Riki
Nakatani
a,
Jin
Sakai
a,
Aishik
Saha
b,
Ayumu
Kondo
a,
Rina
Tomioka
a,
Tokuhisa
Kawawaki
a,
Saikat
Das
*a and
Yuichi
Negishi
*ac
aDepartment of Applied Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. E-mail: saikatdas@rs.tus.ac.jp
bDepartment of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400 076, India
cInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. E-mail: yuichi.negishi.a8@tohoku.ac.jp
First published on 19th November 2024
Silver cluster-assembled materials (SCAMs) are well-defined crystalline extended materials hallmarked by their unique geometric structures, atomically precise designability and functional modularity. In this study, we report for the first time the synthesis of a (3,6)-connected three-dimensional (3D) SCAM, [Ag12(StBu)6(CF3COO)6(TPMA)6]n (designated as TUS 6), TPMA = tris(pyridine-4-ylmethyl)amine, by assembling Ag12 cluster nodes with the help of a tritopic linker TPMA. Besides, we also prepared a two-dimensional (2D) SCAM, [Ag12(StBu)6(CF3COO)6(TPEB)6]n (described as TUS 7), TPEB = 1,3,5-tris(pyridine-4-ylethynyl)benzene, by reticulating Ag12 nodes with a tritopic linker TPEB. Characterized by microscopic and diffraction analyses, the SCAMs revealed distinct morphologies, structural robustness, and phase purity. This paper elucidates how the binding with the organic linkers alters the symmetry of the silver nanoclusters (NCs). Changes in the symmetry of discrete NCs to assembled structures have not been reported yet. This study provides an atomic-level explanation of the transformation of symmetry from NCs to extended structures.
In 2017, Zang et al. reported a silver cluster-assembled material (SCAM) that was constructed by coordinating Ag12 nodes with bidentate pyridyl linkers.31 This SCAM, [Ag12(StBu)8(CF3COO)4(bpy)4]n, bpy = 4,4-bipyridine, demonstrated brilliant stability after prolonged exposure to air and visible light irradiation. The following year, Wang and co-workers employed four different linkers (pyrazine, dipyridine-4-yl-diazene, 4,4′-bipyridine, and 1,4-bis(4-pyridyl)benzene) to connect Ag14 cores, resulting in four SCAMs (SCAM-1,2,3,4).32 Notably, SCAM-4 demonstrated commendable thermal stability and also illustrated a broad range (green to red) of thermochromism. Following these pivotal studies, a significant number of SCAMs constituting Ag NC nodes of variable sizes and diverse organic functionalities have been reported.33–37 SCAMs, composed of different numbers of Ag atoms, viz. Ag10, Ag12, Ag14, Ag20, and Ag27, flaunted distinctive structures, which translated to unique properties and performances.32,37–40 The geometry and connectivity of nodes and linkers dictate the architecture and structural intricacy of SCAMs. Prior reports indicate that SCAMs prepared with pyridine linkers predominantly utilize bidentate and tetradentate linkers, whereas tridentate linkers for SCAM formation have been relatively unexplored.41 The use of a wide pool of linkers, for instance, 3-amino-4,4′-bipyridine,42,43 4,4-azopyridine,35,44,45 tris(4-pyridylphenyl)amine,46 1,4-bis(pyridin-4-ylethynyl)benzene,47 and 9,10-bis(2-(pyridin-4-yl)vinyl)anthracene38 to stitch Ag(I) cluster nodes laid the foundation for orchestrating the properties of SCAMs constructed by means of diverse synthesis strategies such as one-pot synthesis, liquid–liquid interfacial crystallization and ligand-exchange.31,39 Despite significant strides in the designed synthesis of SCAMs, the assembly mechanism from the perspective of the symmetry of the linkers transforming the original symmetry of the nanocluster's core–shell architecture into a new anisotropic symmetry of the extended networks has not been unravelled yet.
In this work, we accomplished the precise coordination of Ag12 nodes and organic linkers into well-ordered, crystalline 3D and 2D SCAM structures. This is the first report of a (3,6)-connected 3D SCAM, [Ag12(StBu)6(CF3COO)6(TPMA)6]n (TUS 6), TPMA = tris(pyridine-4-ylmethyl)amine, utilizing a tridentate linker TPMA. In addition to TUS 6, another 2D SCAM, [Ag12(StBu)6(CF3COO)6(TPEB)6]n (TUS 7), TPEB = 1,3,5-tris(pyridine-4-ylethynyl)benzene, was developed by bridging Ag12 nodes with a different tridentate linker TPEB. By optimizing the crystallization conditions, we achieved controllable dimensionality of SCAMs endowed with high-quality, well-defined crystalline structures. This study imparts with comprehensive insights into the transformation of the isotropic two-fold symmetry of the NC's core–shell structure to an anisotropic six-fold symmetry of assembled structures.
