Article pubs.acs.org/cm
Molecular-like Transformation from PhSe-Protected Au25 to Au23 Nanocluster and Its Application Yongbo Song,†,‡ Hadi Abroshan,‡ Jinsong Chai,† Xi Kang,† Hyung J. Kim,‡,§ Manzhou Zhu,*,† and Rongchao Jin*,‡ †
Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui University, Hefei, Anhui 230601, People’s Republic of China ‡ Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States § School of Computational Sciences, Korea Institute for Advanced Study, Seoul 02455, Korea S Supporting Information *
ABSTRACT: In this work, we report a new size conversion from [Au25(SePh)18]− to [Au23(SePh)16]− nanoclusters under the reductive condition (NaBH4). This novel transformation induced by only reductant has not been reported before in the field of gold nanocluster. The conversion process is studied via MALDI mass spectrometry, and UV−vis spectroscopy. These results demonstrate that the [Au23(SePh)16]− nanocluster is directly obtained by pulling out two units of “Au-SeR” from the [Au25(SePh)18]− nanocluster, which is similar to the “small molecular” reaction. In order to further understand this novel conversion, DFT calculations were performed, in which, with addition of two H− in the [Au25(SeH)18]− model, two Au atoms will depart from the structure of the [Au25(SeH)18]−, which is consistent with the experimental results. Furthermore, the asprepared [Au23(SePh)16]− nanoclusters can be converted into [Au25(PET)18]− nanocluster (PET = SCH2CH2Ph) with excess PET under the reductive condition, which is quite remarkable due to a stronger bond of Au−Se in comparison to Au−S of the final product. Interestingly, the number of the PET ligands on the surface of the 25-atoms nanocluster can be controlled by the addition of the reductant. Based on these results, a circularly progressive mechanism of ligand exchange is proposed. This may offer a new approach to synthesis of new gold nanoclusters and also have significant contribution for understanding and further exploration of the mechanism of ligand exchange.
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isolation from a mixture).3,27 In terms of the synthetic methodologies, three typical routes for preparing metal nanoclusters are usually adopted. The first route is the direct synthesis, which is the simplest approach and involves reduction of the metal−ligand complex. Examples of gold nanoclusters from this method are Au 2 3 (SR) 1 6 , 2 1 Au25(SR)18,12,13 Au30(SR)18,19 and so on. The second route is the size focusing, which comprises two primary steps: (i) kinetically controlled synthesis of a mixture of Aun(SR)m with a properly controlled size range, and (ii) size focusing of such a mixture into a single-sized product. Using this method,
INTRODUCTION Atomically precise metal nanoclusters have recently emerged as a new frontier in nanoscience research.1−4 Of such nanoclusters, gold nanoclusters protected by thiolate have been extensively studied due in part to their excellent stability and attractive properties.5−21 Due to their ultrasmall size, these gold nanoclusters exhibit strong quantum size effects, which endow them with unique physicochemical properties (e.g., multiple absorption bands,13,20 magnetism,22 novel optical properties,23,24 and outstanding catalytic reactivity25,26). All of these extraordinary properties are different from those of the large nanoparticles with surface plasma resonance (SPR), and thus nanoclusters have attracted wide interest. Recent research has made great breakthroughs in the synthesis of atomically precise metal nanoclusters (as well as © 2017 American Chemical Society
Received: January 6, 2017 Revised: March 8, 2017 Published: March 14, 2017 3055
DOI: 10.1021/acs.chemmater.7b00058 Chem. Mater. 2017, 29, 3055−3061
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Chemistry of Materials Au38(SR)24,5 Au64(SR),32,28 Au130(SR)50,10 and Au144(SR)6029 were successfully obtained. Finally, a ligand-exchange method has been developed recently, in which pure metal nanoclusters were used as the primary precursors to be etched by different ligands. A series of distinct nanoclusters (such as Au20(SR)16, Au24(SR)20, Au28(SR)20, Au36(SR)24, Au44(SR)28, Au52(SR)32, Au92(SR)44, and Au133(SR)5214−18,30−32) have been discovered by this approach, which are otherwise difficult to obtain by the previous two methods. Even so, the mechanisms of these methods are still not clear, because they are affected by many influencing factors, such as the temperature, the amount of added ligand, the concentration of solution, and so on. Thus, it is still a great challenge to map out the clear mechanism of these methods, and more research should be done. Herein, we report a novel conversion, in which the [Au25(SePh)18]− nanocluster will be transformed into the [Au23(SePh)16]− nanocluster in the presence of NaBH4. This conversion was explored by UV−vis spectroscopy, MALDI mass spectrometry, and the DFT calculation. Results demonstrate that the two Au atoms and two RSe- ligands are directly pulled out from [Au25(SePh)18]− nanocluster under the reductive condition. Based on this conversion, an extraordinary ligand-exchange induced conversion of [Au23(SePh)16]− to [Au25(PET)18]− nanocluster (PET = SCH2CH2Ph) was achieved, which is quite remarkable due to the stronger bond of Au−Se in comparison to Au−S of the final product. Through controlling the additional driving force (NaBH4), the process of ligand exchange can be regulated, and a circularly progressive mechanism of ligand exchange is proposed.
