Large-Scale Synthesis, Crystal Structure, and Optical Properties of the

Aug 16, 2018 - Solving the atomic structure of large-sized metal nanoclusters is a highly challenging task yet critically important for understanding ...
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Large-Scale Synthesis, Crystal Structure and Optical Properties of the Ag146Br2(SR)80 Nanocluster Yongbo Song, Kelly Lambright, Meng Zhou, Kristin Kirschbaum, Ji Xiang, Andong Xia, Manzhou Zhu, and Rongchao Jin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04233 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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Large-Scale Synthesis, Crystal Structure and Optical Properties of the Ag146Br2(SR)80 Nanocluster Yongbo Song,1,2 Kelly Lambright,3 Meng Zhou,2 Kristin Kirschbaum,3 Ji Xiang,1 Andong Xia,4 Manzhou Zhu1* and Rongchao Jin2* 1

Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui University, Hefei, Anhui 230601, China. 2 Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States. 3 Department of Chemistry and Biochemistry, University of Toledo, Toledo, Ohio 43606, United States. 4 Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. *Emails: (M.Z.) [email protected]; (R.J.) [email protected] Abstract: Solving the atomic structure of large-sized metal nanoclusters is a highly challenging task yet critically important for understanding the properties and developing applications. Herein, we report a stable silver nanocluster―Ag146Br2(SR)80 (where R = 4-isopropylbenzenethiol) with its structure solved by X-ray crystallography. Gram-scale synthesis with high yield has been achieved by a one-pot reaction, which offers opportunities for functionalization and applications. This silver nanocluster possesses a core-shell structure with an Ag51 core surrounded by a shell of Ag95Br2S80. The Ag51 core can be viewed as a distorted decahedron, endowing this nanocluster with quantized electronic transitions. In the surface-protecting layer, five different types of S-Ag coordination modes are observed, ranging from the linear Ag-S-Ag to S-Ag3 (triangle) and S-Ag4 (square). Furthermore, temperature-dependent optical absorption and power independent electron dynamics are conducted to explore the relationship between the properties and structure, demonstrating that the distorted metal core and “flying saucer”-like shape in this nanocluster have significant effects on the electronic behavior. A comparison with multiple sizes of Ag nanoclusters also provides some insights into the evolution from the molecular to metallic behavior. Keywords: atomically precise, silver nanocluster, crystal structure, large-scale synthesis, ultrafast spectroscopy, molecular state

