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Synthesis and Optical Properties of a Dithiolate/ Phosphine-Protected Au Nanocluster 28

Maha A. Aljuhani, Megalamane S. Bootharaju, Lutfan Sinatra, Jean Marie Basset, Omar F. Mohammed, and Osman M. Bakr J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10205 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 17, 2016

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The Journal of Physical Chemistry

Synthesis and Optical Properties of a Dithiolate/PhosphineProtected Au28 Nanocluster Maha A. Aljuhani,†,‡ Megalamane S. Bootharaju,†,$,‡ Lutfan Sinatra,† Jean-Marie Basset,$ Omar F. Mohammed,† and Osman M. Bakr*,† †

KAUST Solar Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia $

KAUST Catalysis Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia ABSTRACT: While monothiols and simple phosphines are commonly exploited for size-controlled synthesis of atomically precise gold nanoclusters (NCs), dithiols or dithiol-phosphine combinations are seldom applied. Herein, we used a dithiol (benzene-1,3dithiol, BDT) and a phosphine (triphenylphosphine, TPP) together as ligands and synthesized an atomically precise gold NC with the formula [Au28(BDT)4(TPP)9]2+. This NC exhibited multiple absorption features and a charge of +2, which are distinctly different from the reported all-thiolated [Au28(SR)20]0 NC (SR: monothiolate). The composition of [Au28(BDT)4(TPP)9]2+ NC was deduced from high-resolution electrospray ionization mass spectrometry (ESI MS) and it was further corroborated by thermogravimetric analysis (TGA). Differential pulse voltammetry (DPV) revealed a HOMO-LUMO gap of 1.27 eV, which is in good agreement with the energy gap of 1.3 eV obtained from its UV-Vis spectrum. The successful synthesis of atomically precise, dithiol-protected Au28 NC would stimulate theoretical and experimental research into bidentate ligands as a new path for expanding the library of different metal NCs, which have so far been dominated by monodentate thiols.

1. INTRODUCTION Ligand-protected atomically precise noble metal nanoclusters (NCs) have been the subject of great interest in the present time not only due to fundamental scientific research, but also due to their fascinating optical, photophysical, electronic, and chemical properties. 1-14 These unique properties – differing entirely from plasmonic nanoparticles (NPs) and bulk metals – made NCs potential candidates for applications in catalysis, 15,16 sensing,17 bioimaging,18,19 energy,20,21 and environment remediation22,23. Particularly, on gold, intense research efforts have culminated in the discovery and characterization of a large number of NC sizes such as Au 11,24 Au18,25,26 Au25,27 Au36,28 Au38,29 Au55,30 Au102,31 Au133,32,33 – to name a few, while similar developments in synthesis of other noble metal NCs such as silver and copper, have more recently commenced after the single crystal X-ray diffraction (XRD) characterization of a few Ag, Cu and other metal NCs including Ag21,34 Ag25,35 Ag29,36 Ag44,37,38 Ag67,39 Ag136,40 Ag374,40 Cu18,41 Cu25,41 Cu3242 and Pd5543. Inspired by these monometallic precise compositions, multimetallic clusters44,45 were also exploited to some extent for acquiring synergistic properties that led to substantial enhancement in photoluminescence 46-48 and catalytic activity44. The space of ligand modification on the surface of metal NCs can further be broadened by looking beyond conventional pure thiol or phosphine ligands. However, in addition to thiols 36 and phosphines 49,50, other classes of monodentate stabilizers including alkynyl, 51

