Nuclearity enlargement from [PW9O34@Ag51] to [(PW9O34)2@Ag72] and 2D and 3D network formation driven by bipyridines

The structural transformations of metal nanoclusters are typically quite complex processes involving the formation and breakage of several bonds, and thus are challenging to study. Herein, we report a case where two lacunary Keggin polyoxometallate templated silver single-pods [PW9O34@Ag51] (SD/Ag51b) fuse to a double-pod [(PW9O34)2@Ag72] by reacting with 4,4’-bipyridine (bipy) or 1,4-bis(4-pyridinylmethyl)piperazine (pi-bipy). Their crystal structures reveal the formation of a 2D 44-sql layer (SD/Ag72a) with bipy and a 3D pcu framework (SD/Ag72c) with pi-bipy. The PW9O349− retains its structure during the cluster fusion and cluster-based network formation. Although the two processes, stripping of an Ag-ligands interface followed by fusion, and polymerization, are difficult to envisage, electrospray ionization mass spectrometry provides enough evidences for such a proposal to be made. Through this example, we expect the structural transformation to become a powerful method for synthesizing silver nanoclusters and their infinite networks, and to evolve from trial-and-error to rational.

T he ubiquity of coordination-dissociation equilibrium of metal clusters in solution with partial retention of their original connectivity within the molecular structure provides a way to change their nuclearities and in some cases, allow for oligomerization to giant homologs [1][2][3][4][5][6] . Because of the subtle balance of bond energies, for example Ag-S, Ag-N, Ag-O versus Ag···Ag, and geometrical shapes, these reactions have given rise to an unexpected chemistry where coordination rules are not always fully obeyed and is now developing in a preparation process hereafter called structural transformation of metal nanoclusters. This process has been used to create clusters in a more controllable way compared to the classical de novo synthesis of cluster from simple reactants [7][8][9][10][11][12][13] . In particular, structural transformation is especially adapted for the assembly of silver clusters because of the versatile coordination geometries of Ag atom (strong coordination bonds) and ubiquitous moderate to weak argentophilic interactions as well as weak hydrogen bonding interactions [14][15][16][17][18] . Such alliance of pre-existing strong and weak interactions in silver clusters favors their subsequent dynamic structure transformation in response to some stimuli while partially retaining the cluster nuclearity, structure and at times its geometry. Currently, the employment of such strategy in the syntheses of silver clusters remains a rare event, although a related one called LEIST (ligand-exchange-induced size/structure transformation) methodology has been developed for reduced Au nanoclusters [19][20][21][22] . Related to structural transformation, Mak's group revealed an induced silver core enlargement from a small Cl@Ag 14 to a large Cl 6 Ag 8 @Ag 30 by the reaction with AgClO 4 23 . Soon afterwards, a discrete Ag 12 cluster was interconnected into a 3D framework by post-synthetic modification using 4,4'-bipyridine (bipy), while the unchanged silver cluster acts as the secondary building unit in the polymerization process 24 . Zang's group reported the reversible configuration transformation of the [V V 10 V IV 2 O 34 ] 10− core in the Ag 30 nanocage upon acid/base stimuli 25 . These sporadic reports exemplified the potential of structural transformation of silver clusters by specific stimuli. Motivated by the above advances, our group also successfully realized the synergetic conversion of both core and shell from [Mo 6 O 22 @Ag 44 ] to [Mo 8 O 28 @Ag 50 ] induced by benzoic acid and deciphered the underlying breakage-growth-reassembly (BGR) transformation mechanism based on a comprehensive characterization using electrospray ionization mass spectrometry (ESI-MS) 26 . This is a very complex and unique structural transformation involving the simultaneous enlargement of both the anionic core template and the outer silver shell.