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Owing to the distinct physicochemical properties such as quantized electronic absorption,1,2 magnetism,3,4 strong fluorescence,5-7 optical limiting capability,8,9 catalytic reactivity,10,11 chirality and dynamics,12 metal nanoclusters have attracted tremendous interest as a new class of functional materials. In fundamental research, it is of great significance to reveal the precise structure of metal nanoclusters, because the structure constitutes the basis for understanding the surface (including the composition, the organo-metal interfacial bonding, and the arrangement of protecting ligands), the relationship between properties and structures, and the mechanisms of growth and evolution.13 Thiolate-protected gold nanoclusters have been extensively studied and significant progress has been obtained recently,14-22 which forms an new addition to metal nanomaterials. The total structure (metal core plus ligands) of the hitherto largest Au246(SR)80 nanocluster revealed that such particles can self-assemble with the same hierarchy, accuracy, and complexity as biomolecules driven by the C-H···π interaction.15 With respect to the emergence of metallic state, ultrafast spectroscopic studies revealed the non-metallic regime (smaller than Au246)26 and the metallic regime (size larger than Au279).14 The research progress has stimulated wide interest in metal nanoclusters.13,23-25 With the significant advances achieved in gold nanocluster research, recently silver nanoclusters have also drawn the attention of nanomaterials chemists and great efforts have been put into the synthesis of atomically precise silver nanoclusters.27-39 Some of them have been successfully characterized by single crystal X-ray diffraction (SC-XRD), such as Ag25, Ag44, Ag67, and a FCC series of Ag14, Ag38, Ag63, as well as Ag62.27-35 On the other hand, revealing the precise structures of large silver nanoclusters (n >100 atoms) is still highly challenging yet critically important because they will offer insight into the evolution of packing modes of silver atoms and the arrangement of surface layer toward plasmonic nanoparticles. For example, the determined Ag134 and Ag374 nanoclusters serves as models to understand the detailed structure distortion within twinned metal nanostructures and how silver nanoclusters span from the molecular to metallic regime.34 Despite the major progresses,27-38 some questions still remain: 1) how do the silver atoms and thiolate ligands form bonds in large nanoclusters that evolve from molecular to metallic state? 2) At what size does the evolution from non-metallic to metallic state occur in silver nanoclusters? To tackle these and many other fundamental questions, the structure determination of large silver nanoclusters is of vital importance. Herein, we report an approach to synthesizing a large-sized silver nanocluster with relatively high stability and the successful determination of its X-ray crystallographic structure. This silver nanocluster contains 146 silver atoms, 2 bromine atoms, and 80 surface-protecting –SR ligands (where HSR = 4-Isopropylbenzenethiol, TIBT for short), hence, formulated as Ag146Br2(TIBT)80 (Ag146 for short). A distorted Ag51 core was observed in this silver nanocluster, which is protected by a Ag95Br2S80 surface. Gram-scale synthesis of this silver nanocluster with high yield has been achieved by a one-pot process. Moreover, temperature-dependent optical absorption spectra and power independent electron dynamics indicate that the Ag146 nanocluster is non-metallic, hence, it implies the emergence of metallic behavior in sizes larger than Ag146. Results and Discussion The Ag146 nanocluster was synthesized by direct reduction of Ag(I)-SR precursor in organic solvent. Briefly, an aqueous solution of AgNO3 was mixed with a dichloromethane solution of 4-isopropylbenzenethiol in a round-bottom flask, which led to the formation of the Ag(I)-SR complex in yellow precipitates. After one hour, tetraphenylphosphonium bromide was added, and a clear solution of the Ag(I)-SR precursor was obtained, to which an aqueous solution of NaBH4 was added to reduce Ag(I) to Ag(0). After ~72 h of reaction, the silver nanoclusters were produced with high yield (~95%) (detailed method in Supporting Information). High quality black cube-like crystals were obtained by diffusion of acetone into a dichloromethane solution of nanoclusters at room temperature (2-3 days). The dark crystals were subject to X-ray diffraction and the atomic structure was solved, which was found to crystallize in space group P-1. 2

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Figure 1 shows the total structure of Ag146 with the metal core’s equatorial diameter being 2.9 nm (Figure 1, top view, the distance between the two farthest S atoms) and 1.4 nm (Figure 1, side view, the distance of Br-Br). The overall morphology of the nanocluster looks like a “flying saucer”. According to the bonding of the silver atoms, the Ag146 nanocluster can be divided into an Ag51 core (Figure 1, marked with green) and a Ag95S80Br2 shell containing surface motifs (Figure 1, marked with gray).

Figure 1. Top (left) and side (right) views of the total structure of Ag146Br2(TIBT)80 nanocluster. Color labels: dark gray/green = Ag, yellow = S, purple = Br; the carbon tails are shown in wire-frame mode. For a more detailed anatomy of the structure, we start with the 51-atom core. As shown in Figure 2, the Ag51 core can be divided into three shells. The first shell contains 7 silver atoms, forming a decahedron with D5h symmetry (Figure 2a, top/side views), which has been observed in the Ag136 nanocluster (Figure S1),34 and the average distance of the Ag-Ag bonds in Ag146 is 2.78 Å. Furthermore, the five silver atoms constitute a pentagon (the sum of inner angles of pentagon: 539.98o) in the waist of the decahedron. The decahedron can also be called as pentagonal bipyramid. The Ag7 kernel is enclosed by a second shell possessing 32 silver atoms (Figure 2b), forming a quasi-decahedral Ag39 core because the top pentagon is rotated with respect to the bottom pentagon, which is in contrast with the Ag39 Ino decahedron revealed in the Ag136 nanocluster (Figure S1b).34 On the other hand, the quasi-decahedral Ag39 in Ag146 nanocluster may be viewed as the transition-state from decahedron to icosahedron, which will help to understand the mutual transformation between the typical arrangement of metal atoms in the nanoparticles. The third part only contains 12 silver atoms at the equator of Ag39, forming a Ag51 decahedron (an incomplete one, which is different from that of Ag136 nanoclusters, in which the Ag39 was encircled by a 15-atom pentagon,34 Figure S1). In the Ag146 structure, due to the relative rotation between the two pentagonal pyramids in quasi-decahedral Ag39, the Ag-Ag peripheral distance in the Ag12 ring is much longer than those in the other two shells. Compared with the Ag136 nanocluster, the distorted decahedral Ag51 core would have significant effect on the electronic structure.

