Structural Isomerization in Cu(I) Clusters: Tracing the Cu Thermal Migration Paths and Unveiling the Structure-Dependent Photoluminescence

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE5 Apr 2022Structural Isomerization in Cu(I) Clusters: Tracing the Cu Thermal Migration Paths and Unveiling the Structure-Dependent Photoluminescence Xi Fan†, Furong Yuan†, Jiaqi Wang, Zhibin Cheng, Shengchang Xiang, Huayan Yang and Zhangjing Zhang Xi Fan† *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, Fujian 350007 State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 †X. Fan and F. Yuan contributed equally to this work.Google Scholar More articles by this author , Furong Yuan† Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, Fujian 350007 †X. Fan and F. Yuan contributed equally to this work.Google Scholar More articles by this author , Jiaqi Wang Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, Fujian 350007 Google Scholar More articles by this author , Zhibin Cheng Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, Fujian 350007 Google Scholar More articles by this author , Shengchang Xiang Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, Fujian 350007 Google Scholar More articles by this author , Huayan Yang Health Science Center School of Biomedical Engineering, Shenzhen University, Shenzhen 518000 Google Scholar More articles by this author and Zhangjing Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Fujian Provincial Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, Fujian 350007 State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202101741 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Revealing structural isomerization in metal clusters would bridge a huge structural gap between small molecular isomerization and solid–solid phase transformation. However, genuine structural isomerism in metal clusters is still rare. In this work, we report the first example of structural isomerism in Cu clusters. By utilizing the coordination flexibility of alkyne to enable the migration of partial Cu atoms in Cu metal cores, two Cu15 cluster complexes ( Cu15-a and Cu15-c) possessing identical composition but different metal core structures have been successfully isolated. Interestingly, although the structure of Cu15-a can be retained in CH2Cl2 solution below 27 °C, it will gradually change to give an intermediate state, Cu15-b, as the temperature rises (at about 31 °C) before it eventually transforms into Cu15-c (at 40∼65 °C). Significantly, atomically precise Cu15-b clearly provides footprints for tracing the thermal migration process of Cu atoms during the thermal transformation from Cu15-a to Cu15-c. In addition, Cu15-a and Cu15-c exhibit diverse crystallization-induced emission enhancement phenomena. Crystalline Cu15-c displays redshifted photoluminescence (820 nm) compared with Cu15-a (726 nm) due to the shorter mean Cu···Cu distance in Cu15-c. Notwithstanding, crystalline Cu15-a exhibits much more intense photoluminescence at room temperature than that in Cu15-c, which might be attributed to the stronger intermolecular C–H⋯π interactions in Cu15-a. These results indicate that cluster isomerism provides valuable opportunities for insight into the structure–property relationships and understanding the complex evolution of phase transformation in nanometallic solids. Download figure Download PowerPoint Introduction Structural isomerization is of fundamental importance in molecular science, whereas bulk or nanoinorganic solids may present phase transformations.1,2 Both structural isomerization and phase transformation represent phenomena in which substances possess the same components but different structures and consequently exhibit dissimilar properties. Phase transformations are initiated by nucleation events and are then propagated discontinuously through atomic migration.3 The processes are difficult to accurately define due to their blurred atomic-level structures.4 In contrast, getting