Stable Electron Acceptor Research Articles

Stabilizing the resonance structure of nonfused-ring electron acceptors via a closed-loop carbazole side chain for efficient and stable organic solar cells

We have unveiled that a closed-loop carbazole side chain is superior to an open-loop diphenylamine side chain in designing both efficient and stable nonfused-ring electron acceptors.

Achievement of partial denitrification in SBR treating acidic wastewater: Performance and mechanism of nitrite accumulation

Partial denitrification (PD) can provide stable electron acceptors for anaerobic ammonium oxidation; however, PD operates mostly under alkaline conditions (pH >7), limiting its application in acidic wastewater treatment. In this study, PD with nitrate-to-nitrite transformation ratio (NTR) of 69.7 % was achieved using a sequencing batch reactor (SBR) by decreasing the pH (7.5 to 5.5) and the ratio of chemical oxygen demand to nitrate nitrogen (COD/NO3−-N) (4.5 to 3.5). Paracoccus (22.68 %) and Thauera (11.53 %) were the dominant microbes in this study. The ratio of COD/N consumed in the SBR was as high as 5.2 ± 0.6, and the content of extracellular polymeric substances (EPS) increased from 36.5 to 128.4 mg g−1 VSS. In a single cycle, a maximum NTR of 86.7 % was obtained at 90 min, and the NTR was maintained at 70 % even when the reaction time was prolonged to 300 min. Further studies confirmed that acidic pH decreased the rate of nitrate reduction, which increased the duration of nitrate presence and inhibited the reduction of nitrite. More importantly, during nitrate reduction, COD was transformed into glycogen and poly-β-hydroxybutyrate, and further converted to EPS, resulting in nitrite reduction limitation due to carbon source scarcity. This study provides a newfound basis for low consumption treatment of acidic nitrogenous wastewater, such as mining and nuclear industry production.

Novel Anolyte Redox Active Organic Molecules for Redox Flow Battery Applications

Non-aqueous redox flow batteries (NARFBs) offer several advantages over traditional aqueous electrolyte-based redox flow batteries, such as higher cell voltage, potentially higher energy density, and flexible operating temperatures. However, the current aqueous chemistries use toxic metals such as Vanadium and Chromium and highly acidic and oxidative acid mixtures. The efforts to develop metal-ligand based chemistries to tap the benefits of NARFBs have met with mixed success and only V(acac)3 based symmetric NARFBs have shown potential for long term operations. Even so, the solubility of V(acac)3 in non-aqueous solvents is low to compete with their aqueous counterpart. Moreover, modification of V(acac)3 to enhance the solubility has resulted in poor redox cyclability. As a result, recent research trends have shifted from the development of Metal-ligand complexes to small redox-active organic molecules (ROMs), which could provide higher solubility. However, due to the developing field, a few ROMs have been reported as stable electron donors and acceptors in their oxidized and reduced states. Therefore, we have focused on developing highly soluble anolyte materials with stable redox states. This presentation will discuss the design principle, synthesis, and electrochemical analysis of novel anolyte ROMs. The additional information about the kinetics, bulk electrolysis, and the performance in the full cell will also be discussed.

Novel Anolyte Redox Active Organic Molecules for Non-Aqueous Redox Flow Batteries

Non-aqueous redox flow batteries (NARFBs) offer several advantages over traditional aqueous electrolyte-based redox flow batteries, such as higher cell voltage, potentially higher energy density, and flexible operating temperatures. However, the current aqueous chemistries use toxic metals such as Vanadium and Chromium and highly acidic and oxidative acid mixtures. The efforts to develop metal-ligand based chemistries to tap the benefits of NARFBs have met with mixed success and only V(acac)3 based symmetric NARFBs have shown potential for long term operations. Even so, the solubility of V(acac)3 in non-aqueous solvents is low to compete with their aqueous counterpart. Moreover, modification of V(acac)3 to enhance the solubility has resulted in poor redox cyclability. As a result, recent research trends have shifted from the development of Metal-ligand complexes to small redox-active organic molecules (ROMs), which could provide higher solubility. However, due to the developing field, a few ROMs have been reported as stable electron donors and acceptors in their oxidized and reduced states. Therefore, we have focused on developing highly soluble anolyte materials with stable redox states. This presentation will discuss the design principle, synthesis, and electrochemical analysis of novel anolyte ROMs. The additional information about the kinetics, bulk electrolysis, and the performance in the full cell will also be discussed. Acknowledgement This work is supported by Laboratory Directed Research & Development (20210680ECR), Los Alamos National Laboratory.