Results
Synthesis. Colorless block crystals of SD/Ag51b are formed from the reaction of (iPrSAg) n , CF 3 COOAg, and Na 9 (A-PW 9 O 34 )·7H 2 O in CH 3 OH/DMF at 25°C. Subsequent addition of bipy to the mother liquor of above reaction system yields light yellow needles of SD/Ag72a (Fig. 1). Also reaction of bipy with crystals of SD/Ag51b dissolved in CH 3 OH leads to SD/Ag72a (Fig. 1). But if the bipy is added before the formation of SD/Ag51b, SD/Ag72a is not obtained. This confirms that the preformed SD/Ag51b is the key precursor to SD/Ag72a while partially retaining its molecular structure (see below). This is also in line with ESI-MS results (see later). We can then stress that structural transformation is indispensable for generating crystalline silver clusters that are unreachable using traditional one-pot reaction. We also tried to respectively add 1,4-bis(pyrid-4-yl)benzene and pi-bipy to the methanol solution of SD/Ag51b, but only latter gave brown-yellow crystals of SD/Ag72c ( Supplementary Fig. 2). A sequence of characterization techniques such as Fourier transform-infrared spectroscopy (FTIR), UV-Vis spectroscopy, DFT calculations, fluorescence spectroscopy, energy-dispersive X-ray spectroscopy (EDS), single-crystal X-ray diffraction (SCXRD), and powder X-ray diffraction (PXRD), were used in this system .
The presence of six coordinated solvent molecules on the surface of Ag 51 shell ( Supplementary Fig. 5) prompted us to explore the possibility of using it as a secondary building unit in extending to polymeric structures. We therefore used bipy as a ditopic connector and successfully isolated a 4 4 -sql network (SD/Ag72a) but with an unexpected modification to the pristine SD/Ag51b cluster where it has gone from a single-pod to a double-pod one. SCXRD of SD/ Ag72a (monoclinic P2 1 /n) reveals a double-pod cluster with face-toface POMs centered by a S 2− ion as template. The molecular crosssectional dimension is 2.1 × 1.0 nm 2 excluding the ligands. The asymmetric unit has a complete peanut-like 72-nuclei silver cluster, which is protected by 41 iPrS − (μ 3 and μ 4 ; Ag-S = 2.29-2.93 Å), 8 CF 3 COO − (μ 1 -κ 1 :κ 0 and μ 2 -κ 1 :κ 1 ; Ag-O = 2.21-2.71 Å), 11 bipy (Ag-N = 2.26-2.38 Å) as well as coordinated CH 3 OH and H 2 O (Fig. 3a). The coordination numbers of silve atoms are now extended to 2 (linear), 3 (T or Y shape), 4 (seesaw or tetrahedron), 5 (square-pyramid) and 6 (octahedron) (Supplementary Fig. 6). The Ag 72 cluster comprises two hemispheres of Ag 29 and Ag 31 sandwiching an equatorial S 2− -centered Ag 12 plane ( Fig. 3b and Supplementary Fig. 7). In each silver hemisphere, a PW 9 O 34 9− is encapsulated as template which supports the outer shell and connects the equatorial S@Ag 12 plane using the lacunary face of the PW 9 O 34 9− . There are 52 of 72 Ag coordinated to two PW 9 O 34 9− through Ag-O bonds (2.29-2.79 Å). The structure of PW 9 O 34 9− in SD/Ag72a is the same as that in SD/Ag51b, indicating the robustness of PW 9 O 34 9− even after experiencing transformation reaction. Importantly, each [(PW 9 O 34 ) 2 @Ag 72 ] cluster is connected to four neighbors through a total of 11 bridging bipy (Fig. 3c) to form a 2D 4 4 -sql network ( Fig. 3d and Supplementary Fig. 8). The two take-away information from these observations are (a) the  structural transformation of a low nuclearity silver cluster to a higher one while retaining its templating POM intact but with a slightly modified Ag shell, and (b) the connection of these clusters, as secondary building units, by the ditopic bipy to a coordination polymer.
