Tailoring the Crystal Structure of Nanoclusters Unveiled High

Article Cite This: Chem. Mater. 2018, 30, 2719−2725

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Tailoring the Crystal Structure of Nanoclusters Unveiled High Photoluminescence via Ion Pairing Megalamane S. Bootharaju,† Sergey M. Kozlov,† Zhen Cao,† Aleksander Shkurenko,‡ Ahmed M. El-Zohry,§ Omar F. Mohammed,§ Mohamed Eddaoudi,‡ Osman M. Bakr,*,†,§ Luigi Cavallo,*,† and Jean-Marie Basset*,† KAUST Catalysis Center, ‡Functional Materials Design, Discovery and Development Research Group (FMD3), Advanced Membranes and Porous Materials Center, and §KAUST Solar Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

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S Supporting Information *

ABSTRACT: The lack of structurally distinct nanoclusters (NCs) of identical size and composition prevented the mechanistic understanding of their structural effects on ion pairing and concomitant optical properties. To produce such highly sought NCs, we designed a new monothiolate-fordithiolate exchange strategy that enabled the selective transformation of the structure of a NC without affecting its metal atomicity or composition. Through this method, a bimetallic [PtAg28(BDT)12(PPh3)4]4− NC (1) was successfully synthesized from [PtAg28(S-Adm)18(PPh3)4]2+ NC (2) (S-Adm, 1-adamantanethiolate; BDT, 1,3-benzenedithiolate; PPh3, triphenylphosphine). The determined X-ray crystal structure of 1 showed a PtAg12 icosahedron core and a partially exposed surface, which are distinct from a face-centered cubic PtAg12 core and a fully covered surface of 2. We reveal through mass spectrometry (MS) that 1 forms ion pairs with counterions attracted by the core charge of the cluster, which is in line with density functional simulations. The MS data for 1, 2, and other NCs suggested that such attraction is facilitated by the exposed surface of 1. The formation of ion pairs increases the photoluminescence (PL) quantum yield of 1 up to 17.6% depending on the bulkiness of the counterion. Unlike small counterions, larger ones are calculated to occupy ≤90% of the volume near the exposed cluster surface and to make the ligand shell of 1 more rigid, which is observed to increase the PL. Thus, the developed synthesis strategy for structurally different NCs of the same size and composition allows us to probe the structure−property relationship for ion pairing and concomitant PL enhancement.

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INTRODUCTION Nanoclusters (NCs), a captivating class of functional nanomaterials comprising a discrete number of metal atoms and ligands, have garnered much attention in the past decade with respect to their fundamental and applied research.1−5 They are considered as a missing link between the two important size regimes of individual molecules and nanoparticles (NPs).6−8 The ultrasmall size ( TBA > CTA > TPP (Figure 4C). In contrast, much smaller tetrametylammonium (TMA) counterions were found to have a minor effect on PL intensity (Figure S8). DFT simulations shed light on the atomistic details of ion pairing with the core charge and the mechanism of PL enhancement by ion pairing and rationalized why bulkiness of counterions matters. First, we simulated NCs 1 and 2 using DFT methods coupled with the linearized Poisson−Boltzmann equation. The latter describes the density of counterions in the electrolyte within the continuous medium approximation, where solvent molecules, counterions, and the charge are smeared over the space. Figure S9 shows that the density of counterions is highest in the vicinity of partially exposed surface facets of 1, whereas 2 attracts counterions to its vertices

Figure 2. Total structure of [PtAg28(BDT)12(PPh3)4]4−, NC 1. The C framework is colored gray; H atoms are not shown for the sake of clarity. Dotted orange arrows show the exposed parts of the cluster surface.

Figure 3. Construction of PtAg28S24P4 in NC 1: (A) Pt-centered Ag12 icosahedron, (B) four trigonal prisms oriented tetrahedrally on the PtAg12 core, (C) AgPS3 motifs protecting the structure B to form (D) PtAg28S12, and (E) μ2 thiols that bridge the edges of trigonal prisms to produce the total structure of (F) PtAg28S24P4. Legend: Pt, blue; Ag, dark green, pink, and light blue; S, orange and yellow; P, green.

