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Crystal and Solution Photoluminescence of MAg (SR) (M = Ag/Pd/Pt/Au) Nanoclusters and Some Implications for the Photoluminescence Mechanisms 24

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Xu Liu, Jinyun Yuan, Chuanhao Yao, Jishi Chen, Lingling Li, Xiaoli Bao, Jinlong Yang, and Zhikun Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01730 • Publication Date (Web): 07 Jun 2017 Downloaded from http://pubs.acs.org on June 15, 2017

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The Journal of Physical Chemistry

Crystal and Solution Photoluminescence of MAg24(SR)18 (M = Ag/Pd/Pt/Au) Nanoclusters and Some Implications for the Photoluminescence Mechanisms Xu Liu†,‡, Jinyun Yuan‡, Chuanhao Yao†, Jishi Chen†, Lingling Li§, Xiaoli Bao§, Jinlong Yang*,‡, Zhikun Wu*,† †

Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, CAS Center for Excellence in Nanoscience, Chinese Academy of Sciences, Hefei 230031, China ‡ Department of Chemistry, University of Science and Technology of China, Hefei 230026, China § Instrumental Analysis Center, Shanghai Jiaotong University, Shanghai 200240, China Supporting Information Placeholder ABSTRACT: Herein we prepare a series of MAg24(SR)18 (M = Ag/Pd/Pt/Au) nanoclusters (NCs) with similar core-innershelloutershell structures and investigate their crystal and solution photoluminescence. The core silver atom replacement by the Pd/Pt/Au atom obviously tunes the geometric and electronic structures of Ag25(SR)18 NC. The crystal photoluminescence intensities sequence hints a core-atom-directing charge transfer from the ligands to the metal kernels. Both the calculated NPA charge and the measured Aginnershell-Sterminal bond length support the proposed mechanism. Further experiments show the solvent influence on the NCs photoluminescence supported by the blueshift of emissions of MAg24(SR)18 NCs and the solventdependent photoluminescence intensity sequences. Especially, for PtAg24(SR)18, the quantum yield is almost 100-fold greater in CH3CN (18.6%) than in CH2Cl2 (0.2%). However, the emission wavelengths of the series of NCs are barely influenced by the solvent type. This work indicates the importance of the core atom and the solvent to the photoluminescence of core-innershelloutershell silver NCs, having important implications for the photoluminescence mechanisms and tuning of noble metal nanoparticles.

INTRODUCTION

The recent emergence of thiolated metal nanoclusters (NCs) with well-defined compositions and structures provides opportunity for in-depth understanding the photoluminescence origin of metal nanoparticles1-20. The photoluminescent NCs also shows great potential for application in a range of fields21-32, especially in biomedicine24,25,28,29,31 owing to their ultrasmall sizes (< ~ 3 nm for the metal diameter), good biocompatibility, easy modification, etc. However, the low photoluminescence quantum yields (generally < 10%) hamper the practical application. How and why can the NCs photoluminescence be enhanced constitutes a core issue for the current research of photoluminescent NCs, which has received the extensive attention in the past years13,26,30,33-39. Among various annotations to this fundamental issue, one major opinion is that the surface ligands can greatly influence the

photoluminescence at least by charge transfer from the ligands to the metal parts of the nanoparticles through the Au-S bonds4,9,10,40. Note that, for the metal-to-ligand or ligand-to-metal transition mechanisms of NC photoluminescence, it was first introduced by Whetten1,2, echoed by Jin10,11, Bakr18, Aikens19 et al. for optical analyses of clusters with structures determined by Xray analysis and by Pelton20, Goodson39, Knappenberger41 et al. for interpreting data for NCs without defined structures. Very recently, theoretical calculations conducted by Aikens and coworkers showed that the photoluminescence of gaseous Au25(SR)18- originates from kernel-based electron transitions rather than charge-transfer or semi-ring states14. Especially, it is unclear whether the mentioned mechanism is applicable to silver NCs or not. Besides, the solvent influence on the silver NCs photoluminescence has been rarely reported so far. To investigate these issues, a family of silver NCs formulated as MAg24(SR)18 (M = Ag/Pd/Pt/Au, SR = 2, 4-dimethylbenzenethiolate or 2, 4DMBT) was successfully prepared and their crystal and solution photoluminescence was carefully studied as shown below.

