Heterocycle- and Amine-Free Electrochromic and Electrofluorochromic Molecules for Energy-Saving See-Through Smart Windows and Displays.

Electrochromic (EC) smart windows are an elegant alternative to dusty curtains, blinds, and traditional dimming devices. The EC energy storage smart windows and displays received remarkable attention in the optoelectronic industry as they hold promise for high energy efficiency, low power consumption, reversibility, and swift response to stimuli. However, achieving these properties remains challenging. Moreover, most EC molecules do not exhibit electrofluorochromism, which is highly essential for smart displays because its EC property can modulate the solar heat entering the building, and its electrofluorochromic (EFC) aspects can create lighting during the night. In this work, a structure-property relationship is utilized to develop new electrochromes that can store the injected charge, and these molecules indeed exhibit electrofluorochromism. The compounds are synthesized from tetrabenzofluorene with two aromatic acceptor units, and avoids the use of widely studied heterocycles and amine derivatives. The electrochromes switches from yellow to dark hue in solution, solid, and gel state. The compounds display exceptional electrochemical stability and reversibility in 1000 cycles and capacity retention of 93-100 % in 300 charging-discharging cycles. The proof-of-concept device fabrication of the self-dimming EC smart window presented here demonstrates that it can furnish visual comfort, modulate transmitted light and glare, and reduce energy usage.

Density Functional Theory Study on [Ni0(1,10-Phenanthroline)]-Catalyzed Reductive Carboxylation of Alkyl and Aryl Halides with CO2: Effect of the Lewis Acid and ß-H Elimination Side Reaction in the Crucial CO2 Insertion Step.

The [Ni0(phen)] (1)-catalyzed reductive carboxylation of propyl and phenyl chloride with a CO2 molecule has been compared using the density functional theory method. The reactivity of 1 in the initial oxidative addition with the propyl and phenyl chloride and in the subsequent single electron transfer step to form the NiI intermediates, [NiI(phen)(R'-CH2)], 4, (R' = CH3-CH2-), and [NiI(phen)(C6H5)], 4', is almost the same. However, an apparent reactivity difference is interpreted in the CO2 insertion step, which involves the NiI and the NiII intermediates. In both propyl and phenyl chloride, the NiI-mediated CO2 insertion is kinetically more preferred than that mediated by the analogues NiII intermediates (3 and 3') by +5.0 and +33.4 kcal/mol, respectively. This trend in energetics clearly shows that the CO2 insertion of phenyl chloride exclusively occurs via the NiI-mediated pathway, whereas in propyl chloride, it follows both NiI- and NiII-mediated pathways. Overall, the catalytic efficacy of 1 is found to be higher in phenyl chloride (by +11.3 kcal/mol) than that in propyl chloride. Furthermore, the effect of a plausible ß-H elimination side reaction in the CO2 insertion step is modeled for propyl chloride. Herein, the ß-H elimination of the NiII propyl species (3) is kinetically more feasible than its CO2 insertion, while the ß-H elimination of NiI propyl (4) is rather difficult compared to its CO2 insertion by +16.3 kcal/mol. This strongly supports the suitability of the NiI intermediate in the CO2 insertion step. In addition, the role of the Lewis acid (X) in the CO2 insertion step is tested by incorporating various Lewis acids (X = MgCl2, ZnCl2, AlCl3, and LiCl) in the NiII propyl (7) and NiI propyl (5) intermediates. The Lewis acids effectively facilitate the CO2 insertion step, and the effect due to MgCl2 is found to be more evident. MgCl2 enhances the CO2 insertion of 5 and 7 by 89 and 84%, respectively, and hence, the NiI-mediated CO2 insertion of propyl halide (ΔG⧧ = +1.4 kcal/mol) is now comparable with that of phenyl halide (ΔG⧧ = +0.9 kcal/mol). This suggests that in the presence of Lewis acids, the catalytic efficacy of 1 is enhanced for the reductive carboxylation of propyl halide and exhibits similar reactivity to that of phenyl halide.

In-situ interfacial compatibilization via edge-sulfurated few layer graphene during the formation of crosslinked graphene-rubber nanocomposites.

