Electron Spin Density Distribution Research Articles

Tandem Electrochemical Oxidation of Polycyclic Aromatic Amines Towards Carbazoles and Phenazine-based Aza-helicenes.

Experimental and theoretical study of the regioselectivity and mechanism of polycyclic aromatic amine (PAA) electrochemical oxidation is important for designing nitrogen doped large π-conjugated functional molecules. Herein, we used binary-, ternary-, and quaternary-fused PAAs as electro-oxidative reaction substrates to investigate the yield changes of carbazole and phenazine based aza-helicene other than oligomers, which were obtained through pyrrole and pyrazine annulation pathways. Combined with the restrained electrostatic potential (RESP) and steric hindrance factor analysis of the substrate, the electron spin density distribution of free radical resonance hybrid and the spin population analysis of the atoms in the structure of each free radical tautomer indicate that the degree of delocalized dispersion of N free radical and the resulting change in the spin density distribution of C free radical tautomers determine the reaction regioselectivity. The potential charge of the K-region, Bay-region, and L-region adjacent to the C(α)-C(β1) bond is higher than that of other regions within the molecule, and the charge in these high RESP regions tends to delocalize more strongly toward electron-deficient N free radicals. Thus, the activity of N-C(α)-C(β1) region is increased, which supports the proposed free radical addition and free radical coupling mechanism for the electro-oxidative reaction of PAA.

Edge engineering of carbon nitride for enhanced sacrificial agent-free photocatalytic H2O2 evolution

Photocatalytic oxygen reduction reaction (ORR) offers a green and sustainable route to produce H2O2. Electronic structure modulation and spin density distribution of the photocatalyst are crucial to the ORR activity. Heteroatom sulfur-doped and functionalized carbon nitride with abundant edge active sites is prepared for photocatalytic H2O2 generation. The as-obtained catalyst provides a faster H2O2 production rate of 416.7 μmol g−1h−1 under visible light (λ ≥ 420 nm) without using sacrificial agents. The enhanced photocatalytic performance is attributed to extended light absorption and improved carrier separation ability for the modulation of the electronic structure. The experimental and theoretic study demonstrates that sulfur doping and terminal group modulation show higher localized spin density, thus promoting the O2 absorption and the higher spin density and charge redistribution via edge active sites make the strong interaction between *OOH and the catalyst, which lowers the energy barrier of *OOH formation, thus facilitating the H2O2 generation.

Anthracene-bridged diarylamine isomers with identical amine–amine distances: Syntheses, (spectro)electrochemistry and theoretical calculations

Two athracene-bridged diarylamine isomers 1 and 2 were constructed and fully characterized. The electrochemical results showed that ΔE of compound 2 was significantly higher than that of 1, which indicated that the mixed-valence compound 2 + had higher thermodynamic stability than 1+ . Spectroelectrochemistry reflected that the NIR absorption of 2 + exhibited a significant blue shift compared with reported compound 1 +. The calculated electronic coupling constant and spin density distribution indicated that 2 + had larger charge delocalization than 1 + under the identical amine-amine distances. The TDDFT predicted β-HOSO→β-LUSO transition from 2 + which further confirmed the existence of N→N·+ IVCT transition.

The intermolecular interactions of ammonia with chlorine and bromine oxides: a theoretical study.

The role of halogen oxides is substantial in the stratosphere, especially for ozone depletion. It is important to make clear the interaction of halogen oxides with other gaseous molecules. This work performed quantum chemical calculations to investigate the intermolecular interactions between XmOn (X = Cl or Br, m, n = 1 or 2) and ammonia. The chlorine and bromine oxides selected in this paper include typical halogen oxides which can influence the atmospheric processes. For each complex, two different types of interactions, halogen and hydrogen bonds were identified. A π-hole interaction was also found in the XO2···NH3 complex. The interaction energy implies that the strength of the halogen bond is far more stronger than the hydrogen bond. A prominent difference exists between the halogen oxides of singlet or doublet state, which can be ascribed to the electron spin density distribution. The nature of the intermolecular interactions was identified by an independent gradient model based on Hirshfeld partition (IGMH) analysis. Symmetry-adapted perturbation theory (SAPT) calculation indicates that electrostatic interaction dominates the halogen-bonded complex, and hydrogen bond is driven by electrostatic interaction and dispersion.

Influence of Polar Mutations on the Electronic and Structural Properties of QA-· in Bacterial Reaction Centers.

