Electron Transfer Quantum Research Articles

Study of The Effect of Concentration on The Efficiency of The Sensitive N749-TiO2 Solar Cell

In this research, we studied the effect of concentration carriers on the efficiency of the N749-TiO2 heterogeneous solar cell based on quantum electron transfer theory using a donor-acceptor scenario. The photoelectric properties of the N749-TiO2 interfaces in dye sensitized solar cells DSSCs are calculated using the J-V curves. For the (CH3)3COH solvent, the N749-TiO2 heterogeneous solar cell shows that the concentration carrier together with the strength coupling are the main factors affecting the current density, fill factor and efficiency. The current density and current increase as the concentration increases and the strength coupling increases as the N749-TiO2 heterogeneous in solar cell. However, the efficiency is more sensitive as the carrier increases and the coupling strength gradually increases from 0.1 × 10−2|eV|2 to 1.5 × 10−2|eV|2 respectivelly. However, the concentration carrier changes of the N749-TiO2 device affect the performance of the dye solar cell and it has a higher efficiency with concentration 4.5 × 10241/m 3 than with less concentration 1.5 × 10241/m 3.

Coenzyme Q10 trapping in mitochondrial complex I underlies Leber’s hereditary optic neuropathy

How does a single amino acid mutation occurring in the blinding disease, Leber's hereditary optic neuropathy (LHON), impair electron shuttling in mitochondria? We investigated changes induced by the m.3460 G>A mutation in mitochondrial protein ND1 using the tools of Molecular Dynamics and Free Energy Perturbation simulations, with the goal of determining the mechanism by which this mutation affects mitochondrial function. A recent analysis suggested that the mutation's replacement of alanine A52 with a threonine perturbs the stability of a region where binding of the electron shuttling protein, Coenzyme Q10, occurs. We found two functionally opposing changes involving the role of Coenzyme Q10. The first showed that quantum electron transfer from the terminal Fe/S complex, N2, to the Coenzyme Q10 headgroup, docked in its binding pocket, is enhanced. However, this positive adjustment is overshadowed by our finding that the mobility of Coenzyme Q10 in its oxidized and reduced states, entering and exiting its binding pocket, is disrupted by the mutation in a manner that leads to conditions promoting the generation of reactive oxygen species. An increase in reactive oxygen species caused by the LHON mutation has been proposed to be responsible for this optic neuropathy.

Nanorod length-dependent photodriven H2 production in 1D CdS-Pt heterostructures.

Colloidal quantum confined semiconductor-metal heterostructures are promising candidates for solar energy conversion because their light absorbing semiconductor and catalytic components can be independently tuned and optimized. Although the light-to-hydrogen efficiencies of such systems have shown interesting dependences on the morphologies of the semiconductor and metal domains, the mechanisms of such dependences are poorly understood. Here, we use Pt tipped 0D CdS quantum dots (with ∼4.6nm diameter) and 1D CdS nanorods (of ∼13.8, 27.8, 66.6, and 88.9nm average rod lengths) as a model system to study the distance-dependence of charge separation and charge recombination times and their impacts on photo-driven H2 production. The H2 generation quantum efficiency increases from 0.2% ± 0.0% in quantum dots to 28.9% ± 0.4% at a rod length of 28nm and shows negligible changes at longer rod lengths. The half-life time of electron transfer from CdS to Pt increases monotonically with rod length, from 0.7 ± 0.1 in quantum dots to 170.2 ± 29.5ps in the longest rods, corresponding to a slight decrease in electron transfer quantum efficiency from 92% to 81%. The amplitude-weighted average lifetime of charge recombination of the electron in Pt with the hole in CdS increases from 4.7 ± 0.4 µs in quantum dots to 149 ± 34 µs in 28nm nanorods, and the lifetime does not increase further in longer rods, resembling the trend in the observed H2 generation quantum efficiency. Our result suggests that the competition of the charge recombination process with the hole removal by the sacrificial electron donor plays a dominant role in the observed nanorod length dependent overall light driven H2 generation quantum efficiency.

Photoinduced electron transfer across the polymer-capped CsPbBr3 interface in a polar medium.

