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Optical properties of gold nanoclusters constructed from Au13 units

Optical properties of gold nanoclusters constructed from Au13 units INTRODUCTIONThe optical properties of metal nanoparticles directly reflect their electronic structures, which are highly dependent on their sizes and shapes. Photo‐excitation of gold nanoparticles (Au NPs, >2.2 nm diameter) leads to a broad absorption band across the UV–vis to near Infrared (NIR) region, which results from the collective oscillation of couduction band electrons, leading to surface plasmon resonance (SPR).[1–2] Generally, an absorption peak around 520 nm can be observed because the Au NPs show a quasi‐continuous conduction band of the electronic structure and SPR effect dominates the photoexcitation process.[3–4] However, the quantum confinement effect becomes significant in gold nanoparticles when the diameter is smaller than 2.2 nm (often known as gold nanoclusters [NCs]). Accordingly, multiple absorption bands due to single‐electron transition are obtained in the UV–vis absorption spectra.[5–7] Gold NCs exhibit size‐dependent electronic, optical, and magnetic properties. There have been considerable research interests in gold NCs in both fundamental science and applications in catalysis, chemical sensing, etc.[8–16]The synthesis at the atomic level in organic molecules is widely reported, while the goal of achieving atomic precision NCs is still challenging.[17–20] Having benefited from the previous research, several strategies have been developed, including size‐focusing and ligand‐exchange‐induced size/structure transformation synthetic methodologies,[21–23] which paved the way for designing and synthesizing the gold NCs with atomic precision and molecular purity. With the help of precise synthesis, the total structure, including the gold core and surface ligands, can be determined by X‐ray crystallography. Therefore, the bonding structures, structure–property relationship, shape control, size effect, ligand–core coordination patterns, etc., can be fully understood.[17] Besides the previous synthesis methods, the assembly of structural building blocks into functional materials is also well developed.[8,24–28]Among those structural building blocks, the icosahedral Au13 motif is often used in NCs protected by different ligands.[26,28–32] The different stacking modes of icosahedral Au13‐cluster (Figure 1A) include linear and cyclic growth patterns, which are also named as vertex‐ and face‐sharing patterns.[26–28] In 1990, Teo et al.[33] reported a 38‐atom cluster, (p‐Tol3P)12Au18Ag20Cl14, with three 13‐atom centered icosahedral units sharing three vertices in a triangle way, and such a kind of cluster was called “clusters of clusters”. Hereafter, the research groups of Tsukuda[26] and Jin[34] demonstrated the synthesis of rod‐shaped gold NCs with 25 gold atoms (Figure 1B), which consisted of two icosahedral Au13 clusters sharing one gold vertex atom.1FIGUREThe structures of (A) Au13, (B) Au25, (C) Au37, and (D) Au60. Only gold atoms are shown in the structures; the center gold atoms of Au13 units are labeled as blue; the vertex gold atoms are labeled as magentaIn 2007, theoretical studies conducted by Nobusada and Iwasa[35] reported that the electronic properties of Au13 unit would remain unchanged in the Au13‐assembled oligomers (including Au25 constructed by two Au13 blocks and Au37 constructed by three Au13 blocks). Jiang et al.[36] performed a theoretical investigation by designing two one‐dimensional nanosystems with a linear chain that are composed of the icosahedral Au13 units fused by vertex and face sharing, respectively. They found that vertex‐sharing linear chain can be made either semiconducting or metallic by changing charge, while face‐sharing nanowire is always metallic. In 2015, Jin et al.[27] successfully synthesized the Au37 NCs and reported the structure (Figure 1C), which comprise three icosahedral Au13 building blocks via linear assembling mode. In the same year, Zhu et al.[28] reported the crystal structure of Au60 NCs (Figure 1D), in which five Au13 units form a closed gold ring by Au‐Se‐Au bonds. For these multiunit structures, it is of importance to figure out how the building blocks are arranged in the cluster‐assembled materials, and whether the optical properties of Au13 building block are preserved in larger cluster‐assembled materials.In this minireview, we focus on the optical properties of several cluster‐assembled materials based on Au13 unit. We first review steady‐state spectral properties, including absorption and photoluminescence spectra, and then we introduce the ultrafast spectroscopic features. We also highlight the photophysical properties of the cluster‐assembled materials (Au13, Au25, Au37, and Au60) and conclude the main spectral features of cluster‐assembled materials fused by Au13 units. Finally, we discuss the main challenge and opportunities for future research in such clusters.OPTICAL ABSORPTIONThe Au13 icosahedral cluster is a commonly used unit, which consists of a center gold atom and a 12‐atom shell of icosahedral geometry (as shown in Figure 1A). Such a small Au13 kernel is widely applied in assembled materials (see Figure 1) so that it is important to understand the electronic properties of Au13 unit before and after the assembly. Teo et al.[37–38] reported that oligomeric clusters can be systematically synthesized in a stepwise way by aggregating the icosahedral units, in which the individual icosahedral clusters serve as the building blocks. Khanna and Castleman et al.[39–41] demonstrated that the aluminum‐based icosahedral clusters form the assembled clusters, which are also named as “superatoms” in their work. In both clusters of clusters and superatoms, the assembled materials are constructed from icosahedral building blocks and the electronic property of each constituent unit is retained.[35]Nobusada et al.[35] theoretically characterized the electronic structures and absorption spectra of a biicosahedral gold cluster [Au25(PH3)10(SCH3)5Cl2]2+ (Au25 I, Figure 2A) and a triicosahedral cluster [Au37(PH3)10(SCH3)10Cl2]+ (Au37 I, Figure 2A). The theoretical absorption spectra of Au25 I and Au37 I were obtained by convoluting the absorption peaks with Lorentz function, and theoretical spectrum of Au25 I (Figure 2A) reproduces the experimental absorption spectrum of [Au25(PPh3)10(SC2H5)5Cl2]2+ (Au25 II). The absorption peak of Au25 I at lower‐energy level (∼702 nm) that is well separated from other absorption peaks at higher‐energy region (<600 nm) is attributed to the electronic transition from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) due to the vertex‐sharing biicosahedral structure (see Figure 2B). The theoretical absorption spectrum of Au37 I is also plotted, and the spectral profile between 400 and 600 nm is comparable to that of Au25 I except that the absorption peaks of Au37 I are shifted to longer wavelength. Moreover, two new peaks at ∼761 and 1230 nm are observed in the absorption spectrum of Au37 I due to the interaction between Au13 units (Figure 2A).2FIGURE(A) Absorption spectrum for Au25 I (top, calculation result), Au25 II (middle, experimental result), and Au37 I (bottom, calculation result). (B) Schematic diagrams of the Kohn–Sham orbitals and main atomic orbital components contributing to each Kohn–Sham orbital (calculation result) for Au25 I (top) and Au37 I (bottom). (C) UV–vis–NIR absorption spectrum of the Au37 II (experimental result). (D) UV–vis absorption spectrum of Au13 I (experiment result). (E) UV–vis‐NIR absorption spectrum of Au60 I (experiment result). (A,B) Adapted with the permission from ref.