Capture of Cesium Ions with Nanoclusters: Effects on Inter- and

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Cite This: Chem. Mater. 2019, 31, 4945−4952

Capture of Cesium Ions with Nanoclusters: Effects on Inter- and Intramolecular Assembly Xiao Wei,†,§ Xi Kang,†,§ Qianqin Yuan,† Chenwanli Qin,† Shan Jin,‡ Shuxin Wang,*,† and Manzhou Zhu*,†,‡ †

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Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials and ‡Institutes of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601, China S Supporting Information *

ABSTRACT: The capture of cations with nanoclusters is a flourishing area in the nanocluster science due to their effects on both molecular chemistry and supramolecular chemistry. The capture of Cs+ is most concerned in this field for its capability of controlling the synthesis and assembly of nanoclusters. However, the atomically precise interaction between Cs+ ions and nanoclusters remains mysterious. In this paper, we report the first X-ray crystal structure of a Cs+-captured nanocluster, formulated as Cs3Ag29(SSR)12(DMF)x (x = 5, 6; SSR, 1,3-benzene dithiol). The capture of Cs+ with Ag29(SSR)12(PPh3)4 peels the PPh3 ligands off from the nanocluster surface, giving rise to Cs3Ag29(SSR)12(DMF)x. The Cs+−cluster interactions not only alter the geometric structure of the Ag29(SSR)12 kernel but also assemble Ag29(SSR)12 clusters into one-dimensional, cluster-based lines. Remarkable differences have been observed by comparing the optical properties of the Cs3Ag29(SSR)12(DMF)x nanocluster in solutions or in crystallized films. Overall, this work is of great significance for revealing both the Cs+-induced intracluster transformation of nanocluster structures and the Cs+-induced intercluster self-assembly. octahedral shape.49 Chakraborty et al. investigated the Cs+ capture with Ag29(SSR)12 (SSR = 1,3-benzene dithiol) by mass spectroscopy.53 Monomers and dimers of Cs+-captured nanoclusters have been optimized by density functional theory calculations.53 However, in spite of the fact that the Cs+ capture has been exploited for several years in affecting the molecular/ supramolecular chemistry of nanoclusters, the atomically precise interaction between Cs+ ions and nanoclusters is still unclear. Besides, the transformation on geometric structures of nanoclusters and the mechanism of the nanocluster assembly that induced by the Cs+-capture remain mysterious until now, which impedes a thorough understanding of the cation capture effects on the molecular/supramolecular chemistry of nanoclusters. In this work, we report the capture of Cs+ ions with nanoclusters and its effects on molecular and supramolecular chemistry of nanoclusters. The Cs+ addition peels the PPh3 ligands off from the [Ag29(SSR)12(PPh3)4]3− (Ag29-PPh3, SSR = 1,3-benzene dithiol) surface, giving rise to an electrically neutral Cs−cluster compound, Cs3Ag29(SSR)12(DMF)x (Ag29Cs, x = 5, 6). The atomically precise interactions between Cs+ and Ag29(SSR)12 are revealed by X-ray crystallography. To the best of our knowledge, this is the first report on the crystal structure of Cs+-captured nanoclusters. The capture of Cs+ ions with the nanocluster is driven by the electrostatic

