Structural Quasi-Isomerism in Au/Ag Nanoclusters

: Atomically precise metal nanoclusters are a new kind of nanomaterials that appeared in recent years; a pair of isomer nanoclusters have the same metal types, numbers of metal atoms, and surface-protected organic ligands but different metal atom arrangements. This article summarizes the structure features of isomer nanoclusters and concentrates on synthesis methods that could lead to isomer structure. The pairs of isomer inorganic nanoclusters’ conversion to each other and their applications in catalyst and photoluminescence are also discussed. We found that the structure conversions are relevant to their stability. However, with the same molecule formulas, different atom arrangements signiﬁcantly inﬂuence their performance in applications. Finally, the existing challenges and some personal perspectives for this novel ﬁeld in the nano-science investigation are proposed. We hope this minireview can offer a reference for researchers interested in inorganic isomer nanoclusters.


Introduction
In chemistry, isomers mean molecules with identical formulas but various structures. The investigation of chemical compounds with isometry can be traced back to 1827 [1]. The structural differences usually cause disparate physical and chemical properties. Because of the diversity of branches of carbon chains and chirality, the structure isomerism and quasi-isomerism are universal phenomena in organic chemistry. Atomically precise metal clusters, usually smaller than 2 nm, have attracted more interest in the last decade [2][3][4]; by now, various kinds of metal clusters have been successfully synthesized, such as Au [5][6][7][8][9][10][11][12][13][14][15][16], Ag [17][18][19][20][21], Pt [21,22], Pd [23][24][25], Cu [26], and mixed metal clusters [27][28][29][30][31][32][33][34]. The crystal structure of metal nanoclusters can be measured using the technology of single-crystal X-ray diffraction. However, there were limited reports concerning the structure isomerism and quasi-isomerism of nanoparticles and nanoclusters due to two main reasons. Firstly, the preparation of nanoclusters at the atomic level is so complex that the prepared nanoparticles are usually polydispersed. Secondly, the different protect organic ligands could cause the difference in formulas, so even though two kinds of nanoclusters could be with the same metal atoms and numbers, they do not have an identical formula and can not be regarded as a couple of exact isomers. Research on isomers has lasted for nearly two hundred years; the reports of structural isomerism and quasi-isomerism in metal nanoclusters only appeared in the last few years [35].
Nanometal catalyst is a significant heterogeneous catalyst. In this regard, metal nanoclusters have many advantages compared to traditional metal nanoparticles because they have more activity sites (caused by the smaller size) and more selectivity (caused by the unique structure) [3]. More importantly, because of specific atomic arrangements, metal Photochem 2022, 2 933 nanoclusters could become a powerful tool for investigating catalytic reaction mechanisms. The emergence of isomers of metal nanoclusters intensifies the application for their use in the study of the catalytic reaction mechanism. Nanoclusters with different formulas usually introduce size effect and ligands effect when they are used in catalytic reactions, and isomeric structures could almost eliminate the influence of both size and ligands effects and only concentrate on structure effect in catalytic reaction progress.
Photoluminescence is another essential application for metal nanoclusters which has attracted interest in both fundamental scientific research and practical applications [36]. Because many factors could impact metal nanocluster photoluminescence, such as nanoclusters aggregation, different ligands, and Au/SR atomic ratio, to find the true influence of the metal core structure, it is crucial to keep these characteristics consistent. In the field of theoretical investigation, it is still a big challenge to reveal the influence of kernel atom structure on metal nanocluster photoluminescence.
Au 38 (PET) 24 (PET, phenylethanethiolate) was the initially reported isomer structure in the field of clusters [8], followed by Au 23 (C≡CBu t ) 15 , [Au 25 (p-MBA) 18 ] − , et al., which have been reported in recent years [37,38]. Besides Au clusters, structural quasi-isomerism of Ag nanoclusters also has been successfully synthesized [39]. In the first section of this literature review, we will focus on the preparation of metal nanoclusters with structure isomers. The mutual transformation between organic isomer structures could happen under certain reaction conditions, and this kind of isomerization reaction could be found in the metal nanoclusters. Usually, an unstable structure could transform into a more stable one irreversibly. We will introduce it in detail in the second part of this paper.
Recently, metal nanoclusters have been widely used in various applications, such as catalysis, sensing, and photoluminescence [3]. Conventional investigation with metal nanoclusters' catalysts usually focuses on the effect of cluster size and different ligands. The appearance of metal nanoclusters with quasi-isomerism could offer an opportunity to achieve an atomic-level understanding of the relationship between metal atom arrangement and character accurately by excluding the influence of different ligands and metal atom numbers. The investigation of metal nanoclusters with isomer structures used for catalytic reactions and photoluminescence is summarized in this article.
To enrich the series of metal nanoclusters, more work must be conducted with isomer structures and extend their applications in catalytic reactions. Primarily, many common metal nanoclusters do not have corresponding isomer structures, such as Au 11 , Au 36 et al. In addition, metal nanoclusters with isomer structures have been used in catalytic reactions but are so limited that more valuable reactions still need to be involved with isomer nanoclusters (biomass conversion, CO 2 RR et al.). Investigation of the influence of catalytic activity on isomer structures is a significant challenge. Therefore, DFT simulations will be used [1,3] to reveal the relationship between catalytic activity and the structure of metal nanoclusters. In situ characterization is a powerful tool used for revealing catalytic reaction mechanisms but was rarely reported in this reaction system. Hence, there is still much work that needs to be done with the aim of further understanding the reaction principle catalyzed by metal nanoclusters that have isomer structures. In the last part of this article, we will provide the research emphasis on structural quasi-isomerism in metal nanoclusters in the near future. Above all, this review aims to summarize the appearance and development of structural quasi-isomerism in metal clusters and their applications in catalytic reactions and photoluminescence while revealing the crucial issue in this field, giving the researchers who are interested in the relevant area fundamental material.

