Modulation of Electrostatic Potential in 2D Crystal Engineered by an Array of Alternating Polar Molecules.

The moiré potential in rotationally misfit two-dimensional (2D) heterostructures has been used to build artificial exciton and electron lattices, which have become platforms for realizing exotic electronic phases. Here, we demonstrate a different approach to create a superlattice potential in 2D crystals by using the near field of an array of polar molecules. A bilayer of titanyl phthalocyanine (TiOPc), consisting of alternating out-of-plane dipoles, is deposited on monolayer MoS2. Time-resolved two-photon photoemission spectroscopy reveals a pair of interlayer exciton states with an energy difference of ∼0.1 eV, which is consistent with the electrostatic potential modulation induced by the TiOPc bilayer as determined by density functional theory calculations. Because the symmetry and the period of this potential superlattice can be changed readily by using molecules of different shapes and sizes, molecule/2D heterostructures can be promising platforms for designing artificial exciton and electron lattices.

How the addition of atomic hydrogen to a multiple bond can be catalyzed by water molecules.

Observational data show complex organic molecules in the interstellar medium (ISM). Hydrogenation of small unsaturated carbon double bond could be one way for molecular complexification. It is important to understand how such reactivity occurs in the very cold and low-pressure ISM. Yet, there is water ice in the ISM, either as grain or as mantle around grains. Therefore, the addition of atomic hydrogen on double-bonded carbon in a series of seven molecules have been studied and it was found that water catalyzes this reaction. The origin of the catalysis is a weak charge transfer between the π MO of the unsaturated molecule and H atom, allowing a stabilizing interaction with H2O. This mechanism is rationalized using the non-covalent interaction and the quantum theory of atoms in molecules approaches.

The Hunt for Chemical Dark Matter across a River-to-Ocean Continuum.

Thousands of mass peaks emerge during molecular characterization of natural dissolved organic matter (DOM) using ultrahigh-resolution mass spectrometry. While mass peaks assigned to certain molecular formulas have been extensively studied, the uncharacterized mass peaks that represent a significant fraction of organic matter and convey biogenic elements and energy have been previously ignored. In this study, we introduce the term dark DOM (DDOM) for unassigned mass peaks and have explored its characteristics and environmental behaviors using a data set of 38 DOM extracts covering the Yangtze River-to-ocean continuum. We identified a total of 9141 DDOM molecules, which exhibited higher molecular weight and greater diversity than the DOM subset with assigned DOM formulas. Although DDOM contributed a smaller fraction of relative abundance, it significantly impacted the molecular weight and molecular composition of bulk DOM. A portion of DDOM with higher molecular weight was found to increase molecular abundance across the river-to-ocean continuum. These compounds could contain halogenated organic molecules and might have a high potential to contribute to the refractory organic carbon pool. With this study, we underline the contribution of dark matter to the total DOM pool and emphasize that more DDOM research is needed to understand its contribution to global biogeochemical cycles and carbon sequestration.

New Insights on the Formation of Nucleation Mode Particles in a Coastal City Based on a Machine Learning Approach.

Atmospheric particles have profound implications for the global climate and human health. Among them, ultrafine particles dominate in terms of the number concentration and exhibit enhanced toxic effects as a result of their large total surface area. Therefore, understanding the driving factors behind ultrafine particle behavior is crucial. Machine learning (ML) provides a promising approach for handling complex relationships. In this study, three ML models were constructed on the basis of field observations to simulate the particle number concentration of nucleation mode (PNCN). All three models exhibited robust PNCN reproduction (R2 > 0.80), with the random forest (RF) model excelling on the test data (R2 = 0.89). Multiple methods of feature importance analysis revealed that ultraviolet (UV), H2SO4, low-volatility oxygenated organic molecules (LOOMs), temperature, and O3 were the primary factors influencing PNCN. Bivariate partial dependency plots (PDPs) indicated that during nighttime and overcast conditions, the presence of H2SO4 and LOOMs may play a crucial role in influencing PNCN. Additionally, integrating additional detailed information related to emissions or meteorology would further enhance the model performance. This pilot study shows that ML can be a novel approach for simulating atmospheric pollutants and contributes to a better understanding of the formation and growth mechanisms of nucleation mode particles.

