ConspectusNegative thermal expansion (NTE) is a counterintuitive property in metamaterials that can be observed upon excitation of certain low-frequency vibrational modes. Conventional materials expand upon heating, showing positive thermal expansion (PTE), whereas NTE materials contract, which is unusual. NTE challenges conventional lattice-dynamical concepts and is of significant importance for a wide range of applications such as in composite material for dental filling, glass-ceramic cooktops, etc. Over the past three decades, major efforts have focused on discovering and tuning NTE compounds. NTE has been reported in a wide range of materials in bulk and low-dimensional systems. To date, the vast majority of experimentally and theoretically identified NTE materials belong to bulk crystalline systems, whereas discovery of two-dimensional (2D) NTE systems is limited. Nevertheless, this phenomenon is remarkably amplified due to reduced dimensionality, enhanced anharmonicity, and unconventional phonon dynamics. Quantum confinement alters electronic, optical, mechanical, and thermal properties significantly in these systems. They sustain large NTE over exceptionally wide temperature ranges, often attributed to rigid-unit modes (RUMs) in framework structures. NTE in 2D materials can arise from several mechanisms including flexural phonons, structural transition, anisotropic bonding, conformational changes, geometric flexibility, electronegativity differences, spin-crossover, etc. We demonstrate a comprehensive and mechanism-oriented overview of NTE in 2D materials, encompassing elemental monolayers such as in graphene and graphyne analogues, h-boron nitride, transition-metal dichalcogenides, metal phosphides, arsenides and other emerging 2D materials. Beyond intrinsic mechanisms, we discuss tunability strategies unique to atomically thin systems, including pore size modulation, heteroatom substitution, defect modification, and magnetic or electronic-state control. Recent studies link NTE in 2D materials to phonon transport and topology-driven lattice responses and reveal trade-offs between thermal expansion and lattice thermal conductivity (TC). Most low-TC 2D systems exhibit pronounced NTE and vice versa. Spontaneous symmetry breaking in 2D-materials is associated with pseudo Jahn-Teller (PJT) distortions. The machine-learning (ML)-based predictions highlight the strong structural dependence of NTE in 2D materials. Despite growing interest in NTE materials across diverse applications, rational structural design and controlled tuning of lattice expansion remain challenging. This Account assesses current limitations and outlines future directions based on high-throughput calculations, ML, and topology-guided design for realizing enhanced NTE in 2D systems.

The Diels-Alder reaction, one of the most renowned pericyclic transformations, is known for its stereospecificity proceeding through a well-defined transition state. Traditionally, it has been regarded as a concerted process leading to a single product. However, recent experimental and computational reports have shown that a single reactant can yield multiple products from the same transition state (TS). To uncover the origin of this behavior, we performed quasi-classical direct dynamics simulations coupled with density functional theory on intramolecular, intermolecular, and palladium (Pd)-catalyzed cycloaddition reactions. All the three reactions are known experimentally to produce both Diels-Alder (DA) and hetero-Diels-Alder (HDA) products. Trajectory analyses reveal evidence of post-transition state bifurcations and dynamical control of selectivity between the DA and HDA products. For the Pd-catalyzed variant, a zwitterionic entropic intermediate was identified, wherein trajectories are transiently stabilized for more than 1000 fs and 500-900 fs before collapsing to the HDA and DA products, 3c and 3d, respectively. This intermediate diverges into three distinct pathways, resulting in a pronounced trifurcation on the potential energy surface. Notably, one of them leads to the formation of a spiro-HDA product, 3e, that has not been reported previously. These findings demonstrate that reaction outcomes are not solely dictated by the statistical transition state theory but are strongly influenced by post-transition state dynamics and entropic effects, providing a deeper mechanistic understanding of how catalysts and molecular motion govern selectivity in complex cycloaddition reactions.