1. INTRODUCTION Owing to their atomically precise structures, monodispersed sizes, and intriguing properties, metal nanoclusters provide an exciting opportunity for investigating the structure−property correlations at the atomic level.1−31 In addition, the research of nanoclusters has been noted to bridge molecular and supramolecular chemistry.32−49 On one hand, the molecular nature of nanoclusters has allowed their properties to be rationalization by reference to their structures.32−40 On the other hand, intercluster interactions that belong to the realm of supramolecular chemistry have been fully researched on the basis of several nanocluster crystal lattices, cluster-based metal−organic frameworks, and cluster-assembled supracrystals.41−49 The capture of cations with nanoclusters has allowed a deeper understanding of the their molecular/supramolecular chemistry.45,49−53 Cations can not only control the syntheses of nanoclusters but also assemble these nanoclusters into cluster-based supracrystals.45,49−53 Among these researches, the cesium ion (Cs+) capture is most concerned for its capacity of (i) acting as counterions of electronegative clusters, (ii) ionizing electroneutral clusters, and (iii) inducing the assembly of nanoclusters.49,53−56 For instance, by changing the counterions of the p-MBA (para-mercaptobenzoic acid) ligand in Ag44(p-MBA)30 from H+ to Cs+, Yao et al. reshaped the rhombohedral supracrystals of protonated Ag44(p-MBA)30 into octahedra.49 The double layer of deprotonated Ag44(p-MBA)30 nanocrystals was sensitive to the charge screening that is affected by the Cs+ concentration, thereby providing a means to regulate these nanocrystals with an octahedral or a concave© 2019 American Chemical Society

Received: May 14, 2019 Revised: June 12, 2019 Published: June 14, 2019 4945

DOI: 10.1021/acs.chemmater.9b01890 Chem. Mater. 2019, 31, 4945−4952

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Chemistry of Materials

with full-matrix least squares on F2 using the SHELXTL software package.59 All nonhydrogen atoms were refined anisotropically, and all of the hydrogen atoms were set in geometrically calculated positions and refined isotropically using a riding model. The solvent has been squeezed by platon.

attraction between the electrically negative [Ag29(SSR)12]3− and the positive Cs+ and is promoted by Cs−S bonds and Cs···π interactions. Such a capture induces obvious transformations on the structure of the Ag29(SSR)12 unit. Besides, in contract with the discrete state of nanoclusters in the Ag29PPh3 crystal lattice, one-dimensional, cluster-based lines generate in the Ag29-Cs crystal lattice. The linear assembly of Ag 29 -Cs results from the interactions among the Ag29(SSR)12 nanoclusters, the Cs+ ions, and the N,Ndimethylformamide (DMF) molecules. Such intercluster interactions remarkably influence the optical absorptions and emissions of Ag29-Cs.

3. RESULTS AND DISCUSSION Ag 29 -PPh 3 comprises an icosahedral Ag 13 kernel, a Ag12(SSR)12 shell, and four Ag-PPh3 vertexes (Figure 1A;