Synthesis of Quasi-Isomerism of Metal Nanoclusters and the Structures
Because of stable structures and smaller sizes, metal nanoclusters have been investigated in detail for many years. However, isomer structures of metal nanoclusters with single metal only appeared in the last few years. Nanocluster structural isomers are clusters with the same atom composition but different configurations. Double-metal nanoclusters could achieve isomer structure by simply changing the replacement location of the minority metal atoms. Compared with that, the preparation of single-metal nanoclusters with isomer structures is more complicated because it requires a real difference for atomic arrangement.
Synthesis of Au 38 nanocluster was realized as early as 1993 [40]. From then on, many efforts have been carried out (such as modifying the synthesis method and introducing distinct organic ligands) to improve the yield and reveal its crystal structure. The first reported isomer nanocluster called Au 38T was a new structure in the Au 38 family [35], and the corresponding old isomer structure was named Au 38Q . The core of Au 38T and Au 38Q constituted twenty-three Au atoms but differed in configurations. In Au 38Q , the core structure contains a pair of enantiomeric clusters. Each cluster has thirteen Au atoms and makes up one icosahedron. Every face of this icosahedron consists of three Au atoms, and two clusters form a rod shape by three Au atoms used in common. In Au 38T , the Au 23 core consists of one icosahedral Au 13 and one Au 10 structure unit by sharing two Au atoms; the Au 13 cluster is like a cap covered on the Au 10 cluster. Besides the core structure, the rest of the fifteen Au atoms covered the Au 23 core also in different manners; each Au atom face-capped onto the Au 3 faces of the Au 23 core for Au 38Q , and some faces are not capped because there were not enough Au atoms. For Au 38T , the Au 23 core was protected by six Au 2 (SR) 3 and three Au (SR) 2 structure units, which can be seen in Figure 1. The Au 38T has a more symmetrical structure than Au 38Q . ters with the same atom composition but different configurations. Double-metal nanoclusters could achieve isomer structure by simply changing the replacement location of the minority metal atoms. Compared with that, the preparation of single-metal nanoclusters with isomer structures is more complicated because it requires a real difference for atomic arrangement.
Synthesis of Au38 nanocluster was realized as early as 1993 [40]. From then on, many efforts have been carried out (such as modifying the synthesis method and introducing distinct organic ligands) to improve the yield and reveal its crystal structure. The first reported isomer nanocluster called Au38T was a new structure in the Au38 family [35], and the corresponding old isomer structure was named Au38Q. The core of Au38T and Au38Q constituted twenty-three Au atoms but differed in configurations. In Au38Q, the core structure contains a pair of enantiomeric clusters. Each cluster has thirteen Au atoms and makes up one icosahedron. Every face of this icosahedron consists of three Au atoms, and two clusters form a rod shape by three Au atoms used in common. In Au38T, the Au23 core consists of one icosahedral Au13 and one Au10 structure unit by sharing two Au atoms; the Au13 cluster is like a cap covered on the Au10 cluster. Besides the core structure, the rest of the fifteen Au atoms covered the Au23 core also in different manners; each Au atom facecapped onto the Au3 faces of the Au23 core for Au38Q, and some faces are not capped because there were not enough Au atoms. For Au38T, the Au23 core was protected by six Au2(SR)3 and three Au (SR)2 structure units, which can be seen in Figure 1. The Au38T has a more symmetrical structure than Au38Q. The Au38T was prepared by a simple one-pot method [35]. Briefly, HAuCl4, TOAB, and phenylethanethiol were dissolved in CH2Cl2 and then reduced by excessive NaBH4. The synthesis of Au38Q is more complicated. Firstly, Aun(SG)m was prepared by the reduction method. Subsequently, Au38Q nanoclusters were obtained by reacting with Aun(SG)m excess PhC2H4SH at 80 °C. This method is commonly called etching. Considering putting various kinds of Au nanoclusters into a certain hash environment, only one type of stable structure will be left to transform into only one stable structure, and others will undergo The Au 38T was prepared by a simple one-pot method [35]. Briefly, HAuCl 4 , TOAB, and phenylethanethiol were dissolved in CH 2 Cl 2 and then reduced by excessive NaBH 4 . The synthesis of Au 38Q is more complicated. Firstly, Au n (SG) m was prepared by the reduction method. Subsequently, Au 38Q nanoclusters were obtained by reacting with Au n (SG) m excess PhC 2 H 4 SH at 80 • C. This method is commonly called etching. Considering putting various kinds of Au nanoclusters into a certain hash environment, only one type of stable structure will be left to transform into only one stable structure, and others will undergo the progress of crushing and recombination. Thus, the Au 38Q nanoclusters were synthesized under a more rigorous situation than their isomer structure. For this reason, the structure of Au 38Q is much more stable than Au 38T, as further proved by the experiment. Au 23 (C≡CBu) 15 (denoted as Au 23 -1 and Au 23 -2) was the first reported Au nanocluster with isomer structures protected by alkynyl ligands [37]. Au 23 -1 and Au 23 -2 have four structure units: one Au 15 core, three V-shaped alkynyl-Au motifs, two linear "Bu-C≡C-Au-C≡C-Bu" motifs, and two bridge -C≡C-Bu ligands, and except for the Au 15 core, other units have precisely uniform structures. In different Au 15 cores, they have the same Au 11 units which were constituted by three octahedral Au 6 units through sharing five Au atoms. Three of the other four Au atoms have the same bonding environment in two Au 15 cores, but the fourth Au atom is not at the same position. The core of Au 23 -1 has C 2 symmetry, which passes through the Au 2 atom and the center of the Au 3 -Au 4 bond. The core of Au 23 -2 also has C 2 symmetry, which passes through the Au 5 atom and the middle of the Au 6 -Au 7 bond (Figure 2a). In Au 23 -1, alkynyl bridge capping on the triangle face 1 (Au 3 ) and linear staple binding over triangle face 2. In Au 23 -2, the positions of both the alkynyl bridge and linear staple changed compared with Au 23 -1 because of the different structures for the Au 15 core (Figure 2b). Unlike Au 38T and Au 38Q, which have different atom arrangement, the pair of Au 23 -1 and Au 23 -2 is much more similar.
the progress of crushing and recombination. Thus, the Au38Q nanoclusters were synthesized under a more rigorous situation than their isomer structure. For this reason, the structure of Au38Q is much more stable than Au38T, as further proved by the experiment.
