Triplet-Mediator Ligand-Protected Metal Nanocluster Sensitizers for Photon Upconversion.

Triplet-triplet annihilation photon upconversion (TTA-UC) is attracting a great deal of attention as a viable approach to exploit unutilized wavelengths of light in solar-driven devices. Recently, ligand-protected metal nanoclusters have emerged as a compelling platform for serving as triplet sensitizers for TTA-UC. In this study, we developed an atomically precise, triplet-mediator ligand (TL)-protected metal nanocluster, Au2Cu6(S-Adm)6[P(DPA)3]2 (Au2Cu6DPA; S-Adm = 1-adamanthanethiolate, DPA = 9,10-diphenylanthracene). In Au2Cu6DPA, the excitation of the Au2Cu6 core rapidly generates a metal-to-ligand charge transfer state, followed by the formation of the long-lived triplet state (approximately 150 µs) at a DPA site in the TL. By combining Au2Cu6DPA with a DPA annihilator, we achieved a red-to-blue upconversion quantum yield (ΦUCg) of 20.7 ± 0.4% (50% max.) with a low threshold excitation intensity of 36 mW cm-2 at 640 nm. This quantum yield almost reaches the maximum limit achievable using a DPA annihilator and establishes a record-setting value, outperforming previously reported nanocrystal and nanocluster sensitizers. Furthermore, strong upconversion emission based on a pseudo-first-order TTA process was observed under 1 sun illumination, indicating that the Au2Cu6DPA sensitizer holds promise for applications in solar-energy-based systems.

Clarifying the Electronic Structure of Anion-Templated Silver Nanoclusters by Optical Absorption Spectroscopy and Theoretical Calculation.

Electronic structures of anion-templated silver nanoclusters (Ag NCs) are not well understood compared to conventional, template-free Ag NCs. In this study, we synthesized three new anion-templated Ag NCs, namely [S@Ag17(S-4CBM)15(PPh3)5]0, [S@Ag18(S-4CBM)16(PPh3)8]0, and [Cl@Ag18(S-4CBM)16(PPh3)8][PPh4], where S-4CBM = 4-chlorobenzene methanethiolate, and single-crystal X-ray crystallography revealed that they have S@Ag6, S@Ag10, and Cl@Ag10 cores, respectively. Investigation of their electronic structures by optical spectroscopy and theoretical calculations elucidated the following unique features: (1) their electronic structures are different from those of template-free Ag NCs described by the superatomic concept; (2) optical absorption in the range of 550-400 nm for S2--templated Ag NCs is attributed to the charge transitions from S2--templated Ag-cage orbitals to the s-shaped orbital in the S2- moiety; (3) the Cl--templated Ag NCs can be viewed as [Cl@Ag18(S-4CBM)16(PPh3)8]0[PPh4]0 rather than the ion pair [Cl@Ag18(S-4CBM)16(PPh3)8]-[PPh4]+; and (4) singlet-coupled singly occupied orbitals are involved in the optical absorption of the Cl--templated Ag NC.

Single Platinum Atom Doping to Silver Clusters Enables Near-Infrared-to-Blue Photon Upconversion.

Photon upconversion (UC) from near-infrared (NIR) to visible has been realized using singlet-to-triplet absorption of sensitizers, which are currently limited to osmium complexes and semiconductor nanocrystals. Motivated by the atomically precise tunability of electronic structure and photophysical properties of noble metal clusters, which often possess absorption bands that extend into the NIR region, we investigated MAg24 (SR)18 (M=Ag, Pt; SR=2,4-dimethylbenzenethiolate) clusters as a new NIR-absorbing sensitizer for triplet-triplet annihilation UC. Combined with a blue light emitter, the NIR excitation (λex =785 nm) of Ag25 (SR)18 results in no UC emission, while PtAg24 (SR)18 exhibits strong UC emission. This enhancement is primarily due to a significant increase in the intersystem crossing quantum yield of the cluster associated with the spin-orbit coupling enhancement in the M@Ag12 core.

Synthesis and Properties of a Cyclohexa-2,7-anthrylene Ethynylene Derivative.