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mg) was added into the aforementioned solution about every 30 min. After ∼15 h, the pure [Au25(PET)18]− nanoclusters will be obtained. It is worth noting that we also can add the ice aqueous solution of NaBH4 about every 60, 90, or 120 min, which will be extended throughout the whole time of the reaction. Characterization. All UV−vis absorption spectra of gold nanoclusters in solution were recorded using an Agilent 8453 diode array spectrophotometer. Solid samples were first dissolved in CH2Cl2 to make a dilute solution, and then transferred to a 1 cm path length quartz cuvette, followed by spectral measurements. Background correction was made using a dichloromethane blank. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry was performed on an Applied Biosystems Voyager DESTR MALDI-TOF equipped with a nitrogen laser (337 nm). The mass spectra of negative ions were collected in the linear mode under an acceleration voltage of 25 kV and a delay time of 350 ns. trans-2-[3(4-tert-Butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was used as the MALDI matrix. X-ray photoelectron spectroscopy was performed using a Thermo ESCALAB 250Xi instrument, and solid samples were analyzed at 296 K. Computation. The Gaussian 09 package and TPSS functional were used to perform DFT optimization and TD-DFT calculations of optical absorption spectra of nanoclusters.33−36 The basis sets 631G** (H, C, and S) and the LANL2DZ (Au) were employed for the calculations.37−41 To investigate the effects of nanocluster reduction by NaBH4, ab initio molecular dynamics simulations (AIMD) were carried out using the Quantum Espresso program.42 The projector augmented wave (PAW) method and the Perdew−Burke−Ernzerhof (PBE) form of the generalized gradient approximation were employed in the simulations.43,44 The gold nanocluster was placed at the center of a cubic box of dimensions 25.0 Å × 25.0 Å × 25.0 Å. Integration in the reciprocal space was carried out at the Γ k-point of the Brillouin zone with kinetic energy cutoff of 450 eV.
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RESULTS AND DISCUSSION For the reaction of [Au25(SePh)18]− nanoclusters with excess NaBH4, the dichloromethane solution of pure [Au25(SePh)18]− nanoclusters (Figure 1, black line, absorption peaks at 430, 500, and 725 nm) was used as the precursor, and an ice-cold aqueous solution of NaBH4 was added into the solution every ∼0.5 h to maintain a reducing environment during the long time reaction (total ∼10 h). As time increases, the color of the solution gradually changed from deep red to deep green,
EXPERIMENTAL SECTION
Chemicals. Selenophenol (PhSeH, 99.9%), 2-phenylethanethiol (PhCH2CH2SH, ≥99.0%), tetrachloroauric(III) acid (HAuCl4·3H2O, ≥99.99% metals basis), tetraoctylammonium bromide (TOAB, ≥98%), sodium borohydride (NaBH4, ≥99.9%) were used. The solvents were toluene (C7H8, ≥99.9%), dichloromethane (CH2Cl2, ≥99.9%), ethanol (C2H5OH, ≥99.9%), and acetonitrile (CH3CN, ≥99.9%). All chemicals were used as received. Synthesis of [Au 25 (SePh) 18 ] − TOA + Nanoclusters. The [Au25(SePh)18]−TOA+ nanoclusters were prepared by a literature method.34 Briefly, 0.2 mmol of HAuCl4·3H2O was dissolved in 1 mL of water and then phase-transferred to 5 mL of toluene with the aid of tetraoctylammonium bromide (TOAB). Then, both C6H5SeH (63 uL, dissolved in 1 mL ice-cold toluene) and NaBH4 (∼12 mg, dissolved in 1 mL of nanopure water) to convert Au(III) to the product. After reaction overnight, the aqueous phase was removed. The mixture in the organic phase was rotavaporated and then washed several times with CH3CH2OH to remove the redundant PhSeH and byproducts. Finally, pure Au25 nanoclusters were obtained through extraction with acetonitrile. Synthesis of [Au23(SePh)16]−TOA+ Nanoclusters. The