Fostering a Holistic Understanding of the Full Volatility Spectrum of Organic Compounds from Benzene Series Precursors through Mechanistic Modeling.

A comprehensive understanding of the full volatility spectrum of organic oxidation products from the benzene series precursors is important to quantify the air quality and climate effects of secondary organic aerosol (SOA) and new particle formation (NPF). However, current models fail to capture the full volatility spectrum due to the absence of important reaction pathways. Here, we develop a novel unified model framework, the integrated two-dimensional volatility basis set (I2D-VBS), to simulate the full volatility spectrum of products from benzene series precursors by simultaneously representing first-generational oxidation, multigenerational aging, autoxidation, dimerization, nitrate formation, etc. The model successfully reproduces the volatility and O/C distributions of oxygenated organic molecules (OOMs) as well as the concentrations and the O/C of SOA over wide-ranging experimental conditions. In typical urban environments, autoxidation and multigenerational oxidation are the two main pathways for the formation of OOMs and SOA with similar contributions, but autoxidation contributes more to low-volatility products. NOx can reduce about two-thirds of OOMs and SOA, and most of the extremely low-volatility products compared to clean conditions, by suppressing dimerization and autoxidation. The I2D-VBS facilitates a holistic understanding of full volatility product formation, which helps fill the large gap in the predictions of organic NPF, particle growth, and SOA formation.

Recent synthetic strategies of small heterocyclic organic molecules with optoelectronic applications: a review.

Over the past few years, there have been tremendous developments in the design and synthesis of organic optoelectronic materials with appealing applications in device fabrication of organic light-emitting diodes, superconductors, organic lasers, organic field-effect transistors, clean energy-producing organic solar cells, etc. There is an increasing demand for the synthesis of green, highly efficient organic optoelectronic materials to cope with the issue of efficiency roll-off in organic semiconductor-based devices. This review systematically summarized the recent progress in the design and synthesis of small organic molecules having promising optoelectronic properties for their potential applications in optoelectronic devices during the last 10-year range (2010-early 2021).

Defects and Defect Passivation in Perovskite Solar Cells.

Perovskite solar cells have made significant strides in recent years. However, there are still challenges in terms of photoelectric conversion efficiency and long-term stability associated with perovskite solar cells. The presence of defects in perovskite materials is one of the important influencing factors leading to subpar film quality. Adopting additives to passivate defects within perovskite materials is an effective approach. Therefore, we first discuss the types of defects that occur in perovskite materials and the mechanisms of their effect on performance. Then, several types of additives used in perovskite solar cells are discussed, including ionic compounds, organic molecules, polymers, etc. This review provides guidance for the future development of more sustainable and effective additives to improve the performance of solar cells.

Regioselective Multisite Atomic-Chlorine Passivation Enables Efficient and Stable Perovskite Solar Cells.

Passivating defects using organic halide salts, especially chlorides, is an effective method to improve power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) arising from the stronger Pb-Cl bonding than Pb-I and Pb-Br bonding. However, Cl- anions with a small radius are prone to incorporation into the perovskite lattice that distorts the lead halide octahedron, degrading the photovoltaic performance. Here, we substitute atomic-Cl-containing organic molecules for widely used ionic-Cl salts, which not only retain the efficient passivation by Cl but also prevent the incorporation of Cl into the bulk lattice, benefiting from the strong covalent bonding between Cl atoms and organic frameworks. We find that only when the distance of Cl atoms in single molecules matches well with the distance of halide ions in perovskites can such a configuration maximize the defect passivation. We thereby optimize the molecular configuration to enable multiple Cl atoms in an optimal spatial position to maximize their binding with surface defects. The resulting PSCs achieve a certified PCE of 25.02%, among the highest PCEs for PSCs, and retain 90% of their initial PCE after 500 h of continuous operation.

The effect of solvent on determining highest occupied molecular orbital energies of semiconducting organic molecules: Insight from a combined computational approach.

