Filling the gaps in icosahedral superatomic metal clusters

ABSTRACT Chemically modified superatoms have emerged as promising candidates in the new periodic table, in which Au13 and its doped MnAu13−n have been widely studied. However, their important counterpart, Ag13 artificial element, has not yet been synthesized. In this work, we report the synthesis of Ag13 nanoclusters using strong chelating ability and rigid ligands, that fills the gaps in the icosahedral superatomic metal clusters. After further doping Ag13 template with different degrees of Au atoms, we gained insight into the evolution of their optical properties. Theoretical calculations show that the kernel metal doping can modulate the transition of the excited-state electronic structure, and the electron transfer process changes from local excitation (LE) to charge transfer (CT) to LE. This study not only enriches the families of artificial superatoms, but also contributes to the understanding of the electronic states of superatomic clusters.


INTRODUCTION
Atomically precise silver nanoclusters have been greatly enriched in terms of both numbers and properties [1 -16 ], as well as the enormous progress in the field of structural growth evolution [17 -21 ].All these deepen our understanding of the property modulation of silver clusters.It is well known that the doping of metal atoms can effectively modulate the optical properties of nanoclusters by adjusting the energy level difference and changing the structural rigidity [22 -26 ].Bakr's research group has demonstrated the construction of a pair of analog clusters (Au 25 and Ag 25 ), which is of great significance for understanding the evolution and properties of different metal nanoclusters [3 ].Based on that, Jin's research group used A g 25 , Au x A g 25 −x , and Au 25 as templates to study the change of the quantum yield (QY) induced by the change of electron vibration coupling via doping [27 ].However, nanoclusters usually consist of a kernel and a motif, and the formation of the kernel structure is expected to play a critical role in the early stage of cluster growth.Therefore, capturing and understanding the kernel structure is indispensable in studying cluster growth and performance.On the other hand, the existence of peripheral motifs in silver nanoclusters blurs the research on the kernel structure, although they contribute to the stability of the clusters.This also introduces complex doping sites, impeding visualization of the effects of different metal doping on kernels [22 -24 ,28 ,29 ].Hence, the construction of novel gold and silver cluster analogs without motifs is required for working out the relationship between the kernel and other properties.
As classical magic-number clusters with closedshell geometry, icosahedral M 13 -type clusters are considered to be thermodynamically stable [30 -44 ] and thus are ideal candidates to be used for the construction of nanocluster analogs.Although Au 13 has been widely studied in the fields of luminescence, chirality, and catalysis [29 -34 ], the homologous Ag 13 has not yet been reported.Interestingly, Ag 13 is observed in the cores of many silver nanoclusters with core-shell structures [3 ,4 ,7 ,17 ,41 -5 4 ], w hich indicates that Ag 13 may be the embryonic state of such silver nanoclusters during their formation and evolution.Hence, capturing Ag 13 helps to understand the growth process from simple silver complexes to complicated silver nanoclusters.Unfortunately, the icosahedron Ag 13 has not been reported so far.
The clusters synthesized from 2,6-(diphenylphosphonyl) pyridine (dpppy) and transition metals are typically high in crystallinity and exhibit exceptional luminescent properties, since these two precursors present distinctive coordination modes, strong chelating ability, and robust structural rigidity [55 -58 ].Herein, we used dpppy as a protective ligand to construct a pair of 13 core icosahedral silver and gold nanoclusters with molecular formulas of [Ag 13 (dpppy) 5 (Cl) 2 ](SbF 6 ) 3 (abbreviated as Ag 13 ) and [Au 13 (dpppy) 5 (Cl) 2 ](SbF 6 ) 3 (abbreviated as Au 13 ) through different reduction strategies (Scheme 1 ).The prepared Ag 13 and Au 13 are identical in size, superatomic electronic structure, charge, and crystal structure.Ag 13 in the solution state was not photoluminescent in contrast with the strong near-infrared (NIR) luminescence observed in that of Au 13 with an ultrahigh quantum yield (QY = 45%) that is quite rare in gold nanoclusters [59 -72 ].A series of alloy nanoclusters ( Au n Ag 13 −n ) were obtained by doping in situ , which exhibited significant gold content-dependent PL intensity, lifetime, and PLQY.More importantly, the uniform nano-micelles ( Au 13 @PLGA ) were obtained by encapsulating Au 13 with methoxypolyethylene glycol-poly(lactic-co-glycolic acid) (mPEG-PLGA), which presents great application potential in cell imaging.