[35]; copyright 2007, American Chemical Society. (C,D) Adapted with the permission from ref.[27]; copyright 2015, American Chemical Society. (E) Adapted with the permission from ref. [28]; copyright 2015, Wiley‐VCH GmbHIn the experimental work reported by Jin et al.[27] the lower‐energy absorption peaks of [Au37(PPh3)10(SC2H4Ph)10X2]+ (X = Cl or Br, Au37 II, gold atoms 3 × 13 − 2 = 37, Figure 2C) are centered at 795 and 1230 nm, which are essentially consistent with the theoretical prediction: the 1230‐nm peak is assigned to the transition from HOMO to LUMO while the 795 nm peak arises from HOMO‐1 to LUMO transition. The absorption spectrum of [Au13(PPh3)10X2]3+ (Au13 I, Figure 2D) consists of a peak of 430 nm and a shoulder around 490 nm, which agrees with the short‐wavelength profiles of Au25 I/II, and Au37 I/II. Therefore, the higher‐energy transitions of Au25 I/II and Au37 I/II should originate from the electronic transition within Au13 clusters. It suggests that the electronic properties of individual icosahedral cluster remain unchanged in the assembled clusters. From the density functional theory (DFT) calculated results (Figure 2B), one can observe the HOMO of Au25 I is mainly composed of the atomic orbital along z axis, and the LUMO is primarily formed from the s orbital of the vertex gold atom at the center. In Au37 I, the HOMO and HOMO‐1 orbitals are composed of the orbitals along the z axis and LUMO is constructed mainly from z orbitals of the two vertex‐sharing gold atoms. As Au13 has spherical symmetry in the x, y, and z axes, the atomic orbitals along the three directions are degenerated. In addition, the molecular orbitals of new clusters Au25 I and Au37 I (HOMO, LUMO for Au25 I and Au37 I, HOMO‐1 for Au37 I) are attributed to the interaction between icosahedral Au13 units induced by electronic symmetry breaking.The cluster‐assembled [Au60Se2(Ph3P)10(SePh)15]+ (Au60 I, Figure 2E) reported by Zhu and co‐workers[28] contains five icosahedral Au13 building blocks with Au‐Se‐Au linkages, and two adjacent Au13 blocks share a vertex atom (Figure 1D), that is, the total numbers of gold atoms are 5 × 13 − 5 = 60. The optical absorption spectrum of Au60 I exhibits five peaks located at 353, 435, 510, 600, and 835 nm. Nobusada et al.[35] reported that the icosahedral Au13 block is a spherically symmetric structure, and the electronic symmetric breaking does not influence the orbital symmetry of Au13 unit, so that the electronic property of the units is retained in assembled Au60 I. Similar to Au13 I, Au25 II, and Au37 II, the absorption peaks of Au60 I shorter than 600 nm can be assigned to the electronic transition within individual Au13 unit, while the absorption peak around 835 nm can be attributed to the HOMO‐LUMO transition of the pentamer structure. Besides the linear optical absorption, the nonlinear optical properties of NCs with Au13 units have also been reported. Optical limiting is a nonlinear effect, meaning the decrease in the transmittance of a sample with high fluence light illumination. The optical limiting properties of Au60 I and Au25 III were also investigated and Au60 I was found to exhibit better performance.[28] The correlation of Au13 assembly and their nonlinear optical properties calls for further investigations.The HOMO‐LUMO absorption peak red‐shifts from Au13 (∼490 nm) to Au25 (∼700 nm) and Au37 (∼1230 nm). However, the HOMO‐LUMO absorption peak of Au60 is around 835 nm, which lies between that of Au25 and Au37. There are two possible explanations for the phenomenon: (1) The electrons in Au60 I is more restricted compared to linear assembled Au25 and Au37 because of the internal strain caused by the cyclic structure; (2) The electronic density of LUMO of Au60 I is more concentrated at the center of pentamer structure because one Se atom connected with five Au13 icosahedral via Se‐(Au)5 bonds simultaneously. The unique absorption features of Au60 I calls for further experimental and theoretical studies in the future.PHOTOLUMINESCENCEThe photoluminescence (PL for short) of gold NCs have attracted much attention because of their promising applications in biosensing, phototherapy, cell labeling, etc.[42–46] The reported PL quantum yields (QYs) of most NCs were very low (<1%).[47–50] It was reported that metal NCs dissolved in water often show relatively stronger PL than their organic solvents‐soluble counterparts.[51] Moreover, rigidifying surface protecting ligands can be applied to enhance PL;[52–54] modifying the gold core size or doping some foreign metal atoms is another strategy;[55–57] aggregation‐induced emission scheme that adopted extensively in organic molecules can also be employed to enhance PL.[58–61]In gold NCs assembled from Au13 units (Au13, Au25, Au37 and Au60), there is no surface motif structures and the PL should originate exclusively from the metal core. In mono‐icosahedral Au13 NCs, DFT calculation performed by Aikens and co‐workers[62] reported that both HOMO and LUMO are delocalized on the Au13 core, and the PL of Au13 originates from the lowest singlet excited state (S1 state). In Au25 and Au37, the HOMO‐LUMO gaps are smaller compared to Au13 so that the PL peaks should be redshifted accordingly. Unlike Au13, the HOMOs of rod‐shaped Au25 and Au37 NCs are distributed along the z axis of the rod while the LUMOs are mainly contributed by vertex gold atoms. Therefore, the PL mechanism in Au25 and Au37 could be more complicated compared to mono icosahedral Au13.Shichibu et al.[63] reported the synthesis and the PL of [Au13(dppe)5Cl2]3+ (Au13 II, dppe: 1,2‐bis(diphenylphosphino) ethane) cluster. They found that the cluster can be luminescent under excitation of 360 nm, displaying wide PL band across visible to NIR centered at ∼766 nm. As shown in Figure 3A, it is observed that the excitation spectrum monitored at 766 nm exhibits the same spectral profile with the absorption spectrum, indicating the emission originates from the gold core Au13. The QY of Au13 II was estimated to be 6.2% relative to anthracene. As we discussed above, ligands modification on gold cluster surface is an optional way to enhance the quantum yield.3FIGURE(A) PL (line a, excited at 360 nm) and excitation (line b, monitoring 766 nm emission) spectra of Au13 II in acetonitrile (0.37 mM) at room temperature, and the absorption spectrum (line c) is provided for comparison. (B) PL spectrum (excited at 485 nm) and excitation spectrum (monitoring 730 nm emission) for Au13 III. (C) The excitation spectrum (monitoring 680 nm emission) of Au25 protected by C6 ligands and the PL emission spectra of Au25 protected different ligands under excitation at 680 nm. (D) PL spectrum of Au25 III under excitation at 808 nm. (E) Proposed PL mechanism of Au25 III. (F) PL spectrum of Au37 II (excitation wavelength was not mentioned in ref. [29]). (A) Adapted with the permission from ref. [63]; copyright 2010, Wiley‐VCH GmbH. (B) Adapted with the permission from ref. [64]; copyright 2019, American Chemical Society. (C) Adapted with the permission from ref. [65]; copyright 2012, American Chemical Society. (D–F) Adapted with the permission from ref. [29]; copyright 2021, Wiley‐VCH GmbHNarouz et al.