Long-chain metal n-alkanethiolates (MTs) are inorganic-organic hybrid materials, characterized by a metal-sulfur inorganic central plane sandwiched between hydrocarbon bilayers. Though MTs are known for over a century now, their crystal structures are conclusively established only recently. Mixed-metal thiolates (MMTs) are structurally similar to MTs and contain heterometals in the inorganic plane. With a very limited number of MMTs explored to date, their structures remain completely unknown. Here we report the synthesis and structural elucidation of a mixed-metal thiolate, AgBi(SC12H25)4, providing direct insight into heterometal arrangement by combining X-ray absorption fine structure spectroscopy and density functional theory calculations. The results establish that the AgBi MMT contains discrete molecular units like [RS-Ag-SR-Bi(SR)2] containing both Ag and Bi metals with an intramolecular heterometallic μ2-thiolate bridging. The units further share strong intermolecular Ag···Bi metallophilic interactions at a very short distance of 2.72 Å, playing a crucial role in the self-assembly. Intermolecular metal-to-metal charge transfer occurring both directly and via the thiolate bridge gives rise to the absorption maxima observed in the UV-Vis spectrum. The intermolecular interactions predominantly arise from the thiolate group (RS-) of one unit interacting with the Ag+ and Bi3+ centers of the adjacent units, in addition to the Ag···Bi metallophilic interaction.

Controlling supramolecular assembly pathways is a powerful strategy for encoding complexity and function in soft materials. Here, we introduce a pentafluorinated azobenzene-peptide conjugate that couples visible-light photoisomerization to large-scale topological switching in an aqueous milieu. The E-isomer undergoes cooperative nucleation-elongation into beta-sheet nanofibers, whereas conversion to the Z-isomer introduces enhanced through-space C delta & centerdot;& centerdot;& centerdot;F delta- interactions to stabilize discrete nanotoroidal assemblies with persistent structural memory. Spectroscopy and kinetic analyses confirm a primary nucleation-elongation mechanism, while seeded polymerization bypasses nucleation barriers to achieve seed-guided supramolecular growth from dormant precursors.19F NMR and DFT reveal C delta & centerdot;& centerdot;& centerdot;F delta-interactions along with other dominant noncovalent interactions, stabilize the Z-form, and molecular dynamics (MD) simulations trace how local dipolar perturbations propagate into curvature-driven reorganization. Microscopy directly visualizes the irreversible fiber-to-toroid transition, establishing light-induced kinetic trapping as a route to nonequilibrium states. Kelvin probe force microscopy further links molecular geometry to nanoscale charge distribution. Taken together, these results demonstrate how molecular conformation, assembly history, and external inputs in tandem dictate supramolecular polymorphism in peptide systems, providing a blueprint for adaptive photoresponsive materials with potential in designing peptide-based hydrogel scaffolds and electroactive biointerfaces.

Two-dimensional transition metal dichalcogenides (TMDs) have emerged as a promising material for thermoelectric applications due to their tunable electron and phonon transport properties. In this work, we investigate the thermoelectric performance of a twisted tungsten diselenide (WSe2) bilayer and compare it with the untwisted configuration using first-principles calculations combined with Boltzmann transport theory. We find that twisting the WSe2 bilayer by 12.53° can significantly reduce the lattice thermal conductivity by a quarter from 26.59 W m-1 K-1 at T = 300 K for the untwisted bilayer. This is primarily due to enhanced anharmonicity and phonon scattering arising from Moiré-induced structural modifications. Although the thermoelectric power factor reduces due to symmetry breaking which enhances electrons scattering rate, a signification reduction in thermal conductivity (∼77%) leads to an improved thermoelectric figure of merit of 0.46 at 300 K and 1.40 at 700 K for the twisted WSe2 bilayer. Our findings highlight the role of twist engineering as an effective strategy to optimize electron and phonon transport in layered TMDs, for the design of high-performance thermoelectric materials.

Negative thermal expansion (NTE) is a counterintuitive phenomenon, in which materials undergo contraction as they are heated. ScF3, a well-known NTE material, has been reported to show NTE coefficients up to 1000 K. Under ambient conditions, ScF3 crystallizes in a cubic symmetry (Pm3̄m space group), the same as that in the ReO3-type structures. Crystal structure predictions (CSPs) show that at P = 1 GPa, a phase transition occurs in cubic ScF3 to form the rhombohedral phase (R3̄C space group). Quasi-harmonic approximation (QHA) calculations under high pressure conditions show that this new phase can show anisotropic NTE coefficients. The stability of this phase persists until 4 GPa. Beyond 4 GPa, the rhombohedral phase further undergoes a phase transition into an orthorhombic phase (Immm space group) with a non-corner-shared polyhedron network. This phase exhibits NTE along only one crystallographic axis, while the other two axes show no NTE response. On further increasing the pressure to 6 GPa, a trigonal-prismatic arrangement of ScF3 is obtained (R32 space group), which shows a reasonably better NTE than the previous phase due to the corner shared framework and remains stable until 9 GPa. All the phases show mechanical stability. Ab initio molecular dynamics (AIMD) simulations show that both the cubic and the rhombohedral phases show bond-length elongation as well as deviation in dihedral angle confirming their NTE.