Structure of Ag 13 , Au 1 Ag 12 and Au 13
Single-crystal X-ray structure revealed Ag 13 crystallizing in the P -1 space group, and the cluster core was found to adopt an icosahedral configuration (Fig. 1 A), in which the ten silver atoms at the waist are protected by five dpppy ligands while the silver atom at the top is capped by a chlorine atom.The icosahedral Ag 13 structure is composed of 20 triangular faces and a single Ag atom wrapped within.The average A g•••A g distance between the central Ag atom and the Ag 12 shell is 2.764 Å, which is shorter than that of the Ag 12 shell (2.906 Å).Au 13 is an analog cluster of Ag 13 , and they are almost identical in terms of the metal atom number, ligands, superatomic electronic configuration, and atomic arrangement (Fig. 1 B).Both of these two nanoclusters have abundant molecular interactions ( Fig. S1).
Electrospray ionization mass spectrometry (ESI-MS) was used to characterize the chemical composition and the charge state of Ag 13 , Au 13 and their alloy nanoclusters in solution.A clean signal was observed for Ag 13 and Au 13 at m / z = 1236.7 and 1622.7,respectively, which is consistent with the simulated data (Fig. 1 C, D).Importantly, the ESI-MS curves of the alloy nanoclusters present a series of molecular ion peaks corresponding to the different numbers of gold atoms doped into the silver clusters.According to the mass between difference peaks, the number of doped gold atoms increased (from one to five) with the gold consumption, keeping the total number of metal atoms at 13 (Fig. 2 A).Moreover, the results of inductively coupled plasma emission spectrometry (ICP) show that the Ag content of 10%, 20% and 30% (Au content) Au n Ag 13 −n gradually decreases, while Au content gradually increases ( Table S1).Although the ICP results for 10% and 20% Au n Ag 13 −n show a small difference in Au content and are very close to that of Au 1 Ag 12 , these slight variations in gold content could result in significant differences in their optical properties, which is consistent with the reported literature [22 -25 ].The full X-ray photoelectron spectroscopy (XPS) spectra of Ag 13 , Au 13 and Au n Ag 13 −n revealed that the composition of the elements in the samples was consistent with the results obtained from single-crystal resolution ( Fig. S2).As shown in Fig. S3A, B, high-resolution Ag 3d and Au 4f spectra of Ag 13 and Au 13 indicated that the presence of Ag 0 in Ag 13 and Au n Ag 13 −n , as well as the presence of Au 0 in Au 13 and Au n Ag 13 −n .In comparison to Ag 13 , the binding energy of Au n Ag 13 −n shifts towards higher binding energy values with increasing Au content ( Fig. S3C), suggesting that Au doping can lead to subtle alterations in the electronic state of Ag.Similar to its effect on the binding energy of Ag, heteroatom doping also induce changes in the binding energy of Au ( Fig. S3D).Due to the difference in ICP and XPS results of Au n Ag 13 −n , we managed to obtain n -dependent structures by adjusting the gold doping ratio ( Fig. S4).Unfortunately, only a structure with one Au atom replacing the Ag atom in the center of Ag 13 , namely Au 1 Ag 12 ( Fig. S5) was harvested, which probably was due to its high thermodynamic stabi lity simi lar to that of Au 1 Ag 24 and Au 1 Ag 28 [24 ,25 ].Notably, we found that Ag 13 , Au 1 Ag 12 and Au 13 are chiral, but in the form of racemates ( Figs S6-S8).Moreover, we tested the time-dependent ultraviolet (UV) absorption spectra of Ag 13 in different solvents at room temperature, and the results showed that its dimethyl sulfoxide (DMSO), dichloromethane (DCM), or acetone (AC) solution has attained certain stability in a short space of time ( Fig. S9).

