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1. Isomerization-induced enhancement of luminescence in Au ₂₈ (SR) ₂₀ nanoclusters

   https://doi.org/10.1039/d0sc01270j  

   Chen, Yuxiang; Zhou, Meng; Li, Qi; Gronlund, Harrison; Jin, Rongchao (August 2020, Chemical Science)

   null (Ed.)

   Understanding the origin and structural basis of the photoluminescence (PL) phenomenon in thiolate-protected metal nanoclusters is of paramount importance for both fundamental science and practical applications. It remains a major challenge to correlate the PL properties with the atomic-level structure due to the complex interplay of the metal core ( i.e. the inner kernel) and the exterior shell ( i.e. surface Au( i )-thiolate staple motifs). Decoupling these two intertwined structural factors is critical in order to understand the PL origin. Herein, we utilize two Au 28 (SR) 20 nanoclusters with different –R groups, which possess the same core but different shell structures and thus provide an ideal system for the PL study. We discover that the Au 28 (CHT) 20 (CHT: cyclohexanethiolate) nanocluster exhibits a more than 15-fold higher PL quantum yield than the Au 28 (TBBT) 20 nanocluster (TBBT: p-tert -butylbenzenethiolate). Such an enhancement is found to originate from the different structural arrangement of the staple motifs in the shell, which modifies the electron relaxation dynamics in the inner core to different extents for the two nanoclusters. The emergence of a long PL lifetime component in the more emissive Au 28 (CHT) 20 nanocluster reveals that its PL is enhanced by suppressing the nonradiative pathway. The presence of long, interlocked staple motifs is further identified as a key structural parameter that favors the luminescence. Overall, this work offers structural insights into the PL origin in Au 28 (SR) 20 nanoclusters and provides some guidelines for designing luminescent metal nanoclusters for sensing and optoelectronic applications. 

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2. Predicting ligand removal energetics in thiolate-protected nanoclusters from molecular complexes

   https://doi.org/10.1039/d0nr07839e  

   McKay, Julia; Cowan, Michael J.; Morales-Rivera, Cristian A.; Mpourmpakis, Giannis (January 2021, Nanoscale)

   null (Ed.)

   Thiolate-protected metal nanoclusters (TPNCs) have attracted great interest in the last few decades due to their high stability, atomically precise structure, and compelling physicochemical properties. Among their various applications, TPNCs exhibit excellent catalytic activity for numerous reactions; however, recent work revealed that these systems must undergo partial ligand removal in order to generate active sites. Despite the importance of ligand removal in both catalysis and stability of TPNCs, the role of ligands and metal type in the process is not well understood. Herein, we utilize Density Functional Theory to understand the energetic interplay between metal–sulfur and sulfur–ligand bond dissociation in metal–thiolate systems. We first probe 66 metal–thiolate molecular complexes across combinations of M = Ag, Au, and Cu with twenty-two different ligands (R). Our results reveal that the energetics to break the metal–sulfur and sulfur–ligand bonds are strongly correlated and can be connected across all complexes through metal atomic ionization potentials. We then extend our work to the experimentally relevant [M 25 (SR) 18 ] − TPNC, revealing the same correlations at the nanocluster level. Importantly, we unify our work by introducing a simple methodology to predict TPNC ligand removal energetics solely from calculations performed on metal–ligand molecular complexes. Finally, a computational mechanistic study was performed to investigate the hydrogenation pathways for SCH 3 -based complexes. The energy barriers for these systems revealed, in addition to thermodynamics, that kinetics favor the break of S–R over the M–S bond in the case of the Au complex. Our computational results rationalize several experimental observations pertinent to ligand effects on TPNCs. Overall, our introduced model provides an accelerated path to predict TPNC ligand removal energies, thus aiding towards targeted design of TPNC catalysts. 

