Aggregation of Surface Structure Induced Photoluminescence Enhancement in Atomically Precise Nanoclusters
2020; Chinese Chemical Society; Volume: 3; Issue: 7 Linguagem: Inglês
10.31635/ccschem.020.202000372
ISSN2096-5745
AutoresXiao Wei, Xi Kang, Shan Jin, Shuxin Wang, Manzhou Zhu,
Tópico(s)Inorganic Chemistry and Materials
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLE1 Jul 2021Aggregation of Surface Structure Induced Photoluminescence Enhancement in Atomically Precise Nanoclusters Xiao Wei†, Xi Kang†, Shan Jin, Shuxin Wang and Manzhou Zhu Xiao Wei† Department of Chemistry, Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei 230601 Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui University, Hefei 230601 , Xi Kang† Department of Chemistry, Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei 230601 Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui University, Hefei 230601 , Shan Jin Department of Chemistry, Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei 230601 Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui University, Hefei 230601 , Shuxin Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei 230601 Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui University, Hefei 230601 and Manzhou Zhu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei 230601 Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of Education, Anhui University, Hefei 230601 https://doi.org/10.31635/ccschem.020.202000372 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Although intermolecular aggregation-induced emission (AIE) has been extensively studied in nanocluster science, it remains challenging to activate AIE via intracluster aggregation (i.e., the aggregation at the single-molecular level). Here, an intracluster AIE system based on the Pt1Ag24(SR)18 nanocluster has been established. Specifically, triggered by the addition of free thiol ligands, the Ag2(SR)3 surface motif structures of Pt1Ag24(SR)18 are aggregated to nanocluster poles, making up new [email protected][3*Ag2(SR)3] surface structures, through which the nanocluster converts into Pt1Ag24(SR)20. The inverse conversion [i.e., from Pt1Ag24(SR)20 to Pt1Ag24(SR)18] has been accomplished under a 427 nm irradiation. Significantly, induced by the intracluster AIE, a 160-fold enhancement of the photoluminescence intensity has been achieved by comparing the emission of Pt1Ag24(SR)18 with Pt1Ag24(SR)20. This study presents a novel AIE case (i.e., intramolecular AIE enhancement) for remarkably boosting the emission of metal clusters at the single-molecular level. Download figure Download PowerPoint Introduction Metal nanoclusters have served as an emerging class of modular nanomaterials1–19 and have attracted intensive research interest due to their atomically precise structures20–32 and intriguing physical–chemical properties.33–47 Among these properties, photoluminescence (PL) is one of the most fascinating, both theoretically and experimentally, due to the plethora of promising PL-based applications.48–60 Particular attention has been focused on understanding mechanisms of the PL of nanoclusters and then controlling the synthesis of more highly emissive nanoclusters.48–60 Aggregation-induced emission (AIE), a photophysical phenomenon wherein nanomaterials that are nonluminescent or weakly luminescent become highly luminescent upon aggregation, has been extensively exploited for preparing strongly emissive cluster-based nanomaterials.61–69 As for the mechanism of the nanocluster AIE, it has been accepted that orderly aggregation would reduce the energy loss of photoexcited clusters through nonradiative transitions, and the energy released through the radiative transition (i.e., PL) would increase accordingly.65 Up to the present, the AIE of metal nanoclusters is mostly accomplished by the restriction of intramolecular motion (RIM), wherein the solvent- or the cation-induced aggregation is commonly exploited.61–69 However, the AIE of most cases is activated by the intercluster aggregation-triggering