precise structural information about isomeric small molecules is much easier. However, due to the huge structural gap between small-molecule and bulk or nanoinorganic solids, research into structural isomerization and research into solid–solid phase transformation have proceeded independently and have not generated much information of relevance to each another. Coinage metal clusters (Cu, Ag, and Au) have recently generated considerable research interest for their promising applications in luminescence signaling and catalysis.5–13 In addition, their inherent attributes of homogeneity and atomically precise structures provide a suitable platform for bridging the gap between small metal–ligand molecules and metal nanosolids.14–20 For metal clusters, structural isomerism means that the clusters possess identical total molecular formulas but distinct structures of the cluster cores. Therefore, investigation into structural isomerization in these clusters not only provides the most precise structure–property correlations but also generates informational relevance to help understand the intricate process of phase transformation in bulk or nanoinorganic solids. Although dozens of quasistructural isomers (isomerism in the cluster core protected by different ligands) have been reported,21–25 it is still quite rare to realize genuine structural isomerism in metal clusters since slight changes in the assembly can lead to significant changes in the metal numbers, counter ions, or peripheral ligands. Remarkably at this point, only a few genuine isomers in coinage metal clusters (mainly Au clusters and Au-based alloy clusters) have been reported, including Au23(C≡CtBu)15,26 Au28(CHT)20,27 Au36(SPh-Me2)24,28 Au38(SC2H4Ph)24,29 Au42(SPhtBu)26,30 and Au13Ag12(PPh3)10X8 (X=Cl or Br).31 Given the fact that cheap Cu compounds may exhibit unique optical and catalytic activity compared with precious metals, the research enthusiasm for Cu clusters is unprecedented.32–39 However, real structural isomerism in Cu clusters has never been reported. The migration of partial atoms in a cluster is an important way to form its isomer. Given that the strength of metallophilic interaction is just comparable to the hydrogen bond,40 the metal core of the Cu cluster is flexible enough to allow the migration of partial Cu atoms. However, the steady coordination modes of commonly used thiolate- and phosphine-protecting ligands in Cu clusters are likely to restrict such migration. Recent ligand engineering in metal clusters has extended the scope to alkynyl ligands using their characteristic σ- and π-coordination modes, exhibiting richer and more flexible coordination modes than the commonly used thiolate and phosphine ligands.41–45 Therefore, without changing the type of ligands and the number of core nuclei, the coordination flexibility of alkynes may enable the migration of partial Cu atoms in the Cu core under external stimuli, which we would expect to realize structural isomerization in Cu clusters (Scheme 1). Scheme 1 | Illustration of the strategy utilizing flexible alkyne and Cu migration to realize structural isomerism in Cu(I) clusters. Download figure Download PowerPoint Following the above consideration, through utilizing the coordination flexibility of alkyne to enable the migration of partial Cu atoms in the Cu metal core, we have successfully obtained the first pair of structural isomers in Cu clusters, whose structures were fully characterized by X-ray diffraction (XRD) analysis. They have identical composition but slightly different metal core structures, namely, Cu15(C≡CtBu)10(BC)5 (denoted as Cu15-a and Cu15-c; tBuC≡CH = 3,3-Dimethyl-1-Butyne, BC = benzoic acid). Interestingly, Cu15-a will irreversibly transform into Cu15-c in CH2Cl2 solution under thermal conditions of 40∼65 °C, although it can be retained in CH2Cl2 solution below 27 °C. Importantly, an intermediate state Cu15-b has unprecedently been isolated, which clearly provides footprints