The discovery of new negative thermal expansion (NTE) metamaterials can be substantially difficult in experiments and computationally expensive in theoretical investigations. In this study, a multistep machine learning (ML) approach was employed to predict NTE maxima (αmax) and thermal expansion coefficient (TEC) in 2D materials. Our predicted target attributes show high correlation with first-principles calculations within quasi-harmonic approximation (QHA). The key idea is to take structural and experimentally tunable features as input variables, which can predict NTE efficiently. Out of the 234 investigated 2D materials, 194 of them can be labeled as NTE materials, as they show NTE within the considered temperature range, T = 0-1000 K. Blind tests were performed using a set of popular materials selected from the 2DMatPedia database to validate the robustness of the model. This work presents a systematic approach toward the rapid screening and prediction of thermal expansion. The identified important features provide guidance for the future design of new 2D NTE materials.

Abstract Precise modulation of charge transfer (CT) characteristics in the excited state of aggregation-induced emission (AIE)-active organic emitters is essential for optimizing their performance mainly in optoelectronic applications such as organic light emitting diodes (OLEDs). In this work, two tetraphenylbuta-1,3-diene (TPB)-based donor–π–acceptor (D–π–A) derivatives, TPB–CHO–CBZ and TPB–CHO–PXZ, are designed by modifying the previously reported TPB–CHO–TPA, with the aim of systematically tuning the CT characteristics of the TPB backbones. Comprehensive photophysical and computational investigations reveal that the variation of the donor strength enables the fine control over the intramolecular CT behavior of these TPB derivatives, thereby influencing their excited-state properties such as solvatochromism, photoluminescence quantum yield, and emission lifetimes in both solution and solid states. Interestingly, temperature-dependent emission results provide an unusual enhancement of emission intensity accompanied by a blue shift for TPB–CHO–PXZ, suggesting the involvement of a twisted intramolecular charge transfer (TICT) state. Polarity and viscosity-dependent transient absorption spectroscopic studies have confirmed the presence of TICT character and its linear dependence of donor strength. Overall, the present findings demonstrate for the first time the significant structure–property correlations in TPB-based AIE systems, contributing to the rational design of high-performance optoelectronic materials for next-generation OLEDs and molecular electronics.

Unusually high coordination number for elements has been of great interest in chemistry. Herein, a new class of hexacoordinated carbon compounds with stoichiometry MCF6 (M = Ca, Sr) at 400 and 500 GPa and K2CF6 at 500 GPa is found using crystal structure predictions assisted by first-principles calculations. . At high pressure, MCF6 (M = Ca, Sr) adopts a trigonal structure (R ) that is isomorphous with several other species with MIIAIVF6 (A = Si, Ge, and Sn) while K2CF6 adopts a trigonal structure (P m1) akin to M2 IAIVF6. Both the species are thermodynamically more stable than the bicomponent MF2/KF & mldr;CF4 mixture at high-pressure region. These solid-state structures are dynamically stable over a wide range of pressure. For all cases, the main building block of the solid-state structures is the [CF6]2- unit in which carbon attains a perfectly octahedral coordination of the six fluoride anions. The present study establishes new fluorine-rich carbon compounds and predicts a new class of perfectly hexacoordinated carbon species. For the first time, the stabilization of [CF6]2- in the solid-state is reported.