asprepared [Au25(SePh)18]− nanoclusters (10 mg) were dissolved in 15 mL of dichloromethane (CH2Cl2) in the bottom flask. And the dichloromethane solution of [Au25(SePh)18]− was cooled to 0 °C in an ice bath over ∼30 min without stirring. After that, 1 mL of ice aqueous solution of NaBH4 (10 mg) was added into the aforementioned solution under vigorous stirring (∼1100 rpm) about every 30 min. After ∼10 h, the pure [Au23(SePh)16]− nanoclusters will be obtained. Synthesis of [Au25(SC2H4Ph)18]−TOA+ Nanoclusters. The asprepared [Au23(SePh)16]− nanoclusters (10 mg) were dissolved in 15 mL of dichloromethane (CH2Cl2) in the bottom flask and then cooled to 0 °C in an ice bath over ∼30 min without stirring. After that, excess 2-phenylethanethiol (PET, ∼100 mol equiv of the [Au23(SePh)16]−) was added into dichloromethane solution under vigorous stirring (∼1100 rpm), and then 1 mL of ice aqueous solution of NaBH4 (10
Figure 1. UV−vis absorption spectra of [Au25(SePh)18]− nanocluster (black line) and its product (green line) after reaction with NaBH4. For clear comparison, UV−vis absorption spectra of [Au25(SR)18]− nanocluster (black dots) and [Au23(SR)16]− (green dots). Inset: schematic diagram of the conversion of [Au25(SePh)18] − to [Au23(SePh)16]− nanocluster (color labels: orange/green, Au; violet, Se; C and H atoms, omitted for clarity). 3056
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Chemistry of Materials indicating some kind of reaction between [Au25(SePh)18]− and NaBH4. As shown in Figure 1, the UV−vis absorption spectrum of the product (green line) shows a peak at 600 nm and a shoulder at 500 nm. To further determine the composition of the product, the MALDI-MS was performed, which shows a single peak at 7027.21 Da (Figure S1, negative mode), which is assigned to the [Au23(SePh)16]− nanocluster (calc m/z, 7028.54 Da). Compared to the optical spectrum of the anionic [Au23(SC6H11)16]− nanocluster,21 which shows peaks at 450 and 570 nm (Figure 1, green dot), the optical spectrum of the present [Au23(SePh)16]− nanocluster shifts to red. This phenomenon is also found in previous work such as the [Au25(SeR)18]− vs [Au25(SR)18]−,45−47 and Au38(SeR)24 vs Au38(SR)24 cases.48 Nevertheless, we carried out TD-DFT calculations to investigate the theoretical absorption of [Au23(SH)16]− and [Au23(SeH)16]− as models for the Au23 nanoclusters with similar gold framework. As shown in Figure S2, the first absorption peak of [Au23(SeH)16]− is red-shifted by 8 nm in comparison to that of [Au23(SH)16]−. These results along with the similar experimental features of UV−vis spectra suggest that the present [Au23(SePh)16]− nanocluster (Figure 1, inset) should have a geometric structure similar to that of the [Au23(SR)16]− counterpart. In order to further make this conclusion more convincing, we also investigated the theoretical absorption of [Au25(SH)18]− and [Au25(SeH)18]− models with the same method. As shown in Figure S3, the first and the second theoretical absorption peaks of [Au25(SeH)18]− are found to be red-shifted by 9 and 28 nm compared to those of [Au25(SH)18]−. These results are in qualitative agreement with the experimental observations of the UV−vis spectra of the nanoclusters (Figure 1).45−47 All these results demonstrate that the present [Au23(SePh)16]− nanocluster does have a geometric structure similar to that of the [Au23(SR)16]− counterpart. Furthermore, other reductants, such as boranetert-butylamine complex, have been used to reacted with the [Au25(SePh)18]− nanocluster, but it is fail to obtain the [Au23(SePh)16]− nanocluster. It may be determined by their different reducing capability, which should be strong enough to conquer the energy barrier needed in the transformation from [Au25(SePh)18]− to [Au23(SePh)16]− nanoclusters. This phenomenon, in which the atomically accurate