Thermally activated delayed fluorescence Au‐Ag‐oxo nanoclusters: From photoluminescence to radioluminescence

Thermally activated delayed fluorescence (TADF) materials have numerous applications in energy conversion and luminescent imaging. However, they are typically achieved as metal‐organic complexes or pure organic molecules. Herein, we report the largest Au‐Ag‐oxo nanoclusters to date, Au18Ag26(R1COO)12(R2C≡C)24(μ4‐O)2(μ3‐O)2 (Au18Ag26, where R1 = CH3‐, Ph‐, CHOPh‐ or CF3Ph‐; R2 = Ph‐ or FPh‐). These nanoclusters exhibit exceptional TADF properties, including a small S1‐T1 energy gap of 55.5 meV, a high absolute photoluminescence quantum yield of 86.7%, and a microseconds TADF decay time of 1.6 μs at ambient temperature. Meanwhile, Au18Ag26 shows outstanding stability against oxygen quenching and ambient conditions. Atomic level analysis reveals the strong π⋯π and C‐H⋯π interactions from the aromatic alkynyl ligands and the enhancement of metal‐oxygen‐metal interactions by centrally coordinated O2−. Modeling of the electronic structure shows spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital, which promote charge transfer from the ligand shell, predominantly carboxylate ligands, to O2−‐embedded metal core. Furthermore, TADF Au‐Ag‐oxo nanoclusters exhibit promising radioluminescence properties, which we demonstrate for X‐ray imaging. Our work paves the way for the design of TADF materials based on large metal nanoclusters for light‐emission and radioluminescence applications.


INTRODUCTION
[3][4][5][6] Compared with traditional fluorescent and phosphorescent materials, TADF materials can remarkably achieve 100% internal quantum efficiency through a reverse intersystem crossing (RISC) process from the lowest triplet excited state (T 1 ) to the lowest singlet excited state (S 1 ). [5,7]The singlet excitons generated from absorbed triplet excitons can emit in an efficient fluorescence process, which will promote the efficient utilization of triplet excitons.Generally, the highly efficient TADF materials require spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) which can result in a very small energy gap (ΔE ST ) between S 1 and T 1 to promote RISC process.Currently, TADF materials are predominantly reported on mononuclear, oligonuclear metal-organic complexes and pure organic molecules with several microseconds to several milliseconds photoluminescent lifetimes. [8,9]Additionally, most of them exhibit diminished TADF behavior in the presence of O 2 due to O 2 has a strong quenching effect on triplet state emission. [10,11]There is an urgent need for novel TADF materials with excellent ambient stability for the development of multifunctional applications.
[21] The intraband transitions of the free electrons in the M(0) core were well-accepted as an important origin of PL of metal nanoclusters. [22,23][44] Inspired by these insights on the origin of PL, an increasing number of metal nanoclusters (e.g., Ag 6 95%, [45] C@Au 6 Ag 6 92%, [46,47] Au@Cu 14 71%, [38] and Ag 64 97% [48] ) possess quantum yields (QYs) that are competitive with those exhibited by quantum dots such as CdS [38,45,46,47] but are often achieved as low-nuclearity.][51] The synthesis of highly efficient TADF nanoclusters sheds light on the process of understanding the mechanism of luminescence and the applications of nanoscale PL particles in photoelectric applications.