In-situ polymer capping of cesium lead bromide (CsPbBr3) nanocrystals with polymethyl acrylate is an effective approach to improve the colloidal stability in the polar medium and thus extends their use in photocatalysis. The photoinduced electron transfer properties of polymethyl acrylate (PMA)-capped CsPbBr3 nanocrystals have been probed using surface-bound viologen molecules with different alkyl chains as electron acceptors. The apparent association constant (Kapp) obtained for the binding of viologen molecules with PMA-capped CsPbBr3 was 2.3 × 107 M-1, which is an order of magnitude greater than that obtained with oleic acid/oleylamine-capped CsPbBr3. Although the length of the alkyl chain of the viologen molecule did not show any impact on the electron transfer rate constant, it influenced the charge separation efficiency and net electron transfer quantum yield. Viologen moieties with a shorter alkyl chain length exhibited a charge separation efficiency of 72% compared with 50% for the longer chain alkyl chain length viologens. Implications of polymer-capped CsPbBr3 perovskite nanocrystals for carrying out photocatalytic reduction in the polar medium are discussed.

Response of Fluorescence and Chlorophyll Physiological Characteristics of Typical Urban Trees to Ozone Stress

In this study, four typical urban landscaping tree species were selected, three open top air chambers with different ozone concentrations were set, and the responses of chlorophyll fluorescence, chlorophyll content and relative conductivity of the trees to ozone stress were studied. The results showed that with the increase in ozone concentration, the maximum photochemical efficiency, electron transfer quantum yield, electron transfer rate (ETR) and chlorophyll content of the different tree species decreased significantly, while the relative conductivity of the different tree species increased significantly. Compared with the ozone concentration of NF, under an ozone concentration of nf40 and nf80, the decline in the rate of Fv/Fm of Koelreuteria paniculata and Ginkgo biloba was 2.47 and 2.28 times that of Pinus bungeana and Platycladus orientalis, respectively, and the increase in the rate of relative conductivity of K. paniculata and G. biloba was 2.11 and 1.28 times that of P. bungeana and P. orientalis, respectively. Under different ozone concentrations, the photochemical efficiency, electron transfer rate, chlorophyll content and relative conductivity of P. bungeana and P. orientalis were higher than those of Ginkgo biloba and K. paniculata, indicating that K. paniculata and G. biloba were more sensitive to ozone. This study is of great significance for improving urban environmental quality and ozone control and also provides a basis for selecting tree species with strong ozone tolerance.

Triplet-State Photochemistry of Dissolved Organic Matter: Triplet-State Energy Distribution and Surface Electric Charge Conditions.

Excited triplet states of chromophoric dissolved organic matter (3CDOM*) are highly reactive species in sunlit surface waters and play a critical role in reactive oxygen species (ROS) formation and pollutant attenuation. In the present study, a series of chemical probes, including sorbic acid, sorbic alcohol, sorbic amine, trimethylphenol, and furfuryl alcohol, were employed to quantitatively determine 3CDOM* and 1O2 in various organic matters. Using a high concentration of sorbic alcohol as high-energy triplet states quencher, 3CDOM* can be first distinguished as high-energy triplet states (>250 kJ mol-1) and low-energy triplet states (<250 kJ mol-1). The terrestrial-origin natural organic matter (NOM) was found to mainly consist of low-energy triplet states, while high-energy triplet states were predominant in autochthonous-origin NOM and effluent/wastewater organic matter (EfOM/WWOM). The 1O2 quantum yields and electron transfer quantum yield coefficients ( fTMP) generated from low-energy triplet states remained constant in all tested organic matters. External phenolic compound showed quenching effects on triplet-state formation and tended to have a higher quenching efficiency for aromatic ketone triplet states, which are the main high-energy triplet states. In comparison with terrestrial-origin NOM, autochthonous-origin NOM and EfOM/WWOM presented lower reaction rate constants for sorbic amines and higher reaction rate constants for sorbic acid, and these differences are likely due to dissimilar surface electric charge conditions. Understanding the triplet-state photochemistry of CDOM is essential for providing useful insights into their photochemical effects in aquatic systems.

Effects of Antimony Stress on Photosynthesis and Growth of Acorus calamus.