[64] reported an N‐heterocyclic carbenes (NHC) ligand protected Au13 superatom (Au13 III), where the Au13 core is surrounded by symmetric distributed nine NHC and three chlorides. The fluorescence quantum yield was determined to be 16% by applying fluorescence excitation−emission matrix spectroscopy (Figure 3B). Furthermore, the excitation spectrum of Au13 III matches very well with the absorption spectrum, which means the intense emission of Au13 III results from the de‐excitation of the excited state directly, which is in accordance with Shichibu's work.[63] X‐ray crystallography data shows that Au13 III is characterized by multiple π–π and CH–π interactions, which restrict the vibrational and rotational motion of ligands, limiting the non‐radiative pathway, which accounts for the enhanced emission. Apart from ligand engineering, doping of metal nanoclusters with heteroatoms offers a new approach for QY enhancement. For example, a substitution of gold with Ru, Rh, or Ir atom for the Au13 nanocluster can drastically enhance the intensity of its phosphorescence, which is ascribed to a rapid intersystem crossing due to the similarity between the singlet and triplet excited states in terms of structure and energy.[57] The mechanism and origin (fluorescence versus phosphorescence) of PL in these Au NCs are still under debate and therefore requires further study.Park et al.[65] performed surface ligand‐dependent optical absorption and PL studies of rod‐shaped Au25 clusters, and they found that the absorption and luminescence spectra of Au25 cluster protected by different ligands are very similar to each other (except the Au25 protected by PhC2 shows an additional shoulder around 780 in emission spectrum, see Figure 3C). Though the absorption and emission peak energies of those Au25 clusters are insensitive to the ligands, the QYs (relative to DTTC) appear to vary from 1 × 10−3 to 1 × 10−4. They thought the observed emission may arise from a sub‐gap emission because the emission energy is substantially smaller than the optical bandgap of 1.82 eV.In a recent work, Li et al.[29] reported a systematic work on the PL of rod‐shaped NCs [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ (Au25 III) and Au37 II and revealed the fundamental electronic transition mechanisms. The emission spectrum of cluster Au25 III lies in the NIR region (Figure 3D,E) with a strong peak at 990 nm. The emission spectra with different excitation wavelengths show no peak shifts, and the excitation spectrum monitored at 970 nm is consistent with the absorption spectrum. These results demonstrated that the PL peak in the NIR region is a gap emission. Furthermore, the QY of the NIR luminescence (relative to p‐FE) is about 8% for Au25 III. Au37 II shows two emission peaks at 1000 nm and ∼1520 nm with a QY of 0.1% (Figure 3F). The relatively low QY in Au37 II should result from strong non‐radiative process, including the excitation electron localization process reported by Zhou et al.[66] The large stokes shift of Au25 III (0.6 eV) indicates that the Au25 rod may experience significant change in geometry and electronic structure upon photoexcitation, which is similar with Au13 NCs.[59]ULTRAFAST EXCITED STATE DYNAMICSUltrafast excited‐state deactivation dynamics, including the relaxation time, coherent oscillations, isotropic and anisotropic dynamics, is of great importance for understanding the excited‐state electronic redistribution process and practical applications of gold NCs.[3,67] By applying femtosecond fluorescence up‐conversion spectroscopy and femtosecond and nanosecond transient absorption spectroscopy (fs‐ and ns‐TA), many efforts have been devoted to revealing the unique ultrafast phenomena of NCs assembled from Au13 units.The excited state dynamics of Au13 II nanocluster was investigated by Zhou et al.[66] in 2017 (see Figure 4A,B). By analyzing the kinetics, they found that the two decay components with excitation of 360 nm can be assigned to internal conversion (LUMO+n to LUMO, 0.4 ps) and relaxation back to ground state (1.72 μs). With 560 nm excitation, the ultrafast internal conversion disappears and only one decay component was observed (1.72 μs). As the icosahedral Au13 cluster has a symmetric superatom structure, the molecular orbitals are degenerated in three directions as we discussed above, so that the excited‐state dynamics is isotropic. Based on the ultrafast dynamics results, the excited‐state deactivation model of Au13 II is summarized in Figure 4C.4FIGUREFs‐TA spectra at all time delays of Au13 II with excitation of (A) 360 and (B) 560 nm, scattering around 560 nm because the pump laser was cut off. (C) Schematic diagram of excited‐stated deactivation processes of Au13 II. (A,B) Adapted with the permission from ref. [66]; copyright 2017, National Academy of SciencesSfeir et al.[68] reported the ultrafast dynamics of Au25 II NCs, where the TA spectra (see Figure 5A) show overlapping from both the excited state absorption (ESA) and ground state bleaching (GSB) signals excited at 415 nm. After global fitting, two decay components were obtained, corresponding to internal conversion (LUMO+n to LUMO, 0.8 ps) followed by the decay to the ground state (2.37 μs). To verify the fitting results from fs‐TA spectroscopy, two control experiments were further performed: (1) With excitation at NIR beam of 775 nm, the initial spectrum is identical to the long‐time scale behavior with UV–vis excitation, which means the first process obtained from TA with excitation at 415 nm is the internal conversion from LUMO+n to LUMO; (2) the ns‐TA spectra after excitation of 420 nm pump is in accordance with 1 ns spectrum of fs‐TA spectra excited at 415 nm, and no evidence of intermediate state is observed.5FIGURE(A) Fs‐TA spectra at all time delays of Au25 II. Evolution associated spectra (EAS) obtained from global analysis on the TA data of (B) Au25 IV. EAS obtained from global analysis on the TA data of (C) Au25 V. Schematic diagram of excited‐stated deactivation processes of (D) Au25 IV and (E) Au25 V. (F) Fs‐TA data of Au37 II with excitation of 1200 nm. (G) Schematic diagram of excited‐stated deactivation processes of Au37 II. (A) Adapted with the permission from ref. [68]; copyright 2011, American Chemical Society. (B–E) Adapted with the permission from ref. [69]; copyright 2021, The Royal Society of Chemistry. (F,G) Adapted with the permission from ref. [66]; copyright 2017, National Academy of SciencesAs the rod‐shaped Au25 II is spatially anisotropic, the anisotropic fs‐TA spectroscopy was also measured. They found the anisotropy near the GSB signal is close to 0.4, which represents the transition being probed aligned parallel to the initial excited transition. By fitting the TA kinetics at HOMO‐LUMO bleaching signal, only one exponential decay component corresponding to rod rotation with a time constant of 1.3 ns was obtained. In a recent work, Kong et al.