Figure 2. Top and side views of the three shells in the Ag51 kernel of the Ag146Br2(TIBT)80 nanocluster: (a) the decahedral 7-atom shell (magenta); (b) the 32-atom shell (gray); (c) the ring-shaped 12-atom (red). The surface-protecting motifs can be divided into four hierarchical parts from the apex to the 3

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equator: Ag30S10, Ag30S20, Ag25S30, Ag10S20 (Figure 3 a-d). Furthermore, there are several kinds of Ag-S motifs on the surface of the Ag146 nanocluster (Figure 3 e-i). For clearly show their coordination modes and roles in the construction, we start from the top Ag15 cap (similarly another Ag15 cap at the bottom), Figure 3a (labeled with light blue). The S atom is exclusively in four-coordination (Figure 3e) with Ag-S distance of 2.69 ± 0.15 Å and the underlying Ag-Ag bonds of 3.05 ± 0.1 Å. This Ag15 part can be viewed as five tetrahedral Ag4S units which are connected via sharing a vertical silver atom between adjacent two Ag4S units. In addition, a Br atom is also observed in the top Ag15 cap (similarly, a Br atom in another Ag15 cap at the bottom) with the coordination of Br-Ag4 (Figure 3a). The second part contains an Ag15 ring on the top and another Ag15 ring at the bottom (Figure 3b, labeled with green). Interestingly, the sulfur atoms resemble the bridges connecting the first and second parts together, with two different coordination modes: i) four-coordinate but the underlying Ag4 is distorted from the square arrangement (Figure 3f) with the longest Ag-Ag distances being 3.24 and 4.42 Å (average of four Ag-Ag bonds: 3.53 Å), and ii) three-coordinate (Figure 3g) with average Ag-Ag of 3.98 Å. Of note, the distorted S-Ag4 may be viewed as a transition from the regular S-Ag4 square motif to the S-Ag3 triangular motif. Furthermore, the fifteen silver atoms play a role in consolidating the 12 silver atoms of the interior core with Ag(core, red)-Ag(surface, green) shown in Figure 3b. The third part of the surface is an Ag25 ring at the equator (Figure 3c, labeled with magenta), and the S atoms are similarly in four- and three-coordination (Figure 3 f-g). Finally, the fourth part consists of ten -S-Ag-S- monomeric staples but with two different configurations (Figure 3 h-i), namely the “W” (Ag-S-Ag angle of 102.93o) and “U” (Ag-S-Ag angle of 133.98o) configurations.

Figure 3. Top and side view of the four shells in the Ag95S80Br2 surface of the Ag146 nanocluster: (a) the 1st Ag30S10Br2 part (light blue); (b) the 2nd Ag30S20 part (green); (c) the 3rd Ag25S30 part (red); (d) the 4th Ag10S20 part; (e)-(i) the different types of S-Ag motifs observed on the surface of the Ag146 nanocluster. It is worth noting that the coordination of the sulfur atom changes from S-Ag2 to S-Ag4 from the equator to the apex. From another point of view, if we consider that the RS- ligand possesses a -1 charge, the silver atoms formally possess +0.5e, +0.33e, and +0.25e from S-Ag2 to S-Ag4. Therefore, as the Ag atoms move from the equator to the core, they show smaller and smaller charges (i.e., Ag0.5+ to Ag0.25+) and the kernel Ag atoms are essentially Ag0. Furthermore, the Ag-S distance in the four parts also follows a regular trend: 2.443 Å 4th < 2.526 Å 3rd < 2.641 Å 2nd < 2.687 Å 1st (where 4th to 1st indicates the sequence from the equator to the apex of the nanocluster, Figure S2). These results may facilitate the understanding of the growth mechanism of thiolate-capped silver nanoclusters. Of note, we found that the crystal contains two co-crystallized nanoclusters of similar structure, 4

