A Silver Nanocluster Assembled by a Superatomic Building Unit

A unique assembly of a two-electron superatom, [Ag10{S2P(OiPr)2}8], as a primary building unit in the construction of a supramolecule [Ag10{S2P(OiPr)2}8]2(μ-4,4′-bpy) through a 4,4′-bipyridine (4,4′-bpy) linker is reported. This approach is facilitated by an open site in the structure that allows for effective pairing. The assembled structure demonstrates a minimal solvatochromic shift across organic solvents with variable polarities, highlighting the influence of self-assembly on the photophysical properties of silver nanoclusters.

M etal nanoclusters (NCs) have gained significant attention due to their remarkable electronic, optical, magnetic, and catalytic properties, 1−4 which are intricately influenced by factors such as size, composition, and surface structure.−11 By the harnessing of self-assembly principles, it becomes possible to guide the organization of metal NCs into larger, wellordered structures.Cluster assembly encompasses the intricate arrangement of metal atoms or small molecular units, leading to the formation of larger nanoscale clusters.−15 However, these intermolecular interactions exhibit relatively weak strength.In contrast, implementing organic linkers in the nanomolecular assembly has emerged as a promising strategy.In the regime of silver cluster-assembled materials (SCAMs), linkers such as pyrazine, 16 1,4-bis(4pyridyl)benzene, 16 dipyridin-4-yldiazene (dpd), 16 4,4′-bipyridine (bpy), [16][17][18]24 1,2-bis(4-pyridyl)acetylene, 19 1,4-bis-(pyridin-4-ylethynyl)benzene (bpeb), 19 3-amino-4,4′-bipyridine, 20 pyridinecarboxylic hydrazide (o-, m-, and p-iah), 21 trans-1,2-bis(4-pyridyl)ethylene, 22 5,10,15,20-tetra(4-pyridyl)porphyrin, 23 2,2′,7,7′-tetra(pyridin-4-yl)-9,9′-spirobi-(fluorene), 11 and (2-thiazolyl)sulfide 25,26 have been employed as more robust and more stable linkages, providing enhanced control over the assembly process.Notably, SCAMs offer many advantages that significantly enhance the functionality, stability, and applicability. Thesebenefits stem from the versatile and tunable nature of organic linkers, which can be engineered to impart specific properties to cluster assemblies. Byenhancement of the structural diversity and complexity, these linkers enable the creation of materials with tailored geometries and dimensionalities.Organic linkers also contribute to the improved stability of SCAMs, protecting metal clusters from environmental degradation and extending their functional lifespan. A oteworthy example is the work by Negishi et al., who utilized a 3D silver(I) cluster-assembled material as a surface-enhanced Raman scattering sensor for the detection of Hg 2+ ions.19 This innovation marks a significant expansion in the application domains of SCAMs, demonstrating their potential beyond traditional uses.Despite these advancements, research in this field is still in its nascent stages, indicating a vast scope for exploration and discovery in utilizing SCAMs for environmental monitoring and beyond.
In general, SCAMs typically feature a secondary building unit (SBU) composed of Ag I atoms, which possess zero electrons.However, a notable exception was reported by Mak et al. in 2018, 16 following their establishment of the first 2D SCAM, [Ag 12 (S t Bu) 8 (CF 3 COO) 4 (bpy) 4 ] n , in 2017. 18In this exceptional case, a two-electron [Ag 14 (C 2 B 10 H 10 S 2 ) 6 ] 0 NC was utilized as the cluster node, which was connected by pyrazine, dpd, bpy, and bpeb ligands, resulting in formation from 1D to 3D frameworks.This discovery marked the first instance of Ag 0 -containing superatomic NCs being employed as SBUs in the construction of SCAMs.The Ag 14 skeleton is composed of an [Ag 6 ] 4+ octahedron with eight face-capping Ag I atoms.The linker ligands connect to these face-capping Ag I atoms, thereby constructing polymeric species.In any case, investigations on a superatomic Ag NC as a SBU remain underexplored.In our previous study, we reported an ultrasmall two-electron Ag NC, [Ag 10 {S 2 P(O i Pr) 2 } 8 ] (denoted as Ag 10 ). 27Interestingly, the presence of an external Ag atom with an unoccupied coordination site suggested the potential for subsequent reactions.Building upon this observation, the current investigation employed bpy as the linker to connect two Ag 10 NCs, resulting in the formation of [Ag 10 {S 2 P-(O i Pr) 2 } 8 ] 2 (μ-bpy) (denoted as Ag 10 bpy).Notably, the Ag 10 NCs, once assembled, exhibited photoluminescence (PL) within the near-infrared (NIR-I) region at ambient temperature in solution along with an elevated QY.This underscores their promising utility in applications such as bioimaging and biosensing. 28,29g 10 bpy was synthesized by mixing Ag 10 and bpy ligands in a tetrahydrofuran (THF) solution with a molar ratio of 1:10.The solution was allowed to stand for 1 week to collect crystals as the product.The resulting yield of crystalline products is ca.85%.It should be noted that decreasing the portion of linkers in the reaction adversely affected the crystal yield.We employed a shorter linker, e.g., pyrazine, in the reaction.Nevertheless, we have not succeeded in obtaining crystals.This challenge may be attributed to the steric hindrance caused by the intermolecular dithiophosphate (dtp) ligands associated with the short contact.In contrast to the previous studies, 16,17,19,20,23 which utilized a Ag I -L (L = thiolate/ acetate) complex as a precursor in the assembly reaction, our approach directly employs two-electron superatom in the synthesis.This methodology facilitates a more precise and controlled synthesis while potentially mitigating the formation of excessive byproducts.
