Two-step Electron Transfer Process Research Articles

Primary charge separation in native and plant pheophytin a-modified reaction centers of Chloroflexus aurantiacus: Ultrafast transient absorption measurements at low temperature

Ultrafast transient absorption (TA) spectroscopy was used to study electron transfer (ET) at 100 K in native (as isolated) reaction centers (RCs) of the green filamentous photosynthetic bacterium Chloroflexus (Cfl.) aurantiacus. The rise and decay of the 1028 nm anion absorption band of the monomeric bacteriochlorophyll a molecule at the BA binding site were monitored as indicators of the formation and decay of the P+BA− state, respectively (P is the primary electron donor, a dimer of bacteriochlorophyll a molecules). Global analysis of the TA data indicated the presence of at least two populations of the P⁎ excited state, which decay by distinct means, forming the state P+HA− (HA is a photochemically active bacteriopheophytin a molecule). In one population (~65 %), P⁎ decays in ~2 ps with the formation of P+HA− via a short-lived P+BA− intermediate in a two-step ET process P⁎ → P+BA−→ P+HA−. In another population (~35 %), P⁎ decays in ~20 ps to form P+HA− via a superexchange mechanism without producing measurable amounts of P+BA−. Similar TA measurements performed on chemically modified RCs of Cfl. aurantiacus containing plant pheophytin a at the HA binding site also showed the presence of two P⁎ populations (~2 and ~20 ps), with P⁎ decaying through P+BA− only in the ~2 ps population. At 100 K, the quantum yield of primary charge separation in native RCs is determined to be close to unity. The results are discussed in terms of involving a one-step P⁎ → P+HA− superexchange process as an alternative highly efficient ET pathway in Cfl. aurantiacus RCs.

Superior peroxidase mimetic activity induced by topological surface states of Weyl semimetal WTe2

Nanozymes, as promising alternatives that integrate the advantage of natural enzymes and nanomaterials are attracted enormous interest due to their low cost, environmental tolerance, high stability, as well as significant catalytic activity. Two-dimensional materials, transition metal dichalcogenides (TMDs) harbor the potential as peroxidase mimic which is attributed to the active edge sites and surface electron transfer capability. However, a challenge faced in peroxidase mimetic of TMDs is the low catalytic activity which originates from the inert structures. Here, we, for the first time, report that the typical Weyl semimetal WTe2 possesses the superior peroxidase-like performance toward H2O2, which derives from the two-step electron-pathway. We demonstrate a novel functionality of the Weyl semimetal through introducing topologically protected surface states (TSSs) for regulating the electron transfer processes. The underlying mechanism for TSSs promoting enzyme mimetic catalysis is attributed to the effective electron bath provided by the robust TSSs. Experiments and the first-principles calculations show that TSSs of WTe2 which serve as effective electron baths are directly involved in the two-step electron transfer process. In the first step, TSSs accept the electrons donated by the substrate which further enhances the substrate’s absorption. Upon H2O2 adsorption, the electrons are transferred out of TSSs and injected into the absorbed H2O2 orbitals. These findings offer a new linkage between the topological matters with TSSs and nanomaterials for enzyme mimicking.

Theory for admittance voltammetry of reversible two step electron transfer process with DC bias at rough and fractal electrodes

We developed a phenomenological theory for DC-bias dependent electrochemical impedance response and admittance voltammetry of reversible two-step electron transfer (EE) process at randomly rough and fractal electrodes. The power spectrum of roughness is used to incorporate the statistical information of surface randomness. For the finite fractal model, statistical morphological parameters of roughness, viz., fractal dimension (DH), topothesy length (ℓτ) and lower cut-off length (ℓ), significantly influence the admittance response. The magnitude of admittance increases with an increase in the roughness of fractal electrode (e.g. increase in the value of DH or ℓτ or decrease in ℓ) for both the electron transfer steps. The magnitude of admittance with the variation of DC potential at fixed temporal frequency unravels the multi-electron transfer process. The two step electron transfer process has two characteristic peaks in their plot. The enhanced roughness of electrode increases the height of the admittance peaks and the regions around it (obtained at different fixed frequencies). Our theory is validated with experimental data for the ethyl viologen system at a moderately rough Pt electrode. Finally, our current methodology for admittance voltammetry has the dual advantage of impedance as well as of voltammetry.