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3. Chirality and Surface Bonding Correlation in Atomically Precise Metal Nanoclusters

   https://doi.org/10.1002/adma.201905488  

   Li, Yingwei; Higaki, Tatsuya; Du, Xiangsha; Jin, Rongchao (March 2020, Advanced Materials)

   Abstract Chirality is ubiquitous in nature and occurs at all length scales. The development of applications for chiral nanostructures is rising rapidly. With the recent achievements of atomically precise nanochemistry, total structures of ligand‐protected Au and other metal nanoclusters (NCs) are successfully obtained, and the origins of chirality are discovered to be associated with different parts of the cluster, including the surface ligands (e.g., swirl patterns), the organic–inorganic interface (e.g., helical stripes), and the kernel. Herein, a unified picture of metal–ligand surface bonding‐induced chirality for the nanoclusters is proposed. The different bonding modes of M–X (where M = metal and X = the binding atom of ligand) lead to different surface structures on nanoclusters, which in turn give rise to various characteristic features of chirality. A comparison of Au–thiolate NCs with Au–phosphine ones further reveals the important roles of surface bonding. Compared to the Au–thiolate NCs, the Ag/Cu/Cd–thiolate systems exhibit different coordination modes between the metal and the thiolate. Other than thiolate and phosphine ligands, alkynyls are also briefly discussed. Several methods of obtaining chiroptically active nanoclusters are introduced, such as enantioseparation by high‐performance liquid chromatography and enantioselective synthesis. Future perspectives on chiral NCs are also proposed. 

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4. Structure–property relationships on thiolate-protected gold nanoclusters

   https://doi.org/10.1039/c8na00246k  

   Cowan, Michael J.; Mpourmpakis, Giannis (January 2019, Nanoscale Advances)

   Since their discovery, thiolate-protected gold nanoclusters (Au n (SR) m ) have garnered a lot of interest due to their fascinating properties and “magic-number” stability. However, models describing the thermodynamic stability and electronic properties of these nanostructures as a function of their size are missing in the literature. Herein, we employ first principles calculations to rationalize the stability of fifteen experimentally determined gold nanoclusters in conjunction with a recently developed thermodynamic stability theory on small Au nanoclusters (≤102 Au atoms). Our results demonstrate that the thermodynamic stability theory can capture the stability of large, atomically precise nanoclusters, Au 279 (SR) 84 , Au 246 (SR) 80 , and Au 146 (SR) 57 , suggesting its applicability over larger cluster size regimes than its original development. Importantly, we develop structure–property relationships on Au nanoclusters, connecting their ionization potential and electron affinity to the number of gold atoms within the nanocluster. Altogether, a computational scheme is described that can aid experimental efforts towards a property-specific, targeted synthesis of gold nanoclusters. 

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5. Controlling magnetism of Au ₁₃₃ (TBBT) ₅₂ nanoclusters at single electron level and implication for nonmetal to metal transition

   https://doi.org/10.1039/c9sc02736j  

   Zeng, Chenjie; Weitz, Andrew; Withers, Gayathri; Higaki, Tatsuya; Zhao, Shuo; Chen, Yuxiang; Gil, Roberto R.; Hendrich, Michael; Jin, Rongchao (October 2019, Chemical Science)

   The transition from the discrete, excitonic state to the continuous, metallic state in thiolate-protected gold nanoclusters is of fundamental interest and has attracted significant efforts in recent research. Compared with optical and electronic transition behavior, the transition in magnetism from the atomic gold paramagnetism (Au 6s 1 ) to the band behavior is less studied. In this work, the magnetic properties of 1.7 nm [Au 133 (TBBT) 52 ] 0 nanoclusters (where TBBT = 4- tert -butylbenzenethiolate) with 81 nominal “valence electrons” are investigated by electron paramagnetic resonance (EPR) spectroscopy. Quantitative EPR analysis shows that each cluster possesses one unpaired electron (spin), indicating that the electrons fill into discrete orbitals instead of a continuous band, for that one electron in the band would give a much smaller magnetic moment. Therefore, [Au 133 (TBBT) 52 ] 0 possesses a nonmetallic electronic structure. Furthermore, we demonstrate that the unpaired spin can be removed by oxidizing [Au 133 (TBBT) 52 ] 0 to [Au 133 (TBBT) 52 ] + and the nanocluster transforms from paramagnetism to diamagnetism accordingly. The UV-vis absorption spectra remain the same in the process of single-electron loss or addition. Nuclear magnetic resonance (NMR) is applied to probe the charge and magnetic states of Au 133 (TBBT) 52 , and the chemical shifts of 52 surface TBBT ligands are found to be affected by the spin in the gold core. The NMR spectrum of Au 133 (TBBT) 52 shows a 13-fold splitting with 4-fold degeneracy of 52 TBBT ligands, which are correlated to the quasi- D 2 symmetry of the ligand shell. Overall, this work provides important insights into the electronic structure of Au 133 (TBBT) 52 by combining EPR, optical and NMR studies, which will pave the way for further understanding of the transition behavior in metal nanoclusters. 

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