL intensity of Au 28ii exhibits a 7.4-fold enhancement compared to that of Au 28i (Figure 9I).This large increase can be attributed to the trimeric staples (Au 3 (SR) 4 ) within Au 28ii .By providing a more rigid surface and reducing mobility, the trimeric staple contributes to the self-locking effect, thus leading to a more efficient PL response.Although the self-locking strategy is still in early development (only with limited successful implementations yet), it represents a unique and potent strategy within the broader toolkit of methods dedicated to fine-tuning and advancing the PL characteristics of gold NCs.
In addition to the aforementioned four strategies, the rigidification of surface Au-ligand motifs of gold NCs can also be accomplished by incorporating them into scaffolds like polymers, [21,99] and metal/covalent-organic frameworks. [170,171]Other methods include dissolving gold NCs in viscous solvents, [142] subjecting them to crystallization, [172] lowering the temperature, [63,138] or increasing the pressure. [173]All of these strategies have been extensively employed to enhance the PL of gold NCs, and mechanistic insights are also obtained.

Tailoring the Core Vibrations
In addition to the surface vibrations, the core vibrations also play a critical role in the hot electron relaxation process in gold NCs, [63,101] thus, a comprehensive understanding of the core vibrations is necessary in order to manipulate the PL properties of gold NCs.In contrast to the surface vibrations in which the relatively high-frequency Au-S vibration (250-400 cm −1 ) is dominant, [174] the core vibrations arise from Au-Au motions, which typically fall between 50 to 200 cm −1 . [175]Such lowfrequency vibrational modes are difficult to measure, hampering the understanding of their roles in affecting the PL properties of gold NCs.
Recently, Liu et al. carried out cryogenic temperaturedependent PL measurements of Au 25 (PET) 18 − , CdAu 24 (PET) 18 , and HgAu 24 (PET) 18 (denoted as Au 25 , CdAu 24 , and HgAu 24 , respectively, or collectively as MAu 24 , where M = Au, Cd, Hg) in thin films and discovered that the heteroatom doping can significantly affect the core vibrations in this system. [125]As shown in Scheme 4A, the structure of Au 25 comprises an Au 13 icosahedral core and six Au 2 (PET) 3 staple motifs. [176]The doping site of Cd or Hg is on the Au 12 icosahedral shell, while the six Au 2 (PET) 3 staple motifs in CdAu 24 and HgAu 24 are the same as those on the parent Au 25 .Generally, the Au-S motif vibrations in this  [125] Copyright 2023, American Chemical Society.
series of NCs can be divided into tangential vibrations and radial vibrations. [177]The polar nature of the Au─S bond in the Au(inner core)−(SR)−Au(motif)−(SR)−Au(motif)−(SR)−Au(inner core) chain makes these vibrations strongly coupled to the electromagnetic field (i.e., light). [63]Hence, these vibrational modes are optically active (called optical phonons).Meanwhile, the core vibrational modes in the MAu 24 series are found to mainly consist of acoustic breathing mode and quadrupolar mode. [178]o understand the interaction between the electron and vibrations in the three MAu 24 NCs, temperature-dependent PL spectra were measured from room temperature down to 20 K.In conventional semiconductor materials, the temperature-dependent PL homogeneous broadening follows a weak electron-phonon coupling model that can be described by Scheme 4B Equation (1).Theoretically, when the temperature is higher than 100 K, the weak-coupling model should lead to a linear increase of the PL linewidth (Γ) as the temperature increases (∝T, the blue dash line in Scheme 4C).However, the PL broadening (orange squares in Scheme 4C) of the MAu 24 series shows a nonlinear trend (only Au 25 is shown for clarity), which does not follow the weak coupling model.Liu et al. proposed that the nonlinear trend indicated a strong electron-phonon interaction, and the strong coupling model (Scheme 4B Equation ( 2)) was found to fit well the temperature-dependent PL linewidth data of the three MAu 24 NCs (∝

√
T, the red line in Scheme 4C, illustrated with Au 25 ).The extracted parameters show that the electron− optical-phonon (i.e., Au-S motif vibrations) interactions are extremely strong for all three samples (Scheme 4D).Interestingly, the electron−acoustic-phonon (i.e., core vibrations) interactions in CdAu 24 and HgAu 24 are significantly suppressed compared to Au 25 (Scheme 4D).The suppression of acoustic phonons in CdAu 24 and HgAu 24 leads to a stronger PL intensity in their thin films. [125]verall, understanding the role of core vibrations in the electron dynamics of gold NCs is crucial to the design and synthesis of highly emissive gold NCs.In addition to metal doping, Zhong et al. [101] discovered that layer-by-layer ligand engineering can also suppress the kernel vibrations and significantly improve the PL (i.e., 0.3% to 90%).Compared to many reports of rigidification of the NC's shell, tailoring the core vibration is still in its infancy, thus, the future effort is still much needed.