The synthesis of a cyclohexa-2,7-(4,5-diaryl)anthrylene ethynylene (1) was achieved for the first time by using 1,8-diaryl-3,6-diborylanthracene and 1,8-diaryl-3,6-diiodoanthracene as key synthetic intermediates. Macrocycle 1 possesses a planar conformation of approximately D6h symmetry, because of the triple-bond linker between the anthracene units at the 2,7-positions. It was confirmed that macrocycle 1, bearing bulky substituents at the outer peripheral positions, behaves as a monomeric form in solution without π-stacking self-association. Macrocycle 1 has an inner-cavity size that allows specific inclusion of [9]cycloparaphenylene ([9]CPP), but not [8]CPP or [10]CPP, through an aromatic edge-to-face CH-π interaction.

Alloy Clusters: Precise Synthesis and Mixing Effects.

Metal alloys exhibit functionalities unlike those of single metals. Such alloying has drawn considerable research interest, particularly for nanoscale particles (metal clusters/nanoparticles), from the viewpoint of creating new functional nanomaterials. In gas phase cluster research, generated alloy clusters can be spatially separated with atomic precision in vacuum. Thus, the influences of increases or decreases in each element on the overall electronic structure of the cluster can be elucidated. However, to further understand the related mixing and synergistic effects, alloy clusters need to be produced on a large scale and characterized by various techniques. Because alloy clusters protected by thiolate (SR) can be synthesized by chemical methods and are stable in both solution and the solid state, these clusters are ideal study materials to better understand the mixing and synergistic effects. Moreover, the alloy clusters thus created have potential applications as functional materials. Therefore, since 2008, we have been working on establishing a precise synthesis method for SR-protected alloy clusters and elucidating their mixing and synergistic effects. Early research focused on the precise synthesis of alloy clusters wherein some of the Au in the stable SR-protected gold clusters ([Au25(SR)18]- and [Au38(SR)24]0) is replaced by Pd, Ag, or Cu. These studies have shown that Pd, Ag, or Cu substitute at different metal sites. We also have examined the as-synthesized alloy clusters to clarify the effect of substitution by each element on the physical and chemical properties of the clusters. However, in early studies, the number of substitutions could not be controlled with atomic accuracy for [Au25- xM x(SR)18]- (M = Ag or Cu). Then, in following research, methods have been established to obtain alloy clusters with control over the composition. We have succeeded in developing a method for controlling the number of Ag substitutions with atomic precision and thereby elucidating the effect of Ag substitution on the electronic structure of clusters with atomic precision. Concurrently, we also studied alloy clusters containing multiple heteroelements with different preferential substitution sites. These results revealed that the effects of substitution of each element can be superimposed on the cluster by combining multiple elemental substitutions at different sites. In addition, we successfully developed methods to synthesize alloy clusters with heterometal core. These findings are expected to lead to clear design guidelines for developing new functional nanomaterials.

Understanding and Practical Use of Ligand and Metal Exchange Reactions in Thiolate-Protected Metal Clusters to Synthesize Controlled Metal Clusters.

It is now possible to accurately synthesize thiolate (SR)-protected gold clusters (Aun (SR)m ) with various chemical compositions with atomic precision. The geometric structure, electronic structure, physical properties, and functions of these clusters are well known. In contrast, the ligand or metal atom exchange reactions between these clusters and other substances have not been studied extensively until recently, even though these phenomena were observed during early studies. Understanding the mechanisms of these reactions could allow desired functional metal clusters to be produced via exchange reactions. Therefore, we have studied the exchange reactions between Aun (SR)m and analogous clusters and other substances for the past four years. The results have enabled us to gain deep understanding of ligand exchange with respect to preferential exchange sites, acceleration means, effect on electronic structure, and intercluster exchange. We have also synthesized several new metal clusters using ligand and metal exchange reactions. In this account, we summarize our research on ligand and metal exchange reactions.

High-resolution separation of thiolate-protected gold clusters by reversed-phase high-performance liquid chromatography.

Thiolate (RS)-protected gold clusters (Aun(SR)m) have attracted much attention as building blocks of functional nanomaterials. Our group has been studying the high-resolution separation of Aun(SR)m clusters using reversed-phase high-performance liquid chromatography. In this perspective, we summarize our recent results on the separation of Aun(SR)m clusters and their doped clusters according to the core size, charge state, ligand composition, and coordination isomers. Additionally, this perspective describes new findings obtained by using high-resolution separation and future prospects for the separation of such types of metal clusters. We believe that the techniques and knowledge gained in this study would contribute to the creation of Aun(SR)m clusters with the desired functions and associated functional nanomaterials.