In this work, we report the facile synthesis and detailed structural characterization of a new series of TADF Au-Ag-oxo nanoclusters protected by alkynyl and carboxylate ligands with absolute QYs as high as 86.7%.The general formula of the nanoclusters is Au 18 Ag 26 (R 1 COO) 12 (R 2 C≡C) 24 (μ 4 -O) 2 (μ 3 -O) 2 (abbrevi-ated hereafter as Au 18 Ag 26 , where R 1 , R 2 = CH 3 , Ph (1a); CH 3 , FPh (1b); Ph, Ph (2a); Ph, FPh (2b); CHOPh, Ph (3a); CHOPh, FPh (3b); CF 3 Ph, Ph (4).In addition, the enhanced room-temperature PL and a small energy separation between the singlet S 1 and triplet T 1 state of ΔE(S 1 −T 1 ) = 447 cm −1 (55 meV) are characterized as TADF properties.Density functional theory (DFT) analysis provided further insights into the electronic structure of these clusters and implied (i) a strong charge redistribution between the Au-Ag-oxo core and ligand shell and (ii) different spatial localization of filled and empty electron orbitals around the HOMO-LUMO energy gap as likely factors contributing to the long-lifetime ultra-bright luminescence.Taking advantage of the high atomic weight content of the Au 18 Ag 26 nanoclusters and their efficient TADF behavior, we demonstrate their X-rayto-visible light converting capabilities and promising X-ray scintillation imaging performance.

Synthesis and structure analysis of Au-Ag-oxo nanoclusters
In general, R 1 COOAg, R 2 C≡CAg and Ag 2 O were firstly added to the turbid solution of R 2 C≡CAu under vigorous stirring.A clear orange red solution was then obtained by centrifugation and orange red rhombic crystals (Figure S1A) were formed in one day.We have successfully synthesized 7 Au-Ag-oxo nanoclusters with different R 1 and R 2 substituents (Figures S2-S8, S10, and S11 and Table S8 -11).Herein, we take Au 18 Ag 26 (CHOPhCOO) 12 (PhC≡C) 24 (μ 4 -O) 2 (μ 3 -O) 2 (Au 18 Ag 26 -3a) as the model to illustrate the atomically precise structure of the Au-Ag-oxo nanoclusters.The Au 18 Ag 26 -3a nanoclusters crystallized in space group P2 1 /n (Figure S9).Au 18 Ag 26 -3a has a van der Waals diameter of about 2.2 nm (including the ligand shell) with a metal core diameter of about 1.4 nm, which is consistent with the particle-size distribution from transmission electron microscopy (TEM) images (Figure S1B).
Within the Au-Ag-oxo metal core of Au 18 Ag 26 -3a, the 18 Au atoms constitute three helical stripes with a crystallographic inversion center (Figure S12), whereas the 26 Ag atoms are wrapping outside (Figure 1A).The metal framework can also be regarded as an Au 12 O 4 @Au 6 Ag 26 core-shell structure (Figure 1B and Figure S13).The 12 Au atoms in the central core are arranged in four layers of Au 3 equilateral triangles possessing C 3 symmetry, with each Au 3 plane having an interstitial oxide ion (O 2-) at the center (on the C 3 symmetry axis) (Figure 1B, Figures S8b and S14).For the planes of interior Au 3 equilateral triangles, O 2− coordinate with 3 Au atoms (μ 3 -O, Figure 1D); for the planes of exterior Au 3 equilateral triangles, O 2− not only coordinate with 3 Au atoms, but also coordinate with the Ag atoms on the threefold axis (idealized or crystallographic, vide infra) to form a tetrahedral coordination for the interstitial oxide ion (μ 4 -O, Figure 1C).It can also be described as having a "layer-by-layer" structure derived from idealized three-layer face-centered-cubic (fcc) arrangements ABC and C′B′A′as depicted in Figure S15.Here the top three ABC layers are related to the bottom three C′B′A′ layers by a crystallographic inversion center (Figure 1B and Figure  S1 and S2), respectively, indicating that they are weak Au-Au interactions.The average Au-O distance is 2.069 Å, suggesting strong interactions between interstitial O 2− and the core Au(I) atoms (four stacked Au 3 triangles). [52,53]In addition, strong interactions between the surface PhC≡C-ligands are observed in the nanocluster crystal structure.As shown in Figure 1E,F and Figures S16-S17, the adjacent phenyl rings are dominated by dislocated stacking π⋯π interactions.Six sets of interacting PhC≡C-are found in the organic shell, and each set contains three alkynyl ligands (Figure 1E,F, marked in different colors) with the center distances between adjacent phenyl rings are 3.630 and 3.653 Å, respectively.Moreover, C-H⋯π interactions are also observed between the two phenyl rings belonging to different sets (Figure 1E,F), resulting in a giant interacting organic shell covering the Au-Ag-oxo core.The strong intracluster interactions effectively increase the rigidity of the organic shell and thus, reduce the nonradiative energy loss, which could be related to the strong PL of the title nanoclusters.

