Ligand Effects on Intramolecular Configuration, Intermolecular Packing, and Optical Properties of Metal Nanoclusters

Abstract

Surface modification has served as an efficient approach to dictate nanocluster structures and properties. In this work, based on an Ag₂₂ nanocluster template, the effects of surface modification on intracluster constructions and intercluster packing modes, as well as the properties of nanoclusters or cluster-based crystallographic assemblies have been investigated. On the molecular level, the Ag₂₂ nanocluster with larger surface steric hindrance was inclined to absorb more small-steric chlorine but less bulky thiol ligands on its surface. On the supramolecular level, the regulation of intramolecular and intermolecular interactions in nanocluster crystallographic assemblies rendered them CIEE (crystallization-induced emission enhancement)-active or -inactive nanomaterials. This study has some innovation in the molecular and intramolecular tailoring of metal nanoclusters, which is significant for the preparation of new cluster-based nanomaterials with customized structures and enhanced performances.

1. Introduction

Metal nanoclusters, bridging between small-sized molecular complexes and large-sized plasmonic nanoparticles, have attracted considerable attention owing to their atomically precise structures and excellent electrical, optical, and catalytical properties directed by the discrete electronic energy level as well as the structure-dependent quantum confinement effect [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. The monodispersed sizes, precise compositions, and accurate configurations of metal nanoclusters make it possible to investigate the relationship between their structures and properties. In addition, the attainable structure–property correlations further enable the rational construction of new nanoclusters with customized performances [17,18,19,20,21,22]. In this context, the regulatable intramolecular structures and intermolecular packing modes render metal nanoclusters or cluster-based nanocomposites prominent nanomaterials for atomic engineering and further practical applications [23,24,25,26,27,28,29].

The past few decades have witnessed great research efforts of the control over intracluster structures/compositions and intercluster aggregates [17,18,19,20,23,24,25,26,27]. Specifically, the intramolecular control of nanoclusters touches upon the manipulation of their metal-ligand compositions and bonding environment at the single molecular level, while the intermolecular control of nanoclusters refers to the manipulation over their aggregating patterns among several cluster molecules in amorphous or crystallographic forms [30]. Several control methods, including (i) intracluster approaches (e.g., ligand exchange [31,32,33,34], heteroatom alloying [35,36,37,38,39], and molecular charge regulation [40,41,42]) and (ii) intercluster approaches (e.g., cluster-based metal-organic framework [43,44,45,46], aggregation-induced emission [47,48,49], and intercluster metallophilic reaction [50,51]), have been exploited to control clusters or their assemblies and to dictate their properties. Of note, the intracluster and intercluster controls are not a binary separation, but an interrelated and inseparable whole to regulate the nanocluster system simultaneously. In this context, the intracluster regulation of nanoclusters may alter their aggregating patterns at the supramolecular level, and vice versa [52]. The profound cognition of the correlation between molecular and supramolecular chemistry of nanoclusters offers great opportunities for the fabrication of novel nanoclusters or cluster-based hybrids with customized properties.

Herein, a new Ag₂₂ nanocluster, formulated as Ag₂₂(S-Adm)₁₀(DPPM)₄Cl₆ (abbreviated as Ag₂₂-L1, where S-Adm = 1-adamantanethiol and DPPM = bis(diphenylphosphino)methane), was synthesized and structure-determined by X-ray single-crystal diffraction. The combination of this Ag₂₂ nanocluster and a previously reported Ag₂₂(SPhMe₂)₁₂(DPPE)₄Cl₄ (abbreviated as Ag₂₂-L2, where SPhMe₂ = 2,5-dimethyl thiophenol and DPPE = 1,2-bis(diphenylphosphino)ethane) constructed a platform to investigate the effects of surface modification on intramolecular constructions and intermolecular packing modes, as well as the properties of nanoclusters or cluster-based crystallographic assemblies. On the molecular level, because of the larger surface steric hindrance of Ag₂₂-L1 relative to Ag₂₂-L2, the Ag₂₂-L1 surface contained more small-steric chlorine but fewer bulky thiol ligands. On the supramolecular level, Ag₂₂-L2 displayed intramolecular and intermolecular interactions in its crystallographic assembly, while these interactions were absent in the Ag₂₂-L1 crystal. Ag₂₂-L2 was CIEE (crystallization-induced emission enhancement) active while Ag₂₂-L1 was CIEE inactive. The optical absorptions and emissions of these two Ag₂₂ nanoclusters were also compared.

