Tailoring the Crystal Structure of Nanoclusters Unveiled High

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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00328 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Chemistry of Materials

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), Ad§ vanced Membranes and Porous Materials Center, and KAUST Solar Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

ABSTRACT: 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 achieve such highly sought NCs, we designed a new monothiolate-for-dithiolate exchange strategy that enabled the selective transformation of the structure of a NC without affecting its metal atomicity and its 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: 1adamantanethiolate; 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 PL quantum yield of 1 up to 17.6% depending on the bulkiness of counterion. Unlike small counterions, larger ones are calculated to occupy up to 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.

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 for 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 (< 2 nm), highmonodispersity, precise molecular formula and distinctly different atomic structure make NC properties strikingly different from their bulk counterparts.9-11 NCs exhibit exotic optical properties such as molecule-like absorption,12 photoluminescence (PL)13 and optical activity,14 in addition to intriguing chemical properties of ligand-15-18 and metal-exchange,19, 20 intercluster21, 22 and ion pairing23, 24 reactions. Furthermore, they offer potential applications in sensing,25 catalysis,26 electrocatalysis,27 energy25 and environmental technologies25. PL is one of the fascinating properties of NCs.28, 29 Despite the lack of complete understanding, it is commonly believed to occur through charge-transfer between metal core and the ligand-shell.30, 31 The PL quantum yield (QY) and ambient stability of NCs are usually low32, 33 and need to be improved for practical applications in imaging and

sensing. PL is known to depend on NC’s size, composition, intercluster distance/aggregation, and solvent.13, 31, 34 It is also known to be strongly affected by the type and the rigidity of the cluster’s ligand-shell.24, 35 In turn, the latter could be affected by the ion pairing between charged ligand-shell and added counterions, which typically occurs when NCs expose charged ligands. Unfortunately, the presence of such ligands (e.g., ligands with ─COO─ termination) greatly complicates the crystallization of NCs and their precise structural characterization.24, 35 Thus, detailed atomistic studies of ion pairing with NCs are still missing. The challenges in crystallization and structure determination are more feasible to overcome for NCs with charge located solely on the metal core.10, 36, 37 One of the other significant parameters that can affect the PL is the structure of a cluster. This dependency has remained largely unexplored due to unavailability of the structurally different clusters of identical size (i.e., metal atomicity) and composition (homoor multi-metallic). That is, such studies have been hardly possible in the absence of suitable synthetic strategies. Development of these NCs will enable the study of the structural effects on the ion pairing and thereby PL. In this work, we designed a new monothiolate-fordithiolate exchange strategy to tune the structure of a Pt-

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doped silver cluster without changing their metal composition and atomicity. Specifically, we obtained a [PtAg28(BDT)12(PPh3)4]4─ NC (1) from an originally synthesized [PtAg28(S-Adm)18(PPh3)4]2+ NC (2) (PPh3: triphenylphosphine, BDT: 1,3-benzenedithiolate, S-Adm: 1-adamantanethiolate) by ligand-exchange (LE) of bulky monothiolate S-Adm with non-bulky dithiolate BDT. Although 1 and 2 have identical metal compositions, the obtained X-ray crystal structure of 1 shows distinctly different geometry, providing an unprecedented opportunity to investigate structure-property relationships for these NCs. In particular, we investigated how the structure of 1 affects its ion pairing with counterions and thereby how ion pairing affects PL. Density functional simulations (DFT) corroborated the key findings and shed light on the atomistic mechanisms of the observed effects. To the best of our knowledge, this is the first example of PL enhancement via an ion pairing between counterions and the core charge of a metal cluster.