[69] conducted a comparative study of the charge state effect on excited‐state dynamics of rod‐shaped [Au25(PPh3)10(SePh)5Cl2]q clusters (q = +1 and +2, Au25 IV and Au25 V, respectively). They found that two decay processes with time constants of 0.9 ps and 2.3 μs, corresponding to internal conversion from higher to lower excited states and the relaxation to ground state, can be identified for Au25 V, while an additional 660 ps decay is observed in Au25 IV due to the presence of single electron (see Figure 5B–E). Transient anisotropic absorption studies revealed a 500 ps rotation process is observed for both Au25 IV and Au25 V, whereas the initial anisotropy is highly dependent on the charge state. By comparing the excited‐state dynamics of Au13 II, Au25 II, and Au25 V, Kong et al. found that the similar decay pathways for Au13 and Au25 (+2), suggesting that the icosahedral units interaction show little effect on excited‐state decay pathways for Au25 (q = +2, Au25 II and Au25 V). However, the Au25 cluster (q = +1, Au25 IV) with one electron occupied orbital show significant influence on excited‐state deactivation process, whereas the in‐depth mechanism requires further theoretical investigations. In addition, Ramakrishna et al. investigated the unusual solvent effects on the optical properties of biicosahedral Au25 nanoclusters using ultrafast transient absorption spectroscopy. They found that the cluster‐solvent hydrogen‐bonding interaction has an influence on the exciton recombination of biicosahedral Au25 nanoclusters.[70–72]The excited‐state dynamics of Au37 is more complicated compared to those of Au13 and Au25 NCs. In 2017, Zhou et al.[66] performed a comprehensive ultrafast spectroscopic study on Au37 II (see Figure 5F). First, a pump energy of 1.03 eV (1200 nm) was chosen because 1.03 eV lies close to the energy gap of Au37 II (0.83 eV), thus excluding excess excitation energy. Combined with ns‐TA, two‐state evolution model (A → B → ground state) with lifetimes of 115 ps and 28 ns, respectively, was confirmed. By conducting pump wavelength‐dependent TA analysis, UV pump/NIR probe TA, and comparing with TA spectra of Au25 III NCs and the theoretical works of Nobusada and coworkers,[35] the A component is assigned to the S1 state with the electron distributed on the two vertex atoms of triicosahedral while B component is attributed to the state that lies between S1 and ground states with electron localized on one of the two vertices gold atoms (see Figure 5G). The excited‐state deactivation schematic diagram of Au13, Au25, and Au37 are shown in Figures 4 and 5.The electron–phonon coupling in nanostructures has been widely observed as periodic oscillations in time‐resolved spectroscopy,[3,73–76] which offers more information on the mechanical properties except for the energy deactivation. According to Sfeir and co‐workers’ work[68] about Au25 II NCs (Figure 6A–D), a modulation frequency of 0.8 THz (26 cm−1) near the boundary between ESA and GSB (650 nm) was obtained, while the coherent oscillation amplitude is close to 0 at GSB and ESA signal. For Au37 II (Figure 6E,F),[66] oscillation with a frequency of 0.6 THz (20 cm−1) was observed at 720, 750, and 820 nm whereas a vibration frequency of 2.1 THz (70 cm−1) was observed at a shorter wavelength (530 nm). One can observe that a similar low‐frequency mode can be observed for both Au25 and Au37 (0.8 THz vs 0.6 THz), whereas the higher frequency mode (2.1 THz) can only be observed for Au37. It was reported that the 2.1 THz vibration is due to the radial breathing mode, and the high frequency mode is the axial vibration within the Au13 unit (see Figure 6), while accurate assignment of the vibration mode requires further theoretical simulations. The coherent vibrations of isolated Au13 and Au60 NCs have not been reported, which calls for future experimental study. The frequency results obtained from ultrafast measurements are summarized in Table 1.6FIGURE(A) Pseudo color fs‐TA spectra of Au25 II (pump 415 nm). (B) Kinetic trace probed at ∼650 nm showing coherent phonon emission for Au25 II. (C) Fourier transform of the trace in (B). (D) Amplitude spectrum (left axis) and phase (right axis) of the coherent phonon oscillation of Au25 II. Kinetic traces probed at (E) longer wavelength and (F) shorter wavelength for Au37 II. (A–D) Adapted with the permission from ref. [68]; copyright 2011, American Chemical Society. (E,F) Adapted with the permission from ref. [66]; copyright 2017, National Academy of Sciences1TABLEThe coherent vibrational frequencies obtained from ultrafast measurements of gold NCs constructed from Au13 unitCompoundsFrequency (THz)Ref.Au13——Au250.8[68]Au370.6, 2.1[66]Au60——SUMMARY AND OUTLOOKIn summary, we have reviewed the optical properties of Au13‐assembled NCs, including Au13, Au25, Au37, and Au60. As the number of Au13 units increases, some general trends in optical absorption are found, which offer valuable information on the self‐assembled Au13 family. The photoluminescence properties and the evolution of excited state dynamics of Au13‐assembled NCs have been discussed. Here, we also provide our perspective on future directions for the optical property studies of Au13‐assembled clusters.First, DFT is a very powerful method for understanding the electronic properties of Au13‐assembled NCs, whereas the simulation of steady‐ and excited‐states electronic structures is insufficient. There have been a few studies on the simulation of excited state dynamics of ligand‐protected Au NCs.[62,77–78] Future theoretical studies on the excited‐state dynamics of Au13‐assembled NCs are required to understand the underlying mechanism.Second, the mechanism of photoluminescence for Au13‐assembled materials calls for systematic investigations. It is important to distinguish the singlet and triplet state (fluorescence and phosphorescence) of gold NCs and probe the origin of the radiative and nonradiative decay pathways, which will help to design luminescent gold NCs.Third, the excited‐state coherent oscillations of Au13‐assembled NCs require further investigations. Besides fs‐TA spectroscopy, some other ultrafast spectroscopic methods, including ultrafast 2D‐electronic spectroscopy and ultrafast Raman spectroscopy, may help to unravel the underlying mechanism of coherent vibrations.Overall, this minireview provides a comprehensive understanding on the optical properties of Au13‐assembled NCs, which should help to broaden their applications and stimulate future research on this topic.ACKNOWLEDGMENTSWe acknowledge the startup funding from University of Science and Technology of China, and the support from Chinese Academy of Sciences (YSBR‐007).ETHICS STATEMENTThis review does not involve any human investigation and animal experiment.CONFLICT OF INTERESTThe authors declare no conflict of interest.AUTHOR CONTRIBUTIONSAll authors contributed to the writing of the manuscript.DATA AVAILABILITY STATEMENTData sharing is not applicable to this article as no new data were created or analyzed in this review.REFERENCESK. M. Mayer, J. H. Hafner, Chem. 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Optical properties of gold nanoclusters constructed from Au13 units

Aggregate , Volume 3 (6) – Dec 1, 2022

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References (73)

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Wiley
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© 2022 The Authors. Aggregate published by SCUT, AIEI, and John Wiley & Sons Australia, Ltd.
