Reversible nanocluster structure transformation between face-centered cubic and icosahedral isomers

The reversible transformation between a FCC and icosahedral configuration has been achieved at the atomic level, based on Pt1Ag28 nanocluster isomers.


Introduction
Nanoclusters, with the advantages of precise compositions and well-dened structures, provide an exciting opportunity to grasp the structure-property correlation at the atomic level. 1,2 The quantum size effect of nanoclusters endows them with a plethora of properties, such as photo-luminescence (PL), catalysis, chirality, and magnetism, to name a few. 1,2 The property manipulation at the atomic level has long been a hot topic, and has allowed a series of nanoclusters with controllable chemical-physical properties to be produced. 1f-h,2g,i,k,l Isomerism is being intensely pursued in nanoscience, and has been exploited for tailoring the performances of nanoparticles. 3 However, understanding the isomerism phenomenon at the atomic level has been largely impeded due to the poly-disperse sizes as well as the uncertain surface coordination modes of nanoparticles. 1a,b,3b The precise structures of nanoclusters make it possible to fully grasp the isomerism phenomenon. 4-6 Although stereoisomerism has been extensively studied in the nanocluster eld (that is, obtaining chiraloptical nanoclusters via the separation of le-and right-handed isomers), 4,5 structural isomerism remains rare. Up to now, only a few structural isomers in the nanocluster range have been observed (e.g., Au 28 (SR) 20 , Au 38 (SR) 24 , and Au 52 (SR) 32 ). 6a,c-f Different structure-dependent properties (e.g., optical absorption, catalytic activity, and thermal stability) have been investigated on the basis of these nanocluster isomers. 5f,6 In addition, experimental and theoretical efforts in exploring the isomerism effect on atomic arrangement and properties of nanoclusters should be continued because such ndings enable us to fully grasp the structure-property correlation, and thus help us to design new nanoclusters with unique functions. 5f,6 Meanwhile, accompanied by the structure determination of nanoclusters resolved by single-crystal X-ray crystallography (SCXC), several efforts have been made for grasping the sizegrowth and structure-transformation modes from small complexes to nanoclusters, then to large nanoparticles. 1l,n,7 Icosahedron and face-centered cubic (FCC) are the two most common congurations of nanocluster kernels. The structural transformation from an icosahedral to FCC conguration or its reverse process has been reported recently in the nanocluster eld. 6d,8 For instance, ligand-exchanging Au 25 (S-C 2 H 4 Ph) 18 with excess t BuPh-SH changes the icosahedral conguration of the Au 13 kernel into a FCC conguration. 6d Besides, the existence of Me 2 PhSH can transform Au 44 (S-Ph t Bu) 28 into Au 44 (S-PhMe 2 ) 26 , where the kernel changes from the FCC to the icosahedral conguration. 8a However, for nanocluster isomer systems, to the best of our knowledge, the structural transformation from the FCC to icosahedron or its reverse process, not to mention the reversible transformation between these two congurations, remains incomplete. Such a shortage impedes the full understanding of nanocluster isomerism in terms of structuretransformation modes and structure-property correlation.
In the current work, a reversible transformation has been accomplished between FCC and icosahedral isomers of Pt 1 Ag 28 nanoclusters. The ligand-exchange method is herein exploited for fullling the reversible transformation between Pt 1 Ag 28 (S-Adm) 18 (PPh 3 ) 4 (Pt 1 Ag 28 -1, where S-Adm represents 1-adamantanethiol) with a FCC Pt 1 Ag 12 kernel and Pt 1 Ag 28 (S-c-C 6 H 11 ) 18 (PPh 3 ) 4 (Pt 1 Ag 28 -2, where S-c-C 6 H 11 represents cyclohexanethiol) with an icosahedral Pt 1 Ag 12 kernel. ESI-MS, PL and EXAFS results are combined to demonstrate that the congurational transformation between the FCC and the icosahedron contains two distinct stages: the motif transformation process and kernel transformation process, where the latter transformation is induced by the former one. Accompanied by the structural transformation (from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2), the emission wavelength red-shis from 672 nm to 720 nm, and the HOMO-LUMO energy gap reduces from 1.86 eV to 1.74 eV.