Figure 3, respectively). Four AgPS3 motifs are located on top of triangular faces of the PtAg12 core with the three S atoms binding to three Ag atoms of the respective core faces and three Ag atoms of three adjacent trigonal prisms (Figure 3D). Phosphorus atoms of AgPS3 motifs attach to Ag atoms via dangling bonds and form part of PPh3 ligands. Furthermore, four trigonal prisms expose 12 top base edges, which are bridged by μ2-S atoms of thiol motifs. Thus, the overall structure is PtAg28S24P4 (Figure 3F). Note that the arrangement of AgPS3 and thiol motifs leaves the faces of trigonal prims exposed to the environment (Figure 3F and dotted arrows in Figure 2). Alternatively, the PtAg28S24P4 framework of 1 can be visualized as a core−shell structure, a bimetallic icosahedron core of PtAg12 encapsulated in a partially closed shell of Ag16S24P4 (Figure S3). Note that a core−shell structure is also adapted by parent cluster 2 (Figure S4) with a similarly sized 2721

DOI: 10.1021/acs.chemmater.8b00328 Chem. Mater. 2018, 30, 2719−2725

Article

Chemistry of Materials

Figure 4. (A) UV−vis absorption (solid lines) and PL emission (dotted lines) spectra of NC 1 and NC1+TOA in DCM. Insets are photographs of DCM solutions of NC 1 and NC1+TOA under UV light. (B) Negative ion-mode ESI MS of NC1+TOA, showing peaks for the cluster paired with up to two TOA ions. (C) PL emission spectra of NC 1 paired with TOA, TBA, CTA, and TPP.

covered by PPh3 ligands. Thus, the location of counterions around a cluster is sensitive to the cluster’s structure. Second, we simulated how much of the free volume around a partially exposed facet of 1 can be occupied by various counterions (the method is described in the Supporting Information and illustrated on Figure S10). Very small TMA and medium-sized TPP counterions are calculated to occupy ν = 29% and ν = 54% of the free volume around a core facet of 1 (Table S2). We also performed such simulations for [NMe4−xOctx]+ (x = 1−3; Me, methyl; Oct, octyl) counterions of increasing bulkiness (see the Supporting Information). [NMe4−xOctx]+ ions are calculated to occupy ν = 60%, ν = 76%, and ν = 89% of the free volume around a core facet of 1 for x values of 1−3, respectively. Thus, counterions are shown to occupy from ∼30 to ∼90% of the free volume around a surface facet of 1 depending on their bulkiness, e.g., on the number of long alkyl chains. Third, bulky counterions in the vicinity of the surface of 1 are shown to rigidify the ligand shell of 1. In particular, we simulated the rotation of a PPh3 ligand around the formed P− Ag bond (see the Supporting Information for details), which is probably one of the most feasible structural perturbations of the ligand shell of 1. The rather low barrier for PPh3 rotation in the absence of any counterion, ∼0.3 eV, does not significantly change upon saturation of 1 with nonbulky TMA counterions (Figure 5). In variation, the presence of bulky TPP counterions increases the barrier for PPh3 rotation to ∼0.5 eV and substantially narrows the low-energy region of the potential energy surface that is accessible at room temperature. More bulky TOA ligands, which occupy more free volume around the facets of 1, are expected to make the ligand shell even more rigid and to hinder the rotation of PPh3 ligands further. Note that the hindered PPh3 rotation in the presence of TOA is also suggested by 31P NMR experiments (Figure S6). Thus, DFT simulations reveal that the particular structure of 1 attracts counterions to the partially exposed surface of the cluster. There, sufficiently bulky counterions occupy a significant amount of the free volume and rigidify the ligand shell. In turn, the rigidity of the ligand shell is known to enhance the PL of gold clusters.24,34,35 For Ag-based clusters,

Figure 5. (A) PPh3 ligand rotation around the P−Ag bond and the color legends of atoms (C atoms of the rotated PPh3 are colored black). (B) Calculated cluster structures for different rotation angles, α. Counterions and other PPh3 ligands are not displayed for the sake of clarity. (C) Energy profiles of PPh3 rotation calculated for NC 1, NC1+TMA, and NC1+TPP.

such an observation along with mechanistic details is described for the first time in this work. Structure−Property Relationships. To explore the transferability of our findings to other systems, we focused on ion pairing of TPP with various clusters and the induced PL enhancement. TPP was found to form ion pairs with NC 1 by ESI MS (Figure S11) and to somewhat enhance its PL (Figure 4C). Despite the same size and composition on the metal core, 2 did not form ion pairs with tetraphenylborate (TPB) counterions (Figure S12), whose size is similar to that of TPP. Hence, the PL of 2 was not enhanced upon TPB treatment (Figure S13). Furthermore, no ion pairing was 2722