TADF properties of Au-Ag-oxo nanoclusters
Single crystals of Au-Ag nanoclusters were dissolved in CHCl 3 and the ultraviolet-visible (UV-vis) absorption spectra were measured (Figure S18), showing a weak absorption peak in the range of 300-550 nm.The resulting solution exhibited bright orange-yellow fluorescence under 365 nm irradiation (Figure S19) with intense emission behavior at 596-612 nm (Figure S20A and Table S3), indicating that the Au-Ag-oxo nanoclusters have a large Stokes shift (ST) (Figures S21 and S22, Table S3).In addition, the photoluminescent decay curves of these Au-Ag-oxo nanoclusters were fitted by a bi-exponential decays, which including a short (τ 1 ) and a long (τ 2 ) component.As shown in Table S4, the contribution of τ 2 component to the average lifetime (τ av ) is significantly greater than that of the τ 1 component.The achieved average lifetime of those Au-Ag-oxo nanoclusters reach the level of μs, specifically 1.57-1.98μs (Figure S20B and Table S3).The large ST and the μs-level of PL lifetime indicate that the emission may be related to triplet excitation. [47,54,55]n addition, the absolute QYs of the nanoclusters in solution were measured under 365-nm excitation, and unexpectedly reached a maximum of 86.7% at room temperature (RT).At present, there are many studies on the fluorescence emission behavior of metal nanoclusters, but most of them focused on the low-nuclearity (<25), and there are few examples with QYs of greater than 80%. [45,46,47]We believe that, in our system, the strong π⋯π interactions (Figure 1E, F, and Figure S16) among the well-parallel-stacked phenyl groups of the surface ligands greatly restricts the motion of the surface ligands, resulting in the observed strong fluorescence.More importantly, the PL of Au 18 Ag 26 -3a shows negligible attenuation in the presence of O 2 , confirming stability against oxygen quenching and ambient conditions (Figure S23).The singlet oxygen ( 1 O 2 ) generation experiments also demonstrated that the photoexcited Au 18 Ag 26 cannot effectively transfer energy/charge to oxygen to generate 1 O 2 (Figure S24). [38,48,56]The quenching of PL in Au 18 Ag 26 -3a is nearly imperceptible primarily due to the fact that the rate of quenching by O 2 is significantly slower, with a k q [O 2 ] air value of approximately 1.6 × 10 3 s -1 ([O 2 ] air = 1.71 mM for CHCl 3 ), which is much smaller than the intrinsic PL decay rate, τ N2 -1 , estimated to be around 4.4 × 10 5 s -1 (Figure S23B). [56]o gain a deeper understanding of the photophysical properties of Au 18 Ag 26 nanoclusters, temperature-dependent PL measurements were performed on Au 18 Ag 26 -3a-PMMA film over a range of 80 to 340 K (Figure 2A,C, Figures S25  and S26).The results showed that the emission energy of Au 18 Ag 26 -3a increased as the temperature increased from 80 to 340 K, leading to a blue shift in the maximum emission from 616 nm (80 K) to 601 nm (340 K) upon excitation at 365 nm (Figure 2C and Figure S26).Additionally, as the temperature rose from 80 to 92 K, the intensity of the emission peaks initially decreased before increasing again as the temperature further rose to 340 K (Figure 2D).At low temperatures (80-92 K), the relaxation processes could