Au23(C≡CBu)15 (denoted as Au23-1 and Au23-2) was the first reported Au nanocluster with isomer structures protected by alkynyl ligands [37]. Au23-1 and Au23-2 have four structure units: one Au15 core, three V-shaped alkynyl-Au motifs, two linear "Bu-C≡C-Au-C≡C-Bu" motifs, and two bridge -C≡C-Bu ligands, and except for the Au15 core, other units have precisely uniform structures. In different Au15 cores, they have the same Au11 units which were constituted by three octahedral Au6 units through sharing five Au atoms. Three of the other four Au atoms have the same bonding environment in two Au15 cores, but the fourth Au atom is not at the same position. The core of Au23-1 has C2 symmetry, which passes through the Au2 atom and the center of the Au3-Au4 bond. The core of Au23-2 also has C2 symmetry, which passes through the Au5 atom and the middle of the Au6-Au7 bond (Figure 2a). In Au23-1, alkynyl bridge capping on the triangle face 1 (Au3) and linear staple binding over triangle face 2. In Au23-2, the positions of both the alkynyl bridge and linear staple changed compared with Au23-1 because of the different structures for the Au15 core ( Figure 2b). Unlike Au38T and Au38Q, which have different atom arrangement, the pair of Au23-1 and Au23-2 is much more similar.
The synthesis of Au23 clusters was using Me2SAuCl as Au precursor mixing with HC≡CBu t and reduced by NaBH4; in order to obtain Au23-2, a certain amount of Ph4P·Cl must be added after NaBH4, so in this reaction system, Ph4PCl was used as the inductive agent, which leads the formation of Au23-1 to transform into Au23-2. During the preparation process, the color change of the solution was the same for both isomer structures, similarly experiencing the transition from colorless to yellow and finally to dark brown. Because the formation of Au23-2 needs an extra inducer, Au23-1 is a more stable structure in the pair of isomers Au23.
The isomer structures of face-centered-cubic (fcc) vs. non-fcc, and non-fcc-structure Au42(TBBT)26 nanocluster (abbreviated Au42N) is the structural isomer of the earlier reported fcc structure Au42(TBBT)26 (abbreviated Au42F) [41]. Although Au42F and Au42N have different atom arrangements, they both have regular symmetrical structures. As can be seen from Figure 3, there are thirty-four Au atoms in the core of Au42F, and with the form of four cuboctahedras, two Au2(TBBT)3 dimers covered the top and bottom of the core, and four Au(TBBT)2 monomers at the middle part. Au42N has a non-fcc core structure formed by twenty-six Au atoms. The core can be further split into three parts: the top and bottom parts have nine Au atoms, and the middle part has eight Au atoms forming a Color code: grey atoms, C; green and orange atoms, Au; highlighted in blue, Au 2 (C≡C-Bu) 2 unit; highlighted in pink, tri-bonded -C≡C-Bu. Reproduced with permission from Ref. [37]. Copyright 2020, American Chemical Society.
The synthesis of Au 23 clusters was using Me 2 SAuCl as Au precursor mixing with HC≡CBu t and reduced by NaBH 4 ; in order to obtain Au 23 -2, a certain amount of Ph 4 P·Cl must be added after NaBH 4 , so in this reaction system, Ph 4 PCl was used as the inductive agent, which leads the formation of Au 23 -1 to transform into Au 23 -2. During the preparation process, the color change of the solution was the same for both isomer structures, similarly experiencing the transition from colorless to yellow and finally to dark brown. Because the formation of Au 23 -2 needs an extra inducer, Au 23 -1 is a more stable structure in the pair of isomers Au 23 .
The isomer structures of face-centered-cubic (fcc) vs. non-fcc, and non-fcc-structure Au 42 (TBBT) 26 nanocluster (abbreviated Au 42N ) is the structural isomer of the earlier reported fcc structure Au 42 (TBBT) 26 (abbreviated Au 42F ) [41]. Although Au 42F and Au 42N have different atom arrangements, they both have regular symmetrical structures. As can be seen from Figure 3, there are thirty-four Au atoms in the core of Au 42F, and with the form of four cuboctahedras, two Au 2 (TBBT) 3 dimers covered the top and bottom of the core, and four Au(TBBT) 2 monomers at the middle part. Au 42N has a non-fcc core structure formed by twenty-six Au atoms. The core can be further split into three parts: the top and bottom parts have nine Au atoms, and the middle part has eight Au atoms forming a concave quadrilateral interface. The core of Au 42N is capped by four Au 3 (TBBT) 4 , four Au(TBBT) 2 , and two TBBT units.
Synthesis of non-fcc-structure and fcc-structure Au42(TBBT)26 both occurred with the method of traditional NaBH4 reduction, but the amount of each reagent was not the same. For example, the molar ratio of TBBT to Au was 7.71 for the synthesis of non-fcc-structure; the ratio changed to 6.1 in preparation of the fcc-structure. The most important distinction was the addition of cadmium cation in the synthesis of non-fcc-structure. Thus, without Cd, the non-fcc-structure of Au42(TBBT)26 could not be obtained. Researchers considered that Cd might influence the kinetics and thermodynamics in forming Au42 nanoclusters, maybe creating some unstable Au/Cd intermediates or influencing the etching rate of thiol ligand, so the detailed mechanisms still need to be further investigated subsequently.
