Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (4): 555-562

The article information

Chen-jie Zeng, Meng Zhou, Gayathri Chakicherla, R. Gil Roberto, Y. Sfeir Matthew, Rongchao Jin
曾晨婕, 周蒙, Chakicherla Gayathri, Roberto R.Gil, Matthew Y.Sfeir, 金荣超
Au10(TBBT)10: the Begining and the End of Aun(TBBT)m Nanoclusters
Au10(TBBT)10:Aun(TBBT)m纳米团簇的起点与终点
Chinese Journal of Chemical Physics, 2018, 31(4): 555-562
化学物理学报, 2018, 31(4): 555-562
http://dx.doi.org/10.1063/1674-0068/31/cjcp1806141

Article history

Received on: June 14, 2018
Accepted on: July 20, 2018
Au10(TBBT)10: the Begining and the End of Aun(TBBT)m Nanoclusters
Chen-jie Zenga, Meng Zhoua, Gayathri Chakicherlaa, R. Gil Robertoa, Y. Sfeir Matthewb, Rongchao Jina     
Dated: Received on June 14, 2018; Accepted on July 20, 2018
a. Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA;
b. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
Author: Rongchao Jin received his B.S. degree in Chemical Physics from the University of Science and Technology of China (USTC) in 1995. At USTC, he worked with Prof. Shufen Li on the combustion of boron particles. He then joined Dalian Institute of Chemical Physics (DICP)/ Chinese Academy of Sciences and worked with Prof. Wenzhao Li on nickel-catalyzed natural gas conversion and received a M.S. degree in Physical Chemistry/Catalysis in 1998. In 1999, he enrolled in Northwestern University and earned his Ph.D. in Chemistry in 2003. In his PhD work, he worked with Prof. Chad A. Mirkin on noble metal nanocrystal synthesis, optical properties, and biological applications. After receiving his Ph.D. degree, he worked at the University of Chicago as a Research Associate with Prof. Norbert F. Scherer and studied femtosecond laser excited second harmonic generation (SHG) from single metal nanoparticles. In the fall of 2006, he joined the chemistry faculty of Carnegie Mellon University and was promoted to Associate Professor in 2012 and Full Professor in 2015. His current research interests are atomically precise nanoparticles, nano-optics, and catalysis..
*Author to whom correspondence should be addressed. Rongchao Jin, E-mail:rongchao@andrew.cmu.edu
Part of the special issue for celebration of "the 60th Anniversary of University of Science and Technology of China and the 30th Anniversary of Chinese Journal of Chemical Physics"
Abstract: Gold(Ⅰ) thiolate compounds (i.e. Au-SR) are important precursors for the synthesis of atomically precise Aun(SR)m nanoclusters. However, the nature of the Au-SR precursor remains elusive. Here, we report that the Au10(TBBT)10 complex is a universal precursor for the synthesis of Aun(TBBT)m nanoclusters (where TBBT=4-tertbutylbenzenethiol/thiolate). Interestingly, the Au10(TBBT)10 complex is also found to be re-generated through extended etching of the Aun(SR)m nanoclusters with excess of TBBT thiol and O2. The formation of well-defined Au10(TBBT)10 complex, instead of polymeric Au-SR, is attributed to the bulkiness of the TBBT thiol. Through 1D and 2D NMR characterization, the structure of Au10(TBBT)10 is correlated with the previously reported X-ray structure, which contains two inter-penetrated Au5(TBBT)5 rings. The photophysical property of Au10(TBBT)10 complex is further probed by femtosecond transient absorption spectroscopy. The accessibility of the precise Au10(TBBT)10 precursor improves the efficiency of the synthesis of the Aun(TBBT)m nanoclusters and is expected to further facilitate excellent control and understanding of the reaction mechanisms of nanocluster synthesis.
Key words: Gold    Cluster    Optical    
Ⅰ. INTRODUCTION