Tuning the LUMO Levels of Z-Shaped Perylene Diimide via Stepwise Cyanation.

The central dogma in constructing organic electron acceptors is to attach electron-withdrawing groups to polycyclic aromatic hydrocarbons. Yet, the full potentials of many organic acceptors were never realized due to synthetic obstacles. By combining the Wittig-Knoevenagel benzannulation, the Pd(0)-catalyzed cyanation, and nucleophilic addition/oxidation cyanation, six polynitrile Z-shaped perylene diimide were synthesized. These stable and soluble electron acceptors possess LUMO energy levels comparable with those of benchmark compounds. Electrochemical investigation reveals that each additional nitrile group reduces the LUMO energy by 0.2 eV.

Synthesis and Physical Study of Perylene and Anthracene Polynitrile as Electron Acceptors.

Attaching electron-withdrawing nitrile groups to π-conjugated systems is an effective approach toward electron acceptors. By combining various cyanation methodologies, perylene and anthracene polynitriles with up to eight nitrile substituents on one aromatic scaffold were synthesized. This strategy produces stable electron acceptors with lowest unoccupied molecular orbital (LUMO) levels comparable to benchmark acceptors. A very stable octanitrile anion was also produced serendipitously. Reactivity patterns of these acceptors were rationalized by density functional theory (DFT) calculation.

Nitric oxide removal from flue gas with ammonium using AnammoxDeNOx process and its application in municipal sewage treatment

A novel AnammoxDeNOx process was designed to simultaneously remove NOx in flue gas and ammonium wastewater, with the aim of exploring the possibility of using NO as a long-term and stable electron acceptor for anammox bacteria. The performance of the AnammoxDeNOx process indicated a NOx removal efficiency from simulated flue gas (including CO2, SO2, O2 and NO2) of 87–96% using simulated ammonium wastewater. With municipal wastewater, the removal efficiencies for NOx were 70–90%, total nitrogen 40–70%, and COD 80–90% (NO concentration: 100–500 ppm). The anammox genus underwent considerable changes from the dominant Candidatus Kuenenia in the stage of domestication to the predominant Candidatus Brocadia, which then became the dominant species in the simulated flue gas and actual municipal wastewater stages.

Tailoring Organic Redox Couples for Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are promising candidates for grid storage, but current systems have not met the stringent cost and/or safety requirements needed for widespread implementation. Replacing vanadium with organic compounds may lower materials cost, and utilizing non-aqueous (aprotic) electrolyte solvents, in place of water, could enable a 2- to 3-fold increase in operating voltage. Both features make non-aqueous RFBs candidates for large-scale stationary storage. A limited number of organic compounds have been reported as stable electron donors and acceptors, with even fewer materials being studied as small molecule two-electron donors and/or two-electron acceptors. Our recent efforts have focused on the development of highly soluble electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability in all relevant states of charge as well as solubility. In particular, we highlight the design, synthesis, and electrochemical analysis of organic redox couples. Results will be presented on the cycling of phenothiazine and naphthoquinone derivatives in flowing full cell battery prototypes.

Doubling up: Increasing Charge Storage in Organic Donors and Acceptors for Non-Aqueous Redox Flow Batteries

A limited number of organic compounds have been reported as stable electron donors and acceptors for non-aqueous redox flow batteries, with even fewer materials studied as two-electron donors and/or acceptors. Yet, assuming that the solubility of the neutral and charged species were retained and that the molecular weight did not significant increase, doubling the charge storage capabilities of the redox molecule would lead to proportionally increases in electrolyte capacity. Pursuant to this goal, this paper reports on design strategies utilized to increase molecular stability in doubly oxidized and reduced states of organic compounds whilst retaining atom economy and high solubility. In particular, we highlight progress in phenothiazine and naphthoquinone derivatives designed to serve as two-electron donors and two-electron acceptors, respectively.

