Theoretical Studies on Structures, Properties and Dominant Debromination Pathways for Selected Polybrominated Diphenyl Ethers

Modeling Electrochemical Vacancy Regeneration in Single-Walled Carbon Nanotubes

Synthesis-induced defects in single-walled carbon nanotubes (SWCNTs) enable diverse catalytic reactions, but the nature of catalytic intermediates and how active species regeneration occurs are unclear. Using a quantum mechanics/molecular mechanics (QM/MM) hybrid methodology based on density functional theory (DFT) and a classical force-field, we explore the reactivity and electrochemical regeneration of a vacancy defect in a zigzag SWCNT. Our findings indicate that hydrolysis of the defect forms a ketone group on one carbon atom and C-H bonds on two adjacent carbons. Applying an electrochemical potential of ESHE = -0.740 V triggers a proton-coupled electron transfer (PCET), converting the ketone to a hydroxyl group. Further reduction at ESHE = -1.08 V induces another PCET, expelling the hydroxyl as water and forming an active carbon with carbene character that can react with hydrogen peroxide and perchlorate. The hydrogen atoms on neighboring carbons prevent further water dissociation, maintaining the catalytic vacancy.

First-Principle Characterization of Structural, Electronic, and Optical Properties of Tin-Halide Monomers

The growing interest in tin-halide semiconductors for photovoltaic applications demands in-depth knowledge of the fundamental properties of their constituents, starting from the smallest monomers entering the initial stages of formation. In this first-principles work based on time-dependent density-functional theory, we investigate the structural, electronic, and optical properties of tin-halide molecules SnXn 2-n, withn = 1 , 2 , 3 , 4 ${n = 1,2,3,4}$ and X=Cl, Br, I, simulating these compounds in vacuo as well as in an implicit solvent. We find that structural properties are very sensitive to the halogen species while the charge distribution is also affected by stoichiometry. The ionicity of the Sn-X bond is confirmed by the Bader charge analysis albeit charge displacement plots point to more complex metal-halide coordination. Particular focus is posed on the neutral molecules SnX2, for which electronic and optical properties are discussed in detail. Band gaps and absorption onset decrease with increasing size of the halogen species, and despite general common features, each molecule displays peculiar optical signatures. Our results are elaborated in the context of experimental and theoretical literature, including the more widely studied lead-halide analogs, aiming to contribute with microscopic insight to a better understanding of tin-halide perovskites.

Role of the solvent polarity on the optical and electronic characteristics of 1-iodoadamantane

The natural absorbance caused by the chromophore and chemical behavior of 1-iodoadamantane is highly influenced by the polarity of different solvent environments. This gives rise to the solvatochromatic shifts in the optical absorption and electronic structure and the experimentally measured UV-vis absorption spectra show significant solvatochromic shifts with respect to the solvent polarity. The absorption shift for both σ to σ*and n to σ* electronic transitions are more dominant in polar solvents than in nonpolar solvents. To obtain a better understanding of the impact of solvent polarity on the 1-iodoadamantane at the molecular level, computational calculations were carried out through implicit solvation. According to this, changes in the HOMO and LUMO energies and electron density distributions of various solvent continuums demonstrate the influence of solvent polarity on the HOMO and LUMO energy levels of the chemical system. This also shows an increment in the HOMO-LUMO gap with respect to the polarity of the solvent.

Calculation of excited states of monolayer TPPA-COF based on first principles

Two-dimensional covalent organic frameworks (COFs) have always been a hot topic in condensed matter physics. Herein, the first 100 excited states of the TPPA-COF are calculated to investigate the optical absorption properties of the materials in the interval. The stable molecular structure of monolayer TPPA-COF is obtained by first-principles calculation, which can be regarded as a hexagon with an aperture size of 18.25 Å. By means of the band structure and density of state analysis, it is found that the monolayer band gap width of the TPPA-COF is 1.52 eV. All excited states of the TPPA-COF exhibit obvious pi → pi* (delocalized π to anti π) local excitation characteristics through analysing the spatial distribution of the electron-hole pairs of the 10 excited states with the highest oscillator strength among the first 100 excited states. In addition, the simulated UV-vis spectra show that the maximum absorption intensity of the TPPA-COF is about 357 684 mol-1 cm-1, indicating that the TPPA-COF is a potential light-absorbing material.

The First-Principle Study on Tuning Optical Properties of MA2Z4 by Cr Replacement of Mo Atoms in MoSi2N4

Recently, with the successful preparation of MoSi₂N_4, an emerging family of two-dimensional (2D) layered materials has been predicted with a general formula of MA₂Z₄ (M: an early transition metal, A: Si or Ge and Z: N, P, or As). In terms of this new type of 2D material, how to effectively tune its light absorption properties is unclear. We systematically discuss the effects of replacing Mo with Cr atoms on the lattice structure, energy bands, and light absorption properties of 2D monolayer MoSi₂N₄ using density functional theory (DFT) and the Vienna Ab initio Simulation Package (VASP). Additionally, the results show that the single replacement of the atom Cr has no significant effect on the lattice structure of the outermost and sub-outer layers but plays a major role in the accumulation of electrons. In addition, the 2D MoSi₂N₄, Mo_0.5Cr_0.5Si₂N₄, and CrSi₂N₄ all have effective electron-hole separation properties. In the visible region, as the excited state increases, the required excitation energy is higher and the corresponding wavelength of light is shorter. It was found that the ultraviolet (UV)-visible spectra are red-shifted when Cr atoms replace Mo atoms in MoSi₂N₄; when Cr atoms and Mo atoms coexist, the coupling between Cr atoms and Mo atoms achieves modulation of the ultraviolet (UV)-visible spectra. Finally, we reveal that doping M-site atoms can effectively tune the light absorption properties of MA₂Z₄ materials. These results provide a strategy for the design of new 2D materials with high absorption properties.