be inhibited, leading to a lower population of emissive singlet states with slower rate of RISC (k RISC ).However, as the temperature increases beyond this range (>92 K), the thermal energy becomes sufficient to activate these relaxation processes, which can overcome the energy barrier between T 1 and S 1 states, increasing the RISC rate.This activation energy facilitates the conversion of triplets to singlets, resulting in a higher population of emissive states.Furthermore, the decay times at various temperatures were successfully measured and plotted versus temperature (Figure 2E,F and Figure S27), which shows nearly a plateau at temperatures below 110 K.At such low temperatures, the population is predominantly frozen in the T 1 state, a constant value of 5.40 μs was observed, representing the phosphorescence decay time (τ) for the T 1 →S 0 transition.Above 110 K, the decay time progressively shortened as the temperature increased due to the thermal population of the higher-lying S 1 state, resulting in a τ (300 K) value of 1.64 μs (Figure 2B).The temperature-dependent photoluminescence decay curves exhibited bi-exponential decays, characterized by a short component (τ 1 ) and a long component (τ 2 ).Table S5 shows that the contribution of τ 2 to the average lifetime (τ av ) is notably higher than that of τ 1 .It also appears that the τ 1 component barely changes with increasing temperature and has a lifetime range of 15-35 ns, but the τ 2 component gradually decreases with increasing temperature from 5.36 to 1.4 μs.Fitting the temperature-dependent PL decay lifetimes of Au 18 Ag 26 -3a to the modified Boltzmann equation (please see the details about equation 1 in the supporting information) gave an activation energy of ΔE(S 1 -T 1 ) = 0.0555 eV and a radiative rate of fluorescence of k r (S 1 -S 0 ) = 1.08 × 10 7 s −1 .The radiative rate of phosphorescence (k r (T 1 -S 0 )) was found to be 1.87 × 10 5 s −1 , corresponding to a phosphorescence decay time of τ(T 1 -S 0 ) = 5.4 μs.This observation is similar to other TADF materials, where RISC from the T 1 state to the S 1 state leads to delayed fluorescence. [2,3,5,8,9,49]t temperatures below 110 K, only long-lasting phosphorescence is observed as the T 1 to S 0 transition has a decay time of 5.4 μs.As the temperature increases, rapid internal conversion (IC) results in a population of the S 1 state with a higher energy than the T 1 state.This results in a 15 nm blue-shift of the emission from Au-Ag-oxo nanoclusters with TADF, which is consistent with the activation energy of 447 cm −1 for the S 1 to T 1 transition, as determined from the analysis of the decay times.The S 1 to S 0 transition rate of TADF material is much higher compared to other organo-metallic TADF materials. [2,3,5,8,9,49]As a result, the TADF decay time at 300 K is significantly reduced to 1.64 μs, which benefits from the small ΔE(S 1 -T 1 ) and the large k r (S 1 -S 0 ) (Figure 2B).Few TADF metal nanoclusters have been reported, and most of them display low QYs and long lifetimes in solution. [45,49,50,51]Compared to these TADF nanoclusters (Table S6), our Au 18 Ag 26 nanoclusters display the largest nuclearity, the highest QY and the shortest TADF lifetime in solution.