Doubling up: Increasing Charge Storage in Organic Donors and Acceptors for Non-Aqueous Redox Flow Batteries

Redox flow batteries (RFBs) are promising candidates for grid storage, with a few large-scale systems currently in operation. However, current systems have not met the stringent cost and/or safety requirements needed for widespread implementation. Replacing vanadium with organic compounds may lower materials cost, and utilizing non-aqueous (aprotic) electrolyte solvents, in place of water, could enable a 2- to 3-fold increase in operating voltage. Both features make non-aqueous RFBs candidates for large-scale stationary storage. Currently a limited number of organic compounds have been reported as stable electron donors and acceptors, with even fewer materials being studied as small molecule two-electron donors and/or two-electron acceptors. Yet if the amount of charged stored within an individual molecule were raised without significantly increasing the molecular weight, then electrolyte capacity could be increased proportionally, assuming solubility of neutral and charged species is retained. Our recent efforts have focused on the development of highly soluble electron donors and acceptors with stable oxidized and reduced states. This presentation will focus on design strategies utilized to increase molecular stability in all relevant states of charge as well as solubility. In particular, we highlight the design, synthesis, and electrochemical analysis of phenothiazine and napthoquinone derivatives designed to serve as two-electron donors and two-electron acceptors, respectively. We show that tactical placement of substituents leads to improved stability of doubly oxidized and doubly reduced species, whilst retaining atom economy and that high solubility.

A High Cyano Content Indacene as a Stable Organic Electron Acceptor

A High Cyano Content Indacene as a Stable Organic Electron Acceptor

Unidirectional photodamage of pheophytin in photosynthesis

The primary reactions in photosynthesis take place in the reaction centers surrounded by light harvesting complexes (Blankenship, 2002; Diner and Rappaport, 2002; Frank and Brudvig, 2004). There are two types of photosynthetic reaction centers. The type I reaction center has iron-sulfur cluster as stable electron acceptor, such as photosystem I complexes, green bacterial and heliobacterial reaction centers, and the type II uses quinone as stable electron acceptor including photosystem II and purple bacterial reaction centers. The electron transfer in type II reaction center in unidirectional via the L-subunit of the reaction centers (Maroti et al., 1985; Hoerber et al., 1986; Martin et al., 1986; Michel-Beyerle et al., 1988). However, the electron transfer in the type I reaction center is different from that in the type II centers, which is bidirectionational (Guergova-Kuras et al., 2001; Li et al., 2006). The three dimensional structures of both types of reaction centers were determined at atomic resolution (Jordan et al., 2001; Ferreira et al., 2004; Loll et al., 2005; Amunts et al., 2007; Umena et al., 2011). The electron transfer pathways in both types of reaction centers are well established (Van Grondelle, 1985; Schatz et al., 1988; Fleming and van Grondelle, 1997; Dekker and Van Grondelle, 2000; Gobets and van Grondelle, 2001; Seibert and Wasielewski, 2003; Van Grondelle and Novoderezhkin, 2006). The primary electron donor in the reaction centers is a pair of chlorophyll molecules, and pheophytin is the primary electron acceptor (Klimov et al., 1977; Holzwarth et al., 2006).

Chloroplast redox imbalance governs phenotypic plasticity: the "grand design of photosynthesis" revisited.

