One-electron Transitions Research Articles

Contrasting the excited state properties of different conformers of trans- and cis-2,2'-bipyridine oligomers in the gas phase.

In this article, we present conformation-dependent photophysical and excited state properties of trans- and cis- BPY oligomers. Oligomers up to tetramers for three conformers, namely, o-, m-, and p-, are constructed and optimized at the B3LYP-D3/def2-SVPD level. The photophysical and excited state properties are interpreted in terms of UV and CD spectra at the RI-ADC(2)/def2-TZVPD level. The UV spectra of oligomers of the m-conformer show high-intensity and red-shifted UV bands compared to o- and p-oligomers. The CD spectra of p-oligomers show intense CD bands compared to o- and p-oligomers in the case of trans-structures. In contrast, oligomers of each conformer of cis-structures show high-intensity CD bands. The excited states of (BPY)2 and (BPY)4 are also characterized by analysis of one-electron transition density matrix considering three descriptors: ωCT, dexc, and PRNTO. The ωCT values of dimers are in the range of 0.06-0.32, which indicates the excited states are mainly LE states, whereas, for (BPY)4, the ωCT values range from 0.17 to 0.53, indicating the possibility of partial CT in the excited states. These observations are also explained using the NTOs and e-h correlation plots.

Excited state properties of an A-D-A non-fullerene electron acceptor: a LC-TD-DFTB study.

Understanding charge transfer processes is essential to estimate the performance of organic photovoltaic technologies. Although experimental production is on the rise, predictability strongly relies on theoretical modeling, which is limited to the size of semiconductors. As a computationally favorable approach, we benchmarked the long-range corrected (LC) time-dependent (TD) formulation of the semi-empirical density functional-based tight-binding method (DFTB) for three polycyclic aromatic hydrocarbons (PAHs) and studied the DTP-IC-4Ph molecule, a PAH-based non-fullerene electron acceptor (NFA) with an A-D-A backbone structure. After a thorough investigation into the long-range parameter (ω) tuning for naphthalene, anthracene and pyrene, the excitation energies, oscillator strengths and Natural Transition Orbitals (NTOs) were compared with the standard ωB97X-D/6-31G(d,p) level of theory and the ADC2/6-31G(d,p) multiconfigurational method. We estimated mobility-related properties of the NFA and considered 1000 thermally accessible configurations to qualitatively reproduce the experimental absorption profile and investigate the energetic disorder. Finally, we conducted a fragment-based analysis using the one-electron transition density matrix (1TDM) to determine the character of the excited states and investigate the effect of side chains on exciton formation. Our results are sensitive to the level of theory and highly dependent on the long-range parameter but suggest that the presence of alkyl chains promotes a higher average charge delocalization and allows for additional hopping mechanisms, favoring the charge transfer dynamics.

Elucidating the conformational change and electronic absorption spectrum of p-dimethylamino-cinnamaldehyde merocyanine across different solvent polarities.

We present a theoretical study on the structural and electronic properties of the p-dimethylamino-cinnamaldehyde (DMACA) merocyanine molecule in solvents of different polarities by combining the free energy gradient and the average solvent electrostatic configuration methods via an iterative procedure based on the sequential quantum mechanics/molecular mechanics hybrid methodology. Studying such a system in solution is a crucial step for understanding the solvent effects on its properties, which can have implications in fields such as optoelectronics and biophysics. We found that the DMACA molecule presents different geometries in nonpolar and polar solvents, changing from a polyene-like structure with a pyramidal dimethylamino group (in gas phase or nonpolar solvents) to a cyanine-like structure with a planar dimethylamino group in water due to the stabilizing effect of hydrogen bonds between DMACA and water. The molecular absorption spectrum showed a significant change, increasing solvent polarity with a large shift of the lower energy band, while the other two low lying bands did not shift significantly. The study accurately described the solvatochromic shift of the lowest-energy band and analyzed the structure of the excited states in terms of the one-electron transition density matrix, which showed that the dominant excited state (associated with the first lower energy band) is characterized by a local excitation on the benzene ring with charge transfer character to the carbon conjugated segment.