A critical size for emergence of nonbulk electronic and geometric structures in dodecanethiolate-protected Au clusters.

We report on how the transition from the bulk structure to the cluster-specific structure occurs in n-dodecanethiolate-protected gold clusters, Au(n)(SC12)m. To elucidate this transition, we isolated a series of Au(n)(SC12)m in the n range from 38 to ∼520, containing five newly identified or newly isolated clusters, Au104(SC12)45, Au(∼226)(SC12)(∼76), Au(∼253)(SC12)(∼90), Au(∼356)(SC12)(∼112), and Au(∼520)(SC12)(∼130), using reverse-phase high-performance liquid chromatography. Low-temperature optical absorption spectroscopy, powder X-ray diffractometry, and density functional theory (DFT) calculations revealed that the Au cores of Au144(SC12)60 and smaller clusters have molecular-like electronic structures and non-fcc geometric structures, whereas the structures of the Au cores of larger clusters resemble those of the bulk gold. A new structure model is proposed for Au104(SC12)45 based on combined approach between experiments and DFT calculations.

Triplet-triplet annihilation-based photon upconversion using nanoparticles and nanoclusters.

The phenomenon of photon upconversion (UC), generating high-energy photons from low-energy photons, has attracted significant attention. In particular, triplet-triplet annihilation-based UC (TTA-UC) has been achieved by combining the excitation states of two types of molecules, called the sensitizer and emitter (or annihilator). With TTA-UC, it is possible to convert weak, incoherent near-infrared (NIR) light, which constitutes half of the solar radiation intensity, into ultraviolet and visible light that are suitable for the operation of light-responsive functional materials or devices such as solar cells and photocatalysts. Research on TTA-UC is being conducted worldwide, often employing materials with high intersystem crossing rates, such as metal porphyrins, as sensitizers. This review summarizes recent research and trends in triplet energy transfer and TTA-UC for semiconductor nanoparticles or nanocrystals with diameters in the nanometer range, also known as quantum dots, and for ligand-protected metal nanoclusters, which have even smaller well-defined sub-nanostructures. Concerning nanoparticles, transmitter ligands have been applied on the surface of the nanoparticles to efficiently transfer triplet excitons formed inside the nanoparticles to emitters. Applications are expanding to solid-state UC devices that convert NIR light to visible light. Additionally, there is active research in the development of sensitizers using more cost-effective and environmentally friendly elements. Regarding metal nanoclusters, methods have been established for the evaluation of excited states, deepening the understanding of luminescent properties and excited relaxation processes.

Tiara-like Hexanuclear Nickel-Platinum Alloy Nanocluster.

Tiara-like metal nanoclusters (TNCs) have attracted a great deal of attention because of their high stability and easy synthesis under atmospheric conditions as well as their high activity in various catalytic reactions. Alloying is one of the methods that can be used to control the physicochemical properties of nanoclusters, but few studies have reported on alloy TNCs. In this study, we synthesized alloy TNCs [NixPt6-x(PET)12, where x = 1-5 and PET = 2-phenylethanethiolate] consisting of thiolate, nickel (Ni), and platinum (Pt). We further evaluated the stability, geometric structure, and electronic structure by high-performance liquid chromatography and density functional theory calculations. The results revealed that NixPt6-x(PET)12 has a distorted structure and is therefore less stable than single-metal TNCs.

Controlled aggregation of Pt/PtH/Rh/RhH doped silver superatomic nanoclusters into 16-electron supermolecules.

The assembly of discrete superatomic nanoclusters into larger constructs is a significant stride towards developing a new set of artificial/pseudo-elements. Herein, we describe a novel series of 16-electron supermolecules derived from the combination of discrete 8-electron superatomic synthons containing interstitial hydrides as vertex-sharing building blocks. The symmetric (RhH)2Ag33[S2P(OPr)2]17 (1) and asymmetric PtHPtAg32[S2P(OPr)2]17 (2) are characterized by ESI-MS, SCXRD, NMR, UV-vis absorption spectra, electrochemical and computational methods. Cluster 1 represents the first group 9-doped 16-electron supermolecule, composed of two icosahedral (RhH)@Ag12 8-electron superatoms sharing a silver vertex. Cluster 2 results from the assembly of two distinct icosahedral units, Pt@Ag12, and (PtH)@Ag12. In both cases, the presence of the interstitial hydrides is unprecedented. The stability of the supermolecules is investigated, and 2 spontaneously transforms into Pt2Ag33[S2P(OPr)2]17 (3) with thermal treatment. The lability of the hydride within the icosahedral framework in solution at low-temperature was confirmed by the VT-NMR.