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which are Ag146Br2(TIBT)80 (discussed above) and Ag150Br2(TIBT)84 (Ag150 for short). The latter has all the same features of Ag146Br2(TIBT)80 but with two additional -RS-Ag-SR-Ag- at the equator (Figure S3-5). According to our model, Ag146 has an occupancy of 60% and Ag150 of 40%. From the structural integrity point of view, the presence of dangling -RS-Ag-SR-Ag- outside the compact Ag146 structure seems strange, and we speculate that the Ag150 might be the Ag146 structure coordinated by residual Ag(I)-SR during the crystallization process. This puzzle remains to be explained. On another note, the Ag146 structure is largely different from the counterpart Au146(SR)57;17 the latter has a FCC-twinned structure. For further verification of the composition of this silver nanocluster, we measured the electrospray ionization (ESI) mass spectrum (Figure S6). As shown in Figure S6, two groups of peaks were observed at ~9000 and ~6500 Da in the mass spectrum, which are assigned to [Ag146-x(TIBT)80-(x+1)Cs]3+ and the [Ag146-x(TIBT)80-(x+1)Cs2]4+ (x = 0-8), losing two Br- atoms (see SI for details). For example, the maximum molecular ion peak locating at 9276.8 Da can be assigned to [Ag146(TIBT)79Cs]3+ (two –Br and one –SR are lost). It is worth noting that no ion peak of the Ag150 nanocluster was observed (the ion peak of [Ag150(TIBT)83Cs]3+ would be at 9622.16 Da but no such peak was observed). Thus, there is only the Ag146 nanocluster in solution. Furthermore, no signal was obtained without adding CsOAc to the nanocluster solution, thus the Ag146 nanoclusters should be neutral, which is consistent with the crystallographic analysis. Metal nanoclusters have many potential applications as functional nanomaterials,36-42 thus large-scale synthesis is important. However, large-scale synthesis has not been achieved for giant metal nanoclusters with n>100. In this work, the Ag146 nanocluster possesses good stability―no observable spectral difference was observed in a dichloromethane of the nanocluster over a month period by monitoring with UV-vis absorption spectroscopy (Figure S7). This stability indicates that the formation of the silver nanoclusters can continuously be accumulated irrespective of the existing nanoclusters in the reaction system, hence making it feasible for easy scaling up of the reaction. We tested scale-up of the above synthetic reaction by 100-fold, which produced 5.08 g of the final product (Figure 4, inset photograph) with ~90% yield from a one-pot reaction (Figure S8). We expect that it would be possible to further scale up to kilogram-scale synthesis by another ~200-fold scale-up. Thus, the easy synthesis method of Ag146 is quite noteworthy, which allows for future developments of practical applications, especially for large-size metal nanoclusters.

Figure 4. UV-vis-NIR absorption spectra of the Ag146Br2(TIBT)80 nanoclusters (blue line: low concentration, and red line: high concentration) in CCl4. Inset: a photograph of 5.08 g nanoclusters in a vial, and the spectrum plotted on photon energy scale (the y-axis is transformed from the wavelength scale spectrum by Abs × λ2 to preserve the oscillator strength). The UV-vis-NIR absorption spectrum of this silver nanocluster was also measured. As shown in 5