Synthesis of Pt 1 Ag 28 -1 nanoclusters
The synthesis of Pt 1 Ag 28 -1 nanoclusters was carried out with reference to our previous work. 8b 2.3 Converting Pt 1 Ag 28 -1 into Pt 1 Ag 28 -2 nanoclusters Specically, 10 mg of Pt 1 Ag 28 -1 nanoclusters was dissolved in 30 mL of CH 2 Cl 2 . 300 mL of C 6 H 11 -c-SH was added and reacted for 2 hours at 40 C. Aer the reaction was complete, the solution was rotavaporated, and then approximately 50 mL of methanol was added to wash the product. The precipitate was dissolved in CH 2 Cl 2 giving rise to the solution of Pt 1 Ag 28 -2 nanoclusters. The yield was about 80% (i.e., 8 mg of Pt 1 Ag 28 -2 was obtained).
2.4 Converting Pt 1 Ag 28 -2 into Pt 1 Ag 28 -1 nanoclusters Specically, 10 mg of Pt 1 Ag 28 -2 nanoclusters was dissolved in 30 mL of CH 2 Cl 2 . 0.1 g of Adm-SH was added and reacted for 3 minutes at room temperature. Aer the reaction was complete, the solution was rotavaporated, and then approximately 50 mL of methanol was added to wash the product. The precipitate was dissolved in CH 2 Cl 2 giving rise to the solution of Pt 1 Ag 28 -1 nanoclusters. The yield was about 90% (i.e., 9 mg of Pt 1 Ag 28 -1 was obtained). Compared with the conversion from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2, the reverse process is quicker.

Crystallization of the Pt 1 Ag 28 -2 nanoclusters
Single crystals of Pt 1 Ag 28 -2 were crystallized by vapor diffusion of ether into the CH 2 Cl 2 solution of the nanoclusters over 7 days. For promoting the crystallization, Na + (BPh 4 ) À counterions (molar ratio between clusters and counter-ions was 1 : 2) were added into the CH 2 Cl 2 solution. Then black crystals were collected and the structure of the Pt 1 Ag 28 -2 nanocluster was determined. The CCDC number of the Pt 1 Ag 28 -2 nanocluster is 1840953.
2.6 Test of the temperature-PL correlation 10 mg of Pt 1 Ag 28 -1 (or Pt 1 Ag 28 -2) was dissolved in 10 mL of CH 2 Cl 2 /2-CH 3 -THF. Then the solutions were cooled to different temperatures and the PL spectra were measured.

Catalytic performance
For the preparation of catalysts, Pt 1 Ag 28 -1 (or Pt 1 Ag 28 -2) clusters were supported on commercial carbon nanotubes (CNTs; Beijing Bo Yu high-tech new material technology Co., Ltd). The CNTs were rst dispersed in toluene, and the nanoclusters were added to the suspension of CNTs under vigorous magnetic stirring. The adsorption of clusters was allowed to proceed overnight. Then the product was separated from the solution by centrifugation. The cluster@CNT composite was dried in a vacuum for 12 h; then, Pt 1 Ag 28 -1@CNT and Pt 1 Ag 28 -2@CNT catalysts (with a 2 wt% cluster loading) were obtained.
For the catalytic activity test, a 10 mL Schlenk bottle was charged with 0.5 mmol of styrene, 1.5 mmol of TBHP (tert-butyl hydroperoxide), 20 mg of cluster@CNT catalyst, 10 mg of K 2 CO 3 , and 2 mL of toluene. Then the suspension was stirred at 50 C for 24 hours. The suspension was then centrifuged to remove solids, and the catalytic product was analysed by gas chromatography with an internal standard.
2.8 XAFS (X-ray absorption ne structure spectroscopy) measurements XAFS measurements at the Pt L 3 -edge (11564 eV) were performed at the beamline BL14W1 station of the Shanghai Synchrotron Radiation Facility (SSRF), China. The storage ring of the SSRF was working at an energy of 3.5 GeV with an average electron current of 300 mA. The hard X-ray was monochromatized with a Si (311) monochromator. EXAFS data were collected in the transmission mode in the energy range from À200 below to 1000 eV above the Pt L 3 -edge. The acquired EXAFS data were processed according to the standard procedures using the ARTEMIS module implemented in the IFEFFIT soware packages.