Of note SD/Ag72a, [(PW 9 O 34 ) 2 @Ag 72 ], is the largest PW 9 O 34templated silver cluster so far, followed by [(PW 9 O 34 ) 2 @Ag 70 ] 31 and [(PW 9 O 34 ) 2 @Ag 67 ] 32 . Their similarities and differences give interesting information in the field of giant silver clusters. Their overall geometric shapes which are dominated by that of the templated POM are almost the same and they are all double-pods with two cores (POM, PW 9 O 34 9− ) wrapped by the shell (Ag-ligands). The major differences appear in the Agligand shells ( Supplementary Fig. 9). To simplify the discussion we define two parts: one is separator between the two pods and the other is the outer hemisphere. For [(PW 9 O 34 ) 2 @Ag 70 ] the separator is an almost flat plane of Ag 10 and the hemispheres are equal consisting of Ag 30 . For [(PW 9 O 34 ) 2 @Ag 67 ] the separator is Ag 12 and the hemispheres are unequal, Ag 28 and Ag 27 . SD/Ag72a has a centrally placed S 2− within an Ag 12 as separator. In spite of the structurally undisturbed anion template, the differences in the overall silver skeletons are likely driven by the capping ligands with different electronic and steric effects, such as iPrS − , CF 3 COO − , bipy for SD/Ag72a, tBuC ≡ C − for [(PW 9 O 34 ) 2 @Ag 70 ] and p-F-PhS − , CF 3 COO − for [(PW 9 O 34 ) 2 @Ag 67 ]. Other factors include the divergence of the bipy and supramolecular interactions.
Structure transformation from SD/Ag51b to SD/Ag72a. The structural transformation procedure indicates the formation of SD/ Ag72a is only possible after the formation of SD/Ag51b; that is introduction of bipy to the mother liquor or following isolation and subsequent reaction ( Supplementary Fig. 1). Furthermore, using bipy as a reactant before the formation of SD/Ag51b gave negative results. Thus, regardless of the route taken, it is important to verify the stability of SD/Ag51b in CH 3 OH, which guarantees the transformation reaction genuinely started from Ag 51 cluster rather than from some fragmented silver species in solution. We performed an ESI-MS study since it has been demonstrated to be a powerful tool to investigate stability and the assembly process of coordination compounds [35][36][37][38][39][40][41][42] . The positive-ion mode ESI-MS of SD/Ag51b dissolved in CH 3 OH (Fig. 4a) presents seven key doubly-charged peaks in the m/z range 2000-7000. The strongest peak (1c: m/z = 5652.40) corresponds to a complete cluster [(PW 9 O 34 )@Ag 51 (iPrS) 25 (CF 3 -COO) 15 ] 2+ (Calcd. m/z = 5652.48), which is formed by losing 2 CF 3 COO − and solvent molecules. All other peaks were assigned with precise formulae (Supplementary Table 1) and matched well with their simulated isotopic distributions (Inset of Fig. 4a). The Δm/z for the pairs 1a-1b, 1b-1c, 1c-1d, 1e-1f, 1f-1g are ca. 110, which equals the mass of CF 3 COOAg divided by charge state (+2), and indicates the progressive gain of CF 3 COOAg. The peak (1e; m/z = 5821.55) can be seen as a solvate addendum of 1d and identified to be [(PW 9 O 34 )@Ag 52 (iPrS) 25 (CF 3 COO) 16 There is no bigger fragment, ruling out the possibility of solvent-induced formation of SD/Ag72a. The ESI-MS of the reaction mother solutions during the synthesis of SD/Ag51b shows the same peaks 1a, 1b, 1c and 1d but of low intensities ( Supplementary Fig. 10), which suggest that formation and fragmentation follow the same fashion to mentioned above. Furthermore, SD/Ag51b can be recrystallized from CH 3 OH ( Supplementary  Fig. 11). These results confirm: (i) SD/Ag51b retains the integrity of its metallic skeleton in CH 3  disassociation of the labile ligands, (iii) SD/Ag72a can only be obtained from SD/Ag51b using a structural transformation way.