Engineering of reduced graphene oxide on nanosheet–g-C3N4/perylene imide heterojunction for enhanced photocatalytic redox performance

g-C3N4-based photocatalysts are recognized as promising candidates for photocatalytic purification of air and solar energy conversions; but their practical application is still limited by the sluggish charge transfer dynamic. Herein, a Z-scheme ternary heterojunction (nanosheet–g-C3N4 [NCN]/perylene imide [PI]/reduced graphene oxide [rGO], NCN/PI/rGO) was successfully constructed. For experimental comparison, NCN/rGO/PI was concurrently synthesized through different reaction sequences. In these ternary heterojunction systems, the introduction order of rGO affects the morphology structure and the interaction between phases and results in two diverse electron transfer modes which determine the different photocatalytic redox performances. The as-obtained NCN/PI/rGO Z-scheme heterostructure exhibited superior photocatalytic activity towards the photocatalytic removal of NO and generation of H2O2 under visible light irradiation. Such photocatalytic activity was about 1.58 and 1.23 times higher than those of NCN and NCN/PI, respectively, in NO removal. Such enhanced photocatalytic properties can be ascribed to the two-step electron transfer process involving the CB electrons in PI combined with the VB holes of NCN via the Z-scheme pathway (process I, PI→NCN) because PI was grown in situ on the NCN through thermal condensation polymerisation. This process enabled intimate contact between NCN and PI and a short charge-transfer distance. The residual electrons in the CB of NCN then flowed into the rGO (process II, PI→NCN→rGO). Thus, the simultaneous occurrence of two electron transfers processes I and II help improve the photocatalytic activity. Constructing NCN/PI/rGO Z-scheme heterostructures is anticipated to be an effective strategy for developing high-performance photocatalysts that facilitate the utilisation of solar energy.

Improved performance of Li/SOCl2 batteries using binuclear metal azaphthalocyanines as electrocatalysts

A total of twenty four binuclear metal azaphthalocyanine compounds in four series of M2(azaPc)2Me, M2(azaPc)2SO2, M2(azaPc)2, M2(azaPc)2CO (M=Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) were synthesized by solid phase method, which were applied for Li/SOCl2 battery as electrocatalysts to enhance its capacity and discharge voltage. In all cases, the electrochemical performances of Li/SOCl2 batteries were improved compared with controls. It is worth mentioning that Mn2(azaPc)2SO2 increased the discharge voltage to 3.3485V, rising by 0.1570V compared with that of the blank. Co2(azaPc)2CO lengthened the discharge time to 1821s, rising by 75.80%. And Co2(azaPc)2 raised battery capacity to 35.24mAh, increasing by 79.56%. Based on cyclic voltammetry measurements, the reaction mechanism was found to involve an irreversible two-step electron transfer process and a hypothesis was conjectured. All the azaphthalocyanine compounds were characterized by element analysis, IR analysis, and UV-Vis absorption.

Polyethylenimine-capped silver nanoclusters as a fluorescence probe for highly sensitive detection of folic acid through a two-step electron-transfer process.

A highly sensitive folic acid (FA) detection method based on the fluorescence quenching of polyethylenimine-capped silver nanoclusters (PEI-AgNCs) was put forward. In the sensing system, FA and PEI-AgNCs were brought into close proximity to each other by electrostatic interaction, and a two-step electron-transfer process, in which the electron was transferred from FA to AgNCs through PEI molecule, led to fluorescence quenching. The fluorescence quenching efficiency of PEI-AgNCs was linearly related to the concentration of FA over the range from 0.1 nM to 2.75 μM. Good linear correlation (R(2) = 0.9981) and a detection limit of 0.032 nM were obtained under optimum conditions. Moreover, the proposed method was used for the determination of FA in real samples with satisfactory results, and those coexistent substances could not cause any significant decrease in the fluorescence intensity of AgNCs. Therefore, the proposed research system is of practical significance and application prospects.