Herein, we report various physico-chemical approaches to probe the nature of the interface between few layers graphene (FLG) and carboxylated nitrile rubber (XNBR) nanocomposites prepared through efficient blending of XNBR latex with an aqueous dispersion of FLG. The extent of physical interaction between FLG and XNBR was investigated using Lorentz-Park and Cunneen-Russell models. The chemical interface between FLG and sulfur crosslinked XNBR was studied using model reactions between sulfur and graphene in presence of zinc 2-mercaptobenzothiazole (ZMBT). We propose that an edge sulfurated FLG is formed, which could chemically bond with XNBR during the vulcanization process. Density Functional Theory (DFT) was employed to unravel the mechanistic insights, which support this hypothesis and suggest a kinetically favorable sulfuration of both XNBR and FLG. The formation of a chemical bond between edge-FLG and XNBR through the proposed intermediacy of sulfurated FLG leads to the observed improvement in mechanical properties of the nanocomposites.

A Density Functional Theory Study on Comparing the Reactivity of [Mn(13-TMC)(OOH)]2+ and [Mn(13-TMC)(O2)]+ for the Sulfoxidation of Thioanisole: Elucidation of Substrate and Non-Redox Metal Ion Effects.

The reactivities of [Mn(13-TMC)(OOH)]2+ (1) and [Mn(13-TMC)(O2)]+ (2) in the sulfoxidation of thioanisole have been compared using density functional theory methods. The orientation of the 13-TMC ligand and substrate and non-redox metal ion effects have been considered to improve the oxidation efficiency of 1 and 2. In 1, the syn- and anti-orientation of the 13-TMC ligand do not change the coordination of the Mn ion. In contrast, the orientation of the 13-TMC ligand regulates the geometry of 2, wherein the syn-13-TMC ligand exhibits the MnIII-peroxo (2hs and 2ls) species, while the anti-13-TMC shows the MnII-superoxo (2'hs and 2'ls) species. However, the MnII-superoxo species are found to be less stable than the MnIII-peroxo complexes by around +26.6 kcal/mol. The ground state geometries of 1 and 2 with the syn-13-TMC ligand are found to be more stable in the high- (S = 2) spin states (1hs and 2hs) than the low- (S = 1) spin complexes (1ls and 2ls), by +15.6 and +25.5 kcal/mol, respectively. The computed mechanistic pathways clearly indicate that the sulfoxidation of thioanisole by 1hs is kinetically (by +16.6 to +46.1 kcal/mol) and thermodynamically (+14.4 to +56.1 kcal/mol) more preferred than 1ls, 2hs, and 2ls species. This is mainly due to the feasible heterolytic O1-O2 bond cleavage followed by the proton transfer step. In addition, the molecular electrostatic potential analysis indicates that the higher oxidation efficacy of 1hs than 2hs is due to the -OOH moiety. The reactivity of 1hs is further enhanced by incorporating electron donating substituents in thioanisole, wherein the p-NH2 thioanisole decreases the ΔG‡ of 1hs by 28%. Interestingly, the incorporation of non-redox metal ions (Mn+ = Sc3+, Y3+, Mg2+, and Zn2+) improves the reactivity of 2hs, wherein the non-redox metal ions tend to bind with the oxygen atoms of 2hs and subsequently shift the one-electron reduction potential (E0(red) vs SCE) toward the positive side. The positive shift in the E0(red) is more evident in 2hs-Y3+ that significantly decreases the ΔG‡ of 2hs by 58.7%, which is in fact lower than the ΔG‡ of 1hs by +2.0 kcal/mol. Hence, in the presence of Y3+, the reactivity of 2hs is comparable with 1hs in the sulfoxidation of thioanisole.

Adaptation of aerobic respiration to low O2 environments.

Aerobic respiration in bacteria, Archaea, and mitochondria is performed by oxygen reductase members of the heme-copper oxidoreductase superfamily. These enzymes are redox-driven proton pumps which conserve part of the free energy released from oxygen reduction to generate a proton motive force. The oxygen reductases can be divided into three main families based on evolutionary and structural analyses (A-, B- and C-families), with the B- and C-families evolving after the A-family. The A-family utilizes two proton input channels to transfer protons for pumping and chemistry, whereas the B- and C-families require only one. Generally, the B- and C-families also have higher apparent oxygen affinities than the A-family. Here we use whole cell proton pumping measurements to demonstrate differential proton pumping efficiencies between representatives of the A-, B-, and C-oxygen reductase families. The A-family has a coupling stoichiometry of 1 H(+)/e(-), whereas the B- and C-families have coupling stoichiometries of 0.5 H(+)/e(-). The differential proton pumping stoichiometries, along with differences in the structures of the proton-conducting channels, place critical constraints on models of the mechanism of proton pumping. Most significantly, it is proposed that the adaptation of aerobic respiration to low oxygen environments resulted in a concomitant reduction in energy conservation efficiency, with important physiological and ecological consequences.