2. Materials and Methods

Chemicals. All reagents were purchased from Adamas Reagent (Shanghai, China) and used without further purification: silver nitrate (AgNO₃, 99%, metal basis), 1-adamantanethiol (HS-Adm, 97%), 2,5-dimethyl thiophenol (HS-PhMe₂, 97%), bis(diphenylphosphino)methane (Ph₂P-CH₂-PPh₂, DPPM, 98%), 1,2-bis(diphenylphosphino)ethane (Ph₂P-C₂H₅-PPh₂, DPPE, 98%), sodium cyanoborohydride (NaBCNH₃, 99.9%), methylene chloride (CH₂Cl₂, HPLC grade), methanol (CH₃OH, HPLC grade), ethyl ether ((C₂H₅)₂O, HPLC grade), and n-hexane (Hex, HPLC grade).

Synthesis of Ag₂₂(S-Adm)₁₀(DPPM)₄Cl₆ (Ag₂₂-L1). Specifically, 60 mg of AgNO₃ (0.36 mmol) and 40 μL of H₂PtCl₆ (0.2 g/mL; 0.015 mmol) were dissolved in 20 mL of CH₃OH and 1 mL of CH₃CN. Then, 40 mg of DPPM (0.1 mmol) and 30 mg of HS-Adm (0.18 mmol) were added. After stirring for 30 min, 100 mg of NaBCNH₃ (1.59 mmol; dissolved in 2 mL of MeOH) was added. The reaction was allowed to proceed for 5 h. After that, the mixture in the organic phase was rotavaporated under vacuum and washed several times by MeOH and Hex. Then, 10 mL of CH₂Cl₂ was used to extract the obtained Ag₂₂-L1 nanocluster. The yield is 30% based on the Ag element (calculated from AgNO₃). Of note, although Pt did not exist in the final Ag₂₂-L1, the absence of Pt sources resulted in the failure of the nanocluster synthesis (Figure S1). Such a phenomenon has also been observed in previous works [53].

Synthesis of Ag₂₂(S-PhMe₂)₁₂(DPPE)₄Cl₄ (Ag₂₂-L2). The preparation of Ag₂₂(S-PhMe₂)₁₂(DPPE)₄Cl₄ was based on the reported method of the Pradeep group [54].

Crystallization of Ag₂₂-L1. In order to accelerate the crystallization process and improve the crystal quality, the counterions (i.e., Cl⁻) in the Ag₂₂-L1 nanocluster were replaced by SbF₆⁻ [55]. The reaction equation was [Ag₂₂(S-PhMe₂)₁₂(DPPE)₄Cl₄]Cl₂ + 2 SbF₆⁻ → [Ag₂₂(S-PhMe₂)₁₂(DPPE)₄Cl₄](SbF₆)₂ + 2Cl⁻. Nanoclusters were crystallized in a CH₂Cl₂/ether system with a vapor diffusion method (Table S1).

3. Results

The Ag₂₂-L1 nanocluster was synthesized by directly reducing the Ag-SR-DPPM complexes by NaBCNH₃ (Scheme S1; see more details in Materials and Methods). The electrospray ionization mass spectrometry (ESI-MS) measurement was performed to verify the molecular composition and to determine the valence state of the Ag₂₂-L1 nanocluster. As shown in Figure S2, the mass result of the nanocluster exhibited an intense peak at 2897.54 Da. The excellent match of the experimental and simulated isotope patterns illustrated that the measured formula was [Ag₂₂(S-Adm)₁₀(DPPM)₄Cl₆]²⁺. The “+2” valence state of the nanocluster matched well with the existence of (SbF₆)⁻ counterions in the crystal lattice, i.e., the molar ratio between the cluster and the counterion was 1:2, as depicted in Figure S3. According to the valence states of Ag₂₂-L1, its nominal electron counts was determined as 4e [56], i.e., 22(Ag) − 10(SR) − 6(Cl) − 2(charge) = 4e, the same as that of Ag₂-L2 [54]. Moreover, the chlorine ligands in Ag₂₂-L1 were proposed to originate from the H₂PtCl₆ or from the CH₂Cl₂ solvent, which has also been discovered in previously determined nanoclusters [57,58,59,60].