Thiolate-protected gold nanoclusters, denoted as Au$_n$(SR)$_m$, have recently emerged as a paradigm of atomically precise nanomaterials [1]. Such nanoclusters are typically made from Au$^{\rm{I}}$-SR precursors; thus, understanding the starting materials is critical for achieving excellent control over the nanocluster synthesis. The structures of the Au$^{\rm{I}}$-SRs compounds have attracted both experimental and theoretical interests [2-5]. In a typical synthesis of Au$_n$(SR)$_m$ nanocluster, the first step is to reduce Au$^{\rm{III}}$ in HAuCl$_4$ with thiols to form colorless Au$^{\rm{I}}$-SR compounds [6, 7], which is further reduced by a stronger reducing reagent (e.g. NaBH$_4$) to form polydisperse Au$_n$(SR)$_m$ nanoclusters. Therefore, the Au$^{\rm{I}}$-SR compounds can be viewed as the precursor or the beginning of the Au$_n$(SR)$_m$ nanoclusters. In order to transform polydisperse Au$_n$(SR)$_m$ nanoclusters into atomically precise ones, a "size-focusing" step is required [8], in which an excess of thiol ligands together with oxygen is applied to etch the initial polydispersed Au$_n$(SR)$_m$ nanoclusters. During the size focusing, the less stable Au$_n$(SR)$_m$ nanoclusters are either transformed to the most stable species or decomposed to Au$^{\rm{I}}$-SR compounds [9, 10]. Therefore, the Au$^{\rm{I}}$-SR compounds can be also viewed as the by-product or the end of the Au$_n$(SR)$_m$ nanoclusters. Despite the significant advancement in the synthesis and crystallization of atomically precise Au$_n$(SR)$_m$ nanoclusters, the structures of Au$^{\rm{I}}$-SR compounds remain elusive, which is largely attributed to their polymeric or oligomeric nature as well as their poor solubility once they are isolated from the reaction solution. In solution, the Au$^{\rm{I}}$$-$Au$^{\rm{I}}$ aurophilic interactions or the van der Waals interactions between the hydrocarbon groups link the polymeric -[-Au$^{\rm{I}}$-S(R)-]$_x$- chains into three-dimensional networks or lamellar structures [11, 12, 13]. Once dried from the solvent, Au$^{\rm{I}}$-SR becomes powders that are unfortunately difficult to be re-dissolved or reused as the precursors.

Among the different types of thiolate ligands, 4-tert-butylbenzenethiol/thiolate (TBBT) has become one of the most popular, powerful, and versatile ligands for the synthesis and crystallization of atomically precise Au$_n$(SR)$_m$ nanoclusters [14-21]. Since its first use in the crystallization of the Au$_{36}$(TBBT)$_{24}$ nanocluster [14], the exploration of TBBT ligand has led to fruitful discoveries, including the chiral structures of Au$_{28}$(TBBT)$_{20}$ and its enantiomer separation [15], a novel Au$_8$(SR)$_8$ octameric ring motif identified in an ultrasmall Au$_{20}$(TBBT)$_{16}$ nanocluster [16], a periodic series of Au$_{8x+4}$(TBBT)$_{4x+8}$ quantum boxes ($x$=3, 4, 5, 6) [17], the atomic structure of self-assembled monolayer on Au(100) plane of Au$_{92}$(TBBT)$_{44}$ [18], the aesthetic hierarchical structural patterns in Au$_{133}$(TBBT)$_{52}$ [19], and recently a giant Au$_{279}$(TBBT)$_{84}$ nanocluster with nascent plasmonic resonance [20, 21]. While the benzene ring in TBBT provides rigidity to the nanoclusters, the bulky tert-butyl tail enables good shielding and solubility. The balance of these two parts makes TBBT an ideal ligand for crystallization of Au$_n$(SR)$_m$ nanoclusters.