Minimizing the use of platinum (Pt) while maintaining hydrogen evolution reaction (HER) activity is a major objective in advancing sustainable hydrogen energy. MoO2 is an interesting host material for low-loading of Pt, largely retaining HER activity, however, its local environment and active state in the presence of Pt is poorly understood. Here, we investigate the electrochemical activation of MoO2 in acidic media (pH = 0, 1, and 2) via potential cycling using a Pt anode. Surface reduction of MoO2 forming undercoordinated MoO2-x and electrodeposition of Pt are the main phenomena that happen during the activation process, enhancing the HER activity of the resultant MoO2-x/Pt. The activation rate of MoO2 follows the order, pH = 0 > pH = 1 > pH = 2. The results show that though an ultralow amount of Pt is sufficient (0.1 mu g/cm(2)) for HER overpotential reduction, a significant amount (7.2 mu g/cm(2)) is required to achieve lower Tafel slope or faster HER kinetics. X-ray photoelectron spectra of the resultant MoO2-x/Pt ((1000, pH=0)) support that surface oxidized Mo6+ species are reduced to a lower oxidation state during the electrochemical activation process. The emergence of distorted Oh/Td geometry accompanied by reduced Mo-O/Mo coordination, revealed by X-ray absorption spectroscopy highlights Pt-induced activation of MoO2 with local structural distortion becoming more prominent at higher Pt loading. Computational (thermodynamics and kinetic) studies corroborate with experimental findings and provide atomic level insights of favorable formation and stabilization of undercoordinated MoOx species with Pt deposition, alongside changes in the rate-limiting step and HER activity.

Abstract Molecular design of an active electrocatalyst can determine the primary product. Precise selection of the active metal center influences the adsorption of reactants, intermediates, and product selectivity and ultimately affects the efficiency of the process. Here, we predict a potent catalyst, namely, Fe@C6N6, with enhanced activity and low overpotential for CO2 reduction reactions (CO2RRs) utilizing the density functional theory (DFT) approach, coupled with machine-learning-based model building. A series of 24 transition-metal single-atom catalysts (SACs) anchored on the C6N6 monolayer were screened, among which 19 exhibit effective CO2 activation. Employing density functional theory (DFT), a systematic study on reaction pathways demonstrates that 13 catalysts favor CH4 formation, while the remaining 6 exhibit selectivity toward CH3OH production. Additionally, 9 among the 19 catalysts are further screened out as they preferentially promote the competing hydrogen evolution reaction (HER). The active SACs demonstrate strong CO2RR activity, with Fe@C6N6 emerging as the most promising catalyst with the lowest limiting potential (0.39 V). To further establish atomic–property relationships, a machine-learning (ML) regression model was developed to predict adsorption energies using statistically significant descriptors and identify key factors governing CO2 adsorption. Furthermore, a classification model developed using the elemental characteristics of the metal atoms shows excellent correlation with the DFT results across the dataset. This combined DFT and machine-learning approach provides mechanistic insights for the rational development of efficient SACs for CO2 electroreduction.

Under pressure, the new rhombohedral phase of ScF 3 shows anisotropic negative thermal expansion.

Designing porous organic polymers (POPs) with aggregation-induced emission (AIE)-active building blocks is of paramount interest owing to their potential to produce highly luminescent POPs. Herein, two emissive POPs (named HVPOP and tBuVPOP) have been developed by taking advantage of the pronounced aggregation-induced emission (AIE) properties of partially twisted aryl-substituted buta-1,3-diene derivatives, which are linked through vinylene bond formation with trimethyl triazine. The synthesized POPs have been thoroughly characterized using FT-IR and solid-state NMR spectroscopy, and their porous nature is confirmed by nitrogen gas adsorption and desorption measurements. Owing to the presence of a bulky tert-butyl group in tBuVPOP, it exhibits relatively higher emission with an excellent photoluminescence quantum yield (27.3%) in the solid state compared to HVPOP (17.9%). Comprehensive photophysical and electrochemical studies of the synthesized POPs, along with their corresponding monomer units, are presented in this study. Additionally, these luminescent POPs are utilized for acid vapor sensing applications in the film state; specifically, tBuVPOP demonstrates an excellent response, displaying visible color changes to the naked eye and significant variations in emission intensity when exposed to various acid vapors over multiple cycles.