In order to investigate whether the flexibility of the linkers contributes to the SCAM structure, we reticulated a rigid linker, 1,3,5-tris(pyridine-4-ylethynyl)benzene (TPEB), with Ag NC vertices to afford single crystals of TUS 7. Regarding the preparation of TUS 7, [AgStBu]n and CF3COOAg were also utilized as metal precursors and dissolved in acetonitrile/ethanol (MeCN/EtOH) (v/v = 1:1). Meanwhile, the TPEB linker was dissolved in dimethylformamide (DMF). Following this, the linker solution was added to the metal precursor solution. After leaving undisturbed for 1 day, colorless hexagonal-shaped crystals were collected from the bottom of the solution mixture in 69.6% yield based on TPEB (Fig. 1 and S1b†).
Close examination of the crystallographic structure of TUS7 revealed that it crystallized in a trigonal system with a space group of R (no. 148) (Table S2†). Although the Ag12 cluster core of TUS 7 resembles that of TUS 6 in shape, the Ag–Ag, Ag–CF3COO− and Ag–N connectivity patterns differ between the two. The two triangular planes above and beneath the intermediate hexagonal layer are equilateral in nature, with the Ag–Ag bond distance measuring 3.020 Å. The connection between the Ag atom(s) present in the triangular plane(s) and the Ag atom(s) present in the middle hexagonal plane alternately display bond lengths of 3.104 Å and 3.228 Å (Fig. 3a, S9 and Table S11†). Since all of the Ag–Ag bond lengths are less than twice the van der Waals radius of Ag, we can categorize the Ag–Ag interactions as argentophilic. The four Ag atoms located on the vertices of the trapezoidal plane formed by two Ag atoms from the top/bottom plane and two atoms from the middle plane are connected to each S atom of the six –StBu stabilizing ligands by μ4-η1, η1, η1, η1 ligation (Fig. S11†), featuring Ag–S bond lengths that span from 2.471 Å to 2.544 Å (Fig. 3b, S10 and Table S13†). Six CF3COO− ligands protect the cluster core, and all of these ligands connect to two Ag atoms, one from the triangular layers and another from the hexagonal layer, by utilizing two O atoms (μ2-η1, η1) (Fig. S13†). The Ag–O bond distance for Ag atoms in the upper or lower layers is 2.401 Å, whereas the corresponding distance for Ag atoms in the intermediate layer is 2.684 Å (Fig. 3c, S12 and Table S15†). Furthermore, the cluster–linker reticulation led to an equal Ag–N bond distance of 2.275 Å (Fig. 3d, S14 and Table S17†). The relatively rigid TPEB linkers reticulate with Ag12 NC nodes (Fig. 3e), generating a 2D 3,6-connected layered structure of TUS 7 (Fig. 5). TUS 7 exhibits an ABC stacking arrangement where the interlayer distance measures 7.063 Å (Fig. S15†).
Fig. 5 2D layered structure of TUS 7 where each layer is formed through the coordinate bonding between Ag12 NC vertices and TPEB linkers. H atoms are removed for clarity. |
The structure of many metal NCs generally consists of a highly isotropic core and a shell (or staple) that stabilizes it, forming a stable core–shell structure. As can be seen from Fig. 6c, the structure of Ag12 NCs31 comprises an Ag8 cuboid core (Fig. 6a) and an Ag4S4 hexagonal shell (Fig. 6b). The small variation in the corner angles of the cuboid core structure indicates that the Ag8 cuboid core closely resembles a cuboid and exhibits a highly isotropic structure (Fig. 6d). This core structure possesses a two-fold symmetry axis. However, as the assembly method progresses, this isotropy and two-fold symmetry decrease, while anisotropy increases, originating from the stress induced by the linker on the nanocluster site and the symmetry (hexagonal symmetry) and distortion-derived anisotropy.
The structure of the SCAM nanocluster discards the Ag8 cuboid core and transforms to a highly anisotropic Ag3–Ag6–Ag3 trilayered structure (Fig. 6e and f). Accompanying this collapse of isotropy, the stress from the linker's symmetry affects the NC site in the SCAM, altering the original NC symmetry. In TUS7, since the linker is of D3h symmetry, the symmetric stress from it acts on the NC site, causing the loss of the two-fold symmetry axis and the emergence of a six-fold symmetry axis in the Ag6 middle layer (indicated by the blue trace in Fig. S16†). In TUS6, as the linker is of C3v symmetry and lacks a symmetry plane perpendicular to the three-fold rotational axis, it retains slight two-fold symmetry but adopts a structure close to six-fold symmetry (red trace in Fig. S16†). This is evident from the variation in the corner angles of the Ag6 middle layer, where the hexagonal angles are extremely small in both TUS6 and TUS7, but very large in the NC (green trace in Fig. 6d). Additionally, the variation in the corner angles of the Ag8 cuboid core mentioned earlier is very small in the NC but large in the SCAMs (orange trace in Fig. 6d). Therefore, it can be concluded that the assembly process entails the collapse of the isotropic two-fold symmetry of the nanocluster's core–shell structure and core, and the formation of new anisotropic six-fold symmetry derived from the linker-induced stress (Fig. 6g).
