Chiral Ligand-Concentration Mediating Asymmetric Transformations of Silver Nanoclusters: NIR-II Circularly Polarized Phosphorescence Lighting

Near infrared (NIR) emitter with circularly polarized phosphorescence (CPP), known as NIR CPP, has emerged as a key part in the research of cutting-edge luminescent materials. However, it remains a challenge to obtain nanoclusters with NIR CPP activity. Here, we propose an asymmetric transformation approach to efficiently synthesize two pairs of chiral silver nanoclusters (R/S-Ag29 and R/S-Ag16) using an achiral Ag10 nanocluster as starting material in the presence of different concentration chiral inducer (R/S)-1,1'-binaphthyl-2,2'-diyl hydrogenphosphate (R/S-BNP). R/S-Ag29, formed in the low-concentration R/S-BNP, exhibits a unique kernel-shell structure consisting of a distorted Ag13 icosahedron and an integrated cage-like organometallic shell with a C3 symmetry, and possesses a superatomic 6-electron configuration (1S2|1P4). By contrast, R/S-Ag16, formed in the high-concentration R/S-BNP, features a sandwich-like pentagram with AgI-pure kernel. Profiting from the hierarchically chiral structures and superatomic kernel-dominated phosphorescence, R/S-Ag29 exhibits infrequent CPP activity in the second near-infrared (975 nm) region, being the first instance of NIR-II CPP observed among CPL-active metal nanoclusters. This study presents a new approach to reduce the difficulty of de novo synthesis for chiral silver nanomaterials, and facilitates the design of CPP-active superatomic nanoclusters in NIR region.

Structure and optical limiting effects of heterometallic Ag6@Ti12 and Ag8@Ti12 oxo clusters regulated by alkynyl ligands

Heterometallic Ag6@Ti12 and Ag8@Ti12 oxo clusters were prepared through a strategy of protecting polynuclear silver cores by a hollow Ti-O module. The introduction of alkyne ligands has shown significant influence on their structures and optical limiting effects.

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

Phosphorescence, characterized by luminescent lifetimes significantly longer than that of biological autofluorescence under ambient environment, is of great value for biomedical applications. Academic evidence of fluorescence imaging indicates that virtually all imaging metrics (sensitivity, resolution, and penetration depths) are improved when progressing into longer wavelength regions, especially the recently reported second near-infrared (NIR-II, 1000-1700 nm) window. Although the emission wavelength of probes does matter, it is not clear whether the guideline of "the longer the wavelength, the better the imaging effect" is still suitable for developing phosphorescent probes. For tissue-specific bioimaging, long-lived probes, even if they emit visible phosphorescence, enable accurate visualization of large deep tissues. For studies dealing with bioimaging of tiny biological architectures or dynamic physiopathological activities, the prerequisite is rigorous planning of long-wavelength phosphorescence, being aware of the cooperative contribution of long wavelengths and long lifetimes for improving the spatiotemporal resolution, penetration depth, and sensitivity of bioimaging. In this Review, emerging molecular engineering methods of room-temperature phosphorescence are discussed through the lens of photophysical mechanisms. We highlight the roles of phosphorescence with emission from visible to NIR-II windows toward bioapplications. To appreciate such advances, challenges and prospects in rapidly growing studies of room-temperature phosphorescence are described.

Dual-Emissive Monoruthenium Complexes of N(CH3)-Bridged Ligand: Synthesis, Characterization, and Substituent Effect

Three monoruthenium complexes 1(PF6)2-3(PF6)2 bearing an N(CH3)-bridged ligand have been synthesized and characterized. These complexes have a general formula of [Ru(bpy)2(L)](PF6)2, where L is a 2,5-di(N-methyl-N'-(pyrid-2-yl)amino)pyrazine (dapz) derivative with various substituents, and bpy is 2,2'-bipyridine. The photophysical and electrochemical properties of these compounds have been examined. The solid-state structure of complex 3(PF6)2 is studied by single-crystal X-ray analysis. These complexes show two well-separated emission bands centered at 451 and 646 nm (Δλmax = 195 nm) for 1(PF6)2, 465 and 627 nm (Δλmax = 162 nm) for 2(PF6)2, and 455 and 608 nm (Δλmax = 153 nm) for 3(PF6)2 in dilute acetonitrile solution, respectively. The emission maxima of the higher-energy emission bands of these complexes are similar, while the lower-energy emission bands are dependent on the electronic nature of substituents. These complexes display two consecutive redox couples owing to the stepwise oxidation of the N(CH3)-bridged ligand and ruthenium component. Moreover, these experimental observations are analyzed by computational investigation.