This study was aimed to explore that effects of Sb on physiological parameters of Acorus calamus and the possibility of using A. calamus as a remediation plant. A. calamus potted experiments were conducted using different concentrations (0, 250, 500, 1000, and 2000 mg/kg) of antimony potassium tartrate (Sb3+) (marked as CK, T1, T2, T3, and T4, respectively) and potassium pyroantimonate (Sb5+) (marked as CK, T′1, T′2, T′3, and T′4, respectively). The effects of Sb stress (Sb3+ and Sb5+) on leaf photosynthetic pigments, biomass, photosynthetic characteristics and chlorophyll fluorescence parameters of potted A. calamus were studied. With the rise of Sb3+ concentration from T1 to T4, the leaf pigment contents (chlorophyll a, b, carotenoid), plant height, dry weight, net photosynthetic rate (Pn), stomatal conductance (Gs), evaporation rate (E), PSII maximum photochemical efficiency (Fv/Fm), and PSII electron transfer quantum yield rate (ΦPSII) of A. calamus all reduced, while intercellular CO2 concentration (Ci) significantly increased. The reduction of Pn was mainly induced by non-stomatal limitation. Chlorophyll a/b ratio increased significantly versus the control, while carotenoid/chlorophyll ratio (Car/Chl) first decreased and then increased. The leaf Chl a, Chl b, Car, plant height, dry weight, Pn, Gs, E, Fv/Fm, and ΦPSII all maximized in T′1 (250 mg/kg), but were not significantly different from the control. As the Sb5+ concentration increased from T′2 to T′4, the above indices all decreased and were significantly different from the control. Moreover, intercellular CO2 concentration (Ci) decreased significantly. The reduction of Pn was caused by non-stomatal limitation, indicating the mesophyll cells were damaged. The Car/Chl ratio was stable within 0–500 mg/kg Sb, but decreased in T3 and T4, and rose in T′3 and T′4. After Sb3+ and Sb5+ treatments, translocation factor varied 19.44–27.8 and 19.44–24.86%, respectively. In conclusion, different form Sb3+ treatment, Sb5+ treatment showed a Hormesi effect, as low-concentration treatment promoted A. calamus growth, but high-concentration treatment inhibited its growth. The two forms of Sb both caused unfavorable effects on A. calamus, but the seedlings did not die and were modestly adaptive and Sb-accumulative. A. calamus, which is easily maintained and cultivated, can serve as a good candidate for phytoremediation of water contaminated with Sb.

Molecular "topological insulators": a case study of electron transfer in the bis(methylene) adamantyl carbocation.

A topological or geometric phase (GP) blockade can be introduced in molecular electron transfer processes if a conical intersection (CI) occurs between two charge configurations. The origin of the blockade is in the destructive interference of two pathways around the CI that acquire opposite GPs. Electron transfer quantum dynamics including and excluding GP have been modelled for the bis(methylene) adamantyl carbocation. These calculations have shown unambiguously that the CI topology and the induced GP are responsible for the transport blockade.

Excited state electron transfer from cobalt coordination compounds anchored to TiO2

There exists an urgent need for stable, earth-abundant inorganic coordination compounds and materials that efficiently harvest sunlight and initiate electron transfer reactions that produce electrical power and/or chemical fuels. It was recently discovered that Co(I) coordination compounds do indeed harvest large fractions of solar photons and efficiently transfer electrons to the acceptor states of TiO2 from metal-to-ligand charge transfer excited states. In some cases the electron transfer quantum yield was ∼46%. Remarkably, and unlike other first row transition metal compounds, the unfilled d-orbitals do not quantitatively quench the excited states and allow for efficient excited state electron transfer. A novel feature of the Co(II/I∗) electron transfer chemistry is a large inner-sphere contribution that results from a change in the coordination number. This short review summarizes the most recent findings that suggest new opportunities for solar energy conversion with first-row transition metal compounds.