X-ray crystallography
The data collection for single crystal X-ray diffraction was carried out on a Bruker Smart APEX II CCD diffractometer under liquid nitrogen ow at 200 K, using graphitemonochromatized Mo Ka radiation (l ¼ 0.71073Å). Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively. The electron density was squeezed by Olex 2. The structure was solved by direct methods and rened with full-matrix least squares on F 2 using the SHELXTL soware package. All non-hydrogen atoms were rened anisotropically, and all the hydrogen atoms were set in geometrically calculated positions and rened isotropically using a riding model.

Theoretical methods
Density functional theory (DFT) calculations were employed to optimize the geometric structures and calculate the Kohn-Sham orbitals of Pt 1 Ag 28 nanoclusters using the Perdew-Burke-Ernzerhof (PBE) GGA functional. 9 The triple-zeta polarized (TZP) basis set with inclusion of the scalar relativistic effect via a zerothorder regular approximation (ZORA) implemented in the ADF package was adopted. 10 In the electronic structure analysis, the Kohn-Sham orbitals were calculated to analyze contributions of different atomic orbital types to molecular orbitals.

Characterization
UV-vis absorption spectra of nanoclusters dissolved in CH 2 Cl 2 were recorded using an Agilent 8453 diode array spectrometer.
PL spectra were measured on an FL-4500 spectrouorometer with the same optical density (OD) of $0.05. Of note, the PL excitation spectrum of Pt 1 Ag 28 -1 or Pt 1 Ag 28 -2 was measured at 600 or 750 nm, respectively, for suppressing the interference from each other (e.g., Pt 1 Ag 28 -1 uoresces at 600 nm, while Pt 1 Ag 28 -2 does not uoresce at this wavelength).
Quantum yields (QYs) were measured with dilute solutions of nanoclusters on a HORIBA FluoroMax-4P.
Transmission electron microscopy (TEM) was conducted on a JEM-2100 microscope with an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyser (DTG-60H, Shimadzu Instruments, Inc.) X-ray photoelectron spectroscopy (XPS) measurement was performed on a Thermo ESCALAB 250, congured with a monochromated Al Ka (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, ood gun to counter charging effects, and analysis chamber base pressure lower than 1 Â 10 À9 mbar.
Inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurements were performed on an Atomscan Advantage instrument made by Thermo Jarrell Ash Corporation.
Electrochemical measurements (differential pulse voltammetry, DPV) of clusters were performed with an electrochemical workstation (CHI 700E) using a Pt working electrode (0.4 mm diameter), a Pt wire counter electrode and an Ag wire quasireference electrode in 0.1 M Bu 4 NPF 6 -CH 2 Cl 2 . The electrolyte solution was deaerated with ultra-high purity nitrogen for 40 min and blanketed under a nitrogen atmosphere during the entire experimental procedure.

Syntheses and crystallization
The Pt 1 Ag 28 -1 nanoclusters were prepared using our previously reported procedure. 8b The Pt 1 Ag 28 -2 nanoclusters were synthesized by reacting pure Pt 1 Ag 28 -1 with excess HS-c-C 6 H 11 at 40 C (see Experimental Methods for more details). Aer $2 hours, all the Pt 1 Ag 28 -1 nanoclusters were completely converted into Pt 1 Ag 28 -2 in a high yield (>80%, Ag atom basis). The as-prepared Pt 1 Ag 28 -2 was crystallized by vapor diffusion of ether into a CH 2 Cl 2 solution of the nanoclusters over 7 days. The Pt 1 Ag 28 -1 structure was reported by us in early work. 8b The structure of Pt 1 Ag 28 -2 is newly determined in the current work. X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) measurements were performed to validate the ratio of Pt/Ag in the bi-metallic Pt 1 Ag 28 -2 nanoclusters (Fig. S1, S2 and Table S1 †), and the results perfectly matched the theoretical value (1/28 of Pt/Ag). Furthermore, the purity of Pt 1 Ag 28 -2 was conrmed by thermogravimetric analysis (TGA). The experimental weight loss of 49.57% (Fig. S3 †) is consistent with the calculated loss (49.26%) of the ligands (i.e., PPh 3 and S-c-C 6 H 11 ) in Pt 1 Ag 28 -2. Both Pt 1 Ag 28 nanoclusters are highly stable at 50 C ( Fig. S4; † clusters are dissolved in CHCl 3 , and in contact with air). In the DPV analysis ( Fig. S5 †), Pt 1 Ag 28 -1 showed two oxidation peaks at 0.76 V (O1) and 1.28 V (O2) and a reduction peak at À1.32 V (R1); in comparison, Pt 1 Ag 28 -2 exhibited three oxidation peaks at 0.75 V (O1), 1.08 V (O2) and 1.45 V (O3) and a reduction peak at À0.92 V (R1). The catalytic performance of both Pt 1 Ag 28 nanoclusters in the oxidation of styrene was evaluated (Fig. S6, † and see the catalysis conditions in Experimental methods). The catalysis was performed at 50 C to ensure that both clusters were stable. Although the catalytic selectivity for epoxide or benzaldehyde is similar in both catalysis, the overall catalytic conversion by Pt 1 Ag 28 -2@CNTs is much higher than that by Pt 1 Ag 28 -1@CNTs (66.48% versus 43.43%; shown in Fig. S6 †). Such a difference may result from the ligand effect between the two nanoclusters.
