Charge Separation Model Research Articles

A protet-based model that can account for energy coupling in oxidative and photosynthetic phosphorylation

Two-stage (e.g. light-dark) phosphorylation experiments showed that there is a stored ‘high-energy’ intermediate linking electron transport and phosphorylation. Large, artificial electrochemical proton gradients (protonmotive forces or pmfs) can also drive phosphorylation, a fact seen as strongly supportive of the chemiosmotic coupling hypothesis that a pmf is the ‘high-energy’ intermediate. However, in such experiments there is an experimental threshold (pmf >170 mV, equivalent to ΔpH ∼2.8) below which no phosphorylation is in fact observed, and 220 mV are required to recreate in vivo rates. This leads to the correct question, which is then whether those values of the pmf generated by electron transport are large enough. Even the lower ones as required for any phosphorylation (leave alone those required to explain in vivo rates) are below the threshold [1, 2], whether measured directly with microelectrodes or via the use of membrane-permeant ions and/or acids/bases (which are always transporter substrates [3], so all such measurements are in fact artefactual). The single case that seemed large enough (220 mV) is now admitted to be a diffusion potential artefact [4]. Many other observables (inadequate bulk H+ in ‘O2-pulse’-type experiments, alkaliphilic bacteria, dual-inhibitor titrations, uncoupler-binding proteins, etc.) are consistent with the view that values of the pmf, and especially of Δψ, are actually very low. A protet-based charge separation model [2], a protonic version analogous to how energy may be stored in devices called electrets, provides a high-energy intermediate that can explain the entire literature, including the very striking demonstration [5] that close proximity is required between electron transport and ATP synthase complexes for energy coupling between them to allow phosphorylation to occur. A chief purpose of this article is thus to summarise the extensive and self-consistent literature, much of which is of some antiquity and rarely considered by modern researchers, despite its clear message of the inadequacy of chemiosmotic coupling to explain these phenomena.

Visible-light-driven CO2 reduction to CO boosted by hydrogenated sphalerite-type CoO atomic layers with exposed polar {1 1 1} surfaces and the photocatalytic mechanism

In order to explain the universality of charge separation driven by spontaneous electric field (Es) between polar crystalline planes, we synthesizd the atomically thick sub-stable sphalerite-type CoO nanosheets with exposed {111} polar crystal faces using a salt template method. Photocatalytic activity of the CoO ultrathin nanosheets for the reduction of CO2 to CO is enhanced by removing O-H groups and Cl− anions located on Co-(111) face through hydrogenation. The CO production rate is 5576.5 µmol g−1 h−1. The apparent quantum yield for CO production is as high as 4.11 %, which is obviusly higher than that of most photocatalysts reported so far. The CoO are thus expected to be applied in the preparation of CO by CO2 reduction. The atomic arrangement of the exposed {111} polar crystalline planes of sphalerite-type CoO is studied by density functional theory calculations. An Es is generated in the CoO ultrathin nanosheets via spontaneous polarization. The enhancement of the photocatalytic activity of the hydrogenated CoO ultrathin nanosheets is primarily attributed to an increase in the Es in CoO ultrathin nanosheets. We put forward thus a photocatalytic mechanism of charge separation induced by Es between Co-(111) and O-(1¯1¯1¯) polar crystalline planes. The finding indicates that the charge separation model driven by Es between the polar surfaces is a general photocatalytic mechanism. The photocatalytic model can contribute to comprehending the morphology and crystal plane-dependent photocatalytic activities and guide the development of high-performance-photocatalysts and optoelectronic devices.

Shape-dependent synthesis and photocatalytic degradation by Cu2O nanocrystals: Kinetics and photocatalytic mechanism

In this study, Cu2O crystals with different morphologies were synthesized to investigate the effect of exposed crystalline facets on photocatalytic degradation efficiency. By adjusting the addition amount of PVP, different morphologies of Cu2O crystals were obtained, such as cubic, decahedral, octahedral, etc. XRD, SEM, and TEM characterizations were used to observe the properties of the synthesized Cu2O in terms of morphology, size, and lattice structure. The results showed that the octahedral cuprous oxide had the strongest photocatalytic degradation effect (78.3%). The study also explored the connection between different crystalline facets and MO microstructure, and the effect of crystalline facet selectivity on the pre-absorption and photocatalytic degradation of MO. Density Functional Theory (DFT) calculations were used to investigate the connection between the energy level structure of different crystalline facets of Cu2O and its photocatalytic activity. Finally, based on the experimental analysis and theoretical calculation, a new charge separation model on crystalline surface was proposed.