Applications
The PL of gold NCs has been found to span 500 to 1700 nm, which covers the visible region (400-700 nm), the first NIR range (700-1000 nm), and the second NIR range (1000-1700 nm).The broad coverage by a single type of material (i.e., gold NCs) is quite unique.[181] Since there are several recent reviews that comprehensively discussed the state-of-the-art of using gold NCs in biomedical applications, [40,45,179,182,183] here we only briefly discuss the applications of gold NCs in NIR-II bioimaging.[186] Additionally, the versatility of gold NCs extends to their utilization in fabricating an array of optoelectronic devices, including but not limited to light-emitting diodes (LEDs), photodetectors, and sensors. [46,50,51,139,140,187,188]The unique optical properties and tunable emission characteristics of gold NCs also make them a promising candidate to push the boundaries of optoelectronic technology and enable innovative advancements in various fields.

NIR-II Bioimaging
Within the NIR-II window (1000-1700 nm), the optical scattering of biological structures is minimized, enabling the real-time noninvasive, high contrast and high-resolution imaging of cells, tissues, and organisms. [189,190]Compared to the conventional NIR-II emissive materials such as organic dyes, QDs, and carbon nanotubes, gold NCs present several advantages such as higher safety and biocompatibility, as well as the ease of surface functionalization.Moreover, gold NCs are of ultrasmall sizes (<2 nm), which allows for an efficient renal clearance. [40,190]These attributes are of paramount importance for their practical applications.
Among the gold NCs, [Au 25 (SR) 18 ] − (denoted as Au 25 ) has drawn significant attention due to its suitability in NIR-II bioimaging applications. [181,191,192]To enable its application in bioimaging, ligands such as glutathione (SG), cysteine, and captopril have been introduced in the synthesis, [192][193][194] which are essential for conferring water solubility to Au 25 -a critical prerequisite for bioimaging applications.Upon photoexcitation, water-soluble Au 25 presents a broad emission band between 900 and 1400 nm, covering both NIR I and NIR II (Figure 10A). [192]Liu et al. found that the long wavelength emission from Au 25 (SG) 18 can easily penetrate the skull of the mouse, facilitating high-resolution imaging of damaged blood vessels in the brain (Figure 10B). [192]However, it is important to mention that the NIR PL quantum yield of Au 25 (SG) 18 falls below 1%, thus, higher QYs are much desired for the applications. [181,192]Literature work suggests that the NIR PL intensity of Au 25 can be enhanced through the incorporation of distinctive ligands, metal doping, and protein binding. [36,192]In a recent breakthrough, Baghdasaryan et al. discovered that Au 25 (SG) 18 can be modified by 4-aminophenylphosphorylcholine (PC for short, Figure 10C).The resulting Au-PC cluster shows an enhanced NIR PL intensity and nearly complete renal excretion from the body within a 24-hour timeframe (Figure 10D).To enhance the PL of Au 25 , Huang et al. [34] found that single atom Cd substitution (i.e., Au 24 Cd 1 ) led to 56-fold enhancement, which was contributed by the Cd 4d state in the HOMO and a redistribution of energy level near the gap of Au 24 Cd 1 .Such NCs may find broader applications in biomedicine.
While gold NCs have demonstrated their potential in NIR-II bioimaging, a few critical issues remain to be addressed.Notably, performing imaging within the range of >1300 nm offers optimal advantages in terms of penetration depth and signal-tonoise ratio. [181]Yet, a prevailing concern is that many of the existing water-soluble gold NCs display only a weak emission tail in the range of >1300 nm.Therefore, designing an efficient method to convert known hydrophobic gold NCs into water-soluble ones is necessary.Additionally, achieving precise targeting of specific cells or tissues requires functionalization of the gold NCs with appropriate ligands/peptides that can recognize and bind to the desired biological targets.These intricate endeavors demand careful considerations and innovations to harness the full potential of gold NCs for advancing the NIR-II bioimaging applications. [175]