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were purchased from Sigma-Aldrich and used without further purification, silver nitrate (AgNO3, 99%, metal basis), triphenylphosphine (PPh3, 99%), 1,3-benzene dithiol (SSR, 99%), sodium borohydride (NaBH4, 99.9%), cesium acetate (CH3COOCs, 99%), methylene chloride (CH2Cl2, HPLC, Aldrich), methanol (CH3OH, HPLC, Aldrich), N,N-dimethylformamide (DMF, Aldrich), ethyl ether ((C2H5)2O, HPLC, Aldrich), and 2methyl-tetrahydrofuran (C4H7O-2-CH3, HPLC, Aldrich). 2.2. Synthesis and Crystallization. 2.2.1. Synthesis of Ag29(SSR)12(PPh3)4. The preparation of Ag29(SSR)12(PPh3)4 was based on the method reported by Bakr et al.57 2.2.2. Synthesis of Cs3Ag29(SSR)12(DMF)x. Ag29(SSR)12(PPh3)4 (10 mg) was dissolved in 5 mL of DMF, and 5 mg of CH3COOCs was added under vigorous stirring at 273 K (ice-bath). After 10 min, the organic layer was separated and poured into 100 mL of CH2Cl2. The precipitate was collected then washed three more times by CH2Cl2. The precipitate was then dissolved in DMF, which produced the Cs3Ag29(SSR)12(DMF)x nanocluster. The yield was 90% based on the Ag element (calculated from the Ag29(SSR)12(PPh3)4). 2.2.3. Crystallization of Cs3Ag29(SSR)12(DMF)x. Single crystal of Cs3Ag29(SSR)12(DMF)x was cultivated at room temperature by vapor diffusing the ethyl ether into a DMF solution of the cluster. After 5 days, red, cubic crystals were collected and the structure of Cs3Ag29(SSR)12(DMF)x was determined. The CCDC number of Cs3Ag29(SSR)12(DMF)x is 1890759. 2.2.4. Test of the Temperature−Photoluminescence (PL) Intensity Correlation. The nanocluster (0.1 mg) was dissolved in 10 mL of the mixture of DMF/C4H7O-2-CH3 (v/v = 1:1). Then, the solutions were cooled to different temperatures and PL spectra were measured. 2.3. Characterization. All UV−vis absorption spectra of the nanoclusters dissolved in DMF were recorded using an Agilent 8453 diode array spectrometer, whose background correction was made using a DMF blank. PL spectra were measured on an FL-4500 spectrofluorometer with the same optical density (OD) of ∼0.05. Absolute quantum yields (QYs) were measured with dilute solutions of nanoclusters (0.05 OD absorption at 445 nm) on a HORIBA FluoroMax-4P. Electrospray ionization mass spectrometry (ESI-MS) measurements were performed by a MicrOTOF-QIII high-resolution mass spectrometer. The sample was directly infused into the chamber at 5 μL/min. For preparing the ESI sample, the nanoclusters were dissolved in DMF (1 mg/mL) and diluted (v/v = 1:2) by methanol. 133 Cs and 31P NMR spectra were acquired using a Bruker 600 Avance III spectrometer equipped with a Bruker BBO multinuclear probe (BrukerBioSpin, Rheinstetten, Germany). 2.4. X-ray Crystallography. The data collection for single-crystal X-ray diffraction was carried out on a Bruker Smart APEX II CCD diffractometer under a liquid nitrogen flow at 160 K using graphitemonochromatized Mo Kα radiation (λ = 0.71073 Å). Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively.58 The electron density was squeezed by PLATON. The structure was solved by direct methods and refined

Figure 1. Transformation from Ag29-PPh3 to Ag29-Cs. (A) Structure of Ag29-PPh3. (B) Structure of Ag29-Cs. Blue, dotted lines in (A) represent the unbonding Ag−Ag, whereas the blue, solid lines in (B) represent the newly generated Ag−Ag bonds in Ag4 pyramid-like units. Color legend: light blue/blue sphere, Ag; dark purple sphere, Cs; yellow sphere, S; violet sphere, P; green sphere, O; gray sphere, C. For clarity, all H atoms, N atoms, and some C atoms are omitted.

also see Figure S1 for a structural anatomy). After the addition of Cs+ ions, the four PPh3 vertexes were peeled off from Ag29PPh3, giving rise to a Ag29(SSR)12 structure that is further stabilized by three Cs+ ions and several DMF molecules (Figure 1B, and vide infra for more structural details). Although the overall Ag29(SSR)12 configuration retained from Ag29-PPh3 to Ag29-Cs, obvious changes have been observed by comparing corresponding bond lengths (Table 1 and Figures S2−S5): (i) in the icosahedral Ag13 kernel, the bonds of both kernel Ag−icosahedral Ag (Figure S2) and icosahedral Ag− icosahedral Ag (Figure S3) in Ag29-Cs are much longer than those in Ag29-PPh3, suggesting a larger icosahedral Ag13 kernel in Ag29-Cs; (ii) the bonds between the icosahedral Ag and the motif Ag in Ag29-Cs are also longer than those in Ag29-PPh3 (edges of the prisms, as shown in Figure S4); and (iii) the biggest difference lies in the interactions between the vertex Ag and the icosahedral Ag (edges of the pyramids, as shown in Figures 1 and S5). In Ag29-Cs, the lengths of the vertex Ag− icosahedral Ag bonds (labeled with a blue, solid line in Figure 1B) range from 2.987 to 3.106 Å (average 3.043 Å). However, no analogous interaction is observed in Ag29-PPh3, distances between them (labeled with a blue, dotted line in Figure 1A) range from 3.493 to 3.643 Å (average 3.523 Å). In this context, the vertex Ag atoms became closer to the icosahedral kernel when the Ag29-PPh3 nanocluster was transformed to Ag29-Cs and the newly generated Ag4 pyramid-like units made the Ag29 metallic kernel more robust. The structural transformations reflect the molecular effects of the Cs+ capture with the Ag29 nanocluster. The capture of Cs+ ions onto the nanocluster surface is driven by the electrostatic attraction between the electrically negative [Ag29(SSR)12]3− and positive Cs+. Figure 2 shows the detailed surface structure of the Ag29-Cs. Each Ag29-Cs consists of one Ag29(SSR)12 nanocluster, three Cs+ ions, and several DMF molecules. These three Cs+ ions locate at five positions (labeled as Cs1−Cs5 in Figure 2) on the Ag29(SSR)12 surface. Cs6 is equal to Cs2 and from the last Ag29-Cs, and S2 is equal to S1 and from the next Ag29-Cs 4946

