Optical Stability

Due to their unique physicochemical properties, gold nanoclusters are considered to be ideal model materials for catalytic and solar energy conversion [10, 188]. However, due to the strong surface energy of the ultrasmall-sized Au cluster, it is very unstable under illumination conditions and can easily agglomerate into large-sized Au nanoparticles [175]. As a result, the photocatalytic reaction mechanism of the material is complicated and the catalytic activity is significantly reduced. Photo-agglomeration of gold clusters has been observed in photocatalytic materials based on gold clusters, and the agglomeration mechanism has been explored [189, 190]. However, determining how to effectively enhance the optical stability of gold clusters remains an unsolved key scientific problem.

In a previous study, Xu et al. discovered the photo-induced aggregation of Au nanoclusters for the first time [191], namely, the photo-induced transformation of small molecular-like Au clusters to larger metallic Au nanoparticles under different illumination conditions. Such a transformation has been demonstrated to be a photocatalytic oxidative process with a diffusion/aggregation mechanism.

Optical Rotation and Circular Dichroism (CD) of Gold Nanoclusters

Chiral phenomena are widely found in nature and are one of the basic and most fascinating properties of natural molecules. The most familiar and vivid phenomenon of chirality is human left and right hands. It is no exaggeration to say that from organic matter to inorganic matter, and from inanimate objects to life phenomena, the shadow of chirality is everywhere. For example, proteins in living organisms are almost always composed of L-amino acids. In 1948, Louis Pasteur separated the two enantiomers in the racemic crystals of tartrate and determined their chirality by measuring the optical rotation signal of the enantiomer solution [192]. Therefore, chirality is often associated with the optical activity. Chiral materials are generally considered to be optically active (optical and circular dichroism]; of course, optically active substances are not all chiral. For example, Xu et al. observed optical signals in the achiral metal- organic framework (MOF), indicating that the optical activity can also be derived from achiral substances under certain conditions [193]. Because of the chiral environment in organisms, most drugs acting on organisms are chiral molecules in order to match the chirality of the target molecules in vivo.

It is worth noting that the concept of chirality does not exist only in organic matter; all substances with optical rotation have chirality, including chiral carbon nanotubes and chiral quantum dots [194]. Generally, chirality is expressed in organic molecules and originates from carbon atoms in the chiral center. H owever, due to the complexity of the cluster structure, the chiral origin of clusters is more elusive than that of common molecules. Therefore, the identification of a large number of clusters in recent years has been helpful for the analysis of the chiral origin of clusters. In particular, considerable progress has been made in the study of chiral thiol ligand-protected gold clusters [10, 194, 195]. In 1998, Whetten synthesized chiral Au28(SG]16 clusters by decomposing polymer Au(I)SG compounds and proposed the concept of a chiral gold cluster for the first time [196]. This section focuses on the chiral origin of gold clusters and their optical properties.

Origin of Chirality of Gold Clusters

Metal nanoparticles/clusters have attracted much attention due to their applications in optical devices, chiral recognition and separation, and biosensors, among others [194, 197]. Over the past decade, with the continuous exploration and efforts of chemists, the synthesis of clusters has shown an explosive momentum of development [10]. Chiral ligands have gradually been introduced into the synthesis process of clusters, and chiral gold clusters have quickly become a popular research area due to their unique physical and chemical properties [7, 10, 198]. In the early stages of research, due to the lack of a crystal structure of chiral clusters, it was speculated that the chirality of gold clusters might originate from the core structure [199, 200]. Theoretical calculations showed that the adsorption of thiolates on the surface of gold nanoparticles may cause distortion of the core structure, resulting in a decrease in the symmetry of the core and leading to chirality [201, 202]. In addition, the introduction of chiral ligands in clusters can also result in the production of chiral optical signals (e.g., circular dichroism).

The chiral origin of gold clusters had been a mystery until the precise atomic structure was obtained. In 2007, a milestone work unveiled the mystery of the atomic-level precision structure of ligand-protected gold clusters [33]. Jadzinsky et al. successfully synthesized and resolved the single-crystal structure of Au102 using achiral p-mercaptobenzoic acid (p-MBA), which was the first identified chiral Au„(SR)m nanocluster [33]. They found that the single-crystal structure of Au102 is a racemate composed of a levorotatory isomer and a dextrorotatory isomer. Although the Au79 kernel of the truncated decahedral in Au102 exhibits DSh symmetry, the random arrangement of the surface -S-Au-S- staple motifs at the waist of the decahedron causes the symmetry to be destroyed, resulting in chirality. Similarly, the unit cell of Au38(2-PET)24 (2-PET: 2-phenylethylthiolate) reported by Qian et al. also contains a pair of enantiomeric structures [203]. By analyzing its crystal structure, the chirality of Au38 was determined to be mainly due to the outer Auis shell. The chiral Au38 cluster has both an intrinsic chiral core and a chiral induction of the outer chiral ligand. The chiral signal (CD signal) of Au38 under dual influence changes significantly, which further proves that the metal atomic arrangement of the inner core and the chirality of the ligand shell play crucial roles [200, 203]. In fact, gold clusters synthesized by achiral ligands are often racemates. To obtain optically active chiral clusters, these gold clusters in the form of racemates must be separated by high-performance liquid chromatography (HPLC) [204]. In addition, gold clusters with optical rotation and specific chirality can be directly synthesized by chiral ligands. As the structures of a large number of Au„(SR)m clusters were resolved by single-crystal X-ray diffraction, many of the clusters were found to be chiral [10]. The chirality of these clusters is mainly due to the chiral arrangement of staple motifs and bridging thiolates on the surface of the kernel [52, 58, 66,194]. It is worth mentioning that a cluster that does not have chirality itself will exhibit chiral features when a chiral ligand is introduced into the synthesis, for example, Au25 [205]. Zeng and Jin summarized three possible chiral sources of these gold clusters [194]: (1) chiral configuration of gold core atoms, (2) chiral arrangement of Au-S staple motifs on the gold kernel interface, and (3) intrinsic chiral or chiral arrangement of the outer R groups.