Photosensitization
As we illustrated above, the intersystem crossing (photoexcited S 1 to T 1 state) in gold NCs is typically rapid and effective, which renders these NCs suitable for efficient triplet sensitization. [60,184],196] TTA-UC stands as a photophysical process of transforming two low-energy photons into a high-energy photon under lowintensity incoherent light irradiation (such as solar light, as opposed to lasers). [184]In this mechanism (Figure 11A), a photosensitizer (singlet sensitizer in ground state, 1 S) first absorbs incident low-energy photons, leading to the generation of a singlet excited state ( 1 S*) and then triplet excited state ( 3 S*) through ISC.The triplet energy is then transferred to the emitter through triplet energy transfer (TET).Subsequently, TTA occurs between two sensitized 3 E* molecules, causing one molecule to return to its ground state ( 1 E) and the other to transition to a high-energy singlet excited state ( 1 E*).Finally, the electron in the 1 E* state returns to the ground state ( 1 E) through radiative relaxation, releasing the upconverted fluorescence. [184]The HOMO-LUMO absorption peak of most gold NCs falls within the region of red  [192] Copyright 2019, Wiley-VCH.C) Postfunctionalization of Au 25 (SG) 18 and schematic representation of Au-PC conjugate structure.D) Wide-field NIR-II fluorescence images (excited by an 808 nm laser at a power density of 70 mW cm −2 , exposure time 40 ms, 1100 nm long pass filter) of intravenously injected Au-PC conjugate (4×, ≈1.2 mg, upper panel), Au-SG cluster (4×, ≈1.2 mg, lower panel) probes at different time points (six to seven weeks old female Balb/c, n = 3).Reproduced with permission. [181]Copyright 2022, Springer Nature.
to NIR wavelengths.Therefore, gold NCs are ideal candidates for red-to-blue or NIR-to-visible TTA-UC, a crucial technique that can enhance the efficiency of solar energy capture. [197]For example, Mitsui et al. achieved a NIR (785 nm) to visible (570 nm) TTA-UC utilizing the Au 25 rod as the triplet sensitizer and rubrene as the emitter, but the upconversion quantum yield was very low (Φ UCg = 0.13%). [60]More recently, Arima et al. employed an Au 4 Cu 4 NC as the sensitizer and 9,10-diphenylanthracene (DPA) as the emitter, resulting in a red-to-blue UC with a substantially higher Φ UCg of 14% (Figure 11B).The investigation of TTA-UC is an emerging direction in the field of gold NCs, necessitating more endeavors.Especially, the energy of triplet state in gold NCs is typically small (i.e., below 1.3 eV), which makes it challenging to find suitable emitters that match the triplet energy from gold NCs.Additionally, achieving a high efficiency of TET is pivotal for enhancing upconversion quantum yield, and TET efficiency is influenced by the distance between the sensitizer and the emitter.Therefore, future investigations aimed at building chemical bonds between gold NCs and emitters may be promising for enhancing the efficiency of NIR-to-visible upconversion.
Gold NCs can also serve as sensitizers for generating singlet oxygen, [108,113] which is central to phototherapy, photocatalysis, and other scenarios.Recently, Li et al. introduced an innovative concept, proposing that nanoclusters can act as photosensitizers to promote singlet oxygen generation, which subsequently induces protein crosslinking (Figure 11C). [185]Based on this concept, Li et al. designed a novel photoresist composed of Au 22 (SG) 18 NC and silk fibroin, which was successfully applied in 3D nanoprinting to create honeycomb-like nanostructures (Figure 11D).Moreover, gold NCs (i.e., Au 25 rod) can act as an initiator for radical polymerization and two-photon acid generators for cationic polymerization (Figure 11C).Using appropriate monomers, 3D printing based on both types of polymerization processes can result in cluster-polymer nanolattices with unprecedented structural complexity. [185]Gold NC-based photoresists are a novel research direction with rich opportunities.Future research may focus on selecting suitable combinations of gold NCs and organic monomers, which will allow for the fabrication of complex nanostructures with good mechanical properties.
Gold NCs can also act as an intercellular photosensitizer for light-harvesting purposes in organisms. [186]For instance, Zhang et al. incorporated Au 22 (SG) 18 into a non-photosynthetic bacterium, Moorella thermoacetica (M.thermoacetica, Figure 11E).The Au 22 -M.thermotactic system successfully achieved photosynthesis of acetic acid from CO 2 (Figure 11F).The efficiency of Au 22 as a photosensitizer was found to be higher than conventional CdS quantum dot photosensitizer (Figure 11F).Such artificial photosynthetic biohybrid systems hold great potential in solar energy harvesting.