The Pt 1 Ag 28 -2 nanoclusters crystallize in the P2 1 /c space group. The structure of Pt 1 Ag 28 -2 is shown in Fig. 1A and the structural anatomy is shown in Fig. 1B-E (see Fig. S7 † for the complete structure). Specically, a single Pt atom is arranged in the innermost position of the overall structure (Fig. 1B), which is further capped by an Ag 12 cage, forming a Pt 1 Ag 12 kernel with an icosahedral conguration (Fig. 1C). Furthermore, this Pt 1 Ag 12 kernel is surrounded by four Ag 3 (S-c-C 6 H 11 ) 6 motifs. Of note, each Ag 3 (S-c-C 6 H 11 ) 6 motif links with three other motifs via sharing the terminal thiolates, which constitutes a cage-like Ag 12 (S-c-C 6 H 11 ) 18 motif structure fully surrounding the Pt 1 Ag 12 kernel (Fig. 1D). Moreover, four Ag-PPh 3 architectures occupy the four vacancies of the aforementioned Pt 1 Ag 12 @Ag 12 (S-c-C 6 H 11 ) 18 structure, giving rise to the nal Pt 1 Ag 28 -2 structure. It should be noted that the capped Ag 12 (SR) 18 @(Ag-PPh 3 ) 4 structure of Pt 1 Ag 28 -2 is similar to that of Pt 1 Ag 28 -1. 8b The TEM images of Pt 1 Ag 28 -1 and Pt 1 Ag 28 -2 indicate that the clusters are uniform in size of about 1.6 nm (Fig. S8 †), and this agrees with that determined by X-ray structural analysis.

Reversible structure transformation
In order to elucidate the structural differences induced by the ligand exchange (i.e., S-c-C 6 H 11 versus S-Adm), a comparison of the kernels of Pt 1 Ag 28 -1 and Pt 1 Ag 28 -2 is provided in Fig. 2. Both Pt 1 Ag 28 nanoclusters comprise a Pt 1 Ag 12 kernel; however, the FCC conguration of the Pt 1 Ag 12 kernel in Pt 1 Ag 28 -1 turns into the icosahedral conguration when the nanocluster converts to Pt 1 Ag 28 -2. The opposite process is also conrmed by reacting Pt 1 Ag 28 -2 with excess HS-Adm. Previously, two nanoclusters following the isomerism phenomenon are almost arranged in the same molecular conguration, e.g., Au 21 (S-Adm) 15 and Au 21 (S-t Bu) 15 isomers with a FCC conguration, 11,12 Au 28 (S-Ph t Bu) 20 and Au 28 (S-c-C 6 H 11 ) 20 isomers with a FCC conguration, 6d,e Au 52 (S-Ph t Bu) 32 and Au 52 (S-PhC 2 H 4 ) 32 isomers with a FCC conguration, 6c Au 9 Ag 12 (S-Adm) 4 (dppm) 6 Cl 6 and Au 9 -Ag 12 (S-t Bu) 4 (dppm) 6 Cl 6 with an icosahedral conguration, 6f and so on.  Signicantly, these two Pt 1 Ag 28 isomers exhibit reversibility in terms of the kernel conguration between the FCC and icosahedron, which is observed for the rst time in the nanocluster range (Fig. 2). Table 1 lists the comparison of bond lengths of Pt(core)-Ag(edge), Ag(edge)-Ag(edge), Ag(edge)-S(motif) and Ag(vertex)-P(vertex) between these two Pt 1 Ag 28 isomers (see Fig. S9-S12 † for the highlights of these types of bonds). Specically, in Pt 1 Ag 28 -1, the bond lengths between the central Pt atom and the Ag atoms on the Pt 1 Ag 12 shell range from 2.768 to 2.797Å (average: 2.783Å), whereas the Pt(core)-Ag(edge) distances in Pt 1 Ag 28 -2 shorten to 2.744-2.791Å (average: 2.763 A). In contrast, the average Ag(edge)-Ag(edge) bond length of 2.801Å in Pt 1 Ag 28 -1 signicantly increases to 2.915Å (with a 4.07% difference) in Pt 1 Ag 28 -2. Furthermore, the average Ag(edge)-S bond length displays a 0.92% elongation in Pt 1 Ag 28 -2 compared with that of the Pt 1 Ag 28 -1 nanocluster. Moreover, the average Ag(vertex)-P bond length (2.400Å) in Pt 1 Ag 28 -2 is also slightly longer than that in Pt 1 Ag 28 -1 (2.356Å).