However, these three sources of chirality are based only on the speculation of the experimental results, and the true face of chiral origin is yet to be further studied. Chiral gold nanoclusters have laid the foundation for an in-depth understanding of the chiral origin of nanomaterials. In future work, the practical application of chiral gold nanoclusters, such as chiral sensing, enantioselective catalysis, chiral optics, and chiral recognition in biological systems, should be pursued.

Optical Properties of Chiral Gold Clusters

Chiral gold clusters exhibit a remarkable Cotton effect in the visible range, and their optical properties are mainly reflected in optical rotation and circular dichroism. The optical rotation of chiral gold clusters may result in a potential negative refractive material in the visible region, in which the refraction direction of the light wave is opposite to that of the conventional refraction. The incident and refracted waves are located on the same side of the normal direction of the interface.

Isotropic or cubic symmetry systems do not exhibit dichroism, and so anisotropy is a necessary condition for producing dichroism. Currently, in the identified gold clusters with atomic precision structures, many of the structures exhibit dichroism via a destroyed symmetry due to chirality. CD spectroscopy is widely used to study the chirality of matters and is usually associated with the optical activity. Although a large number of identified gold clusters appear as optically active species, because most of their crystal structures are in the form of racemates, the optically active structures can be obtained only by separating them. The chiral HPLC method is usually used for the separation of optical isomers. Biirgi et al. have conducted a significant amount of research work on the chirality of ligand-covered metal clusters [7, 195, 199, 204, 206-210]. The chirality and optical activity of the thiol ligand-protected gold clusters were outlined in a review published prior to those by Biirgi et al.

Au38(SCH2CH2Ph)24was the first ligand-protected gold cluster for enantiomeric separation using the chiral HPLC method [204]. Biirgi et al. measured the CD spectrum of the fractions of the collected Au38 enantiomers. The CD curves of the two enantiomers of Au38 exhibit a good symmetry relationship. By comparing the CD spectra of Au38 protected by different ligands, it was found that the ligand had little effect on the curve of the CD spectrum. The enantioseparation of

Au40(SCH2CH2Ph)24 has also been carried out by chiral HPLC [210]. It is worth noting that the enantiomeric resolution of Au40 is not completely separated by HPLC.

As reported by Jin et al., Au28(TBBTJ2o also contains a pair of enantiomers that were separated by chiral HPLC, and the CD spectra ofthe isomerswere also measured [58]. However, the effectiveness of the commonly used HPLC method is still unsatisfactory, which limits the practical application ofthe optical activity of chiral clusters [208]. Au102(p-MBA)44 [33] was the first identified thiol-protected gold cluster; however, the enantiomers in its crystal structure were not isolated until 2014 by Knoppe etal. [208]. Rather than chiral HPLC, a chiral phase transfer method was used to separate the enantiomers of Au102 [208]. In addition, Tang et al. obtained the enantiopure Аи20(РРз)4С14 [PP3: tris(2-(diphenylphosphino)ethyl)phosphine] using a supramolecular assembly strategy with a-cyclodextrin (a- CD) [211]. Theoretical studies indicate that the preferential self- assembly of а-CD with the organic ligand on the surface of the right- handed Au20 is responsible for this enantioseparation. In addition to the creation by enantioseparation, it is also possible to synthesize a homochiral cluster directly. For example, a homochiral Au13 nanocluster was synthesized enantioselectively by chiral ligands with stereogenic centers at the phosphorus atoms [212].

In future research, researchers may be able to focus on the following two aspects: obtaining highly optically active homochiral gold clusters and exploring the mechanism of chiral transformation. To achieve the first goal, it is necessary to find novel and efficient synthetic methods. For example, new and efficient methods must be developed for the enantioseparation of chiral clusters, and new ways to synthesize highly optically active homochiral clusters should be explored. For the latter goal, recent work by Hakkinen and Malola on the Au38 chiral transition may provide some instructive insights [213].

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