EXPERIMENTAL METHODS SYNTHESES. Ag25(SR)18: Silver nitrate (~ 80 mg) was first dissolved in 5 mL of methanol, after which 2,4dimethylbenzenethiol (190 μL) was injected. Subsequently, the reaction mixtures after the addition of 30 mL of CH2Cl2 was stirred for 30 min. under an ice-bath. A freshly prepared solution of PPh4Br (16 mg) was added prior to the drop-wise addition of ice-cold aqueous NaBH4 (40 mg) under stirring. The color of the solution changed slowly from light yellow to dark, and the reaction was carried out for 6 h in an ice-bath. Then the resultant solution was centrifuged, and the resulting supernatant was concentrated to ~ 5 mL through rotary evaporation under vacuum. The crude product was washed with methanol twice to remove excess ligands. MAg24(SR)18 (M = Pd/Pt): Initially, ~ 38 mg of AgNO3 was dissolved in 3 mL of methanol, followed by the addition of 90 μL of 2,4-dimethylbenzenethiol and 20 mL of CH2Cl2. Subsequently, 0.02 mmol of Pd2+ (or Pt4+, 0.1 eqv. of AgNO3) was added to the flask. The mixture was stirred in an icebath for 30 min. A freshly prepared solution of PPh4Br (7 mg in 0.5 mL of methanol) was added before the drop-wise addition of

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ice-cold aqueous NaBH4 (20 mg in 1 mL of DI water) under constant stirring, and the above reaction was carried out for 6 h. The purifying process of the crude product was similar to that of Ag25(SR)18. AuAg24(SR)18: About 20 mg of Ag25(SR)18 was dissolved in 5 mL of CH2Cl2, followed by the addition of 4 µL of CH2Cl2 solution of AuClPPh3 (8 mg/mL) under constant stirring. The reaction was continued for 4 h at ambient temperature, along with gradual change of the solution color from reddish brown to green. After the reaction, the purified process is identical to that of Ag25(SR)18.

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Figure 1. Structural anatomy of core-innershell-outershell Ag25(SR)18 NC: black = core Ag; green = innershell Ag; magenta = outershell Ag; yellow = S. C and H are omitted for clarity.

CHARACTERIZATION. For PdAg24(SR)18 and PtAg24(SR)18, the X-ray crystallography was performed on a Bruker D8 VENTURE CMOS photo 100 diffractometer with helios mx multilayer monochromator Cu Kα radiation (λ = 1.54178 Å), for AuAg24(SR)18, the X-ray diffraction was collected on a Smart APEX II instrument from Bruker AXS (Mo Kα, λ = 0.71073 Å). UV/vis/NIR spectra with a range of 190-900 nm were recorded on a UV2600 spectrophotometer. For DPV measurement, a conventional three-electrode system was employed for the experiments. Carbon rods and an SCE (with saturated KCl solution) electrode are used as a counter electrode (CE) and reference electrode (RE), respectively. Working electrode (WE) was a Pt disk electrode (1 mm in diameter). The experiment was carried out at 0.01 V/s in degassed CH2Cl2 containing 0.1 M Bu4NPF6 with 1 mm diameter Pt working, SCE reference, and carbon rod counter electrodes. Solid photoluminescence of the clusters were probed by Lifetime and Steady State Spectrometer (FLS 980, Edinburgh Instruments Ltd.) with a 450W xenon lamp as the excitation source, the step and dwell time was set at 1 nm and 0.1 s, respectively, and the slit for both excitation and emission monochromators was kept at 15 nm. Photoluminescence properties of the clusters in solution were characterized on F-4600 FL-Spectrophotometer (Hitachi High-Technologies Corporation, Japan) with a xenon lamp as the excitation source, the scan speed was set at 1200 nm/s, and the band pass for both excitation and emission monochromators was kept at 10 nm. Before the test, all dilute solutions were calibrated based upon the UV/vis/NIR spectra to avoid AIE effect12, i.e., the absorption value of every sample at 470 nm was below 0.1 in optical spectrum (Table S2). Photoluminescence images of the as-prepared NCs were carried out on A1Rsi laser confocal scanning microscope imaging system.