To synthesize various metal nanoclusters with isomer structures, theoretical studies have been used to predict the structures of a series of Au clusters. Xu et al. viewed Au4 tetrahedron as a unit structure in building the Au8n+4(SR)4n+8 type nanoclusters, and the growth of Au kernels in thiolate-protected Au nanoclusters could be considered as the sequent addition of a basic unit. Then, the structure of Au36(DMBT)24-1D and Au36(DMBT)24-2D were predicted by a grand unified model and density functional theory calculation before being successfully synthesized [42]. Analyzed by single-crystal X-ray crystallography, both Au36(DMBT)24-1D and Au36(DMBT)24-2D can be viewed as a core with twenty Au atoms covered by eight other external staple motifs. In Au36(DMBT)24-2D, two groups of Au4 units arranged in a staggered mode in the core, and the surface staple motifs can be divided into three categories: two monomeric Au1(SR)2, two trimeric Au3(SR)4, and four dimeric Au2(SR)3. In contrast to Au36(DMBT)24-2D, the helical Au4 tetrahedron unit in Au36(DMBT)24-1D had the one-dimensional growth.
Different from Au38, Au23, and Au42, which are pairs of isomer structures that were synthesized respectively, Au36(DMBT)24-1D and Au36(DMBT)24-2D nanoclusters were synchronously synthesized with a two-step method. Firstly, the precursor was obtained by mixing HAuCl4 with organic ligands and reduced by NaBH4. Subsequently, the Au precursor was further etched by DMBT for 48 h at room temperature. Then, two isomer structures were obtained by thin-layer chromatography separation and crystallized in organic Synthesis of non-fcc-structure and fcc-structure Au 42 (TBBT) 26 both occurred with the method of traditional NaBH 4 reduction, but the amount of each reagent was not the same. For example, the molar ratio of TBBT to Au was 7.71 for the synthesis of non-fcc-structure; the ratio changed to 6.1 in preparation of the fcc-structure. The most important distinction was the addition of cadmium cation in the synthesis of non-fcc-structure. Thus, without Cd, the non-fcc-structure of Au 42 (TBBT) 26 could not be obtained. Researchers considered that Cd might influence the kinetics and thermodynamics in forming Au 42 nanoclusters, maybe creating some unstable Au/Cd intermediates or influencing the etching rate of thiol ligand, so the detailed mechanisms still need to be further investigated subsequently.
To synthesize various metal nanoclusters with isomer structures, theoretical studies have been used to predict the structures of a series of Au clusters. Xu et al. viewed Au 4 tetrahedron as a unit structure in building the Au 8n+4 (SR) 4n+8 type nanoclusters, and the growth of Au kernels in thiolate-protected Au nanoclusters could be considered as the sequent addition of a basic unit. Then, the structure of Au 36 (DMBT) 24 -1D and Au 36 (DMBT) 24 -2D were predicted by a grand unified model and density functional theory calculation before being successfully synthesized [42]. Analyzed by single-crystal X-ray crystallography, both Au 36 (DMBT) 24 -1D and Au 36 (DMBT) 24 -2D can be viewed as a core with twenty Au atoms covered by eight other external staple motifs. In Au 36 (DMBT) 24 -2D, two groups of Au 4 units arranged in a staggered mode in the core, and the surface staple motifs can be divided into three categories: two monomeric Au 1 (SR) 2 , two trimeric Au 3 (SR) 4 , and four dimeric Au 2 (SR) 3 . In contrast to Au 36 (DMBT) 24 -2D, the helical Au 4 tetrahedron unit in Au 36 (DMBT) 24 -1D had the one-dimensional growth.
Different from Au 38 , Au 23 , and Au 42 , which are pairs of isomer structures that were synthesized respectively, Au 36 (DMBT) 24 -1D and Au 36 (DMBT) 24 -2D nanoclusters were synchronously synthesized with a two-step method. Firstly, the precursor was obtained by mixing HAuCl 4 with organic ligands and reduced by NaBH 4 . Subsequently, the Au precursor was further etched by DMBT for 48 h at room temperature. Then, two isomer structures were obtained by thin-layer chromatography separation and crystallized in organic solution, respectively. In this pair of isomer Au 36 nanoclusters, Au 36 (DMBT) 24 -1D is the more stable one. Au 28 (CHT) 20 structural isomers (Au 28i and Au 28ii for short) were prepared by a new quasi-antigalvanic method [43]. Similar to Au 36 , the synthesis of Au 28 isomers was also conducted with a synchronous method. Au 23 (SC 6 H 11 ) 16 was firstly synthesized as the precursor. Then, 1.7 equivalents of AuCHT (CHT: cyclohexanethiol) complex were mixed with an Au 23 precursor and dissolved in CH 2 Cl 2 . After stirring for 24 h at room temperature and separated by column chromatography on silica gel, Au 28i and Au 28ii were obtained. The structure of Au2 8ii is similar to that earlier reported: a four-tetrahedral Au 14 core is protected by four Au 3 (SR) 4 trimers and two Au(SR) 2 dimers. Au 28i has the same Au 14 core structure as Au 28i but is covered by two Au 3 (SR) 4 trimers and four Au 2 (SR) 3 dimers. The structure of Au 28i was not directly obtained from crystals of genuine Au 28i because it cannot be formed under different conditions. After changing the protect ligand from CHT to CPT, single crystals of CPT-protected Au 28i were obtained and analyzed by SCXD. UV-vis-NIR spectrometry confirmed that the Au 28i structure was not varied before and after ligand exchange.