Quantum Biological Switch Based on Superradiance Transitions

A linear chain of connected electron sites with two asymmetric sinks, one attached to each end, is used as a simple model of quantum electron transfer in photosynthetic bio-complexes. For a symmetric initial population in the middle of the chain, it is expected that electron transfer is mainly directed towards the strongest coupled sink. However, we show that quantum effects radically change this intuitive "classical" mechanism, so that electron transfer can occur through the weaker coupled sink with maximal efficiency. Using this capability, we show how to design a quantum switch that can transfer an electron to the left or right branch of the chain, by changing the coupling to the sinks. The operational principles of this quantum device can be understood in terms of superradiance transitions and subradiant states. This switching, being a pure quantum effect, can be used as a witness of wave--like behaviour of excitations in molecular chains. When realistic data are used for the photosystem II reaction center, this quantum biological switch is shown to retain its reliability, even at room temperature.

Feasibility study of electron transfer quantum well infrared photodetectors for spectral tuning in the long-wave infrared band

An electron transfer quantum well infrared photodetector (QWIP) consisting of repeating units of two coupled quantum wells (QWs) is capable of exhibiting a two color voltage dependent spectral response. However, significant electron transfer between the coupled QWs is required for spectral tuning, which may require the application of relatively high electric fields. Also, the band structure of coupled quantum wells is more complicated in comparison to a regular quantum well and, therefore, it is not always obvious if an electron transfer QWIP can be designed such that it meets specific performance characteristics. This paper presents a feasibility study of the electron transfer QWIP and its suitability for spectral tuning. Self consistent calculations have been performed of the bandstructure and the electric field that results from electron population within the quantum wells, from which the optical characteristics have been obtained. The band structure, spectral response, and the resonant final state energy locations have been compared with standard QWIPs. It is shown that spectral tuning in the long-wave infrared band can be achieved over a wide wavelength range of several microns while maintaining a relatively narrow spectral response FWHM. However, the total absorption strength is more limited in comparison to a standard QWIP, since the higher QW doping densities require much higher electric fields for electron transfer.

Noise-assisted quantum electron transfer in photosynthetic complexes

Electron transfer (ET) between primary electron donors and acceptors is modeled in the photosystem II reaction center (RC). Our model includes (i) two discrete energy levels associated with donor and acceptor, interacting through a dipole-type matrix element and (ii) two continuum manifolds of electron energy levels ("sinks"), which interact directly with the donor and acceptor. Namely, two discrete energy levels of the donor and acceptor are embedded in their independent sinks through the corresponding interaction matrix elements. We also introduce classical (external) noise which acts simultaneously on the donor and acceptor (collective interaction). We derive a closed system of integro-differential equations which describes the non-Markovian quantum dynamics of the ET. A region of parameters is found in which the ET dynamics can be simplified, and described by coupled ordinary differential equations. Using these simplified equations, both sharp and flat redox potentials are analyzed. We analytically and numerically obtain the characteristic parameters that optimize the ET rates and efficiency in this system.

Non‐Hermitian approach for modeling of noise‐assisted quantum electron transfer in photosynthetic complexes

AbstractWe model the quantum electron transfer (ET) in the photosynthetic reaction center (RC), using a non‐Hermitian Hamiltonian approach. Our model includes (i) two protein cofactors, donor and acceptor, with discrete energy levels and (ii) a third protein pigment (sink) which has a continuous energy spectrum. Interactions are introduced between the donor and acceptor, and between the acceptor and the sink, with noise acting between the donor and acceptor. The noise is considered classically (as an external random force), and it is described by an ensemble of two‐level systems (random fluctuators). Each fluctuator has two independent parameters, an amplitude and a switching rate. We represent the noise by a set of fluctuators with fitting parameters (boundaries of switching rates), which allows us to build a desired spectral density of noise in a wide range of frequencies. We analyze the quantum dynamics and the efficiency of the ET as a function of (i) the energy gap between the donor and acceptor, (ii) the strength of the interaction with the continuum, and (iii) noise parameters. As an example, numerical results are presented for the ET through the active pathway in a quinone‐type photosystem II RC.