Figure 2. a) Structural framework of MAg24(SR)18 (M = Ag/Pd/Pt/Au) NCs: yellow = S; green and black = Ag; blue= Pd; red = Pt; magenta = Au. C and H are omitted for clarity. b) Photoluminescence spectra of MAg24(SR)18 (M = Ag/Pd/Pt/Au) NCs in crystal state. X-ray crystallography suggests that MAg24(SR)18 (M = Ag/Pd/Pt/Au) NCs adopt homogenous core-innershell-outershell structures, see Figure 1 for the anatomy of this kind of structure with Ag25(SR)18 (Note: AgAg24(SR)18 is written as Ag25(SR)18 for convenience) as an example. Their structures can be briefly described as an icosahedral M@Ag12 kernel (core-innershell) capped by six Ag2(SR)3 dimeric staples (outershell), as shown in Figure 2a and S2, indicating that mono-doped MAg24(SR)18 retains the framework of Ag25(SR)18 with the core Ag atom substituted by the Pd/Pt/Au atom. Note that, for mono-Au/Pt doped MAg24(SR)18, it is not difficult to ascertain the central position of the heteroatom in the structure of doped NCs using Xray analysis owing to the evident discrepancy between Ag and Au/Pt. For PdAg24(SR)18, however, it is not easy to judge the doped position of Pd atom only by X-ray analysis because of high similarity between Pd and Ag. Fortunately, Zheng’s group manifested mono-Pd/Pt centrally doped MAg24(2,4-SPhCl2)18 NCs using X-ray crystallography coupled with theoretical calculations49. By comparing the UV/vis/NIR spectra (see Figure S3) and crystal structures between MAg24(2,4-SPhCl2)18 and MAg24(2,4-DMBT)18 (M = Pd/Pt), respectively, we can conclude that the Pd atom is embedded in the core of the PdAg24(2,4DMBT)18 structure.

COMPUTATIONAL METHODS. All calculations were carried out using Density Functional Theory (DFT) with the pure functional PBE42,43, and all electron basis set 6-31g (d, p) for H, C, and S, pseudopotential basis set LANL2DZ for Ag, Au, Pd and Pt as implemented in the Gaussian 09 program package. The PBE functional has been verified to be reliable for gold cluster systems44,45. Natural population analysis (NPA) was performed with the natural bond orbital (NBO) program (version 3.1) 46.

RESULTS AND DISCUSSION

Although mono-doped MAg24(SR)18 adopts a structural framework akin to Ag25(SR)18, the M@Ag12 icosahedron experiences a slight variation compared with Ag@Ag12. As shown in Table S1, the average Aucore-Aginnershell distance (2.7702 Å) in Au@Ag12 is a little longer than the Agcore-Aginnershell distance (2.7643 Å) in Ag@Ag12, while the average Pdcore-Aginnershell (2.7495 Å) in Pd@Ag12 and Ptcore-Aginnershell (2.7541 Å) in Pt@Ag12 are both slightly shorter than the average distance of Agcore-Aginnershell. Undoubtedly, this variation of metal bond length originated from the central atom substitution. This variation is further illustrated by the comparisons of Aginnershell-Aginnershell distance between the doped M@Ag12 icosahedrons and homogeneous Ag25(SR)18 (see Table S1). Meanwhile, the replacement of the central Ag atom by Au/Pt/Pd causes a notable change in electronic structure of Ag25(SR)18, testified by

Two reported synthetic protocols with some modifications were employed to prepare these silver NCs47-49. In brief, MAg24(SR)18 (M = Ag/Pd/Pt) NCs were obtained by respectively reducing Agthiolate complexes with or without the existence of Pd/Pt salt, which is called one-pot synthesis, whereas AuAg24(SR)18 NCs with molecular purity was obtained by reacting the as-prepared Ag25(SR)18 with AuClPPh3, which is called galvanic reduction by Bakr et al47. All the MAg24(SR)18 NCs were crystallized in the mixture solvents of dichloromethane/hexane.

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Although an abundance of research effort was devoted to photoluminescence properties of noble metal NCs, surprisingly little attention has been paid to the solvent influence. To probe this, five solvents (including toluene, dichloromethane, acetone acetonitrile and dimethylsulfoxide, which polarity is increased in order) are chosen for the investigation. It is found that both the emission wavelength and the emission intensity sequence are changed when solvents are involved. For example, in acetonitrile, blueshifts of ~ 200 nm were observed for the maximum emission wavelengths of MAg24(SR)18 NCs in contrast to those in crystal state, and the emission intensity sequence is as follows: PtAg24(SR)18 > AuAg24(SR)18 > PdAg24(SR)18 > Ag25(SR)18, see Figure 3, differing from their crystal photoluminescence sequence. To our surprise, when the solvent acetonitrile is substituted by another solvent, the photoluminescence intensity sequence varies, whereas the emission peaks are almost unchanged relative to those in acetonitrile, as shown in Figure S7-10 (for dichloromethane and toluene, the photoluminescence intensity sequences become AuAg24(SR)18 > Ag25(SR)18 > PtAg24(SR)18 > PdAg24(SR)18 and PtAg24(SR)18 > AuAg24(SR)18 > Ag25(SR)18 > PdAg24(SR)18, respectively).