The crystal structure of Ag 10 bpy shows a pair of [Ag 10 {S 2 P-(O i Pr) 2 } 8 ] molecules connected by bpy as a linker through the external Ag atom (Figure 1a).It crystallized at space group P1̅ and showed two half-molecules (clusters I and II) in the asymmetric unit.Because the bond distances in the two molecules are very similar, only the distance of cluster I will be mentioned below.Relevant distances are summarized in Table S2.The entire molecule possesses C i symmetry, where the inversion center is located at the center of the bpy linker, equally divided into two six-membered rings.The Ag 10 framework in Ag 10 bpy retains the geometry of a tetracapped trigonal bipyramid and an extended capping Ag ext (Figure 1b).The Ag•••Ag distances in the two tetrahedra of the bipyramid are similar [avg.2.8932(9) Å in yellow Td; avg.2.8775(9) Å in green Td], while that in the capping tetrahedra is slightly longer [avg.2.9567(9) Å in cyan Td].In comparison to Ag 10 , the average Ag•••Ag distance within each tetrahedron in the metal framework is marginally shorter [2.8562(13) Å in yellow Td, 2.8553(13) Å in green Td, and 2.9495(13) Å in cyan Td], indicating a more pronounced argentophilic interaction.Notably, an empty site was observed within the Ag ext S 3 motif in Ag 10 (Figure 1c).This unique vacancy suggests the potential introduction of organic solvents or heteroligands to this specific position.The connection of the bpy linker leads to an elongation between Ag ext and its trigonal bottom (2.593 Å in Ag 10 bpy; 2.353 Å in Ag 10 ), resulting in a unique μ 4 -Ag ext in a pyramidal geometry (Figure 1d), thus offering a fixed distance between two Ag 10 motifs.The N−Ag ext and Ag ext •••Ag ext distances in Ag 10 bpy are 2.395(7) and 11.871(1) Å, respectively.The coordination modes of the dtp ligands in Ag 10 bpy (Figure 1c) are consistent with those in Ag 10 (Figure 1e).Specifically, the ligands on P1, P3, P4, and P5 maintain a tetrametallic tetraconnective (η 4 :μ 2 , μ 2 ) mode and that on P8 is in a trimetallic tetraconnective (η 3 :μ 2 , μ 2 ) mode, while those on P2 and P6 adopt a trimetallic triconnective (η 3 :μ 1 , μ 2 ) mode and that on P7 is in a bimetallic diconnective (η 2 :μ 1 , μ 1 ) mode.The sum of the rotation angles in the Ag ext S 3 motif in both Ag 10 (Figure 1f) and Ag 10 bpy (Figure 1g) reveals noteworthy distinctions.The former case shows that the cumulative angle closely approximates 360°, indicative of the Ag ext S 3 motif's close alignment with a coplanar arrangement, thereby facilitating a solvent molecule proximity.Conversely, in Ag 10 bpy, the introduction of bpy ligands results in a pyramidal geometry.
The absorption spectrum of Ag 10 bpy exhibits two prominent bands at 389 and 516 nm, accompanied by a shoulder at 347 nm (Figure 2a).This pattern bears similarity to that of the previously reported Ag 10 (348, 392, and 520 nm). 27The emission maximum of Ag 10 bpy at 749 nm closely resembles that of Ag 10 .The predominant portion of the emitted light range is situated within the NIR-I region.Despite this similarity, there is a slight reduction in the quantum yield (QY) for Ag 10 bpy, which stands at 2.3%, in contrast to discrete Ag 10 with a QY of 6%.This reduction might be attributed to the linker in Ag 10 bpy, which connects the Ag ext atoms, increasing the distance between Ag ext and nearby Ag atoms by about 0.2 Å.This increased distance, averaging 3.202 Å in Ag 10 bpy compared to 3.022 Å in Ag 10 , could lead to decreased argentophilicity, promoting energy release through nonradiative vibrational relaxation pathways.Additionally, a temperature-dependent blue shift of 63 nm is observed in Ag 10 bpy when the temperature is lowered from 298 to 77 K.The PL decay curve for Ag 10 bpy aligns well with a singleexponential fitting curve.The emission lifetime of Ag 10 bpy is 2.4 ns at room temperature (RT; Figure S4) and 14.5 ns at 77 K (Figure S5), exhibiting the fluorescence origin of the emission.The photophysical data are summarized in Table S3.Overall, the absorption and emission spectra exhibit consistency after the assembly of Ag 10 NCs, showing subtle shifts that uphold the electronic characteristics of the two-electron superatoms.
