Controlling nanoparticles with atomic precision, somewhat like the way organic chemists control small molecules by organic chemistry principles, is highly desirable for nanoparticle chemists. Recent advances in the synthesis of gold nanoparticles have opened the possibility to precisely control the number of gold atoms in a particle. In this Perspective, we will discuss a size-focusing methodology that has been developed in the synthesis of a number of atomically monodisperse ultrasmall gold nanoparticles (also called nanoclusters). We focus our discussion on thiolate-stabilized Au nanoclusters (referred to as Au n (SR) m , where n and m are the respective number of metal atoms and ligands). The underlying principle of this size-focusing process is primarily related to the peculiar stability of certain sized Au n (SR) m nanoparticle, that is, "survival of the robustest", much like the natural law "survival of the fittest". We expect that this universal size-focusing method will ultimately allow for preparing a full series of size-discrete, atomically monodisperse nanoparticles that span the size regimes of both nonplasmonic nanoclusters and plasmonic nanocrystals. These well-defined nanoparticles will be of major importance for both fundamental science research and technological applications.

Thiolate‐protected Aun clusters are exploited as homogeneous and supported catalysts for two chemical processes: i) the selective oxidation of styrene by O2 to benzaldehyde and ii) the chemoselective hydrogenation of α,β‐unsaturated ketone to α,β‐unsaturated alcohol. The results demonstrate that a quantum‐size effect in Aun clusters plays a critical role in their catalysis. This work highlights the importance of pursuing quantum‐sized gold clusters for catalysis.

A conveniently synthesized photochromic compound, BTB-1, containing an unprecedented six-membered 2,1,3-benzothiadiazole unit as the center ethene bridge, possesses good photochromic performance, with a high cyclization quantum yield and moderate fatigue resistance in solution or an organogel system. The fluorescence of BTB-1 can be modulated by solvato- and photochromism. However, the analogue BTB-2, in which the dimethylthiophene substituents are relocated to the 5,6-positions of benzothiadiazole, does not show any detectable photochromism. To the best of our knowledge, this is the first example of six-membered bridge bisthienylethenes (BTEs) in which the photochromism can be controlled by the substitution position. The photochromism difference is elucidated by the analysis of resonance structure, the Woodward-Hoffmann rule, and theoretical calculations on the ground-state potential-energy surface. In a well-ordered single-crystal state, BTB-1 adopts a relatively rare parallel conformation, and forms an interesting two-dimensional structure due to the presence of multiple directional intermolecular interactions, including C--HN and C--HS hydrogen-bonding interactions, and pi-pi stacking interactions. This work contributes to several aspects for developing novel photochromic BTE systems with fluorescence modulation and performances controlled by substitution position in different states (solution, organogel, and single crystal).

Nanogold has been found to be an effective catalyst for many chemical reactions. However, mechanistic studies have thus far only met with limited success, largely due to the unavailability of welldefined catalysts. We are motivated to create atomically precise gold (Au) nanoclusters in the hope of unraveling some fundamental aspects of nanogold catalysis. In this feature article, we summarize recent works on the catalytic promise of a new class of materials: ultrasmall (<2 nm), semiconducting Au nanoclusters protected by thiolates, referred to as Au n (SR) m , where n and m represent the number of gold atoms and thiolate ligands, respectively. The recent research is focused on the synthesis and structural determination of atomically precise Au n (SR) m nanoclusters as well as exploring their catalytic properties. The correlation of the X-ray crystal structures of the Au n (SR) m nanoclusters with their catalytic properties will ultimately permit a deep understanding of the origin of nanogold catalysis and will also benefit the future design of new catalysts with high selectivity and activity.

We report chromatographic isolation of a new thiolate-protected gold cluster species from the approximately 8 kDa Au(n)(SR)(m) clusters. This new cluster is separated by size-exclusion chromatography from Au(38)(SC(2)H(4)Ph)(24) (a previously reported main component in the approximately 8 kDa gold-thiolate species). Based on detailed MALDI and ESI mass spectrometry analyses, the new cluster possesses a core mass of approximately 8.6 kDa and its formula is determined to be Au(40)(SC(2)H(4)Ph)(24). The Au(40)(SR)(24) species is also found to exist in other thiolate systems, including -SR=SC(6)H(13) and SC(5)H(11), indicating that Au(40)(SR)(24) is a ubiquitous cluster as is Au(38)(SR)(24).