We investigated the ultrafast electron dynamics of the Au 18 Ag 26 -3a-PMMA film using femtosecond and nanosecond transient absorption (TA) measurements to gain a deeper understanding of the excited-state process on the early time scale.As seen in Figure 2G, two excited state absorptions (ESAs) were observed at 550 and 700 nm after photoexcitation at 400 nm, which begin to decay after a time delay of 0.5 ps.The wide range of ESA band at 450-750 nm indicates that Au 18 Ag 26 -3a has dense excited states.The kinetic traces for these bands, shown in Figures S28 and S29A, exhibit a sub-ps decay of 756 fs and 557 fs for 550 and 700 nm, respectively, which can be attributed to IC and vibrational relaxation (VR) to the S 1 state. [57,58]The second time component, 808 and 1006 ps, is due to the combination of delayed VR process and radiative decay at semi-relaxed state (Figure S28). [57]The longer remaining components were determined using ns-TA, as shown in Figure 2H,I and Figure S29, two distinct unimolecular decay mechanisms were identified to match TADF: the prompt fluorescence (PF) and the delayed fluorescence (DF).The delayed component has a decay time of 1.13 ± 0.10 μs and 1.14 ± 0.23 μs for 550 and 700 nm, respectively.The prompt component has a decay time of 33.13 ± 3.2 ns and 43.11 ± 2.0 ns and can be assigned to the fluorescence of Au 18 Ag 26 , which is consistent with the short component (τ 1 : 15-35 ns) obtained for the excited-state decay lifetime.The key photophysical rates can be determined using the experimental prompt and delayed emission lifetimes and the photoluminescence quantum efficiency of Au 18 Ag 26 -3a-PMMA film, yielding k ISC = 1.95 × 10 7 s −1 and k RISC = 9.43 × 10 5 s −1 , which are consistent with other TADF molecules. [2,3,5,8,9,49]According to the emission QYs and the decay times, one can figure out the radiative decay rate k r = Φ PL /τ (300 K) = 3.75 × 10 5 s −1 , suggesting that RT emission contains contributions of delayed fluorescence and phosphorescence, [9] which can also be reflected by a continuous increase in emissive energy from 300 to 350 K.The high radiative TADF decay rate leads to an emission quantum yield of 60% at ambient temperature.The ratio of I(S 1 )/I(T 1 ) ≈ 2.25 at ambient temperature suggests that the emission spectrum is dominated by 70% TADF and 30% phosphorescence from T 1 . [9]The correspondence between the activation energy and the spectral shift upon temperature increase further supports the assignment of the ambient temperature emission as TADF, although it is noted that the emission at ambient temperature may not only represent TADF but also contain some T 1 →S 0 (phosphorescence) contribution (Figure S30).

Radioluminescence of Au-Ag-oxo nanoclusters
In addition to PL properties, many highly TADF materials usually exhibit excellent radioluminescence (RL) properties. [4,59,60]Owing to the small triplet-singlet energy separation, TADF materials significantly facilitates luminescent harvesting of radiation-induced triplet excitons, [4,59,60] which can be developed as radiation scintillators to convert high-energy radiation (X-rays and gamma rays) into visible light photons.[62] Metal nanoclusters are very attractive for the design of X-ray responsive materials and have great potential in X-ray imaging. [63,64]On the one hand, metal nanoclusters have a high atomic number (Z) and good luminescence efficiency, resulting a considerable X-ray absorption capacity. [63,65]On the other hand, metal nanoclusters are  (G and H) The optical (G) and X-ray photographs (H) of a standard X-ray resolution pattern plate (lp/mm).(I and J) Photographs of a ball pen with a metal spring inside before (I) and after (J) X-ray exposure.(K and L) Photographs of an electronic chip before (K) and after (L) X-ray exposure.low-temperature solution-processable and highly soluble, which can be incorporated into organic polymers for preparing high flexibility and high transparency hybrid films. [63,66]ay more attention in the metal nanoclusters scintillator may open a new world for the application of TADF metal nanoclusters.To study the radioluminescence performance of Au 18 Ag 26 -3a, the X-ray absorption capability was first determined, which is comparable to that of Bi 4 Ge 3 O 12 (BGO) and Cs 3 Cu 2 I 5 scintillators (Figure 3A).Under X-ray excitation, Au 18 Ag 26 -3a shows a radioluminescence spectra peaked at 600 