Photocatalytic Decomposition of Azo Dyes and Phenols Using Polymer Composites Containing Nanostructured Poly(Titanium Oxide) Doped with Gold or Silver Nanoparticles under Light Irradiation in a Wide Wavelength Range

The photocatalytic properties of poly(titanium oxide) (PTO) dispersed in optically transparent polymeric matrices of different natures under the action of both UV and visible light on aqueous solutions of azo dyes and phenols were investigated. PTO in materials forms clusters of mixed polymorphic modification—anatase and rutile—with an average size ~12 nm. With a one-electron transition Ti4+ + e− → Ti3+ accompanied by a reversible break of the Ti-O bond, the formation of electron-hole pairs and, consequently, active oxygen species occurs in PTO under UV irradiation. The PTO band gap in nanocomposites is 3.11–3.35 eV. Its doping with gold and silver nanoparticles with sizes from ~10 to ~30 nm reduces the PTO band gap by up to 2.11 eV, which leads to the operating wavelength range extension of the materials. It provides the enhancement of nanocomposites’ photocatalytic properties under UV irradiation and is the reason for their high activity under visible light action. It was found that azo dyes decompose by ~90% in this case. A phenol and para-nitrophenol conversion of 80–90% was proven at ~60 min upon their aqueous solutions’ visible-light irradiation at the nanocomposite concentration in a solution of 0.5 g/L.

Synthesis of C<sub>60</sub> fullerene hybrid molecules with catecholamines under the action of ultrasound

For the first time, morpholine monoadducts of С60-fullerene were obtained in the reactions of fullerene with biogenic amines (norepinephrine, adrenaline) under the action of ultrasound. The reactions take place in air in ( g 2.0000 and Δ H 1/2 3.2 G) was detected by EPR, a key intermediate in the synthesis of the C60-adrenaline cycloadduct, which was obtained as a result •-a toluene/DMF medium at room temperature. The radical anion C60- of a one-electron transition from the adrenaline molecule to the C60 framework.

Properties of poly(titanium oxide)-containing polymeric materials exhibiting UV-induced superhydrophilicity under simulated climate test conditions

Optically transparent organic-inorganic terpolymers containing nanostructured poly (titanium oxide) (PTO or (≡TiO)n) in polymeric matrixes of poly (hydroxiethylmethacrylate) (pHEMA) and its copolymers with acrylonitrile (AN), 2-ethylhexyleacrylate, buthylmethacrylate, vinylbuthyl ether and exhibiting UV-induced reversible surface hydrophilization had been subjected by climatic tests in the Q-SUN Xe-2 light weathering chamber, which simulates the exposure of the material under ambient conditions for 20 months of its operation. Adhesion, physical-mechanical and thermal-physical properties of PTO-containing materials had studied before and after climatic testing. The materials retain the capacity for the one-electron transition Ti4+ + e‾ ⇆ Ti3+ under UV-irradiation after environmental testing, resulting in both photochromic properties and reversible surface hydrophilization. The photocatalytic activity of materials in stearic acid decomposition at terpolymer's surface of composition [(≡TiO)n]: [HEMA]: [AN] = 1:5:1 had proven.

BREMS: Partial-wave calculation of spectra and angular distributions of electron-atom bremsstrahlung at electron energies less than 30 MeV (New Version Announcement)

BREMS: Partial-wave calculation of spectra and angular distributions of electron-atom bremsstrahlung at electron energies less than 30 MeV (New Version Announcement)

Cation effect on the electrochemical reduction of polyoxometalates in room temperature ionic liquids