Electrospray ionization mass spectrometry (ESI-MS) was performed to verify the purity of each Pt 1 Ag 28 isomer. As shown in Fig. 2C and D, the reaction between Pt 1 Ag 28 -1 and HS-c-C 6 H 11 decreases the mass value from 3637.64 Da to 3169.36 Da, which is assigned to Pt 1 Ag 28 -2. The magnication of the peak suggests a +2 charge state of Pt 1 Ag 28 -2, since this peak evidences a characteristic isotopic pattern with peaks separated by an m/z of 0.5 Da (in the positive mode). In this context, the overall charge state of these two Pt 1 Ag 28 isomers is the same +2. Of note, the +2 charge state of Pt 1 Ag 28 -2 matches the crystal data, because two negative (BPh 4 ) À counterions were observed in the crystal structure of Pt 1 Ag 28 -2. In addition, the gap between the mass peaks of two Pt 1 Ag 28 isomers is calculated as 936.56 Da (i.e., (3637.64 À 3169.36) Â 2), which is in accordance with 18-fold the molecular weight gap between HS-Adm and HS-c-C 6 H 11 ligands.
It is accepted that the structures of nanoclusters play a decisive role in their chemical/physical properties. 1a,b,1f-h The optical properties of Pt 1 Ag 28 isomers are compared here to investigate the precise structure-property correlation. First of all, the time-dependent variation of UV-vis absorption from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2 was recorded. As shown in Fig. 3A, the absorptions at 545 and 443 nm in Pt 1 Ag 28 -1 red-shi to 575 and 455 nm, respectively, in Pt 1 Ag 28 -2. Furthermore, the 333 and 305 nm absorption peaks in Pt 1 Ag 28 -1 become more pronounced and red-shi to 350 and 315 nm, respectively. A total of four iso-absorption points are observed, centering at 307, 338, 427 and 454 nm. These iso-absorption points illustrate the high level of conversion from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2. The solution of Pt 1 Ag 28 -2 appears green to the naked eye, while the Pt 1 Ag 28 -1 solution is orange (Fig. 3A, insets). The photon energy plots of the two Pt 1 Ag 28 isomers were recorded (Fig. 3B), which demonstrate that the energy gap (between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) of Pt 1 Ag 28 -2 (1.74 eV) is much smaller than that of Pt 1 Ag 28 -1 (1.86 eV); this is possibly due to the conguration transformation from the FCC to the icosahedron. In addition, accompanied by the ligand-exchange process from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2, gradual reduction was detected in the HOMO-LUMO gap (Fig. 3B). Fig. 3C and D show the PL performances of the Pt 1 Ag 28 isomers. First of all, the PL excitation spectra are almost identical to the absorption spectra, which is reminiscent of the behavior of quantum-dots and some nanoclusters. 8b,13 In addition, the emission peak wavelength is not dependent on the excitation wavelength but remains at 672 nm for Pt 1 Ag 28 -1 and at 720 nm for Pt 1 Ag 28 -2. Furthermore, the emission of each isomer (i.e., 1.84 eV of Pt 1 Ag 28 -1 and 1.72 eV of Pt 1 Ag 28 -2) is very close to the HOMO-LUMO gap energy derived from the optical absorption spectrum. The extremely small difference in energy illustrates that the uorescence corresponds to the HOMO-LUMO transition of each Pt 1 Ag 28 isomer.