gold nanocluster can be converted into another one with new size only induced by reductant, is first observed in the gold nanoclusters. One question may attract our interest: how does the process occur? In order to gain insight into the reaction between [Au25(SePh)18]− and NaBH4, we monitored the entire reaction process by UV−vis absorption spectroscopy and MALDI mass spectrometry. As shown in Figure 2A, the black curve is the UV−vis absorption spectrum of the solution before reaction. After reacting for 2 h, a shoulder at 600 nm was observed, and the peak at 725 nm became less obvious, which indicates that the starting [Au25(SePh)18]− nanocluster has reacted with NaBH4. With time going on, the UV−vis absorption spectrum of the solution only shows a peak at 600 nm with a shoulder at 500 nm, and the original peaks (430, 500, and 725 nm) disappeared gradually, which indicates that the [Au25(SePh)18]− nanoclusters have totally converted into the [Au23(SePh)16]− nanoclusters. These results only demonstrate that the [Au25(SePh)18]− nanoclusters are gradually disappearing as the [Au23(SePh)16]− nanoclusters are forming. It is still unclear for the real relationship between [Au23(SePh)16]− and [Au25(SePh)18]− nanoclusters.
Figure 2. (A) Time-dependent UV−vis spectra of the transformation from [Au25(SePh)18]− to [Au23(SePh)16] nanoclusters. (B) Corresponding MALDI-MS of different times in parallel with UV−vis spectral measurements.
Considering this, the MALDI mass spectra were also collected. As shown in Figure 2B, the mass spectrum of the solution exhibits typical ion peaks of the [Au25(SePh)18]− nanoclusters at the beginning of the reaction. Interestingly, there is a fragment peak at 7026.98 Da, which is assigned to a formula of [Au23(SePh)16], indicating that under the high voltage (e.g., 25 kV) in MALDI analysis, the [Au25(SePh)18]− can dissipate two PhSe-Au units, and accordingly be converted into [Au23(SePh)16]. With prolonged time of solution phase reaction, the abundance of the peak at 7733.56 Da gradually becomes weaker and weaker (Figure 2B); so do the other fragments except the peak at 7026.98 Da, which becomes more and more obvious (all the mass spectra were obtained under the same conditions). And there are not any other new peaks except for the peak at 7026.98 Da. After ∼10 h, all the characteristic peaks of the initial [Au25(SePh)18]− nanoclusters disappeared and a single peak at 7026.98 Da was observed (Figure 2B, green line). Adding to the changes of the UV−vis spectra, the peak at 7026.98 Da is not the fragment of the [Au25(SePh)18]− nanocluster but the [Au23(SePh)16]− nanocluster ion peak. Based on these results, we can conclude that, under the reductant environment, the [Au25(SePh)18]− nanocluster was directly transformed into a [Au23(SePh)16]− nanocluster by spitting out two Au atoms and two PhSeligands. Interestingly, this transformation only induced by the reductant is very similar to the redox reaction in the small molecular science. In this point, the [Au25(SePh)18]− nanocluster can be viewed as an oxidizing agent (the average electron of each Au atoms is 8/25 e), and the [Au23(SePh)16]− nanocluster can be viewed as a reduced product (the average electron of each Au atoms is 8/23 e). This attracts our eyes: how does the NaBH4 drive this transformation to happen? In order to gain further insight into the effects of a reducing agent, i.e., NaBH4, on the structure of [Au25(SePh)18]− nanocluster, DFT geometry optimization of the model nanocluster Au25(SeH)18 with the net charges of −1, −2, and −3 was performed (Scheme 1). The results indicate that the addition of the first and second electron to [Au25(SeH)18]−, i.e., [Au25(SeH)18]− → [Au25(SeH)18]2− → [Au25(SeH)18]3−, Scheme 1. Pathway of Adding Free Electron into the [Au25(SeH)18]− Model