Although cyclic voltammetry (CV) measurements in solution have been widely used to determine the highest occupied molecular orbital energy (EHOMO ) of semiconducting organic molecules, an understanding of the experimentally observed discrepancies due to the solvent used is lacking. To explain these differences, we investigate the solvent effects on EHOMO by combining density functional theory and molecular dynamics calculations for four donor molecules with a common backbone moiety. We compare the experimental EHOMO values to the calculated values obtained from either implicit or first solvation shell theories. We find that the first solvation shell method can capture the EHOMO variation arising from the functional groups in solution, unlike the implicit method. We further applied the first solvation shell method to other semiconducting organic molecules measured in solutions for different solvents. We find that the EHOMO obtained using an implicit method is insensitive to solvent choice. The first solvation shell, however, produces EHOMO values that are sensitive to solvent choices and agrees with published experimental results. The solvent sensitivity arises from a hierarchy of three effects: (1) the solute electronic state within a surrounding dielectric continuum, (2) ambient temperature or solvent atoms changing the solute geometry, and (3) electronic interactions between the solute and solvents. The implicit method, on the other hand, only captures the effect of a dielectric environment. Our findings suggest that EHOMO obtained by CV measurements should account for the influence of solvent when the results are reported, interpreted, or compared to other molecules.

A Novel Aqueous Asymmetric Supercapacitor based on Pyrene-4,5,9,10-Tetraone Functionalized Graphene as the Cathode and Annealed Ti3 C2 Tx MXene as the Anode.

Asymmetric supercapacitors (ASCs), employing two dissimilar electrode materials with a large redox peak position difference as cathode and anode, have been designed to further broaden the voltage window and improve the energy density of supercapacitors. Organic molecule based electrodes can be constructed by combining redox-active organic molecules with conductive carbon-based materials such as graphene. Herein, pyrene-4,5,9,10-tetraone (PYT), a redox-active molecule with four carbonyl groups, exhibits a four-electron transfer process and can potentially deliver a high capacity. PYT is noncovalently combined with two different kinds of graphene (Graphenea [GN] and LayerOne [LO]) at different mass ratios. The PYT-functionalized GN electrode (PYT/GN 4-5) possesses a high capacity of 711 F g-1 at 1 A g-1 in 1 M H2 SO4 . To match with the PYT/GN 4-5 cathode, an annealed-Ti3 C2 Tx (A-Ti3 C2 Tx ) MXene anode with a pseudocapacitive character is prepared by pyrolysis of pure Ti3 C2 Tx . The assembled PYT/GN 4-5//A-Ti3 C2 Tx ASC delivers an outstanding energy density of 18.4 Wh kg-1 at a power density of 700 W kg-1 . The PYT-functionalized graphene holds great potential for high-performance energy storage devices.

Carboxyl-Substituted Organic Molecule Assembled with MXene Nanosheets for Boosting Aqueous Na+ Storage.

Aqueous alkali-ion batteries have enormous promise as a kind of safe, reliable, and sustainable energy technologies for power supplies. Although organic molecules with tunable and diverse configurations are potential electroactive materials, their inadequate redox activity and electron affinity hinder the practical application for aqueous alkali-ion storage. Herein, a novel electron-withdrawing carboxyl-substituted dipyridophenazine (CDPPZ) organic molecule is designed and synthesized for aqueous Na+ storage. Significantly, the introduction of carboxyl functional groups not only serves as additional redox-active sites for reversible Na+ coordination, but also causes the rearrangement of intramolecular electron cloud density to reduce the energy level, thereby ensuring the high redox activity and superior electron affinity of the CDPPZ molecule. For portable electronics, a self-supporting, adhesive-free, and flexible CDPPZ@MXene electrode is further constructed by incorporating highly redox-active CDPPZ molecule with MXene nanosheets, which delivers a fast, stable, and unrivaled aqueous Na+ storage capability with a high reversible capacity of 172.6 mAh cm-3 and excellent redox stability over 4000 cycles. In situ dynamic analysis combined with theoretical calculations illustrates the Na+ storage mechanism and corresponding coordinated pathway. Finally, a high-performance flexible aqueous Na-ion battery is fabricated with exceptional energy/power density and remarkable cycling lifespan, further confirming its promising application prospect.

