Disentangling the Optoelectronic Behavior of Lead Iodide Governed by Two-Dimensional Electron Confinement.

We present a joint experimental and theoretical study for complete spectroscopic characterization and optoelectronic properties of lead iodide. Experimentally, we combine X-ray diffraction experiments to elucidate the structure with photoelectron spectroscopy to explore its electronic structure. Computationally, simulations are performed in the frame of density functional theory. We show that PbI2 presents a two-dimensional layered structure and exhibits a large transient photocurrent effect under visible light illumination, which are compatible with the surface photovoltage scenario. The transient photocurrent has an extremely long lifetime: when the sample is lightened with visible light, it shows very long relaxation times and, consequently, huge charge carrier diffusion lengths. We explain this anomalous behavior with the slow carrier mobility of holes and electrons caused by the 2D electron confinement in the layered material. Our results can be used as a simple model for understanding the optoelectronic properties of more complex 2D hybrid perovskites.

Uncovering low-frequency vibrations in surface-enhanced Raman of organic molecules.

Accessing the terahertz (THz) spectral domain through surface-enhanced Raman spectroscopy (SERS) is challenging and opens up the study of low-frequency molecular and electronic excitations. Compared to direct THz probing of heterogenous ensembles, the extreme plasmonic confinement of visible light to deep sub-wavelength scales allows the study of hundreds or even single molecules. We show that self-assembled molecular monolayers of a set of simple aromatic thiols confined inside single-particle plasmonic nanocavities can be distinguished by their low-wavenumber spectral peaks below 200 cm-1, after removal of a bosonic inelastic contribution and an exponential background from the spectrum. Developing environment-dependent density-functional-theory simulations of the metal-molecule configuration enables the assignment and classification of their THz vibrations as well as the identification of intermolecular coupling effects and of the influence of the gold surface configuration. Furthermore, we show dramatically narrower THz SERS spectra from individual molecules at picocavities, which indicates the possibility to study intrinsic vibrational properties beyond inhomogeneous broadening, further supporting the key role of local environment.

Bicarbonate-binding catalysis for the enantioselective desymmetrization of keto sulfonium salts.

Herein, an enantioselective desymmetrization of cyclic keto sulfonium salts through enantioselective deprotonation/ring opening process by anion-binding catalysis is presented. We report a squaramide/HCO3- complex as catalytic active species which is able to stereo-differentiate two enantiomeric protons, triggering the ring opening event taking advantage of the great tendency of sulfonium salts to act as leaving groups. Thus, this desymmetrization methodology give rise to ß-methylsulfenylated sulfa-Michael addition type products with excellent yields and very good enantioselectivities. The bifunctional organocatalyst has been demonstrated to be capable of activating simultaneously the base and the keto sulfonium salt by DFT calculations and experimental proofs.

Determining Factors to Understand the External Quantum Efficiency Values: Study Carried Out with Copper(I)-I and 1,2-Bis(4-pyridyl)ethane Coordination Polymers as Downshifters in Photovoltaic Modules.

Downshifters refer to compounds with the capacity to absorb UV photons and transform them into visible light. The integration of such downshifters has the potential to improve the efficiency of commercial photovoltaic modules. Initially, costly lanthanide derivatives and organic fluorescent dyes were introduced, resulting in a heightened module efficiency. In a novel research direction guided by the same physicochemical principles, the utilization of copper(I) coordination compounds is proposed. This choice is motivated by its simpler and more economical synthesis, primarily due to copper being a more abundant and less toxic element. Our proposal involves employing 1,2-bis(4-pyridyl) ethane (bpe), an economically viable commercial ligand, in conjunction with CuI to synthesize coordination polymers: [CuI(bpe)]n(1), [Cu3I3(bpe)3]n(2), and [CuI(bpe)0.5]n(3). These polymers exhibit the ability to absorb UV photons and emit light within the green and orange spectra. To conduct external quantum efficiency studies, the compounds are dispersed on glass and then encapsulated with ethylene vinyl acetate through heating to 150 °C. Interestingly, during these procedural steps, the solvents and temperatures employed induce a phase transformation, which has been thoroughly examined through both experimental analysis and theoretical calculations. The outcomes of these studies reveal an enhancement in external quantum efficiency with [Cu3I3(bpe)3]n(2), at a cost significantly lower (between 340 and 350 times) than that associated with lanthanide DS complexes.

Scalable Synthesis and Electrocatalytic Performance of Highly Fluorinated Covalent Organic Frameworks for Oxygen Reduction.