Influence of NH3 flow rate on pyridine-like N content and NO electrocatalytic oxidation of N-doped multiwalled carbon nanotubes

Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) have been prepared by pyrolysis of pyridine and iron phthalocyanine over an iron catalyst at 850 °C at various ammonia gas (NH3) flow rates. X-ray photoelectron spectroscopy results reveal that the pyridine-like nitrogen (N) content can be controlled by changing the flow rate of NH3, and that pyridine-like N plays an important role: it can increase the electrocatalytic activity and the rate of nitric oxide (NO) electrooxidation and decrease the activation energy of NO electrooxidation. Cyclic voltammetry results demonstrate that the N-MWCNTs sample grown with 200 mL/min NH3 flow has the maximum N content of 3.22 atomic %, and its content of pyridine-like N that is chemically active is also the highest among all the N-MWCNTs samples. Electrochemical impedance spectroscopy results indicate that two-step electron transfer process occurs at the N-MWCNT-modified electrode, and the control step is different in various potential regions. The stability of NO electrooxidation at the N-MWCNT-modified electrode is examined, and the reaction mechanism is discussed.

Hybrid complexes: Pt(ii)-terpyridine linked to various acetylide-bodipy subunits

By tailoring the chemistry of the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (bodipy) core in order to change its spectroscopic properties, we linked some modified bodipy dyes to Pt(ii)-terpyridine centers thereby preparing three novel hybrid complexes. In one of such species, two pyrene subunits are also present. The absorption spectra and luminescence properties (both at room temperature in acetonitrile solution and at 77 K in butyronitrile rigid matrix) of the new hybrid complexes and of their bodipy parent species have been investigated, and compared with those of previously studied compounds. Absorption spectra showed that the different chromophoric subunits roughly keep their own features in the multicomponent systems, indicating the supramolecular nature of the compounds. Efficient photoinduced intercomponent energy transfer takes place in the various hybrid Pt(ii)-terpyridine-bodipy compounds, by taking advantage of different mechanisms: at room temperature in fluid solution very efficient energy transfer from the (3)MLCT state(s) of the Pt residue to the lowest-lying (3)bodipy is mediated by a charge transfer state via a two-step electron transfer process. Direct energy transfer from (3)MLCT state(s) to the (3)bodipy states takes place at 77 K, most likely via the Dexter mechanism. When two pyrene fragments are grafted to the boron, cascade energy transfer from the pyrene to the (1)bodipy also occurs by the Förster mechanism. For one of the new compounds, (3)bodipy emission with a maximum at 738 nm is also observed at cryogenic temperature.

Electrochemical gate-controlled electron transport of redox-active single perylene bisimidemolecular junctions

We report a scanning tunneling microscopy (STM) experiment in an electrochemicalenvironment which studies a prototype molecular switch. The target molecules were perylenetetracarboxylic acid bisimides modified with pyridine (P-PBI) and methylthiol (T-PBI)linker groups and with bulky tert-butyl-phenoxy substituents in the bay area. At a fixedbias voltage, we can control the transport current through a symmetric molecular wireAu|P-PBI(T-PBI)|Au by variation of the electrochemical ‘gate’ potential. The current increases by up to twoorders of magnitude. The conductances of the P-PBI junctions are typically a factor 3larger than those of T-PBI. A theoretical analysis explains this effect as a consequence ofshifting the lowest unoccupied perylene level (LUMO) in or out of the bias window whentuning the electrochemical gate potential VG. The difference in on/off ratios reflects thevariation of hybridization of the LUMO with the electrode states with the anchor groups.IT–ES(T) curves of asymmetric molecular junctions formed between a bare AuSTM tip and a T-PBI (P-PBI) modified Au(111) electrode in an aqueouselectrolyte exhibit a pronounced maximum in the tunneling current at−0.740, which is close to the formal potential of the surface-confined molecules. Theexperimental data were explained by a sequential two-step electron transfer process.