The most intriguing question that arises from the structural transformation is how SD/Ag51b transforms to SD/Ag72a after adding bipy to the solution? ESI-MS provide such insights into the evolution of solution species. Introduction of bipy into a methanol solution of SD/Ag51b initiated a synchronous cluster destruction and growth, which are revealed by two separated groups of +3 charged peaks before and after m/z = 4000, respectively (Fig. 4b). Below m/z = 4000, a total of ten peaks originating from 15 different species were identified (Fig. 4c). All contain a single PW 9 O 34 9− template enwrapped by a shell smaller than Ag 51 , (PW 9 O 34 )@Ag [37][38][39][40][41][42][43][44][45] . Among them, the most dominant species of 2i" can be assigned to [(PW 9 O 34 )@Ag 44 (iPrS) 22 (CF 3 COO) 10 (H 2 O) 2 ] 3+ . Each grouped peak in 2f-2j contains two overlapped species with one iPrS − in former replaced by one CF 3 COO − in latter. Above m/z = 4000, another 15 species overlapping in seven peaks were assigned to double PW 9 O 34 9− templated clusters of nuclearity higher than 51, (PW 9 O 34 ) 2 @Ag 60-66 (Fig. 4d). They are very likely intermediates towards Ag 72 . The main species in this group are (PW 9 O 34 ) 2 @Ag 62 having different iPrS − :CF 3 COO − ratios (2 m', 2 m", and 2 m"'). All thirty species were precisely assigned based on simulated and observed isotopic patterns (Supplementary Table 2). The result indicates the lability of the ligands allows for their rapid exchange which promotes the silver shell of SD/Ag51b to be dynamic to the formation of the intermediates toward the final SD/Ag72a. In comparison with the plenary POMs, the PW 9 O 34 9− possesses more bare-oxygen vacancies and a higher negative charge, therefore, the W 6 face at the base of the pumpkin-like Ag 51 cluster may be more active and have higher affinity with the silver atoms than the W 3 face. Correspondingly, the silver atoms attached to the W 6 face are also endowed with high reactivity. These highly active regions may be preferentially chosen the way of face-fusion to form a stable entity. Combined with the above observations, we proposed a bipy-induced breakage-fusion conversion mechanism for this system (Fig. 5). The successful isolation of SD/Ag72a is caused by the disappearance of the smaller SD/Ag51b and this kind of fusion represents a simple model to explain usual coalescence of surfactant-protected nanoparticles from discrete nanocrystals or nanoclusters. The structural transformation was further elucidated by the HAADF-STEM, where the size of the nanoparticles increased from approximately 1.3 to 2.1 nm and some aggregation behavior of them were observed after adding bipy to the methanol solution of SD/Ag51b ( Supplementary  Fig. 12).
The 31 P NMR was also performed to verify this question ( Supplementary Fig. 13). A single resonance peak at δ = −10.24 ppm corresponding to PO 4 3− was observed for SD/Ag51b dissolved in methanol, corroborating with one crystallographically independent PW 9 O 34 9− . However, the resonance signal of PO 4 3− completely disappear in the −350 to 250 ppm range after adding bipy with the color changing from colorless to yellow. When ejecting the NMR tube from instrument, we surprisingly found abundance of tiny crystals precipitated from the solution, which were confirmed to be SD/Ag72a by unit cell checking on a single crystal X-ray diffractometer ( Supplementary Fig. 14). The formation of insoluble SD/Ag72a decreased the concentration of PO 4 3− -contained species in solution, as a result, no signal can be detected after conversion reaction. All these results unambiguously evidenced the occurrence of cluster conversion reaction by addition of bipy.