for tracing the thermal migration process of Cu atoms during the thermal-transformation from Cu15-a to Cu15-c. Both Cu15-a and Cu15-c are nonemissive in CH2Cl2 solution at ambient temperature (≤27 °C), but exhibit diverse crystallization-induced emission enhancement (CIEE) phenomena. Crystalline Cu15-c displays redshifted photoluminescence (820 nm) compared with Cu15-a (726 nm) due to the shorter mean Cu⋯Cu distance in Cu15-c. Nonetheless, crystalline Cu15-a displays more intense room-temperature photoluminescence than that in Cu15-c, which should be attributed to the stronger intermolecular C–H⋯π interactions in Cu15-a. Experimental Methods Synthesis of Cu15-a Cu2O (0.15 g, 1.1 mmol) and BC (0.7 g, 5.7 mmol) were added to acetonitrile (9 mL), and then BuC≡CH (0.2 mL, 0.48 mmol) was added under N2 atmosphere. The resultant mixture was heated at 85 °C for 5 days. After being cooled to room temperature (27 °C) for 1 day, the mixture was washed by acetonitrile, and red-orange rodlike crystals of Cu15-a were obtained (yield: 47% based on BuC≡CH). Elemental analysis for C95H115Cu15O10, Calcd (%): C, 48.14; H, 4.89; N, 0. Found: C, 48.09; H, 4.75; N, <0.3. Synthesis of Cu15-c Cu2O (0.15 g, 1.1 mmol) and BC (0.7 g, 5.7 mmol) were added to acetonitrile (9 mL), and then BuC≡CH (0.20 mL, 0.48 mmol) was added under N2 atmosphere. The resultant mixture was heated at 85 °C for 5 days. After being cooled to room temperature (27 °C) for 3 days, the mixture of large red-orange rodlike crystals of Cu15-a and orange quasihexagonal flake crystals of Cu15-c were obtained. Alternatively, Cu15-c can be formed in the following manner with a pure phase and higher yield by slightly changing the synthetic condition: Cu2O (0.15 g, 1.1 mmol) and BC (0.7 g, 5.7 mmol) were added to acetonitrile (9 mL), and then BuC≡CH (0.20 mL, 0.48 mmol) was added under N2 atmosphere. The resultant mixture was heated at 85 °C for 5 days. After being cooled to the temperature of 40 °C for 2 days, the mixture was washed by acetonitrile, and red crystals of Cu15-c were obtained (yield: 59% based on BuC≡CH). Elemental analysis for C95H115Cu15O10, Calcd (%): C, 48.14; H, 4.89; N, 0. Found: C, 48.06; H, 4.93; N, <0.3. Recrystallization of Cu15-a in CH2Cl2 Under N2 atmosphere, 0.025 g samples of Cu15-a were completely dissolved in 0.5 mL CH2Cl2 below 27 °C. The solution was transferred into a 2 mL small glass vial covered with a porous membrane. The small glass vial was then placed in a sealed 23 mL glass vial containing 1 mL methanol to slowly evaporate the CH2Cl2 at room temperature under N2 atmosphere. After 2 days, red-orange rodlike crystals of Cu15-a were recrystallized (yield: 91% based on Cu15-a). Recrystallization of Cu15-c in CH2Cl2 Under N2 atmosphere, 0.025 g samples of Cu15-c were completely dissolved in 0.5 mL CH2Cl2. The solution was transferred into a 2 mL small glass vial covered with a porous membrane. The small glass vial was then placed in a sealed 23 mL glass vial containing 1 mL methanol to slowly evaporate the CH2Cl2 at room temperature under N2 atmosphere. After 2 days, quasihexagonal flake crystals of Cu15-c were recrystallized (yield: 88% based on Cu15-c). Isolation of Cu15-b from Cu15-a in CH2Cl2 Under N2 atmosphere, 0.025 g samples of Cu15-a were completely dissolved in 0.5 mL CH2Cl2 in a tightly capped glass vial. The solution was heated at 31 °C for 10 min or longer and then transferred into a 2 mL small glass vial covered with porous membrane. The small glass vial was then placed in a sealed 23 mL glass vial containing 1 mL methanol to slowly evaporate the CH2Cl2 at the temperature of 31 °C under N2 atmosphere. After 2 days, red-orange block crystals of Cu15-b were recrystallized (yield: 23% based on Cu15-a). C95H115Cu15O10, Calcd (%): C, 48.14; H, 4.89; N, 0. Found: C, 48.21; H, 4.91; N, <0.3. Isolation of Cu15-c from Cu15-a in CH2Cl2 Under N2 atmosphere, 0.025 g samples of Cu15-a were completely dissolved in 0.5 mL CH2Cl2 in a tightly capped