Separation of precise compositions of noble metal clusters protected with mixed ligands.

This report describes the precise and systematic synthesis of PdAu24 clusters protected with two types of thiolate ligands (-SR1 and -SR2). It involved high-resolution separation of metal clusters containing a distribution of chemical compositions, PdAu24(SR1)18-n(SR2)n (n = 0, 1, 2, ..., 18), to individual clusters of specific n using high-performance liquid chromatography. Similar high-resolution separation was achieved for a few ligand combinations as well as clusters with other metal cores, such as Au25 and Au38. These results demonstrate the ability to precisely control the chemical composition of two types of ligands in thiolate-protected mono- and bimetallic metal clusters. It is expected that greater functional control of thiolate-protected metal clusters, their regular arrays, and systematic variation of their properties can now be achieved.

Toward the creation of stable, functionalized metal clusters.

Nanomaterials which exhibit both stability and functionality are currently considered to hold the most promise as components of nanotechnology devices. Thiolate (RS)-protected gold nanoclusters (Aun(SR)m) have attracted significant attention in this regard and, among these, the magic clusters are believed to be the best candidates since they are the most stable. We have investigated the effects of heteroatom doping, protection by selenolate ligands and protection by photoresponsive thiolates on the stability and physical/chemical properties of these clusters. Through such studies, we have attempted to establish methods of modifying magic Aun(SR)m clusters as a means of creating metal clusters that are both robust and functional. This paper summarizes our studies towards this goal and the obtained results.

Synthesis and luminescence properties of two silver cluster-assembled materials for selective Fe3+ sensing.

Silver cluster-assembled materials (SCAMs) are emerging light-emitting materials with molecular-level structural designability and unique photophysical properties. Nevertheless, the widespread application scope of these materials is severely curtailed by their dissimilar structural architecture upon immersing in different solvent media. In this work, we report the designed synthesis of two unprecedented (4.6)-connected three-dimensional (3D) luminescent SCAMs, [Ag12(StBu)6(CF3COO)6(TPEPE)6]n (denoted as TUS 1), TPEPE = 1,1,2,2-tetrakis(4-(pyridin-4-ylethynyl)phenyl)ethene and [Ag12(StBu)6(CF3COO)6(TPVPE)6]n (denoted as TUS 2), TPVPE = 1,1,2,2-tetrakis(4-((E)-2-(pyridin-4-yl)vinyl)phenyl)ethene, composed of an Ag12 cluster core connected by quadridentate pyridine linkers. Attributed to their exceptional fluorescence properties with absolute quantum yield (QY) up to 9.7% and excellent chemical stability in a wide range of solvent polarity, a highly sensitive assay for detecting Fe3+ in aqueous medium is developed with promising detection limits of 0.05 and 0.86 nM L-1 for TUS 1 and TUS 2 respectively, comparable to the standard. Furthermore, the competency of these materials to detect Fe3+ in real water samples reveals their potential application in environmental monitoring and assessment.

A new two-dimensional luminescent Ag12 cluster-assembled material and its catalytic activity for reduction of hexacyanoferrate(III).

Silver cluster-assembled materials (SCAMs) have garnered significant interest as promising platforms for different functional explorations. Their atomically precise structures, intriguing chemical/physical properties, and remarkable luminescent capabilities make them highly appealing. However, the properties of these materials are primarily determined by their structural architecture, which is heavily influenced by the linker molecules used in their assembly. The choice of linker molecules plays a pivotal role in shaping the structural characteristics and ultimately determining the unique properties of SCAMs. To this end, the first SCAM with an intriguing (3,6)-connected kgd topology, [Ag12(StBu)6(CF3COO)6(TPBTC)6]n (termed TUS 3), TPBTC = benzene-1,3,5-tricarboxylic acid tris-pyridin-4-ylamide, has been synthesized by reticulating C6-symmetric Ag12 cluster cores with C3-symmetric tripodal pyridine linkers. Due to the structutural architecture of the linker molecule, TUS 3 posseses a luminescent porous framework structure where each two-dimensional (2D) layers are non-covalently linked with each other to form a three dimensional (3D) framework and ultimately offers uniaxial open channels. The compact mesoporous structural architecture not only gives the excellent surface area but also offers impressive stability of this material even in water medium. Taking advantage of these properties, TUS 3 shows brilliant catalytic activity in the reduction of hexacyanoferrate(III) using sodium borohydride in aqueous solutions.