Reaction centers from Rhodobacter sphaeroides with residue M265 mutated from isoleucine to threonine, serine, and asparagine (M265IT, M265IS, and M265IN, respectively) in the QA-· state are studied by high-resolution electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance spectroscopy methods to investigate the structural characteristics of these mutants influencing the redox properties of the QA site. All three mutants decrease the redox midpoint potential (Em) of QA by ∼0.1 V, yet the mechanism for this drop in Em is unclear. In this work, we examine (i) the hydrogen bonding interactions between QA-· and residues histidine M219 and alanine M260, (ii) the electron spin density distribution of the semiquinone, and (iii) the orientations of the ubiquinone methoxy substituents. 13C measurements show no significant contribution of methoxy dihedral angles to the observed decrease in Em for the QA mutants. Instead, 14N three-pulse ESEEM data suggest that electrostatic or hydrogen bond formation between the mutated M265 side chain and His-M219 Nδ may be involved in the observed lowering of the QA midpoint potential. For mutant M265IN, analysis of the proton hyperfine couplings reveals a weakened hydrogen bond network, resulting in an altered QA-· spin density distribution. The magnetic resonance study presented here is most consistent with an electrostatic or structural perturbation of the His-M219 Nδ hydrogen bond in these mutants as a mechanism for the ∼0.1 V decrease in QA Em.

Two-dimensional HYSCORE spectroscopy reveals a histidine imidazole as the axial ligand to Chl3A in the M688HPsaA genetic variant of Photosystem I

Recent studies on Photosystem I (PS I) have shown that the six core chlorophyll a molecules are highly coupled, allowing for efficient creation and stabilization of the charge-separated state. One area of particular interest is the identity and function of the primary acceptor, A0, as the factors that influence its ultrafast processes and redox properties are not yet fully elucidated. It was recently shown that A0 exists as a dimer of the closely-spaced Chl2/Chl3 molecules wherein the reduced A0− state has an asymmetric distribution of electron spin density that favors Chl3. Previous experimental work in which this ligand was changed to a hard base (histidine, M688HPsaA) revealed severely impacted electron transfer processes at both the A0 and A1 acceptors; molecular dynamics simulations further suggested two distinct conformations of PS I in which the His residue coordinates and forms a hydrogen bond to the A0 and A1 cofactors, respectively. In this study, we have applied 2D HYSCORE spectroscopy in conjunction with molecular dynamics simulations and density functional theory calculations to the study of the M688HPsaA variant. Analysis of the hyperfine parameters demonstrates that the His imidazole serves as the axial ligand to the central Mg2+ ion in Chl3A in the M688HPsaA variant. Although the change in ligand identity does not alter delocalization of electron density over the Chl2/Chl3 dimer, a small shift in the asymmetry of delocalization, coupled with the electron withdrawing properties of the ligand, most likely accounts for the inhibition of forward electron transfer in the His-ligated conformation.

Electronic Structure of the Primary Electron Donor P700+• in Photosystem I Studied by Multifrequency HYSCORE Spectroscopy at X- and Q-Band.

The primary electron donor P700 of the photosystem I (PSI) is a heterodimer consisting of two chlorophyll molecules. A series of electron-transfer events immediately following the initial light excitation leads to a stabilization of the positive charge by its cation radical form, P700+•. The electronic structure of P700+• and, in particular, its asymmetry with respect to the two chlorophyll monomers is of fundamental interest and is not fully understood up to this date. Here, we apply multifrequency X- (9 GHz) and Q-band (35 GHz) hyperfine sublevel correlation (HYSCORE) spectroscopy to investigate the electron spin density distribution in the cation radical P700+• of PSI from a thermophilic cyanobacterium Thermosynechococcus elongatus. Six 14N and two 1H distinct nuclei have been resolved in the HYSCORE spectra and parameters of the corresponding nuclear hyperfine and quadrupolar hyperfine interactions were obtained by combining the analysis of HYSCORE spectral features with direct numerical simulations. Based on a close similarity of the nuclear quadrupole tensor parameters, all of the resolved 14N nuclei were assigned to six out of total eight available pyrrole ring nitrogen atoms (i.e., four in each of the chlorophylls), providing direct evidence of spin density delocalization over the both monomers in the heterodimer. Using the obtained experimental values of the 14N electron-nuclear hyperfine interaction parameters, the upper limit of the electron spin density asymmetry parameter is estimated as RA/Bupper = 7.7 ± 0.5, while a tentative assignment of 14N observed in the HYSCORE spectra yields RB/A = 3.1 ± 0.5.