Mechanism of Mixed-Valence Fe2.5+···Fe2.5+ Formation in Fe4S4 Clusters in the Ferredoxin Binding Motif

Most low-potential Fe₄S₄ clusters exist in the conserved binding sequence CxxCxxC (CⁿCⁿ⁺³Cⁿ⁺⁶). Fe(II) and Fe(III) at the first (Cⁿ) and third (Cⁿ⁺⁶) cysteine ligand sites form a mixed-valence Fe^2.5+···Fe^2.5+ pair in the reduced Fe(II)₃Fe(III) cluster. Here, we investigate the mechanism of how the conserved protein environment induces mixed-valence pair formation in the Fe₄S₄ clusters, F_X, F_A, and F_B in photosystem I, using a quantum mechanical/molecular mechanical approach. Exchange coupling between Fe sites is predominantly determined by the shape of the Fe₄S₄ cluster, which is stabilized by the preorganized protein electrostatic environment. The backbone NH and CO groups in the conserved CxxCxxC and adjacent helix regions orient along the Fe^Cn···Fe^C(n+6) axis, generating an electric field and stabilizing the Fe^Cn(II)Fe^C(n+6)(III) state in F_A and F_B. The overlap of the d orbitals via -S- (superexchange) is observed for the single Fe^Cn(II)···Fe^C(n+6)(III) pair, leading to the formation of the mixed-valence Fe^2.5+···Fe^2.5+ pair. In contrast, several superexchange Fe(II)···Fe(III) pairs are observed in F_X due to the highly symmetric pair of the CDGPGRGGTC sequences. This is likely the origin of F_X serving as an electron acceptor in the two electron transfer branches.

Dual Sensitization via Electron and Energy Harvesting in a Nanohybrid for Improvement of Therapeutic Efficacy

We demonstrate experimental evidence of the effect of surface plasmon resonance of noble metal nanoparticles (NPs) on the activity of a well-known biomedicinal drug in the proximity of a semiconductor having a wide band gap for enhanced photodynamic therapy (PDT) efficacy. We have chosen riboflavin (Rf) (or vitamin B₂) as a model photosensitizer, attached with ZnO NPs and further attached with gold (Au) NP-decorated ZnO to increase the efficiency. The synthesized nanohybrids are characterized with the help of different microscopic, optical spectroscopic, and density functional theory (DFT)-based techniques. The DFT and time-dependent DFT-based calculations validate the experimental findings. A detailed ultrafast spectroscopic study has been carried out further to study the excited-state charge dynamics in the interface of the nanohybrids. The occurrence of a Förster resonance energy transfer (FRET) between Rf and Au has been found to be the key reason for the increased efficiency in the Rf-ZnO-Au nanohybrid over the Rf-ZnO one. The dipolar coupling between Au and Rf in the Rf-ZnO-Au nanohybrid further facilitates the generation of reactive oxygen species (ROS) in comparison to Rf-ZnO under blue-light irradiation. The greater efficiency in ROS generation by the Rf-ZnO-Au nanohybrid has been utilized for antimicrobial action against methicillin-resistant S. aureus (MRSA). Overall, the present study highlights the dual sensitization for achieving enhanced electron injection efficiency in the Rf-ZnO-Au nanohybrid in order to use it as an antibacterial agent that could be translated in PDT.

A review on experimental design for pollutants removal in water treatment with the aid of artificial intelligence

Water pollution occurs mainly due to inorganic and organic pollutants, such as nutrients, heavy metals and persistent organic pollutants. For the modeling and optimization of pollutants removal, artificial intelligence (AI) has been used as a major tool in the experimental design that can generate the optimal operational variables, since AI has recently gained a tremendous advance. The present review describes the fundamentals, advantages and limitations of AI tools. Artificial neural networks (ANNs) are the AI tools frequently adopted to predict the pollutants removal processes because of their capabilities of self-learning and self-adapting, while genetic algorithm (GA) and particle swarm optimization (PSO) are also useful AI methodologies in efficient search for the global optima. This article summarizes the modeling and optimization of pollutants removal processes in water treatment by using multilayer perception, fuzzy neural, radial basis function and self-organizing map networks. Furthermore, the results conclude that the hybrid models of ANNs with GA and PSO can be successfully applied in water treatment with satisfactory accuracies. Finally, the limitations of current AI tools and their new developments are also highlighted for prospective applications in the environmental protection.