Polyoxometalates (POMs) are compounds that undergo multiple successive one-electron redox transitions, making them convenient model reactants to study ion solvation effects. Room temperature ionic liquids (RTILs) are solvents made entirely of ions, and are expected to have interactions with the highly negatively charged POM reduction products. In this work, 12 RTILs with a range of different anions ([FSI]−=bis(fluorosulfonyl)imide, [TFSI]−=bis(trifluoromethylsulfonyl)imide, [BETI]−=bis(pentafluoroethylsulfonyl)imide, [BF4]−, [PF6]−) and cations (imidazolium, pyrrolidinium, sulfonium, ammonium, phosphonium) were employed as solvents to study the kinetics and thermodynamics of [S2W18O62]4− reduction, to shed light on solvation effects and ion-pairing effects caused by different RTIL structures. Up to six reversible reduction processes (producing highly negatively charged [S2W18O62]10−) were observed. For the RTILs that showed multiple processes, a clear trend in both the thermodynamics (inferred from the reduction peak potentials) and kinetics (inferred from the peak-to-peak separation) was observed, in the order: imidazolium < sulfonium ≈ ammonium < pyrrolidinium < phosphonium, supporting strong interactions of the negatively charged POM reduction products with the cation. Two related POMs, [P2W18O62]6− and [PW12O40]3−, were also studied in the optimum RTIL found for [S2W18O62]4− ([C2mim][FSI]=1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide), revealing fast kinetics and asymmetric peaks for [PW12O40]3−. This work demonstrates the importance of understanding the solvation effects of RTIL ions for highly charged electrogenerated products, allowing tuning of the RTIL structure to achieve the optimum kinetics and thermodynamics for an electrochemical process.

Alkaline earth cations binding mode tailors excited-state charge transfer properties of guanine quadruplex: A TDDFT study

Quadruplexes formed by nucleic acids and their derivates tend to chelate different monovalent and bivalent cations, which simultaneously affect their excited electronic states properties. Cation binding to every and every other cavity of the central ion channel could be exploited for tuning exited-state charge transfer properties. In this work we utilize set of descriptors constructed on the basis of the one-electron transition density matrix obtained using linear-response TDDFT to study excited states properties of four crystallized tetramolecular quadruplexes that chelate alkaline earth cations (Ca2+, Sr2+ and Ba2+). Here, we show that alkaline earth cations situated at adjacent vacancies promote existence of the nucleobase-metal charge separation (CS) states, contrary to the structures with cations that occupy every second available vacancy. We argued that stabilization of these CS states is due to the strong electric field that stabilizes d orbitals of the cations which accept an excited-electron. Moreover, CS content is increased and redshifted below the first bright transition when number of the chelated cations is increased. Hydration effects stabilized CS states and increased their relative content. We also identified electron detachment states in the broad energy range for the Ca2+ containing system. These findings are valuable for understanding and development of the novel nanostructures based on the quadruplex scaffold with adjustable optical properties.

Electrostatics and water occlusion regulate covalently-bound flavin mononucleotide cofactors of Vibrio cholerae respiratory complex NQR.

Proteins like NADH:ubiquinone oxidoreductase (NQR), an essential enzyme and ion pump in the physiology of several pathogenic bacteria, tightly regulate the redox properties of their cofactors. Although flavin mononucleotide (FMN) is fully reduced in aqueous solution, FMN in subunits B and C of NQR exclusively undergo one-electron transitions during its catalytic cycle. Here, we perform ab initio calculations and molecular dynamics simulations to elucidate the mechanisms that regulate the redox state of FMN in NQR. QM/MM calculations show that binding site electrostatics disfavor anionic forms of FMNH2 , but permit a neutral form of the fully reduced flavin. The potential energy surface is unaffected by covalent bonding between FMN and threonine. Molecular dynamics simulations show that the FMN binding sites are inaccessible by water, suggesting that further reductions of the cofactors are limited or prohibited by the availability of water and other proton donors. These findings provide a deeper understanding of the mechanisms used by NQR to regulate electron transfer through the cofactors and perform its physiologic role. They also provide the first, to our knowledge, evidence of the simple concept that proteins regulate flavin redox states via water occlusion.

How the Size and Density of Charge-Transfer Excitons Depend on Heterojunction’s Architecture

We have characterized the size, intensity, density, and distribution of charge-transfer (CT) excitons as a function of the acceptor–donor architecture of prototypical organic interfaces. This characterization was done by computational analysis of 17 models of varying numbers, positions, and orientations of the donor and acceptor molecules. The models’ building blocks were phenyl-C61-butyric acid methyl ester (PCBM) fullerene acceptors and dual-band donor polymers composed of thiophene, benzothiadiazole, and benzotriazole subunits. The electronic structure of the donor–acceptor complexes was computed with the time-dependent long-range-corrected density-functional tight-binding method and analyzed with the fragment-based one-electron transition density matrix. In all models, the complexes with edge-on orientation have denser spectra of low-energy CT states lying below the absorption bands compared to the complexes with face-on orientation. This CT-state distribution in edge-on complexes provides a gate to efficiently populate cold CT excitons. Moreover, the cold CT excitons have a higher degree of charge separation in the edge-on than in the face-on complexes. The CT amount and the CT exciton size generally increase with the energy of the CT states, although the electron remains localized on a single molecule in cold CT states. Delocalization over two PCBM molecules was observed for high-energy CT states. The exciton size also depends on the orientation. Larger excitons are produced by the delocalization of the electrons perpendicularly to the interface. When the delocalization is parallel, the smaller electron–hole distances yield moderately sized CT excitons.