Here, we report an interesting finding that, instead of having a commonly observed polymeric nature, the Au$^{\rm{I}}$-TBBT compounds formed during the synthesis or the etching of Au$_n$(TBBT)$_m$ nanoclusters have a well-defined molecular nature, e.g. Au$_{10}$(TBBT)$_{10}$ (Scheme 1). The Au$_{10}$(TBBT)$_{10}$ was first discovered as a byproduct during the size transformation process from Au$_{25}$(SCH$_2$CH$_2$Ph)$_{18}$ to Au$_{28}$(TBBT)$_{20}$ or Au$_{20}$(TBBT)$_{16}$ [22]. Pure Au$_{10}$(TBBT)$_{10}$ was obtained if the transformation reaction was allowed to proceed for longer time (e.g. a couple of days) under air. Inspired by this observation, we suspect that Au$_{10}$(TBBT)$_{10}$ is a stable form of the Au$^{\rm{I}}$-TBBT compound and may also exist as the precursor during the synthesis of Au$_n$(TBBT)$_m$ nanoclusters. After checking the Au$^{\rm{I}}$-TBBT compounds generated in different synthetic methods, we find that the Au$_{10}$(TBBT)$_{10}$ is indeed the universal precursor. Based on this finding, we further design a simple and fast reaction to synthesize large quantity of Au$_{10}$(TBBT)$_{10}$ in high yield. The as-synthesized Au$_{10}$(TBBT)$_{10}$ can be readily dissolved in different solvents and reduced by NaBH$_4$. The accessibility of a well-defined and precise Au$_{10}$(TBBT)$_{10}$ precursor improves the efficiency of the synthesis of Au$_n$(SR)$_m$ nanoclusters and provides a better control of the reaction as well. We perform NMR characterizations to correlate the structure of Au$_{10}$(TBBT)$_{10}$ with a previously identified crystal structure [23]. We further study the photophysical properties of the Au$_{10}$(TBBT)$_{10}$ complex by transient absorption spectroscopy, which could provide a good reference to the photophysical properties of surface relaxation in Au$_n$(TBBT)$_m$ nanoclusters. Author note: $n$ and $m$ should be subscript and italic.

Scheme 1 Synthesis of Au$_{10}$(TBBT)$_{10}$ complex by top-down etching or bottom-up assembly.
Ⅱ. EXPERIMENTS A. Chemicals

Tetrachloroauric(Ⅲ) acid (HAuCl$_4\cdot$3H$_2$O, $>$99.99% metals basis, Aldrich), tetraoctylammonium bromide (TOAB, $\geq$98%, Fluka), 4-tert-butylbenzenethiol (TBBT, 97%, Alfa Aesar). Solvents: methanol (HPLC grade, $\geq$99.9%, Aldrich), dichloromethane (HPLC grade, $\geq$99.9%, Aldrich), toluene, (HPLC grade, $\geq$99.9%, Aldrich). All chemicals were used as received.

B. Synthesis of Au$_{\mathbf{10}}$(TBBT)$_{\mathbf{10}}$ 1. Bottom-up assembly method 1

0.085 g of HAuCl$_4$ in 5 mL of H$_2$O was mixed with 0.136 g of TOAB in 15 mL of toluene. After vigorous stirring for 15 min, the colorless aqueous phase was discarded. Then, 180 ${\rm{\mu }}$L of TBBT thiol was added to the toluene phase. The color of the reaction solution changed from deep orange into faint yellow within one hour, indicating that the Au$^{\rm{III}}$ was reduced to Au$^{\rm{I}}$ by TBBT thiol. The Au$^{\rm{I}}$-TBBT compound was separated by drying the toluene and then precipitated and washed with methanol. 0.075 g sample was obtained, corresponding to a yield of 96% (Au based).

2. Bottom-up assembly method 2

0.1 g of HAuCl$_4$ was dissolved in 10 mL of methanol. 210 ${\rm{\mu }}$L of TBBT was added. The reaction mixture was stirred for 1 h. The yellowish precipitate was collected and washed with methanol for several times to remove unreacted thiol. The yield of Au$_{10}$(TBBT)$_{10}$ complex was $>$95%.

3. Top-down etching method

5 mg of Au$_{25}$(SC$_2$H$_4$Ph)$_{18}$ was dissolved in 1 mL of toluene. 1 mL of TBBT thiol was then added. The reaction mixture was stirred at 80 $℃$ for two days in air. Methanol was added to precipitate and wash the yellowish product.