DOI: 10.1021/acs.chemmater.9b01890 Chem. Mater. 2019, 31, 4945−4952

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Chemistry of Materials Table 1. Comparison of Bond Lengths in Ag29-Cs and Ag29-PPh3 Crystal Structures kernel Ag−icosahedral Ag

icosahedral Ag−icosahedral Ag

icosahedral Ag−motif Ag (prism)

icosahedral Ag− motif Ag (pyramid)

cluster

range (Å)

avg. (Å)

range (Å)

avg. (Å)

range (Å)

avg.(Å)

range (Å)

avg. (Å)

Ag29-Cs Ag29-PPh3 diff.

2.769−2.803 2.755−2.772

2.788 2.765 +0.83%

2.800−3.080 2.834−2.975

2.937 2.907 +1.03%

3.064−3.223 3.077−3.158

3.132 3.111 +0.68%

2.987−3.106 3.493−3.643

3.043 3.523 −15.77%

nanoclusters in a unit cell (vide infra in Figure 3), different x values are observed, x = 5 for one type and x = 6 for another.

Figure 3. One-dimensional, linear assembly of Ag29-Cs nanoclusters in the crystal lattice. (A) [Cs3Ag29(SSR)12(DMF)x]2 unit. The two Ag29 nanocluster units are in differently twisting angles. (B) Two adjacent one-dimensional [Cs3Ag29(SSR)12(DMF)x]2n lines. (C−E) Packing of the Ag29-Cs in crystal lattices: view from the x-axis (C), zaxis (D), and y-axis (E). Color legend: light blue/gray sphere, Ag; dark purple sphere, Cs; yellow/red sphere, S; green sphere, O. For clarity, all H atoms, C atoms, N atoms, and some Cs+ ions and DMF molecules are omitted. Each O atom represents a DMF molecule.

Figure 2. Surface structure of Ag29-Cs. (A) Overall structure of Ag29Cs. (B) Interactions among the Ag29(SSR)12, Cs1, Cs2, and DMF molecules. (C) Interactions between Ag29(SSR)12 and Cs3−Cs5. (D) Interactions between Ag29(SSR)12 and Cs6. Color legend: light blue sphere, Ag; dark purple sphere, Cs; yellow/red sphere, S; green sphere, O; gray sphere, C. For clarity, all H atoms and N atoms are omitted and each O atom represents a DMF molecule.