The structures of nanoclusters are determinant of their physical and chemical properties. 1a,b,f-h In this context, the signicant red-shi of the emission wavelength from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2 ($50 nm) is traceable to the kernel transformation from the FCC to icosahedron. Considering that the kernel transformation is induced by the ligand exchange, we were motivated to track this conguration variation by matching the ESI-MS changes and the corresponding emission variations. Here, the ESI-MS and uorescence variations from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2 are detected since the corresponding transformation is much slower than the reverse process, and thus is easier to monitor (see Fig. S13-S15 † for the UV-vis, ESI-MS and PL variations from Pt 1 Ag 28 -2 to Pt 1 Ag 28 -1). First of all, ESI-MS measurements were performed to monitor the ligandexchange degree of each stage from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2 (Fig. 3E), and the time-dependent ligand-exchange process in mild and stepwise modes was deduced. Furthermore, the uorescence variation can be divided into two periods (Fig. 3F, see the lateral comparison in Fig. S16 †): (Period 1) from stage 1 to stage 5: each uorescence curve only exhibits a single characteristic peak, although the emission red-shis from 672 nm of stage 1 to 674, 675, 677, and 678 nm of stages 2-5, respectively; (Period 2) from stage 6 to stage 9: each emission curve contains two overlapping peaks, where the front peak decreases accompanying the ligand-exchange process, but the rear one increases gradually. Finally, the uorescence curve just exhibits a single peak at 720 nm, which is the emission of pure Pt 1 Ag 28 -2 (stage 9). Combining the variations of ESI-MS and uorescence, the emission just displays a slight red-shi when the number of ligands exchanged on Pt 1 Ag 28 -1 is less than 9-10 (stages 1-5, depicted by the blue arrow in Fig. 3F). When exchanging more ligands onto the nanocluster, a new peak emerges at 712 nm Table 1 Comparison of different bond lengths in Pt 1 Ag 28 -1 and Pt 1 Ag 28 -2 crystal structures (stage 6). This new peak gradually red-shis and increases to the nal emission at 720 nm (stages 6-9, depicted by the red arrow in Fig. 3F), in which process the peak at $672 nm fades away. According to these phenomena, we propose that the transformation from Pt 1 Ag 28 -1 to Pt 1 Ag 28 -2 is a two-step process (Fig. 4): (i) the motif transformation process: the outmost motif structures alter gradually when inadequate S-c-C 6 H 11 ligands are exchanged on the surface of the Pt 1 Ag 28 nanocluster. In this process the FCC conguration of the Pt 1 Ag 12 kernel remains (corresponding to stages 1-5); (ii) the kernel transformation process: sharp transformation of the kernel conguration from the FCC to icosahedron occurs as long as enough foreign ligands have been exchanged on the nanocluster surface, and nally all Pt 1 Ag 28 nanoclusters contain the icosahedral Pt 1 Ag 12 kernel (corresponding to stages 6-9). In addition, considering that the exchanged ligands do not come into contact with the innermost Pt 1 Ag 12 kernel directly, we speculate that the kernel transformation occurs as a result of the motif transformation.