Intramolecular Through-Space Double-Electron Transfer Between A Pair of Redox-Active Guanidine Units Aligned by Dithiolate Bridges.

Using unconventional synthesis protocols, two redox-active triguanidine units are connected by a dithiolate bridge, aligning the two redox-active units in close proximity. The reduced, neutral and the tetracationic redox states with two dicationic triguanidine units are isolated and fully characterized. Then, the dicationic redox states are prepared by mixing the neutral and tetracationic molecules. At low temperatures, the dications are diamagnetic (singlet ground state) with two different triguanidine units (neutral and dicationic). At room temperature, the triplet state with two radical monocationic triguanidine units is populated. At low temperature (210 K), chemical exchange by intramolecular through-space electron-transfer between the two triguanidine units is evidenced by EXSY NMR spectroscopy. Intramolecular through-space transfer of two electrons from the neutral to the dicationic triguanidine unit is accompanied by migration of the counterions in opposite direction. The rate of double-electron transfer critically depends on the bridge. No electron-transfer is measured in the absence of a bridge (in a mixture of one dicationic and one neutral triguanidine), and relatively slow electron transfer if the bridge does not allow the two triguanidine units to approach each other close enough. The results give detailed, quantitative insight into the factors that influence intramolecular through-space double-electron-transfer processes.

Conformational energies of reference organic molecules: benchmarking of common efficient computational methods against coupled cluster theory.

We selected 145 reference organic molecules that include model fragments used in computer-aided drug design. We calculated 158 conformational energies and barriers using force fields, with wide applicability in commercial and free softwares and extensive application on the calculation of conformational energies of organic molecules, e.g. the UFF and DREIDING force fields, the Allinger's force fields MM3-96, MM3-00, MM4-8, the MM2-91 clones MMX and MM+, the MMFF94 force field, MM4, ab initio Hartree-Fock (HF) theory with different basis sets, the standard density functional theory B3LYP, the second-order post-HF MP2 theory and the Domain-based Local Pair Natural Orbital Coupled Cluster DLPNO-CCSD(T) theory, with the latter used for accurate reference values. The data set of the organic molecules includes hydrocarbons, haloalkanes, conjugated compounds, and oxygen-, nitrogen-, phosphorus- and sulphur-containing compounds. We reviewed in detail the conformational aspects of these model organic molecules providing the current understanding of the steric and electronic factors that determine the stability of low energy conformers and the literature including previous experimental observations and calculated findings. While progress on the computer hardware allows the calculations of thousands of conformations for later use in drug design projects, this study is an update from previous classical studies that used, as reference values, experimental ones using a variety of methods and different environments. The lowest mean error against the DLPNO-CCSD(T) reference was calculated for MP2 (0.35 kcal mol-1), followed by B3LYP (0.69 kcal mol-1) and the HF theories (0.81-1.0 kcal mol-1). As regards the force fields, the lowest errors were observed for the Allinger's force fields MM3-00 (1.28 kcal mol-1), ΜΜ3-96 (1.40 kcal mol-1) and the Halgren's MMFF94 force field (1.30 kcal mol-1) and then for the MM2-91 clones MMX (1.77 kcal mol-1) and MM+ (2.01 kcal mol-1) and MM4 (2.05 kcal mol-1). The DREIDING (3.63 kcal mol-1) and UFF (3.77 kcal mol-1) force fields have the lowest performance. These model organic molecules we used are often present as fragments in drug-like molecules. The values calculated using DLPNO-CCSD(T) make up a valuable data set for further comparisons and for improved force field parameterization.

Enigma of Urban Gaseous Oxygenated Organic Molecules: Precursor Type, Role of NOx, and Degree of Oxygenation.