Optoelectronic Devices
Luminescent gold NCs can be applied for several types of optoelectronic devices, including LEDs, [51,140] solar cells, [47,48,198], and sensors [188,199] .LED is one of the most important optoelectronic applications for luminescent materials.Niesen et al. first incorporated Au 25 (SG) 18 NCs as emitters in a thin-film LED structure. [51]The fabricated device (Figure 12A) consists of a thin layer of Au 25 (SG) 18 sandwiched between an electron injecting  [184] Copyright 2023, American Chemical Society.C) Photoresist composition and printing setup (Left).Schematic of photopolymerization of different classes of monomers during two-photon lithography (Right).D) Silk fibroin honeycomb structures.Reproduced with permission. [185]Copyright 2022, The American Association for the Advancement of Science.E) Schematic of the M. thermoacetica@Au 22 (SG) 18 hybrid system, and the photo-generated electron transfer process.F) Normalized photosynthetic production of acetic acid by M. thermoacetica, M. thermoacetica@Au 22 (SG) 18 , and M. thermoacetica@CdS.Reproduced with permission. [186]Copyright 2018, Springer Nature.layer (ZnO) and a hole transport layer (NPB)/NPB: MoO 3 ).However, the external quantum efficiency (EQE) of the fabricated LED amounts to only 0.012% (Figure 12B) due to the limited quantum yield and insufficient conductivity of the Au 25 (SG) 18 film.More recently, Ma et al. synthesized a TADF Au(I)-Cu(I) NC with intense visible emission (PLQY = 93.0%). [140]The Au(I)-Cu(I) NC is further mixed with mCP (9,9'-(1,3-phenylene)bis-9H-carbazole) to form the emitting layer (Figure 12C), resulting in remarkably efficient organic light-emitting diodes (OLEDs) with an achieved EQE as high as 20.8% (Figure 12D).TADF NCs-based OLEDs are capable of harvesting both singlet and triplet excitons, which means that such devices can convert more excitons into light emission, leading to higher quantum efficiency.Nevertheless, most current studies have achieved only visible-emitting LED.NIR TADF OLED based on gold NCs is a potential future direction.Another interesting application of gold NCs is for use as efficient waveguides for energy transmission.Wang et al. recently discovered that the microrod crystals of ligand-protected PtAg 18 and Au x Ag 19-x NCs present excellent optical waveguide properties (Figure 12E).The optical loss coefficient is much smaller than conventional organic and inorganic waveguide materials (Figure 12F), indicating their great potential in optical energy transmission.