RIM, and it remains challenging to activate the AIE of metal clusters via a directly intracluster aggregation-triggering RIM (i.e., the aggregation at the single-molecular level). Such a deficiency impedes a full understanding of AIE of metal nanoclusters. In this study, based on the Pt1Ag24(SR)18 cluster template (SR = 2,4-SPhCl2), the AIE of metal nanoclusters has been accomplished at the single-molecular level, activating by the intracluster aggregation-triggering RIM. The presence of free thiol ligands assembled the Ag2(SR)3 surface motif structures of Pt1Ag24(SR)18 to the nanocluster poles, giving rise to new [email protected][3*Ag2(SR)3] surface structures, and the nanocluster converts into Pt1Ag24(SR)20. The Pt1Ag24(SR)20 could be inversely converted Pt1Ag24(SR)18 under a 427 nm irradiation. Electrospray ionization mass spectrometry (ESI-MS) and UV–vis measurements were performed to track the nanocluster reversible transformation. As a result of the surface structure aggregation, the Pt1Ag24(SR)20 nanocluster exhibited a high PL intensity, which was 160 times that of the Pt1Ag24(SR)18 nanocluster. Experimental Methods Materials All reagents were purchased from Sigma-Aldrich, Shanghai, China and used without further purification: silver nitrate (AgNO3; 99%, metal basis), potassium tetrachloroplatinate (II) (K2PtCl4; 99%, metal basis), 2,4-dichlorobenzenethiol (2,4-SPhCl2; 99%), 4-chlorothiophenol (HSPhtCl; 99%), tetraphenylphosphonium bromide (PPh4Br; 98%), sodium borohydride (NaBH4; 98%), triethylamine (C6H15N; 99.5%), dichloromethane (CH2Cl2; HPLC (high-performance liquid chromatography grade solvent), Sigma-Aldrich), methanol (CH3OH, HPLC, Sigma-Aldrich), N,N-dimethylformamide (DMF; HPLC, Sigma-Aldrich), hexane (C6H6; HPLC, Sigma-Aldrich), and ethyl ether [(CH3CH2)O; HPLC, Sigma-Aldrich]. Synthesis of Pt1Ag24(SR)18 The preparation of Pt1Ag24(SR)18 was based on the reported method of the Zheng group.70 Conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20 To the DMF solution of Pt1Ag24(SR)18, different mole ratios of free thiol were added in. After sixfold mole of free thiol was fed, all Pt1Ag24(SR)18 were converted into the Pt1Ag24(SR)20. The yield is about 90% based on the Ag element [calculated from the Pt1Ag24(SR)18] for the synthesis of Pt1Ag24(SR)20. Conversion from Pt1Ag24(SR)20 to Pt1Ag24(SR)18 The DMF solution of Pt1Ag24(SR)20 was irradiated by the visible lamp with a wavelength of 427 nm. After 80 s, all Pt1Ag24(SR)20 were converted into the Pt1Ag24(SR)18. The yield was about 90% based on the Ag element [calculated from the Pt1Ag24(SR)20] for the synthesis of Pt1Ag24(SR)18. Crystallization of Pt1Ag24(SR)20 Single crystals of Pt1Ag24(SR)20 were cultivated at room temperature by vapor diffusing the (CH3CH2)O into the CH2Cl2 solution of Pt1Ag24(SR)20. After 3 days, red crystals were collected, and the structure of Pt1Ag24(SR)20 was determined. The CCDC (Cambridge Crystallographic Data Centre) number of Pt1Ag24(SPhCl2)20 is 1975650; the CCDC numbers of Pt1Ag24(SR′)20 are 2008420 and 2008615 (SR′ is a mixture of SPhCl2 and SPhtCl). Characterizations The UV–vis absorption spectra of nanoclusters were recorded using an Agilent 8453 diode array spectrometer. The emission spectra were measured on a FL-4500 spectrofluorometer with the same optical density. Optical absorption and PL emission of nanoclusters in the solution state were measured in the DMF. Optical absorption and PL emission of nanocluster films were measured on quartz plates with the same painting operation of different clusters. Absolute quantum yields (QYs) were measured with dilute solutions of nanoclusters (0.05 OD absorption at 445 nm) on a HORIBA FluoroMax-4P. ESI-MS measurements were performed by MicrOTOF-QIII high-resolution mass spectrometer. The sample was directly infused into the chamber at 5 μL/min. For preparing the ESI samples, nanoclusters were dissolved in CH2Cl2 (1 mg/mL) and diluted (v/v = 1∶2) by CH3OH. Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyzer (DTG-60 H, Shimadzu Scientific Instruments Inc., Columbia, Maryland, USA). TGA data were collected from 10 mg of clusters. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 configured with a monochromatized Al Kα (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, flood gun to counter charging effects, and analysis chamber base pressure lower than 1 × 10−9 mbar. Inductively coupled plasma atomic emission spectrometry (ICP-AES) measurements were performed on an AtomScan Advantage instrument made by Thermo Jerrell Ash Corporation (Waltham, Massachusetts, USA). X-Ray crystallography The data collection for single-crystal X-ray diffraction was carried out on a Stoe StadiVari diffractometer under nitrogen flow, using graphite-monochromatized Cu Kα radiation (λ = 1.54186 Å). Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively. The electron density was squeezed by PLATON. The structure was solved by direct methods and refined with full-matrix least-squares on F2 using the SHELXTL software package (Madrid, Spain). All nonhydrogen atoms were refined anisotropically, and all the hydrogen atoms were set in geometrically calculated positions and refined isotropically using a riding model. Results and Discussion Syntheses and characterizations The Pt1Ag24(SR)18 (SR = 2,4-SPhCl2) nanocluster was prepared using the previously reported procedure.70,71 The Pt1Ag24(SR)20 was synthesized by reacting the Pt1Ag24(SR)18 nanocluster with excess thiol ligand (Figures 1a and 1b, also see Experimental Methods section for more details). Reversibly, the conversion from Pt1Ag24(SR)20 to Pt1Ag24(SR)18 was accomplished by irradiating the Pt1Ag24(SR)20 with 427 nm UV light (Figures 1a and 1b, also see Experimental Methods section for more details). The conversion rates of both from Pt1Ag24(SR)18 to Pt1Ag24(SR)20 and from Pt1Ag24(SR)20 to Pt1Ag24(SR)18 were very high (>90%, Ag atom basis). Figure 1 | Reversible conversion between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters. (a and b) Thiol addition-induced conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20, and 427 nm irradiation-induced conversion from Pt1Ag24(SR)20 to Pt1Ag24(SR)18. Color legends: dark green sphere, Pt; magenta sphere, Ag on the icosahedron; light blue sphere, Ag on motif shell; red sphere, S. For clarity, all C, H, and Cl atoms and some bonds are omitted. (c) Optical absorptions of Pt1Ag24(SR)18 with various ratios of thiol ligand to nanocluster. (d) Time-dependent optical absorptions of the 427 nm irradiation-induced conversion from Pt1Ag24(SR)20 to Pt1Ag24(SR)18. Download figure Download PowerPoint The crystal structures of Pt1Ag24(SR)18 and Pt1Ag24(SR)20 are shown in Figures 1a and 1b, and the crystal lattices of Pt1Ag24(SR)18 and Pt1Ag24(SR)20 are shown in Supporting Information Figure S1 and Table S1. Each unit cell of Pt1Ag24(SR)18 contains four cluster compounds and eight PPh4 counterions. Thus, the chemical formula is [Pt1Ag24(SR)18](PPh4)2 ( Supporting Information Figure S1a). By comparison, each unit cell of Pt1Ag24(SR)20 comprises one cluster compound and four PPh4 counterions. Thus, the chemical formula of the nanocluster is [Pt1Ag24(SR)20](PPh4)4 ( Supporting Information Figure S1b). A combination of ICP and XPS measurements was performed to validate the ratio of Pt/Ag in the bimetallic Pt1Ag24(SR)20 nanocluster ( Supporting Information Figure S2 and Table S2), and the results perfectly matched the theoretical value (1/24 of Pt/Ag). In addition, the purity of Pt1Ag24(SR)20 was confirmed by the TGA measurement; the experimental weight loss of 64.13% ( Supporting Information Figure S3) was consistent with the calculated loss (63.86%) of the ligands and counterions (i.e., 2,4-SPhCl2 and PPh4) in Pt1Ag24(SR)20. UV–vis measurement was performed to track the reversible conversions between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters (Figures 1c and 1d). The DMF solution of Pt1Ag24(SR)18 exhibited two intense optical absorptions at 455 and 575 nm and a shoulder band at 400 nm, whereas the DMF solution of Pt1Ag24(SR)20 displayed one