Oxidation of volatile organic compounds (VOCs) forms oxygenated organic molecules (OOMs), which contribute to secondary pollution. Herein, we present measurement results of OOMs using chemical ionization mass spectrometry with nitrate as the reagent ion in Shanghai. Compared to those in forests and laboratory studies, OOMs detected at this urban site were of relatively lower degree of oxygenation. This was attributed to the high NOx concentrations (∼44 ppb), which overall showed a suppression on the propagation reactions. As another result, a large fraction of nitrogenous OOMs (75%) was observed, and this fraction further increased to 84% under a high NO/VOC ratio. By applying a novel framework on OOM categorization and supported by VOC measurements, 50 and 32% OOMs were attributed to aromatic and aliphatic precursors, respectively. Furthermore, aromatic OOMs are more oxygenated (effective oxygen number, nOeff = 4-6) than aliphatic ones (nOeff = 3-4), which can be partly explained by the difference in initiation mechanisms and points to possible discrimination in termination reactions. This study highlights the roles of NOx in OOM formation in urban areas, as well as the formation of nitrogenous products that might show discrimination between aromatic and aliphatic VOCs.

Molecular Composition of Anthropogenic Oxygenated Organic Molecules and Their Contribution to Organic Aerosol in a Coastal City.

Organic aerosols (OA) have gained attention as a substantial component of atmospheric aerosols owing to their impact on atmospheric visibility, climate, and human health. Although oxygenated organic molecules (OOMs) are essential contributors to OA formation, the sources, transformations, and fates of the OOMs are not fully understood. Herein, anthropogenic OOMs (AOOMs), anthropogenic volatile organic compounds (AVOCs), and OA were concurrently measured in Xiamen, a coastal city in southeastern China. Our results show that the AOOMs exhibited a high nitrogen content (76%) and a low oxidation degree. Strong photochemical processes of aromatic VOCs were the predominant sources of AOOMs. Also, NOx concentrations and the occurrence of multigeneration OH radical oxidations were the critical factors that might influence the formation of AOOMs. Finally, the newly developed aerosol dynamic model's results show that more than 35% of the OA mass growth rate is attributed to the gas-particle partitioning of AOOMs. Further sensitivity testing demonstrates that the contribution of AOOMs to OA growth is significantly enhanced during high-particulate-concentration periods, especially under low-temperature conditions. This study emphasizes the vital role of photochemically produced AOOMs derived from AVOCs in OA growth in a coastal urban atmosphere.

Molecular Characterization of Oxygenated Organic Molecules and Their Dominating Roles in Particle Growth in Hong Kong.

Oxygenated organic molecules (OOMs) are critical intermediates linking volatile organic compound oxidation and secondary organic aerosol (SOA) formation. Yet, the understanding of OOM components, formation mechanism, and impacts are still limited, especially for urbanized regions with a cocktail of anthropogenic emissions. Herein, ambient measurements of OOMs were conducted at a regional background site in South China in 2018. The molecular characteristics of OOMs revealed dominant nitrogen-containing products, and the influences of different factors on OOM composition and oxidation state were elucidated. Positive matrix factorization analysis resolved the complex OOM species to factors featured with fingerprint species from different oxidation pathways. A new method was developed to identify the key functional groups of OOMs, which successfully classified the majority species into carbonyls (8%), hydroperoxides (7%), nitrates (17%), peroxyl nitrates (10%), dinitrates (13%), aromatic ring-retaining species (6%), and terpenes (7%). The volatility estimation of OOMs was improved based on their identified functional groups and was used to simulate the aerosol growth process contributed by the condensation of those low-volatile OOMs. The results demonstrate the predominant role of OOMs in contributing sub-100 nm particle growth and SOA formation and highlight the importance of dinitrates and anthropogenic products from multistep oxidation.

Recent advances in room temperature phosphorescence of chiral organic materials.

Room temperature phosphorescence (RTP) in purely organic materials is an uncommon phenomenon of emission, which can be characterized by a long persistent luminescence after removal of the excitation source. In the recent years, RTP organic materials have received a considerable attention due to their high application potential in various advancing technologies, ranging from optoelectronic to biomedical applications. In parallel, many progresses have been achieved on the rationalization of this process and led to the emergence of innovative strategies aiming to achieve highest performances both in terms of phosphorescence efficiency and lifetime. While the topic is still on an ascendant development, the generation of circularly polarized phosphorescent (CPP) emission from purely organic molecules is by far much less explored and remains an impressive challenge. Still, the perspective of CPP materials appears as an interesting opportunity to answer several comprehensives issues existing in the field. In this article, we define, in a straightforward way, basic principles and key notions for the generation of RTP and CP luminescence (CPL) guiding the design toward CPP materials. After this brief insight, recent advances in the field of chiral organic RTP materials are discussed with an emphasis on their CP-RTP properties. Based on this development, the conclusion drawn allows establishing the next challenges and future opportunities standing in the field.