XAFS measurements
Our repeated attempts to crystallize the samples from stages 2-8 were unsuccessful, which might be because such Pt 1 Ag 28 samples are protected by different proportions of S-Adm and S-c-C 6 H 11 ligands (Fig. 3E, stages 2-8). XAFS measurements were performed for grasping the structural variation of Pt 1 Ag 28 nanoclusters in different stages. XAFS results of stages 1, 3, 5, 7, and 9 are recorded since such results are capable of revealing the structural variations that correspond to the initial PL red-shi (672 nm / 678 nm) and the further abrupt changes (678 nm / 712 nm). As shown in Fig. 5 and Table 2 (see Fig. S17-S21 † for more information), the tted number of Pt-Ag bonds in each Pt 1 Ag 28 sample is 12, which is in accordance with the Pt 1 Ag 12 kernel in both FCC-Pt 1 Ag 28 and icosahedral-Pt 1 Ag 28 nanoclusters. Furthermore, the invariable bond lengths of Pt-Ag (remaining as 2.75 AE 0.01Å, Table 2) in these samples validate the extremely small difference (cal. 0.72%, Table 1) in average Pt(core)-Ag(edge) bond lengths in Pt 1 Ag 28 -1 and Pt 1 Ag 28 -2.
Importantly, both the k 2 c(k) oscillation and Fourier transform curves of stages 1-5 demonstrate that the Pt-Ag bonds are almost invariable in these stages ( Fig. 5 and Table 2); however, the lower intensity of the Fourier transformed EXAFS k 2 c(k) oscillations in stages 1-5 than in stages 7-9 indeed shows the higher disorder degree of Pt-Ag bonds in the former, i.e., the   larger Debye-Waller factor s 2 listed in Table 2. Generally, the Debye-Waller factor s 2 is a sum of two components, thermal disorder (s T 2 ) and structural disorder (s S 2 ), i.e., s 2 ¼ s T 2 + s S 2 .
From the single crystal XRD analysis (Table 1), the Pt-Ag bond lengths in Pt 1 Ag 28 -1 span a narrower range of 0.029Å (from 2.768 to 2.797Å, average 2.783Å) than that (0.047Å, varying from 2.744 to 2.791Å, average 2.763Å) in Pt 1 Ag 28 -2, indicating a smaller structural disorder s S 2 in the FCC isomer. The larger s 2 but smaller s S 2 of the Pt-Ag bonds in Pt 1 Ag 28 -1 than in icosahedral Pt 1 Ag 28 -2 suggests that the thermal disorder s T 2 is larger in Pt 1 Ag 28 -1. It is known that when the measurement temperature T is close to or higher than the Einstein temperature of a bond, the thermal disorder s T 2 could be approximated by the correlated Einstein model: Einstein temperature and k eff is the effective spring constant of the bond. 14 Therefore, the larger thermal disorder s T 2 in Pt 1 Ag 28 -1 indicates that the Pt-Ag interaction (reected by the effective spring constant) is weaker in Pt 1 Ag 28 -1 than in Pt 1 Ag 28 -2, or the Pt-Ag bond is strengthened in Pt 1 Ag 28 -2. This deduction is supported by the fact that, from the XRD analysis, the average Pt-Ag bond length (2.783Å) in Pt 1 Ag 28 -1 is longer by 0.02Å than that (2.763Å) in Pt 1 Ag 28 -2. More support could also be afforded by the Kohn-Sham molecular orbitals (MO) of both clusters yielded by DFT calculations (Fig. S22 †). The bonding HOMO orbitals of Pt 1 Ag 28 -1 are composed of 2.16% Pt 6sp, 2.29% Pt 5d, 26.03% Ag 5sp, 20.3% Ag 4d, and 41.67% S 3p orbitals, while the HOMO orbitals of Pt 1 Ag 28 -2 are composed of 4.08% Pt 6sp, 1.24% Pt 5d, 39.79% Ag 5sp, 21.46% Ag 4d, and 26.97% S 3p orbitals. The smaller amount of Pt orbital components in Pt 1 Ag 28 -1 than in Pt 1 Ag 28 -2 (4.45% versus 5.22%) suggests the weaker Pt-Ag bonding in the former. This is also consistent with the consideration based on antibonding LUMO orbitals, where Pt 1 Ag 28 -1 has a larger amount of Pt orbital components than Pt 1 Ag 28 -2 (8.04% versus 7.33%). EXAFS results indeed illustrate that the kernel transformation occurs within the ligand-exchange process from stage 5 to stage 7. Such a process (stage 5 / stage 7) is also regarded as the break point where signicant PL variation occurs (Fig. 3F). In this context, by combining the ESI-MS, PL and EXAFS variation results, it has been unambiguously demonstrated that the nanocluster conguration transformation (from FCC to icosahedron) contains two discrete steps: the motif transformation and the kernel transformation. Specically, in the early stage of the ligand-exchange process (stages 1-5), only the motif transformation occurs, which hardly affects the inner Pt-Ag bonds; when enough foreign ligands are exchanged on the Pt 1 Ag 28 nanocluster (stages 5-9), remarkable transformation in the kernel occurs and the FCC conguration turns into the icosahedral conguration.