UV/vis/NIR spectra, DPV and HOMO-LUMO calculations together (see Figures S3-4,6). The structure similarity among MAg24(SR)18 NCs makes it possible to study the core atom influence on the photoluminescence of the 25-metal-atom NCs. To avoid any possible interference, the crystal photoluminescence of these silver NCs was examined by Lifetime and Steady State Spectrometer (see the supporting information for the details). To the best of our knowledge, this is the first time to characterize the crystal photoluminescence of doped metal NCs. Interestingly, the four NCs exhibit IR photoluminescence centered at the range from 1014 to 1042 nm (the corresponding energy ranges from 1.17 to 1.21 eV), see Figure 2b and S5, which may correlate to the HOMO-LUMO transition (the calculated HOMO-LUMO gaps vary from 1.38 to 1.59 eV, see Figure S6), the discrepancy between the photoluminescence energy and the HOMO-LUMO gap may be due to calculation deviation, some energy loss during the excited-state relaxation process[5], etc. The doping of Ag25(SR)18 by Pd/Pt/Au heteroatom leads to a marginal blueshift of the maximum emission without remarkable discrepancy among various doped cases (for Pd/Pt/Au doping, the blueshift is 28/32/23 nm, respectively). On the other hand, the heteroatom doping induces a distinct change of photoluminescence intensity of Ag25(SR)18: the Au/Pt doping enhances the photoluminescence intensity by ~ 270%/83%, while the Pd doping decreases the photoluminescence by ~ 40%. The photoluminescence intensities of MAg24(SR)18 NCs follow a sequence of PdAg24(SR)18 < Ag25(SR)18 < PtAg24(SR)18 < AuAg24(SR)18, and such a sequence is in good agreement with the order of the electron affinity (i.e., the capability of attracting electron) of the core atom，see Table 1, hinting that the core-atom-directing charge transfer might contribute to the emission intensity of these NCs. Note that, the correlation between solid-state photoluminescence intensities and electronegativities might be coincidental in case that dipole-dipole coupling between the NCs is strong. Table 1. Electron affinity of the Pd/Ag/Pt/Au atom50,51. Element

Pd

Ag

Pt

Au

Electron affinity/eV

0.562

1.302

2.128

2.309

Figure 3. Photoluminescence spectra of MAg24(SR)18 (M = Ag/Pd/Pt/Au) NCs in acetonitrile. Undoubtedly, the above variation of photoluminescence intensity sequence is derived from the interaction between the solvent molecules and the NCs molecules, i.e., the solvent imparts influence to the photoluminescence of MgAg24(SR)18 NCs. Especially, the photoluminescence of PtAg24(SR)18 shows dramatic solvent effect: the quantum yield is almost 100-fold greater in acetonitrile (18.6%) than in dichloromethane (0.2%), as shown in Table 2. Besides, as mentioned above, these solvents scarcely vary the maximum emission wavelengths but remarkably influence the photoluminescence intensities, and the most intensive photoluminescence is achieved in acetonitrile for every NC, see Figures S11-14, indicating that the photoluminescence intensity is not solely determined by the solvent polarity53 and the solution photoluminescence is rather complex. Further experiments indicate the emissions of PtAg24(SR)18 and AuAg24(SR)18 are slightly dependent of the excitation wavelengths ranging from 400 to 580 nm, see Figure S15-16. The repeated observations that these four NCs have close emission wavelengths but obviously different photoluminescence intensities in both crystal and solution indicate that herein the excited-state relaxation dynamics rather than the excited-state relaxation thermodynamics might mainly account for the emission intensity. Of note, among the four NCs, PtAg24(SR)18 shows the most extensive photoluminescence in solution except for the case that dichloromethane is employed as the solvent (see Figure S13), and even it can be imaged in the visible region not only in crystal state but also in acetonitrile while others can’t, see Figure 4, indicating