selenolate,52,53 and carbonyl54,55 have also been utilized to functionalize NCs. Significant advances have been made to obtain thiolate-protected NCs,56 particularly monothiolated ones,3,57,58 but dithiol ligands for the formation of NCs are rarely used,36,59 because they do not usually yield discrete stable clusters. As a result, there may be many dithiol-protected metal NCs remaining to be unearthed. The relative rigidity of dithiol ligands, in relation to monothiols, presents an opportunity to enhance the stability of the NC. However, this rigidity creates geometric restrictions and insolubility issues that may destabilize the NCs. Therefore, the use of a phosphine as a co-ligand may remediate the latter drawbacks, as was observed in a recent report in silver NCs, where the used triphenylphosphine (TPP) stabilized and increased the yield of benzene-1,3-dithiol (BDT)-protected silver NC, a superatom with a formula of [Ag29(BDT)12(TPP)4]3-.36 With this motivation in mind and the lack of pure dithiolprotected gold clusters, in this work, we explored the synthesis of gold NCs using BDT and TPP. We find that BDT alone did not produce gold NCs. However, when we introduced TPP as a co-ligand to BDT, in the form of AuClTPP (acting as a source for both gold and TPP, vide infra), we obtained a 28-atom gold NC protected with four BDT and nine TPP ligands i.e., [Au28(BDT)4(TPP)9]2+. The 2+ charge-state of this cluster is in contrast to the charge-state of the reported Ag29 cluster i.e., 3-. Furthermore, the absorption spectrum and HOMO-LUMO gap of the Au28 cluster were also compared with those of an all-monothiol-protected

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The Journal of Physical Chemistry [Au28(SR)20]0 cluster,60 which suggested electronic and structural variations between the two clusters as a result of different ligand protection. 2. EXPERIMENTAL METHODS 2.1. Chemicals All chemicals including chloro(triphenylphosphine)gold (I) (AuClPPh3 or AuClTPP), tetrachloroauric(III) acid trihydrate (HAuCl4 .3H2O), sodium borohydride (NaBH4, 99.99% metals basis), benzene-1,3-dithiol (BDT) and tetrabutylammonium hexafluorophosphate (Bu4NPF6) were purchased from SigmaAldrich and used without further purification. Solvents including methanol, dichloromethane (DCM), and tetrahydrofuran (THF) were received from Sigma-Aldrich. Distilled water (H2O) was collected from Milli-Q (Millipore apparatus). 2.2. Synthesis of [Au28(BDT)4(TPP)9]2+ NCs AuClTPP (60 mg) was dissolved in THF (10 mL) and BDT (27 µL) was added to this solution under magnetic stirring. After five minutes, NaBH4 (10.5 mg in 0.5 mL of Milli-Q water) solution was added and the reaction was continued for 3 h. The organic solvent was removed using a rotary evaporator. Then, the obtained precipitate of NCs was washed with methanol for several times to remove the excess ligands. This NC can also be synthesized in a mixture of DCM (10 mL) and methanol (5 mL) by maintaining the quantities of Au and BDT precursors as mentioned above. We attempted to synthesize Au NCs with pure BDT protection by following the above synthesis conditions by replacing AuClTPP with HAuCl 4 .3H2O. However, BDT alone did not form gold clusters as the reaction solution remained colorless for more than two days after NaBH4 reduction of a mixture of HAuCl4 .3H2O and BDT. 2.3. Characterization The UV-Vis absorption spectra of NCs were recorded using a Cary 5000 UV-Vis-NIR spectrophotometer (Varian Inc.). Mass spectrometry of Au28 cluster was carried out using a Bruker MicroTOF-II mass spectrometer. The NC was dissolved in DCM or THF and electrosprayed using a stainless steel needle syringe. The instrument parameters used were of mass range: 100-10000 Da; capillary voltage: 3.0-4.0 kV; sample flow rate: 500 μL/h; dry gas temperature: 80-110 °C; nebulizer gas: 0.1-0.4 bar, and dry gas: 0.5-1.4 L/min. Thermogravimetric analysis (TGA) was conducted using a TA Q500 apparatus. The NC was heated at a rate of 10 0C/minute up to 1000 0C under N2 flow (50 mL/minute). Energy dispersive X-ray analysis (EDS) was performed using EDAX analysis equipped in SEM quanta 200 from FEI, Inc. The clean powder of Au28 NC was deposited on a double-sided carbon tape with aluminum foil inside. Differential pulse voltammetry (DPV) was performed using Gamry Reference 3000 potentiostat in 0.1 M Bu4NPF6 dichloromethane solutions that were degassed for 15 minutes and then blanketed with N2 during the measurement. The working electrode was 3 mm Platinum electrode. Reference and counter electrodes were Ag quasi reference and Platinum wire electrode, respectively. The working electrode was polished with Al2O3 slurries followed by sonication in Milli-Q water. Au28 NCs solution with concentration around 5 mg/mL was used for the measurement at 0-5 0C using an ice-bath.