glass vial. The solution was heated at 40∼65 °C for 10 min or longer and then transferred into a 2 mL small glass vial covered with porous membrane. The small glass vial was then placed in a sealed 23 mL glass vial containing 1 mL methanol to slowly evaporate the CH2Cl2. After 2 days, quasihexagonal flake crystals of Cu15-c were recrystallized (yield: 85% based on Cu15-a). Isolation of Cu15-c from Cu15-b in CH2Cl2 Under N2 atmosphere, 0.025 g samples of Cu15-b were completely dissolved in 0.5 mL CH2Cl2 in a tightly capped glass vial. The solution was heated at 40∼65 °C for 10 min or longer and then transferred into a 2 mL small glass vial covered with porous membrane. The small glass vial was then placed in a sealed 23 mL glass vial containing 1 mL methanol to slowly evaporate the CH2Cl2 under N2 atmosphere. After 2 days, quasihexagonal flake crystals of Cu15-c were recrystallized (yield: 93% based on Cu15-b). Results and Discussion Synthesis and characterization of the isomers Mixing CuI and alkynyl can generally form intractable polymeric materials46,47 so mono-carboxylate ligands can be used to cut off the polymerization, thereby facilitating the formation of alkynyl-protected Cu clusters. One-pot solvothermal reaction of Cu2O with tBuC≡CH and BC in CH3CN at 85 °C for 5 days and then 27 °C for 1 day gave rise to red-orange rodlike crystals of Cu15-a (Scheme 2). Single-crystal XRD (SCXRD) analysis at 300 K reveals that Cu15-a is characterized by a Cu15 kernel stabilized by 5 BC− and 10 tBuC≡C− ligands (Figure 1a). Since no NaBH4 was added in reaction, the cluster can’t possess any hydrides (H−) within its kernel, which indicates that Cu in Cu15-a is CuI and Cu15-a contains no free valence electron (15 − 5 – 10 = 0). The Cu15 core can be seen as two puckered hexagonal Cu6 rings (Cu1∼Cu6 and Cu8∼Cu13) strung together by a linear “tBu–C≡C–Cu–C≡C–tBu” motif, and the remaining two Cu atoms are located next to the two Cu6 rings (Figure 1c). Extensive Cu⋯Cu distances of 2.558–3.263 Å are found in Cu15-a ( Supporting Information Table S2). The distances of 17 couples of Cu⋯Cu are less than the sum of the van der Waals radii (2.8 Å) of two Cu atoms, revealing the occurrence of cuprophilic interactions. Ten alkynyl ligands exhibit various coordination modes, including two kinds of μ3 bridging modes (μ3-η1, η1, η1 × 3, μ3-η1, η1, η2 × 4) and two kinds of unusual μ4 bridging modes (μ4-η1, η1, η1, η2 × 1, μ4-η1, η1, η1, η2 × 2) ( Supporting Information Figure S1a). The lengths of Cu–C bonds range from 1.887 to 2.496 Å, respectively ( Supporting Information Table S3). Scheme 2 | Synthetic routes for Cu15-a, Cu15-c, and their intermedia Cu15-b. Download figure Download PowerPoint When the above reaction time under 27 °C was extended from 1 to 3 days, except for rodlike crystals of Cu15-a, a new kind of quasihexagonal flake crystal Cu15-c was also obtained (Scheme 2). Indeed, Cu15-c was also obtained in the pure phase by one-pot solvothermal reaction of Cu2O with tBuC≡CH and BC in CH3CN at 85 °C for 5 days and then 40 °C for 2 days (Scheme 2), while no Cu15-a came out in such condition. Full structural analysis indicates that Cu15-c processes identical composition and similar structure with Cu15-a (Figures 1b and 1d). However, the hexagonal Cu6 ring of Cu1∼Cu6 in Cu15-c is like a “squashed version” of that in Cu15-a (Figures 1e and 1f) although their Cu8∼Cu13 rings are relatively comparable ( Supporting Information Figure S2). The Cu⋯Cu distances in Cu15-c range from 2.503 to 3.105 Å, and 22 pairs of them exhibit obvious coprophilic interactions due to short distances (below 2.8 Å) ( Supporting Information Table S2). Therefore, the kernel of Cu15-c is more compact than that of Cu15-a. Besides two kinds of μ3 bridging modes (μ3-η1, η1, η1 × 4, μ3-η1, η1, η2 × 4), two kinds of unusual μ4 bridging modes (μ4-η1, η1, η1, η2 × 1, μ4-η1, η1, η2, η2 × 1) of tBu–C≡C− exist in Cu15-c ( Supporting Information Figure S1b) with the Cu–C distances