RESULTS AND DISCUSSION Size-preserved LE transformation of PtAg28 NCs A promising alternative to the direct synthesis of NCs from metal and ligand sources is their post synthetic transformation through LE.38 Usually, LE alters both the structure and the size of the NC,17, 39-41 or in exceptional cases preserves both38 of them. Thus, the desired clusters with the same size but different structures have not been achieved with LE. Here, we used the non-bulky dithiolates to replace the bulky monothiolates of a doped NC in an attempt to preserve the size of the NC while altering its structure after LE-induced transformation. For this study, we chose a bimetallic NC2 [PtAg28(SAdm)18(PPh3)4]2+, which was prepared by following a known protocol16 (see Supporting Information, SI for details). Then, this cluster was converted to NC1 (see Experimental section for details) with a transformation yield of 76.3 wt% through the single-phase LE with BDT by the addition of BDTH2 (1,3-benzenedithiol). The formation and purity of 2 were confirmed by the high resolution electrospray ionization mass spectrometry (ESI MS) and UV-vis absorption spectroscopy (Figures 1A and S1A, respectively). Upon LE of bulky S-Adm monothiolates of 2 with a relatively less bulky dithiolates BDT (insets of Figure 1A), the absorption features at 441 and 530 nm were blue-shifted to 426 and 505 nm, respectively (Figure S1A), indicating significant changes in the cluster. However, the ESI MS (Figures 1B and S1B) suggested the composition of the LE product to be [PtAg28(BDT)12(PPh3)4]4─ NC (1). Note that unlike similar studies of Au-based clusters,42, 43 our study is the first to report a complete exchange of monothiolate ligands with dithiolates while preserving the NC size.

Figure 1. Positive- and negative-mode ESI MS of (A) NC2 and (B) NC1, respectively. The molecular structures of S-Adm monothiolate and BDT dithiolate are shown as insets of A. Inset of B shows a characteristic sequential loss of PPh3 from the molecular ion 1.

Single crystal X-ray [PtAg28(BDT)12(PPh3)4]4─

structure

of

NC1

The molecular formula of 1 requires a total of 24 S binding sites on the cluster, while its parent cluster 2 contains only 18 of them, despite the same size and composition of the metal core. We investigated how the structure of 1 allows it to accommodate six more S atoms by using single-crystal X-ray diffraction (SC-XRD). The suitable orange-red single crystals were grown by the solvent evaporation method (see Experimental section for details). SCXRD (Figure 2) confirmed the molecular formula of 1 obtained from ESI MS. The unit cell of 1 was found to contain four clusters (Figure S2) and reveals trigonal P-3c1 symmetry (Table S1), whereas crystals of 2 crystallized in the monoclinic C2/c space group.16

Pt Ag S P


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Chemistry of Materials Figure 2. Total structure of [PtAg28(BDT)12(PPh3)4] ─, NC1. C framework is displayed in gray; H atoms are not shown for clarity. Dotted orange arrows show the exposed parts of the cluster surface. 4

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Detailed structure analysis revealed that 1 consists of a Pt-centered Ag12 icosahedron core (Figure 3A), in sharp contrast to the face-centered cubic arrangement of PtAg12 core in 2. The average Pt-Ag and Ag-Ag distances in the PtAg12 core of 1 are 2.75 and 2.89 Å, respectively. The latter value is close to 2.88 Å in bulk Ag and suggests strong binding between the respective Ag atoms. Each of 12 additional Ag atoms connects to one Ag atom from the core, forming four trigonal prisms oriented tetrahedrally with four triangular faces of PtAg12 as their bases (Figure 3B). The prisms are roughly right [the pertinent angles are 108.74(9)–110.18(9)°] and their average height is elongated to 3.164 Å. The structure of 1 is stabilized by 3D AgPS3 and 2D bridging thiol motifs (Figure 3C and E, 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). Phosphorous atoms of AgPS3 motifs attach to Ag atoms via dangling bonds and form part of PPh 3 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).

Figure 3. Construction of PtAg28S24P4 in NC1: (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; (E) µ2 thiols bridge the edges of trigonal prisms to produce the total structure of (F) PtAg28S24P4. Color legends: Pt – blue; Ag – dark green, pink, and light blue; S – orange and yellow; P – green.