In this study, we present a novel approach for the synthesis of covalent organic frameworks (COFs) that overcomes the common limitations of non-scalable solvothermal procedures. Our method allows for the room-temperature and scalable synthesis of a highly fluorinated DFTAPB-TFTA-COF, which exhibits intrinsic hydrophobicity. We used DFT-based calculations to elucidate the role of the fluorine atoms in enhancing the crystallinity of the material through corrugation effects, resulting in maximized interlayer interactions, as disclosed both from PXRD structural resolution and theoretical simulations. We further investigated the electrocatalytic properties of this material towards the oxygen reduction reaction (ORR). Our results show that the fluorinated COF produces hydrogen peroxide selectively with low overpotential (0.062 V) and high turnover frequency (0.0757 s-1 ) without the addition of any conductive additives. These values are among the best reported for non-pyrolyzed and metal-free electrocatalysts. Finally, we employed DFT-based calculations to analyse the reaction mechanism, highlighting the crucial role of the fluorine atom in the active site assembly. Our findings shed light on the potential of fluorinated COFs as promising electrocatalysts for the ORR, as well as their potential applications in other fields.

Emergence of π-Magnetism in Fused Aza-Triangulenes: Symmetry and Charge Transfer Effects.

On-surface synthesis has paved the way toward the fabrication and characterization of conjugated carbon-based molecular materials that exhibit π-magnetism such as triangulenes. Aza-triangulene, a nitrogen-substituted derivative, was recently shown to display rich on-surface chemistry, offering an ideal platform to investigate structure-property relations regarding spin-selective charge transfer and magnetic fingerprints. Herein, we study electronic changes upon fusion of single molecules into larger dimeric derivatives. We show that the closed-shell structure of aza-triangulene on Ag(111) leads to closed-shell dimers covalently coupled through sterically accessible carbon atoms. Meanwhile, its open-shell structure on Au(111) leads to coupling via atoms displaying a high spin density, resulting in symmetric or asymmetric products. Interestingly, whereas all dimers on Au(111) exhibit similar charge transfer properties, only asymmetric ones show magnetic fingerprints due to spin-selective charge transfer. These results expose clear relationships among molecular symmetry, charge transfer, and spin states of π-conjugated carbon-based nanostructures.

Structural and Theoretical Study of Copper(II)-5-fluoro Uracil Acetate Coordination Compounds: Single-Crystal to Single-Crystal Transformation as Possible Humidity Sensor.

This paper describes the synthesis and characterization of seven different copper(II) coordination compounds, as well as the formation of a protonated ligand involving all compounds from the same reaction. Their synthesis required hydrothermal conditions, causing the partial in situ transformation of 5-fluoro uracil-1-acetic acid (5-FUA) into an oxalate ion (ox), as well as the protonation of the 4,4'-bipyridine (bipy) ligand through a catalytic process resulting from the presence of Cu(II) within the reaction. These initial conditions allowed obtaining the new coordination compounds [Cu2(5-FUA)2(ox)(bipy)]n·2n H2O (CP2), [Cu(5-FUA)2(H2O)(bipy)]n·2n H2O (CP3), as well as the ionic pair [(H2bipy)+2 2NO3-] (1). The mother liquor evolved rapidly at room temperature and atmospheric pressure, due to the change in concentration of the initial reagents and the presence of the new chemical species generated in the reaction process, yielding CPs [Cu(5-FUA)2(bipy)]n·3.5n H2O, [Cu3(ox)3(bipy)4]n and [Cu(ox)(bipy)]n. The molecular compound [Cu(5-FUA)2(H2O)4]·4H2O (more thermodynamically stable) ended up in the mother liquor after filtration at longer reaction times at 25 °C and 1 atm., cohabiting in the medium with the other crystalline solids in different proportions. In addition, the evaporation of H2O caused the single-crystal to single-crystal transformation (SCSC) of [Cu(5-FUA)2(H2O)(bipy)]n·2n H2O (CP3) into [Cu(5-FUA)2(bipy)]n·2n H2O (CP4). A theoretical study was performed to analyze the thermodynamic stability of the phases. The observed SCSC transformation also involved a perceptible color change, highlighting this compound as a possible water sensor.

Chiral and Catalytic Effects of Site-Specific Molecular Adsorption.

The changes of properties and preferential interactions based on subtle energetic differences are important characteristics of organic molecules, particularly for their functionalities in biological systems. Only slightly energetically favored interactions are important for the molecular adsorption and bonding to surfaces, which define their properties for further technological applications. Here, prochiral tetracenothiophene molecules are adsorbed on the Cu(111) surface. The chiral adsorption configurations are determined by Scanning Tunneling Microscopy studies and confirmed by first-principles calculations. Remarkably, the selection of the adsorption sites by chemically different moieties of the molecules is dictated by the arrangement of the atoms in the first and second surface layers. Furthermore, we have investigated the thermal effects on the direct desulfurization reaction that occurs under the catalytic activity of the Cu substrate. This reaction leads to a product that is covalently bound to the surface in chiral configurations.