This article introduces a heavy-atom-free, acceptor-donor-acceptor conjugated organic photosensitizer (ADA-S), exhibiting an exceptional singlet oxygen (1O2) quantum yield (Φ∆) ∼1. The ADA-S structure integrates two thionated naphthalene-monoamide acceptors with a central thiophene donor moiety. To elucidate the impact of the extended conjugation on the photosensitizing efficiency, control molecules such as a less conjugated analogue (AD-S), an acceptor-only unit (NMIS-Br) or an un-thionated derivative (ADA-O) were studied. Through the 1,3-diphenylisobenzofuran (DPBF) assay, it was evident that ADA-S significantly outperforms all controls in the 1O2 generation upon photoirradiation. Transient absorption spectroscopy studies, ranging from femtosecond-to-microsecond timescales revealed that extended conjugation is crucial for enhancing the triplet state longevity, which directly correlates with the observed enhancement in the 1O2 production. In sharp contrast, similar ADA-type conjugated chromophore (ADA-O), lacking thionation of the carbonyl groups, failed to produce any detectable 1O2, indicating a remarkable impact of the sulphur atoms on the enhanced spin-orbit coupling, facilitating much faster intersystem crossing. Time-dependent density functional theory calculations further supported these experimental findings, establishing ADA-S as an outstanding new organic photosensitizer. Furthermore, an amphiphilic derivative of ADA-S was prepared, which showed spontaneous self-assembly in water producing nanoparticles, in which the photosensitizer remained in the dormant state due to π-stacking. Such organic nanoparticles showed excellent cellular uptake and intra-cellular swelling/ disassembly probed by FRET, facilitating reactivation of the photosensitization ability of the ADA-S chromophore. Consequently, highly efficient light-triggered cell death was noticed by reactive oxygen species.

Pressure-induced polymerization (PIP) provides a route towards the controlled synthesis of topological polymeric compounds from the basic molecular units. In general, PIP is a green chemical process that does not require solvents, additives or catalysts. Siloxane, a backbone polymer containing alternating silicon-oxygen linkages with organic side chains, is commercially known as silicone oil. Herein, we report a new class of polymeric compound formed from molecular acetone using crystal structure predictions assisted by first-principles calculations at high pressure (P ≥ 20 GPa). Acetone forms a polymeric compound with alternating carbon-oxygen linkages, [-(Me)2C-O-C(Me)2-]n analogous to the polydimethylsiloxane (PDMS) structures. This polymeric structure is dynamically stable over a wide range of pressures and thermally stable at room temperature. The present finding establishes the first example of carbon-based PDMS-type structure and should motivate further exploration in novel polymerization protocols. At an intermediate pressure P = 10 GPa, acetone self-dimerizes into the aldol (diacetone alcohol, DAA). While aldol formation from acetone traditionally requires external acid or base catalysts, pressure in itself suffices for this reaction. The self-dimerization of acetone occurs via a hydrogen-bonded keto-enol co-crystal formed in situ under pressure.

Stilbenes are prototypical photoswitches due to their pronounced photoinduced structural changes and robust thermal stability. However, a narrow window of excitation in the UV range, formation of photostationary states (PSS), low quantum yields, and photochemical side-products have significantly constrained their practical applications. Here, we leverage nanoconfinement-driven chemistry to achieve ultrafast, high-efficiency cis-to-trans isomerization of stilbenes in aqueous media. The selective encapsulation of cis-stilbene derivatives compared to their trans-versions within the water-soluble nanocage Pd6L412+ induces host-guest charge-transfer interactions, enabling visible-light-mediated photoisomerization without PSS formation. Broadband femtosecond transient absorption spectroscopy provided direct signatures of cis-stilbene radical cation and allowed us to track the isomerization dynamics, occurring on ultrafast time scales in tens of picoseconds. Computationally derived optical signatures along with low-temperature electron paramagnetic resonance spectroscopy further validate the radical cation-mediated isomerization pathway. This work presents the first sensitizer-free strategy for driving stilbene photoisomerization out of equilibrium using visible light in aqueous environments, broadening the scope of charge-transfer-driven photochemical transformations.