3. RESULTS AND DISCUSSION The purified product of Au NCs was dissolved in dichloromethane (DCM) and its solution appeared brown (inset of Figure 1). The optical absorption spectrum (Figure 1) of this solution showed multiple peaks at 450, 580, and 725 nm, which are characteristic to molecular NPs. The absence of a sharp UV-Vis peak around 520 nm for plasmonic gold NPs and the observed molecular optical signatures together suggest that the synthesized product is indeed a NC.

0.20 0.15 Absorbance

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0.10 0.05 0.00

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Figure 1. UV-Vis absorption spectrum of [Au28(BDT)4(TPP)9]2+ NCs dissolved in DCM. Inset is a photograph of the DCM solution of [Au 28(BDT)4(TPP)9]2+ NCs.

To determine the molecular formula of the NC, highresolution electrospray ionization mass spectrometry (ESI MS) was performed. ESI MS of the cluster, in positive ion mode, shows a predominant peak at m/z 4218 along with two other less predominant peaks at m/z 4087 and 3956 (Figure 2A). In addition to these peaks, [Au(TPP)2]+ and [Au3(BDT)1(TPP)3]+ complexes were identified in low-mass range (Figure S1 in the Supporting Information). An expansion of m/z 4218 peak showed multiple peaks separated by m/z 0.5, indicating the charge of the cluster to be 2+. After several trials to match the m/z 4218 peak with possible compositions by considering the Au, TPP, and BDT, we arrived with a best possible formula for the cluster, [Au28(BDT)4(TPP)9]2+. The proposed composition of the cluster was further verified by comparing the experimental mass spectrum with the simulated one for [Au28(BDT)4(TPP)9]2+, which shows a perfect match with one another (Figure 2B). We also investigated the origin of other peaks at m/z 4087 and 3956. The species corresponding to those peaks are also doubly charged. The difference between consecutive peaks at m/z 4218, 4087 and 3956 corresponds to m/z 131. The total mass of m= 262 (i.e., 131×2) is equal to the molecular mass of single TPP. From this observation, it is clear that the peaks down to [Au28(BDT)4(TPP)9]2+ originated from the parent cluster of [Au28(BDT)4(TPP)9]2+ by a sequential loss of TPP ligands, which is a common feature in Ag and Au clusters containing labile phosphines. 39,61,62 However, we confirmed the composition of the fragment [Au28(BDT)4(TPP)8]2+ by comparing its simulated spectrum with the experimental one (Figure 2C). The exact


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match of these spectra is a further validation that [Au28(BDT)4(TPP)8]2+ is actually a fragment of the [Au28(BDT)4(TPP)9]2+ cluster.

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Au28(C6H4S2)4(C18H15P)9, M ,8433.79

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Figure 2. (A) ESI MS of [Au28(BDT)4(TPP)9]2+ NCs in positive mode. B and C are comparisons of experimental and corresponding simulated mass spectra of [Au28(BDT)4(TPP)9]2+ NC and its fragment, [Au28(BDT)4(TPP)8]2+. Note: experimental spectra were shifted by m/z: -0.11 to match with corresponding simulated spectra. 60

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Figure 3. TGA curve of [Au28(BDT)4(TPP)9]2+ cluster.

