Rapid Charge Recombination Research Articles

Efficient Charge Transport in DNA Diblock Oligomers

The realization of highly efficient photoinduced charge separation across the pi-stacked base pairs in duplex DNA remains elusive. The low efficiencies (<5%) typically observed for charge separation over a dozen or more base pairs are a consequence of slow charge transport and rapid charge recombination. We report here a significant (5-fold or greater) enhancement in the efficiency of charge separation in diblock purine oligomers consisting of two or three adenines followed by several guanines, when compared to oligomers consisting of a single purine or alternating base sequences. This approach to wire-like behavior is attributed to both slower charge recombination and faster charge transport once the charge reaches the G-block in these diblock systems.

Mechanism of Charge Separation in DNA by Hole Transfer through Consecutive Adenines

To investigate the mechanism of charge separation in DNA with consecutive adenines adjacent to a photosensitizer (Sens), a series of naphthalimide (NI) and 5-bromouracil ((br)U)-modified DNAs were prepared, and the quantum yields of formation of the charge-separated states (Phi) upon photo-excitation of the Sens NI in DNA were measured. The Phi was modulated by the incorporation site of (br)U, which changes the oxidation potential of its complementary A through hydrogen bonding and the hole-transfer rates between adenines. The results were interpreted as charge separation by means of the initial charge transfer between NI in the singlet excited state and the second- and third-nearest adenine to the NI. In addition, the oxidation of the A nearest to NI leads to the rapid charge recombination within a contact ion pair. This suggests that the charge-separation process can be refined to maximize the Phi by putting a redox-inactive spacer base pair between a photosensitizer and an A-T stretch.

Reversible Bridge-Mediated Excited-State Symmetry Breaking in Stilbene-Linked DNA Dumbbells

The excited-state behavior of synthetic DNA dumbbells possessing stilbenedicarboxamide (Sa) linkers separated by short A-tracts or alternating A-T base-pair sequences has been investigated by means of fluorescence and transient absorption spectroscopy. Electronic excitation of the Sa chromophores results in conversion of a locally excited state to a charge-separated state in which one Sa is reduced and the other is oxidized. This symmetry-breaking process occurs exclusively via a multistep mechanism-hole injection followed by hole transport and hole trapping-even at short distances. Rate constants for charge separation are strongly distance-dependent at short distances but become less so at longer distances. Disruption of the A-tract by inversion of a single A-T base pair results in a pronounced decrease in both the rate constant and efficiency of charge separation. Hole trapping by Sa is highly reversible, resulting in rapid charge recombination that occurs via the reverse of the charge separation process: hole detrapping, hole transport, and charge return to regenerate the locally excited Sa singlet state. These results differ in several significant respects from those previously reported for guanine or stilbenediether as hole traps. Neither charge separation nor charge recombination occur via a single-step superexchange mechanism, and hole trapping is slower and detrapping faster when Sa serves as the electron donor. Both the occurrence of symmetry breaking and reversible hole trapping by a shallow trap in a DNA-based system are without precedent.

Parameters affecting the chemical work output of a hybrid photoelectrochemical biofuel cell

A hybrid photoelectrochemical biofuel cell employing the photoanode architecture of a dye-sensitized solar cell has been assembled. A porphyrin dye sensitizes a TiO(2) semiconductor over the visible range to beyond 650 nm. Photoinduced charge separation at the dye-TiO(2) interface results in electron migration to a cathode, and the holes generated on surface bound dyes oxidize soluble electron mediators. The increased [Ox] : [Red] ratio of the mediator drives the solution-based enzymatic oxidation of appropriate substrates. In this report we investigate how the accumulation of anodic and cathodic products limits cell performance. The NAD(+)/NADH and benzoquinone/hydroquinone redox couples were studied as sacrificial electron donors in the absence of appropriate enzymes or substrates. Comparatively poor cell performance was observed using the benzoquinone/hydroquinone couple. This effect is explained in terms of rapid charge recombination by electron donation from the electrode to benzoquinone in solution, as compared to much less recombination with NAD(+). With the NAD(+)/NADH couple the cell performance is relatively independent of the redox poise of the anode solution, but limited by accumulation of reduction products in the cathodic compartment. Using the NAD(+)/NADH couple, the photochemical reforming of ethanol to hydrogen was demonstrated under conditions where the process would be endergonic in the dark.