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2692-4560
DOI
10.1002/agt2.207
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Abstract

INTRODUCTIONThe optical properties of metal nanoparticles directly reflect their electronic structures, which are highly dependent on their sizes and shapes. Photo‐excitation of gold nanoparticles (Au NPs, >2.2 nm diameter) leads to a broad absorption band across the UV–vis to near Infrared (NIR) region, which results from the collective oscillation of couduction band electrons, leading to surface plasmon resonance (SPR).[1–2] Generally, an absorption peak around 520 nm can be observed because the Au NPs show a quasi‐continuous conduction band of the electronic structure and SPR effect dominates the photoexcitation process.[3–4] However, the quantum confinement effect becomes significant in gold nanoparticles when the diameter is smaller than 2.2 nm (often known as gold nanoclusters [NCs]). Accordingly, multiple absorption bands due to single‐electron transition are obtained in the UV–vis absorption spectra.[5–7] Gold NCs exhibit size‐dependent electronic, optical, and magnetic properties. There have been considerable research interests in gold NCs in both fundamental science and applications in catalysis, chemical sensing, etc.[8–16]The synthesis at the atomic level in organic molecules is widely reported, while the goal of achieving atomic precision NCs is still challenging.[17–20] Having benefited from the previous research, several strategies have been developed, including size‐focusing and ligand‐exchange‐induced size/structure transformation synthetic methodologies,[21–23] which paved the way for designing and synthesizing the gold NCs with atomic precision and molecular purity. With the help of precise synthesis, the total structure, including the gold core and surface ligands, can be determined by X‐ray crystallography. Therefore, the bonding structures, structure–property relationship, shape control, size effect, ligand–core coordination patterns, etc., can be fully understood.[17] Besides the previous synthesis methods, the assembly of structural building blocks into functional materials is also well developed.[8,24–28]Among those structural building blocks, the icosahedral Au13 motif is often used in NCs protected by different ligands.[26,28–32] The different stacking modes of icosahedral Au13‐cluster (Figure 1A) include linear and cyclic growth patterns, which are also named as vertex‐ and face‐sharing patterns.[26–28] In 1990, Teo et al.[33] reported a 38‐atom cluster, (p‐Tol3P)12Au18Ag20Cl14, with three 13‐atom centered icosahedral units sharing three vertices in a triangle way, and such a kind of cluster was called “clusters of clusters”. Hereafter, the research groups of Tsukuda[26] and Jin[34] demonstrated the synthesis of rod‐shaped gold NCs with 25 gold atoms (Figure 1B), which consisted of two icosahedral Au13 clusters sharing one gold vertex atom.1FIGUREThe structures of (A) Au13, (B) Au25, (C) Au37, and (D) Au60. Only gold atoms are shown in the structures; the center gold atoms of Au13 units are labeled as blue; the vertex gold atoms are labeled as magentaIn 2007, theoretical studies conducted by Nobusada and Iwasa[35] reported that the electronic properties of Au13 unit would remain unchanged in the Au13‐assembled oligomers (including Au25 constructed by two Au13 blocks and Au37 constructed by three Au13 blocks). Jiang et al.[36] performed a theoretical investigation by designing two one‐dimensional nanosystems with a linear chain that are composed of the icosahedral Au13 units fused by vertex and face sharing, respectively. They found that vertex‐sharing linear chain can be made either semiconducting or metallic by changing charge, while face‐sharing nanowire is always metallic. In 2015, Jin et al.[27] successfully synthesized the Au37 NCs and reported the structure (Figure 1C), which comprise three icosahedral Au13 building blocks via linear assembling mode. In the same year, Zhu et al.[28] reported the crystal structure of Au60 NCs (Figure 1D), in which five Au13 units form a closed gold ring by Au‐Se‐Au bonds. For these multiunit structures, it is of importance to figure out how the building blocks are arranged in the cluster‐assembled materials, and whether the optical properties of Au13 building block are preserved in larger cluster‐assembled materials.In this minireview, we focus on the optical properties of several cluster‐assembled materials based on Au13 unit. We first review steady‐state spectral properties, including absorption and photoluminescence spectra, and then we introduce the ultrafast spectroscopic features. We also highlight the photophysical properties of the cluster‐assembled materials (Au13, Au25, Au37, and Au60) and conclude the main spectral features of cluster‐assembled materials fused by Au13 units. Finally, we discuss the main challenge and opportunities for future research in such clusters.OPTICAL ABSORPTIONThe Au13 icosahedral cluster is a commonly used unit, which consists of a center gold atom and a 12‐atom shell of icosahedral geometry (as shown in Figure 1A). Such a small Au13 kernel is widely applied in assembled materials (see Figure 1) so that it is important to understand the electronic properties of Au13 unit before and after the assembly. Teo et al.[37–38] reported that oligomeric clusters can be systematically synthesized in a stepwise way by aggregating the icosahedral units, in which the individual icosahedral clusters serve as the building blocks. Khanna and Castleman et al.[39–41] demonstrated that the aluminum‐based icosahedral clusters form the assembled clusters, which are also named as “superatoms” in their work. In both clusters of clusters and superatoms, the assembled materials are constructed from icosahedral building blocks and the electronic property of each constituent unit is retained.[35]Nobusada et al.[35] theoretically characterized the electronic structures and absorption spectra of a biicosahedral gold cluster [Au25(PH3)10(SCH3)5Cl2]2+ (Au25 I, Figure 2A) and a triicosahedral cluster [Au37(PH3)10(SCH3)10Cl2]+ (Au37 I, Figure 2A). The theoretical absorption spectra of Au25 I and Au37 I were obtained by convoluting the absorption peaks with Lorentz function, and theoretical spectrum of Au25 I (Figure 2A) reproduces the experimental absorption spectrum of [Au25(PPh3)10(SC2H5)5Cl2]2+ (Au25 II). The absorption peak of Au25 I at lower‐energy level (∼702 nm) that is well separated from other absorption peaks at higher‐energy region (<600 nm) is attributed to the electronic transition from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) due to the vertex‐sharing biicosahedral structure (see Figure 2B). The theoretical absorption spectrum of Au37 I is also plotted, and the spectral profile between 400 and 600 nm is comparable to that of Au25 I except that the absorption peaks of Au37 I are shifted to longer wavelength. Moreover, two new peaks at ∼761 and 1230 nm are observed in the absorption spectrum of Au37 I due to the interaction between Au13 units (Figure 2A).2FIGURE(A) Absorption spectrum for Au25 I (top, calculation result), Au25 II (middle, experimental result), and Au37 I (bottom, calculation result). (B) Schematic diagrams of the Kohn–Sham orbitals and main atomic orbital components contributing to each Kohn–Sham orbital (calculation result) for Au25 I (top) and Au37 I (bottom). (C) UV–vis–NIR absorption spectrum of the Au37 II (experimental result). (D) UV–vis absorption spectrum of Au13 I (experiment result). (E) UV–vis‐NIR absorption spectrum of Au60 I (experiment result). (A,B) Adapted with the permission from ref.