Luminescence properties of Ag 13 , Au n Ag 13 −n and Au 13
Although Ag 13 and Au 13 are analogs, their optical properties are quite different.From the absorption spectra, it is clear that the main peak of Ag 13 is located at 403 nm, while it has shifted to 386 nm after gold doping, and further blue shifted to 348 nm when Ag 13 becomes its analog Au 13 (Fig. 2 B and Fig. S10).As for the luminescent property, Ag 13 exhibited red emission in its solid state, while almost no signal can be detected in its solution state ( Fig. S11), which is reminiscent of the aggregation-induced emission (AIE) behavior.The luminescence of Ag 13 DMSO solution can be re-i l luminated by adding deionized water ( Fig. S12) or lowering the temperature to 83 K ( Fig. S13), indicating that the interaction between the silver clusters and the solvent quenches their luminescence.After mixing Ag 13 with polymathic methacrylate (PMMA) to form a film ( Ag 13 @PMMA), its emission also showed the intrinsic nature of Ag 13 in the solid state ( Fig. S14).Au 13 displayed NIR luminescence in both solid and solution states with emission peaks located at 798 nm and 763 nm, respectively ( Fig. S15).Excitingly, the QY of Au 13 was as high as 45% in the solution state, much higher than that in the solid state (20%, Table S2), which could be attributed to self-quenching as a result of intermolecular interactions in the solid state [73 ].
Although both Ag 13 and Au 13 have red luminescence and microsecond lifetimes in the solid state ( Figs S16, S17), their solution luminescence properties are significantly different ( Fig. S18).To further investigate the change of luminescence in solution during the evolution from Ag 13 to Au 13 , doping was used to construct a series of alloy nanoclusters.The emission center of alloy clusters gradually shifted from 680 nm to 720 nm with increasing gold content, and their intensity gradually increased and reached the maximum at 30% doping, which was sti l l much weaker than that of Au 13 (Fig. 2 C).Alloy clusters also showed bright luminescence in the solid state ( Fig. S19), and their emission wavelength gradually red shifted from 645 to 700 nm with the increase in gold content ( Fig. S20).Moreover, the luminescence of the solution was i l luminated and gradually enhanced, and the photoluminescence lifetime and QY for Au 13 also gradually increased and reached 3.5 μs and 45%, respectively ( Table S2 and Fig. S21).The microsecond lifetime implied inherent phosphorescence properties ( Ag 13 , Au 13 and Au n Ag 13 −n ), which were further confirmed by their luminescence quenching both in solid and solution in the presence of oxygen ( Figs S22-S25).C, and S27-S29), it was found that the highest occupied molecular orbitals (HOMO, P x orbital) and HOMO-1 (P y orbital) are doubly degenerated in all of the three clusters, while the P z orbital (HOMO-2) has much lower energy than P x and P y , which might be due to the influence of the  Cl p orbitals [31 ].As shown in Fig. S26D-F, the UV spectra of the three clusters ( Ag 13 , Au 1 Ag 12 and Au 13 ) calculated by time-dependent DFT (TD-DFT) agree well with the measurements.According to the energy arrangement of the molecular orbitals (MOs) ( Fig. S26G-I), the absorption of Ag 13 at 310 nm (a: HOMO-4→ LUMO + 1, HOMO-1→ LUMO + 31.LUMO stands for the lowest unoccupied molecular orbital.) is mainly produced by the combined effect of metal-metal electron transfer (MMCT) and metal-ligand electron transfer (MLCT), while the absorption at 403 nm (b: HOMO-2→ LUMO + 3) and the shoulder peaks at 485 (c: HOMO-1→ + 1 and HOMO-LUMO + 2) are mainly due to MMCT.Comto Ag 13 , the absorption of 1 Ag in 386 nm is mainly attributed to ligand-metal electron transfer (LMCT, a: HOMO-2→ LUMO + 7; b: HOMO-1→ LUMO + 13; c: HOMO-1→ LUMO + 11), while the absorption at 450 nm is mainly attributed to MMCT (d: HOMO-1→ LUMO + 1, HOMO-1→ LUMO + 2).The peak of Au 13 at 348 nm, is mainly attributed to electrons from HOMO-4→ LUMO + 3, HOMO-3→ LUMO + 4. The shoulder peak at 450 nm is mainly attributed to electrons jumping from HOMO-1→ LUMO + 1, and HOMO-1→ LUMO + 2. The results of the calculations demonstrated that, despite the similar structures of Ag 13 , Au 1 Ag 12 and Au 13 , their electron transfer pathways are distinct, which is the basis for their varied optical properties.