Structural characterization of the SCAMs was conducted using different analytical techniques. The specific surface areas of the SCAMs were assessed by degassing the adsorbents at a temperature of 50 °C for 8 hours, followed by recording the N2 adsorption/desorption isotherms under 77 K over the relative pressure (P/P0) range of 0–1. Type-I isotherms were observed, characterized by their concavities to the P/P0 axis and a steep rise in N2 uptake at very low relative pressures (Fig. 7(a) and 8(a)). Application of the Brunauer–Emmett–Teller (BET) equation furnished specific surface areas of 330.2 and 88.4 m2 g−1 for TUS 6 and TUS 7, respectively (Fig. S17 and S18†). To evaluate the phase purity of TUS 6 and TUS 7 crystals, we performed PXRD and compared with simulation results generated from SCXRD analyses. Reasonable matching was observed that implied the preservation of structural crystallinity even after the removal of the mother solvents (Fig. 7b and 8b). Optical microscopy and scanning electron microscopy (SEM) inspection divulged distinct morphologies for the SCAMs. Octahedral and hexagonal-shaped crystals were observed for TUS 6 and TUS 7, respectively (Fig. 7d and 8d). After this, we explored the elemental composition and electronic structures of the SCAMs via X-ray photoelectron spectroscopy (XPS). As evident from the survey spectra (Fig. 7c and 8c), the presence of Ag, S, N, F, and O could be traced, which was found to be in agreement with the energy dispersive X-ray (EDX) findings (Fig. 7d and 8d). To scrutinise in more detail, high-resolution XPS measurements were undertaken for each element to observe their respective electronic states (Fig. S19 and S20†). The Ag peaks detected at 373.7 and 374.4 eV for TUS 6 and TUS 7, respectively, can be attributed to Ag 3d3/2. On the other hand, the peaks at 367.6 and 368.4 eV for TUS 6 and TUS 7, respectively, correspond to Ag 3d5/2. The splitting of 6.0 eV between the Ag 3d3/2 and Ag 3d5/2 peaks validated that the oxidation state of Ag is +1 for both the SCAMs. The S binding energy spectra showcased two peaks: 162.5 and 161.3 eV for TUS 6 and 163.6 and 162.3 eV for TUS 7, which can be traced to S 2p1/2 and S 2p3/2, respectively. These binding energies corroborate the existence of Ag–S–C bonds. Additionally, the collected O 1s binding energy spectra revealed peaks at 530.8 eV for TUS 6 and 531.4 eV for TUS 7, which substantiate the formation of Ag–O bonds. Regarding the N 1s spectra, the peaks were located at 398.8 eV (TUS 6) and 399.4 eV (TUS 7), reinforcing the presence of coordination bonds between silver clusters and linkers. Next, we screened the stability of the SCAMs against different solvents, which attested to their high chemical stabilities (Fig. 7e and 8e). To assess the thermal stabilities at elevated temperatures, the SCAMs were subjected to thermogravimetric analysis (TGA). No appreciable thermal decomposition of the SCAMs could be detected before 120 °C, reflective of substantial improvements in structural robustness against temperature relative to discrete Ag NCs38 (Fig. S21 and S22†). PXRD analyses of the SCAMs after heating at different temperatures also corroborated the maintenance of the crystalline structures up to 120 °C (Fig. S23 and S24†).
Diffuse reflectance spectroscopy (DRS) measurements revealed that TPMA exhibited a pronounced absorption peak at around 255 nm and a small peak at 325 nm. In contrast, TUS 6 displayed peaks at approximately 200, 280, 325, and 385 nm (Fig. 7f). The shift in the absorbance of TUS 6 from that of the TPMA linker suggests that the observed peak arises exclusively from the SCAM structure and not only from the linker. Upon excitation at 255 nm, a pronounced PL emission peak was detected at 471 nm, which could be assigned to the TPMA linker (Fig. 7g). Comparative analysis of the DRS spectra for TPEB and TUS 7 demonstrated that TPEB features absorption maxima around 285 nm and 300 nm. On the other hand, TUS 7 exhibited a strong peak around 200 nm and a weaker peak around 300 nm (Fig. 8f). In the case of the TPEB solid, the PL peak at 370 nm originates from the linker, and the shoulder peak around 450 nm may correspond to the excimer emission of TPEB (Fig. 8g). When excited at 298 nm, TUS 7 exhibited emission maxima at 426 and 466 nm. In TUS 7, the PL peaks around 400–500 nm can be assigned to enhanced excimer emission from the stacked linker. As a result, the PL in TUS 6 and TUS 7 arises from their linkers. Based on these observations, it can be inferred that the generation of the SCAM leads to significant changes in the peak shapes, indicative of alterations in electronic states brought about by the formation of the extended structures.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2373129 and 2373236. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nr03992k |
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