DOI: 10.1021/acs.chemmater.8b00328 Chem. Mater. 2018, 30, 2719−2725

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Chemistry of Materials observed between TPP and [Ag44(SR)30]4−,36 despite its similar charge with 1 (Figure S14). Also, the [PtAg24(SR)18]2− cluster44−46 with a PtAg12 icosahedron metal core, which is close in size and isoelectronic (8e−) to 1, did not exhibit ion pairing ability with TPP. However, we observed (see below) ion pairing between TOA and undoped [Ag29(BDT)12(PPh3)4]3− clusters (3),37 which are isostructural and isoelectronic to 1 (Figure S15). From these observations, we suggest that the structure of the NC controls its ion pairing ability, whereas its charge as well as the size and the composition of the metal core are less important. The particular structural feature of 1 that distinguishes it from other considered clusters, [PtAg 2 8 (S-Adm) 1 8 (PPh 3 ) 4 ] 2 + , 1 6 [Ag44(SR)30]4−,36 and [PtAg24(SR)18]2−,44 is the partially exposed surface structure. The latter allows counterions to closely approach the charged metallic core of the NC, which leads to the stronger interaction. When the structure of the cluster enables ion pairing, the cluster charge steers the number of the attracted counterions and the magnitude of the resulting PL enhancement. Indeed, the charge on 1 is −4, whereas the charge on isostructural undoped cluster 3 is −3. As a result, ion pairs of 3 with only one TOA are detected by ESI MS (Figure S15), whereas up to two TOAs paired with 1 could be detected (Figure 4). In turn, the PL intensity and QY of NC1+TOA are observed to increase by 18- and 5-fold, respectively, when compared to those of NC3+TOA (Figure S16A). DFT simulations confirm that the ion pairing ability of NC3 is significantly weaker than that of NC 1, i.e., that binding of counterions to 3 is significantly less exothermic than that to 1 (Figure S17). Although the binding energy between the clusters and the counterions is very strong in vacuum (i.e., during ESI MS measurements), it drastically decreases in polar solvents. For example, NC 1 could attract four TPP counterions only in solvents with a dielectric constant ε of less than ∼10. This finding is in line with the observation of PL enhancement by ion pairing only in the relatively nonpolar solvent DCM, with an ε of 8.93 (Figure 4C), but not in the more polar solvent dimethylformamide (DMF), with an ε of 36.7 (Figure S18). Finally, we have performed nanosecond transient absorption (ns-TA) experiments to obtain the lifetimes of excited states of ion pairs NC1+TOA and NC3+TOA. The excited state decay profile of NC1+TOA was fitted into two exponential decays (60 ns, 14%, and 3.12 μs, 86%). The average lifetimes for NC3+TOA and NC1+TOA were found to be 136 ns and 1.8 μs, respectively (Figure S16B). The increase in the lifetime of NC1+TOA, by more than 13 times, may be attributed to the enhanced ion pairing capability due to the increased magnitude of cluster charge upon Pt doping. The enhancement of excited state lifetime further confirmed the strong rigidification of the ligand shell of NC1+TOA, in agreement with the reported gold NCs,24 when they were paired with TOA ions through the ligand charge.

former cluster allowed it to form ion pairs with counterions via electrostatic attraction to the cluster’s core charge. Ion pairing with sufficiently bulky counterions enhanced the PL QY by more than 10 times to 17.6%. DFT simulations reveal that bulky counterions attracted by the cluster with a partially exposed surface occupy a significant volume in the vicinity of the cluster and rigidify the ligand shell, consistent with 31P NMR measurements. For a given structure, the charge of the cluster is found to significantly affect the magnitude of PL enhancement by influencing the number of attracted counterions. Our synthesis methodology will encourage the creation of similar structurally different pairs of NCs with the same size and composition for other metals as well as further development of NCs with novel properties such as the accessible surface structure and thermally robust cluster framework possessed by the discussed PtAg28 cluster. The observed PL enhancement by ion pairing with core charge will stimulate future studies of the interactions between NCs and their environment.