intense absorption at 400 nm and several shoulder bands at 435, 485, and 575 nm (Figures 1c and 1d). A total of four iso-absorption points were observed in the conversion, centering at 395, 420, 480, and 505 nm (Figure 1c); these iso-absorption points confirm the high level of conversion. Besides, considering the same size of both nanoclusters as M25, we proposed that their different optical absorptions resulted from the surface structure effect (but not the size effect). As evidenced by the UV–vis tracking from Pt1Ag24(SR)18 to Pt1Ag24(SR)20, when more than fivefold moles (Mthiol/Mcluster) of thiol ligands were added into the nanocluster solution, all Pt1Ag24(SR)18 nanoclusters were completely converted into the Pt1Ag24(SR)20 (Figure 1c). As for the conversion from Pt1Ag24(SR)20 to Pt1Ag24(SR)18 induced by a 427 nm irradiation (with a 427 nm UV light), the UV–vis tracking demonstrated that such a conversion could be achieved with a rapid rate (within 70 s, as depicted in Figure 1d). It is suggested that photons from the 427 nm irradiation exhibited higher energy than the Eg of Pt1Ag24(SR)20 [e.g., highest occupied molecular orbital (HOMO)– lowest unoccupied molecular orbital (LUMO) energy gap],72 and thus the Pt1Ag24(SR)20 nanocluster was activated and then transformed to Pt1Ag24(SR)18. Collectively, a combination of thiol addition and 427 nm irradiation operations completed the reversible conversion between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters (Figure 1). Structure analyses The crystal structures of Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters are shown in Figures 1a and 1b and Supporting Information Figure S4. The Pt1Ag24(SR)18 nanocluster follows a spherical configuration, whereas the Pt1Ag24(SR)20 is in ellipsoidal ( Supporting Information Figure S4). Structurally, (1) For the Pt1Ag24(SR)18 nanocluster: the total structure of Pt1Ag24(SR)18 contains an icosahedral Pt1Ag12 kernel, which is stabilized by a spherical Ag12(SR)18 shell, and such a shell can be split into six Ag2(SR)3 dimeric staple-like motifs ( Supporting Information Figure S5).70,71 In this context, the overall framework of Pt1Ag24(SR)18 can be viewed as "Pt1Ag12 + 6*Ag2(SR)3." (2) For the Pt1Ag24(SR)20 nanocluster: the overall Pt1Ag24(SR)20 structure comprises a Pt1Ag12 kernel and two Ag6(SR)10 surface caps ( Supporting Information Figure S6), where the Pt1Ag12 kernel is in a same icosahedral configuration with that of Pt1Ag24(SR)18. Accordingly, the overall framework of Pt1Ag24(SR)18 can be viewed as "Pt1Ag12 + 2*Ag6(SR)10." Each Ag6(SR)10 surface structure is composed of three irregular hexagons by sharing the Ag–SR–Ag edges and then terminated by three SR ligands ( Supporting Information Figure S6). The two Ag6(SR)10 surface structures are totally the same and cover the top and bottom of the Pt1Ag12 kernel separately; however, there are no interactions between the two Ag6(SR)10 surface caps ( Supporting Information Figure S6). The overall configuration of the Pt1Ag24(SR)20 nanocluster is in threefold axisymmetry, and the C3 symmetry axis passes through the two polar S and the innermost Pt atoms ( Supporting Information Figure S7). Among the 12 surface Ag atoms of Pt1Ag24(SR)18, half of them are μ2-Ag (i.e., binding with two thiol ligands), and the others are in a μ3 coordination mode (i.e., binding with three thiol ligands).70 By comparison, all 12 surface Ag atoms in Pt1Ag24(SR)20 are in a μ3 coordination mode, restraining the surface vibrations of Pt1Ag24(SR)20 and rendering their surface structures more robust than Pt1Ag24(SR)18. The comparisons of the corresponding bonds between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters are shown in Supporting Information Table S3. With respect to the icosahedral Pt1Ag12 kernel, both the Pt(core)–Ag(icosahedral shell) and the Ag(icosahedral shell)–Ag(icosahedral shell) bonds in Pt1Ag24(SR)20 are shorter than those in Pt1Ag24(SR)18, demonstrating the shrinkage of the icosahedral Pt1Ag12 kernel upon the transformation from Pt1Ag24(SR)18 