Photoelectrochemical Analysis of Anion-Doped TiO2 Colloidal and Powder Thin-Film Electrodes

The photoelectrochemical characteristics of anion-doped powder and colloidal electrodes were investigated under UV and visible illumination. The solar absorption of was improved by synthesizing reduced forms of using anions of the main group elements (C, N, and P) to substitute for oxygen in the titania lattice. Analytical methods, including scanning electron microscopy, X-ray diffraction, elemental analysis, Raman, and UV–visible spectroscopy were employed to characterize these materials. The photoactivity of the electrodes was evaluated using a three-electrode configuration quartz photoelectrochemical cell using simulated solar irradiance. All anion-doped electrodes investigated, specifically C-, N-, and P-doped, increased the visible absorption of n-, leading to higher photoactivity under solar light. The addition of methanol to the electrolyte reduced the rapid charge recombination and hence enhanced the photocurrents. Although the efficiency is not yet comparable to single-crystal n-, optimization of the parameters studied here showed that significant increases in photoactivity could be achieved over the native powder.

Space charge behaviour in epoxy laminates under high constant electric field

The development of space charge in insulating materials is one of the main causes of their electrical ageing. The pulsed electro-acoustic method is often used to determine space charge distribution, but the signal analysis in the case of laminate structures is much more complex to analyse. In this paper the authors describe and use a simulated signal in order to study laminates made of epoxy resin and fibre mat. The relatively large conductivity of the fibres compared with that of the resin seems to produce a rapid charge dissociation and recombination in the fibres. Under voltage the presence of fibres close to an electrode seems to promote charge injection.

Photochemistry of charge-transfer complexes in a viologen periodic mesoporous organosilica: time evolution from femtoseconds to minutes.

Persistent radical cations can be generated in V∝︁PMO, a periodic mesoporous organosilica incorporating a viologen in its silicate framework. Photoexcitation of V∝︁PMO forms a geminate viologen–halide ion pair that can undergo rapid charge recombination or electron hopping to form the nongeminate ion pair, which persists for minutes (see picture). In contrast, viologen–halide CT complexes in solution undergo complete charge recombination on the sub-nanosecond timescale.

Effects of Interaction of Photosensitizer with DNA and Stacked G Bases on Photosensitized One-Electron Oxidation of DNA

Three naphthalimide (NI) derivatives (NIN, NIP, and NIO) as photosensitizers for the G-selective oxidation were synthesized, and the mechanism of the photosensitized one-electron oxidation of DNA by the NI derivatives was studied. NIN possessing a cationic side chain exhibited strong association with the oligodeoxynucleotides (ODNs) due to an electrostatic interaction between the phosphate groups of ODN and the cationic group of NIN, while no association was observed for NIP possessing an anionic side chain due to electrostatic repulsion among the phosphate groups. The DNA-binding properties of NIO with a neutral side chain were intermediate between those of NIN and NIP. The effects of the electrostatic interaction and repulsion between the NI derivatives and ODN and stacked G bases on the photosensitized one-electron oxidation of ODN were studied by the nanosecond transient absorption measurement and HPLC analysis. The yield of the NI derivatives in the triplet excited state (3NI*) observed immediately after a 355-nm laser flash decreased with an increase in the binding constants, and almost no transient absorption was observed when NIN was used as a photosensitizer. This result indicates that rapid charge separation occurs between the NI in the singlet excited state (1NI*) and the adjacent nucleobase, followed by a rapid charge recombination between the NI radical anion and the adjacent nucleobase radical cation when NIN is bound to ODN. Therefore, the sequence dependence of the one-electron oxidation of ODN was investigated using NIP, which does not bind to ODN. By monitoring the one-electron reduced form of NIP (NIP•-) produced by the electron transfer from ODN to 3NIP*, we found that the charge-separation efficiency increased with the sequence of sequential G's, such as GG and GGG, because of the oxidation potential of G decreased by the stacking effect of G's. To clarify the relationship between the formation of the transient intermediates observed during the LFP and actual amount of ODN oxidative damage, we performed the quantitative analysis of the ODN oxidative lesion caused by the NI derivatives using HPLC. With respect to the sequence of ODN, larger amounts of G consumed by the photosensitized one-electron oxidation were observed with ODN possessing multiple G, GG, and GGG compared to a single G. In contrast to the LFP experiments, a similar amount of oxidative damage was observed for three synthetic NI derivatives, indicating that the association of the NI derivatives to ODN apparently has no effect on the one-electron oxidative process. This might be due to the O2 involved in the one-electron oxidation process. These results are discussed in the context of the concentration of O2 in the solution.