Electron Spin Current and Spin-Dependent Galvanomagnetic Phenomena in Metals

A system of coupled kinetic equations for the electron density and spin density distribution functions has been obtained based on the quantum kinetic equation. The collision integrals in the kinetic equations are written for an arbitrary scattering potential, taking into account the spin-orbit interaction of conduction electrons with scattering defects. As applied to the spin system of conduction electrons, the idea of a “reduced” description of transport phenomena is developed. The description of the system in terms of distribution functions depending on the quasi-momentum of electrons reduces to a description in terms of macroscopic mean quantities: electron density, spin density, electron flux, and spin current.

Oxidized divinyl oligoacene-bridged diruthenium complexes: bridged localized radical characters and reduced aromaticity in bridge cores.

A series of bimetallic ruthenium vinyl complexes 1-5 bridged by oligoacenes were synthesized and characterized in this study. Comparative cyclic voltammetry results from 1-5 indicated that the first oxidation potential decreased gradually with the extension of conjugate ligands. Upon oxidation to singly oxidized species 1+-5+, rather small ν(CO) changes in the infrared (IR) spectra and the characteristic bands of metal-to-ligand charge transfer absorptions in the near IR (NIR) region predicted via time-dependent DFT calculations suggested that strong bridged ligands participate in redox processes. NIR absorptions were not observed in complexes 4+ and 5+ possibly because of instability in their twisted and noncoplanar geometry. Electron paramagnetic resonance results and spin density distribution demonstrated that the bridged localized degrees of 1+-5+ successively increased with the extension of oligoacene from benzene to tetracene. Further comparative analysis of neutral molecules and monocations to the aromaticity and π-electron density of bridge cores indicated a step-by-step transformation process from an aromatic to quinoidal radical upon oxidation.

Elucidating the Role of Zinc-Bacteriochlorophyll A' in the Primary Photochemistry of Chloroacidobacterium thermophilum Reaction Centers

Chloracidobacterium (Cab.) thermophilum is a microaerophilic, chlorophototrophic species in the phylum of Acidobacteria that contains homodimeric Type I reaction centers (RC). Photosynthetic RCs in bacteria convert light energy into chemical energy using (bacterio)chlorophyll ((B)Chl) cofactors. In addition to participating in energy transfer in the light-harvesting complexes, the (B)Chl molecules also act as the primary electron donor and primary electron acceptor in Type I RCs. Previously, reversed-phase high-performance liquid chromatography of Cab. thermophilum RCs revealed the presence of (B)Chl a, Chl a, and Zn2+-(B)Chl a' in a ratio of about 12.8:8.0:2.0. Moreover, it was shown that Cab. thermophilum uses chlorosomes and FMO for light harvesting, and synthesizes (B)Chl c in addition to (B)Chl a, Chl a, and Zn2+-(B)Chl a'. It has been demonstrated that Chl a is the primary electron acceptor in Cab. thermophilum RC and since Zn2+-(B)Chl a' is only present in the RC, it was suggested that it could function as the primary electron donor. In this study, we utilize high-resolution, two-dimensional (2D) 14N and 67Zn hyperfine sublevel correlation (HYSCORE) spectroscopy to determine the electronic and geometric structure of the primary donor cation, P840+, in the Cab. thermophilum RC. Additionally, we perform density functional theory (DFT) calculations to determine the electron spin-density distribution and electron-nuclear hyperfine coupling parameters of P840+ for comparison with the parameters that are obtained from the experimental 2D HYSCORE measurements. Our studies indicate that that the primary donor, P840, of the Cab. RC is comprised of Zn2+-sBChl a' molecules.

Spin density encodes intramolecular singlet exciton fission in pentacene dimers

The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.

Influence of the Electrochemical Properties of the Bacteriochlorophyll Dimer on Triplet Energy-Transfer Dynamics in Bacterial Reaction Centers.

Energetics, protein dynamics, and electronic coupling are the key factors in controlling both electron and energy transfer in photosynthetic bacterial reaction centers (RCs). Here, we examine the rates and mechanistic pathways of the P+HA- radical-pair charge recombination, triplet state formation, and subsequent triplet energy transfer from the triplet state of the bacteriochlorophyll dimer (P) to the carotenoid in a series of mutant RCs (L131LH + M160LH (D1), L131LH + M197FH (D2), and L131LH + M160LH + M197FH (T1)) of Rhodobacter sphaeroides. In these mutants, the electronic structure of P is perturbed and the P/P+ midpoint potential is systematically increased due to addition of hydrogen bonds between P and the introduced residues. High-resolution, broad-band, transient absorption spectroscopy on the femtosecond to microsecond timescale shows that the charge recombination rate increases and the triplet energy transfer rate decreases in these mutants relative to the wild type (WT). The increase of the charge recombination rate is correlated to the increase in the energy level of P+HA- and the increase in the P/P+ midpoint potential. On the other hand, the decrease in rate of triplet energy transfer in the mutants can be explained in terms of a lower energy of 3P and a shift in the electron spin density distribution in the bacteriochlorophylls of P. The triplet energy-transfer rate follows the order of WT > L131LH + M197FH > L131LH + M160LH > L131LH + M160LH + M197FH, both at room temperature and at 77 K. A pronounced temperature dependence of the rate is observed for all of the RC samples. The activation energy associated to this process is increased in the mutants relative to WT, consistent with a lower 3P energy due to the addition of hydrogen bonds between P and the introduced residues.