Nested Keplerian architecture of [Cu58H20(SPr)36(PPh3)8]2+ nanoclusters.

This article explores the challenges in synthesizing highly symmetric Cu(I)-thiolate nanoclusters and reports a nested Keplerian architecture of [Cu58H20(SPr)36(PPh3)8]2+ (Pr = CH2CH2CH3). The structure is made up of five concentric polyhedra of Cu(I) atoms, which create enough space to accommodate five ligand shells all within a range of 2 nm. This fascinating structural architecture is also linked to the unique photoluminescence properties of the nanoclusters.

Key factors for connecting silver-based icosahedral superatoms by vertex sharing.

Metal nanoclusters composed of noble elements such as gold (Au) or silver (Ag) are regarded as superatoms. In recent years, the understanding of the materials composed of superatoms, which are often called superatomic molecules, has gradually progressed for Au-based materials. However, there is still little information on Ag-based superatomic molecules. In the present study, we synthesise two di-superatomic molecules with Ag as the main constituent element and reveal the three essential conditions for the formation and isolation of a superatomic molecule comprising two Ag13-xMx structures (M = Ag or other metal; x = number of M) connected by vertex sharing. The effects of the central atom and the type of bridging halogen on the electronic structure of the resulting superatomic molecule are also clarified in detail. These findings are expected to provide clear design guidelines for the creation of superatomic molecules with various properties and functions.

Pt17 nanocluster electrocatalysts: preparation and origin of high oxygen reduction reaction activity.

We recently found that [Pt17(CO)12(PPh3)8]z (Pt = platinum; CO = carbon monoxide; PPh3 = triphenylphosphine; z = 1+ or 2+) is a Pt nanocluster (Pt NC) that can be synthesized with atomic precision in air. The present study demonstrates that it is possible to prepare a Pt17-supported carbon black (CB) catalyst (Pt17/CB) with 2.1 times higher oxygen reduction reaction (ORR) activity than commercial Pt nanoparticles/CB by the adsorption of [Pt17(CO)12(PPh3)8]z onto CB and subsequent calcination of the catalyst. Density functional theory calculation strongly suggests that the high ORR activity of Pt17/CB originates from the surface Pt atoms that have an electronic structure appropriate for the progress of ORR. These results are expected to provide design guidelines for the fabrication of highly active ORR catalysts using Pt NCs with a diameter of about 1 nm and thereby enabling the use of reduced amounts of Pt in polymer electrolyte fuel cells.

Improved activity for the oxygen evolution reaction using a tiara-like thiolate-protected nickel nanocluster.

Practical electrochemical water splitting and carbon-dioxide reduction are desirable for a sustainable energy society. In particular, facilitating the oxygen evolution reaction (OER, the reaction at the anode) will increase the efficiency of these reactions. Nickel (Ni) compounds are excellent OER catalysts under basic conditions, and atomically precise Ni clusters have been actively studied to understand their complex reaction mechanisms. In this study, we evaluated the geometric/electronic structure of tiara-like metal nanoclusters [Nin(PET)2n; n = 4, 5, 6, where PET refers to phenylethanethiolate] with the same SR ligand. The geometric structure of Ni5(SR)10 was determined for the first time using single-crystal X-ray diffraction. Additionally, combined electrochemical measurements and X-ray absorption fine structure measurements revealed that Ni5(SR)10 easily forms an OER intermediate and therefore exhibits a high specific activity.

Evidence for triplet-state-dominated luminescence in biicosahedral superatomic molecular Au25 clusters.

In photoluminescence (PL) quenching and triplet fusion upconversion experiments with fluorescent organic-molecule quenchers, it was revealed that a rod-shaped, phosphine- and thiolate-protected biicosahedral Au25 cluster (a representative di-superatomic molecule) exhibits only phosphorescence, not fluorescence, at room temperature with an intersystem crossing quantum yield of almost 100%. By virtue of these photophysical properties, this cluster can be used as a triplet sensitizer that undergoes direct singlet-triplet transitions in the near-infrared (NIR) region (730-900 nm), inducing photon upconversion from NIR to visible light.