In this context, the final molecular formula of the nanocluster is determined as Cs3Ag29(SSR)12(DMF)x (x = 5 or 6). The determination of such atomically precise Cs+−cluster interactions is of great significance because they unambiguously identify how Cs+ ions bind onto the nanocluster surface. Bakr et al. reported the crystallization of the Ag29-PPh3 by slow evaporation of a DMF solution of the cluster, which produced crystals packed in a cubic lattice with a space group of Pa3̅ (Table 2).57 Recently, by a crystallization method of vapor diffusion of methanol into a DMF solution of the cluster, Pradeep and co-workers obtained the crystals of Ag29-PPh3 packed in a trigonal lattice with a space group of R3̅ (Table 2).44 The crystal of Ag29-Cs was obtained by vapor diffusing the ethyl ether into a DMF solution of the cluster. The Ag29Cs crystallized in an orthorhombic lattice with a Pbcn space group, which was different for Ag29-PPh3 in either of the two crystallized patterns (Table 2). Crystal structures of these three nanoclusters (i.e., Ag29-Cs, Ag29-PPh3-cubic, and Ag29-PPh3trigonal) contain a similar Ag29(SSR)12 unit; however, compared to the symmetric arrangement of the PPh3 ligands on Ag29-PPh3, the asymmetric arrangement of the Cs+ and DMF molecules on Ag29-Cs reduces the packing symmetry of nanoclusters in the crystal lattice. As a result, the cubic (or trigonal) crystal system of the Ag29-PPh3-cubic (or Ag29PPh3-trigonal) alters into a less symmetrical orthorhombic lattice system of the Ag29-Cs. Owing to the connections of the Cs2−S2 and Cs6−S1 (Figure 2), the Ag29-Cs nanocluster molecules are assembled into one-dimensional cluster-based lines in the crystal lattice (Figure 3), which reflect the supramolecular effects of the Cs+

(Figure 2A). Accordingly, the Ag29 nanoclusters have been orderly assembled in the crystal lattice (vide infra). Atom occupation of Cs1 or Cs2 is 100%, whereas it is only 50/35/ 15% for Cs3/Cs4/Cs5, respectively (Figure 2). The same occupation has been observed for the fixed DMF on Cs3−Cs5. In this context, the sum of Cs3, Cs4, Cs5, and their fixed DMF molecules is (Cs−DMF)1 (Figure 2C). Structurally, the capture of Cs1 with the cluster is driven by a Cs−S bond, a Cs−DMF−Ag unit, and a Cs···π interaction (Figure 2B). The Cs···π interaction is strong, ubiquitous in crystal structures of Cs-captured ringlike aromatics.60−62 Cs2 has no direct interaction with the corresponding Ag29 nanocluster but just links with the Cs1 via two DMF molecules. Besides, Cs2 links with a S atom and two C atoms from the next Ag29-Cs (Figure 2B,D). As to Cs3−Cs5, each Cs+ ion interacts with several neighboring carbon atoms and a DMF molecule fixing on the vertex Ag atom (Figure 2C). Specifically, the Cs···π interaction forms between Cs3 and neighboring benzene ring, whereas Cs4 or Cs5 only interacts with two or four carbon atoms in corresponding benzene rings. In addition, Cs1/Cs3/Cs4/Cs5 anchors one free DMF molecule, whereas Cs2 captures three free DMF molecules. Of note, the sum of DMF molecules bonding with Cs3, Cs4, and Cs5 is 1 because the proportions of Cs3, Cs4, and Cs5 are 50, 35, and 15%, respectively. Single-crystal X-ray diffraction demonstrates that the average x in Cs3Ag29(SSR)12(DMF)x is 5.8. More specifically, for the two types of Ag 29-Cs 4947

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Chemistry of Materials Table 2. Comparison of the Unit Cell Parameters of Ag29-Cs and Ag29-PPh3 Crystals unit cell a, b, c (Å) α, β, γ (deg) volume (Å3) crystal system space group density (g cm−3) Z

Ag29-Cs 35.1264(11), 27.9091(10), 29.5657(12) 90, 90, 90 28 984.6(18) orthorhombic Pbcn 2.590 8

Ag29-PPh3-cubic 34.2011(8), 34.2011(8), 34.2011(8) 90, 90, 90 40 006(3) cubic Pa3̅ 2.116 8

Ag29-PPh3-trigonal 27.4634(6), 27.4634(6), 46.6552(16) 90, 90, 120 30 474.7(17) trigonal R3̅ 2.041 6