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Figure 4, the optical spectrum not only exhibits four distinct peaks at 446, 490, 580 and 785 nm in the UV-vis region (Figure 4, blue line), but also shows a band at ~1200 nm (1.0 eV) in the NIR region, which can be clearly seen with a higher concentration of solution (Figure 4, red line). The NIR absorption of large Ag nanoclusters may be potentially useful for solar thermal conversion and biomedicine.43 Based on the optical absorption, the optical HOMO-LUMO gap is determined to be 0.5 eV (Figure 4, inset spectrum on the photon energy scale). It is worth noting that this Ag146 nanocluster possesses 64 free valence electrons (64 = 146 – 80 – 2), which does not fall in the conventional magic numbers such as 8e, 18e, 54e and 92e; thus, this Ag146 nanocluster cannot be viewed as a superatom and we attribute this to the particle shape effect.21,44 To gain deeper insight into the electronic structure of the Ag146 nanocluster, we further performed measurements of temperature dependent absorption spectra between 300 and 80 K. At low temperatures, the absorption peaks become sharpened and their intensities increase distinctly (Figure 5A). Unlike the Au25(SR)18 nanocluster reported by Ramakrishna and coworkers45⎯which showed significant shifts in positions of both low and high energy absorption peaks, the low-energy absorption peaks around 1.58 eV and 2.10 eV of the Ag146 nanocluster did not show any shift with decreasing temperatures, while the high energy peaks at 2.44 eV and 2.73 eV indeed shifted to 2.51 eV and 2.79 eV, respectively. The peak shift in absorption in gold nanoclusters is ascribed to the electron-phonon interactions.45 Here we performed a similar analysis on the Ag146 nanocluster, with the absorption maxima at 2.44 eV vs. temperature being fitted with an equation:46 ⎡ ⎛ hv E (T ) = E (0) − C hυ ⎢coth ⎜ ⎜ 2kT ⎢⎣ ⎝

⎞ ⎤ ⎟ − 1⎥ ⎠ ⎥⎦

where, E(0) is the absorption position at 0 K, C is the e-p coupling constant and hv is the average phonon mode which contributes to the electron-phonon interaction. From the fitting, the average phonon frequency is determined to be 7.2 meV (56 cm-1).

Figure 5. Temperature-dependent absorption spectra of Ag146Br2(TIBT)80 nanoclusters. (A) Absorption intensity as a function of temperature from 300 K to 80 K. (B) The high-energy absorption maxima as a function of temperature and data fitting with the O’Donnell-Chen equation. The spike around 1.40 eV in (A) is due to the artifact of the instrument. As the low energy absorption peaks (1.58 eV and 2.1 eV) do not shift with temperature, it is very likely that they arise from the metal core orbitals. The metal core has relatively lower phonon frequency, thus such phonon effects are not observable in the absorption spectra for the 80-300 K range. In contrast, the high energy absorption peaks show significant shifts. In the Ag146 nanocluster, the inner core has 51 silver atoms while the surface structure is made up of 95 silver atoms. Unlike the staple motifs observed in the cases of Au25(SR)18 and Au38(SR)24 nanoclusters, the surface silver atoms may give rise to lower phonon frequency modes.45 Therefore, the 56 cm-1 phonon mode could arise from both the silver metal core and the large staple motif. Detailed assignment of the absorption peaks and the phonon mode requires further theoretical calculations in future work. 6

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Furthermore, the electron dynamics of metal nanoclusters is of great importance for unravelling the origin of surface plasmon resonance and metallic bonding.26,47,48 Herein, femtosecond transient absorption (TA) spectroscopy was also performed to probe the photophysics of this silver nanocluster. Upon photoexcitation at 430 nm, the TA spectra is dominated by broad excited state absorption (ESA) overlapped with ground state bleaching (GSB) around 445 nm, 490 nm, 675 nm and 785 nm (Figure 6A), and the TA signal decays to zero within 2 ns. Global fitting analysis was performed to extract the excited state species and three decay components are required to obtain a good fitting quality (Figure 6B and C). With excitation at 1200 nm and 1500 nm, the 800 fs component is absent and two decay components (3 ps, ~300 ps) are required to fit the decay. Therefore, the 850 fs component with 430 nm excitation should be assigned to the internal conversion from Sn to S1 state. On the other hand, as the bandgap of the nanocluster is relatively small (about 0.5 eV), the ~300 ps component should be the relaxation from the S1 state to the ground state. The 3.5 ps component can be assigned to the relaxation within the metal core (Figure S9, see SI for details).