[35]; copyright 2007, American Chemical Society. (C,D) Adapted with the permission from ref.[27]; copyright 2015, American Chemical Society. (E) Adapted with the permission from ref. [28]; copyright 2015, Wiley‐VCH GmbHIn the experimental work reported by Jin et al.[27] the lower‐energy absorption peaks of [Au37(PPh3)10(SC2H4Ph)10X2]+ (X = Cl or Br, Au37 II, gold atoms 3 × 13 − 2 = 37, Figure 2C) are centered at 795 and 1230 nm, which are essentially consistent with the theoretical prediction: the 1230‐nm peak is assigned to the transition from HOMO to LUMO while the 795 nm peak arises from HOMO‐1 to LUMO transition. The absorption spectrum of [Au13(PPh3)10X2]3+ (Au13 I, Figure 2D) consists of a peak of 430 nm and a shoulder around 490 nm, which agrees with the short‐wavelength profiles of Au25 I/II, and Au37 I/II. Therefore, the higher‐energy transitions of Au25 I/II and Au37 I/II should originate from the electronic transition within Au13 clusters. It suggests that the electronic properties of individual icosahedral cluster remain unchanged in the assembled clusters. From the density functional theory (DFT) calculated results (Figure 2B), one can observe the HOMO of Au25 I is mainly composed of the atomic orbital along z axis, and the LUMO is primarily formed from the s orbital of the vertex gold atom at the center. In Au37 I, the HOMO and HOMO‐1 orbitals are composed of the orbitals along the z axis and LUMO is constructed mainly from z orbitals of the two vertex‐sharing gold atoms. As Au13 has spherical symmetry in the x, y, and z axes, the atomic orbitals along the three directions are degenerated. In addition, the molecular orbitals of new clusters Au25 I and Au37 I (HOMO, LUMO for Au25 I and Au37 I, HOMO‐1 for Au37 I) are attributed to the interaction between icosahedral Au13 units induced by electronic symmetry breaking.The cluster‐assembled [Au60Se2(Ph3P)10(SePh)15]+ (Au60 I, Figure 2E) reported by Zhu and co‐workers[28] contains five icosahedral Au13 building blocks with Au‐Se‐Au linkages, and two adjacent Au13 blocks share a vertex atom (Figure 1D), that is, the total numbers of gold atoms are 5 × 13 − 5 = 60. The optical absorption spectrum of Au60 I exhibits five peaks located at 353, 435, 510, 600, and 835 nm. Nobusada et al.[35] reported that the icosahedral Au13 block is a spherically symmetric structure, and the electronic symmetric breaking does not influence the orbital symmetry of Au13 unit, so that the electronic property of the units is retained in assembled Au60 I. Similar to Au13 I, Au25 II, and Au37 II, the absorption peaks of Au60 I shorter than 600 nm can be assigned to the electronic transition within individual Au13 unit, while the absorption peak around 835 nm can be attributed to the HOMO‐LUMO transition of the pentamer structure. Besides the linear optical absorption, the nonlinear optical properties of NCs with Au13 units have also been reported. Optical limiting is a nonlinear effect, meaning the decrease in the transmittance of a sample with high fluence light illumination. The optical limiting properties of Au60 I and Au25 III were also investigated and Au60 I was found to exhibit better performance.[28] The correlation of Au13 assembly and their nonlinear optical properties calls for further investigations.The HOMO‐LUMO absorption peak red‐shifts from Au13 (∼490 nm) to Au25 (∼700 nm) and Au37 (∼1230 nm). However, the HOMO‐LUMO absorption peak of Au60 is around 835 nm, which lies between that of Au25 and Au37. There are two possible explanations for the phenomenon: (1) The electrons in Au60 I is more restricted compared to linear assembled Au25 and Au37 because of the internal strain caused by the cyclic structure; (2) The electronic density of LUMO of Au60 I is more concentrated at the center of pentamer structure because one Se atom connected with five Au13 icosahedral via Se‐(Au)5 bonds simultaneously. The unique absorption features of Au60 I calls for further experimental and theoretical studies in the future.PHOTOLUMINESCENCEThe photoluminescence (PL for short) of gold NCs have attracted much attention because of their promising applications in biosensing, phototherapy, cell labeling, etc.[42–46] The reported PL quantum yields (QYs) of most NCs were very low (<1%).[47–50] It was reported that metal NCs dissolved in water often show relatively stronger PL than their organic solvents‐soluble counterparts.[51] Moreover, rigidifying surface protecting ligands can be applied to enhance PL;[52–54] modifying the gold core size or doping some foreign metal atoms is another strategy;[55–57] aggregation‐induced emission scheme that adopted extensively in organic molecules can also be employed to enhance PL.[58–61]In gold NCs assembled from Au13 units (Au13, Au25, Au37 and Au60), there is no surface motif structures and the PL should originate exclusively from the metal core. In mono‐icosahedral Au13 NCs, DFT calculation performed by Aikens and co‐workers[62] reported that both HOMO and LUMO are delocalized on the Au13 core, and the PL of Au13 originates from the lowest singlet excited state (S1 state). In Au25 and Au37, the HOMO‐LUMO gaps are smaller compared to Au13 so that the PL peaks should be redshifted accordingly. Unlike Au13, the HOMOs of rod‐shaped Au25 and Au37 NCs are distributed along the z axis of the rod while the LUMOs are mainly contributed by vertex gold atoms. Therefore, the PL mechanism in Au25 and Au37 could be more complicated compared to mono icosahedral Au13.Shichibu et al.[63] reported the synthesis and the PL of [Au13(dppe)5Cl2]3+ (Au13 II, dppe: 1,2‐bis(diphenylphosphino) ethane) cluster. They found that the cluster can be luminescent under excitation of 360 nm, displaying wide PL band across visible to NIR centered at ∼766 nm. As shown in Figure 3A, it is observed that the excitation spectrum monitored at 766 nm exhibits the same spectral profile with the absorption spectrum, indicating the emission originates from the gold core Au13. The QY of Au13 II was estimated to be 6.2% relative to anthracene. As we discussed above, ligands modification on gold cluster surface is an optional way to enhance the quantum yield.3FIGURE(A) PL (line a, excited at 360 nm) and excitation (line b, monitoring 766 nm emission) spectra of Au13 II in acetonitrile (0.37 mM) at room temperature, and the absorption spectrum (line c) is provided for comparison. (B) PL spectrum (excited at 485 nm) and excitation spectrum (monitoring 730 nm emission) for Au13 III. (C) The excitation spectrum (monitoring 680 nm emission) of Au25 protected by C6 ligands and the PL emission spectra of Au25 protected different ligands under excitation at 680 nm. (D) PL spectrum of Au25 III under excitation at 808 nm. (E) Proposed PL mechanism of Au25 III. (F) PL spectrum of Au37 II (excitation wavelength was not mentioned in ref. [29]). (A) Adapted with the permission from ref. [63]; copyright 2010, Wiley‐VCH GmbH. (B) Adapted with the permission from ref. [64]; copyright 2019, American Chemical Society. (C) Adapted with the permission from ref. [65]; copyright 2012, American Chemical Society. (D–F) Adapted with the permission from ref. [29]; copyright 2021, Wiley‐VCH GmbHNarouz et al.