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 the parent cluster 2 (Figure S4) with similarly sized face-centered cubic (fcc) Pt1Ag12 core, but completely different Ag16S18P4 ligand-shell composed of four Ag4PS6 units sharing six S atoms. Thus, the core and the surface structure of PtAg28 clusters were clearly tuned by exchanging monothiolates with dithiolate ligands (Figures 2, 3 and S4). PL enhancement by ion pairing The successful synthesis and total structure determination of 1, a NC compositionally similar, but structurally different from a known NC2, allowed us to explore their structure-property relationship. The as-synthesized 1 exhibits a weak PL (QY: 1.5%) with a broad peak at 750 nm (PtAg28 on Figure 4A and its inset). Interestingly, after introducing tetraoctylammonium (TOA) counterions to 1 (denoted as NC1+TOA), the PL intensified (QY: 17.6%) as shown in Figure 4A inset, in addition to a blue-shift of the peak position to 680 nm. This blue-shift of PL peak was in agreement with the blue-shift of the absorption onset from ~690 nm to ~600 nm, which is due to rigidification of the ligand-shell24 in NC1+TOA (vide infra). We investigated into the mechanism of the PL enhancement via mass spectrometry. The ESI MS of NC1+TOA identified the peaks of the cluster with attachment of at least two TOA counterions (Figures 4B and S5). The decrease of cluster charge from 4- to 2- (for two TOA attachments) suggested electrostatic binding of TOA to 1. Moreover, 31P nuclear magnetic resonance (NMR) peaks systematically shift and broaden upon addition of TOA to 1 (Figure S6). These findings suggest that TOA ions are located close to the cluster surface and that they hinder the rotation of PPh3 ligands. Note that such hindered motion of ligands is known to be sufficient for significant PL enhancement in certain Au clusters.24, 35 The ion-pairing (NC1+TOA) was corroborated by a decrease in the Zeta potential of cluster from ─10.1 mV to +2.48 mV after TOA+ addition. Furthermore, the dichloromethane (DCM) solution of NC1+TOA ion-pairs were processed into thin films on substrates such as thin layer chromatography (TLC) plate and cellulose paper (Figure S7) and they were dried under ambient conditions. The NCs remained intact upon deposition and their intense PL could be perceived by the naked eye, suggesting their potential for applications in sensing metal ions and chemicals. Furthermore, NC1 was found to be thermally robust (stable in DMF solution at least for five hours at 80 0C in air), suggesting its possible applications in homogeneous catalysis.


Chemistry of Materials Intens. [%]

4─ [PtAg 28(BDT)12] 1224.9414

NC1+TOA

NC1

B

80

PL Intensity

NC1 NC1+TOA

Absorbance

A

60

3─ [PtAg 28(BDT)12(TOA)] 1788.7819

400

C PL Intensity

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600

800

Wavelength (nm)

1000 40

NC1+TOA NC1+TBA NC1+CTA NC1+TPP

20

NC1

500

[PtAg28(BDT)12(TOA)2]2─

600

700

Wavelength (nm)

800

0 2000

2000

3000

3000

m/z

4000

4000

5000

5000

6000

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

Other counterions, such as tetrabutylammonium (TBA), cetyltrimethylammonium (CTA) and tetraphenylphosphonium (TPP) were also found to enhance PL intensity of 1. The magnitude of the enhancement was higher for bulkier counterions: TOA > TBA > CTA > TPP (Figure 4C). In contrast, much smaller tetrametylammonium (TMA) counterions were found to have minor effect on PL intensity (Figure S8). Density functional simulations shed light on the atomistic details of ion pairing with the core charge, the mechanism of PL enhancement by ion pairing and rationalized why bulkiness of counterions matters. First, we simulated NCs 1 and 2 using density functional methods coupled with 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 the highest in the vicinity of partially exposed surface facets of 1, whereas 2 attracts counterions to its vertices 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 SI and illustrated on Figure S10). Very small TMA and mediumsized TPP counterions are calculated to occupy ν = 29 % and ν = 54% of the free volume around are a core facet of 1 (Table S2). We also performed such simulations for [NMe4-xOctx]+ (x = 1 to 3, Me: methyl, Oct: octyl)

counterions of increasing bulkiness (see SI). [NMe4+ xOctx] are calculated to occupy ν = 60 %, ν = 76 % and ν = 89 % of the free volume around a core facet of 1 for x = 1 to 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 SI 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 non-bulky 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 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).


7000

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Chemistry of Materials 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, [PtAg28(S-Adm)18(PPh3)4]2+,16 [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 TOA 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 5folds, respectively, when compared to NC3+TOA (Figure S16A).