Structurally, the Ag₂₂-L1 nanocluster contained an Ag₁₀ kernel which comprised two distorted trigonal bipyramidal Ag₅ units via an edge–edge vertical assembling mode (Figure 1A,B). Then, two Ag₂(S-Adm)₃(DPPM)₁Cl₁ surface units capped the Ag₁₀ kernel from the same side via Ag-S or Ag-Cl interactions, giving rise to an Ag₁₄(S-Adm)₆(DPPM)₂Cl₂ structure (Figure 1C,D). The other unprotected side of the Ag₁₀ kernel was further stabilized by two Ag₂(S-Adm)₂(DPPM)₁Cl₂ surface units, making up a Ag₁₈(S-Adm)₁₀(DPPM)₄Cl₆ structure (Figure 1E,F). Finally, four Ag atoms acting as bridges linked these surface units via S-Ag-S interactions, yielding the final Ag₂₂(S-Adm)₁₀(DPPM)₄Cl₆ framework (Figure 1G,H). Because of the asymmetry of surface units in Ag₂₂-L1, especially the asymmetrical arrangement of peripheral thiol and chlorine ligands, no symmetrical element was observed in the Ag₂₂-L1 nanocluster framework (Figure 1I and Figure S4).

The overall constructions of Ag₂₂-L1 and Ag₂₂-L2 nanoclusters were almost the same. However, because of the different steric hindrances of ligands in these two nanoclusters (i.e., S-Adm and DPPM in Ag₂₂-L1; S-PhMe₂ and DPPE in Ag₂₂-L2), these two nanoclusters displayed some structural differences:

(i) For the kernel structure: the average Ag-Ag bond length in bipyramidal Ag₅ of Ag₂₂-L1 was 2.824 Å, much shorter than that in Ag₂₂-L2 (i.e., 2.933 Å). In addition, the average Ag-Ag bond lengths between these two Ag₅ bipyramids were 2.870 and 2.937 Å in Ag₂₂-L1 and Ag₂₂-L2, respectively. In this context, due to the larger surface steric hindrance of Ag₂₂-L1 relative to Ag₂₂-L2, the Ag₁₀ kernel of the former nanocluster was compressed.

(ii) For the surface environment: the biggest structural difference between the two Ag₂₂ nanoclusters lay in their surface ligand environments in terms of the proportion of the chlorine in peripheral ligands. Specifically, the Ag₂₂-L1 nanocluster contained 10 thiol and 6 chlorine ligands, while Ag₂₂-L2 included 12 thiol and 4 chlorine ligands (Figure 2). As shown in Figure 2A,B, a thiol ligand at the specific location on the Ag₂₂-L2 surface was substituted by a chlorine ligand in Ag₂₂-L1. Another thiol ligand at the symmetrical position was also replaced by chlorine. Such a substitution from bulky thiol to small-steric chlorine was reasonable by considering that the more compact surface environment on Ag₂₂-L1, resulting from the bulkier DPPM and S-Adm ligands relative to DPPE and S-PhMe₂, was unable to host as many bulky thiol ligands as Ag₂₂-L2 (Figure 2C,D). Moreover, several intramolecular noncovalent C-H···π and π···π interactions were observed in the Ag₂₂-L2 structure, which was advantageous to the compact packing of its surface ligands [54]. By comparison, none of such noncovalent interactions was observed in Ag₂₂-L1, which might be another reason that more small-steric chlorine but fewer bulky thiol ligands were arranged on the Ag₂₂-L1 nanocluster surface.