C. Characterization

Matrixassisted laser desorption ionization (MALDI) mass spectrum was recorded on the PerSeptive-Biosystems Voyager DE super-STR timeof-flight (TOF) mass spectrometer. The matrix was trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenyldidene] malononitrile18 (DCTB). Electrospray ionization (ESI) mass spectrum was recorded on a Waters Q-TOF mass spectrometer equipped with Z-spray source. The source temperature was kept at 70 $℃$. The sample was directly infused into the chamber at 5 ${\rm{\mu }}$L/min. The spray voltage was kept at 2.20 kV and the cone voltage at 60 V. The ESI sample was dissolved in toluene and diluted (1:2 in volume ratio) by dry methanol containing 50 mmol/L CsOAc to impart charges to the clusters through the formation of Cs+cluster adducts in ESI. Thermal gravimetric analysis (TGA) was performed on the TG/DAT6300 analyzer (Seiko Instruments, Inc.) under a N$_2$ atmosphere (flow rate $\sim$50 mL/min). $\sim$5 mg of sample was used for analysis. UV-Vis spectrum was recorded on a Hewlett-Packard (HP) Agilent 8453 diode array spectrophotometer at room temperature using CH$_2$Cl$_2$ as solvent. NMR analysis was performed on a Bruker Avance 500 spectrometer operating at 500.13 MHz for $^1$H. For data collection, $\sim$10 mg of Au$_{10}$(TBBT)$_{10}$ nanocluster was dissolved in CDCl$_3$. The following experiments were performed for NMR analysis and signal assignment: 1D $^1$H NMR, 2D correlation spectroscopy ($^1$H-$^1$H COSY), and ($^1$H, $^{13}$C) heteronuclear single quantum correlation spectroscopy (HSQC).

D. Femtosecond transient absorption spectroscopy

Femtosecond transient absorption spectroscopy were carried out using a commercial Ti:Sapphire laser system (Spitfire Spectra Physics). The $\sim$100 fs laser pulses in the ultraviolet region were generated by a 3.5 mJ regenerative amplifier system (Spitfire, Spectra-Physics) and optical parametric amplifier (OPA, TOPAS). A small portion of the laser fundamental was focused into a sapphire plate to produce supercontinuum in the visible range (400$-$800 nm), which overlapped in time and space with the pump. Multiwavelength transient spectra were recorded using dual spectrometers (signal and reference) equipped with array detectors whose data rates exceed the repetition rate of the laser (1 kHz). Solutions of both clusters in 1 mm path length cuvettes were excited by the tunable output of the OPA (pump). During the measurement, Teflon coated magnetic bar was used to stir the sample constantly and the UV-Vis remained the same before and after the fs experiments.

Ⅲ. RESULTS AND DISCUSSION A. Synthesis and characterizations of Au$_{\mathbf{10}}$(TBBT)$_{\mathbf{10}}$ complex

There are multiple ways to synthesize the Au$_{10}$(TBBT)$_{10}$ complex. The Au$_{10}$(TBBT)$_{10}$ complex during the reaction of 5 mg of Au$_{25}$(SCH$_2$CH$_2$Ph)$_{18}$ nanoclusters with 1 mL of TBBT thiol at 80 $℃$ for more than 2 days. Previously, we reported a ligand-exchange induced size/structure transformation (LEIST) reaction [22]. One example of LEIST reaction is the transformation of Au$_{25}$(PET)$_{18}$ into Au$_{28}$(TBBT)$_{20}$ by reacting with TBBT thiol at 80 $℃$ for just 2 h [15]. However, this reaction was continued for two days, the dark brown color of the solution gradually faded into faint yellow. The fading color indicates an increasing number of gold atoms in the nanoclusters were oxidized from Au$^0$ to Au$^{\rm{I}}$. After the brown color completely disappeared, the final product was isolated by precipitation and washing with methanol.