Eight-Electron Superatomic Cu31 Nanocluster with Chiral Kernel and NIR-II Emission

Owing to the inherent instability caused by the low Cu(I)/Cu(0) half-cell reduction potential, Cu(0)-containing copper nanoclusters are quite uncommon in comparison to their Ag and Au congeners. Here, a novel eight-electron superatomic copper nanocluster [Cu₃₁(4-MeO-PhC≡C)₂₁(dppe)₃](ClO₄)₂ (Cu₃₁, dppe = 1,2-bis(diphenylphosphino)ethane) is presented with total structural characterization. The structural determination reveals that Cu₃₁ features an inherent chiral metal core arising from the helical arrangement of two sets of three Cu₂ units encircling the icosahedral Cu₁₃ core, which is further shielded by 4-MeO-PhC≡C^- and dppe ligands. Cu₃₁ is the first copper nanocluster carrying eight free electrons, which is further corroborated by electrospray ionization mass spectrometry, X-ray photoelectron spectroscopy and density functional theory calculations. Interestingly, Cu₃₁ demonstrates the first near-infrared (750-950 nm, NIR-I) window absorption and the second near-infrared (1000-1700 nm, NIR-II) window emission, which is exceptional in the copper nanocluster family and endows it with great potential in biological applications. Of note, the 4-methoxy groups providing close contacts with neighboring clusters are crucial for the cluster formation and crystallization, while 2-methoxyphenylacetylene leads only to copper hydride clusters, Cu₆H or Cu₃₂H₁₄. This research not only showcases a new member of copper superatoms but also exemplifies that copper nanoclusters, which are nonluminous in the visible range may emit luminescence in the deep NIR region.

The first polyoxoniobate-templated silver cluster with temperature-dependent luminescent emission

The compound [(Nb₆O₁₉)@Ag₃₄(^tBuCC)₂₄(CH₃COO)₂] (Ag₃₄) was synthesized using the solvothermal method combined with volatilization. This was the first case, to the best of our knowledge, of isolating a silver cluster containing a polyoxoniobate (PONb) template. The luminescence, solution behavior and solid-state stability of Ag₃₄ were studied in detail. Electrospray ionization mass spectrometry indicated that Ag₃₄ can maintain the integrity of its skeleton in solution. Detection of temperature could be a potential application of its unique luminescent behavior. We expect this work to inspire further fabrications of PONb-templated high-nuclearity silver clusters.

Synthesis of Two Neutral Silver Alkynyl Nanoclusters by a Single Divalent Tetrahedral Anion Template and a Study of Their Optical Features

The synthesis of nanoclusters from simple structural units is usually a challenging process because of the complexity and unpredictability of the self-assembly process of these types of compounds. Herein, two new neutral 19-nuclearity silver nanoclusters based on alkynyl ligands with the formulas [(CrO₄)@Ag₁₉(C≡C^tBu)₈(Ph₂PO₂)₆(tfa)₃(CH₃OH)₂] (1) and [(SO₄)@Ag₁₉(C≡C^tBu)₈(Ph₂PO₂)₆(tfa)₃(CH₃OH)₂] (2), in which tfa = trifluoroacetate, were synthesized, and their structures were investigated by single-crystal and powder X-ray diffraction, electrospray ionization mass spectrometry, elemental analyses, and Fourier transform infrared spectroscopy. The surface ligands of Ph₂PO₂H and trifluoroacetate were assembled through hydrogen bonding, metal-aromatic interactions, and coordination bonding around 19 silver atoms as the metal skeletons of the nanoclusters. Sulfate and chromate anions, as a template within the metal skeleton of clusters through bonding with silver atoms, stabilized the structure. In addition, the UV-vis absorption spectroscopy, luminescence properties, and thermal stability of the nanoclusters were investigated.

Trinuclear cationic silver nanoclusters based-on bis-(phosphine) ligands and stabilized by CF3SO3− anions

Three trinuclear cationic silver nanoclusters based-on bis-(phosphine) ligands and stabilized by CF₃SO₃⁻ anions, displayed excellent photocurrent responses and electrochemical properties.

The interesting luminescence behavior and rare nonlinear optical properties of the {Ag55Mo6} nanocluster

The reversible thermochromic luminescence and rare third-order NLO properties of the {Ag55Mo6} nanocluster reported were studied experimentally, and the contributions of Ag+, CC− and MoO42− groups to NLO properties were proved by DFT calculations.