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at 400, 445, and 670 nm with a shoulder at 780 nm (Figure S7A), indicating that pure [Au25(PET)18]− nanocluster is obtained, which is also proved by the MALDI mass spectrometry (Figure S7B). The successful reverse transformation of [Au23(SePh)16]− to [Au25(PET)18]− is quite remarkable in consideration of the starting stronger Au−Se bond than the final weaker Au−S bond. It raises a number of interesting questions. First of all, how does the ligand-exchange occur (i.e., the detailed reaction pathway)? There are two possible routes, (i) SN2 reactions the Au−S bond is formed before the Au−Se bond breaks, which is also called associative ligand substitution, and (ii) SN1 reactionsthe Au−Se bond is fully broken before the Au−S bond forms, which is also named dissociative ligand substitution. Second, why does the reaction occur (i.e., the driving force leading to the weaker RS−Au bond displacement of the stronger RSe−Au bond)? Third, what specific role does NaBH4 play in the processbreaking or weakening the Au−Se bond or others? To address these questions, we are motivated to carry out a detailed investigation of the conversion by UV−vis absorption spectroscopy and MALDI mass spectrometry. As shown in Figure 3A, upon the addition of PET, the optical absorption peak of [Au23(SePh)16]− nanoclusters (600 nm)
energetically stabilizes and destabilizes the nanocluster, respectively (ΔE1 = −1.84 eV/mol and ΔE2 = +1.46 eV/ mol). In the calculations, the self-energy of the added electrons was assumed to be zero; therefore, our results should be considered as qualitative estimates for the relative stability of [Au25(SeH)18]2− and [Au25(SeH)18]3−, rather than absolute energy change for a sequential reduction of [Au25(SeH)18]−. We also note that the HOMO and LUMO levels of the [Au25(SeH)18]− become destabilized by, respectively, 2.24 and 2.81 eV/mol after the addition of the first electron to form [Au25(SeH)18]2−. The addition of the second electron destabilizes the HOMO and LUMO levels by 0.77 and 11.0 eV/mol, respectively. It is worth mentioning that energies of both HOMO and LUMO of [Au25(SeH)18]3− are found to be positive, in contrast to those of [Au25(SeH)18]1− and [Au25(SeH)18]2− (Figure S4). These results altogether suggest that two electrons are needed for considerable destabilization of [Au25(SePh)18]− and its decomposition to a different gold nanocluster. We further conducted AIMD simulations of [H2Au25(SeH)18]3−. Of note, two hydride anions (H−) are added to [Au25(SeH)18]1− to model reduced states of the nanocluster (Figure S5). Interestingly and consistent with the DFT results mentioned above, AIMD simulations show that the addition of two hydride anions has a considerable effect on the structural stability of the nanocluster; it leads to a loss of two reduced gold atoms in the form of H−Au to yield [Au23(SeH)18]3−. We believe that the remaining nanocluster in an aqueous solution will lose two anionic -SeR ligands and become [Au23(SeH)16]1−, which eventually transforms to the optimized [Au23(SeH)16]1− nanocluster via an exothermic step of 0.85 eV/mol. These results are all in good agreement with our experimental observation. Furthermore, a little precipitation at the bottom was observed in the process of transformation. The precipitation was collected and subjected to X-ray photoelectron spectroscopic analysis (XPS). As shown in Figure S6, the Au 4f peak (83.9 eV, red line) is very close to that of Au (0) (84.0 eV), which indicates that the two Au(I) atoms split from the [Au25(SePh)18]− core were reduced into Au(0) under the reducing condition (i.e., the presence of NaBH4). The reaction of this step can be written as
Figure 3. (A) Time-dependent UV−vis spectra and (B) MALDI mass spectra of the transformation from [Au 23 (SePh) 1 6 ] − to [Au25(PET)18]− nanocluster.