Au 25 (SR) 18 is a common structure in the family of Au nanoclusters and has been widely used in various kinds of catalytic reactions [6,44], but the strict isomer structures with Au 25 (SR) 18 were reported only recently [38]. As described in Figure 4, the traditional structure of Au 25 (SR) 18 (Au 25R , Figure 4a) changed into a new isomer structure (Au 25G , Figure 4b) by collective rotation of the icosahedral Au 13 core (Figure 4b) [45]. It is worth noting that the novel structure of Au 25 (SR) 18 was firstly predicted by molecular dynamics simulations and further confirmed by DFT. The synthesis of Au 25 (SR) 18 could be simply described as a NaBH 4 reduction method, and the foundation of isomer structure was caused by the characterization of electrospray ionization ion mobility mass spectrometry. The peak observed at 74.1 ms could be ascribed to Au 25 (SR) 18 , which was predicted by the calculations earlier.
Photochem 2022, 2, FOR PEER REVIEW 6 solution, respectively. In this pair of isomer Au36 nanoclusters, Au36(DMBT)24-1D is the more stable one. Au28(CHT)20 structural isomers (Au28i and Au28ii for short) were prepared by a new quasi-antigalvanic method [43]. Similar to Au36, the synthesis of Au28 isomers was also conducted with a synchronous method. Au23(SC6H11)16 was firstly synthesized as the precursor. Then, 1.7 equivalents of AuCHT (CHT: cyclohexanethiol) complex were mixed with an Au23 precursor and dissolved in CH2Cl2. After stirring for 24 h at room temperature and separated by column chromatography on silica gel, Au28i and Au28ii were obtained. The structure of Au28ii is similar to that earlier reported: a four-tetrahedral Au14 core is protected by four Au3(SR)4 trimers and two Au(SR)2 dimers. Au28i has the same Au14 core structure as Au28i but is covered by two Au3(SR)4 trimers and four Au2(SR)3 dimers. The structure of Au28i was not directly obtained from crystals of genuine Au28i because it cannot be formed under different conditions. After changing the protect ligand from CHT to CPT, single crystals of CPT-protected Au28i were obtained and analyzed by SCXD. UV-vis-NIR spectrometry confirmed that the Au28i structure was not varied before and after ligand exchange.
Au25(SR)18 is a common structure in the family of Au nanoclusters and has been widely used in various kinds of catalytic reactions [6,44], but the strict isomer structures with Au25(SR)18 were reported only recently [38]. As described in Figure 4, the traditional structure of Au25(SR)18 (Au25R, Figure 4a) changed into a new isomer structure (Au25G, Figure 4b) by collective rotation of the icosahedral Au13 core (Figure 4b) [45]. It is worth noting that the novel structure of Au25(SR)18 was firstly predicted by molecular dynamics simulations and further confirmed by DFT. The synthesis of Au25(SR)18 could be simply described as a NaBH4 reduction method, and the foundation of isomer structure was caused by the characterization of electrospray ionization ion mobility mass spectrometry. The peak observed at 74.1 ms could be ascribed to Au25(SR)18, which was predicted by the calculations earlier. The chiral isomer phenomenon can be widely found in the field of organic chemistry; however, inorganic compounds with chiral isomer structures were rarely reported. Mark et al. proved that introducing the chiral organic ligands in the synthesis of Ag clusters could produce the chiral effect in Ag atom arrangement [46]. The chiral ligands L/D and PL/PD lead to the formation of two pairs of enantiomeric silver clusters, Ag6L6/D6 and Ag6PL6/PD6 ( Figure 5). Every cluster exhibits a distorted silver octahedron with six faces capped by chiral ligands that alternate in their orientation. Each Ag atom is coordinated with two sulfur atoms and one nitrogen atom from three ligands: every ligand ligates The chiral isomer phenomenon can be widely found in the field of organic chemistry; however, inorganic compounds with chiral isomer structures were rarely reported. Mark et al. proved that introducing the chiral organic ligands in the synthesis of Ag clusters could produce the chiral effect in Ag atom arrangement [46]. The chiral ligands L/D and PL/PD lead to the formation of two pairs of enantiomeric silver clusters, Ag 6 L 6 /D 6 and Ag 6 PL 6 /PD 6 ( Figure 5). Every cluster exhibits a distorted silver octahedron with six faces capped by chiral ligands that alternate in their orientation. Each Ag atom is coordinated with two sulfur atoms and one nitrogen atom from three ligands: every ligand ligates three silver atoms. In further investigation, the R/S-NYA ligands were also used in the synthesis of Ag nanoclusters with chiral isomer structures [47]. R/S-Ag 17 was prepared by a reduction reaction with triethylamine serving as a reducing agent.
Single-crystal structural analysis revealed that R/S-Ag 17 is composed of an Ag 17 core and 12 R/S-NYA ligands. The Ag 17 core can be considered as three layers with a 7-3-7 arrangement. Six edge-sharing silver triangles are located on the top and bottom. Three silver atoms form a triangle in the middle of the core structure. The method for synthesis of all Ag clusters could be simply described as dissolving silver nitrate and ligands in organic solvent and crystallization in the dark. Thus, the formation of chiral isomer Ag nanoclusters must be caused by various chiral ligands.