Molecular sensors based on the photoelectric effect of bacteriorhodopsin: Origin of differential responsivity

Bacteriorhodopsin is a popular advanced material. When oriented in a membrane or a thin film deposited on a transparent electrode, it responds to sudden changes of the illumination level with spikes of fast photoelectric signal, of which the amplitude and polarity reflect the extent and sense of changes (a phenomenon known as differential responsivity). The present article explain how light-induced rapid charge separation and recombination in the membrane leads to differential responsivity. This article also evaluates several alternative proposed mechanisms appearing in the literature.

Component analysis of the fast photoelectric signal from model bacteriorhodopsin membranes part 4. A method for isolating the B2 component and the evidence for its polarity reversal at low pH

Thin films formed from oriented purple membranes of Halobacterium halobium exhibit fast photoelectric signals upon pulsed light stimulation. These signals are generated by light-induced rapid charge separation and recombination. The fast photosignal consists of several components. We have previously developed methodology to separate these components on the basis of their pH dependence. We used the Trissl-Montal method of membrane reconstitution to generate a photosignal which contains the pH-independent B1 component and the pH-dependent B2 component, whereas we used a modified multilayered thin film method to obtain a pure B1 signal. The B2 component is prominent in the medium to high pH range (pH 7–11) and has a polarity opposite to that of B1. As the pH decreases, the B2 amplitude declines progressively. However, the B2 amplitude does not just eventually become zero but rather undergoes a polarity reversal when the pH declines below 2.7, i.e. the B2 component acquires the same polarity as the B1 component below pH 2.7. Its amplitude grows as pH decreases further. Experimentally, the polarity reversal is demonstrated by a pH titration, in which successive signals were recorded as the pH is reduced in small decrements (about 0.2–0.3 pH units). The null point of titration, where the B2 component is completely absent (pH 2.7), is determined by the superposition of the signal (from a Triss-Montal film) and the standard pure B1 signal from a multilayered thin film after normalization. The justification of this titration procedure is based on an equivalent circuit analysis previously established.

Coordinated Photoinduced Electron and Proton Transfer in a Molecular Triad

Excitation of carotenoid-porphyrin-quinone (C-P-Q) triads yields the porphyrin first excited singlet state, which decays by electron transfer to give a C-P[sup [center dot]+]-Q[sup [center dot]-] charge-separated state. Competing with rapid charge recombination is electron transfer from the carotenoid to produce a long-lived C[sup [center dot]+]-P-Q[sup [center dot]-] species. High quantum yields of the final state require tuning of electronic and thermodynamic factors to favor forward electron transfer over charge recombination. Triad 1 illustrates a new strategy for slowing charge recombination based on coupling photoinduced electron transfer to a change in proton chemical potential. The quantum yields and lifetimes of the final charge-separated states in the triads were assessed by monitoring the transient carotenoid radical cation absorptions. The results demonstrate that the yield of charge separation in multicomponent molecular photovoltaics can be increased by a coordinated electron and proton transfer process. It is also interesting that in 1 a substantial fraction of the intramolecular redox potential produced by photoinduced electron transfer is transformed into proton chemical potential. Elaboration of this concept could lead to photoinduced generation of proton motive force in a heterogeneous system. 24 refs., 3 figs.

Photoinduced electron transfer reactions in quinone-linked zinc porphyrin arrays

Intramolecular photoinduced electron transfer reactions of conformationally restricted quinone-linked zinc porphyrin monomer, dimer, and trimers have been studied by picosecond time-resolved absorption spectroscopy. Both charge separation and charge recombination are very rapid in the monomeric model, while charge recombination is partly impeded in the dimeric model. In the trimeric models, the singlet excited state of the distal coplanar diporphyrin is efficiently quenched by the attached quinone and long-lived charged separated states are formed most probably by electron transfer from the diporphyrin part to the monomeric porphyrin part competing with rapid charge recombination reaction.