nm, which is the same as the photoluminescence spectra under UV excitation (Figure S31).The integrated RL intensity of Au 18 Ag 26 -3a corresponds to the standard scintillation (BGO), which has a light yield of 10,000 photons MeV −1 (Figure 3B).Also, the RL intensities of Au 18 Ag 26 -3a exhibited an excellent linear response at higher dose rates from 19.8 μGy s −1 to 1.29 mGy s −1 (Figure 3C), resulting in a low X-ray dose rates of 950.4 nGy s −1 (Figure 3D).Moreover, the X-ray radiation stability was further evaluated (Figure 3E).The RL intensity of TADF Au 18 Ag 26 -3a nanoclusters remained >93% under ionizing radiation (146.5 μGy s −1 ) continuously applied for 900 s (Figure 3E), which showed the excellent radiation resistance of Au 18 Ag 26 -3a nanoclusters.In addition, the intensity of the UV-vis spectra does not decrease significantly between 300 and 500 nm, which demonstrates the stability of the nanoclusters under continuous X-ray irradiation (Figure S32).A uniform largearea (4 × 4 cm 2 ) Au 18 Ag 26 -3a-PMMA film with a thickness of ∼40 μm was prepared to further investigate the potential applications in X-ray imaging (Figures S33-S35).The light transmittance in the visible light range is up to 80% (Figure S33), indicating that the Au 18 Ag 26 -3a-PMMA film possesses negligible light scattering effect.It is noteworthy that the poor optical transparency caused by light scattering usually results in reduced X-ray imaging spatial resolution. [67]Moreover, the X-ray image of standard line chart shows a high spatial resolution with distinguishable line space of ∼10 lp mm −1 , which was further demonstrated by the modulation transfer function using slanted-edge method (Figure 3F-H and Figure S36).Such a high spatial resolution is even comparable to that of commercial CsI (Tl) scintillator. [65,67]In addition, a ball pen with a metal spring inside and an electronic chip were used as imaging objects, as shown in Figure 3I-L, the outline of the inside metal spring and inner structure could be clearly presented on the film through the plastic case.We attribute the high spatial resolution to the homogeneous distribution and large Stokes shift of Au 18 Ag 26 -3a nanoclusters that mitigates the light scattering effect and self-absorption behavior of emitted X-ray.

Mechanism of luminescent properties of Au-Ag-oxo nanoclusters
It is generally believed that PL intensity of M(I) nanoclusters appears to be stronger than M(0)/M(I) nanoclusters. [45,46,47,68]This special charge balance system is bound to affect the photoluminescence properties of the material, particularly Au-Ag-oxo metal nanoclusters usually accompanied by excellent luminescence. [51,54,69]The addition of negatively charged atoms, such as C 4− , N 3− , P 3− , O 2− and S 2− , helps to enhance the aggregation of Au and Ag, forming the unique regioselective distribution of two different metal ions and changing the physical and chemical properties of the resulting nanocluster. [52,53,70]In addition, the above studies have shown that the π⋯π interactions between surface ligands can drive the different crystallization behaviors of nanocluster molecules and affect the rigidity of the surface ligands, thereby enhancing the fluorescence properties of metal nanoclusters. [32]FT calculations performed directly on the crystal structure of the Au 18 Ag 26 nanocluster confirm significant charge transfer inside the nanocluster (Table S7).The accumulated positive Bader charge for the metal atoms is 17.5 |e|, compensated partially by the total negative charge of −3.6 |e| of the oxo-species.In the ligand shell, the 24 PhC≡C and 12 CH 3 COO ligands have total charges of −6.0 |e| and −7.9 |e|, respectively.The HOMO-LUMO energy gap is significant, 1.76 eV, confirming the stability of the nanocluster.The computed UV-vis absorption spectrum is in a qualitative agreement with the measured one and shows an optical gap at about 705 nm corresponding exactly to the HOMO-LUMO gap (Figure S37).This implies that the lowest optical transitions involve the frontier orbitals around the gap as shown by the dipole transition contribution map analysis close to the optical band edge (Figure S38).We analyzed the electronic density of states around the HOMO-LUMO energy gap by decomposing it to atom/ligand types (Figure 4A-D) and formed the combined spatial density from a few orbitals closest to the gap (Figure 4D).The orbitals