to Pt1Ag24(SR)20. Besides, both the Ag(icosahedral shell)–S(motif shell) and the Ag(motif shell)–S(motif shell) bonds in Pt1Ag24(SR)20 are also shorter than the corresponding bonds in Pt1Ag24(SR)18. Such differences in bond lengths can be rationalized in terms of the different surface structures between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters wherein the addition of two polar SR ligands in Pt1Ag24(SR)20 involves the formation of some new Ag–S bonds that might induce the shrinkage of its overall configuration. Surface structure aggregation On account of nearly the same Pt1Ag12 kernel in both Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters, the presence or the absence of the two polar SR ligands primarily affects the surface structures of two clusters. The comparison of the surface structures between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 is depicted in Figure 2. Figure 2 | Aggregation of Ag2(SR)3 surface motif structures to nanocluster poles. (a) Structural anatomy of Pt1Ag24(SR)18. (b) Structural anatomy of Pt1Ag24(SR)20. (a and b) The addition of two thiol ligands induced the aggregation of six Ag2(SR)3 surface motif structures to the nanocluster poles. (c and d) Structural comparison between three Ag2(SR)3 surface units in Pt1Ag24(SR)18 and [email protected][Ag2(SR)3] surface unit in Pt1Ag24(SR)20. The combination of Ag2(SR)3 surface units is triggered by the addition of the polar thiol ligand (labeled in purple). Color legends: dark green sphere, Pt; magenta sphere, Ag on the icosahedron; light blue/gray/orange sphere, Ag on different Ag2(SR)3 motif structures; yellow/light green/red sphere, S on different Ag2(SR)3 motif structures; purple sphere, the introduced S. For clarity, all C, H, and Cl atoms and some bonds are omitted. Download figure Download PowerPoint The six Ag2(SR)3 dimeric staple motifs at the surface of Pt1Ag24(SR)18 can be divided into two groups (Figures 2a and 2c), and each group contains three Ag2(SR)3 motif structures: Ag2(SR)3 with Ag in light blue and S in yellow, Ag2(SR)3 with Ag in gray and S in light green, and Ag2(SR)3 with Ag in orange and S in red. These three Ag2(SR)3 motif structures are separated, to an extent, as the corresponding Ag–S distances are so long (4.587, 4.604, and 5.121 Å) that no bond exists (Figure 2c). These two groups of 3*Ag2(SR)3 are spherically symmetric around the icosahedral Pt1Ag12 kernel (Figures 2a and 2c). Interestingly, the addition of SR ligands to the poles of Pt1Ag24(SR)18 induces the aggregation of each group of three Ag2(SR)3 motif structures into an orderly organized [email protected][Ag2(SR)3] structure (Figures 2b and 2d; the additional SR ligands are labeled in purple), in this process the cluster chemical formula turns to Pt1Ag24(SR)20 [Pt1Ag24(SR)18 + 2SR]. The three Ag2(SR)3 motif structures are assembled through the interactions between Ag in motifs and the newly added SR; the corresponding bond lengths are 2.537, 2.547, and 2.567 Å, respectively (Figure 2d, labeled in purple). Besides, upon the surface structure aggregation, the three Ag2(SR)3 motif structures are slightly deformed to mutually interact via Ag (in one motif)–S (in adjacent motif) bonds. The corresponding bond lengths are 2.620, 2.629, and 2.630 Å, respectively, much shorter than those in Pt1Ag24(SR)18 (Figures 2c and 2d, labeled in blue). The two [email protected][Ag2(SR)3] surface structures are spherically symmetric in two poles of the Pt1Ag24(SR)20 around the innermost Pt1Ag12 kernel (Figures 2b and 2d). Altogether, upon the conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20, the Ag2(SR)3 surface motif structures are orderly aggregated to the two poles of the overall structure. For further validating the thiol addition at the pole position, the feeding HS-PhCl2 in the conversion was altered to HS-PhtCl. Resultantly, the Pt1Ag24(SPhCl2)18 nanocluster still converted into Pt1Ag24(SR′)20, where SR′ represented a mixture of S-PhCl2 and S-PhtCl. As for the crystal structure of the new Pt1Ag24(SR′)20 