The quest to meet global energy requirements through the development of stable and cost-effective electrocatalysts that promote the hydrogen evolution reaction (HER), has gained tremendous interest. The modulation of electrocatalyst-electrolyte interaction is a pivotal step towards achieving enhanced HER performances. The present work emphasizes the design of a distinctly stable polymer-metal nanostructure, aiming to accelerate hydrogen production by tuning the active electronic environment of the conducting polymer, which serves as a site-selective platform for the nanoparticles. Spatially uniform and confined palladium oxide nanoparticles (PdO-NPs) were integrated on the surface of symmetrical organic acid doped polyaniline nanotubes (PANI_NTs) to generate a hybrid catalyst. Repeating amine sites provided a uniform platform for the adsorption/formation of PdO-NPs, and subsequent electronic structure modification of active Pd sites resulted in enhanced electrocatalytic activity. After optimization, the hybrid catalyst exhibited a low overpotential of 67 mV at 10 mA cm(-2) current density and remarkable stability for 15 000 accelerated degradation test (ADT) cycles with negligible depletion (1.36%) compared with a commercially available Pd/C catalyst (76.09% depletion after 6000 ADT cycles) in 0.5 M H2SO4. Chronoamperometric studies utilising a custom-designed electrochemical flow cell demonstrated the long-term durability (>100 h) of the catalyst with higher current density (350 mA cm(-2)). Through controlled experiments, including electrochemical studies, X-ray photoelectron spectroscopy, and in situ infrared spectroscopy, the driving force behind the remarkable performance was identified by mapping the active sites, charge transfer kinetics and reaction mechanisms; the conclusions were further substantiated by first-principles based DFT calculations.

An ever-increasing demand for nontoxic, high-performance electric batteries has motivated the exploration of novel aqueous electrolyte systems. One such system, hexahydrate aluminum nitrate [Al(H2O)6](NO3)3 solvated in water, is being investigated as an important candidate in this venture. While this system presents an interesting interplay of multiple interacting species, detailed theoretical studies of this complex aqueous system seem to be lacking. Here, we carry out such a theoretical study with the aim to understand the structure and dynamics for this system. We first perform a state-of-the-art quantum chemical calculation to develop the force field of this system. Subsequently, we employ molecular dynamics simulations to calculate various relevant quantities, such as the partial radial distribution functions among the species involved, and the transport properties like diffusion, of a 1 M solution. In order to validate our computational approach and the force field employed, we conduct NMR measurements to determine the diffusion coefficients of the different species involved. The computed diffusion coefficients show excellent agreement with NMR experimental values, reinforcing the accuracy of the developed force field. Furthermore, our results reveal that the diffusion of free water in this system is reduced by more than a factor of 2 compared to that in pure water, highlighting the effects of strong intermolecular correlations among hexahydrate aluminum nitrate and water molecules. Our calculated radial distribution functions further confirm the presence of pronounced structural correlations, while trajectory analyses provide valuable insights into the intricate dynamical behavior of the system where species move on different time scales.

A series of bay-substituted PDI-based conjugated copolymers, which are composed of a linker as the donor moiety and perylene core as the strong p-electron acceptor moiety, have been designed to investigate the modulation of the photophysical properties with increasing donor-acceptor strength. After successful synthesis, all copolymers (namely, Benz-PDI, Btz-PDI, TzTz-PDI and NH-PDI) have been characterized by 13C NMR, MALDI-ToF and FT-IR analyses. The copolymers are moderately stable upon heating and are semi-crystalline in nature (except TzTz-PDI and NH-PDI). The optical characteristics of the copolymers are determined by absorption and emission studies. The redox properties of these copolymers are observed to change with increasing donor-acceptor strength, which is further supported by DFT calculations. Initial electrochemical studies reveal that the designed copolymers have the potential to be used as pseudocapacitive materials for energy storage applications. Among the four copolymers, charge-discharge studies indicate that NH-PDI produces the highest specific capacitance of 363 F g-1 with outstanding cyclic stability of 15 000 cycles under a three-electrode setup. A symmetric supercapacitor device made of NH-PDI produced a specific capacitance of 134.2 F g-1 with relatively higher values of specific energy density (E) and specific power density (Pmax) at a current density of 0.5 A g-1. The excellent result of PDI-embedded conjugated donor-acceptor polymers reveals a novel design strategy as well as the subsequent application of a new type of a stable polymeric electrode material for high-performance supercapacitors.