just below the gap have weight on metal atoms, acetate ligands, and to a minor extent on the PA-metal interface.On the other hand, acetate ligands do not contribute to the density just above the gap.As with conventional organic TADF materials, the Au-Ag-oxo metal core acts as an acceptor and the surface ligand (especially acetate ligands) acts as a donor.This cre-ates an interesting spatial redistribution of the charge in the hypothetical LUMO-HOMO transition, if the main mechanism for the emitted photon in the luminescence involves the orbitals around the gap.Hence, the spatially separating HOMO and LUMO orbitals greatly promoted the ligand-tometal charge transfer, resulting in small ΔE ST values between S 1 and T 1 . [71]The optimized S 1 and T 1 states (electrons being excited from HOMO to LUMO) are also visualized in Figure S39, exhibiting a qualitative similarity to the trends discussed for the frontier of the ground state calculation.In both S 1 and T 1 states, hole states are slightly more localized to the ligands and outer layers of the nanocluster than the electron states.Consequently, high emission would occur through electron transfer from the electron state back to the hole state, consistently indicating a redistribution of the charge.Furthermore, DFT calculations confirmed that the coordination of R 1 COO − is crucial for the construction and emission properties of Au 18 Ag 26 , which is the same as [(C)(AuPPhpy 2 ) 6 Ag 6 (CF 3 COO) 3 ] 5+ nanocluster. [47]For traditional non-TADF metal nanoclusters, the PL intensity increased with increasing charge donating capability of ligands.But for Au 18 Ag 26 nanoclusters, the QYs are closely related to the strength of the electron-withdrawing group R 1 -(CF 3 Ph-→ CHOPh-→ Ph-→ CH 3 -) in acetate ligands, resulting 75.5% and 85.6% for CH 3 -and CF 3 Ph-respectively (Table S3).It is interesting to note that the phenyl rings of PA ligands do not contribute to this process.Au 18 Ag 26 can display excellent TADF properties, which are benefit from a small overlap of HOMO and LUMO.For CF 3 Phfunctionalized Au-Ag-oxo nanoclusters, the introduction of CF 3 Ph-groups contributes to the formation of π-extended donors, which can reduce the HOMO−LUMO overlap to achieve comparable energy levels for S 1 and T 1 , resulting in small ΔE ST values and high QYs.Overall, partial modification with CF 3 Ph-groups at the appropriate position of the donor moiety can improve the luminescence efficiency of TADF nanoclusters.
In the ultraviolet absorption and conversion process, the PL of Au 18 Ag 26 may be attributed to ligand-to-metal charge transfer (LMCT) and/or ligand-to-metal-metal charge transfer (LMMCT), which induces radiative relaxation through a metal-centered triplet. [24,47]When the nanoclusters are excited by UV light, the electrons are excited from the ground state to the LMCT and/or LMMCT state due to the charge separation between the surface ligands and the metal atoms.The excited electrons undergo multiple processes including rapid IC, VR, intersystem crossing (ISC), and RISC, and then decay to the emitting state, characterized by the recombination of electrons and holes in the metal atom.This recombination leads to the emission of a photon characterized by our TADF Au-Ag-oxo metal nanoclusters. [15,47,57,58]Figure 4E schematically illustrates the proposed X-ray-irradiated luminescence, which differs from the photoluminescence.During the X-ray absorption and conversion process, X-ray photons directly interact with heavy atoms (Au and Ag) in the Au-Ag-oxo nanoclusters through the photoelectric effect and Compton scattering, resulting in the ejection of high-energy electrons.These hot electrons then interact with atoms in the material, leading to a cascade of secondary electrons through energy loss.This process continues to ionize or excite other molecules.After losing sufficient energy, electrons are finally captured by ligands to form hole-electron pairs.The excited nanoclusters characteristic of MLCT emit RL. [4,59,61,62] Subsequently, the charge recombination between the electrons and holes produces ∼25% singlet excitons and ∼75% triplet excitons.Owing to the small ΔE ST between S 1 and T 1 of Au 18 Ag 26 , the triplet excitons can efficiently transfer to singlet excitons through a RISC process, which can result in emitting an efficient fluorescence process with a higher internal quantum efficiency.