Conclusion and Future Perspectives
Over the past years, substantial endeavors have been dedicated to unraveling the PL mechanisms and tailoring the PL properties of gold NCs.In this review, we have summarized the primary PL mechanisms (e.g., fluorescence, phosphorescence, TADF, and dual emission) inherent to gold NCs and established the essential criteria for differentiation of each type of PL.Subsequently, we delve into recent strides made in tailoring the PL characteristics of gold NCs.This involves manipulation of the HOMO-LUMO  [51] Copyright 2013, Wiley-VCH.C) Device architecture of cluster-based OLED.D) Current density-voltage-luminance characteristics of the fabricated OLED.Reproduced with permission. [140]Copyright 2023, Springer Nature.E) Left panel: Experimental set-up for the optical waveguide equipment.Right panel: PL image of microrod crystals of Pt 1 Ag 18 .F) The ratio of the intensities at the tip and body of microrod crystals of Pt 1 Ag 18 versus the distance between the excitation and emission spots.Reproduced with permission. [188]Copyright 2023, The American Association for the Advancement of Science.
gap and rigidification of the nanocluster's outer shell structure and core phonons.The comprehension at both atomic (e.g., heterometal doping) and molecular (e.g., ligands) levels regarding the PL mechanism, coupled with the impressive versatility in tailoring the PL properties such as the emission wavelength and linewidth, renders gold NCs highly promising candidates for diverse applications.Some recent advances in applying gold NCs are highlighted, such as bio-imaging, photosensitization, and optoelectronic devices.
Despite the tremendous progress in understanding the PL of gold NCs, several fundamental questions remain and require dedicated research endeavors in the future.We briefly discuss them as follows.
First, the PL mechanism of several classic gold NCs (i.e., the bi-icosahedral Au 38 , the fcc series of Au 28 , Au 36 , Au 44 , and Au 52 , and the hcp Au 30 ) remains unclear. [62,200,201]A careful revisit of the electron dynamics and emission process of these NCs is necessary.The current understanding of the correlation between the kernel packing pattern and the photoexcited carrier lifetime is not yet sufficient. [55]Moreover, as we addressed in this review, current strategies for increasing the PLQY focus on the suppression of non-radiative relaxation, but the understanding of how to improve the radiative relaxation rate is lacking.According to Einstein's spontaneous emission theory, both the dipole moment and the energy gap will affect the radiative rate, but this remains to be proved in the new materials of gold NCs.
Second, the vast majority of obtained gold NCs are spherical or only slightly anisotropic. [202]A general trend of these spherelike gold NCs is that the HOMO-LUMO gap transition probability diminishes significantly as the gap decreases (i.e., extending into NIR).Weak HOMO-LUMO absorption typically suggests weak emission.This trend, together with the energy gap law, results in a challenge in achieving high PLQY in the long wavelength range (i.e., >1300 nm).However, rod-like gold NCs usually have much stronger HOMO-LUMO transition rates compared to sphere-like gold NCs with similar HOMO-LUMO gaps. [20,99,143]herefore, rod-shaped gold NCs can potentially achieve relatively strong NIR emission in the long wavelength range. [20]In addition, Li et al. found that Au 42 rods possess an exceptional photothermal effect at NIR excitation, which holds great potential in cancer therapy. [143]Nevertheless, only a few cases of rod-shaped gold NCs with sufficient aspect ratios are available in the current library of NCs and their QYs are not high enough yet.Thus, it calls for future efforts in the development of new synthetic methods and studies of PL mechanisms for anisotropic gold NCs.
Third, TADF materials hold great potential for applications in OLED and other light-emitting devices. [128,203]Gold NCs are new promising materials, but the current understanding and available cases of TADF gold NCs are still very limited.The Zang group achieved several successes in highly efficient, visibleemitting TADF clusters, but the design and synthesis of NIR TADF NCs remain a great challenge.
Fourth, biomedical and sensing applications require watersoluble gold NCs, but most of the known and well-studied gold NCs are only organic soluble, which hampers their applications.Thus, facile, and universal approaches to transform organic soluble gold NCs into water-soluble ones should be developed in future work.
Fifth, the photostability of gold NCs is of paramount importance for their real-world application.Generally, most of the gold NCs are stable under visible light, but the coexistence of UV light and oxygen may lead to damage or oxidization of some NCs. [47,204]herefore, a comprehensive photostability study is necessary for gold NCs.
Last but not least, DFT calculations [10,97,98,205,206] of the PL properties of gold NCs are critically needed, especially with the triplet states taken into consideration.The simulations of electron dynamics in the NCs are generally quite challenging, but future efforts should overcome the challenges and increase the accuracy for direct comparison with the experiment. [55,56,124,207]verall, future investigations into gold NCs are anticipated to serve a dual purpose: advancing the fundamental comprehension of nanoscience while also exploring novel applications based on the unique properties of atomically precise and tailored nanocluster materials.We believe that the experiment-theory coupled approach will significantly boost the PL research, map out the structure-PL correlation, and ultimately achieve the grand goal of atomic-level design of customized NCs with tailored PL properties for specific applications.

Figure 2 .
Figure 2. A) Crystal structure of [Au 8 (dppp) 4 L 2 ]X 2 NCs (L = Cl, X = PF 6 ).B) UV-vis absorption spectrum (solid line) and emission spectrum (dash line) of Au 8 in CH 2 Cl 2 (upper panel).UV-vis absorption spectrum (black dash line) and PL excitation spectrum (red solid line) of Au 8 in CH 2 Cl 2 (lower panel).C) Picosecond-range PL decay profiles of Au 8 in CH 2 Cl 2 and MeOH at 25 °C.Reproduced with permission.[100]Copyright 2017, American Chemical Society.D) Schematic illustration of the triple-ligands layer-by-layer self-assembly on the Au 10 NCs.E) UV-vis absorbance and luminescence spectra of triple layer protected Au 10 NCs.F) The excited-state electron dynamics of triple-layer protected Au 10 NCs.Reproduced with permission.[101]Copyright 2023, Springer Nature.G) PL spectra of Au 13 in acetonitrile under Ar atmosphere (solid lines) and O 2 atmosphere (dotted lines).Reproduced with permission.[58]Copyright 2022, Wiley-VCH.H) PL spectra of Au 13 (14.9μM) in deaerated N, N-dimethylformamide (DMF) with increasing concentration of perylene.I) PL spectrum obtained for the deaerated DMF solution of Au 13 (14.9μM) and perylene (1.0 mM) with 640 nm CW laser excitation.Reproduced with permission.[103]Copyright 2022, American Chemical Society.