Preparation, Supramolecular Organization, and On-Surface Reactivity of Enantiopure Subphthalocyanines: From Bulk to 2D-Polymerization.

The development of chiral materials is severely limited by the challenge to achieve enantiopure derivatives with both configurational stability and good optoelectronic properties. Herein we demonstrate that enantiopure subphthalocyanines (SubPcs) fulfill such demanding requirements and bear the prospect of becoming components of chiral technologies. Particularly, we describe the synthesis of enantiopure SubPcs and assess the impact of chirality on aspects as fundamental as the supramolecular organization, the behavior in contact with metallic surfaces, and the on-surface reactivity and polymerization. We find that enantiopure SubPcs remarkably tend to organize in columnar polar assemblies at the solid state and highly ordered chiral superstructures on Au(111) surfaces. At the metal interface, such SubPcs are singled out by scanning tunneling microscopy. DFT calculations suggest that SubPcs undergo a bowl-to-bowl inversion that was shown to be dependent on the axial substituent. Finally, we polymerize by means of on-surface synthesis a highly regular 2D, porous and chiral, π-extended polymer that paves the way to future nanodevice fabrication.

Total Electron Detachment and Induced Cationic Fragmentation Cross Sections for Superoxide Anion (O2-) Collisions with Benzene (C6H6) Molecules.

In this study, novel experimental total electron detachment cross sections for O2- collisions with benzene molecules are reported for the impact energy range (10-1000 eV), as measured with a transmission beam apparatus. By analysing the positively charged species produced during the collision events, relative total ionisation cross sections were derived in the incident energy range of 160-900 eV. Relative partial ionisation cross sections for fragments with m/z ≤ 78 u were also given in this energy range. We also confirmed that heavier compounds (m/z > 78 u) formed for impact energies between 550 and 800 eV. In order to further our knowledge about the collision dynamics governing the fragmentation of such heavier molecular compounds, we performed molecular dynamics calculations within the framework of the Density Functional Theory (DFT). These results demonstrated that the fragmentation of these heavier compounds strongly supports the experimental evidence of m/z = 39-42, 50, 60 (u) cations formation, which contributed to the broad local maximum in the total ionisation observed from 550 to 800 eV. This work reveals the reactivity induced by molecular anions colliding with hydrocarbons at high energies, processes that can take place in the interstellar medium under various local conditions.

Unexpected benzene oxidation in collisions with superoxide anions.

Superoxide anions colliding with benzene molecules at impact energies from 200 to 900 eV are reported for the first time to form massive complexes. With the aid of quantum chemistry calculations, we propose a mechanism in which a sudden double ionization of benzene and the subsequent electrostatic attraction between the dication and the anion form a stable covalently bonded C6H6O2+ molecule, that evolves towards the formation of benzene-diol conformers. These findings lend support to a model presenting a new high energy anion-driven chemistry as an alternative way to form complex molecules.

Order from a Mess: The Growth of 5-Armchair Graphene Nanoribbons.

The advent of on-surface chemistry under vacuum has vastly increased our capabilities to synthesize carbon nanomaterials with atomic precision. Among the types of target structures that have been synthesized by these means, graphene nanoribbons (GNRs) have probably attracted the most attention. In this context, the vast majority of GNRs have been synthesized from the same chemical reaction: Ullmann coupling followed by cyclodehydrogenation. Here, we provide a detailed study of the growth process of five-atom-wide armchair GNRs starting from dibromoperylene. Combining scanning probe microscopy with temperature-dependent XPS measurements and theoretical calculations, we show that the GNR growth departs from the conventional reaction scenario. Instead, precursor molecules couple by means of a concerted mechanism whereby two covalent bonds are formed simultaneously, along with a concomitant dehydrogenation. Indeed, this alternative reaction path is responsible for the straight GNR growth in spite of the initial mixture of reactant isomers with irregular metal-organic intermediates that we find. The provided insight will not only help understanding the reaction mechanisms of other reactants but also serve as a guide for the design of other precursor molecules.

Synergy Effects in Heavy Metal Ion Chelation with Aryl- and Aroyl-Substituted Thiourea Derivatives.

Detection and removal of metal ion contaminants have attracted great interest due to the health risks that they represent for humans and wildlife. Among the proposed compounds developed for these purposes, thiourea derivatives have been shown as quite efficient chelating agents of metal cations and have been proposed for heavy metal ion removal and for components of high-selectivity sensors. Understanding the nature of metal-ionophore activity for these compounds is thus of high relevance. We present a theoretical study on the interaction between substituted thioureas and metal cations, namely, Cd2+, Hg2+, and Pb2+. Two substituent groups have been chosen: 2-furoyl and m-trifluoromethylphenyl. Combining density functional theory simulations with wave function analysis techniques, we study the nature of the metal-thiourea interaction and characterize the bonding properties. Here, it is shown how the N,N'-disubstituted derivative has a strong affinity for Hg2+, through cation-hydrogen interactions, due to its greater oxidizing capacity.