[64] reported an N‐heterocyclic carbenes (NHC) ligand protected Au13 superatom (Au13 III), where the Au13 core is surrounded by symmetric distributed nine NHC and three chlorides. The fluorescence quantum yield was determined to be 16% by applying fluorescence excitation−emission matrix spectroscopy (Figure 3B). Furthermore, the excitation spectrum of Au13 III matches very well with the absorption spectrum, which means the intense emission of Au13 III results from the de‐excitation of the excited state directly, which is in accordance with Shichibu's work.[63] X‐ray crystallography data shows that Au13 III is characterized by multiple π–π and CH–π interactions, which restrict the vibrational and rotational motion of ligands, limiting the non‐radiative pathway, which accounts for the enhanced emission. Apart from ligand engineering, doping of metal nanoclusters with heteroatoms offers a new approach for QY enhancement. For example, a substitution of gold with Ru, Rh, or Ir atom for the Au13 nanocluster can drastically enhance the intensity of its phosphorescence, which is ascribed to a rapid intersystem crossing due to the similarity between the singlet and triplet excited states in terms of structure and energy.[57] The mechanism and origin (fluorescence versus phosphorescence) of PL in these Au NCs are still under debate and therefore requires further study.Park et al.[65] performed surface ligand‐dependent optical absorption and PL studies of rod‐shaped Au25 clusters, and they found that the absorption and luminescence spectra of Au25 cluster protected by different ligands are very similar to each other (except the Au25 protected by PhC2 shows an additional shoulder around 780 in emission spectrum, see Figure 3C). Though the absorption and emission peak energies of those Au25 clusters are insensitive to the ligands, the QYs (relative to DTTC) appear to vary from 1 × 10−3 to 1 × 10−4. They thought the observed emission may arise from a sub‐gap emission because the emission energy is substantially smaller than the optical bandgap of 1.82 eV.In a recent work, Li et al.[29] reported a systematic work on the PL of rod‐shaped NCs [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ (Au25 III) and Au37 II and revealed the fundamental electronic transition mechanisms. The emission spectrum of cluster Au25 III lies in the NIR region (Figure 3D,E) with a strong peak at 990 nm. The emission spectra with different excitation wavelengths show no peak shifts, and the excitation spectrum monitored at 970 nm is consistent with the absorption spectrum. These results demonstrated that the PL peak in the NIR region is a gap emission. Furthermore, the QY of the NIR luminescence (relative to p‐FE) is about 8% for Au25 III. Au37 II shows two emission peaks at 1000 nm and ∼1520 nm with a QY of 0.1% (Figure 3F). The relatively low QY in Au37 II should result from strong non‐radiative process, including the excitation electron localization process reported by Zhou et al.[66] The large stokes shift of Au25 III (0.6 eV) indicates that the Au25 rod may experience significant change in geometry and electronic structure upon photoexcitation, which is similar with Au13 NCs.[59]ULTRAFAST EXCITED STATE DYNAMICSUltrafast excited‐state deactivation dynamics, including the relaxation time, coherent oscillations, isotropic and anisotropic dynamics, is of great importance for understanding the excited‐state electronic redistribution process and practical applications of gold NCs.[3,67] By applying femtosecond fluorescence up‐conversion spectroscopy and femtosecond and nanosecond transient absorption spectroscopy (fs‐ and ns‐TA), many efforts have been devoted to revealing the unique ultrafast phenomena of NCs assembled from Au13 units.The excited state dynamics of Au13 II nanocluster was investigated by Zhou et al.[66] in 2017 (see Figure 4A,B). By analyzing the kinetics, they found that the two decay components with excitation of 360 nm can be assigned to internal conversion (LUMO+n to LUMO, 0.4 ps) and relaxation back to ground state (1.72 μs). With 560 nm excitation, the ultrafast internal conversion disappears and only one decay component was observed (1.72 μs). As the icosahedral Au13 cluster has a symmetric superatom structure, the molecular orbitals are degenerated in three directions as we discussed above, so that the excited‐state dynamics is isotropic. Based on the ultrafast dynamics results, the excited‐state deactivation model of Au13 II is summarized in Figure 4C.4FIGUREFs‐TA spectra at all time delays of Au13 II with excitation of (A) 360 and (B) 560 nm, scattering around 560 nm because the pump laser was cut off. (C) Schematic diagram of excited‐stated deactivation processes of Au13 II. (A,B) Adapted with the permission from ref. [66]; copyright 2017, National Academy of SciencesSfeir et al.[68] reported the ultrafast dynamics of Au25 II NCs, where the TA spectra (see Figure 5A) show overlapping from both the excited state absorption (ESA) and ground state bleaching (GSB) signals excited at 415 nm. After global fitting, two decay components were obtained, corresponding to internal conversion (LUMO+n to LUMO, 0.8 ps) followed by the decay to the ground state (2.37 μs). To verify the fitting results from fs‐TA spectroscopy, two control experiments were further performed: (1) With excitation at NIR beam of 775 nm, the initial spectrum is identical to the long‐time scale behavior with UV–vis excitation, which means the first process obtained from TA with excitation at 415 nm is the internal conversion from LUMO+n to LUMO; (2) the ns‐TA spectra after excitation of 420 nm pump is in accordance with 1 ns spectrum of fs‐TA spectra excited at 415 nm, and no evidence of intermediate state is observed.5FIGURE(A) Fs‐TA spectra at all time delays of Au25 II. Evolution associated spectra (EAS) obtained from global analysis on the TA data of (B) Au25 IV. EAS obtained from global analysis on the TA data of (C) Au25 V. Schematic diagram of excited‐stated deactivation processes of (D) Au25 IV and (E) Au25 V. (F) Fs‐TA data of Au37 II with excitation of 1200 nm. (G) Schematic diagram of excited‐stated deactivation processes of Au37 II. (A) Adapted with the permission from ref. [68]; copyright 2011, American Chemical Society. (B–E) Adapted with the permission from ref. [69]; copyright 2021, The Royal Society of Chemistry. (F,G) Adapted with the permission from ref. [66]; copyright 2017, National Academy of SciencesAs the rod‐shaped Au25 II is spatially anisotropic, the anisotropic fs‐TA spectroscopy was also measured. They found the anisotropy near the GSB signal is close to 0.4, which represents the transition being probed aligned parallel to the initial excited transition. By fitting the TA kinetics at HOMO‐LUMO bleaching signal, only one exponential decay component corresponding to rod rotation with a time constant of 1.3 ns was obtained. In a recent work, Kong et al.