Evidence for a [17] π-Electronic Full-Fledged Non-innocent Gallium(III)-Corrole Radical.

One-electron oxidation of a GaIII -corrole with N(4-BrC6 H4 )3 SbCl6 resulted in an air-stable non-innocent GaIII -corrole radical. The single-crystal X-ray crystallography of the 2,17-bis-formyl-5,10,15-tris(pentafluorophenyl)corrolato gallium(III)(chloride) radical ([3-Cl]. ) revealed delocalization of the unpaired electron, which was further confirmed by electron spin resonance (ESR) spectroscopy and spin density distribution plot. In addition, the nucleus-independent chemical shift (NICS), anisotropy-induced current density (AICD) and harmonic oscillator model of aromaticity (HOMA) supported a [17] π-electron-conjugated (or antiaromatic) radical.

Determination of the Complete Spin Density Distribution in 13C-Labeled Protein-Bound Radical Intermediates Using Advanced 2D Electron Paramagnetic Resonance Spectroscopy and Density Functional Theory.

Determining the complete electron spin density distribution for protein-bound radicals, even with advanced pulsed electron paramagnetic resonance (EPR) methods, is a formidable task. Here we present a strategy to overcome this problem combining multifrequency HYSCORE and ENDOR measurements on site-specifically 13C-labeled samples with DFT calculations on model systems. As a demonstration of this approach, pulsed EPR experiments are performed on the primary QA and secondary QB ubisemiquinones of the photosynthetic reaction center from Rhodobacter sphaeroides 13C-labeled at the ring and tail positions. Despite the large number of nuclei interacting with the unpaired electron in these samples, two-dimensional X- and Q-band HYSCORE and orientation selective Q-band ENDOR resolve and allow for a characterization of the eight expected 13C resonances from significantly different hyperfine tensors for both semiquinones. From these results we construct, for the first time, the most complete experimentally determined maps of the s- and pπ-orbital spin density distributions for any protein organic cofactor radical to date. This work lays a foundation for understanding the relationship between the electronic structure of semiquinones and their functional properties, and introduces new techniques for mapping out the spin density distribution that are readily applicable to other systems.

Carbon-Based Oxamate Cobalt(III) Complexes as Bioenzyme Mimics for Contaminant Elimination in High Backgrounds of Complicated Constituents.

Complex wastewater with massive components is now a serious environmental issue facing humanity. Selective removal of low-concentration contaminants in mixed constituents holds great promise for increasing water supplies. Bioenzymes like horseradish peroxidase exhibit oxidizing power and selectivity. Here, we manufactured its mimic through immobilizing non-heme oxamate anionic cobalt(III) complex ([CoIII(opba)]−, opba = o-phenylenebis(oxamate)) onto pyridine (Py) modified multiwalled carbon nanotubes ([CoIII(opba)]−-Py-MWCNTs, MWCNTs = multiwalled carbon nanotubes), where MWCNTs captured substrates and Py functioned as the fifth ligand. We chose typical azo dye (C.I. Acid Red 1) and antibiotic (ciprofloxacin) as model substrates. Without •OH, this catalyst could detoxify target micropollutants efficiently at pH from 8 to 11. It also remained efficient in repetitive tests, and the final products were non-poisonous OH-containing acids. Combined with radical scavenger tests and electron paramagnetic resonance result, we speculated that high-valent cobalt-oxo active species and oxygen atom transfer reaction dominated in the reaction pathway. According to density functional theory calculations, the electron spin density distribution order showed that electron-withdrawing ligand was beneficial for inward pulling the excess electron and lowering the corresponding energy levels, achieving an electrophilic-attack enhancement of the catalyst. With target removal property and recyclability, this catalyst is prospective in water detoxication.

Time-Resolved Electron Paramagnetic Resonance and Theoretical Investigations of Metal-Free Room-Temperature Triplet Emitters.