Figure 6. The ultrafast transient absorption spectroscopy characterization of the Ag146 nanocluster. (A) Transient absorption data map pumped at 430 nm; (B) Decay associated spectra (DAS) obtained from global fitting; (C) Kinetic traces at selected wavelengths and the corresponding fits; (D) Kinetic traces probed at the GSB around 780 nm at different pump fluences. Moreover, it is found that the electron dynamics is independent on the pump fluence (Figure 6D), suggesting that this silver nanocluster shows non-metallic behavior.26 Their excitonic behavior is also evidenced by the relatively long-lived excited state lifetime and the existence of internal conversion between different electronic states. In contrast with the comparable size Ag141 nanocluster reported recently⎯which show surface plasmon resonance (SPR) at ~460 nm,49 the Ag146 nanocluster here exhibits discrete absorption bands, a significant optical bandgap (0.5 eV), and molecular-like electron dynamics. The difference in their electronic structures can be explained by their differences in shape and structure. The Ag141 nanocluster shows a spherical-shape, while Ag146 exhibits a “flying saucer”-like shape. The emergence of SPR would be affected by the particle shape, and the anisotropic shape of Ag146 may delay the onset of metallic behavior. Moreover, Ag141 has a Ag71 metal core while our Ag146 nanocluster only has 51 silver atoms in the metal core. A smaller metal core in Ag146 explains its molecular-like behavior since the absorption spectra of large metal nanoclusters are mainly contributed by the metal core as in the case of gold nanoclusters.50 7

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To further study the evolution from the molecular to the metallic state, a series of sizes of silver nanoparticles have been synthesized (detailed methods in Supporting Information), Figure 7. Among the seven sizes, the Ag25, Ag44, and Ag67 were reported previously,27-29 whereas the other four are obtained for the first time. As shown in Figure 7a, matrix-assisted laser desorption/ionization mass spectra (bottom to top) show that the mass peaks of the silver nanoparticles are at 0.6K (Ag25), 9.1K (Ag44), 11.8K (Ag67), 22K (Ag~130), 26K (Ag146), 34K (Ag~200), and 39K (Ag~230), respectively (where K = 1,000 Da). Figure 7b shows the optical absorption spectra of these silver nanoparticles. The spectra of the Ag25, Ag44, Ag67, Ag~130, and Ag146 exhibit multiple bands, and no SPR peak is observed, which are characteristic of the molecular-state nanoparticles. As the size increases, the optical properties of the silver nanoparticles exhibit distinct changes, in which the Ag~200 and Ag~230 nanoparticles exhibit a prominent SPR peak at 460 and 445 nm, respectively. Our results indicate that the metallic state emerges earlier in silver nanoparticles compared with the gold nanoparticles.14,26

Figure 7. Different-sized silver nanoparticles spanning the molecular and metallic states. (a) Matrix-assisted laser desorption/ionization mass spectra. (b) Steady-state ultraviolet–visible spectra. Conclusion In summary, we have successfully synthesized a large silver nanocluster formulated as Ag146Br2(TIBT)80 by direct reaction of the Ag(I)-SR complex with NaBH4, and gram-scale synthesis is achieved for this nanocluster. X-ray crystallography reveals that the framework of Ag146 contains a decahedral Ag51 core (incomplete), which is protected by an Ag95Br2S80 surface. The hierarchical structure of the surface provides some insights into the nucleation of nanoclusters from the metal-sulfur complex. Interestingly, this silver nanocluster exhibits an observable optical bandgap and power independent electron dynamics, which indicate the molecular-like nature of this nanocluster. The successful structure determination and molecular-like optical properties of Ag146 will be a basis for future work on large-sized (n > 100 atoms) nanoclusters.