ranging from 1.883 to 2.367 Å ( Supporting Information Table S3). Based on the same components and distinct cluster core structures, Cu15-a and Cu15-c can be identified as a pair of structural isomers, which represent the first example in Cu clusters. Figure 1 | The total molecular structures of (a) Cu15-a and (b) Cu15-c. Structural dissection of the Cu15 cores in (c) Cu15-a and (d) Cu15-c: they are both comprised of two puckered hexagonal Cu6 rings strung together by a linear “tBu–C≡C–Cu–C≡C–tBu” motifs, and the remaining two Cu atoms are located next to the two Cu6 rings. Structural comparison of the puckered hexagonal Cu6 rings (Cu1∼Cu6) in (e) Cu15-a and (f) Cu15-c. Atom color codes: blue Cu1∼Cu6; green Cu8∼Cu13; pink Cu7, Cu14 and Cu15; red O; gray C. Download figure Download PowerPoint The clusters in the single-crystals of Cu15-a and Cu15-c are packed together through supramolecular interactions like C–H⋯π and Van der Waals forces. Their packing fashions are similar, and each cluster (for example, label A) is adjacent to 12 clusters (for example, label B∼M) (Figures 2a and 2c). However, their supramolecular interactions are different from each other. Five benzene rings in each cluster of Cu15-a (label A) have C–H⋯π intermolecular interactions with eight nearby benzene rings distributed in five surrounding Cu15-a clusters (labels C, F, H, I, and K) (Figures 2a and 2b and Supporting Information Figure S3). Correspondingly, five benzene rings in each cluster of Cu15-c (label A) have C–H⋯π intermolecular interactions with only four nearby benzene rings distributed in three surrounding Cu15-a clusters (labels E, G, and H) (Figures 2c and 2d and Supporting Information Figure S4). In addition, the mean distance of such C–H⋯π in Cu15-a (3.026 Å) is shorter than that in Cu15-c (3.050 Å). Therefore, intermolecular interactions among clusters in Cu15-a are stronger than those in Cu15-c although the kernel of Cu15-c is more compact than that of Cu15-a. Single crystals of Cu15-a came out earlier than Cu15-c at 27 °C as mentioned above, which may be attributed to the stronger intermolecular interactions of Cu15-a facilitating the crystallization process. Figure 2 | The packing fashions of (a) Cu15-a and (b) its C–H⋯π intermolecular interactions between two adjacent clusters. The packing fashions of (c) Cu15-c and (d) its C–H⋯π intermolecular interactions between two adjacent clusters. Download figure Download PowerPoint Thermal-driven structural transformation from Cu15-a to Cu15-c The above-mentioned one-pot synthesis conditions show that Cu15-a and Cu15-c have different preferences for the synthetic temperature (27 °C for pure Cu15-a and 40 °C for pure Cu15-c). This important feature motivates us to achieve the thermally driven structural transformation between this pair of isomers. Matrix-assisted laser desorption ionization mass spectrometry shows that Cu15-a will not decompose in CH2Cl2 below 27 °C ( Supporting Information Figure S5), and recrystallized single-crystal structural analysis confirms that its structure remains unchanged (Cif name: Cu15-a-R, Supporting Information Figure S7), indicating the stability of Cu15-a in CH2Cl2 below 27 °C. However, after heating Cu15-a in CH2Cl2 at 40∼65 °C for 10 min or longer, the solution was slowly volatilized to obtain new hexagonal crystals (Scheme 2), which were confirmed to be Cu15-c in the pure phase by SCXRD and powder XRD (PXRD; Cif name: Cu15-c-R, Supporting Information Figure S8). Therefore, Cu15-a can thermally transform to Cu15-c in CH2Cl2 at 40∼65 °C. The reverse transformation from Cu15-c to Cu15-a failed under various investigated conditions as confirmed by SCXRD and matrix-assisted laser desorption ionization mass spectrometry ( Supporting Information Figure S6), indicating that Cu15-a can only be irreversibly transformed to Cu15-c. Thus, Cu15-a is metastable, and tBu–C≡C–Cu motifs are flexible so that the Cu atoms in Cu15-a can process thermally driven migration to form