A New Model for Charge Separation by Proton Transfer During Collision Between Ice Particles in Thunderstorms

AbstractField and laboratory observations strongly support the view that the development of electric fields in thunderstorms is caused by charge separation during rebounding collisions between ice crystals and graupels, followed by their gravitational separation. Although several plausible microphysical mechanisms have been proposed to explain the physics of charge separation, none have been found to be fully consistent with the experimental evidence, and a fresh approach is necessary. We propose a new mechanism for charge separation in ice‐ice collisions based on the fundamental molecular mechanism of charge transport in ice, involving the diffusion of H+ ions (excess protons or positive ionic defects) via a proton hopping relay along the hydrogen‐bond network of ice and the trapping and release of H+ ions from L‐orientational defects. The collision of two ice particles leads to the formation of a transient crystalline ice bridge at the contact point, which is spontaneously driven by the tendency for ice sintering, permitting rapid H+ diffusion between the two particles. Charge separation is achieved by the asymmetry in the concentrations of H+ ions or L‐orientational defects between the two ice surfaces. The proposed H+ transport mechanism successfully explains the direction and magnitude of charge transfer as well as its dependence on the relative growth rates of the two ice surfaces observed in laboratory studies. In addition, it offers a molecular‐level interpretation of the empirical rule that, during a collision, the faster‐growing ice surface is positively charged at the expense of negative charging of the slower‐growing or sublimating ice surface.

Quantum feedback control in quantum photosynthesis

A model of charge separation in quantum photosynthesis as a model of quantum feedback control in a system of interacting excitons and vibrons is introduced. Quantum feedback in this approach describes the Landau--Zener transition with decoherence. The model explains irreversibility in the process of charge separation for quantum photosynthesis -- direct transitions for this quantum control model will have probabilities close to one and reverse transitions will have probabilities close to zero. This can be considered as a model of quantum ratchet. Also this model explains coincidence of energy of the vibron paired to the transition and Bohr frequency of the transition.

Controlling Symmetry Breaking Charge Transfer in BODIPY Pairs.

Symmetry breaking charge transfer (SBCT) is a process in which a pair of identical chromophores absorb a photon and use its energy to transfer an electron from one chromophore to the other, breaking the symmetry of the chromophore pair. This excited state phenomenon is observed in photosynthetic organisms where it enables efficient formation of separated charges that ultimately catalyze biosynthesis. SBCT has also been proposed as a means for developing photovoltaics and photocatalytic systems that operate with minimal energy loss. It is known that SBCT in both biological and artificial systems is in part made possible by the local environment in which it occurs, which can move to stabilize the asymmetric SBCT state. However, how environmental degrees of freedom act in concert with steric and structural constraints placed on a chromophore pair to dictate its ability to generate long-lived charge pairs via SBCT remain open topics of investigation.In this Account, we compare a broad series of dipyrrin dimers that are linked by distinct bridging groups to discern how the spatial separation and mutual orientation of linked chromophores and the structural flexibility of their linker each impact SBCT efficiency. Across this material set, we observe a general trend that SBCT is accelerated as the spatial separation between dimer chromophores decreases, consistent with the expectation that the electronic coupling between these units varies exponentially with their separation. However, one key observation is that the rate of charge recombination following SBCT was found to slow with decreasing interchromophore separation, rather than speed up. This stems from an enhancement of the dimer's structural rigidity due to increasing steric repulsion as the length of their linker shrinks. This rigidity further inhibits charge recombination in systems where symmetry has already enforced zero HOMO-LUMO overlap. Additionally, for the forward transfer, the active torsion is shown to increase LUMO-LUMO coupling, allowing for faster SBCT within bridging groups.By understanding trends for how rates of SBCT and charge recombination depend on a dimer's internal structure and its environment, we identify design guidelines for creating artificial systems for driving sustained light-induced charge separation. Such systems can find application in solar energy technologies and photocatalytic applications and can serve as a model for light-induced charge separation in biological systems.

Photogenerated Charge Separation between Polar Crystal Facets Under a Spontaneous Electric Field

AbstractUntil now, the photogenerated charge separation mechanism is not clear in photocatalysis. Based on the structures at the atomic scale of the polar crystal facets with photocatalytic activities, a general separation model of photogenerated charge between polar facets under a spontaneous electric field is presented. This charge separation model is proven by photo‐induced selective redox reactions on polar planes, photovoltaic and rectification effects in the polar direction orientated thin films, and electric field assisted assembly and alignment of semiconductor nanowires grown along the polar direction. This polar structure is similar to a semiconductor p–n junction, which can control the movement of electronic carriers and can be used to fabricate high‐efficiency photocatalysts and electronic and optoelectronic devices based on a new principle.