Figure 3 .
Figure 3. A) UV-vis absorption and PL spectra of the Au 25 rod measured under deaerated conditions (the red solid line) and aerated conditions (the black dashed line).B) Emitter-concentration-dependent PL decay curves of the Au 25 rod in deaerated THF solution.The inset shows the corresponding PL spectra.Reproduced with permission.[60]Copyright 2022, Royal Society of Chemistry.C) PL spectra of MAu 12 in acetonitrile under Ar atmosphere (solid lines) and O 2 atmosphere (dotted lines).Inset shows the plot of the PLQYs of MAu 12 under Ar atmosphere.D) Time-resolved PL decays of MAu 12 under the Ar atmosphere.Black lines represent the fitting results with single exponential functions.Inset shows the plot of the PL lifetimes of MAu 12 under the Ar atmosphere.Reproduced with permission.[58]Copyright 2022, Wiley-VCH.E) PL processes for [RuAu 12 (dppm) 6 ] 2+ at temperatures of more than 140 K (left) and less than 130 K (right).Reproduced with permission.[24]Copyright 2021, American Chemical Society.F) Energy level diagrams showing the photophysical processes of Ag 29 -TBA (left) and Ag 29 + Ag NCs (right).Reproduced with permission.[119]Copyright 2023, American Chemical Society.

Figure 4 .
Figure 4. A) A full depiction of a three-state model.B) Absorption spectra measured at room temperature in toluene, fluorescence spectra at room temperature, and phosphorescence spectra at 77 K of CzDBA.C) Temperature-dependent transient PL decay curves of CzDBA.Reproduced with permission.[134]Copyright 2018, Springer Nature.D) PL decay curves of (Carbene)M(Cz) (M = Cu, Ag and Au) complexes.E) Ultrafast timeresolved emission decay traces for (Carbene)Cu(Cz) in toluene solution at room temperature.F)Temperature-dependent lifetimes of (Carbene)M(Cz) in polystyrene.Reproduced with permission.[135]Copyright 2019, American Chemical Society.

Scheme 3 .
Scheme 3. Schematic description of the temperature-dependent trend of A) radiative rate, B) relative PL intensity, and C) PL peak position in TADF NCs.The plots here are arbitrary and illustrative only (i.e., not from real samples).

Figure 5 .
Figure 5. A) UV-vis absorption spectra of Au 22 NCs.B) Temperature-dependent emission spectra of Au 22NCs with excitation at 530 nm.Reproduced with permission.[37]Copyright 2019, Wiley-VCH.C) Temperature-dependent emission spectra of Au 38 with excitation at 400 nm.D) Decay profiles of Au 38 at different temperatures.Reproduced with permission.[138]Copyright 2023, Springer Nature.

Figure 6 .
Figure 6.A) Emission pathway for the coexistent fluorescence and phosphorescence.B) UV-vis absorption spectrum (green line) and PL spectrum (grey line) of Au 42 .C) Decay profiles of PL I and PL II.Reproduced with permission.[99]Copyright 2022, American Chemical Society.D) Emission pathway of dual phosphorescence.E) UV-vis absorption spectrum (green) and PL spectrum (orange) of Au 20 .F) Decay profiles of PL I and PL II.Reproduced with permission.[141]Copyright 2023, The American Association for the Advancement of Science.G) Emission pathway of dual fluorescence.H) UV-vis absorption spectrum (black line) and PL spectrum (grey line) of Au 24 .I) Decay profiles of PL I and PL II.Reproduced with permission.[142]Copyright 2020, Springer Nature.