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

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 significant amount of the free volume and rigidify the ligand-shell. In turn, the rigidness of the ligand-shell is known to enhance the PL of gold clusters24, 34, 35. For Agbased clusters such an observation along with mechanistic details are 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 NC1 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 observed between TPP and [Ag44(SR)30]4-,36 despite its similar charge with 1 (Figure S14). Also [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

DFT simulations confirm that the ion-pairing ability of NC3 is significantly weaker than that of NC1, i.e., that binding of counterions to 3 is significantly less exothermic than 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, NC1 could attract four TPP counterions only in solvents with dielectric constant ε < ~10. This finding is in line with the observation of PL enhancement by ion pairing only in the relatively nonpolar solvent DCM, with ε = 8.93 (Figure 4C), but not in the more polar solvent dimethylformamide (DMF), with ε = 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 high 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.

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 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 ten times to 17.6%. DFT reveal that bulky counterions attracted by the cluster with partially exposed surface occupy a significant volume in the cluster vicinity 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 creating similar structurally different pairs of NCs with the same size and composition for other metals as well as further development of NCs with such novel properties 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.

EXPERIMENTAL SECTION Materials. The chemicals including silver nitrate (AgNO3), hexachloroplatinic acid (H2PtCl6·6H2O), 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 Millipore apparatus. 4─

Synthesis and purification of [PtAg28(BDT)12(PPh3)4] NCs (1). About 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 minimum methanol. Crystallization of NC1. The as-prepared dark orange solution (500 µL) of 1 in acetone was transferred to a glass tube and then it was placed in a small closed box. The solvent was allowed to evaporate slowly in the dark. After four 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, two equivalents of NaBPh4 and TPPB were added to one equivalent each of [PtAg28(S-Adm)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 four and three equivalents of the desired counterions (e.g., TOA, TBA, TPP and CTA), respectively.

COMPUTATIONAL SECTION

VASP.47 Geometry optimization of ion-pairs and their components was performed using PBE48 exchange correlation functional augmented with D3 Grimme corrections,49 plane wave basis set with 400 eV cut-off energy, and projected augmented wave technique50 to treat core electrons until forces on atoms became less than 0.3 eV/nm. The separation between adjacent clusters was larger than 1 nm. Poisson-Boltzmann equation was solved self-consistently with DFT using VASPSol51 for ε(DCM) = 8.93 and Debye length of 0.3 nm (only for data in Figures 5 and ST1). Potential energy profile of PPh3 rotation is calculated through a constrained geometry optimization (see SI). Molecular dynamics. 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 SI). ADF. Binding energies 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).

ASSOCIATED CONTENT Details of the synthesis of various NCs, SCXRD of [PtAg28(BDT)12(PPh3)4]4─ NC (1), characterization of ion-pairs through ESI MS, UV-vis, PL as well as details of performed simulations (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] [email protected] [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT 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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Jin, R.; Zeng, C.; Zhou, M.; Chen, Y. Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities. Chem. Rev. 2016, 116, 1034-10413. Joshi, C. P.; Bootharaju, M. S.; Bakr, O. M. Tuning Properties in Silver Clusters. J. Phys. Chem. Lett. 2015, 6, 3023-3035. Chang, W. T.; Lee, P. Y.; Liao, J. H.; Chakrahari, K. K.; Kahlal, S.; Liu, Y. C.; Chiang, M. H.; Saillard, J. Y.; Liu, C. W. Eight-Electron Silver and Mixed Gold/Silver Nanoclusters Stabilized by Selenium Donor Ligands. Angew. Chem. Int. Ed. 2017, 56, 10178-10182. Salorinne, K.; Malola, S.; Wong, O. A.; Rithner, C. D.; Chen, X.; Ackerson, C. J.; Hakkinen, H. Conformation and dynamics of the ligand shell of a water-soluble Au102 nanoparticle. Nat. Commun. 2016, 7, 10401. Wan, X. K.; Cheng, X. L.; Tang, Q.; Han, Y. Z.; Hu, G.; Jiang, D. e.; Wang, Q. M. Atomically Precise Bimetallic Au19Cu30 Nanocluster with an Icosidodecahedral Cu30 Shell and an Alkynyl–Cu Interface. J. Am. Chem. Soc. 2017, 139, 9451-9454.


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Chemistry of Materials

Table of Contents artwork

Counterion Ion pairing Nanocluster with exposed surfaces

Ligand-shell rigidification Fluorescence enhancement


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Tailoring the Crystal Structure of Nanoclusters Unveiled High

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