became very weak, and at the same time two broad peaks at 500 and ∼700 nm were seen within 1 h, indicating that some of the [Au23(SePh)16]− nanoclusters have been converted into [Au25(SePh)18−m(PET)m]− nanoclusters. With prolonged time, these two peaks blue shift gradually and become more and more obvious. After ∼15 h, these two peaks locate at 445 and 670 nm, which indicates the [Au25(PET)18]− nanoclusters were obtained. Corresponding to the UV−vis absorption spectra, the MALDI mass spectra were also collected. As shown in Figure 3B, at the beginning of the reaction, [Au23(SePh)16]− shows its intact molecular ion peak at 7026.98 Da. After adding the PET for 1 h, the single peak evolved into five peaks from 6000 to 8000 Da in the mass spectra, which indicates the Au23 core has been converted into a Au25 core. With prolonged time, the intact molecular ion peak (marked with an asterisk (*)) moves toward the lower m/z range. After 3 h, a broader peak (marked with double asterisk (**)) was observed in the mass spectrum. By zooming this broader peak, we were amazed to find that this is a group of peaks with a uniform spacing of 19 (Figure 3B inset), which corresponds to the mass difference between -SePh
[Au 25(SePh)18 ]− + NaBH4 → [Au 23(SePh)16 ]− + 2Au(0) + 2(‐SePh)
It is worth noting that if the thiolate-capped Au25 nanocluster is used to react with the NaBH4, no changes can be observed; i.e., the [Au25(PET)18]− nanoclusters can stably exist even though much more reductant (NaBH4) is added, which may be dictated by their different geometric and electronic structures, such as the bond lengths of Au-ligands (Au−Se (2.434 Å) vs Au−S (2.301 Å)), the less electron in the core of [Au25 (SePh) 18] (−0.1 |e|) than that of [Au25 (PET) 18 ]− (−0.27 |e|) nanocluster, and the more negative HOMO and LUMO energies in [Au25(SePh)18] (−7.37 and −6.31 eV) than those in [Au25(PET)18]− (−7.11 and −5.88 eV) nanocluster.45 Application. Interestingly, if we added excess PET into the solution of the [Au23(SePh)16]− nanoclusters under the reductive condition (NaBH4), the color of the solution immediately changed from green to brown, which indicates that the [Au23(SePh)16]− nanoclusters have reacted with the PET. After ∼15 h, the UV−vis absorption exhibits three peaks 3058
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work.49,50 As shown in Scheme 2, the [Au25(SePh)18]− nanocluster is converted into the [Au23(SePh)16]− nanocluster
and -SC2H4Ph ligands (MW(-SC2H4Ph) = 136 Da, MW(SePh) = 155 Da, Δm/z = 19 when z = 1). And the peaks are assigned as the intact molecular ion peak of [Au25(SePh)18‑m(PET)m] (4 ≤ m ≤ 16); i.e., the peak located at 7508.84 Da is assigned as the formula of [Au25(PET)12(SePh)6] (calc, 7059.42 Da). These results indicate that some original PhSe- ligands have been replaced by the PET. With prolonged time, peak * disappeared gradually and peaks ** became more and more obvious and shifted to lower m/z range gradually. After ∼15 h, the group of peaks (**) turned into a single peak at 7392.78 Daindicative of pure [Au25(PET)18]− nanoclusters (calc, 7393.94 Da), which is consistent with the UV−vis absorption spectrum. However, if we only add the excess PET without the reductant, something is different from the complete exchange discussed above. As shown in Figure 4A, at the beginning, the
Scheme 2. Reaction Pathway for the Conversion of [Au25(SePh)18]− to [Au25(PET)18]− Nanoclustersa
a
The cycle contains two steps: step I, structure distortion; step II, structure distortion and ligand exchange.