Universality of bipy-induced transformation. Followed by above successful case, we further evidenced the universality of structural transformation strategy and isolated SD/Ag72c as a pcu framework by adding pi-bipy to the methanol solution of SD/Ag51b. SCXRD analysis shows that the asymmetric unit in SD/Ag72c is very similar to that in SD/Ag72a (Supplementary Fig. 15). Differently, each [(PW 9 O 34 ) 2 @Ag 72 ] cluster in SD/Ag72c is connected to six neighbors through a total of 9 bridging pi-bipy ligands (Fig. 6a) to form a 3D pcu framework (Fig. 6b).
UV-Vis absorption spectra and luminescence properties. The solid-state UV/Vis spectra of SD/Ag51b and SD/Ag72a at room temperature are shown in Supplementary Fig. 16. Both exhibit single intense absorption centered at 341 and 352 nm, respectively. Given their structural relevance, the structurally simplified model SD/Ag51b was used as a representative to analyze their electronic transitions by means of TD-DFT calculations (See details in Supplementary Information). The absorptions above 400 nm can be ascribed to the ligand and metal charge transfer to the inner core (herein named after L[C]CT and M[C]CT, respectively), i.e. transitions from the surface ligands (mainly 3p orbitals of S atoms in the thiol ligands and 2p orbitals of O atoms in the carboxylic ligands) and Ag atoms (mainly 4d orbitals) to the W = O π* orbitals (formed by 5d orbitals of W atoms and 2p orbitals of O atoms) in the inner PW 9 O 34 core; such type of transitions were also previously found in some molybdatetemplated Ag clusters 26 . Another two types of transitions, such as those from Ag 4d to its 5 s/p orbitals (MMCT, metal to metal   Table 3 and Data 1). The calculated UV spectrum shows an absorption maximum at 338 nm, which is in fair agreement with the experimental value (300 nm) of SD/Ag51b (Supplementary Fig. 17). The experimental variable-temperature (113-293 K) solid state emission spectra of SD/Ag51b and SD/Ag72a under the excitation of 468 nm are shown in Fig. 7. Both emit luminescence in the near-infrared (NIR) region when the temperature is cooling to 263 K with emission peaking at 787 and 737 nm, respectively. Their emission peaks gradually blue-shift to 740 nm for SD/Ag51b and 716 nm for SD/Ag72a upon cooling to 113 K along with the increase of intensity. The quantum yields of SD/ Ag51b and SD/Ag72a at 113 K were measured to be 1.26% and 2.17%, respectively. The average lifetimes of the emissions are also determined to be on the microsecond scale (τ = 219.4 μs for SD/ Ag51b and τ = 208.1 μs for SD/Ag72a), respectively (Supplementary Fig. 18), indicating their phosphorescence character 43 . Supplementary Fig. 19 shows good linearity correlation between maximum emission intensity (I max ) and temperature (T) in the ranges of 113-233 K (SD/Ag51b) and 113-293 K (SD/Ag72a), which indicate both of them may be promising materials for lowtemperature molecular luminescent thermometer. Due to the restriction of theoretical method and computational cost, it is nearly impossible to utilize TD-DFT or other ab-initio method to identify emission mechanism (e.g. electronic transitions, quantum yield) for such larger clusters of SD/Ag51b and SD/Ag72a with  ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-29370-w more than 300 non-H atoms. Fortunately, studying electronic transitions of UV absorption is helpful to understand emission property. Based on the calculated absorption results, the emissions of SD/Ag51b and SD/Ag72a should be associated with the above three types of charge transfers, though the exact excitation/relaxation pathway is still difficult to be determined. According to the previous findings, however, these NIR emissions is more likely to be attributed to the ligand-to-metal charge transfer (LMCT) from S/O p to Ag 5 s/p orbitals perturbed by Ag···Ag interaction 44,45 . The lowest-energy electronic excitation of the model SD/Ag51b in gaseous phase, relevant to its fluorescent emission, is calculated to occur at 474 nm and mainly contributed by the transition from HOMO to LUMO. However, the absorption spectrum of SD/Ag51b both experimentally and theoretically in solid state shows a longer-wavelength tail beyond 500 nm. It is demonstrated that a lower excitation energy is sufficient to trigger the transition of S 0 → S 1 in solid state and then gives rise to a longer-wavelength emission by further relaxation of S 1 . In this regard, the NIR emissions in SD/Ag51b and SD/Ag72a are reasonable and can be partially rationalized by the intermolecular interactions in solid state, which often leads to significant red-shift for the emission by forming possible excimers [46][47][48] .