Differential effects of glutamate-286 mutations in the aa(3)-type cytochrome c oxidase from Rhodobacter sphaeroides and the cytochrome bo(3) ubiquinol oxidase from Escherichia coli.

Both the aa(3)-type cytochrome c oxidase from Rhodobacter sphaeroides (RsCcO(aa3)) and the closely related bo(3)-type ubiquinol oxidase from Escherichia coli (EcQO(bo3)) possess a proton-conducting D-channel that terminates at a glutamic acid, E286, which is critical for controlling proton transfer to the active site for oxygen chemistry and to a proton loading site for proton pumping. E286 mutations in each enzyme block proton flux and, therefore, inhibit oxidase function. In the current work, resonance Raman spectroscopy was used to show that the E286A and E286C mutations in RsCcO(aa3) result in long range conformational changes that influence the protein interactions with both heme a and heme a(3). Therefore, the severe reduction of the steady-state activity of the E286 mutants in RsCcO(aa3) to ~0.05% is not simply a result of the direct blockage of the D-channel, but it is also a consequence of the conformational changes induced by the mutations to heme a and to the heme a(3)-Cu(B) active site. In contrast, the E286C mutation of EcQO(bo3) exhibits no evidence of conformational changes at the two heme sites, indicating that its reduced activity (3%) is exclusively a result of the inhibition of proton transfer from the D-channel. We propose that in RsCcO(aa3), the E286 mutations severely perturb the active site through a close interaction with F282, which lies between E286 and the heme-copper active site. The local structure around E286 in EcQO(bo3) is different, providing a rationale for the very different effects of E286 mutations in the two enzymes. This article is part of a Special Issue entitled: Allosteric cooperativity in respiratory proteins.

Blocking the K-pathway still allows rapid one-electron reduction of the binuclear center during the anaerobic reduction of the aa3-type cytochrome c oxidase from Rhodobacter sphaeroides.

The K-pathway is one of the two proton-input channels required for function of cytochrome c oxidase. In the Rhodobacter sphaeroides cytochrome c oxidase, the K-channel starts at Glu101 in subunit II, which is at the surface of the protein exposed to the cytoplasm, and runs to Tyr288 at the heme a3/CuB active site. Mutations of conserved, polar residues within the K-channel block or inhibit steady state oxidase activity. A large body of research has demonstrated that the K-channel is required to fully reduce the heme/Cu binuclear center, prior to the reaction with O2, presumably by providing protons to stabilize the reduced metals (ferrous heme a3 and cuprous CuB). However, there are conflicting reports which raise questions about whether blocking the K-channel blocks both electrons or only one electron from reaching the heme/Cu center. In the current work, the rate and extent of the anaerobic reduction of the heme/Cu center were monitored by optical and EPR spectroscopies, comparing the wild type and mutants that block the K-channel. The new data show that when the K-channel is blocked, one electron will still readily enter the binuclear center. The one-electron reduction of the resting oxidized ("O") heme/Cu center of the K362M mutant, results in a partially reduced binuclear center in which the electron is distributed about evenly between heme a3 and CuB in the R. sphaeroides oxidase. Complete reduction of the heme/Cu center requires the uptake of two protons which must be delivered through the K-channel.

Accommodation of two diatomic molecules in cytochrome bo: insights into NO reductase activity in terminal oxidases.