Theoretical calculations of
To gain deeper insights into the impact of different core atomic compositions on the mechanism of the phosphorescence, we selected the two most classic Ag 13 and Au 13 clusters for excited state structure optimization.Simultaneously, we theoretically constructed alloy clusters Au 1 Ag 12 and Au 3 Ag 10 , with additional details provided in Figs S30 and S31.Based on the optimized S 1 and T 1 /T 2 excited state geometric structures of Au n Ag 13 −n ( n = 0, 1, 3, 13), the fluorescence and phosphorescence radiative transition properties are studied.Fig. 3 shows the distribution of the hole and electron pairs during the S 1 → S 0 and T 1 /T 2 → S 0 vertical transitions for Au n Ag 13 −n ( n = 0, 1, 3, 13) by natural transition orbital (NTO) analysis.As for A g 13 , Au 1 A g 12 and Au 3 Ag 10 , the calculated vertical emission energy of T 1 to S 0 agrees well with the experimental results (Fig. 3 A-C and Table S3).For Au 13 (Fig. 3 D), the calculated vertical emission energy of S 1 to S 0 is 1.35 eV (919 nm), and T 1 to S 0 is 1.27 eV (974 nm) which is much smaller than that of experimental results (1.57eV, 798 nm).But the vertical emission energy of T 2 → S 0 is 1.77 eV (702 nm), which is close to the experimental result (1.57eV, 798 nm).The phosphorescent emission of Au 13 observed in the experiment appears to be mainly attributed to the T 2 → S 0 transition.The generation of T 2 phosphorescence can be explained by a large E (T 2 − T 1 ) value (0.55 eV) and T 2 | ˆ H soc |S 0 (124.16cm −1 ), causing T 2 phosphorescence to compete favorably with internal conversion (IC).
The large E (S 2 − S 1 ) value (0.67 eV) leads to slow S 2 → S 1 internal conversion, and strong spin-orbit coupling between the S 2 and high-lying triplet states T m (T 4 , T 3 , T 2 ) leads to fast S 2 → T m ( Tables S4, S5).Therefore, the phosphorescence mechanism of Au 13 can be better understood from The long lifetime ( τ p ) and high quantum efficiency ( p ) conflict in principle [74 ].It is thus desirable to gain a better understanding of the factors governing p .The intersystem crossing (ISC) rate is mainly determined by the spin −orbit coupling (SOC) S | ˆ H soc | T and the energy difference E ST between S and T states.The ISC rate can be qualitatively estimated through El-Sayed's rules [75 ].According to these rules, the ISC rate is relatively large if the transition between the S and T states involves a change of molecular orbital type ( Tables S10-S13).In Ag 13   structure of the S 0 , S 1 /S 2 , and T 1 /T 2 states of Au n Ag 13 −n ( n = 0, 1, 3, 13) is listed in Tables S6-S9.