We first examined the formula of the yellowish powder product with MALDI-MS. Three $m/z$ peaks at 3453, 2197, and 2005 are observed (FIG. 1(A)). The peaks are correlated to Au$_{10}$(TBBT)$_9$ with theoretical value of 3457, Au$_7$(TBBT)$_5$ with theoretical value of 2205, and Au$_6$(TBBT)$_5$ with theoretical value of 2008, respectively. The Au$_{10}$(TBBT)$_9$ is assigned to the molecular ionization peak Au$_{10}$(TBBT)$_{10}$. The loss of one TBBT ligand during the matrix-assisted laser desorption/ionization process is commonly observed in the Au$_n$(TBBT)$_m$ nanoclusters [24]. The Au$_6$(TBBT)$_5$ and Au$_7$(TBBT)$_5$ are additional fragments of Au$_{10}$(TBBT)$_{10}$. The fragmentation pattern of loosing Au$_4$(SR)$_4$ is also frequently observed for thiolate-protected gold nanoclusters [25]. To confirm the formula, we further characterized the product with ESI-MS. As shown in FIG. 1(B), an intense $m/z$ peak at 3754 is observed, corresponding to the adduct [Au$_{10}$(TBBT)$_{10}$Cs]$^+$; note that cesium acetate is added to facilitate the ionization of the neutral complex via adduct formation. The experimental isotope pattern matches perfectly with the simulated one (FIG. 1(B), red line). Interestingly, two more peaks (at $m/z$=5566 and 7377) are also observed in ESI-MS, which correspond to [Au$_{30}$(TBBT)$_{30}$Cs$_2$]$^{2+}$ and [Au$_{20}$(TBBT)$_{20}$Cs]$^{+}$, respectively. This does not mean that there are Au$_{30}$(TBBT)$_{30}$ or Au$_{20}$(TBBT)$_{20}$ nanoclusters in the product. Instead, it is the dimerization and trimerization of Au$_{10}$(TBBT)$_{10}$ during the electrospray ionization process. Thermogravimetric analysis of the product shows a weight loss of 45.49% at 260 $℃$ (FIG. 1(C)), which is in good agreement with the calculated weight loss of Au$_{10}$(TBBT)$_{10}$ (45.58%). The UV-Vis absorption spectrum of the product shows a peak at 345 nm, a hump at 378 nm, and the absorption onset at $\sim$450 nm (FIG. 1(D)). The absorption spectrum of Au$_{10}$(TBBT)$_{10}$ is slightly redshifted compared to the Au$_{10}$(glutathione)$_{10}$ counterpart reported previously [26]. This is mainly due to the conjugation effect from TBBT ligands, which delocalizes the electrons and decreases the energy gap [27].

FIG. 1 Characterization of the Au$_{10}$(TBBT)$_{10}$ complex. (A) MALDI-MS, (B) ESI-MS, insets are the isotope patterns of each peak, the colored profiles are the isotope simulation based on the formula, (C) TGA, and (D) UV-Vis.

The formation of Au$_{10}$(TBBT)$_{10}$ complexes during the prolonged etching of Au$_{25}$(SCH$_2$CH$_2$Ph)$_{18}$ nanoclusters implies that the Au$_{10}$(TBBT)$_{10}$ is a stable form of the Au$^{\rm{I}}$-TBBT compound. Au$_{10}$(TBBT)$_{10}$ can be viewed as the end point of the etching reaction, and the etching reaction can be viewed as a "top-down" method to synthesize the Au$_{10}$(TBBT)$_{10}$ complex. Based on the observation, an interesting question is whether the "bottom-up" process, i.e. directly reacting HAuCl$_4$ with TBBT thiol, would also produce the same Au$_{10}$(TBBT)$_{10}$ complex? In other words, would Au$_{10}$(TBBT)$_{10}$ also be the precursor for the synthesis of Au$_n$(TBBT)$_m$ nanoclusters? To answer this question, we repeated a typical synthesis of Au$^{\rm{I}}$-SR precursors by mixing 0.085 g of HAuCl$_4$ in 5 mL of H$_2$O with 0.137 g of TOAB in 15 mL of toluene. After phase transformation of Au$^{\rm{III}}$ from the aqueous to the toluene phase, the aqueous phase is discarded and 180 ${\rm{\mu }}$L of TBBT thiol is added to the toluene phase. The color of the reaction solution gradually changes from deep orange to faint yellow within 1 h, indicating that the Au$^{\rm{III}}$ is reduced to Au$^{\rm{I}}$ by TBBT thiol. The UV-Vis and MALDI of the product are indeed identical to those of the Au$_{10}$(TBBT)$_{10}$ obtained by the top-down etching method.

The bottom-up self-assembling synthesis of Au$_{10}$(TBBT)$_{10}$ complex was found to be independent of the reaction solvent. Thus, we further simplified the synthesis by simply mixing one portion of HAuCl$_4$ with five portions of TBBT thiol in methanol. The yellowish precipitate of Au$_{10}$(TBBT)$_{10}$ is formed immediately, with a yield larger than 95%. Using methanol as the reaction solvent avoids the phase transition compound such as TOAB, since HAuCl$_4$ can be readily dissolved in methanol. Also, the polar environment of methanol facilitates the reduction and coordination of TBBT with Au salt, which significantly enhances the reaction rate. These advantages make the methanol a facile solvent for the synthesis of Au$_{10}$(TBBT)$_{10}$ complex.