the stable Cu15-c. Our one-step preparation of isomers precipitated out the only crystals of Cu15-c at 40 °C, which might be attributed the metastability of Cu15-a at this condition. Tracing the Cu migration paths For fundamental scientific research, tracing the Cu migration paths is of major importance to understanding the evolution of isomeric transformation, thus further helping us to understand the intricate processes of phase transformations in bulk or nanoinorganic solids. Recently, high-resolution electrospray ionization mass spectrometry (HRESI-MS) has been used to study the assembly mechanism of metal clusters,18,48–50 which is obviously unsuitable for investigating the isomeric transformation in clusters because they have the same composition. Liquid UV can be used to monitor the conversion process of isomers26,27,30,31 but is unable to clarify the structures of the intermediates. Therefore, the previously reported isomeric transformations only experimentally detected the cluster structures of the initial and final states while the intermediates could only be predicted by theoretical calculations, lacking the real intermediate images to clearly track the migration paths of atoms. The most effective and intuitive way to track these migration paths is to obtain the single crystals of the intermediates,51–53 which is an unprecedentedly challenging task and has never been achieved in the previously reported transformations between isomeric clusters. Two main reasons might account for such an obstacle: (1) the process of isomeric conversion may be too fast to be captured; and (2) there are various substances in the transformation process so that a single intermediate state does not have enough yield, and the intermolecular forces among intermediate clusters are weak, hampering the crystallization of the clusters. Structural transformation between this pair of isomers is driven through thermal energy, indicating that the conversion process may be controlled by temperature. In addition, the benzene groups of the protecting ligands in the clusters may facilitate the crystallization through C–H⋯π and π–π intermolecular interactions.54 Encouraged by these precious advantages, we subsequently made great efforts to capture the intermediate states by treating Cu15-a in CH2Cl2 at various temperatures. Fortunately, we have successfully isolated a new cluster Cu15-b at the intermediate temperature of about 31 °C (Scheme 2). Cu15-b possesses the same composition and and has a similar structure to the two isomers ( Supporting Information Figure S9), and its Cu1–Cu6 hexagonal Cu6 ring is more puckered than Cu15-a but less squashed than Cu15-c ( Supporting Information Figure S10). The reason why Cu15-b cannot be isolated during the one-pot synthesis is probably because crystalline Cu15-b is more favorable in CH2Cl2 than CH3CN. It is worth noting that Cu15-b can be further converted into Cu15-c in CH2Cl2 at 40∼65 °C (Scheme 2). This evidence clearly demonstrates that the transformation from Cu15-a to Cu15-c can be controlled by temperature: the structure of Cu15-a in CH2Cl2 gradually changes to give an intermediate state Cu15-b as the temperature rises to about 31 °C and eventually turns into Cu15-c. The structure of solid or crystalline Cu15-a does not change during the heating process from 27∼65 °C, indicating that the CH2Cl2 solution plays a decisive role in stimulating the structural transformation process. Attempts to capture single crystals of more intermediate states probably failed due to the difficulty in their crystallization. Significantly, the isolation of atomically precise Cu15-b clearly provides footprints for tracing the thermal migration process of Cu atoms during the thermal-transformation from Cu15-a to Cu15-c. To make the Cu migration paths clear, we mainly describe the obvious migrating movements of the two hexagonal Cu6 rings (Cu1∼Cu6 and Cu8∼Cu13) relative to Cu7 in the following: Firstly, Cu6 moves away from Cu1 