Femtosecond Decay and Electron Transfer Dynamics of the Organic Sensitizer D149 and Photovoltaic Performance in Quasi-Solid-State Dye-Sensitized Solar Cells

In this work, the femtosecond upconversion spectroscopy was used for the study of ultrafast decay and electron transfer dynamics of the very promising all-organic sensitizer indoline dye, D149, in inert environments as well as on TiO2 substrates. The inert environments were toluene and acetonitrile solutions as well as poly(methyl methacrylate) (PMMA) films and Al2O3 nanostructured substrates. These were used to investigate relaxation and aggregation induced decays. TiO2 substrates were used to probe interfacial electron transfer dynamics. The excitation was at the S2 band while the dynamics of both S2 and S1 bands were detected. The dynamics of the S2 band in solutions and PMMA film, show a rapid S2–S1 internal conversion, ranging from 0.20 to 0.45 ps depending on the environment. This component becomes <0.10 ps in Al2O3 and in TiO2. The dynamics of the S1 band probe both hot and cold S1 species. In solutions and PMMA, the hot species relax to cold ones within a few tens of picoseconds while the cold ones decay to the ground state within a few hundreds of picoseconds. In Al2O3, the dynamics become much faster because of aggregation induced quenching. In TiO2, the S1 dynamics exhibit three mechanisms of 0.1, 1.3, and 8.6 ps time constants indicating a multiphasic injection mechanism. The electron transfer quantum yield and rate from the S1 band were calculated, using the time-resolved data, and were found to be 0.90 and 3.82 × 1011 s–1, respectively. Finally, quasi-solid-state dye-sensitized solar cells, with D149 as sensitizer, were constructed, and their efficiency was found 5.66%. Similar cells, using the ruthenium complexes N719 and black dye as sensitizers, were also constructed for comparison. It was found that D149 showed higher efficiency than black dye but lower than N719.

Photoinduced Electron Transfer of Oxazine 1/TiO2 Nanoparticles at Single Molecule Level by Using Confocal Fluorescence Microscopy

Kinetics of photoinduced electron transfer (ET) from oxazine 1 dye to TiO(2) nanoparticles (NPs) surface is studied at a single molecule level by using confocal fluorescence microscopy. Upon irradiation with a pulsed laser at 630 nm, the fluorescence lifetimes sampled among 100 different dye molecules are determined to yield an average lifetime of 2.9 +/- 0.3 ns, which is close to the value of 3.0 +/- 0.6 ns measured on the bare coverslip. The lifetime proximity suggests that most interfacial electron transfer (IFET) processes for the current system are inefficient, probably caused by physisorption between dye and the TiO(2) film. However, there might exist some molecules which are quenched before fluorescing and fail to be detected. With the aid of autocorrelation analysis under a three-level energy system, the IFET kinetics of single dye molecules in the conduction band of TiO(2) NPs is evaluated to be (1.0 +/- 0.1) x 10(4) s(-1) averaged over 100 single molecules and the back ET rate constant is 4.7 +/- 0.9 s(-1). When a thicker TiO(2) film is substituted, the resultant kinetic data do not make a significant difference. The trend of IFET efficacy agrees with the method of fluorescence lifetime measurements. The obtained forward ET rate constants are about ten times smaller than the photovoltage response measured in an assembled dye-sensitized solar cell. The discrepancy is discussed. The inhomogeneous and fluctuation characters for the IFET process are attributed to microenvironment variation for each single molecule. The obtained ET rates are much slower than the fluorescence relaxation. Such a small ET quantum yield is yet feasibly detectable at a single molecule level.

Quantum electron transfer processes induced by thermo-coherent state

When the reactant surface is not in a thermal equilibrium, but in a thermo-coherent state we have derived the rate and discussed about the quantum features of the rate. In the limit of very low and very high temperature the expressions are derived analytically and compared with the case of thermal distribution. We have investigated the dependence of temperature on the rate due to displacement, distortion of the harmonic potential energy surfaces of the reactant and product manifold.

Closed-Form Expressions of Quantum Electron Transfer Rate Based on the Stationary-Phase Approximation

Closed-form rate expressions are derived on the basis of the stationary-phase approximation for the Fermi golden rule expression of the quantum electron-transfer (ET) rate. First, on the basis of approximate solutions of the stationary-phase points near DeltaG = 0, -lambda, and lambda, where DeltaG is the reaction free energy and lambda is the reorganization energy, three closed-form rate expressions are derived, which are respectively valid near each value of DeltaG. Numerical tests for a model Ohmic spectral density with an exponential cutoff demonstrate good performance of the derived expressions in the respective regions of their validity. In particular, the expression near DeltaG = -lambda, which differs from the semiclassical approximation only by a prefactor quadratic in DeltaG, works substantially better than the latter. Then, a unified formula is suggested, which interpolates the three approximate expressions and serves as a good approximation in all three regions. We have also demonstrated that the interpolation formula can serve as a good quantitative means for understanding the temperature dependence of the quantum ET rate.