nanoclusters ( Supporting Information Figure S8), the pole position was 100% occupied by the introduced S-PhtX ligand, demonstrating that the polar ligands in Pt1Ag24(SR′)20 indeed came from the feeding thiol. Of note, several other thiols in Pt1Ag24(SR′)20 were also substituted (or partly substituted) by S-PhtCl, which is rational considering the possible ligand-exchange process. Indeed, the thiol addition-induced conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20 required at least fivefold moles (Mthiol/Mcluster) of the feeding thiol ligands (Figure 1c), in which process the ligand exchange was inevitable. In this context, we proposed that the requirement of the excess thiol addition (more than 2 moles; Figure 1c) resulted from the different reaction capacities between (1) the ligand exchange and (2) the thiol addition upon the Pt1Ag24(SR)18 framework. That is, the initially added thiol ligands gave priority to activate the ligand-exchange process, and the excess thiol ligands further induced the thiol addition and the transformation from Pt1Ag24(SR)18 to Pt1Ag24(SR)20. The ESI-MS measurements were then performed to track the conversion from Pt1Ag24(SPhCl2)18 to Pt1Ag24(SPhCl2)20 (Figure 3). As depicted in Figure 3a, such a thiol addition-induced transformation from Pt1Ag24(SR)18 to Pt1Ag24(SR)20 was divided into five stages—Stage 1, in the absence of free thiol ligands, only the mass peak of [Pt1Ag24(SR)18]2− existed; Stage 2, mass peaks of [Pt1Ag24(SR)19]3− and [Pt1Ag24(SR)20]4− were detected; Stage 3, the mass peak of [Pt1Ag24(SR)18]2− weakened gradually, and meanwhile the signal of [Pt1Ag24(SR)20]4− enhanced remarkably; Stage 4, the mass peak of [Pt1Ag24(SR)20]4− evolved to be the main signal; and Stage 5, either [Pt1Ag24(SR)19]3− or [Pt1Ag24(SR)18]2− was hard to observe, but only the mass peak of [Pt1Ag24(SR)20]4− existed. The excellent match between experimental and calculated isotope patterns of these nanoclusters verified their chemical formulas (Figures 3b–3d). The magnification of these peaks evidenced the "−2," "−3," and "−4" charge state of Pt1Ag24(SR)18, Pt1Ag24(SR)19, and Pt1Ag24(SR)20, respectively, because such peaks showed a characteristic isotopic pattern with peaks separated by m/z of 1/2, 1/3, and 1/4 Da in the negative mode (Figures 3b–3d). In this context, the free electron counts of all Pt1Ag24 species are the same as 8e, that is, 24 (Ag) – 18 (SR) + 2 (charge) = 8e of [Pt1Ag24(SR)18]2−, 24 (Ag) – 19 (SR) + 3 (charge) = 8e of [Pt1Ag24(SR)19]3−, and 24 (Ag) – 20 (SR) + 4 (charge) = 8 of [Pt1Ag24(SR)20]4−, all displaying closed-shell electronic structures. Figure 3 | ESI-MS tracking of the conversion from Pt1Ag24(SPhCl2)18 to Pt1Ag24(SPhCl2)20. (a) ESI-MS results throughout the conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20 induced by the addition of free thiol ligands. (b) Experimental (in black) and calculated (in red) isotope patterns of [Pt1Ag24(SR)18]2−. (c) Experimental (in black) and calculated (in red) isotope patterns of [Pt1Ag24(SR)19]3−. (d) Experimental (in black) and calculated (in red) isotope patterns of [Pt1Ag24(SR)20]4−. (e) Proposed reversible conversion among [Pt1Ag24(SR)18]2−, [Pt1Ag24(SR)19]3−, and [Pt1Ag24(SR)20]4− cluster compounds triggered by the SR addition or the SR dissociation. (f) Tracking of the [Pt1Ag24(SR)19]3− cluster compound. Insets: (1) disproportionation from two [Pt1Ag24(SR)19]3− into one [Pt1Ag24(SR)18]2− and one [Pt1Ag24(SR)20]4−; (2) generation of [Pt1Ag24(SR)19]3− in the ESI-MS measurement from [Pt1Ag24(SR)18]2− or [Pt1Ag24(SR)20]4−; (3) TLC separations of the cluster compounds that correspond to Stages 1, 3, and 5 in Figure 3a. ESI-MS, electrospray ionization mass spectrometry; TLC, thin-layer chromatography. Download figure Download PowerPoint Considering that (1) the UV–vis tracking of the conversion exhibited several iso-absorption points and (2) the ESI-MS tracking displayed a complementary relationship between [Pt1Ag24(SR)18]2− and [Pt1Ag24(SR)20]4−, we speculated that the conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20 was a thiol addition-triggered surface structure aggregation process, but not a destruction–reorganization process of the cluster overall configuration. That is, the thiol addition process could be summarized as: [Pt1Ag24(SR)18]2− + 2(SR) − ↔ [Pt1Ag24(SR)19]3− + (SR)− ↔ [Pt1Ag24(SR)20]4− (Figure 3e). Of note, throughout the conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20, the content of Pt1Ag24(SR)19 was very low (Figure 3a). Although many efforts have been made to separate the Pt1Ag24(SR)19 from the cluster mixture (e.g., Stage 3 in Figure 3), we cannot obtain it. As depicted in Figure 3f, the thin-layer chromatography (TLC) separations of the cluster compounds (corresponding to the Stages 1, 3, and 5) suggested the absence of the Pt1Ag24(SR)19 in each stage, even in Stage 3, wherein the TLC signal of Pt1Ag24(SR)19 should locate between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 (see Supporting Information Figure S9 for the ESI-MS confirmation of Pt1Ag24(SR)18 and Pt1Ag24(SR)20). Two reasons were proposed to account for the absence of Pt1Ag24(SR)19 (Figure 3f): (1) the Pt1Ag24(SR)19 was extremely unstable and would convert to Pt1Ag24(SR)18 and Pt1Ag24(SR)20 through disproportionation; (2) the Pt1Ag24(SR)19 only existed in ESI-MS measurements and was generated from Pt1Ag24(SR)18 with SR addition or from Pt1Ag24(SR)20 with SR dissociation. Optical property and thermal-stability comparison It is accepted that the structures of metal clusters play a decisive role in their physical–chemical properties, such as optical absorption and PL emission.34,39,48–60 Generally, photoexcited cluster compounds can release their energy through two pathways including non-radiative transitions (mainly through molecular vibrations) and radiative transitions (mainly through the PL).65 As to the conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20, the surface structure aggregation may reduce the energy release of photoexcited clusters through molecular vibrations; accordingly, the PL intensity of nanoclusters is anticipated to be enhanced. Figure 4a depicts the optical absorptions and PL emissions of Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters in the DMF solution: the optical absorptions are in solid lines and the PL emissions are in dotted lines. Both Pt1Ag24(SR)18 and Pt1Ag24(SR)20 cluster solutions emit at 715 nm. Upon the conversion from Pt1Ag24(SR)18 to Pt1Ag24(SR)20, a 160-fold enhancement on PL intensity has been monitored. The PL QY of the DMF solution of Pt1Ag24(SR)20 is 8.5%, whereas the PL QY of Pt1Ag24(SR)18 solution is <0.1%. Under weak UV lights, the PL of Pt1Ag24(SR)20 is strong enough to be perceived with the naked eye, whereas the emission of Pt1Ag24(SR)18 is hard to observe (Figure 4a, insets). Such a PL boost is reasonable in terms of the aggregation of surface structures induced emission enhancement. Of note, most of the previously reported AIEs of cluster-based materials are activated by the intercluster aggregation-triggering RIM.61–69 In comparison, this study presents another AIE approach, that is, intracluster aggregation-triggering RIM, which accomplishes the AIE at the single-molecular level. Figure 4 | Comparison of the optical properties between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters. (a) Optical absorptions and PL emissions of Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters in the solution state. Insets: digital photo of each cluster in solutions under UV lights. (b) Optical absorptions and normalized PL emissions of Pt1Ag24(SR)18 and Pt1Ag24(SR)20 crystalline films. PL, photoluminescence. Download figure Download PowerPoint The PL excitation spectrum of Pt1Ag24(SR)20 was recorded, and all excitation peaks were identical with the optical absorption ( Supporting Information Figure S10). The fluorescence lifetimes between Pt1Ag24(SR)18 and Pt1Ag24(SR)20 nanoclusters were compared ( Supporting Information Figure S11). The fluorescenc
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