Sunlight, the ultimate energy source for life on our planet, enters the biosphere as a direct consequence of the evolution of photoautotrophy. Photoautotrophs must balance the light energy absorbed and trapped through extremely fast, temperature-insensitive photochemistry with energy consumed through much slower, temperature-dependent biochemistry and metabolism. The attainment of such a balance in cellular energy flow between chloroplasts, mitochondria and the cytosol is called photostasis. Photoautotrophs sense cellular energy imbalances through modulation of excitation pressure which is a measure of the relative redox state of QA, the first stable quinone electron acceptor of photosystem II reaction centers. High excitation pressure constitutes a potential stress condition that can be caused either by exposure to an irradiance that exceeds the capacity of C, N, and S assimilation to utilize the electrons generated from the absorbed energy or by low temperature or any stress that decreases the capacity of the metabolic pathways downstream of photochemistry to utilize photosynthetically generated reductants. The similarities and differences in the phenotypic responses between cyanobacteria, green algae, crop plants, and variegation mutants of Arabidopsis thaliana as a function of cold acclimation and photoacclimation are reconciled in terms of differential responses to excitation pressure and the predisposition of photoautotrophs to maintain photostasis. The various acclimation strategies associated with green algae and cyanobacteria versus winter cereals and A. thaliana are discussed in terms of retrograde regulation and the “grand design of photosynthesis” originally proposed by Arnon (1982).

Modulating the Redox Potential of the Stable Electron Acceptor, QB, in Mutagenized Photosystem II Reaction Centers

One of the unique features of electron transfer processes in photosystem II (PSII) reaction centers (RC) is the exclusive transfer of electrons down only one of the two parallel cofactor branches. In contrast to the RC core polypeptides (psaA and psaB) of photosystem I (PSI), where electron transfer occurs down both parallel redox-active cofactor branches, there is greater protein-cofactor asymmetry between the PSII RC core polypeptides (D1 and D2). We have focused on the identification of protein-cofactor relationships that determine the branch along which primary charge separation occurs (P(680)(+)/pheophytin(-)(Pheo)). We have previously shown that mutagenesis of the strong hydrogen-bonding residue, D1-E130, to less polar residues (D1-E130Q,H,L) shifted the midpoint potential of the Pheo(D1)/Pheo(D1)(-) couple to more negative values, reducing the quantum yield of primary charge separation. We did not observe, however, electron transfer down the inactive branch in D1-E130 mutants. The protein residue corresponding to D1-E130 on the inactive branch is D2-Q129 which presumably has a reduced hydrogen-bonding interaction with Pheo(D2) relative to the D1-E130 residue with Pheo(D1). Analysis of the recent 2.9 Å cyanobacterial PSII crystal structure indicated, however, that the D2-Q129 residue was too distant from the Pheo(D2) headgroup to serve as a possible hydrogen bond donor and directly impact its midpoint potential as well as potentially determine the directionality of electron transfer. Our objective was to characterize the function of this highly conserved inactive branch residue by replacing it with a nonconservative leucine or a conservative histidine residue. Measurements of Chl fluorescence decay kinetics and thermoluminescence studies indicate that the mutagenesis of D2-Q129 decreases the redox gap between Q(A) and Q(B) due to a lowering of the redox potential of Q(B). The resulting increased yield of S(2)Q(B)(-) charge recombination in the D2-Q129 mutants leads to an increased susceptibility to photoinhibitory light presumably due to (3)P(680)-mediated oxidative damage. The results indicate that the D2-Q129 residue plays a critical role in stabilizing the charge-separated state in PSII and further documents the structural and functional asymmetry between the two cofactor branches in PSII.

Diazaboroles with quinone units: hydrogen bonding network and n-type FETs involving a three-coordinate boron atom

A series of monodiazaborole and bis(diazaborole) derivatives with a quinone moiety have been prepared as stable electron acceptors. They show dark colors in the solid states and reversible reduction potentials in the cyclic voltammograms. X-Ray structure analysis of one derivative revealed a π-stacking structure accompanied with a hydrogen-bonding network. Trifluoromethyl substituted derivatives afforded n-type FET characteristics with an electron mobility of 10−2 cm2 V−1s−1. An ambipolar FET characteristic was also observed in the biphenyl derivative. The planar three-coordinate boron units were used as π-linkers to afford n-type and ambipolar FET characteristics.