Quantum versus classical electron transfer energy as reaction coordinate for the aqueous Ru2+/Ru3+ redox reaction

Applying density functional theory (DFT)-based molecular dynamics simulation methods we investigate the effect of explicit treatment of electronic structure on the solvation free energy of aqueous Ru2+ and Ru3+.Our approach is based on the Marcus theory of redox half reactions, focussing on the vertical energy gap for reduction or oxidation of a single aqua ion. We compare the fluctuations of the quantum and classical energy gap along the same equilibrium ab initio molecular dynamics trajectory for each oxidation state. The classical gap is evaluated using a standard point charge model for the charge distribution of the solvent molecules (water). The quantum gap is computed from the full DFT electronic ground state energies of reduced and oxidized species, thereby accounting for the delocalization of the electron in the donor orbital and reorganization of the electron cloud after electron transfer (ET). The fluctuations of the quantum ET energy are well approximated by gaussian statistics giving rise to parabolic free energy profiles. The curvature is found to be independent of the oxidation state in agreement with the linear response assumption underlying Marcus theory. By contrast, the diabatic free energy curves evaluated using the classical gap as order parameter, while also quadratic, are asymmetric reflecting the difference in oxidation state. The response of these two order parameters is further analysed by a comparison of the spectral density of the fluctuations and the corresponding reorganization free energies.

Challenges in the theory of electron transfer at passive interfaces

Electron transfer (ET) has been described as the simplest of chemical reactions, because it generally does not result in the change in identity of a chemical species, but merely results in a change in the oxidation state of a single atomic center. In principle, ET is the ideal vehicle within which to develop the theory of rate processes, because of the inherent simplicity of the reaction dynamics. However, the development of an effective theory that can predict reaction rates from first principles has proven to be elusive and scientists are no closer to that goal than they were three decades ago. However, starting with the groundbreaking work of Gurney in 1931, which was hailed as one of the first triumphs of quantum mechanics, QET (quantum electron transfer) theory has provided an unassailable explanation of Tafel’s law; the exponential dependence of the rate of a charge transfer reaction on the applied potential. This paper provides a brief review of QET theory with the particular goal of accounting for Tafel’s law on passive surfaces. The inverse Tafel’s law, where the current decreases exponentially with increasing voltage for an anodic (oxidation) reaction, is also discussed. In this case, the decrease in the current is due to the thickening of an oxide layer on the surface and the resulting decrease in the quantum mechanical tunneling probability of charge carriers across the film. Both phenomena (the normal and inverse Tafel’s law) occur in electrochemical and corrosion systems and, indeed, often occur in the same system, such as the reduction of a cathodic depolarizer on a passive surface. Finally, the use of the inverse law as a tool for probing the dynamics of formation of thin oxide films on metal surfaces is discussed with emphasis on the oxidation of hydrogen on oxidized platinum in alkaline solution at ambient and elevated temperatures.

Solvent nuclear quantum effects in electron transfer reactions. III. Metal ions in water. Solute size and ligand effects

Solvent nuclear quantum effects in redox electron transfer (ET) reactions between metal ions in aqueous solution are studied via a molecular dynamics simulation analysis. The impacts of the solute size and charge variations together with the solvent ligand effects are examined by comparing with our previous study on a moderate size donor–acceptor system that assumed typical organic fluorescer-quencher molecules [J. Chem. Phys. 106, 116 (1997)]. It is shown that the spectral density function of the solvent coupling to ET, and consequently the quantum ET rate and its energy gap law, are strongly dependent on these variations of the system parameters. Two kinds of decomposition analysis, one into spatial contributions from inner- and outer-sphere solvations, and the other into motional frequency contributions from solvent intramolecular vibrations and intermolecular collective modes, are presented.