Charged neutron stars and observational tests of a dark force weaker than gravity

We discuss the possibility of exploring an unbroken U(1) gauge interaction in the dark sector by means of gravitational waves. Dark sector states charged under the dark force can give a macroscopic charge to astronomical bodies. Yet the requirement of having gravitationally bounded stars limits this charge to negligible values if the force has a sizeable strength. Gravitational tests are only possible if the dark force is weaker than gravity. By solving the Einstein-Maxwell field equations, we study in detail an explicit model for dark charge generation and separation in a neutron star. Charged states originate from the decay of neutrons inside the star into three dark fermions; we show that in this model the equation of state is consistent with limits on neutron star masses and tidal deformability. We find that while the dark force can be observed in binary mergers (making them an optimal observational test even though with limited precision), it is screened in binary pulsars (for which more precise data exist). The emitted radiation in the inspiral phase of a binary system is modified and the dark force tested at the level of the uncertainty of the experimental detection. The test covers a region where current limits on deviations from Newton inverse-squared law come from geophysical and laser-ranging observations.

Charge separation and strong adsorption-enhanced MoO3 visible light photocatalytic performance

This report aims to systematically study the photocatalytic properties of α-MoO3 and h-MoO3. Here, α-MoO3 nanorods and h-MoO3 hexagonal prism were successfully synthesized via a facile hydrothermal method. The products were characterized in detail, and the results showed that the products are very pure with excellent morphology. The photocatalytic properties of MoO3 were evaluated via methylene blue and rhodamine B dyes as well as tetracycline. The results indicate that the photocatalytic efficiency of α-MoO3 is better than that of h-MoO3, and α-MoO3 has the best photocatalytic activities at 24 h because of the synergistic adsorption and photocatalysis and dye-sensitized mechanism. We further proposed a charge separation model between (002) and (00_2) polar surfaces via density functional theory computation to explain the enhanced photocatalytic activities. The excellent photocatalytic efficiency of α-MoO3 suggests that it can be used for photodegradation of wastewater.

High-Pressure Modulation of Primary Photosynthetic Reactions.

Photochemical charge separation is key to biological solar energy conversion. Although many features of this highly quantum-efficient process have been described, others remain poorly understood. Herein, ultrafast fluorescence barospectroscopy is used for the first time to obtain insights into the mechanism of primary charge separation in a YM210W mutant bacterial reaction center under novel surrounding modulating conditions. Over a range of applied hydrostatic pressures reaching 10 kbar, the rate of primary charge separation monotonously increased and that of the electron transfer to secondary acceptor decreased. While the inferred free energy gap for charge separation generally narrowed with increasing pressure, a pressure-induced break of a protein-cofactor hydrogen bond observed at ∼2 kbar significantly (by 219 cm-1 or 27 meV) increased this gap, resulting in a drop in fluorescence. The findings strongly favor a model for primary charge separation that incorporates charge recombination and restoration of the excited primary pair state, over a purely sequential model. We show that the main reason for the almost threefold acceleration of the primary electron transfer rate is the pressure-induced increase of the electronic coupling energy, rather than a change of activation energy. We also conclude that across all applied pressures, the primary electron transfer in the mutant reaction center studied can be considered nonadiabatic, normal region, and thermally activated.

Ligand Charge Separation To Build Highly Stable Quasi-Isomer of MOF-74-Zn.

Built from an unusual high-charge-density ligand 2,5-dioxido-1,4-benzenedicarboxylate (dobdc4-), MOF-74 M (M2dobdc) have unsurpassed gas uptake and separation properties. It is thus intriguing to mimic or replicate such ligand properties in other chemical systems. Here, we show a ligand charge separation (LCS) model that could offer one pathway toward this goal. Two new materials (CPM-74 and -75, corresponding to MOF-74 and IRMOF-74-II, respectively) are presented here to illustrate this concept and its feasibility. Specifically, the dobdc4- ligand in MOF-74 can be conceptually broken down into OH- and obdc3- (H3obdc = 2-hydroxyterephthalic acid), which leads to CPM-74, Zn2(OH)(obdc), that is nearly isomeric with MOF-74-Zn. Different from MOF-74, CPM-74 is made from homohelical rod packing. Moreover, CPM-74 has high hydrothermal and thermal stability uncommon for Zn-MOFs. It contains open Zn sites on 4-coordinated Zn2+ and its isosteric heat of adsorption for CO2 is 22% higher than that of MOF-74-Zn at low pressures.