Discussion
The key message of this work is the successful application of the structural transformation from an isolated PW 9 O 34 9− templated single-pod Ag 51 cluster to a double-pod Ag 72 cluster by the stimulus of bipy while partially retaining the original bonding, shape and the template. Importantly, the linkers bipy and pi-bipy connect the generated Ag 72 into a 2D corrugated polymeric sheet and a 3D framework, respectively. Thus, the Ag 51 cluster can be viewed as an intermediate en-route to larger progeny. The structural transformation involves the increase of the nuclearity of silver cluster by trapping more POM templates and the extension of the discrete cluster to infinite 2D and 3D coordination networks by installing the exotic linkers. A breakagefusion conversion mechanism was also established via reaction monitoring by ESI-MS. This work not only establishes a wellcontrolled method for synthesizing larger silver nanoclusters but also deepens our understanding on the structural variability and chemical reactivity of this class of silver clusters.

Methods
Synthesis of (iPrSAg) n . (iPrSAg) n was prepared by the following reported procedure 6 . The solution of AgNO 3 (30 mmol, 5.1 g) in 75 mL acetonitrile was mixed with 100 mL ethanol containing iPrSH (30 mmol, 2.8 mL) and 5 mL Et 3 N under stirring for 3 h in the dark at room temperature, then the yellow powder of (iPrSAg) n was isolated by filtration and washed with ethanol and ether, then dried in the ambient environment (yield: 97 %).
Synthesis of Na 9 (A-PW 9 O 34 )·7H 2 O. Na 9 (A-PW 9 O 34 )·7H 2 O was prepared by the following reported procedure 49 . Na 2 WO 4 ·2H 2 O (0.36 mol, 120 g) was dissolved in l50 g of water and phosphoric acid (85%) was added dropwise with stirring (0.06 mol, 4.0 mL). After addition of the acid is complete, the measured pH was 8. Single crystal X-ray diffraction analyses. Single crystals of SD/Ag51b, SD/Ag72a and SD/Ag72c with appropriate dimensions were chosen under an optical microscope and quickly coated with high vacuum grease (Dow Corning Corporation) to prevent decomposition. Each crystal was mounted on CryoLoop™ loop and the cell parameters and intensity data were recorded on a Rigaku Oxford Diffraction XtaLAB Synergy-S diffractometer equipped with a HyPix-6000HE Hybrid Photon Counting (HPC) detector operating in shutterless mode and an Oxford Cryosystems Cryostream 800 Plus at 100 K using Cu Kα (λ = 1.54184 Å) for SD/Ag51b and SD/Ag72c and Mo Kα (λ = 0.71073 Å) for SD/Ag72a from PhotonJet micro-focus X-ray Source. Data were processed using the CrystAlis Pro software suite 50 . These structures were solved using the charge-flipping algorithm, as implemented in the program SUPERFLIP 51 and refined by full-matrix least-squares techniques against F o 2 using the SHELXL program 52 through the OLEX2 interface 53 . Hydrogen atoms at carbon were placed in calculated positions and refined isotropically by using a riding model. Appropriate restraints or constraints were applied to the geometry and the atomic displacement parameters of the atoms in the cluster. All structures were examined using the Addsym subroutine of PLATON 54 to ensure that no additional symmetry could be applied to the models. Pertinent crystallographic data collection and refinement parameters are collated in Supplementary Table 4. Selected bond lengths and angles are collated in Supplementary Data 2.