induced by the reductant (NaBH 4 ). And then the [Au23(SePh) 16 ]− nanocluster will be converted to the [Au25(PET)m(SePh)18−m] nanoclusters induced by excess PET. These two steps consist of one cycle, in which only some PhSe- ligands are replaced by the PET. Sequentially, the [Au25(PET)m(SePh)18−m] nanoclusters will be induced into [Au23(PET)m(SePh)16−m] nanoclusters under the reductive environment, and in the same way, the [Au23(PET)m(SePh)16−m] nanocluster will be transformed into [Au25(PET)m+m1(SePh)18−m−m1] nanoclusters. Compared with the first cycle, more thiolate ligands attach to the surface of the Au25 nanoclusters. This kind of iterative process does not stop until all the PhSe- ligands are replaced by PET ligands. Furthermore, it is worth noting that the number of the PET ligands can be controlled by adjusting the amount of the NaBH4, which will be beneficial to the functionalization of the metal nanoclusters
Figure 4. (A) Time-dependent UV−vis spectra and (B) MALDI mass spectra of the transformation reaction between [Au23(SePh)16]− nanoclusters and excess PET without adding the NaBH4.
change of the UV−vis spectra is similar to before (Figure 3A). But, the peaks of the solution located at ∼480 and ∼700 nm instead of 445 and 670 nm at last (∼24 h), which indicates that the pure [Au25(PET)18]− nanoclusters were not produced. Consistent with the UV−vis spectra, the MALDI mass spectra exhibit an intact molecular ion peak at 7659.25 Da, which is assigned to [Au25(PET)4(SePh)14] (note: this is a group of peaks and the main peak is located at 7659.25 Da; see Figure S8). However, if additional NaBH4 solution is added into the solution sequentially, the ligand-exchange process will continue, and at last, pure [Au25(PET)18]− nanoclusters will be obtained. From these results, three conclusions can be obtained: (1) under the excess PET condition, the [Au23(SePh)16]− nanoclusters are not stable and will convert into [Au25(SePh)18−m(PET)m]− nanoclusters (in other words, it is the stability to drive the ligand exchange to happen); (2) the original PhSe- ligands are replaced one by one, and in the intact molecular ion peak of the [Au25(SePh)18−m(PET)m]−, the number of ligands (PhSe- and PET) is always 18, which indicate that the substitution pattern should be the SN2 reaction; (3) the reductant (NaBH4) is not only an activating agent to convert [Au25(SePh) 18]− into [Au 23 (SePh)16 ]− nanoclusters at the beginning but also plays an important role in the whole process of ligand exchange. Furthermore, we also achieved, under the excess PET and the NaBH4 condition, the [Au25(SePh)18]− nanocluster can be converted into [Au25(PET)18]− nanocluster (Figure S9). Taking into account all the results, we proposed a circularly progressive process from [Au 2 5 (SePh) 1 8 ] − to the [Au25(PET)18]− nanocluster, in which the Au23 nanocluster is the important “intermediate”. This is different from previous
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CONCLUSION In summary, we have successfully mapped out the details of the transformation mechanism from [Au25(SePh)18]− to the [Au23(SePh)16]− nanocluster with UV−vis absorption and MALDI-MS techniques, in which the [Au25(SePh)18]− nanocluster will lose two Au atoms and two PhSe- and convert into [Au23(SePh)16]− nanocluster directly. And this phenomenon is also revealed by coupling experiments with density functional theory (TD-DFT) calculations. Based on this novel transformation, an extraordinary ligand exchange from [Au25(SePh)18]− to [Au25(PET)18]− nanocluster was achieved. And a new ligand-exchange mechanism was proposed. These results will provide a new approach to synthesize new metal nanoclusters and also help us further understand the mechanism of the ligand exchange. Furthermore, considering the previous work (Au24(SeR)20, ring-like Au60, and the rod-like Au25 with different charges), changing the ligands from thiolate to selenolate has a great significance for tailoring the properties, size, and structure of the gold nanoclusters, which will offer more opportunities to further understand the cluster science. 3059
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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.7b00058. Detailed information given in Figures S1−S9 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(M.Z.) E-mail:
[email protected]. *(R.J.) E-mail:
[email protected]. ORCID
Hyung J. Kim: 0000-0003-4334-1879 Manzhou Zhu: 0000-0002-3068-7160 Rongchao Jin: 0000-0002-2525-8345 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from NSFC Grants 21631001, 21372006, and U1532141, the Ministry of Education, the Education Department of Anhui Province, and the 211 Project of Anhui University.
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REFERENCES
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