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The electrochemical properties of [Au28(BDT)4(TPP)9]2+ NCs were studied (see characterization section for details) by differential pulse voltammetry (DPV) to estimate the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap. From the DPV result (Figure 4), several peaks in the positive and negative potential regions could be assigned to oxidation and reduction peaks, respectively. The first oxidation peak (O1) and the first reduction peak (R1) of [Au28(BDT)4(TPP)9]2+ clusters are at +0.92 and 0.61 V, respectively. This difference between O1 and R1 is the electrochemical gap of the NCs, which is 1.53 V. The actual HOMO-LUMO gap can be obtained by subtracting the electrochemical gap from the charging energies.63 Charging energies are estimated from the difference between oxidation peak 1 and 2 (O2-O1= 1.18-0.92 = 0.26 V). Therefore, the HOMOLUMO gap of Au28 cluster is estimated to be 1.27 V. This value is in good agreement with the optical gap (1.3 eV or 953 nm) obtained from UV-Vis spectrum by extrapolating the absorbance to zero i.e., absorption onset (Figure 1).

The composition of the [Au 28(BDT)4(TPP)9]2+ cluster was additionally verified by thermogravimetric analysis (TGA). A total weight loss of 34.54% was observed (Figure 3) corresponding to the total ligand content of four BDT and nine TPP. The close match of the experimental (34.54%) and the estimated (34.62%) ligand content of the [Au28(BDT)4(TPP)9]2+ cluster corroborates the assigned composition of the cluster. Energy dispersive Xray spectroscopy (EDS) analysis of [Au 28(BDT)4(TPP)9]2+ powder was performed to gain more insight into the composition of the synthesized NCs. The EDS analysis (Figure S2) showed the presence of Au, S, P and Cl elements, consistent with the formula of the cluster determined using ESI MS (Figure 2) and TGA (Figure 3). The presence of Cl suggests the possibility of Cl ─ as a counterion of the [Au28(BDT)4(TPP)9]2+ cluster, in which the exact number of Cl- ions needs to be verified by the single-crystal X-ray crystallographic studies in our future work. Figure 4. DPV of [Au28(BDT)4(TPP)9]2+ clusters.

It is worth noting that an all-monothiol-stabilized [Au28(SR)20]0 cluster was reported including its crystalstructure.60 However, [Au28(BDT)4(TPP)9]2+ NC is predicted to exhibit different atomic arrangements from [Au28(SR)20]0 as both of these NCs displayed distinctly different absorption features (Figure S3) and unique charge states. The HOMO-



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LUMO gap of [Au28(BDT)4(TPP)9]2+ i.e., ~1.3 eV, which was found to be lower than that of [Au28(SR)20]0 i.e., ~1.78 eV. It is also interesting to note that the combined use of BDT and TPP ligands resulted in the superatomic anionic cluster of [Ag29(BDT)12(TPP)4]3- with silver36, while these ligands form a cationic cluster of [Au28(BDT)4(TPP)9]2+ with gold (in this work). This study clearly reveal the use of phosphine coligands can offer unique pathways to tune the NC size, charge, structure, and properties of different metals at a defined atomic level. 4. CONCLUSIONS In summary, we designed a synthetic strategy involving a dithiol and a phosphine as ligands to successfully synthesize a cationic gold NCs with a molecular formula [Au28(BDT)4(TPP)9]2+. The composition of the cluster was determined through ESI MS and it was further characterized using TGA and elemental analysis. DPV was used to estimate the HOMO-LUMO gap of the cluster, which was in good agreement with its optical energy gap. The optical spectrum of the [Au28(BDT)4(TPP)9]2+ cluster is markedly different from the all-monothiol-protected [Au28(SR)20]0 cluster, indicating key structural differences, which need to be verified by crystal-structure determination. Both of these Au28 clusters with different ligand coverage provide a window to compare structure-dependent properties and understanding their origins both experimentally and computationally.

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ASSOCIATED CONTENT

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Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. ESI MS, EDS and UV-Vis absorption spectra of Au28 NCs.

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AUTHOR INFORMATION

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Corresponding Author *E-mail: [email protected]

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Author Contributions ‡M.A.H.

and M.S.B. contributed equally to this work. (19)

Notes The authors declare no competing financial interest. (20)

ACKNOWLEDGMENT The work reported here was supported by KAUST. (21)

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