Markovian model for photoinduced charge separation in dendritic molecules

We study theoretically electron-transfer reactions in families of dendrimers DPm⊃Py2Fn which have been studied experimentally by Li et al. (2006). For these families, and for additional architectures, we consider two limiting scenarios depending on the number of bipyridines (with one, two, or three fullerenes) ligated around the core DPm dendritic molecule. Using the theory of finite Markov processes, our results show that electron transfer on the largest dendrimer DP48⊃Py2F3 is the most efficient.

Enhanced Visible-Light Photocatalytic H2 Evolution in Cu2O/Cu2Se Multilayer Heterostructure Nanowires Having {111} Facets and Physical Mechanism.

It is rather challenging to develop photocatalysts based on narrow-band-gap semiconductors for water splitting under solar irradiation. Herein, we synthesized the Cu2O/Cu2Se multilayer heterostructure nanowires exposing {111} crystal facets by a hydrothermal reaction of Se with Cu and KBH4 in ethanol amine aqueous solution and subsequent annealing in air. The photocatalytic H2 production activity of Cu2O/Cu2Se multilayer heterostructure nanowires is dramatically improved, with an increase on the texture coefficient of Cu2O(111) and Cu2Se(111) planes, and thus the exposed {111} facets may be the active surfaces for photocatalytic H2 production. On the basis of the polar structure of Cu2O {111} and Cu2Se {111} surfaces, we presented a model of charge separation between the Cu-Cu2Se(111) and O-Cu2O(1̅ 1̅ 1̅) polar surfaces. An internal electric field is created between Cu-Cu2Se(111) and O-Cu2O(1̅ 1̅ 1̅) polar surfaces, because of spontaneous polarization. As a result, this internal electric field drives the photocreated charge separation. The oxidation and reduction reactions selectively occur at the negative O-Cu2O(1̅ 1̅ 1̅) and the positive Cu-Cu2Se(111) surfaces. The polar surface-engineering may be a general strategy for enhancing the photocatalytic H2-production activity of semiconductor photocatalysts. The charge separation mechanism not only can deepen the understanding of photocatalytic H2 production mechanism but also provides a novel insight into the design of advanced photocatalysts, other photoelectric devices, and solar cells.

EMP control and characterization on high-power laser systems

Giant electromagnetic pulses (EMP) generated during the interaction of high-power lasers with solid targets can seriously degrade electrical measurements and equipment. EMP emission is caused by the acceleration of hot electrons inside the target, which produce radiation across a wide band from DC to terahertz frequencies. Improved understanding and control of EMP is vital as we enter a new era of high repetition rate, high intensity lasers (e.g. the Extreme Light Infrastructure). We present recent data from the VULCAN laser facility that demonstrates how EMP can be readily and effectively reduced. Characterization of the EMP was achieved using B-dot and D-dot probes that took measurements for a range of different target and laser parameters. We demonstrate that target stalk geometry, material composition, geodesic path length and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3D particle-in-cell simulations is used to inform our conclusions about the effects of stalk geometry on EMP, providing an opportunity for comparison with existing charge separation models.

Evidences of sensitization mechanism for PbSe thin films photoconductor

Over the past decades, sensitization has been considered as a key process that determined the performance of lead selenide (PbSe) photoconductive detectors. However, insufficiency of evidences about the changes on material properties and structure in micro level becomes a barrier on comprehensive explanation of the sensitization mechanism. In this work, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) technologies were employed to characterize the materials properties evolution process of PbSe during sensitization. Changes on microstructure was identified in details by high-resolution transmission electron microscope (HRTEM). Accordingly, we provided a profile of the sensitization mechanism in micro level. With these evidences, significant impacts on the photoconductive performances of PbSe could be well explained via the charge separation model.

