Rakesh Kumar Gupta

Thermally activated delayed fluorescence (TADF) in coinage metal nanoclusters (CMNCs) has introduced new paradigms in the design of optoelectronic materials, uniting atomic precision with tunable quantum and photophysical properties. This review provides a critical and comprehensive analysis of TADF mechanisms in gold, silver, copper, and doped/alloy nanoclusters, emphasizing quantum confinement, strong spin–orbit coupling, and engineered singlet–triplet energy gaps that enable efficient reverse intersystem crossing and high photoluminescence quantum yields. Recent advances in TADF-active CMNCs are highlighted, including ligand architecture, core composition, supramolecular assembly, and heterometallic doping, which collectively afford unprecedented control over emission color, dynamics, and stability. Comparative perspectives versus organic and metal–organic TADF emitters are presented, underscoring the distinctive advantages and ongoing challenges of CMNCs. Current and emerging applications in OLEDs, X-ray scintillation, chiral photonics, and bioimaging are discussed, along with future research directions and critical barriers to the realization of CMNCs in next-generation optoelectronic devices.

The surface oxygen vacancy defects generated from the abstraction of lattice oxygen atoms of bulk ceria accompanied by the reduction of Ce ^IV to Ce ^III , are considered to play a pivotal role in photocatalysis. As ideal molecular models of nanoceria, atom‐precise cerium‐oxo clusters (COCs) hold great promise in determining the accurate Ce ^III /Ce ^IV ratio and disclosing the underlying catalytic mechanism. However, the number of COCs, especially those mixed‐valent COCs with high surface Ce ^III concentrations, is very limited on account of their formidable synthetic challenges. Herein, we report the first silsesquioxane‐protected COC, Ce13 , featuring a fluorite‐type Ce ₁₃ O ₈ core, which is structurally reminiscent of bulk ceria. The cluster exhibits a substantially high Ce ^III /Ce ^IV ratio with one central Ce ^IV encapsulated by twelve surface‐exposed Ce ^III atoms, serving as a molecular analogue of highly reduced ceria surfaces. This cluster shows strong light‐harvesting ability that covers the entire visible range and demonstrates high photocatalytic activity and selectivity for oxidative coupling of various amines, where superoxide radical was involved using O ₂ as the oxidant at room‐temperature under visible light. The surface Ce ^III sites are proposed to significantly enhance the adsorption and activation of O ₂ . These results highlight the potential of Ce13 as an efficient sustainable photocatalyst for organic transformations.

Nanofibrous architectures, a prominent class of one-dimensional nanomaterials, continue to captivate researchers in the exploration of novel nanofibrous materials and their diverse applications in nanotechnology. Creating crystalline nanofibers with inorganic walls capped by organic ligands is both critical and challenging, requiring precise structural control and consistent internal diameters. Herein, through anion-directed assembly, we demonstrate the controlled creation of semiconducting silver nanofibers with tunable and uniform internal diameters, exemplified by [V₃O₁₀@Ag₂₂(ⁱPrS)₁₄(CH₃COO)₂Cl]_n (Ag22-NF), [Cl₃@Ag₂₁(ⁱPrS)₁₄(PhCOO)₄]_n (Ag21-NF), and [Ag₆(ⁱPrS)₄(PhCOO)₂]_n (Ag6-NF). Importantly, Ag22-NF encapsulates V₃O₁₀^5- ions within a ∼ 6.52 Å diameter; Ag21-NF contains Cl^- and silver ions within a ∼ 5.69 Å diameter; and Ag6-NF is a hollow, template-free nanofiber with a diameter of ∼ 4.57 Å. These nanofibers were further fabricated through mechanical exfoliation of three distinct silver single crystals, preserving their well-ordered structural integrity and alignment with the bulk crystal architecture. As a concept-of-application, the field-effect transistor devices based on Ag6-NF exhibit high ratios of on to off current at ∼ 10⁴-10⁷, demonstrating the outstanding semiconducting performance. This work not only unveils a diverse array of connecting building units that generate nanofibers within three distinct assemblies but also elucidates the profound influence of template size on their optical properties, thereby exemplifying the intricacies of anion-directed design in controlled nanoscale synthesis.

Metal nanoclusters (MNCs) are an emerging class of atomically precise nanomaterials with sizes comparable to the Fermi wavelength of free electrons, exhibiting discrete energy levels, molecular-like behaviors, and tunable physicochemical properties. Among these properties, photothermal conversion—the process of transforming absorbed light into thermal energy—has garnered considerable interest due to its vital importance in applications such as solar energy harvesting, photothermal therapy, and catalysis. This review begins by summarizing recent progress in synthetic strategies for MNCs, including kinetic control, seeded growth, in situ two-phase ligand exchange, and metal exchange, which help overcome challenges such as polydispersity, low yield, restricted surface functionality, and lengthy synthesis times. Subsequently, a comprehensive analysis is provided on the photothermal conversion behaviors of various MNC systems (e.g., coinage metal nanoclusters, Ti NCs, and Mo NCs) reported in the past five years, with in-depth discussion of their structural characteristics, absorption properties, photothermal conversion efficiencies, and underlying conversion mechanisms. Finally, the review addresses current challenges and prospects for advancing MNC-based photothermal technologies via atomic-level engineering and interdisciplinary approaches. Through this in-depth and systematic review, we endeavor to provide scholars dedicated to metal nanocluster research—as well as experts engaged in photothermal conversion and its diverse applications—with valuable scientific insights. We are confident that this contribution will not only catalyze innovative breakthroughs but also unlock exciting new frontiers within this vibrant and rapidly evolving field of study.