Conformational Control of Rates of Intramolecular Electron Transfer

Abstract A series of porphyrins having a “viologen” appended via a short chain to a meso-aryl ring has been studied by time-resolved absorption and emission techniques. The viologen quenches porphyrin fluorescence but not the triplet state lifetime. Complex fluorescence decay profiles, which are observed for all compounds, are attributed to the presence of families of discrete ground-state conformers. Fluorescence quenching is interpreted in terms of intramolecular electron transfer, for which there is a modest thermodynamic driving force. However, charge separation (CS) is followed by rapid charge recombination (CR) such that long-lived redox products are not observed. Such behavior is consistent with recently introduced theoretical models dealing with conformational control of electron transfer rates. The rates of CS are almost insensitive to changes in temperature or solvent dynamics for alkanols.

Extraction of vitamin K-1 from Photosystem I particles by treatment with diethyl ether and its effects on the A −1 EPR signal and System I photochemistry

Treatment of Photosystem I particles, prepared by digitonin treatment of spinach chloroplasts, with dry diethyl ether extracted all the vitamin K-1 (2.2 molecules/P-700 in the original Photosystem I particles) and carotenoids as well as 60% of antenna chlorophylls. P-700 and FeS centers (F X , F A and F B) were not extracted, and were photochemically oxidized and reduced, respectively, upon continuous illumination. This preparation showed the A − 0 EPR signal but no A − 1 signal even after long illumination at 210 or 230 K, in which condition A − 1 was accumulated in the untreated Photosystem I particles. Laser flash excitation induced only a small absorption change, corresponding to 20% of the total P-700, at 429 nm in the extracted preparation, suggesting that charge recombination took place on a submicrosecond time scale. Addition of vitamin K-1 or K-3 but not that of benzyl viologen increased the extent of the slow decay phase of P-700 +. The extraction also induced a new fluorescence band peaking at 692 nm, the intensity of which increased upon addition of dithionite. This fluroescence band is possibly related to the rapid charge recombination between A − 0 and P-700 +. These results strongly suggest that A 1 is vitamin K-1 and that it functions to mediate electron transfer between A 0 and FeS centers. Similar results were also obtained with the ‘P-700 enriched particles’ with 7–16 antenna chlorophyll a P-700 which can be obtained by extraction of Photosystem I particles with diethyl ether containing (80% saturated) water.

Site of salicylaldoxime interaction with photosystem II

The reversible inhibition of Photosystem II by salicylaldoxime was studied in spinach D-10 particles by fluorescence, optical absorption, and electron spin resonance spectroscopy. In the presence of 15 mM salicylaldoxime, the initial fluorescence yield was raised to the level of the maximum fluorescence, indicating efficient charge recombination between reduced pheophytin (Ph) and P680(+). In agreement with the rapid (ns) backreaction expected between Ph(-) and P680(+), the optical absorption transient at 820 mm was not observed. When the particles were washed free of salicylaldoxime, the optical absorption transient resulting from the rereduction of P680(+) was restored to the µs timescale. These results, along with the previously observed inhibition of electron transport reactions and diminution of the 515-nm absorption change in chloroplasts [Golbeck, J.H. (1980) Arch Biochem Biophys 202, 458-466], are consistent with a site of inhibition between Ph and QA in Photosystem II. ESR Signal IIf and Signal Its were abolished in the presence of 25 mM salicylaldoxime, but both signals could be recovered by washing the D-10 particles free of the inhibitor. The loss of Signal Ilf is most likely a consequence of the inhibition between Ph and QA; the rapid charge recombination between Ph(-) and P680(+) would preclude electron transfer from an electron donor on the oxidizing side of Photosystem II. The loss of Signal Its may be due to a change in the environment of the donor complex such that the semiquinone radical giving rise to Signal Its interacts with a nearby reductant.

The existence of a high photochemical turnover rate at the reaction centers of System II in Tris-washed chloroplasts

In Tris-washed chloroplasts the kinetics of the primary electron acceptor X 320 of reaction center II has been investigated by fast repetitive flash spectroscopy with a time resolution of ≈ 1 μs. It has been found that X 320 is reduced by a flash in ⩽ 1 μs. The subsequent reoxidation in the dark occurs mainly by a reaction with a 100–200 μs kinetics. The light-induced difference spectrum confirms X 320 to be the reactive species. From these results it is concluded that in Tris-washed chloroplasts the reaction centers of System II are characterized by a high photochemical turnover rate mediated either via rapid direct charge recombination or via fast cyclic electron flow.