and Cu7 (Figure 3a), resulting in the dissociation of the Cu6-Cu1 and Cu6-Cu7 bonds (Figure 3b). Meanwhile, the migration of Cu10 and Cu11 causes the acute angle ∠Cu10Cu11Cu7 to be stretched into an obtuse angle (Figures 3d and 3e). Then Cu3 moves away from Cu2, and Cu4∼Cu6 rotates clockwise around Cu7 (Figure 3b), resulting in the dissociation of the Cu2–Cu3 bond but the association of Cu4–Cu5 (Figure 3c). At the same time, Cu13 moves close to Cu8 and Cu7 (Figure 3e), resulting in the association of Cu13–Cu8 and Cu13–Cu7 bonds (Figure 3f). All these migration processes are accompanied by the changes in the coordination modes of the alkynyl groups with Cu to give the final Cu15-c ( Supporting Information Figures S1 and S11). Figure 3 | Illustration of migration process of Cu atoms during the thermal-transformation from Cu15-a to Cu15-b and finally Cu15-c. The migration movements of the hexagonal Cu6 ring (Cu1∼Cu6, blue color) relative to Cu7 (pink color) in (a) Cu15-a, (b) Cu15-b, and (c) Cu15-c. The migration movements of the hexagonal Cu6 ring (Cu8∼Cu13, green color) relative to Cu7 (pink color) in (d) Cu15-a, (e) Cu15-b, and (f) Cu15-c. The illustrated structural characterizations of Cu15-a, Cu15-b, and Cu15-c above were all collected at 300 K to eliminate the influence of temperature on the determination of crystal structure. Download figure Download PowerPoint Stabilities of the isomers The stability of clusters in the solid state are crucial in practical applications. Therefore, the stability of the three clusters mentioned above were investigated. Although CuI clusters are generally unstable under ambient air, and Cu15-a is metastable in CH2Cl2,37,55 the solid crystalline samples of Cu15-a and Cu15-c isolated from CH3CN can maintain their crystallinities unchanged for at least one month in the ambient environment, which demonstrates the high stability of crystalline samples in O2 as confirmed by the comparison of X-ray photoelectron spectroscopy, SCXRD, and PXRD ( Supporting Information Figures S12–S17). The unique stability in the ambient environment may be caused by the coordination effect between Cu and organic ligands, which then form dense organic protection shells to prevent oxidation by O2 in air.56 In addition, intermolecular interactions including C–H⋯π may also facilitate maintaining the crystallinity and stability.57 Unfortunately, the intermediate Cu15-b isolated from CH2Cl2 was unstable after being exposed to the air for hours, probably because that low boiling point and volatile CH2Cl2 guest weakened the stability of the crystals.58,59 Unveiling the structure-dependent photoluminescence The photoluminescence of Cu clusters is mainly determined by their structures. Considering that the structural isomers Cu15-a and Cu15-c isolated from CH3CN exhibit excellent ambient stability, we decided to investigate their structure-dependent photoluminescence although they share many identical physicochemical properties ( Supporting Information Figures S21 and S22). Both Cu15-a and Cu15-c are nonemissive in the CH2Cl2 solution at the ambient temperature of 27 °C (Figures 4a and 4b). However, Cu15-a and Cu15-c exhibit distinct CIEE phenomena (Figures 4a and 4b). Upon excitation at 515 nm, the crystalline state of Cu15-a displays intense photoluminescence in the near-infrared (NIR) region with an emission maximum of 726 nm at 27 °C while the crystalline Cu15-c (excitation at 500 nm) exhibits much weaker but more redshifted photoluminescence (820 nm) compared with Cu15-a (Figures 4a and 4b). With reference to the spectroscopic studies on CuI alkynyl clusters of Yam, Sun, and Mak, the NIR emissions can be tentatively assigned to the 3LMCT (tBu–C≡C→CuX) excited state mixed with cluster-centered (3CC) characters perturbed by cuprophilic interactions.33,60,61 Notwithstanding, the different luminous behaviors greatly aroused our interest in further investigating the structure-dependent photoluminescence in Cu15-a and Cu15-c.