We note that the Au$_{10}$(TBBT)$_{10}$ complex, when dissolved in solvents such as toluene or THF, can be readily reduced by NaBH$_4$ to form polydispersed Au$_n$(TBBT)$_m$ nanoclusters. The result is similar to the direct reduction of the Au$_{10}$(TBBT)$_{10}$ complex generated in situ in the common synthetic routes. Therefore, the accessibility of a precise precursor (Au$_{10}$(TBBT)$_{10}$ complex) can largely simplify the synthesis of the Au$_n$(TBBT)$_m$ nanoclusters.

The formation of a well-defined Au$_{10}$(TBBT)$_{10}$ complex during the top-down etching or bottom up assembly process is in sharp contrast to the formation of less-defined Au$^{\rm{I}}$-SR polymers or oligomers in other thiolate system [6, 7, 9, 10]. The formation of Au$_{10}$(TBBT)$_{10}$ complex can be attributed to the bulkiness of the TBBT ligand, which prevents the intermolecular assembly of the Au$^{\rm{I}}$-SR complexes as well as provides a shielding for the intermolecular Au$^{\rm{I}}$-Au$^{\rm{I}}$ aurophilic interactions. The increase of the solubility, when the bulkiness of the thiolate ligand increases, is consistent with the observations in other metal-thiolate systems [28, 29].

B. Structural analysis and correlation by NMR

We note that the synthesis and structure of Au$_{10}$(TBBT)$_{10}$ were previously reported by Wiseman et al. [23]. In their method, Au$_{10}$(TBBT)$_{10}$ was obtained by reacting TBBT thiol with [AuCl(C$_2$H$_5$SC$_2$H$_4$OH)] compound in aqueous solution. The structure of Au$_{10}$(TBBT)$_{10}$ was found to be two interlocked Au$_5$(TBBT)$_5$ pentameric rings, forming a catenane structure (FIG. 2(A)). To see whether the Au$_{10}$(TBBT)$_{10}$ complexes synthesized in the current work has the same structure as Wiseman's, we performed a NMR study, which can provide deep insight into the symmetric environment of ligands. As shown in FIG. 2(B), the ten TBBT ligands in the Au$_{10}$(TBBT)$_{10}$ complex have three different chemical environments: (Ⅰ) two located at the outmost points of the pentagon ring (red), (Ⅱ) four in the middle (green), and (Ⅲ) the other four at the center (blue). Based on the structure, the NMR pattern should display three sets of peaks in the ratio of 1:2:2.

FIG. 2 (A) The structure of Au$_{10}$(TBBT)$_{10}$ complex. (B) Three environments of tert-butylphenyl groups. (C, D) 1D NMR of TBBT thiol (black) and Au$_{10}$(TBBT)$_{10}$ complex in aromatic and tert-butyl regions. (E) H-COSY. (F) H-C HSQC.

The 1D-NMR spectrum of free TBBT thiol and Au$_{10}$(TBBT)$_{10}$ complexes are compared in FIG. 2 (C, D). The pure TBBT thiol shows peaks at 7.29/7.26, 7.24/7.22, and 1.30 ppm, which are corresponding to the $\alpha$-Η, $\beta$-Η on the aromatic ring, and the tert-butyl-H, respectively (black lines in FIG. 2 (C, D). When anchored in the Au$_{10}$(TBBT)$_{10}$, the TBBT ligands are split into three groups with the integral ratio of 1:2:2, as reflected in both the aromatic region (FIG. 2(C)) and tertbutyl region (FIG. 2(D)). The splitting of peaks into three groups as well as the degeneracy in each group matches with the prediction (FIG. 2(B)), therefore, the structure of Au$_{10}$(TBBT)$_{10}$ reported here should be identical to the Wiseman's structure [23]. We have also checked the NMR of the Au$^{\rm{I}}$-TBBT compounds synthesized by different methods, and they are all identical, which indicates they have the same structure.