CONCLUSION
In this work, we obtained a series of TADF Au-Agoxo nanoclusters, showing a maximum absolute quantum yield up to 86%.The atomically precise molecular formula of the largest Au-Ag-oxo nanoclusters is Au 18 Ag 26 (R 1 COO) 12 (R 2 C≡C) 24 (μ 4 -O) 2 (μ 3 -O) 2 , and the core is Au 12 O 4 layered structure with four interstitial O 2− .These TADF nanoclusters showed enhanced roomtemperature PL and a small ΔE(S 1 −T 1 ) value of 447 cm −1 (55 meV).With the help of DFT and structural analysis, the excellent luminescent properties were shown to originate from the ultrastrong π⋯π interactions by surface organic ligand, metal aggregation by the interstitial O 2− , strong intracluster charge transfer, and spatial differences in the localization of single-electron states around the HOMO-LUMO energy gap.In addition, the flexible and uniform Au 18 Ag 26 −PMMA film was achieved, which show notable X-ray-to-visible light conversion capabilities and promise for X-ray imaging.Our work will motivate further development of TADF materials based on large nanoclusters, enabling future design and tailoring of such functional materials for X-ray imaging applications.

F I G U R E 1
The molecular structure of Au 18 Ag 26 -3a.(A) The Au-Ag-oxo metal core with three Au helical stripes highlighted in large size and different colors.(B) The Au-Ag-oxo metal core with Au atoms highlighted in ABCC′B′A′ stacking.(C and D) The coordination modes of μ 4 -O (C) and μ 3 -O (D).Au = yellow, orange, dark orange, Ag = green, O = red, C = gray, H = white.(E and F) Side and top views of the coordination modes of surface ligands with π⋅⋅⋅π stacking (E) and C-H⋯π interactions (F).Carbons belong to different ligand sets are marked in different colors, and the μ 3 -O and μ 4 -O are highlighted in large size.The phenyl rings of carboxylate ligands are omitted for clarity.S15), resulting in an overall ABCC′B′A′ stacking.Due to the O 2− coordination, the distances between the Au atoms in the Au 3 surface layer are much greater than that in the bulk Au (2.884 Å), being 3.30 Å and 3.45 Å for interior and exterior Au 3 equilateral triangles (Table

F I G U R E 2
Emission and transient absorption spectra of Au 18 Ag 26 -3a in PMMA film.(A) Normalized emission and excitation spectra.(B) Excited-state lifetime decay at 300 K. (C) Normalized temperature-dependent emission in the range of 80 to 340 K. (D and E) Temperature-dependent emission intensity (D) and excited-state lifetimes (E) in the range of 80 to 340 K. (F) Temperature dependence of the in the range of 80 to 340 K. Plot of emission decay lifetime against temperature (80 to 340 K); the red line represents the fit according to the thermally activated delayed fluorescence (TADF) equation (see Equation (1) in the supporting information).(G and H) Time evolution of transient absorption spectra 0.5 ps-2 ns (G) and 5 ns-5 μs (H) after excitation at 400 nm.(I) Kinetic trace probed at 550-nm wavelength in μs-scale time window.

F I G U R E 4
Density functional theory (DFT) and proposed mechanism of luminescence (A) Electronic density of states of Au 18 Ag 26 -1a, decomposed into the atom/ligand types.The center of the HOMO-LUMO energy gap is at zero.(B, C, and D) Localization of electron density from the frontier orbitals around the HOMO-LUMO gap of Au 18 Ag 26 -1a (the brown highlight in Figure 4A).Five highest occupied orbitals HOMO-4 to HOMO (B); four lowest unoccupied orbitals LUMO to LUMO+3 (C); combined density (B) + (C) (D).(E) Proposed mechanism of radioluminescence of TADF Au-Ag-oxo nanoclusters.