Bacterial heme-copper terminal oxidases react quickly with NO to form a heme-nitrosyl complex, which, in some of these enzymes, can further react with a second NO molecule to produce N(2)O. Previously, we characterized the heme a(3)-NO complex formed in cytochrome ba(3) from Thermus thermophilus and the product of its low-temperature illumination. We showed that the photolyzed NO group binds to Cu(B)(I) to form an end-on NO-Cu(B) or a side-on copper-nitrosyl complex, which is likely to represent the binding characteristics of the second NO molecule at the heme-copper active site. Here we present a comparative study with cytochrome bo(3) from Escherichia coli. Both terminal oxidases are shown to catalyze the same two-electron reduction of NO to N(2)O. The EPR and resonance Raman signatures of the heme o(3)-NO complex are comparable to those of the a(3)-NO complex. However, low-temperature FTIR experiments reveal that photolysis of the heme o(3)-NO complex does not produce a Cu(B)-nitrosyl complex, but that instead, the NO remains unbound in the active-site cavity. Additional FTIR photolysis experiments on the heme-nitrosyl complexes of these terminal oxidases, in the presence of CO, demonstrate that an [o(3)-NO.OC-Cu(B)] tertiary complex can form in bo(3) but not in ba(3). We assign these differences to a greater iron-copper distance in the reduced form of bo(3) compared to that of ba(3). Because this difference in metal-metal distance does not appear to affect the NO reductase activity, our results suggest that the coordination of the second NO to Cu(B) is not an essential step of the reaction mechanism.

A new ruthenium complex to study single-electron reduction of the pulsed O(H) state of detergent-solubilized cytochrome oxidase.

The first step in the catalytic cycle of cytochrome oxidase, the one-electron reduction of the fully oxidized enzyme, was investigated using a new photoactive binuclear ruthenium complex, [Ru(bipyrazine)2]2(quaterpyridine), (Ru2Z). The aim of the work was to examine differences in the redox kinetics resulting from pulsing the oxidase (i.e., fully reducing the enzyme followed by reoxidation) just prior to photoreduction. Recent reports indicate transient changes in the redox behavior of the metal centers upon pulsing. The new photoreductant has a large quantum yield, allowing the kinetics data to be acquired in a single flash. The net charge of +4 on Ru2Z allows it to bind electrostatically near CuA in subunit II of cytochrome oxidase. The photoexcited state Ru(II*) of Ru2Z is reduced to Ru(I) by the sacrificial electron donor aniline, and Ru(I) then reduces CuA with yields up to 60%. A stopped-flow-flash technique was used to form the pulsed state of cytochrome oxidase (the "OH" state) from several sources (bovine heart mitochondria, Rhodobacter sphaeroides, and Paracoccus denitrificans). Upon mixing the fully reduced anaerobic enzyme with oxygenated buffer containing Ru2Z, the oxidized OH state was formed within 5 ms. Ru2Z was then excited with a laser flash to inject one electron into CuA. Electron transfer from CuA --> heme a --> heme a3/CuB was monitored by optical spectroscopy, and the results were compared with the enzyme that had not been pulsed to the OH state. Pulsing had a significant effect in the case of the bovine oxidase, but this was not observed with the bacterial oxidases. Electron transfer from CuA to heme a occurred with a rate constant of 20,000 s-1 with the bovine cytochrome oxidase, regardless of whether the enzyme had been pulsed. However, electron transfer from heme a to the heme a3/CuB center in the pulsed form was 63% complete and occurred with biphasic kinetics with rate constants of 750 s-1 and 110 s-1 and relative amplitudes of 25% and 75%. In contrast, one-electron injection into the nonpulsed O form of the bovine oxidase was only 30% complete and occurred with monophasic kinetics with a rate constant of 90 s-1. This is the first indication of a difference between the fast form of the bovine oxidase and the pulsed OH form. No reduction of heme a3 is observed, indicating that CuB is the initial electron acceptor in the one-electron reduced pulsed bovine oxidase.

Evolutionary migration of a post-translationally modified active-site residue in the proton-pumping heme-copper oxygen reductases.

In the respiratory chains of aerobic organisms, oxygen reductase members of the heme-copper superfamily couple the reduction of O2 to proton pumping, generating an electrochemical gradient. There are three distinct families of heme-copper oxygen reductases: A, B, and C types. The A- and B-type oxygen reductases have an active-site tyrosine that forms a unique cross-linked histidine-tyrosine cofactor. In the C-type oxygen reductases (also called cbb3 oxidases), an analogous active-site tyrosine has recently been predicted by molecular modeling to be located within a different transmembrane helix in comparison to the A- and B-type oxygen reductases. In this work, Fourier-transform mass spectrometry is used to show that the predicted tyrosine forms a histidine-tyrosine cross-linked cofactor in the active site of the C-type oxygen reductases. This is the first known example of the evolutionary migration of a post-translationally modified active-site residue. It also verifies the presence of a unique cofactor in all three families of proton-pumping respiratory oxidases, demonstrating that these enzymes likely share a common reaction mechanism and that the histidine-tyrosine cofactor may be a required component for proton pumping.