The Ag₂₂-L1 cluster entities were crystallized in a triclinic crystal system with a P-1 space group, whereas the Ag₂₂-L2 cluster entities were crystallized in a tetragonal crystal system with an I4₁/a space group. Both nanoclusters followed a lamellar eutectic packing pattern between R-nanocluster and S-nanocluster enantiomers in the crystal lattice; however, due to their distinct crystal systems, the interlayer distances were different: 26.561 Å of Ag₂₂-L1, and 28.957 Å of Ag₂₂-L2 (Figure 3 and Figure S5). Of note, there are equal R-nanocluster and S-nanocluster enantiomers in the crystal lattice, and the crystalline material of the nanocluster was racemic. Furthermore, owing to the existence of several benzene-rings in the Ag₂₂-L2 nanoclusters, strong intracluster and intercluster interactions occurred, including C-H···π interaction and π-π stacking [54]. In vivid contrast, these interactions were absent within the Ag₂₂-L1 nanocluster or among Ag₂₂-L1 cluster entities (Figure S6).

The Ag₂₂-L1 nanocluster (dissolved in CH₂Cl₂) exhibited three intense absorptions centered at 368, 494, and 635 nm (Figure 4A). By comparison, the UV-vis spectrum of Ag₂₂-L2 displayed several peaks at 445, 512, and 670 nm (Figure 4A). The blue shifts in the optical absorptions of Ag₂₂-L1 relative to Ag₂₂-L2 resulted from the different electronic structures of the two Ag₂₂ nanoclusters. The CH₂Cl₂ solution of Ag₂₂-L1 emitted at 650 nm, while the emission of Ag₂₂-L2 was located around 670 nm (Figure 4B). The 20 nm blue-shift and 1.2-fold enhancement of the emission of Ag₂₂-L1 relative to that of Ag₂₂-L2 resulted from their different electronic structures. Indeed, these two nanoclusters displayed different optical absorptions, demonstrating their distinguishable electronic excitations and HOMO-LUMO energy gaps (HOMO: the highest occupied molecular orbital; LUMO: the lowest unoccupied molecular orbital). In addition, the different electronic excitations endowed these two nanoclusters with distinct emissions.

The Ag₂₂-L2 nanocluster was CIEE active owing to the presence of extensive intramolecular and intermolecular interactions in its crystal lattice [54]. In this context, the emission intensity of Ag₂₂-L2 in the crystalline state was remarkably higher than that of the nanocluster in the solution or the amorphous state. By comparison, the Ag₂₂-L1 was CIEE inactive since no significant enhancement in emission intensity was observed (Figure 4C). Actually, the Ag₂₂-L1 in the amorphous or crystalline state was almost non-emissive. Such a striking contrast was reasonable considering that the intramolecular and intermolecular interactions were absent in the crystal lattice of Ag₂₂-L1, as mentioned above. The investigation of the Ag₂₂ nanocluster system promoted the understanding of the crystalline packing mode and the CIEE of cluster-based nanomaterials.

4. Conclusions

In summary, a new Ag₂₂ nanocluster, formulated as Ag₂₂(S-Adm)₁₀(DPPM)₄Cl₆, has been synthesized and structurally determined, which constituted an Ag₂₂ cluster system together with the previously reported Ag₂₂(S-PhMe₂)₁₂(DPPE)₄Cl₄. Based on this Ag₂₂ cluster system, the effects of surface modification on intracluster constructions and intercluster packing modes, as well as the properties of nanoclusters or cluster-based crystallographic assemblies were investigated. The Ag₂₂ nanocluster with larger surface steric hindrance was inclined to load more small-steric chlorine but fewer bulky thiol ligands on its surface. Moreover, the Ag₂₂ nanocluster, which embodied several intramolecular and intermolecular interactions in cluster crystallographic assemblies, was CIEE active; by comparison, the Ag₂₂ nanocluster without such interactions was CIEE inactive. This work provides new insight into the surface modification of metal nanoclusters and its effects on intramolecular configuration, intermolecular packing, and optical properties.