Reassessing Alkyne Coupling Reactions While Studying the Electronic Properties of Diverse Pyrene Linkages at Surfaces.

The combination of alkyne and halogen functional groups in the same molecule allows for the possibility of many different reactions when utilized in on-surface synthesis. Here, we use a pyrene-based precursor with both functionalities to examine the preferential reaction pathway when it is heated on an Au(111) surface. Using high-resolution bond-resolving scanning tunneling microscopy, we identify multiple stable intermediates along the prevailing reaction pathway that initiate with a clearly dominant Glaser coupling, together with a multitude of other side products. Importantly, control experiments with reactants lacking the halogen functionalization reveal the Glaser coupling to be absent and instead show the prevalence of non-dehydrogenative head-to-head alkyne coupling. We perform scanning tunneling spectroscopy on a rich variety of the product structures obtained in these experiments, providing key insights into the strong dependence of their HOMO-LUMO gaps on the nature of the intramolecular coupling. A clear trend is found of a decreasing gap that is correlated with the conversion of triple bonds to double bonds via hydrogenation and to higher levels of cyclization, particularly with nonbenzenoid product structures. We rationalize each of the studied cases.

Ultrafast Dynamics of Electronic Resonances in Molecules Adsorbed on Metal Surfaces: A Wave Packet Propagation Approach.

We present a wave packet propagation-based method to study the electron dynamics in molecular species in the gas phase and adsorbed on metal surfaces. It is a very general method that can be employed to any system where the electron dynamics is dominated by an active electron and the coupling between the discrete and continuum electronic states is of importance. As an example, one can consider resonant molecule-surface electron transfer or molecular photoionization. Our approach is based on a computational strategy allowing incorporating ab initio inputs from quantum chemistry methods, such as density functional theory, Hartree-Fock, and coupled cluster. Thus, the electronic structure of the molecule is fully taken into account. The electron wave function is represented on a three-dimensional grid in spatial coordinates, and its temporal evolution is obtained from the solution of the time-dependent Schrödinger equation. We illustrate our method with an example of the electron dynamics of anionic states localized on organic molecules adsorbed on metal surfaces. In particular, we study resonant charge transfer from the π* orbitals of three vinyl derivatives (acrylamide, acrylonitrile, and acrolein) adsorbed on a Cu(100) surface. Electron transfer between these lowest unoccupied molecular orbitals and the metal surface is extremely fast, leading to a decay of the population of the molecular anion on the femtosecond timescale. We detail how to analyze the time-dependent electronic wave function in order to obtain the relevant information on the system: the energies and lifetimes of the molecule-localized quasistationary states, their resonant wavefunctions, and the population decay channels. In particular, we demonstrate the effect of the electronic structure of the substrate on the energy and momentum distribution of the hot electrons injected into the metal by the decaying molecular resonance.

8-Mercaptoquinoline as a Ligand for Enhancing the Photocatalytic Activity of Pt(II) Coordination Complexes: Reactions and Mechanistic Insights.

A family of quinoline-platinum(II) complexes as efficient photocatalysts is presented. Their key characteristic is their easy preparation by coordination of the readily available 8-hydroxy- or 8-thio-quinoline ligands, which are well known for their strong chelating ability to different metal ions. In the different photochemical transformations investigated, such as cross-dehydrogenative coupling, oxidation of arylboronic acids, and asymmetric alkylation of aldehydes, 8-mercaptoquinoline-Pt(II) complex proved to be the most general catalyst. Moreover, quenching experiments showed that, contrary to related methods reported in the literature, these complexes followed an oxidative quenching mechanism in all transformations studied. Besides, simulations performed with high-level ab initio methods of the complexes have helped to understand their photocatalytic activity.

Antimonene: Mechanical Isolation of Highly Stable Antimonene under Ambient Conditions (Adv. Mater. 30/2016).

On page 6332, J. Gómez-Herrero, F. Zamora, and co-workers describe the isolation of antimonene, a new allotrope of antimony that consists of a single layer of atoms. They obtain antimonene flakes by the scotch tape method; these flakes are highly stable in ambient conditions and even when immersed in water. The 1.2 eV gap calculated in this study suggests potential applications in optoelectronics.

Mechanical Isolation of Highly Stable Antimonene under Ambient Conditions.

Antimonene fabricated by mechanical exfoliation is highly stable under atmospheric conditions over periods of months and even when immersed in water. Density functional theory confirms the experiments and predicts an electronic gap of ≈1 eV. These results highlight the use of antimonene for optoelectronics applications.