Temperature-dependent PL
The PL QY of Pt 1 Ag 28 -2 is only 2.7% at room temperature, which is much lower than that of Pt 1 Ag 28 -1 (QY $ 4.9%). However, Pt 1 Ag 28 -2 can emit bright-red light at low temperature (QY $ 100% at 98 K or lower temperature). Accordingly, the uorescence intensity increased signicantly (a 63-fold increase by comparing the 98 K data with the 293 K data) when the temperature is reduced to 98 K (Fig. 6A-C), and the UV-vis absorption presents a 1.8-fold enhancement (Fig. 6D). In this context, the PL QY of Pt 1 Ag 28 -2 increases to almost 100%. In detail, when the temperature is higher than 250 K, the PL intensity is so weak that it cannot be observed by the naked eye (Fig. 6B, inset 275 K). When the temperature is continually reduced, the PL intensity enhances rapidly, and an obvious emission can be observed at 173 K (Fig. 6B, inset 173 K). Furthermore, the emission of Pt 1 Ag 28 -2 nanoclusters is considerably bright when the temperature is reduced to 98 K or lower (Fig. 6B, inset 98 K). The enhanced PL intensity of Pt 1 Ag 28 -2 is induced by the restrained thermal vibration (non-radiative  transition) of the nanoclusters at low temperature, and thus the energy loss is just uorescence (radiative transition). 15 For comparison, the PL variation of Pt 1 Ag 28 -1 accompanying the reduction of the temperature is exhibited in Fig. S23. † Temperature-dependent PL spectra were also measured on the intermediate ligand-exchange Pt 1 Ag 28 product. Specically, we chose the product of stage 7 (see Fig. 3E and F) because the PL spectrum of this product displays the most obvious overlapping peaks. As shown in Fig. 6E, accompanying the reduction of temperature from 293 K to 77 K, the overlapped peaks become sharper and more separated. Meanwhile, the emission intensity appears to be signicantly enhanced in the temperature reduction process. In this context, the peak-tting becomes more easy and precise. Fig. 6F exhibits the overlapped curves from the PL spectrum of the intermediate ligand-exchange product (stage 7) measured at 77 K. Two independent curves were separated from the multi-peak PL spectrum, and they center at 680 and 713 nm, respectively. Of note, the tting-peak positions are retained for these two separated curves; however, following the temperature reduction, the relative intensity of the tted curve at $680 nm becomes stronger relative to the other one, which demonstrates the different emission performance of the FCC-Pt 1 Ag 28 and icosahedral-Pt 1 Ag 28 nanoclusters with the temperature reduction.

Conclusions
In summary, a ligand-exchange method was exploited to reversibly transform the Pt 1 Ag 28 -1 nanoclusters with a FCC conguration and the Pt 1 Ag 28 -2 nanoclusters with an icosahedral conguration. This is the rst time that the isomerism phenomenon with reversible congurations has been observed. ESI-MS, PL and EXAFS results were combined to illustrate that the conguration transformation (between FCC and icosahedron) is a two-step process, including the outmost motif transformation process and the innermost kernel transformation process, where the latter transformation is induced by the former one. Based on these Pt 1 Ag 28 isomers, the corresponding structure-optical property correlation was evaluated. UV-vis absorption, together with PL emission, demonstrates a reduced HOMO-LUMO gap of Pt 1 Ag 28 -2 compared with that of Pt 1 Ag 28 -1. Overall, this work presents a nanocluster isomer system with a reversibly transforming conguration, which hopefully draws great attention of structural and theoretical chemists to fully understand the structural transformations as well as the structure-property correlation of metal nanoclusters.

Conflicts of interest
There are no conicts to declare.