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Methods Synthesis of the Ag146Br2(TIBT)80 Nanocluster. 1 mL aqueous solution of AgNO3 (50 mg, 0.294 mmol) and 10 mL dichloromethane were added into a 25 mL round-bottom flask. Then, the solution was vigorously stirred (~1100 rpm) with a magnetic stir bar. After ~5 min, 700 μL (4.5 mmol) 4-isopropylbenzenethiol was added into the solution, and the color of the solution immediately changed from colorless to yellow (a slurry solution), indicating that the RS-Ag(I) complex was formed. After that, Ph4PBr (150 mg, 0.358 mmol) was added to the solution. After ~5min, 5 mL aqueous solution of NaBH4 (37.8 mg, 1.00 mmol) was rapidly added to the solution. Then, the reaction was allowed to proceed for ~72 h. After that, the aqueous phase was separated from the organic phase, and the dichloromethane solution was rota-evaporated. Then the black precipitate was washed several times with ethanol/toluene to remove the redundant thiolates, Ph4PBr and by-products. Finally, ~52 mg pure Ag146Br2(TIBT)80 nanoclusters were obtained with ~95% yield (Ag atom basis). Gram-Scale Synthesis of Ag146Br2(TIBT)80. The large-scale synthesis of the Ag146 nanocluster was gradually amplified by 10-fold, 20-fold and 100-fold. Herein, the 100-fold synthesis method is discussed. Briefly, 10 mL aqueous solution of AgNO3 (5 g, 29.4 mmol) and 1000 mL dichloromethane were added into a 2.5 L round-bottom flask. Then, the solution was vigorously stirred (~300 rpm) with a mechanical stirrer. After ~5 min, 15 mL (96.4 mmol) of 4-isopropylbenzenethiol was added to the solution and the color of the solution immediately changed from colorless to yellow (slurry), indicating that the RS-Ag complex was formed. After that, Ph4PBr (10 g, 23.85 mmol) was added into the solution. After ~10min, 50 mL aqueous solution of NaBH4 (3.78 g, 100 mmol) was added into the solution. Then, the reaction was allowed to proceed for ~72 h. After that, the aqueous phase was separated from the organic phase and the dichloromethane solution was rota-evaporated. Then, the black precipitate was washed several times with ethanol/toluene to remove the redundant thiolates, Ph4PBr and by-products. Finally, ~5.08 g silver nanoclusters were obtained with ~90% yield. Synthesis of Other Silver Nanoclusters. The syntheses of Ag25, Ag44 and Ag67 nanoclusters followed the protocols reported previously.28,29,31 The Ag~130, Ag~200 and Ag~230 nanoclusters were obtained by direct reduction of RS-Ag complexes (details of syntheses are provided in the Supporting Information). Single-Crystal Analysis of the Ag146Br2(TIBT)80 Nanocluster. Single crystal X-ray diffraction was performed on a Bruker Apex-II CCD area detector using IµS CuKα radiation (λ = 1.54178 Å). A dark crystal was mounted onto a MiTeGen capillary with Krytox ® GPL 107 oil. Data collection was performed under a dry nitrogen stream at 100 (2) K. Data were collected with a total exposure time of 137.40 hours. The frames were integrated with the Bruker SAINT software package. The integration of the data using a triclinic unit cell yielded a total of 367,029 reflections to a maximum θ angle of 37.50° (1.27 Å resolution), of which 47,827 were independent (average redundancy 7.674, completeness = 99.3%, Rint = 9.86%) and 36,660 (76.65%) were greater than 2σ(F2). Data were corrected for absorption effects using the Multi-Scan method (SADABS). Ultrafast Transient Absorption Spectroscopy. The femtosecond transient absorption measurements with ~90 fs time-resolution were measured using a homemade femtosecond broadband pump–probe setup. Briefly, a regeneratively amplified Ti:sapphire laser (Coherent Legend Elite) produced 40 fs, 1 mJ pulses at a 500 Hz repetition rate with a spectrum centered at 800 nm and a bandwidth of 40 nm (FWHM). The output from the amplifier was split by a 90/10 beamsplitter into pump and probe beams. A portion of the 800 nm fundamental light was used to pump a collinear optical parametric amplifier of white light continuum (TOPAS-C, Light Conversion, Lithuania) to provide the excitation pulse. A small portion of the laser fundamental was focused into a sapphire plate to produce supercontinuum in the visible region, which overlapped in time and space with the pump. 9

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ASSOCIATED CONTENT The authors declare no competing financial interests. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.Z.); [email protected] (R.J.) Acknowledgments M.Z. acknowledges financial support by NSFC (21372006, U1532141, 21631001), the Ministry of Education, the Education Department of Anhui Province, 211 Project of Anhui University. R.J. acknowledges the support from the National Science Foundation (DMR-1808675) and Air Force Office of Scientific Research under AFOSR Award No. FA9550-15-1-9999 (FA9550-15-1-0154) and DURIP (FA9550-16-1-0218). Supporting Information Available: Details of the synthesis, stability test, crystallization and SC-XRD analysis, supporting Figures S1-S9, Table S1-3 and crystallographic structure of the Ag146Br2(SR)80 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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