2D-NMRs including H-H COSY and HSQC further correlate the $\alpha$-H, $\beta$-H in the same ligands (FIG. 2(E, F)). Group 1 ligands are the most shifted, with $\alpha$-H, $\beta$-H at 7.70/7.68, 6.92/6.90 respectively; group 2 are at 7.61/7.59, 7.28/7.25; and group 3 are the least shift, and the chemical shifts are at 7.55/7.53, 7.32/7.28. The tert-butyl-H also has three groups at 1.33, 1.31, and 1.17 ppm, respectively. The group 3 TBBT ligands can be easily assigned to environment I in FIG. 2(B), based on its half intensity compared with other groups. Group 2 is the most likely in the environment Ⅱ, and group 1 is the most likely to be in environment Ⅲ, since the environment Ⅲ ligands are buried in the middle of the complex and have the strongest interaction with the Au$-$S pentagonal ring. Thus, the environment Ⅲ ligands are expected to have the largest change in chemical shift. Also, the phenyl rings are parallel to the Au$-$S pentagonal ring in environment Ⅲ and are structurally confined (e.g. limited rotation freedom). More detailed correlation requires future theoretical calculations.

C. Photophysical property

Understanding the photophysics of gold-thiolate complexes is of great importance to their photoluminescence and energy-related applications [30]. In the steady state UV-Vis spectrum of Au$_{10}$(TBBT)$_{10}$ complex, the high energy transitions ($\lambda$$ <$300 nm) originate from intraligand (IL) transitions, while absorbance at 345 and 378 nm should arise from ligand to metal charge transfer (LMCT, Au$\leftarrow$S) modified by Au$^{\rm{I}}$$-$Au$^{\rm{I}}$ interaction [31-33]. Femtosecond transient absorption spectroscopy was further employed to investigate the photophysics of Au$_{10}$(TBBT)$_{10}$ complex. Upon photoexcitation at $\lambda$=365 nm, consistent positive signal all over the visible detection range can be observed (FIG. 3(A)). As Au$_{10}$(TBBT)$_{10}$ has almost no absorbance in the visible region, no ground state bleaching (GSB) signal can be observed so that the transient signal originates solely from excited state absorption (ESA). In the initial 36 ps, the ESA at $\sim$750 nm decays to give rise to the ESA at around 520 and 650 nm (FIG. 3(B)). In the following 2.5 ns, ESA at all wavelengths decays dramatically by 90% and only a flat, featureless transient signal can be observed at 2.6 ns. Decay associated spectra (DAS) obtained by global analysis of the transient absorption data and singular value decomposition (SVD) exhibit three decaying components, 14 ps, 350 ps, and $>$1 ns (FIG. 3 (C, D)). Gold-thiolate complexes show ligand to metal charge transfer (LMCT) characteristics in their steady state UV-Vis spectra [31-33], so photo-excitation at 365 nm can directly generate LMCT excited state. As $\lambda$=365 nm is located on the red side of the maximum absorption, it has no excess electronic or vibrational energy to promote Au$_{10}$(TBBT)$_{10}$ to higher excited states. Therefore, there is no vibrational cooling process and the first 14 ps process can be assigned to the stabilization and equilibrium of LMCT state.

FIG. 3 (A) Transient absorption (fs-TA) data of Au$_{10}$(TBBT)$_{10}$ complex dissolved in toluene. (B) Transient absorption spectra as a function of time delay from 0.5 ps to 36 ps. Scattering due to the laser pulse was excluded around 800 nm for (A) and (B). (C) Decay associated spectra (DAS) obtained from the global analysis of the TA data. (D) Kinetic traces (dots) and corresponding fit (solid line) at selected wavelengths, which shows the quality of the fitting.

The fs-TA measurements were repeated in dichromethane and the global analysis gave three time constants of 14 ps, 290 ps, and $>$1 ns. Comparison on the kinetic traces in two solvents (toluene and dichromethane) suggests that the second decay process is sensitive to solvent polarity (FIG. 4(A)) and thus can be ascribed to non-radiative relaxation. Based on the above results, the schematic energy diagram of Au$_{10}$ complex can be summarized in FIG. 4(B). Upon photoexcitation, intersystem crossing (ISC) from $^1$LMCT to $^3$LMCT occurs in a very short time ($ <$100 fs) due to the strong spin-orbital coupling of gold atoms, which is not resolved in our TA measurement. The $^3$LMCT state is then stabilized in 14 ps to form a $^3$LMCT$^*$ state, which experiences a two exponential decay (350 ps and $>$1 ns). The strong non-radiative decay in Au$_{10}$(TBBT)$_{10}$ complex may arise from its low rigidity, which accounts for the extremely weak luminescent. Future theoretical calculation may help to further identify the observed decaying components.