The large structural relaxation of Au 13 leads to more energy dissipation.Femtosecond-nanosecond transient absorption (fs-ns TA) was used to deeply analyze the excited state behavior of A g 13 , Au n A g 13 −n and Au 13 .After irradiating by a 360 nm laser pulse, the excited electrons in the high energy state (403 nm) were reduced for Ag 13 (Fig. 4 A) while increased for Au n Ag 13 −n (Fig. 4 B, and Figs S35A, S36A) and Au 13 (Fig. 4 C), indicating that there was an electron transfer process between them.The ground state bleaching (GSB) signal absorption of Ag 13 (Fig. 4 D) and alloy clusters ( Figs S35B, S36B) can be observed.As for Au 13 , although no corresponding GSB absorption was observed at 348 nm due to the selection of laser pulse, the change of its trajectory also indicated that it has a set of negative signals corresponding to the absorption (Fig. 4 F).The non-negative signals at ∼480 nm may be caused by the overlap between excited state absorption (ESA) and GSB at 500 nm.The global fitting analysis demonstrated that Ag 13 had a decay constant of 3.3 ps and 127 ns (Fig. 4 D), in which the 3.3 ps process was assigned to IC.The 127 ns process was attributed to the return of the excited electrons from the T 1 state to the S 0 state.However, Ag 13 has a fast non-radiative transition in solution, so the luminescence is quenched and the triplet state lifetime is short.Moreover, the decay constants of the 10% alloy cluster and Au 13 were 250 ps, 238 ns (Fig. 4 E), and 10 ps, 3.4 μs (Fig. 4 F), respectively.Combined with DFT and TD-DFT calculation results, we believe that the 250 ps and 238 ns process of 10% alloy cluster can be attributable to the electron trans-fer from the ligand to metal core (LMCT) and electron return to the ground state after reaching a newly excited state through LMCT, respectively.The 10 ps process of Au 13 is considered to be a slower IC process.The process of 3.4 μs belongs to the electron reaching the newly excited state and finally returning to the ground state via radiative transition, producing luminescence with a high quantum yield.
Due to its NIR photoluminescence with an ultrahigh QY feature, the Au 13 clusters produced can be utilized for bioimaging.First, we employed mPEG-PLGA to encapsulate Au 13 clusters to enhance their water dispersibility and biocompatibility.As shown in Fig. 5 A, the Au 13 nanoclusters aggregated into uniform nano-micelles ( Au 13 @PLGA ) with an average particle size of ∼49.2 nm in Dulbecco's phosphate-buffered saline (PBS).In addition, the emission spectrum of Au 13 @PLGA sti l l exhibits excellent NIR photoluminescence, similar to that of Au 13 , indicating that the encapsulation of PLGA does not deteriorate the structure of Au 13 ( Fig. S37) .Moreover, the transmission electron microscopy (TEM) images further demonstrated that free Au 13 ( ∼1 nm, Fig. S38) was assembled with mPEG-PLGA to form uniform nanoparticles with sizes of 30-50 nm (Fig. 5 B).After incubating with different concentrations (from 0 to 10 μM) of Au 13 @PLGA for 24 h, the survival rate of HeLa cells exceeded 90% (Fig. 5 C), indicating its good biocompatibility.Au 13 @PLGA (4 μM) was then used for cell imaging.The confocal image exhibited significant intracellular red luminescence in HeLa cells after culturing for 2 h (Fig. 5

CONCLUSION
In summary, we synthesized a pair of gold-silver analogue clusters ( Ag 13 and Au 13 ) as wel l as their al loy clusters ( Au n Ag 13 −n ), where the icosahedral Ag 13 was first reported as a whole rather than as a kernel.The Ag 13 solution exhibited non-luminescence, while the analogue Au 13 solution displayed bright NIR luminescence with a QY up to 45%.The DFT calculation shows that the evolution of Ag 13 and Au 13 leads to the change of nanoclusters' energy level and structural rigidity with the increase of gold atom content, which leads to the subsequent variation of optical properties M series nanoclusters.This work reported the icosahedral Ag 13 structure for the first time and the evolution from Ag 13 to the analogous nanocluster Au 13 , uncovering the changes in optical properties during this process and revealing the intrinsic underlying causes.