Utilization of triplets is important for preparing organic light-emitting diodes with high efficiency. Very recently, both electrophosphorescence and electrofluorescence could be observed at room temperature for thienyl-substituted phenazines without any heavy metals ( Ratzke et al. J. Phys. Chem. Lett. , 2016 , 7 , 4802 ). It was found that the phosphorescence efficiency depends on the orientation of fused thiophenes. In this work, the thienyl-substituted phenazines are investigated in more detail by time-resolved electron paramagnetic resonance (EPR) and quantum chemical calculations. Spin dynamics, zero-field splitting constants, and electron-spin structures of the excited triplet states for the metal-free room-temperature triplet emitters are correlated with phosphorescence efficiency. Complete active space self-consistent field (CASSCF) calculations clearly show that the electron spin density distributions of the first excited triplet states are strongly affected by the molecular geometry. For the phosphorescent molecules, the electron spins are localized on the phenazine unit, in which the sulfur atom of the fused thiophene points upward. The electron spins are delocalized onto the thiophene unit just by changing the orientation of the fused thiophenes from upward to downward, resulting in the suppression of phosphorescence. Time-resolved EPR measurements and time-dependent density functional theory (TD-DFT) calculations demonstrate that the electron spins delocalized onto the thiophene unit lead to the acceleration of nonradiative decays, in conjunction with the narrowing of the singlet-triplet energy gap.

Formaldehyde adsorption on graphane

Using first principles calculations, we have studied the adsorption of a formaldehyde molecule on a hydrogenated graphene substrate, by a free radical initiated reaction. This kind of reaction begins at a hydrogen vacancy on the graphane layer, in which, the oxygen atom of the formaldehyde molecule attaches. Then, the nearest hydrogen atom is abstracted by the carbon atom of the formaldehyde, forming a stable molecule, and leaving behind a new dangling bond on the graphane substrate. Our calculations show that the reaction has an energy barrier of the order of 0.56eV, larger than in the case of silicane, indicating that graphane is not a very good substrate for formaldehyde adsorption. Analysis of the electronic structure and spin density distributions help us to understand the proposed reaction.

Probing the Electronic Structure of Bacteriochlorophyll Radical Ions-A Theoretical Study of the Effect of Substituents on Hyperfine Parameters.

In reaction centers (RCs) of photosynthesis, a light-induced charge separation takes place creating radical cations and anions of the participating cofactors. In photosynthetic bacteria, different bacteriochlorophylls (BChl) are involved in this process. Information about the electronic structure of the BChl radical cations and anions can be obtained by measuring the electron spin density distribution via the electron-nuclear hyperfine interaction using EPR and ENDOR techniques. In this communication, we report isotropic hyperfine coupling constants (hfcs) of the BChl b and g radical cations and anions, calculated by density functional theory, and compare them with the more common radical ions of BChl a and with available experimental data. The observed differences in the computed hyperfine data are discussed in view of a possible distinction between these species by EPR/ENDOR methods. In addition, 14 N nuclear quadrupole coupling constants (nqcs) computed for BChl a, b, g, and also for Chl a in their charge neutral, radical cation and radical anion states are presented. These nqcs are compared with experimental values obtained by ESEEM spectroscopy on several different radical ions.

Combined W-Band Light-Induced ESR/ENDOR/TRIPLE and DFT Study of PPVtype/PC61BM Ion Radicals

The combined W-band pulsed light-induced multiresonance ESR/ENDOR/TRIPLE technique and DFT calculation have been applied to study the 1H hyperfine coupling and electron spin density distribution on PPV cation and PC61BM anion radicals in (PPVs):PC61BM blends. The sign of calculated electron spin density populations of protons delocalized on polymer and fullerene side chain has been confirmed experimentally by means of TRIPLE in frozen solution at the temperature of 20 K.

Interstate Crossing-Induced Chemiexcitation Mechanism as the Basis for Imidazopyrazinone Bioluminescence

Imidazopyrazinone dioxetanone is a central molecule in bioluminescence, as it is the scaffold responsible for light emission in many marine species. Herein, we have theoretically studied the thermolysis of its neutral and anionic forms. Our results allowed us to explain imidazopyrazinone bioluminescence with the Interstate Crossing-Induced Chemiexcitation (ICIC) mechanism, and explain why neutral forms emit light efficiently to the contrary of anionic ones. Both forms decompose via a stepwise-biradical pathway, but the biradical is formed either via an electron transfer (anion) or due to homolytic bond cleavage (neutral species). Absence of electron transfer leads to an entropic trap, a flat region where infinite possibilities for chemiexcitation exist. However, the resulting products can be in either triplet or singlet excited states. The triplet to singlet ratio is determined by the electron spin density distribution, which controls the rates of intersystem crossing.