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EXPERIMENTAL SECTION

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COMPUTATIONAL SECTION

Materials. The chemicals, including silver nitrate (AgNO3), hexachloroplatinic acid (H 2 PtCl 6 ·6H 2 O), 1,3-benzenedithiol (BDTH2), triphenylphosphine (PPh3), 1-adamantanethiol (AdmSH), 4-fluorobenzenethiol (FTP), 2,4-dimethylbenzenethiol (DMBT), tetraphenylphosphonium bromide (TPPB), tetraoctylammonium bromide (TOAB), tetrabutylammonium bromide (TBAB), cetyltrimethylammonium bromide (CTAB), tetramethylammonium bromide (TMAB), tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate {[Ru(bpy)3](PF6)2}, sodium tetraphenylborate (NaBPh4), and sodium borohydride (NaBH4), were purchased from Sigma-Aldrich and used without further purification. Solvents, including methanol (MeOH), dichloromethane (DCM), dimethylformamide (DMF), acetone, ethanol, and acetonitrile, were of HPLC grade, and dichloromethane-d2 (CD2Cl2) was received from Sigma. Deionized (DI) water was obtained from a Millipore apparatus. Synthesis and Purification of [PtAg28(BDT)12(PPh3)4]4− NCs (1). Approximately 18 mg of [PtAg28(S-Adm)18(PPh3)4]2+ cluster 2 was dissolved in 10 mL of acetone and treated simultaneously with 30 mg of PPh3 and 12 μL of BDT. The reaction mixture was stirred under ambient conditions for 20 h in the dark. The dark orange solution of [PtAg28(BDT)12(PPh3)4]4− cluster 1 was centrifuged to discard the insoluble byproducts. The supernatant of the cluster was vacuum-dried and washed with a minimum amount of methanol. Crystallization of NC 1. The as-prepared dark orange solution (500 μL) of 1 in acetone was transferred to a glass tube, and then the tube was placed in a small closed box. The solvent was allowed to evaporate slowly in the dark. After 4 days, the orange-red single crystals suitable for SC-XRD were obtained. Ion Pairing of NCs with Counterions. Different counterions of interest were added to the cluster solution in such a way that the charge on the cluster is fully neutralized. For example, 2 equiv of NaBPh4 and TPPB were added to 1 equiv each of [PtAg28(SAdm)18(PPh3)4]2+ and [PtAg24(SR)18]2− NCs, respectively. Similarly, other NCs, [PtAg28(BDT)12(PPh3)4]4− and [Ag29(BDT)12(PPh3)4]3−, were paired with 4 and 3 equiv of the desired counterions (e.g., TOA, TBA, TPP, and CTA), respectively.

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CONCLUSION We achieved a structural transformation between two NCs of the same size and composition through a developed monothiolate-for-dithiolate LE strategy. Namely, we obtained a [PtAg28(BDT)12(PPh3)4]4− NC with a partially exposed surface from a [PtAg28(S-Adm)18(PPh3)4]2+ NC with a surface fully covered by bulky monothiolates. This achievement allowed us to perform a unique structure−property relationship study for these NCs. The partially exposed surface of the

VASP.47 Geometry optimization of ion pairs and their components was performed using the PBE48 exchange-correlation functional augmented with D3 Grimme corrections,49 the plane wave basis set with a 400 eV cutoff energy, and the projected augmented wave technique50 to treat core electrons until forces on atoms became 1 nm. The Poisson−Boltzmann equation was solved self-consistently with DFT using VASPSol51 for an ε(DCM) of 8.93 and a Debye length of 0.3 nm (only for data in Figure S9). The potential energy profile of PPh3 2723

DOI: 10.1021/acs.chemmater.8b00328 Chem. Mater. 2018, 30, 2719−2725

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Chemistry of Materials rotation is calculated through a constrained geometry optimization (see the Supporting Information). Molecular Dynamics. The volume occupied by counterions in ion pairs was derived from 3000 selected snapshots from the trajectory of a given counterion calculated with the OPLS force field52 at 300 K (see the Supporting Information). ADF. Binding energies for binding of counterions to the clusters were calculated in single-point fashion on the geometries optimized in VASP. We used the PBE exchange-correlation functional, TVP basis set, and COSMO solvent model53 (only for data in Figure S17B).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00328. Details of the synthesis of various NCs, SC-XRD of [PtAg28(BDT)12(PPh3)4]4− NC (1), characterization of ion pairs via ESI MS, UV−vis data, PL, and details of performed simulations (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Aleksander Shkurenko: 0000-0001-7136-2277 Ahmed M. El-Zohry: 0000-0003-2901-5815 Omar F. Mohammed: 0000-0001-8500-1130 Mohamed Eddaoudi: 0000-0003-1916-9837 Osman M. Bakr: 0000-0002-3428-1002 Luigi Cavallo: 0000-0002-1398-338X Jean-Marie Basset: 0000-0003-3166-8882 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this work was provided by KAUST. For computer time, this research used the resources of the Supercomputing Laboratory at KAUST.

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REFERENCES

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DOI: 10.1021/acs.chemmater.8b00328 Chem. Mater. 2018, 30, 2719−2725

Article

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DOI: 10.1021/acs.chemmater.8b00328 Chem. Mater. 2018, 30, 2719−2725