Photoelectron Spectra of Gd2O2- and Nonmonotonic Photon-Energy-Dependent Variations in Populations of Close-Lying Neutral States.

Photoelectron spectra of Gd2O2- obtained with photon energies ranging from 2.033 to 3.495 eV exhibit numerous close-lying neutral states with photon-energy-dependent relative intensities. Transitions to these states, which fall within the electron binding energy window of 0.9 and 1.6 eV, are attributed to one- or two-electron transitions to the ground and low-lying excited neutral states. An additional, similar manifold of electronic states is observed in an electron binding energy window of 2.1-2.8 eV, which cannot be assigned to any simple one-electron transitions. This study expands on previous work on the Sm2O- triatomic, which has a more complex electronic structure because of the 4f6 subshell occupancy of each Sm center. Because of the simpler electronic structure from the half-filled 4f7 subshell occupancy in Gd2O2 and Gd2O2-, the numerous close-lying transitions observed in the spectra are better resolved, allowing a more detailed view of the changes in relative intensities of individual transitions with photon energy. With supporting calculations on numerous possible close-lying electronic states, we suggest a potential description of the strong photoelectron-valence electron interactions that may result in the photon-energy-dependent changes in the observed spectra.

Excitation energies expressed as orbital energies of Kohn-Sham density functional theory with long-range corrected functionals.

A new simple and conceptual theoretical scheme is proposed for estimating one-electron excitation energies using Kohn-Sham (KS) solutions. One-electron transitions that are dominated by the promotion from one initially occupied orbital to one unoccupied orbital of a molecular system can be expressed in a two-step process, ionization, and electron attachment. KS with long-range corrected (LC) functionals satisfies Janak's theorem and LC total energy varies almost linearly as a function of its fractional occupation number between the integer electron points. Thus, LC reproduces ionization energies (IPs) and electron affinities (EAs) with high accuracy and one-electron excitation energies are expressed as the difference between the occupied orbital energy of a neutral molecule and the corresponding unoccupied orbital energy of its cation. Two such expressions can be used, with one employing the orbital energies for the neutral and cationic systems, while the other utilizes orbital energies of just the cation. Because the EA of a molecule is the IP of its anion, if we utilize this identity, the two expressions coincide and give the same excitation energies. Reasonable results are obtained for valence and core excitations using only orbital energies.

Ionization and electron capture in Li3+ + H2O collisions

SynopsisTotal cross sections for ionization and electron capture in Li3+ + H2O collisions are calculated at energies 20 ⩽ E ⩽ 750 keV/u. Three different models are applied: the classical trajectory Monte Carlo method, an expansion in terms of asymptotic-frozen-molecular-orbitals and the numerical solution of the time dependent Schrödinger equation. One-electron transition probabilities from the three methods show general good agreement and with those from the calculation of Luna et al Phys Rev. A 93, 052705.

Biocompatible compositions based on chitosan and copolymer (lactide–titanium oxide) for engineering of tissue substitutes for wound healing

The optically transparent biocompatible, biodegradable, wound-healing materials were obtained based on chitosan and poly(lactide–titanium oxide). The optical transmittance of these films was more than 70% in the visible light region. The decreasing of the films transmittance by ~ 5–10% was observed under its UV irradiation as a result of the one-electron transition Ti4+ + e− ⇄ Ti3+. The tensile strength of the samples was up to 117 MPa. The investigation of the materials biocompatibility on experimental animals demonstrated the positive blood parameters and the absence of the inflammation process in the animals’ organisms, allergic reactions and stress after implantation of film and non-toxicity of the composite. The materials can be bioutilized and are biodegradable. The fibroblast cells (hTERT BY-5ta) adhesion and proliferation on the films surface were demonstrated in vitro. The films exhibited the UV-induced antibacterial properties.