Figure 4. Optical properties. (A) Comparison of optical absorptions and emission spectra (nanoclusters were dissolved in DMF) of Ag29-PPh3 (black lines) and Ag29-Cs (red lines) nanoclusters. (B) Comparison of optical absorptions and emission spectra (nanoclusters were in a crystallized film) of Ag29-PPh3 (black lines) and Ag29-Cs (red lines) nanoclusters.

capture with the Ag29 nanocluster. For clarify, all Cs+ ions and DMF molecules (except for Cs1, Cs2, Cs6, and the DMF molecules between them) are omitted in Figure 3. As shown in Figure 3A, the Ag29 clusters are assembled through a (−Cs+− DMF−Cs+−S−Ag−Ag−S−)2n line. Because of the twisted line, two adjacent Ag29(SSR)12 clusters are not equivalent but packed in different twisting angles. In this context, the onedimensional cluster-based line can be formulated as [Cs3Ag29(SSR)12(DMF)x]2n (Figure 3A). Of note, such a line can be just obtained when the Ag29-Cs nanoclusters are crystallized because the Cs+ ions and DMF molecules will be in a free form when nanoclusters are dissolved in solutions. In the crystal lattice of Ag29-Cs, each two neighboring cluster-based lines are packed with a mirror symmetry (Figure 3B). The distance (from kernel Ag to kernel Ag, as shown in Figure 3B) between two adjacent lines is 17.291 Å. The assembly of the nanoclusters in the crystal lattice follows an ABAB layer-by-layer packing mode (Figure 3C−E). By comparison, the Ag29-PPh3 nanoclusters in the crystal lattice are discrete; that is, no assembly occurs (Figure S6).57 Besides, the crystallized packings of Ag29-Cs nanoclusters in the crystal lattice are, visually, different when viewed from different axes (Figure 3C−E), whereas are the same in the crystal lattice of Ag29-PPh3 (Figure S6B−D). Cation effects are also investigated. However, only Cs+ has effects on the inter- and intramolecular assembly of Ag29 nanoclusters and the Ag29-PPh3 nanocluster maintains its structure in the presence of Li+, Na+, or K+ cations (Figure S7). ESI-MS spectra of both Ag29-PPh3 and Ag29-Cs were detected (Figure S8). Mass spectra of Ag29-PPh3 showed five peaks that matched with [Ag 2 9 (SSR) 1 2 (PPh 3 ) 4 ] 3 − , [Ag 2 9 (SSR) 1 2 (PPh 3 ) 3 ] 3 − , [Ag 2 9 (SSR) 1 2 (PPh 3 ) 2 ] 3 − , [Ag 29 (SSR) 12 (PPh 3 ) 1 ] 3− , and [Ag 29 (SSR) 12 ] 3− (Figure S8A,B), respectively, in agreement with the previously reported “dissociation−aggregation pattern” of the PPh3 ligands in Ag29PPh3.57,63 By comparison, the mass spectra of Ag29-Cs only