[69] conducted a comparative study of the charge state effect on excited‐state dynamics of rod‐shaped [Au25(PPh3)10(SePh)5Cl2]q clusters (q = +1 and +2, Au25 IV and Au25 V, respectively). They found that two decay processes with time constants of 0.9 ps and 2.3 μs, corresponding to internal conversion from higher to lower excited states and the relaxation to ground state, can be identified for Au25 V, while an additional 660 ps decay is observed in Au25 IV due to the presence of single electron (see Figure 5B–E). Transient anisotropic absorption studies revealed a 500 ps rotation process is observed for both Au25 IV and Au25 V, whereas the initial anisotropy is highly dependent on the charge state. By comparing the excited‐state dynamics of Au13 II, Au25 II, and Au25 V, Kong et al. found that the similar decay pathways for Au13 and Au25 (+2), suggesting that the icosahedral units interaction show little effect on excited‐state decay pathways for Au25 (q = +2, Au25 II and Au25 V). However, the Au25 cluster (q = +1, Au25 IV) with one electron occupied orbital show significant influence on excited‐state deactivation process, whereas the in‐depth mechanism requires further theoretical investigations. In addition, Ramakrishna et al. investigated the unusual solvent effects on the optical properties of biicosahedral Au25 nanoclusters using ultrafast transient absorption spectroscopy. They found that the cluster‐solvent hydrogen‐bonding interaction has an influence on the exciton recombination of biicosahedral Au25 nanoclusters.[70–72]The excited‐state dynamics of Au37 is more complicated compared to those of Au13 and Au25 NCs. In 2017, Zhou et al.[66] performed a comprehensive ultrafast spectroscopic study on Au37 II (see Figure 5F). First, a pump energy of 1.03 eV (1200 nm) was chosen because 1.03 eV lies close to the energy gap of Au37 II (0.83 eV), thus excluding excess excitation energy. Combined with ns‐TA, two‐state evolution model (A → B → ground state) with lifetimes of 115 ps and 28 ns, respectively, was confirmed. By conducting pump wavelength‐dependent TA analysis, UV pump/NIR probe TA, and comparing with TA spectra of Au25 III NCs and the theoretical works of Nobusada and coworkers,[35] the A component is assigned to the S1 state with the electron distributed on the two vertex atoms of triicosahedral while B component is attributed to the state that lies between S1 and ground states with electron localized on one of the two vertices gold atoms (see Figure 5G). The excited‐state deactivation schematic diagram of Au13, Au25, and Au37 are shown in Figures 4 and 5.The electron–phonon coupling in nanostructures has been widely observed as periodic oscillations in time‐resolved spectroscopy,[3,73–76] which offers more information on the mechanical properties except for the energy deactivation. According to Sfeir and co‐workers’ work[68] about Au25 II NCs (Figure 6A–D), a modulation frequency of 0.8 THz (26 cm−1) near the boundary between ESA and GSB (650 nm) was obtained, while the coherent oscillation amplitude is close to 0 at GSB and ESA signal. For Au37 II (Figure 6E,F),[66] oscillation with a frequency of 0.6 THz (20 cm−1) was observed at 720, 750, and 820 nm whereas a vibration frequency of 2.1 THz (70 cm−1) was observed at a shorter wavelength (530 nm). One can observe that a similar low‐frequency mode can be observed for both Au25 and Au37 (0.8 THz vs 0.6 THz), whereas the higher frequency mode (2.1 THz) can only be observed for Au37. It was reported that the 2.1 THz vibration is due to the radial breathing mode, and the high frequency mode is the axial vibration within the Au13 unit (see Figure 6), while accurate assignment of the vibration mode requires further theoretical simulations. The coherent vibrations of isolated Au13 and Au60 NCs have not been reported, which calls for future experimental study. The frequency results obtained from ultrafast measurements are summarized in Table 1.6FIGURE(A) Pseudo color fs‐TA spectra of Au25 II (pump 415 nm). (B) Kinetic trace probed at ∼650 nm showing coherent phonon emission for Au25 II. (C) Fourier transform of the trace in (B). (D) Amplitude spectrum (left axis) and phase (right axis) of the coherent phonon oscillation of Au25 II. Kinetic traces probed at (E) longer wavelength and (F) shorter wavelength for Au37 II. (A–D) Adapted with the permission from ref. [68]; copyright 2011, American Chemical Society. (E,F) Adapted with the permission from ref. [66]; copyright 2017, National Academy of Sciences1TABLEThe coherent vibrational frequencies obtained from ultrafast measurements of gold NCs constructed from Au13 unitCompoundsFrequency (THz)Ref.Au13——Au250.8[68]Au370.6, 2.1[66]Au60——SUMMARY AND OUTLOOKIn summary, we have reviewed the optical properties of Au13‐assembled NCs, including Au13, Au25, Au37, and Au60. As the number of Au13 units increases, some general trends in optical absorption are found, which offer valuable information on the self‐assembled Au13 family. The photoluminescence properties and the evolution of excited state dynamics of Au13‐assembled NCs have been discussed. Here, we also provide our perspective on future directions for the optical property studies of Au13‐assembled clusters.First, DFT is a very powerful method for understanding the electronic properties of Au13‐assembled NCs, whereas the simulation of steady‐ and excited‐states electronic structures is insufficient. There have been a few studies on the simulation of excited state dynamics of ligand‐protected Au NCs.[62,77–78] Future theoretical studies on the excited‐state dynamics of Au13‐assembled NCs are required to understand the underlying mechanism.Second, the mechanism of photoluminescence for Au13‐assembled materials calls for systematic investigations. It is important to distinguish the singlet and triplet state (fluorescence and phosphorescence) of gold NCs and probe the origin of the radiative and nonradiative decay pathways, which will help to design luminescent gold NCs.Third, the excited‐state coherent oscillations of Au13‐assembled NCs require further investigations. Besides fs‐TA spectroscopy, some other ultrafast spectroscopic methods, including ultrafast 2D‐electronic spectroscopy and ultrafast Raman spectroscopy, may help to unravel the underlying mechanism of coherent vibrations.Overall, this minireview provides a comprehensive understanding on the optical properties of Au13‐assembled NCs, which should help to broaden their applications and stimulate future research on this topic.ACKNOWLEDGMENTSWe acknowledge the startup funding from University of Science and Technology of China, and the support from Chinese Academy of Sciences (YSBR‐007).ETHICS STATEMENTThis review does not involve any human investigation and animal experiment.CONFLICT OF INTERESTThe authors declare no conflict of interest.AUTHOR CONTRIBUTIONSAll authors contributed to the writing of the manuscript.DATA AVAILABILITY STATEMENTData sharing is not applicable to this article as no new data were created or analyzed in this review.REFERENCESK. M. Mayer, J. H. Hafner, Chem. 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Published: Dec 1, 2022

Keywords: absorption; Au 13; gold nanoclusters; optical properties; photoluminescence; ultrafast dynamics

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