Theoretical investigation of Q A −· -ligand interactions in bacterial reaction centers ofRhodobacter sphaeroides

Density functional theory was used to calculate magnetic resonance parameters for the primary stable electron acceptor anion radical (QA−·) in its binding site in the bacterial reaction center (bRC) ofRhodobacter sphaeroides. The models used for the calculations of the QA−· binding pocket included all short-range interactions of the ubiquinone with the protein surroundings in a gradual manner and thus allowed a decomposition and detailed analysis of the different specific interactions. Comparison of the obtained hyperfine and quadrupole couplings with experimental data demonstrates the feasibility and reliability of calculations on such complex biologically relevant systems. With these results, the interpretation of previously published 3-pulse electron spin echo envelope modulation data could be extended and an assignment of the observed double quantum peak to a specific amino acid is proposed. The computations provide evidence for a slightly altered binding site geometry for the QA ground state as investigated by X-ray crystallography with respect to the QAt-· anion radical state as accessible via EPR spectroscopy. This new geometry leads to improved fits of the W-band correlated-coupled radical pair spectra of QA−·-P865+· compared to orientation data from the crystal structure. Finally, a correlation of the14N quadrupole parameters of His219 with the hydrogen bond geometry and a comparison with previous systematic studies on the influence of hydrogen bond geometry on quadrupole coupling parameters (J. Fritscher: Phys. Chem. Chem. Phys. 6, 4950–4956, 2004) is presented.

Effects of long‐term chilling on growth and photosynthesis of the C4 gramineae Paspalum dilatatum

Dallis grass (Paspalum dilatatum Poir.) is a C4/NADP‐ME gramineae, previously classified as semi‐tolerant to cold, although a complete study on this species acclimation process under a long‐term chilling and controlled environmental conditions has never been conducted. In the present work, plants of the variety Raki maintained at 25/18°C (day/night) (control) were compared with plants under a long‐term chilling at 10/8°C (day/night) (cold‐acclimated) in order to investigate how growth and carbon assimilation mechanisms are engaged in P. dilatatum chilling tolerance. Although whole plant mean relative growth rate (mean RGR) and leaf growth were significantly decreased by cold exposure, chilling did not impair plant development nor favour the investment in biomass below ground. Cold‐acclimated P. dilatatum cv. Raki had a lower leaf chlorophyll content, but a higher photosynthetic capacity at optimal temperatures, its range being shifted to lower values. Associated with this higher capacity to use the reducing power in CO2 assimilation, cold‐acclimated plants further showed a higher capacity to oxidize the primary stable quinone electron acceptor of PSII, QA. The activity and activation of phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) were not significantly affected by the long‐term chilling. Cold‐acclimated P. dilatatum cv. Raki apparently showed a lower transfer of excitation energy from the light‐harvesting complex of photosystem II to the respective reaction centre and enhancement of radiationless energy‐dissipating mechanisms at suboptimal temperatures. Overall, long‐term chilling resulted in several effects that comprise responses with an intermediate character of both chilling‐tolerant and –sensitive plants, which seem to play a significant role in the survival and acclimation of P. dilatatum cv. Raki at low temperature.

Photosynthetic energy conversion under extreme conditions—II: the significance of lipids under light limited growth in Antarctic sea ice diatoms

Low photosynthetic active radiation is a strong determinant in the development and growth of sea ice algae. The algae appear to have universal mechanisms to overcome light limitation. One important process, which is induced under light limitation, is the desaturation of chloroplast membrane lipids. In order to discover whether this process is universally valid in sea ice diatoms, we investigated three species coexisting in chemostats illuminated with 15 and 2 μmol photons m −2 s −1 at −1 °C. Growth under 2 μmol photons m −2 s −1 caused a 50% increase in monogalactosyldiacylglycerols (MGDG) thylakoid membrane related 20:5 n-3 fatty acids. This fatty acid supports the fluidity of the thylakoid membrane and therefore the velocity of electron flow, which is indicated by increasing rate constants for the electron transport between Q A (first stable electron acceptor) and bound Q B (second stable electron acceptor) (11.16±1.34 to 23.24±1.35 relative units). Two μmol photons m −2 s −1 furthermore resulted in higher amounts of non-lipid bilayer forming MGDG in relation to other bilayer forming lipids, especially digalactosydiacylglycerol (DGDG). The ratio of MGDG:DGDG increased from 3.4±0.3 to 5.7±0.3. The existence of bilayer thylakoid membranes with high proportions of non. bilayer forming lipids is only possible when sufficient thylakoid pigment–protein complexes are present. If more thylakoid pigment–protein complexes are present in membranes, as found under extreme light limitation, less bilayer forming lipids such as DGDG are required to stabilize the bilayer structure. Differences in protein contents between both light intensities were not found. Consequently pigment contents which nearly doubled under 2 μmol photons m −2 s −1 must be responsible in balancing the potential stability loss resulting from an increase in MGDG:DGDG ratio.