Atomic-Resolution Imaging of Small Organic Molecules on Graphene.

Here, we demonstrate atomic-resolution scanning transmission electron microscopy (STEM) imaging of light elements in small organic molecules on graphene. We use low-dose, room-temperature, aberration-corrected STEM to image 2D monolayer and bilayer molecular crystals, followed by advanced image processing methods to create high-quality composite images from ∼102-104 individual molecules. In metalated porphyrin and phthalocyanine derivatives, these images contain an elementally sensitive contrast with up to 1.3 Å resolution─sufficient to distinguish individual carbon and nitrogen atoms. Importantly, our methods can be applied to molecules with low masses (∼0.6 kDa) and nanocrystalline domains containing just a few hundred molecules, making it possible to study systems for which large crystals cannot easily be grown. Our approach is enabled by low-background graphene substrates, which we show increase the molecules' critical dose by 2-7×. These results indicate a new route for low-dose, atomic-resolution electron microscopy imaging to solve the structures of small organic molecules.

Benchmark study on deep neural network potentials for small organic molecules.

There has been tremendous advancement in machine learning (ML) applications in computational chemistry, particularly in neural network potentials (NNP). NNPs can approximate potential energy surface (PES) as a high dimensional function by learning from existing reference data, thereby circumventing the need to solve the electronic Schrödinger equation explicitly. As a result, ML accelerates chemical space exploration and property prediction compared to quantum mechanical methods. Novel ML methods have the potential to provide efficient means for predicting the properties of molecules. However, this potential has been limited by the lack of standard comparative evaluations. In this work, we compare four selected models, that is, ANI, PhysNet, SchNet, and BAND-NN, developed to represent the PES of small organic molecules. We evaluate these models for their accuracy and transferability on two different test sets (i) Small organic molecules of up to eight-heavy atoms on which ANI and SchNet achieve root mean square error (RMSE) of 0.55 and 0.60 kcal/mol, respectively. (ii) On random selection of molecules from the GDB-11 database with 10-heavy atoms, ANI achieves RMSE of 1.17 kcal/mol and SchNet achieves RMSE of 1.89 kcal/mol. We examine their ability to produce smooth meaningful surface by performing PES scans for bond stretch, angle bend, and dihedral rotations on relatively large molecules to assess their possible application in molecular dynamics simulations. We also evaluate their performance for yielding minimum energy structures via geometry optimization using various minimization algorithms. All these models were also able to accurately differentiate different isomers of the same empirical formula C10H20 . ANI and PhysNet achieve an RMSE of 0.29 and 0.52 kcal/mol, respectively, on C10H20 isomers.

Oxygen Reduction Activity of B←N-Containing Organic Molecule Affected by Asymmetric Regulation.

Organic molecular catalysts have received great attention as they have the merits of well-controlled molecular structures for the development of catalytic chemistry. Herein, the electronic distribution of active sites is regulated by asymmetrically introducing S-heterocycle on one side of the molecular core. As a result, the asymmetric as-PYT and as-BNT show higher oxygen reduction performance than their symmetric counterparts without (s-PY, s-PY2T) or with two S-heterocycle units (s-BN, s-BN2T). Density functional theory calculations reveal that the carbon atoms (site-12) at symmetric s-BN and s-BN2T are the catalytic active sites, while for asymmetric as-BNT, it has changed to amino-N atom (site-14). Due to the non-uniform charge distribution and increased dipole moment of as-BNT caused by asymmetric molecular configuration, the kinetics of catalytic reaction has changed significantly. The catalytically active sites of specific N atoms are further verified experimentally and theoretically by using sterically hindered phenyl groups. This work provides a simple but efficient method to design metal-free oxygen reduction electrocatalysts.