Photochem 2022, 2, FOR PEER REVIEW 7 three silver atoms. In further investigation, the R/S-NYA ligands were also used in the synthesis of Ag nanoclusters with chiral isomer structures [47]. R/S-Ag17 was prepared by a reduction reaction with triethylamine serving as a reducing agent. Single-crystal structural analysis revealed that R/S-Ag17 is composed of an Ag17 core and 12 R/S-NYA ligands. The Ag17 core can be considered as three layers with a 7-3-7 arrangement. Six edge-sharing silver triangles are located on the top and bottom. Three silver atoms form a triangle in the middle of the core structure. The method for synthesis of all Ag clusters could be simply described as dissolving silver nitrate and ligands in organic solvent and crystallization in the dark. Thus, the formation of chiral isomer Ag nanoclusters must be caused by various chiral ligands. Figure 5. The enantiomers of Ag6L6/D6 and Ag6PL6/PD6. Color code: yellow atoms, S; green atoms, Ag; blue atoms, N; grey atoms, C. Reproduced with permission from Ref. [46]. Copyright 2020, Science.
In conclusion, various strict isomer structures of metal nanoclusters with single metal have been successfully synthesized by now. Among all obtained isomer clusters, the structure of Au28ii(CHT)20 could not be directly observed; it could only be proved indirectly, and the isomer phenomenon of Au25(SR)18 is detected by electrospray ionization ion mobility mass spectrometry. Every other cluster has a crystal structure. As can be seen from the results of single-crystal structural analysis, some pairs have a similar structure. Some other pairs are totally different.
Interestingly, both pairs of isomer Ag nanoclusters currently have the structure of a chiral isomer, but this kind of phenomenon could not be found in the system of isomer Au nanoclusters. The synthesis could proceed synchronously for different pairs of isomer metal nanoclusters, respectively. When the synchronous method was used, a pair of isomer structures must be separated by a chromatographic method. No matter what method was used, the obtained two isomer structures usually have different stability for Au nanoclusters. The respective synthesis method must be used for chiral isomer Ag nanoclusters because of the demand for different chiral ligands. However, distinct chirality influencing Ag nanoclusters' stability is yet to be reported. Figure 5. The enantiomers of Ag 6 L 6 /D 6 and Ag 6 PL 6 /PD 6 . Color code: yellow atoms, S; green atoms, Ag; blue atoms, N; grey atoms, C. Reproduced with permission from Ref. [46]. Copyright 2020, Science.
In conclusion, various strict isomer structures of metal nanoclusters with single metal have been successfully synthesized by now. Among all obtained isomer clusters, the structure of Au 28ii (CHT) 20 could not be directly observed; it could only be proved indirectly, and the isomer phenomenon of Au 25 (SR) 18 is detected by electrospray ionization ion mobility mass spectrometry. Every other cluster has a crystal structure. As can be seen from the results of single-crystal structural analysis, some pairs have a similar structure. Some other pairs are totally different.
Interestingly, both pairs of isomer Ag nanoclusters currently have the structure of a chiral isomer, but this kind of phenomenon could not be found in the system of isomer Au nanoclusters. The synthesis could proceed synchronously for different pairs of isomer metal nanoclusters, respectively. When the synchronous method was used, a pair of isomer structures must be separated by a chromatographic method. No matter what method was used, the obtained two isomer structures usually have different stability for Au nanoclusters. The respective synthesis method must be used for chiral isomer Ag nanoclusters because of the demand for different chiral ligands. However, distinct chirality influencing Ag nanoclusters' stability is yet to be reported.

Reversion of the Isomerism Structure
Isomerization reaction is a kind of common organic reaction that is usually induced by solid catalysts. The discovery of isomerization reaction in the field of single-metal nanoclusters such as the isomer structures was successfully synthesized [35]. Au 38T and Au 38Q constituted the first reported Au nanoclusters with isomer structures. The reversion between these two structures also has been investigated in detail. At a temperature of −10 • C when dissolved in toluene, Au 38T remained unchanged for one month. The absorption spectrum proved that, when the storage temperature was elevated to 50 • C, Au 38T could transform into Au 38Q . Although many methods have been tried, the retrorse transformation could not happen, so we concluded that the structure of Au 38Q is more stable due to synthesis with a rigorous etching method. Similar to this conversion process, the unstable Au 36 (DMBT) 24 -2D is irreversibly converted to a more stable Au 36 (DMBT) 24 -1D, which also needs a relatively high temperature of 60 • C. As a result, the reaction speed is much faster than Au 38 isomers (only need 2 h) [42].
Compared to Au 38 and Au 36 isomers, which need to raise the temperature for isomerization reactions, the conversion of Au 23 -2 to Au 23 -1 (a pair of isomer structures with a molecular formula of Au 23 (C≡CBu t ) 15 ) could happen spontaneously at room temperature [37]. In the first 2 h, no change was detected by the UV-vis absorption spectrum for Au 23 -2. This led to the reduction in absorption peaks of 500-600 nm to 400 nm. The absorption between 400 and 500 nm was enhanced gradually. After 7 h, the spectrum showed two distinct peaks at 412 and 598 nm, which proved the successful conversion into Au 23 -1; the structure of Au 23 -1 was further confirmed by MALDI-TOF-MS, and no other kinds of nanoclusters were detected. As discussed, the synthesis of Au 23 -2 needs Ph 4 P·Cl as the inductive agent, which finally changes Au 23 -2 into Au 23 -1, forming a more stable structure and proving this inductive effect to be unsustainable.
Because of the character of the oscillator, a pair of Au 28 (CHT) 20 structural isomers (Au 28i and Au 28ii ) could repeatedly and reversibly transform into each other [43]. The conversion reaction of Au 28i and Au 28ii could be tuned by the solvent. Crystals of Au 28ii dissolved in CH 2 Cl 2 show the same absorption as Au 28i even after several days. When a solubility-poor solvent (acetonitrile or methanol) is added, the formed crystals show the same absorption as Au 28ii , and this pathway transformation can be repeated at least 10 times. Therefore, the Au 28i and Au 28ii are two isomers that have been reported initially. The stability property for Au 28 (CHT) 20 isomers is exceptional, Au 28i is more stable in solution, but Au 28ii prefers to exist in a crystal state.