Following its assembly with linker ligands, the Ag 10 bpy molecule experiences a discernible change in its molecular shape.This structural modification potentially gives rise to an alteration in the molecular dipole moment. 30To evaluate this hypothesis, we conducted a solvent-dependent absorption spectral analysis.The solvent-dependent UV−vis spectra (Figure 2b,c) show discernible shifts in the absorption bands when dissolved in solvents with varying polarities.The polarity of the solvents (Figure S6) is quantified using the polarity parameter (E T ), which is defined by the molar transition energy (measured in kilocalories per mole). 31Band A exhibits a heightened sensitivity to variations in the solvent polarity.This behavior is logically consistent with its classification as a part of the ligand-to-metal charge-transfer band, particularly due to the ligands being situated at the outermost layer and thus being more prone to interact with solvent molecules.On the other hand, bands B and C initially exhibit blue-shifting and then red-shifting with increasing solvent polarity.The observation of a blue shift as the solvent polarity increases aligns with the behavior exhibited by the eight-electron NCs Au 2 2 − x Ag x Cd 1 (SAdm) 1 5 X (x ∼ 3; X = Br/Cl), Au 22 Cd 1 (SAdm) 15 Br, and Au 19 Ag 4 (SAdm) 15 . 30It is noted that the previous study did not employ a solvent of higher polarity, with the most polar solvent used being CH 2 Cl 2 .Band C primarily involves a 1S → 1P x transition, wherein the orientation of its 1P x orbital is oriented toward Ag ext , rendering it susceptible to influences from bpy ligands or solvents attached to this site.Band C displays a slight shift (∼18 meV) in Ag 10 bpy, whereas Ag 10 exhibits more shifts (24 meV).In contrast, other NCs characterized by low dipole moments (μ < 4 D), such as Au 30 (S t Bu) 18 , 32 [Au 25 (SC 2 H 4 Ph) 18 ] − , 33 [Au 25 (SC 2 H 4 Ph) 18 ] 0 , 34 and Au 21 (SAdm) 15 , 35 show smaller peak shifts (<14 meV). 28Our observations suggest that the dipole moment of Ag 10 is higher than that of Ag 10 bpy.In essence, merging separate entities with their own distinct dipole moments can lead to a new structure in which these moments partially cancel each other out.The assembly of Ag 10 (C 1 ) into Ag 10 bpy (C i ) results in a more symmetrical molecular shape, leading to a reduction in the molecule's dipole moment and a consequent decrease in its susceptibility to solvent polarity.
The solvent-dependent emission spectra reveal a consistent trend, wherein Ag 10 exhibits a more pronounced peak shift of 54 meV compared to that of Ag 10 bpy (21 meV).In addition to reducing the dipole moment after assembly, another reason may be that the linker blocks the open site on Ag ext , avoiding the interaction of various solvent molecules with this site.The emission in Ag 10 originates from the transition of 1P x to 1S.Consequently, solvent molecules can significantly influence Ag 10 with its vacant site, altering the distance from the superatomic core and thereby resulting in a prominent solvatochromic shift at RT.
In summary, this study presents a unique assembly approach employing superatomic Ag NCs, specifically [Ag 10 {S 2 P-(O i Pr) 2 } 8 ], as building blocks.The surface characteristics of the Ag 10 NC reveal an accessible binding site on the Ag ext atom, enabling the attachment of organic linkers and yielding the formation of [Ag 10 {S 2 P(O i Pr) 2 } 8 ] 2 (μ-4,4′-bpy).Notably, the solvent-dependent UV−vis absorption and emission spectra underscore the substantial influence of the solvent polarity.This research not only introduces innovative approaches to designing supramolecular architectures utilizing superatomic building blocks but also opens a novel avenue for manipulating the photophysical properties of atomically precise Ag NCs.The findings highlight the resilience of superatomic electronic properties, showcasing their capacity for fine-tuning in an atypical way.Further investigations will be warranted to deepen our understanding of the underlying factors governing the successful formation of the targeted assembled architecture.The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c00139.

Figure 1 .
Figure 1.(a) Total structure of Ag 10 bpy.(b) Ag 10 skeleton in Ag 10 bpy.(c) Enlarged view of the area near the Ag ext S 3 motif in Ag 10 (d) and Ag 10 bpy.(e) Ag 10 {S 2 P(O i Pr) 2 } 8 motif in Ag 10 bpy (the isopropoxy groups and bpy were omitted for clarity).(f) Sum of the rotation angle at the Ag ext S 3 motif in Ag 10 and (g) Ag 10 bpy.Thermal ellipsoids were drawn at 30% probability.

Figure 2 .
Figure 2. (a) Absorption and emission spectra of Ag 10 and Ag 10 bpy in THF at RT.(b) Solvent-dependent absorption spectra of Ag 10 and (c) Ag 10 bpy at RT.(d) Solvent-dependent emission spectra of Ag 10 and (e) Ag 10 bpy at RT.