Figure 9 .
Figure 9. A) Schematic diagram of binding TOA to Au 22 (SG) 18 (left) and (right) PL spectra of pure Au 22(SG) 18 and TOA-bonded Au 22 NCs.Reproduced with permission.[122]Copyright 2015, American Chemical Society.B) Schematic diagram of intramolecular cross-linking of ligands by the formation of bis-Schiff linkages on the Au 22 (SG) 18 surface.C) PL spectra of Au 22 (SG) 18 and PDA-Au 22 (SG) 18 NCs.D) Effect of the content of surface-bound Au(I)-SG complexes on the PDA-induced PL enhancement of Au NCs.Reproduced with permission.[123]Copyright 2022, Springer Nature.E) Schematic diagram of host-guest assembly of CB(7) and CB(8) with FGGC-Au 22 and the corresponding PL photographs.Reproduced with permission.[165]Copyright 2020, Royal Society of Chemistry.F) Schematic diagram of AIE of non-luminescent oligomeric Au(I)-thiol complexes and Au(0)@Au(I)-thiol core-shell NCs.Reproduced with permission.[52]Copyright 2012, American Chemical Society.G) Schematic diagram of the self-locking effect in Au 38 S 2 (SR) 20 NCs.Reproduced with permission.[120]Copyright 2021, American Chemical Society.H) Schematic diagram of structural differences of Au 28i and Au 28ii NCs.I) PL spectra of Au 28i and Au 28ii NCs.Reproduced with permission.[166]Copyright 2020, American Chemical Society.

Scheme 4 .
Scheme 4. A) Structures of Au 25 , CdAu 24 , and HgAu 24 .B) The weak coupling model and strong coupling model.C) The linewidth of the steady-state PL spectra as a function of temperature for Au 25 .D) Emission mechanism for the MAu 24 (M = Au, Cd, and Hg) NCs.Reproduced with permission.[125]Copyright 2023, American Chemical Society.

Figure 10 .
Figure 10.A) Schematic diagram of the brain imaging setup and NIR-IIa imaging window.B) Blood vessel damage in brain injury model.Reproduced with permission.[192]Copyright 2019, Wiley-VCH.C) Postfunctionalization of Au 25 (SG) 18 and schematic representation of Au-PC conjugate structure.D) Wide-field NIR-II fluorescence images (excited by an 808 nm laser at a power density of 70 mW cm −2 , exposure time 40 ms, 1100 nm long pass filter) of intravenously injected Au-PC conjugate (4×, ≈1.2 mg, upper panel), Au-SG cluster (4×, ≈1.2 mg, lower panel) probes at different time points (six to seven weeks old female Balb/c, n = 3).Reproduced with permission.[181]Copyright 2022, Springer Nature.

Figure 11 .
Figure 11.A) Schematic of the mechanism of TTA-UC.B) Excitation power density (I ex ) dependence of emission spectra of Au 4 Cu 4 (30 μM) with DPA (40 mM) in deaerated THF under continuous wave 640 nm laser excitation.The right plot shows the dependence of internal UC quantum yield (Φ UCg ) on I ex at 640 nm.Reproduced with permission.[184]Copyright 2023, American Chemical Society.C) Photoresist composition and printing setup (Left).Schematic of photopolymerization of different classes of monomers during two-photon lithography (Right).D) Silk fibroin honeycomb structures.Reproduced with permission.[185]Copyright 2022, The American Association for the Advancement of Science.E) Schematic of the M. thermoacetica@Au 22 (SG) 18 hybrid system, and the photo-generated electron transfer process.F) Normalized photosynthetic production of acetic acid by M. thermoacetica, M. thermoacetica@Au 22 (SG) 18 , and M. thermoacetica@CdS.Reproduced with permission.[186]Copyright 2018, Springer Nature.

Figure 12 .
Figure 12.A) Schematic of the NC-LED layer stack and energy level diagram of the NC-LED.B) EQE (▪) and optical power density (○) of an Au NC LED.Reproduced with permission.[51]Copyright 2013, Wiley-VCH.C) Device architecture of cluster-based OLED.D) Current density-voltage-luminance characteristics of the fabricated OLED.Reproduced with permission.[140]Copyright 2023, Springer Nature.E) Left panel: Experimental set-up for the optical waveguide equipment.Right panel: PL image of microrod crystals of Pt 1 Ag 18 .F) The ratio of the intensities at the tip and body of microrod crystals of Pt 1 Ag 18 versus the distance between the excitation and emission spots.Reproduced with permission.[188]Copyright 2023, The American Association for the Advancement of Science.