The phenomenon of isomer structure reversibly transforming into each other also could be found in the pair of isomers Au 25R and Au 25G [48]. When CTA + ions were introduced to the Au 25R solution, the interactions between CTA + ions and nanoclusters' surface changed the surface charge of nanoclusters. Zeta potential was used to monitor the state of intermolecular interactions, and the initial zeta potential was −45 mV, which means, under alkaline conditions, the surface of the negatively charged nanoclusters was caused by deprotonated carboxylic groups. After the addition of CTAB, zeta potential changed to +43 mV, indicating that the adsorbed CTA + ions are a double-layer structure. The first layer was used to balance the negative surface charge and further transferred to the positive charge by the absorption of the second layer. The color of the solution gradually changed from reddish brown to dark green by two weeks after the addition of CTA + ions, which indicated a synthesized novel species. ESI-MS revealed that these metal clusters have the same molecular formula as Au 25 (p-MBA) 18 . When Au 25G was dissolved in methanol, decoupling of CTA + ions would happen, which could be seen from the zeta potential, which changed from positive to negative. Then, the color of the solution shifted back to reddish brown after 4 h at room temperature, changing the structure back to Au 25R , which could be further proved by its absorption characteristics. Hence, the stability of Au 25G needs persistent induction by surface-covered CTA + ions.
Au 25 isomerization reaction caused by CTA + ions will stretch the inner metallic core of Au nanoclusters and induce the isomerization process [49]. Other isomerization reactions with Au nanoclusters are all correlated with the stability of nanoclusters. Commonly, unstable structures transform into related stable structures irreversibly. Among them, Au 28 pairs are special because the isomerization reaction with Au 28i and Au 28ii was realized by different stability in different phases. In all of the reported Au isomer structures, only the pair of Au 42 (TBBT) 26 nanoclusters is not found with the isomerization reaction, which may be caused by two isomer structures having the same stability. It is worth noting that isomerization reactions are not reported with chiral isomer Ag nanoclusters. Similarly, the reason may be the same stability for chiral structures.

Catalytic Applications and Photoluminescence
Metals nanoparticles have played an essential role in industrial catalysis for a long time, to improve the catalytic activity and selectivity, revealing that the catalytic activity sites are major tasks for theoretical investigation [49]. Because traditional metal nanoparticles are usually polydisperse and have unknown surface structures, metal nanoclusters are ideal catalyst models for studying the reaction mechanism because of certain structures. By now, metal nanoclusters have been widely used in many different types of reactions such as cross-coupling, selectivity hydrogenation, et al. They could be used in correlating the catalytic properties with specific structures, and also could exhibit excellent catalytic selectivity in some cases [5]. Besides metal atoms arrangement, the kind of organic ligands covering the surface of metal clusters could also influence the catalytic activity, so different core metal structures with other ligands are not perfect catalyst models for investigation of the reaction mechanisms. In essence, the metal nanoclusters with isomer structures could solve this problem efficiently. Tian et al. used a pair of Au isomer structures, which is called Au 38T and Au 38Q in the reaction of 4-nitrophenol hydrogenation with NaBH 4 [35]. In this reaction system, 4-nitrophenol can be reduced into 4-aminophenol in 44% yield with 0.1 mol% Au 38T catalyst for half an hour; however, no product was detected when Au 38Q was used in the same reaction condition. The ultraviolet-visible-near-infrared spectrum revealed that the structure of Au 38T remained unchanged even after 18 catalytic cycles. The catalyst transformed to a more stable Au 38Q and lost all of the catalytic activity, which was observed after 21 cycles. The authors supposed that the high catalytic activity of Au 38T may be caused by its surface that was not as protected by organic ligands as densely as Au 38Q .
Photoluminescence is an important item in the fields of both practical application and theoretical investigation. The photoluminescence of metal nanoclusters is usually influenced by many factors, and the isomers with different core metal structures will provide an excellent opportunity for investigating the kernel atom arrangement influence on the photoluminescence. Wu et al. reported that Au 42F has stronger emission than Au 42N (a pair of isomer structures with a molecular formula of Au 42 (TBBT) 26 ), and the photoluminescence quantum yield of Au 42F is about twice as high as Au 42N [36]. Furthermore, density function theory calculation reveals that the charge states of total Au atoms in Au 42N and Au 42F are +2.678 and +2.898, respectively (electrically neutral for the whole cluster); thus, the kernel metal structures influence the charge distribution between the Au atoms and ligands. As reported earlier, the more positive charge state in the metal core prefers the charge transfer from ligands to metal part by Au-S bonds, resulting in more excellent photoluminescence performance. Au 42F and Au 42N have essential differences in Au atom arrangement. Ag isomer clusters only have a chiral transformation, and enantiomeric silver clusters have the same characteristics of photoluminescence [47].
By now, there is not so much literature on applications of isomer metal nanoclusters. From limited reports, we could conclude that, with the same molecular formulas, the application performance could also be influenced by core atom arrangements, except for chiral isomer, which could not impact the photoluminescence property of Ag nanoclusters. Among these reports, the discovery of specific catalytic performance with isomer Au clusters is especially important because it is the first time that directly correlates the structure and stability of metal clusters with catalyst activity. Meanwhile, it has brand new guiding significance for further investigation of catalytic reaction mechanism with nano-metal particles.