FIG. 4 (A) Comparison of TA kinetics traces of Au$_{10}$(TBBT)$_{10}$ complex dissolved in toluene (solid line) and DCM (dashed line). (B) Proposed energy diagram of Au$_{10}$(TBBT)$_{10}$ complex.
Ⅳ. CONCLUSION

In conclusion, we have identified that the Au$_{10}$(TBBT)$_{10}$ complex is the universal precursor for the Au$_n$(TBBT)$_m$ nanoclusters. The Au$_{10}$(TBBT)$_{10}$ complex is also the end product of the etching of Au$_n$(SR)$_m$ nanoclusters with excess of TBBT thiol and O$_2$. The formation of well-defined Au$_{10}$(TBBT)$_{10}$ complex instead of polymeric Au$^{\rm{I}}$-TBBT species is attributed to the bulkiness of the TBBT thiol. Through NMR characterization, the structure of Au$_{10}$(TBBT)$_{10}$ is correlated with the previously reported structure which is composed of two inter-penertrated Au$_5$(TBBT)$_5$ rings. The photophysical properties of Au$_{10}$(TBBT)$_{10}$ complex are further mapped out by transient absorption spectroscopy, which reveals the exited state absorption in the visible light range with three decaying components at 14 ps, 350 ps, and $>$1 ns The accessibility of the well-defined and precise Au$_{10}$(TBBT)$_{10}$ complex improves the efficiency of the Au$_n$(TBBT)$_m$ nanocluster synthesis, and is expected to further facilitate the control and understanding of the nucleation and growth mechanisms as well as the photophysical properties of the Au$_n$(TBBT)$_m$ nanoclusters.

Ⅴ. ACKNOWLEDGEMENTS

This work was supported by the U.S. National Science Foundation (DMR-1808675). This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract (No.DE-SC0012704).

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Au10(TBBT)10:Aun(TBBT)m纳米团簇的起点与终点
曾晨婕a, 周蒙a, Chakicherla Gayathria, Roberto R.Gila, Matthew Y.Sfeirb, 金荣超a     
a. 美国卡内基梅隆大学化学系, 宾夕法尼亚州, 匹兹堡 15213;
b. 美国布鲁克海文国家实验室, 功能纳米材料中心, 纽约, 厄普顿 11973
摘要: 一价金与巯基形成的配合物(Au-SR)是合成具有原子精确度的Aun(SR)m纳米团簇的重要前驱体.然而,目前研究中Au-SR前驱体的属性还模糊不清.本文发现Au10(TBBT)10配合物是一种广泛存于Aun(TBBT)m纳米团簇合成中的前驱体(TBBT=对叔丁基苯巯酚),即硼氢化钠还原Au10(TBBT)10可制备一系列尺寸的团簇.有趣的是,Au10(TBBT)10配合物还可以通过大量的TBBT巯基和氧气刻蚀Aun(SR)m纳米团簇来重新获得,所以Au10(TBBT)10既是Aun(TBBT)m纳米团簇的合成起点同时也是团簇分解的终点物种.TBBT巯基的位阻效应是形成精确的Au10(TBBT)10配合物而非无定形的Au-SR高分子的原因.通过一维和二维核磁共振的表征,Au10(TBBT)10配合物的结构与先前报道的晶体结构获得了关联.Au10(TBBT)10由两个互相穿插的Au5(TBBT)5五元环组成.通过飞秒吸收光谱表征,Au10(TBBT)10配合物的光物理性质得到进一步的研究.此项对Au10(TBBT)10精确前驱体的研究有助于提高Aun(TBBT)m纳米团簇的合成效率,并且进一步促进对纳米团簇合成的反应机理的理解和控制.
关键词:     团簇    前驱体    光物理性质