Highly Robust Tetranuclear Cobalt-Based 3D Framework for Efficient C2H2/CO2 and C2H2/C2H4Separations

A novel noninterpenetrated tetranuclear cobalt(II)-based metal-organic framework, (NH₄)₂·[Co₄(μ₃-OH)₂(ina)₂(pip)₃]·4EtOH·H₂O (simplified as NbU-10·S), constructed by mix linkers was synthesized by a hydrothermal method. Interestingly, the presence of a hydrophobic benzene ring in the organic linker makes NbU-10·S exhibit high stability in high temperature and even in aqueous solution over a wide pH range of about 4-13. Magnetic studies showed that the tetranuclear cobalt(II) units in NbU-10·S show dominant antiferromangetic properties. However, in the absence of Lewis basic functional sites and open metal sites in the material, NbU-10 still displays high C₂H₂/CO₂ and C₂H₂/C₂H₄ selectivity in ideal adsorbed solution theory calculations and dynamic breakthrough experiments. Moreover, density functional theory calculations were performed to identify the adsorption characteristics of different gas molecules.

All-inorganic open frameworks based on gigantic four-shell Ln@W8@Ln8@(SiW12)6 clusters

A series of POM-based all-inorganic open frameworks, H₉[Ln₉W₈(μ₄-O)₁₂(μ₂-O)₂₄(H₂O)₂₄](SiW₁₂O₄₀)₃·60H₂O (1-Ln, Ln = La, Pr and Nd), have been fabricated and structurally characterized for the first time. The unique architectures are constructed from nanoscale four-shell Ln@W₈@Ln₈@(SiW₁₂)₆ clusters. Additionally, the porous nature of the frameworks has also been investigated.

A one-dimensional infinite silver alkynyl assembly [Ag8(C[triple bond, length as m-dash]C t Bu)5(CF3COO)3(CH3CN)] n : synthesis, crystal structure and properties

A high-yield silver alkynyl assembly [Ag₈(CC^tBu)₅(CF₃COO)₃(CH₃CN)]_n (1) constructed from [AgCC^tBu]_n ligand, CF₃COOAg and CH₃CN auxiliary ligands with a one-dimensional infinite chain structure has been obtained in one pot. Compound 1 has been well-defined and characterized. The photocurrent properties and the temperature-sensitive luminescent properties of 1 have been investigated.

A high-yield one-dimensional infinite silver alkynyl assembly [Ag₈(CC^tBu)₅(CF₃COO)₃(CH₃CN)]_n (1) displays excellent photocurrent properties and temperature-sensitive luminescence properties.

Core-dependent properties of copper nanoclusters: valence-pure nanoclusters as NIR TADF emitters and mixed-valence ones as semiconductors

While valence-pure copper alkynyl nanoclusters show near-infrared TADF, the mixed-valence ones exhibit semiconductivity.

We report herein that copper alkynyl nanoclusters show metal-core dependent properties via a charge-transfer mechanism, which enables new understanding of their structure–property relationship. Initially, nanoclusters 1 and 2 bearing respective Cu(i)₁₅ (C1) and Cu(i)₂₈ (C2) cores were prepared and revealed to display near-infrared (NIR) photoluminescence mainly from the mixed alkynyl → Cu(i) ligand-to-metal charge transfer (LMCT) and cluster-centered transition, and they further exhibit thermally activated delayed fluorescence (TADF). Subsequently, a vanadate-induced oxidative approach to in situ generate a nucleating Cu(ii) cation led to assembly of 3 and 4 featuring respective [Cu(ii)O₆]@Cu(i)₄₇ (C3) and {[Cu(ii)O₄]·[VO₄]₂}@Cu(i)₄₆ (C4) cores. While interstitial occupancy of Cu(ii) triggers inter-valence charge-transfer (IVCT) from Cu(i) to Cu(ii) to quench the photoluminescence of 3 and 4, such a process facilitates charge mobility to render them semiconductive. Overall, metal-core modification results in an interplay between charge-transfer processes to switch TADF to semiconductivity, which underpins an unusual structure–property correlation for designed synthesis of metal nanoclusters with unique properties and functions.

Electrically transmissive alkyne-anchored monolayers on gold

Well-ordered, tightly-packed (surface coverage 0.97 × 10-9 mol cm-2) monolayer films of 1,4-bis((4-ethynylphenyl)ethynyl)benzene (1) on gold are prepared via a simple self-assembly process, taking advantage of the ready formation of alkynyl C-Au σ-bonds. Electrochemical measurements using [Ru(NH3)6]3+, [Fe(CN)6]3-, and ferrocenylmethanol [Fe(η5-C5H4CH2OH)(η5-C5H5)] redox probes indicate that the alkynyl C-Au contacted monolayer of 1 presents a relatively low barrier for electron transfer. This contrasts with monolayer films on gold of other oligo(phenylene ethynylene) derivatives of comparable length and surface coverage, but with different contacting groups. Additionally, a low voltage transition (Vtrans = 0.51 V) from direct tunneling (rectangular barrier) to field emission (triangular barrier) is observed. This low transition voltage points to a low tunneling barrier, which is consistent with the facile electron transport observed through the C-Au contacted self-assembled monolayer of 1.

Tailoring the photoluminescence of atomically precise nanoclusters

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.