Synthesis of the Materials with a Switchable Wettability Based on Photosensitive Terpolymers Containing Poly(Titanium Oxide)

In this paper, we synthesized the organic–inorganic terpolymers based on poly(titanium oxide) ((≡TiO)n), hydroxyethyl methacrylate (HEMA), and various organic vinyl monomers (styrene, vinylbutyl ether, butyl methacrylate, isobornyl acrylate, 2-ethylhexyl acrylate, acrylonitrile, and methyl methacrylate) by using a polycondensation-polymerization method. Independent of the composition, the terpolymers are optically transparent in the visible spectral range (transparency is T ~ 90%). Using X-ray phase analysis, the terpolymers were established to have an amorphous structure, and the poly(titanium oxide) chains are self-organized into anatase-type nanoclusters. The dimming of terpolymers occurred under the effect of UV irradiation as a result of the reversible photochromic one-electron transition Ti4+ + $$\bar {e}$$ $$ \rightleftarrows $$ Ti3+. The change in the nature of monomers of terpolymers and the molar ratio of their components allows controlling the rates of both direct and reverse reactions and also leads to the modification of the strength characteristics of the materials. The biggest change in light transmission under UV exposure for 180 min is observed in terpolymers with styrene, butyl methacrylate, and acrylonitrile monomers at a molar ratio of the components [(≡TiO)n] : [HEMA] : [M] = 1 : 5 : 1; the corresponding changes are 55, 70, and 60%. Higher rates of the Ti3+ → Ti4+ + $$\bar {e}$$ reaction (clearing) are observed for materials of the indicated composition, as well as for terpolymers with vinylbutyl ether monomers. The effect of photoinduced superhydrophilicity was found for terpolymers; i.e., a ~60° reduction in the contact angle of their surface under UV exposure was observed.

A vanadate-based white light emitting luminescent material for temperature sensing.

Calcium magnesium vanadate-europium vanadate powders with a homogeneous distribution have been prepared by a sol–gel method followed by a sintering process. The as-prepared powders show both broadband emission around 520 nm and sharp peak emission at 617 nm under UV light excitation, which are ascribed to the one-electron charge transfer transition in the VO4 tetrahedra and the typical 5D0 → 7F2 transition of Eu3+. Energy transfer occurs between the vanadate and Eu ions. The emission color of the products can be tuned by controlling the Eu concentration and temperature. White light emission can be obtained at the Eu concentration of 15% and at room temperature. The temperature related luminescence properties have been studied for the sample with 15 mol% Eu. The intensity ratio between the broadband emission (due to VO4 tetrahedra) and the sharp peak emission (due to Eu3+ ions) decreases as the temperature increases in a linear relationship. The relative sensitivity (SR) of this luminescent temperature sensor has been calculated and a maximum has been gained at 455 K with the value equal to 1.83% K−1.

BREMS: A program for calculating spectra and angular distributions of bremsstrahlung at electron energies less than 3 MeV