As a type of well-defined model nanocatalyst, metal nanoclusters with specific configurations have also been employed in catalytic selective oxidations [50,51]. Adding a monodopant to a metal particle at a particular position might modify the electronic properties of these materials and alter their catalytic activity. A novel type of "pigeon-pair" cluster known as [Au 13 Ag 12 (PPh 3 ) 10 Cl 8 ]·[Au 12 Ag 13 (PPh 3 ) 10 Cl 8 ] 2+ was synthesized by Qin et al. by doping a mono-Ag atom at the center site of Au 13 Ag 12 (PPh 3 ) 10 Cl 8 nanoclusters with a rod-shape [52,53]. A variation in catalytic performance may emerge from the single-atom exchange between nanoclusters with identical structures since the electronic characteristics are disturbed [54,55]. To test the effect of a single-atom exchange on the catalytic process, Au 13 Ag 12 and Au 13 Ag 12 Au 12 Ag 13 clusters were both supported on TiO 2 and used in the photocatalytic conversion of ethanol. The reaction took place under UV irradiation at 30 • C. Figure 6 shows that, compared to the Au 13 Ag 12 clusters (23%), the Au 13 Ag 12 .Au 12 Ag 13 clusters had a greater conversion of ethanol, and their selectivity of ethanal (79%) was only marginally higher than that of the Au 13 Ag 12 clusters (72%). Given how similarly distributed the products are amongst the two clusters, it is reasonable to assume that the conversion pathway and catalytic reaction mechanism should be the same for both catalysts. In essence, the identical structure of the M 25 clusters with a single atom change from Au to Ag results in a considerable variation in the catalytic activity driven by different electronic characteristics.
clusters is especially important because it is the first time that directly correlates the structure and stability of metal clusters with catalyst activity. Meanwhile, it has brand new guiding significance for further investigation of catalytic reaction mechanism with nanometal particles.
As a type of well-defined model nanocatalyst, metal nanoclusters with specific configurations have also been employed in catalytic selective oxidations [50,51]. Adding a monodopant to a metal particle at a particular position might modify the electronic properties of these materials and alter their catalytic activity. A novel type of "pigeon-pair" cluster known as [Au13Ag12(PPh3)10Cl8]·[Au12Ag13(PPh3)10Cl8] 2+ was synthesized by Qin et al. by doping a mono-Ag atom at the center site of Au13Ag12(PPh3)10Cl8 nanoclusters with a rod-shape [52,53]. A variation in catalytic performance may emerge from the singleatom exchange between nanoclusters with identical structures since the electronic characteristics are disturbed [54,55]. To test the effect of a single-atom exchange on the catalytic process, Au13Ag12 and Au13Ag12Au12Ag13 clusters were both supported on TiO2 and used in the photocatalytic conversion of ethanol. The reaction took place under UV irradiation at 30 °C. Figure 6 shows that, compared to the Au13Ag12 clusters (23%), the Au13Ag12.Au12Ag13 clusters had a greater conversion of ethanol, and their selectivity of ethanal (79%) was only marginally higher than that of the Au13Ag12 clusters (72%). Given how similarly distributed the products are amongst the two clusters, it is reasonable to assume that the conversion pathway and catalytic reaction mechanism should be the same for both catalysts. In essence, the identical structure of the M25 clusters with a single atom change from Au to Ag results in a considerable variation in the catalytic activity driven by different electronic characteristics.

Conclusions and Perspectives
In this mini-review, we summarized development in recent years of the investigation of the structural quasi-isomerism with Au and Ag nanoclusters, including their metal atoms arrangements, synthesis methods, structure conversion, and applications, as listed in Table 1. The synthesis of isomer metal nanoclusters needs different craft processes, or thin-layer chromatography separation for one batch produced nanoclusters. Usually, a pair of isomer Au clusters have different stability, and the unstable one could be converted into the more stable one irrevocably under a particular situation. Au nanoclusters with isomer structures could exhibit different characteristics in catalysis and photoluminescence, proving this novel nanomaterial to have enormous potential for both practical application and theoretical investigation. To expand species and applications for isomer clusters, much work still needs to be carried out. The fresh AgCl dispersed in CH 2 Cl 2 was added in to a solution containing Au 13 Ag 12 cluster.
Photocatalysis of ethanol (I) Exploitation of new structures of quasi-isomerism of metal nanoclusters. In view of isomer structures, Au nanoclusters could be divided into three basic categories: (i) those which have been successfully synthesized and detected the single-crystal structure, (ii) one of a pair isomer structures that have not gained the specific crystal structure, and (iii) the other kinds of Au nanoclusters for which the isomer structures have yet to be discovered. By now, only a few amounts of Au nanoclusters could be classified into the first and second kinds, and in the second species, it is challenging to say isomer structures have been synthesized because the clusters do not have definite crystal structures. The reports of isomer Au clusters are limited, and the same kinds of common Au nanoclusters (such as Au 11 , Au 13 et al.) have not been found with isomer structures. To enlarge the family of isomer metal clusters and further establish an isomer metal nanocluster library, it is necessary to undergo a long progress and requires a great amount of effort. As reported in works of literature, the possible methods may include thin-layer chromatography separation for known metal nanoclusters, the addition of inducing agents, and fine-tuning the synthesis formula.
(II) Exploitation of new catalytic applications of quasi-isomerism of nanoclusters. Because of uniform particle size and specific metal atom arrangement, metal nanoclusters could exhibit excellent performance in catalytic reactions. Until now, the only report of isomer metal clusters used in the catalytic reaction is the pair of Au 38 application for 4-nitrophenol hydrogenation. As expected, the stability of nanocluster could influence their performance, and an unstable one has more activity under the same reaction system but would transform into a more stable structure during the reaction process gradually. Until recently, metal nanoparticles have been used in many kinds of valuable catalytic reactions, such as methane oxidation, biomass conversion, electrocatalytic CO 2 hydrogenation, and selectivity hydrogenation for unsaturated aldehyde et al. All of them require high stability of nano-catalysts, which have to be carried out under hyperthermy [56].
(III) DFT simulations and in situ spectroscopy on the investigation of mechanism with isomer metal clusters used in catalytic reactions.