exhibited two intense peaks; the excellent match of the experimental and simulated isotope patterns illustrated that the measured formulas were [Ag29(SSR)12]3− and [CsAg29(SSR)12]2−, respectively (Figure S8C). No peaks of [Ag29(SSR)12(PPh3)n]3− (n = 1−4) were observed, demonstrating that the PPh3 ligands on Ag29-PPh3 have been fully peeled off after the Cs+ addition. The mass signal of [CsAg29(SSR)12]2− verified the capture of Cs+ with the Ag29 nanocluster. However, only one Cs+ ion was detected to be captured by the nanocluster. The unattained mass peak of the overall Ag29-Cs molecule resulted from the weak interactions between the Ag29(SSR)12 nanocluster and the Cs+ ions as well as the DMF molecules when the Ag29-Cs was in solutions. 133 Cs and 31P NMR were performed to validate the capture of Cs+ ions and the dissociation of PPh3 ligands on the Ag29 nanocluster surface, respectively (Figures S9 and S10). As depicted in Figure S9, the 133 Cs NMR spectrum of CH3COOCs exhibited an intense signal at 72.70 ppm and this signal shifted to high field (69.35 ppm) when the Cs+ ions were captured with the Ag29(SSR)12. Besides, the intense 31P NMR signal of Ag29-PPh3 at 26.20 ppm disappeared after the PPh3 ligands were peeled off from the nanocluster surface (Figure S10). Optical properties of Ag 29-PPh 3 and Ag29-Cs were compared (Figure 4). Both DMF solutions of Ag29-PPh3 and Ag29-Cs displayed an intense optical absorption at 445 nm, and three shoulder bands at 325, 365, and 510 nm, demonstrating that the dissociation of PPh3 from Ag29-PPh3 and the capture of Cs+ with Ag29-PPh3 can hardly affect the energy levels of the nanocluster (Figure 4A, nanocluster were dissolved in DMF). Upon crystallization, the optical absorptions of both Ag29PPh3 and Ag29-Cs nanoclusters displayed a red shift (∼10 nm, from 445 to 455 nm) compared to those of the nanoclusters in solutions. In addition, the absorption peaks became broader (Figure 4B). The broadening and the red shift of the absorptions arose from the electronic coupling between the 4948

DOI: 10.1021/acs.chemmater.9b01890 Chem. Mater. 2019, 31, 4945−4952

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Chemistry of Materials

Figure 5. Temperature-dependent PL properties. Temperature-dependent emissions of (A) Ag29-PPh3 and (D) Ag29-Cs. The PL intensity at the fixed points of (B) 642 nm for Ag29-PPh3 and (E) maximum emission for Ag29-Cs at different temperatures. The derivative results for the temperature-dependent PL intensity of (C) Ag29-PPh3 and (F) Ag29-Cs.

PPh3 ligands were bonded on this nanocluster; therefore, the stage of “the restriction of ligand dissociation and aggregation” was absent. The PL intensity of Ag29-Cs presented a 200-fold enhancement by comparing the 89 K data with the 293 K data (Figure 5E), and the optical absorption exhibited a 2.2-fold enhancement in this temperature-lowering process (Figure S11B). In this context, the PL QY of Ag29-Cs was almost 100% when the temperature was lower than 89 K. Besides, the emission wavelength of Ag29-PPh3 was maintained to be 640 nm during the temperature-lowering process (Figure 5A), whereas a significant red shift (15 nm, from 640 to 655 nm) was observed for Ag29-Cs (Figure 5D). The shift in emission of Ag29-Cs might arise from the temperature-dependent states of the DMF molecules fixed on the nanocluster surface (i.e., solvent state at high temperature, but frozen glass state at low temperature) induced by the presence of Cs+, which has been observed previously in the cluster-based metal−organic frameworks.67 As for Ag29-PPh3, due to the absence of Cs+ ions that fixed DMF molecules on the nanocluster surface (that is, DMF molecules were more like to be in a free state in crystals of Ag29-PPh3), the temperature-dependent states of these DMF molecules had almost no effect on the nanocluster. Accordingly, no shifts in emission occurred with the cooling process of Ag29-PPh3.