This work describes a program that implements the method developed by Tseng and Pratt in the 1970s for exact screened calculations of atomic-field bremsstrahlung. The calculation method is based on the relativistic partial-wave formulation describing interaction of the incident electron with an arbitrary central potential of the neutral target atom. The set of Fortran-90 codes BREMS has been written with the aim of creating a comprehensive library of spectra and shape functions of unpolarized atomic-field bremsstrahlung for all chemical elements and for the approximate range of the incident electron energy from 10 eV to 3 MeV on a dense energy grid, using the best theory available. Several examples of using this software and its verification against the published data calculated using the same theory are provided. BREMS can be run on modern personal computers, with processing times from several minutes to several hours, depending on the user-specified values of the atomic number, incident electron energy, and photon energy, as well as on the accuracy requested. Program summaryProgram title: BREMSProgram Files doi: http://dx.doi.org/10.17632/mvd57skzd9.1Licensing provisions: GNU General Public License v3Programming language: Fortran 90Nature of problem: The atom is described as a static spherically symmetrical charge distribution of infinite mass. The elementary process of bremsstrahlung is described as a one-electron transition in the field of the neutral atom with a point-like nucleus. The atom is described by a central potential. The singly and doubly differential cross sections of bremsstrahlung are calculated using the relativistic partial wave formulation, which is currently the best theory for calculating the cross sections of unpolarized atomic-field bremsstrahlung.Solution method: The set of codes BREMS solves the Dirac equation with one of three possible types of the interaction potential: 1) the point-Coulomb potential (unscreened nucleus), 2) the Thomas–Fermi–Czavinsky potential, 3) the Kohn–Sham potential. It is also possible to use an arbitrary interaction potential specified by the user in tabular format. The wave functions are expanded in partial wave series and the radial integrals are calculated numerically. The photon energy spectrum and the angular distribution of bremsstrahlung photons are expressed in terms of the mentioned radial matrix elements, resulting in an infinite series, which must be truncated at a predefined value of the absolute value of the quantum number κ. A partial-wave interpolation method is applied in order to reduce the number of radial integrals that have to be calculated. The truncation error is decreased by extrapolation of the differential cross sections as functions of the number of terms retained in the truncated series.Additional comments: (i) Restrictions: Although there is no explicit limitation on the incident electron energy in the code of BREMS, it is currently applicable only at electron energies that are not greater than approximately 3 MeV. At the low-energy end of the spectrum, the single-electron approximation may not be sufficiently accurate due to an increased importance of the many-electron effects at electron energies of the order of 100 eV or lower.(ii) Unusual features: Multiple precision (MP) and quadruple precision (QP) arithmetic are used in several stages of the calculation in order to eliminate the rounding errors due to strong cancellation of terms in the alternating series. The capability of the MP system to change the precision level at runtime is used to optimize the precision level automatically and thus to decrease the computation time.

Carbo-biphenyls and Carbo-terphenyls: Oligo(phenylene ethynylene) Ring Carbo-mers.

Ring carbo-mers of oligo(phenylene ethynylene)s (OPEn, n=0-2), made of C2 -catenated C18 carbo-benzene rings, have been synthesized and characterized by NMR and UV-vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT-optimized structures show that the C18 rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo-terphenyls (n=2) are reversibly reduced at ca. -0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo-benzene (-0.83 V/SCE), thus revealing efficient inter-ring π-conjugation. An accurate linear fit of E1/2red1 vs. the DFT LUMO energy suggests a notably higher value (-0.30 V/SCE) for a carbo-quaterphenyl congener (n=3). Increase with n of the effective π-conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT-calculated excited states, one of them restricting to a HOMO→LUMO main one-electron transition.

Myeloperoxidase-catalyzed oxidation of cyanide to cyanate: A potential carbamylation route involved in the formation of atherosclerotic plaques?

Protein carbamylation by cyanate is a post-translational modification associated with several (patho)physiological conditions, including cardiovascular disorders. However, the biochemical pathways leading to protein carbamylation are incompletely characterized. This work demonstrates that the heme protein myeloperoxidase (MPO), which is secreted at high concentrations at inflammatory sites from stimulated neutrophils and monocytes, is able to catalyze the two-electron oxidation of cyanide to cyanate and promote the carbamylation of taurine, lysine, and low-density lipoproteins. We probed the role of cyanide as both electron donor and low-spin ligand by pre-steady-state and steady-state kinetic analyses and analyzed reaction products by MS. Moreover, we present two further pathways of carbamylation that involve reaction products of MPO, namely oxidation of cyanide by hypochlorous acid and reaction of thiocyanate with chloramines. Finally, using an in vivo approach with mice on a high-fat diet and carrying the human MPO gene, we found that during chronic exposure to cyanide, mimicking exposure to pollution and smoking, MPO promotes protein-bound accumulation of carbamyllysine (homocitrulline) in atheroma plaque, demonstrating a link between cyanide exposure and atheroma. In summary, our findings indicate that cyanide is a substrate for MPO and suggest an additional pathway for in vivo cyanate formation and protein carbamylation that involves MPO either directly or via its reaction products hypochlorous acid or chloramines. They also suggest that chronic cyanide exposure could promote the accumulation of carbamylated proteins in atherosclerotic plaques.