Photosynthetic energy conversion under extreme conditions—I: important role of lipids as structural modulators and energy sink under N-limited growth in Antarctic sea ice diatoms

The availability of dissolved nutrients such as nitrate under extreme low temperatures is a strong determinant in the development and growth of ice diatoms. Consequently we investigated regulation of photosynthesis in a mixed culture of three diatom species, which grew in chemostats at −1 °C, 15 μmol photons m −2 s −1 under N-limitation. When nitrogen is limiting, pigment–protein complexes are one of the most affected structures under low-light conditions. The loss of integral polar thylakoid components destabilized the bilayer structure of the membrane with consequences for lipid composition and the degree of fatty acid desaturation. N-Limitation caused a decrease in monogalactosydiacylglycerol (MGDG) and a simultaneous increase in bilayer forming digalactosyldiacylglycerol (DGDG). Their ratio MGDG:DGDG decreased from 3.4±0.1 to 1.1±0.4, while 20:5 n-3 fatty acids of chloroplast related phospholipid classes such as phosphatidylglycerol (PG) increased under N-limitation. These data reveal that lipids are important components, required to sustain membrane structure under a deficiency of integral membrane bound proteins and pigments. Nonetheless, energy conversion at photosystem II is still affected by N-limitation despite this structural regulation. Photosynthetic quantum yield ( F v/ F m) and electron transport rates decreased under N-limitation caused by an increasing amount of electron acceptors (second stable electron acceptor=Q B) which had slower reoxidation kinetics. The energy surplus under these conditions is stored in triacylglycerols, the main energy sink in Antarctic sea ice diatoms under N-limitation.

An evaluation of the potential triggers of photoinactivation of photosystem II in the context of a Stern-Volmer model for downregulation and the reversible radical pair equilibrium model.

Photoinactivation of photosystem II (PS II) is a light-dependent process that frequently leads to break-down and replacement of the D1 polypeptide. Photoinhibition occurs when the rate of photoinactivation is greater than the rate at which D1 is replaced and results in a decrease in the maximum efficiency of PS II photochemistry. Downregulation, which increases non-radiative decay within PS II, also decreases the maximum efficiency of PS II photochemistry and plays an important role in protecting against photoinhibition by reducing the yield of photoinactivation. The yield of photoinactivation has been shown to be relatively insensitive to photosynthetically active photon flux density (PPFD). Formation of the P680 radical (P680+), through charge separation at PS II, generation of triplet-state P680 (3P680*), through intersystem crossing and charge recombination, and double reduction of the primary stable electron acceptor of PS II (the plastoquinone, Q(A)) are all potentially critical steps in the triggering of photoinactivation. In this paper, these processes are assessed using fluorescence data from attached leaves of higher plant species, in the context of a Stern-Volmer model for downregulation and the reversible radical pair equilibrium model. It is shown that the yield of P680+ is very sensitive to PPFD and that downregulation has very little effect on its production. Consequently, it is unlikely to be the trigger for photoinactivation. The yields of 3P680* generated through charge recombination or intersystem crossing are both less sensitive to PPFD than the yield of P680+ and are both decreased by down regulation. The yield of doubly reduced Q(A) increases with incident photon flux density at low levels, but is relatively insensitive at moderate to high levels, and is greatly decreased by downregulation. Consequently, 3P680* and doubly reduced Q(A) are both viable as triggers of photoinactivation.