Silsesquioxanes as versatile organic-inorganic hybrids with terminal siloxyl sites have been utilized to construct diverse metallasilsesquioxane complexes, yet their niche in silver nanoclusters remains largely unexplored. Herein, we report the first silsesquioxane-protected superatomic silver nanocluster, [Ag₄₀(Ph₄Si₄O₈)₆(^tBuC≡C)₈] (Ag40), acquired through an "adaptive multipath synthesis" that circumvents the long-standing limitations imposed by hard and soft acids and bases principles in Ag(I) coordination chemistry. Single-crystal X-ray diffraction analysis reveals a kernel-shell architecture: a face-centered-cubic Ag₁₆⁸⁺ kernel, encapsulated by a [Ag₂₄(Ph₄Si₄O₈)₆] cage and ^tBuC≡C^- ligands. Synthetic methodology studies indicate that the macrocyclic all-cis-T₄: Ph₄Si₄O₈^4- ligands can be accessed via three distinct routes: in situ generation, pre-synthesis, and hierarchical-synthesis, all of which involve the hydrolytic condensation of PhSi(OR)₃ (R = Me, Et). Of these, the hierarchical approach addresses the limitations of the other two routes (poor stereochemical control in the in situ route and delayed supersaturation-induced slow crystallization in the pre-synthesis route), improving both the reproducibility and the growth efficiency of the Ag40 crystals. The mass spectrometry analysis not only provides compelling evidence for in situ transformation from cis,cis-T₃ to all-cis-T₄ ligands but also reveals the assembly pathways for the Ag₄₀ nanocluster either in pre-synthesis or in hierarchical-synthesis, underscoring the dynamic transformation of cyclic oligomeric silsesquioxanes and its critical role in capturing the ultrasmall subvalent silver kernel. Notably, Ag40 exhibits superatomic 1D→1P transition-dominated red phosphorescent emission, persisting in nondegassed solutions. As the highest-nuclearity oligosiloxane-capped metal cluster and first superatomic silver-silsesquioxane species, Ag40 establishes a paradigm for silica-supported superatom synthesis and advances hard-base-protected coinage metal nanoclusters.

ConspectusAtomically precise metal nanoclusters (NCs) are a class of nanomaterials composed of a specific number of metal atoms stabilized by well-defined organic ligands. These NCs exhibit molecular-like electronic states and offer exceptional control over their optoelectronic properties. Recent advancements have extended their photoluminescence deep into the near-infrared II (NIR-II) window (950-1700 nm), a spectral region that provides significant advantages for biomedical imaging and photonic applications, including reduced tissue scattering, minimal autofluorescence, and enhanced penetration depth. In comparison to conventional quantum dots and larger plasmonic nanoparticles, atomically precise metal NCs offer unprecedented tunability in terms of emission wavelength, quantum yield, and photostability, facilitated by the modulation of size, composition, and ligand shell chemistry.In this Account, we highlight cutting-edge strategies, including ligand engineering, core-shell engineering, and alloying, which enable fine-tuning of NIR-II photoluminescence in metal NCs. We also explore the photophysical mechanisms underlying NIR-II emission, such as core-ligand charge transfer, metal-centered transitions, and the role of surface electronic states in radiative recombination efficiency. Advanced spectroscopic techniques, such as time-resolved photoluminescence and transient absorption, are discussed for their ability to probe excited-state lifetimes and energy transfer processes that control the emission properties. Finally, we critically address the current limitations in quantum yield enhancement, long-term photostability, and biocompatibility while outlining future directions for developing hybrid materials and multifunctional NC platforms and advancing NIR-II photonic technologies. Our Account aims to offer molecular-level insights and guide the rational design of next-generation atomically precise metal NCs as versatile materials for advanced NIR-II photoluminescence.

The excited-state properties of metal nanoclusters have attracted considerable attention for potential applications in optoelectronics and biomedicine. However, the theoretical exploration of these properties, particularly emission mechanisms, remains challenging due to the high computational cost of excited-state structure optimization. Herein, we investigate the geometric and electronic structural changes upon photoexcitation in the magic series gold nanocluster Au_8n+4(SR)_4n+8 (R = H or phenyl, n = 3-6) using time-dependent density functional theory (TDDFT) and its approximate variant, DFT plus tight binding (TD-DFT+TB). Our results demonstrate that approximate methods, which combine full DFT-ground-state descriptions with the tight-binding approximation in linear-response calculations, offer a cost-effective and reliable approach for predicting size and ligand effects on the excited-state properties of gold nanoclusters. Key parameters such as the Stokes shift, charge-transfer character, and energy gap between the first singlet (S₁) and triplet (T₁) states show strong dependence on cluster size and the nature of the ligand shell, and these trends are well captured by the approximate methods. These results emphasize their potential for the efficient design of gold nanoclusters with tailored optical functionalities.