Experiment details. iPrSH (Adamas-beta®) and CF 3 COOAg (Adamas-beta ® ) were purchased from Shanghai Titan Scientific Co., Ltd. All other chemicals and solvents used in the syntheses were of analytical grade and used without further purification. IR spectra were recorded on a Perkin Elmer Spectrum Two in the frequency range of 4000-450 cm −1 . Powder X-ray diffraction (PXRD) patterns were recorded for microcrystalline powdered samples using a Rigaku Oxford Diffraction XtaLAB Synergy-S diffractometer using Cu radiation (λ = 1.54184 Å). The PXRD patterns were processed with the CrysAlis Pro software suite using the Powder function 50 . 31 P NMR spectra were recorded in J. Young NMR tube on Bruker Avance 300 MHz spectrometer. UV-Vis spectra were performed on UV−Vis spectrophotometer (Evolution 220, ISA-220 accessory, Thermo Scientific). Temperature-dependent photoluminescence measurements were carried out in an Edinburgh spectrofluorimeter (FLS920) coupled with an Optistat DN cryostat (Oxford Instruments), and the ITC temperature controller and a pressure gauge were used to realize the variable-temperature measurement in the range of 113-293 K. Spectra were collected at different temperatures after a 5 min homoiothermy. Time-resolved photoluminescence lifetime measurements were measured on Edinburgh spectrofluorimeter (FLS920) using a time-correlated single-photon counting technique. Mass spectra were recorded on an Agilent 6224 (Agilent Technologies, USA) ESI-TOF-MS spectrometer. Sample solutions are infused by a syringe pump at 240 μL/h. Data were acquired using the following settings: electrospray ionization in positive mode, capillary voltage was set at 3.5 kV (-) and fragmentor at 200 V. The nebulizer was set to 15 psi and the nitrogen drying gas was set to a flow rate of 4 L/min. Drying gas temperature was maintained at 150°C. The data analyses of mass spectra were performed based on the isotope distribution patterns using Agilent MassHunter Workstation Data acquisition software (Version B.05.00). The reported m/z values represent monoisotopic mass of the most abundant peak within the isotope pattern. The HAADF-STEM experiments were carried out at 300 kV using Thermofisher Spectra 300 scanning transmission electron microsope with a probe Cs-corrector.
Computational details. DFT calculations were performed with the Gaussian 16 suite of programs 55 . To reduce the computational cost, the ligand-simplified Ag 51 cluster [(PW 9 O 34 )@Ag 51 (CH 3 S) 25 (CF 3 COO) 17 (HCONH 2 ) 3 (H 2 O) 3 ] was used as a model for SD/Ag51b. The ligand simplification by changing iPrS to CH 3 S was done from the crystal structure of SD/Ag51b with GaussView and the resulting model cluster was directly used for the subsequent TD-DFT calculation. The B3PW91 hybrid density functional 56 was employed in the TD-DFT calculation. The Los Alamos valence double-zeta with Hay-Wadt ECPs (LanL2DZ) basis set 57 containing relativistic effects was employed for Ag and W atoms, and 6-31G basis set was used for other non-metal atoms. A total of 300 singlet states were chosen and the root was set as 1. Data for orbital composition analysis with Mulliken partition are from Gaussian 16 calculations and further processed with Multiwfn software 58 . The most probable transitions were determined based on the oscillator strength values and weights. The optical absorption spectrum was convoluted with a Gaussian line shape with a half-width at half-height of 0.25 eV.

Data availability
The data supporting the findings reported herein can be found in the manuscript, its supplementary Information, or from the authors upon request. The time-dependent density functional theory calculations of SD/Ag51b and the selected bond lengths (Å) and angles ( o ) for silver nanoclusters are provided with Supplementary Data 1 and 2, respectively. The X-ray crystallographic coordinates for structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition number CCDC: 2091982-2091984 for SD/Ag51b, SD/Ag72a, and SD/Ag72c. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.