Theoretical calculations show that the overall NPA charges of the Ag12 innershells are 1.31, 1.35, 1.61 and 1.66 for PdAg24(SR)18, Ag25(SR)18, PtAg24(SR)18 and AuAg24(SR)18, respectively. Importantly, the NPA charge sequence is also consistent with the photoluminescence intensity sequence, which in another aspect provides clue for the charge transfer from the ligands to the M@Ag12 kernel via Ag-S bonds in the emission (the high NPA charge value means high affinity to the delocalized electrons of bonding sulfur). The comparison of Aginnershell-Sterminal distances between the innershell Ag atom and the terminal S atom in the Ag2(SR)3 staple indeed affords double support for this proposal. It is revealed by single crystal X-ray diffraction that the mentioned Aginnershell-Sterminal bond lengths average 2.4987, 2.4794, 2.4771 and 2.4690 Å for PdAg24(SR)18, Ag25(SR)18, PtAg24(SR)18 and AuAg24(SR)18, respectively, thus the Ag-S bond length sequence (PdAg24(SR)18 > Ag25(SR)18 > PtAg24(SR)18 > AuAg24(SR)18) is exactly opposite to the photoluminescence intensity sequence (PdAg24(SR)18 < Ag25(SR)18 < PtAg24(SR)18 < AuAg24(SR)18). And it is known that short bond length generally means strong interaction and is beneficial to the charge-transfer of delocalized electrons52. Taken together, these results demonstrate that the core atom greatly influences the photoluminescence intensity of 25-metal-atom NCs, and provide novel evidences for the charge-transfer photoluminescence mechanism1,2,4,9-11,18-20,40.

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and the surrounded solvent and has important implications for the photoluminescence mechanisms and tuning of noble metal nanoparticles.

that PtAg24(SR)18 is of the biggest potential for practical application among the four NCs. Table 2. The quantum yield of MAg24(SR)18 (M = Ag/Pd/Pt/Au) NCs in different solvents ( calibrated to Au24(SCH2Ph)20 ) 30. Solvent

Ag25

PdAg24

PtAg24

AuAg24

Toluene Dichloromethane Acetone Acetonitrile Dimethylsulfoxide

0.3% 0.6% 0.2% 0.6% 0.2%

0.04% 0.05% 0.3% 0.7% 0.4%

5.3% 0.2% 8.6% 18.6% 12.3%

3.1% 3.5% 1.8% 3.2% 1.0%

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Experimental details and data (PDF). X-ray Crystallographic data of PdAg24(SR)18, PtAg24(SR)18, AuAg24(SR)18(CIF).

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

Author Contributions X.L., J.-Y.Y. contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We acknowledge Natural Science Foundation of China (nos. 21171170, 21222301, 21528303, 21603234, 21601193, 21501182, 21401064), National Basic Research Program of China (grant no. 2013CB934302), the Ministry of Human Resources and Social Security of China, the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2014FXCX002), Hefei Science Center, CAS (user of potential: 2015HSCUP003), the CAS/SAFEA International Partnership Program for Creative Research Teams and the “Hundred Talents Program” of the Chinese Academy of Sciences.

Figure 4. Photoluminescence images of Ag25(SR)18 (a) (e), PdAg24(SR)18 (b) (f), PtAg24(SR)18 (c) (g), AuAg24(SR)18 (d) (h) NCs in crystal state and in acetonitrile,respectively. The inset is bright field image. Note that, the color shown in (c) (d) (g) is not the real case.

CONCLUSIONS In summary, we prepared a family of MAg24(SR)18 (M = Ag/Au/Pt/Pd) NCs adopting similar core-innershell-outershell structures with different core atoms. The core doping of Ag25(SR)18 by the Pd/Pt/Au atom results in the tuning of geometric and electronic structures for Ag25(SR)18 NC, testified by the combination of X-ray crystallography, UV/vis/NIR spectra, DPV and HOMO-LUMO calculations. Especially, a phenomenon contrary to the John-Teller-like effect was observed in the geometric tuning of Ag25(SR)18 NC by doping. For the first time, the crystal photoluminescence of doped metal NCs was investigated. It is found that the emission peaks slightly blueshift after the core Ag replaced by Pd/Pt/Au, but the photoluminescence intensities of MAg24(SR)18 NCs in crystal state notably increase with the increase of the core atom electron affinity, which may imply a charge-transfer photoluminescence mechanism. Theoretical calculation and the Aginnershell-Sterminal distance measurement by X-ray diffraction also back up the charge-transfer process in emitting. Further experiments reveal that the solvent plays an important role in the photoluminescence intensity rather than the emission wavelength of these NCs, and the photoluminescence intensity sequence is solvent-dependent. The multiple observations that MAg24(SR)18 NCs have remarkably distinct photoluminescence intensities but bear almost unchanged emission wavelengths in the investigated conditions indicate that herein the excited-state relaxation dynamics might be more responsible for the emission intensity of these NCs in comparison with the excited-state relaxation thermodynamics (reflected on the emission wavelength). It is worth noting among the four NCs, PtAg24(SR)18 exhibits the biggest potential for practical application due to the highest QYs in most cases (the highest QY of 18.6% was achieved for PtAg24(SR)18 in CH3CN). Overall, this work demonstrates the significant photoluminescence influences of the investigated metal NCs from the metal core atom

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