neighboring nanoclusters in crystal lattices via interactions between the transition dipole moment of the individual absorbing nanocluster and the induced dipole moments in neighboring nanoclusters.57,64,65 Both nanocluster solutions emitted at 640 nm when illuminated at 445 nm (Figure 4A). By comparison, the emission wavelengths of the crystallized films of both nanoclusters exhibited remarkable red shifts relative to those of the solutions; as depicted in Figure 4B, Ag29-PPh3 film emitted at 700 nm and Ag29-Cs emitted at 670 nm. The photoluminescence (PL) intensity of Ag29-Cs in DMF reflected a 20% enhancement relative to that of the Ag29PPh3 (Figure 4A), and the PL quantum yields (QYs) of the DMF solutions of Ag29-PPh3 and Ag29-Cs nanoclusters were 0.91 and 1.15%, respectively. However, the PL QY of Ag29-Cs crystallized film was 7.07%, much lower than that of the Ag29PPh3 (12.14%). The significant differences in emission spectra of Ag29-PPh3 in different forms (crystal film and solution) were expected to arise from a combined effect of the electronic coupling and lattice-origin, nonradiative decay pathways occurring through electron−phonon interactions.57,64,65 For the Ag29-Cs nanocluster, in addition to the explanation of Ag29-PPh3, the differences can also be explained in terms of the different Cs+−cluster interactions, strong Cs+−cluster interactions in crystals of Ag29-Cs and weak interactions in solutions. Temperature-dependent fluorescence of Ag29-PPh3 or Ag29Cs was monitored (Figure 5). The temperature-dependent fluorescence of Ag29-PPh3 showed two stages: (stage i) from 293 to 251 K, the PL intensity showed a 25-fold enhancement (PL QY from 0.9 to 22.5%). Such an enhancement resulted from the restriction of the dissociation−aggregation pattern of the PPh3 ligands on Ag29-PPh3;63 (stage ii) from 251 to 107 K, the PL intensity enhanced significantly (a 280-fold boost by comparing the 107 K data with the 293 K data, shown in Figure 5B) and the optical absorption presented a 2.5-fold enhancement (Figure S11A). Accordingly, the PL QY of Ag29PPh3 was almost 100% when the temperature was lower than 107 K (Figure 5A−C). The PL boost in stage ii was induced by the reduced energy consumption of thermal vibrations of nanoclusters, and thus, the excitation energy could only be released by the PL approach.63,66 In strong contrast, the temperature-dependent fluorescence of Ag29-Cs just showed one stage (Figure 5D−F) because no

4. CONCLUSIONS In summary, based on the Ag29 nanocluster, effects of the Cs+ capture on molecule and supramolecular chemistry of nanoclusters have been evaluated. The addition of Cs+ ions into the solution of Ag29(SSR)12(PPh3)4 peeled the PPh3 ligands off from the cluster surface, giving rise to a Cs+− DMF−cluster compound, formulated as Cs3Ag29(SSR)12(DMF)x. ESI-MS and NMR spectroscopy techniques verified the conjunction between [Ag29(SSR)12]3− and Cs+. X-ray crystallography suggested that the Cs+ capture on the nanocluster surface was driven by Cs−S and Cs···π interactions. Because of these interactions, Cs3Ag29(SSR)12(DMF)x displayed differently crystallized patterns with Ag29(SSR)12(PPh3)4. Upon crystallization, Cs+ ions and DMF molecules were combined to serve as intercluster linkers to assemble the Ag29(SSR)12 nanoclusters, making up one-dimensional cluster-based lines in the crystal lattice of Cs3Ag29(SSR)12(DMF)x. Obvious differences have been observed by comparing the optical properties (optical 4949

DOI: 10.1021/acs.chemmater.9b01890 Chem. Mater. 2019, 31, 4945−4952

Article

Chemistry of Materials

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absorptions and emissions) of Cs3Ag29(SSR)12(DMF)x in solutions to those in the crystallized film. Our findings are of great significance for revealing both the Cs+-induced intracluster transformation of nanocluster structures and the Cs+induced intercluster self-assembly.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b01890.

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Structural anatomies of the nanoclusters, and ESI, NMR and optical spectra; temperature-dependent optical absorptions; crystal data and structure refinement (PDF) Crystallographic data for Ag29-Cs (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.W.). *E-mail: [email protected] (M.Z.). ORCID

Shuxin Wang: 0000-0003-0403-3953 Manzhou Zhu: 0000-0002-3068-7160 Author Contributions §

X.W. and X.K. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support by NSFC (U1532141, 21631001, 21871001, 21803001), the Ministry of Education, the Education Department of Anhui Province (KJ2017A010), 211 Project of Anhui University.

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REFERENCES

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