Visible-light photocatalysis in CdTe nanoflakes with exposed {111} facets and charge separation between polar CdTe {111}surfaces

The search for active semiconductor photocatalysts that can utilize adequately solar-energy remains to be an open issue. CdTe nanoflakes and nanowires with a range of texture coefficients of (111) have been synthesized. Photocatalytic activities of the CdTe nanocrystals in degradation of malachite green, methyl orange, acid red 88 and ciprofloxacin can be enhanced by increasing the texture coefficients of (111) and reducing texture coefficient of (220). The exposed polar {111} facets were found to be the active facets, and a charge separation model between polar Cd-CdTe (111) and Te-CdTe (1¯1¯1¯) surfaces is proposed. An internal electric field is created between polar Cd-CdTe (111) and Te-CdTe (1¯1¯1¯) surfaces by the spontaneous polarization, and the internal electric field drives separation of the photoinduced electrons and holes. Therefore, the CdTe nanoflakes with exposed polar {111} facets synthesized by using oleylamine as a morphology controlling agent show superior photocatalytic activity, compared with CdTe nanowires, nanocrystals with irregular shapes and commercial powders as well as CdS branched nanostructures. The charge separation model provides a clear opinion into charge transfer in the semiconductor nanocrystals with exposed active facets, and affords guidance to design more effective photocatalysts, solar cells and other photoelectronic devices.

Spatial charge separation between wurtzite CdS polar (0001) and (000[formula omitted]) facets and their enhanced visible-light photocatalytic activity

The films assembled from wurtzite CdS nanocrystals with average size of 8.0 nm and clean {0001} facets have been synthesized by heating Cd(NO3)2-thiourea aqueous solution on Si substrate at 300 °C for 20 min. The CdS films show superior photoreactivity, in comparison with CdS dendrites and flowerlike architectures assembled from nanoflakes with exposed {0001} facets. The superior photocatalytic activity is attributed to the active {0001} facets. Structure of the wurtzite CdS {0001} facets was studied using periodic density functional theory. The exposed CdS {0001} facets were found to be polar surfaces, thus a charge separation model between polar Cd-CdS (0001) and S-CdS (0001¯) surfaces was proposed. The internal electric field between the polar Cd-CdS (0001) and S-CdS (0001¯) surfaces provides an effective driving force for the separation of photogenerated electrons and holes. The reduction and oxidation reactions selectively occur on the positive Cd-CdS (0001) and negative S-CdS (0001¯) planes. The good charge separation results in high photocatalytic activity. The charge separation model may provide new insights into the understanding of charge transfer in the semiconductor nanomaterials with reactive facets and the design of advanced photocatalysts and photoelectronic devices.

Stokhasticheskaya model razdeleniya zaryadov v fotovozbuzhdennykh molekulyarnykh triadakh

Stokhasticheskaya model razdeleniya zaryadov v fotovozbuzhdennykh molekulyarnykh triadakh

Effect of Intramolecular High-Frequency Vibrational Mode Excitation on Ultrafast Photoinduced Charge Transfer and Charge Recombination Kinetics.

A model of photoinduced ultrafast charge separation and ensuing charge recombination into the ground state has been developed. The model includes explicit description of the formation and evolution of nonequilibrium state of both the intramolecular vibrations and the surrounding medium. An effect of the high-frequency intramolecular vibrational mode excitation by a pumping pulse on ultrafast charge separation and charge recombination kinetics has been investigated. Simulations, in accord with experiment, have shown that the effect may be both positive (the vibrational mode excitation increases the charge-transfer rate constant) and negative (opposite trend). The effect on charge separation kinetics is predicted to be bigger than that on the charge recombination rate but nevertheless the last is large enough to be observable. The amplitude of both effects falls with decreasing vibrational relaxation time constant, but the effects are expected to be observable up to the time constants as short as 200 fs. Physical interpretation of the effects has been presented. Comparisons with the experimental data have shown that the simulations, in whole, provide results close to that obtained in the experiment. The reasons of the deviations have been discussed.

Mutations to R. sphaeroides Reaction Center Perturb Energy Levels and Vibronic Coupling but Not Observed Energy Transfer Rates.

The bacterial reaction center is capable of both efficiently collecting and quickly transferring energy within the complex; therefore, the reaction center serves as a convenient model for both energy transfer and charge separation. To spectroscopically probe the interactions between the electronic excited states on the chromophores and their intricate relationship with vibrational motions in their environment, we examine coherences between the excited states. Here, we investigate this question by introducing a series of point mutations within 12 Å of the special pair of bacteriochlorophylls in the Rhodobacter sphaeroides reaction center. Using two-dimensional spectroscopy, we find that the time scales of energy transfer dynamics remain unperturbed by these mutations. However, within these spectra, we detect changes in the mixed vibrational-electronic coherences in these reaction centers. Our results indicate that resonance between bacteriochlorophyll vibrational modes and excitonic energy gaps promote electronic coherences and support current vibronic models of photosynthetic energy transfer.