Role of Adenine in Thymine-Dimer Repair by Reduced Flavin-Adenine Dinucleotide

We present a study of excited-state behavior of reduced flavin cofactors using femtosecond optical transient absorption spectroscopy. The reduced flavin cofactors studied were in two protonation states: flavin-adenine dinucleotide (FADH2 and FADH-) and flavin-mononucleotide (FMNH2 and FMNH-). We find that FMNH- exhibits multiexponential decay dynamics due to the presence of two bent conformers of the isoalloxazine ring. FMNH2 exhibits an additional fast deactivation component that is assigned to an iminol tautomer. Reduced flavin cofactors also exhibit a long-lived component that is attributed to the semiquinone and the hydrated electron that are produced in photoinduced electron transfer to the solvent. The presence of adenine in FADH2 and FADH- further changes the excited-state dynamics due to intramolecular electron transfer from the isoalloxazine to the adenine moiety of cofactors. This electron transfer is more pronounced in FADH2 due to pi-stacking interactions between two moieties. We further studied cyclobutane thymine dimer (TT-dimer) repair via FADH- and FMNH- and found that the repair is much more efficient in the case of FADH-. These results suggest that the adenine moiety plays a significant role in the TT-dimer repair dynamics. Two possible explanations for the adenine mediation are presented: (i) a two-step electron transfer process, with the initial electron transfer occurring from flavin to adenine moiety of FADH-, followed by a second electron transfer from adenine to TT-dimer; (ii) the preconcentration of TT-dimer molecules around the flavin cofactor due to the hydrophobic nature of the adenine moiety.

Behaviors of Excitation Cross Sections in the Intermediate Velocity Regime on Ion-Atom Collisions

We studied the behaviors of excitation cross sections and electron capture cross sections as functions of collision velocity using previous data of various collision systems. The structures peculiar to the intermediate velocity regime were recognized on the excitation cross sections. Close correlation of the excitation cross section to the electron capture cross section and a relaxation time longer than that for the electron capture process characterize the excitation process in the intermediate velocity regime. A model consisting of a two-step electron transfer process well explains the characteristics of the excitation cross sections in the intermediate velocity regime.

Experimental and model angular distributions of one- and two-electron capture processes in 0.5-20 eV/u Ar4+-Ar collisions.

We have measured and calculated state-resolved angular distributions of one- and two-electron-capture processes in 0.5--20 eV/u ${\mathrm{Ar}}^{4+}$-Ar collisions. The experimental energy-gain spectra show single-electron capture to six unresolved 4p LS terms, while no trace of a 4s population was found for laboratory collision energies below 200 eV. Semiclassical trajectory calculations, using a seven-channel Landau-Zener model for the probability flux on the multivalued deflection functions, show that the oscillations in the experimental 4p angular differential cross section are due to multiple rainbow scattering and that interference effects play a limited role. Experimental and calculated 4p angular distributions compare nicely over the whole energy range. For energies above 150 eV, true double-capture and transfer ionization with angular distributions consistent with a two-step electron-transfer process appear within the \ifmmode\pm\else\textpm\fi{}9\ifmmode^\circ\else\textdegree\fi{} experimental acceptance.

Numerical methods for the solution of the rotating disc electrode system

Finite-difference and orthogonal collocation methods are applied to the solution of the rotating disc electrode system. Both methods reduce the ordinary differential equation system, describing the steady-state condition at the rotating disc, to a set of simultaneous equations, which are readily solved using simple matrix operations, and both may be generally applied to coupled kinetic and convective diffusion problems. Orthogonal collocation is found to be by far the more efficient of the two methods considered and its efficiency can be optimized by using a change of the space variable, chosen to best suit the concentration function of which the numerical solution is sought. The method is applied to a specific second-order kinetic problem, including inequality of diffusion coefficients, of relevance to the two-step electron transfer process in micellar solution.