Figure 1 .
Figure 1.The structures of (A) Ag 13 and (B) Au 13 (omitting the hydrogen atom for clarity).The mass spectra of (C) Ag 13 and (D) Au 13 .The insets show the experimental (black trace) and simulated (red trace) isotopic patterns of the molecular ion peaks.

Ag 13 ,
Au n Ag 13 −n and Au 13To figure out the relationship between electronic structure and optical properties during the evolution, density functional theory (DFT) calculations were conducted for the different model clusters.All three clusters have eight 'free' valence electrons, leading to a superatomic configuration of 1S 2 |1P 6 according to the Aufba u rule.By analyzing the DFT calculated results ( Figs S26A-

Figure 3 .
Figure 3. Energy diagrams of (A) Ag 13 , (B) Au 1 Ag 12 , (C) Au 3 Ag 10 and (D) Au 13 .Images of the hole and electron pairs.The Sr index is defined as the full space integration of a function (Sr(r)) describing the overlap between electron and hole distributions, and the D index is the distance between a hole and an electron center of mass.
and Au 13 , both S 1 and T 1 display obvious local excitation (LE) features due to the small D values ( D index < 0.5 Å).Obviously, both the S 1 | ˆ H soc |T 1 and T 1 | ˆ H soc |S 0 are less than 17 cm −1 in Ag 13 , indicating a slow ISC process of S 1 → T 1 and T 1 → S 0 [70 ], thus leading to the low p (exp.1.5%) and long τ p (exp.4.4 μs), respectively.These SOC constants ( S 2 | ˆ H soc |T 4 , 3 , 2 and T 1 , 2 | ˆ H soc |S 0 ) in Au 13 are enhanced up to 8 0 ∼13 0 cm −1 , which is responsible for the short τ p and high p .Turning to Au 1 Ag 12 , a significant charge transfer (CT) character is observed in the T 1 states ( D index = 4.73 Å), in addition to a predominant CT nature for their S 1 states ( D index = 5.19 Å).Consequently, E ST is small ( ∼0.01 eV), but S 1 | ˆ H soc |T 1 (23.97 cm −1 ) and T 1 | ˆ H soc |S 0 (40.49cm −1 ) < 41 cm −1 , thus leading to an increase in p (exp.9%) and decrease in τ p (exp.3.8 μs), respectively.Owing to the change in transition characters of the singlet and triplet states in the Au 3 Ag 10 cluster, S 1 | ˆ H soc |T 1 is enhanced up to 45.55 cm −1 , and T 1 | ˆH soc |S 0 up to 213.50 cm −1 , respectively.On the other hand, the CT triplet state becomes energetically close to the LE triplet state (0.03 eV), which is responsible for the short τ p (exp.5.2 μs) and high p (exp.38%).The emission and UV absorption spectra of M 13 in different solvents further confirm that the emission of Au 13 originates from the LE state, while the emission of alloy clusters originates from the CT state ( Figs S32 and S33)[76 -78 ].On the other hand, Au 13 exhibits a large structural change from T 1 /T 2 to S 0 , as shown in Fig.S34, whereas Ag 13 , Au 1 Ag 12 and Au 3 Ag 10 retain nearly the same structure for the T 1 and S 0 states.For Au 13 , its structural deformation comes mainly from the core.A detailed comparison of the bond lengths of the equilibrium

Figure 5 .
Figure 5. (A) Dynamic light scattering results of Au 13 @PLGA in PBS.Inset: images of Au 13 @PLGA under natural light (top) and UV (bottom) irradiation.(B) TEM image of Au 13 @PLGA .(C) Viability of HeLa cells after treatment with different concentrations of Au 13 @PLGA for 24 h.(D) Confocal image of HeLa cells incubated with Au 13 @PLGA for 2 h.