Viologens-based redox mediators with tunable HOMO–LUMO energy gap for highly effective hydrogen peroxide electrosynthesis

In comparison with the developing nano-carbon catalysts,some small organic molecules are also emerging as catalysts with typical features,however,their working mechanism is still unclear.Here,we synthesized a series of viologen-based heterogeneous catalysts with the same molecular skeleton but different substituent groups through anion exchange engineering.These viologen-based molecules were used as a model catalyst to investigate the underlying structure–function relationship for small molecules-based H_(2)O_(2) electrosynthesis.Differing from the commonly reported carbon-based electrocatalysts,viologens can produce H_(2)O_(2) in a synergistic manner,which means that viologens can not only directly catalyze oxygen reduction but also serve as a redox mediator.We found that the ring current and H_(2)O_(2) selectivity of viologens deliver an increasing trend with the increase of the alkyl chain length of alkyl-substituted viologens and further increase when using benzyl as the substituent group.As a result,a benzyl-substituted viologen(BV)delivers the best electrocatalytic performance among the samples,including the highest H_(2)O_(2) selectivity of 96.9%at 0.6 V and the largest ring current density of about 13.6 mA·mmol-1.Furthermore,density functional theory(DFT)calculations disclose that the carbon atoms bonded with positively charged N are the active sites and the small highest occupied molecular orbital(HOMO)–lowest unoccupied molecular orbital(LUMO)energy gap of BV is beneficial to the synergistic mechanism for H_(2)O_(2) production.This work sheds new insight into the efficient H_(2)O_(2) production in a synergistic manner for small molecules-based electrocatalysts.

Pyrene-tethered bismoviologens for visible light-induced C(sp^(3))-P and C(sp^(2))-P bonds formation

Developing efficient photosensitizers for C-P bond construction is highly important and remains a challenge due to the urgently needed for the synthesis of modified nucleosides,nucleotides,and other phosphine-containing ligands.Herein,two pyrene-tethered bismoviologen derivatives(Py-BiV^(2+))were designed and synthesized for visible-light-induced C-P bonds formation.The photochemical and electrochemical properties of Py-BiV^(2+)were studied systemically,certifying fine-tunable opto-electronic properties through the number of pyrene groups(4,n=1;6,n=2).The prepared Py-BiV^(2+)showed strong light absorption,while retaining good redox features and chromic response features that were inherent to viologens.4 exhibited accelerated photoinduced electron transfer in the presence of the electron donor(pyrene)and the generated 4(radical cation)showed higher stability.Accordingly,Py-BiV^(2+)directly served as photosensitizers for the first time in the visible-light-induced C(sp^(3))-P and C(sp^(2))-P bonds formation.As expected,these novel viologen derivatives exhibited good catalytic performance and good substrate expansibility under ambient conditions.

Viologen-based flexible electrochromic devices

Electrochromic technology has gained significant attention in various fields such as displays,smart windows,biomedical monitoring,military camouflage,human-machine interaction,and electronic skin due to its ability to provide reversible and fast color changes under applied voltage.With the rapid development and increasing demand for flexible electronics,flexible electrochromic devices(FECDs)that offer smarter and more controllable light modulation hold great promise for practical applications.The electrochromic material(ECM)undergoing color changes during the electrochemical reactions is one of the key components in electrochromic devices.Among the ECMs,viologens,a family of organic small molecules with 1,1'-disubstituted-4,4'-dipyridinium salts,have garnered extensive research interest,due to their well-reversible redox reactions,excellent electron acceptance ability,and the ability to produce multiple colors.Notably,viologen-based FECDs demonstrate color changes in the liquid or semisolid electrolyte layer,eliminating the need for two solid electrodes and thus simplifying the device structure.Consequently,viologens offer significant potential for the development of FECDs with high optical contrast,fast response speed,and excellent stability.This review aims to provide a comprehensive overview of the progress and perspectives of viologen-based FECDs.It begins by summarizing the typical structure and recent exciting developments in viologen-based FECDs,along with their advantages and disadvantages.Furthermore,the review discusses recent advancements in FECDs with additional functionalities such as sensing,photochromism,and energy storage.Finally,the remaining challenges and potential research directions for the future of viologen-based FECDs are addressed.

Viologen-based host–guest supramolecule with tunable intramolecular/intermolecular electron transfer chromism and dynamic fluorescence

Viologen,as a type of strong electron acceptor,is prone to undergo electron transfer(ET)and change color under external stimuli.However,due to the easy aggrega-tion of viologen molecules,they usually suffer from poorfluorescence emission in the condensed phase.Herein,a new viologen derivative of VioCl_(2)⋅2Cl(1^(2+)⋅2Cl)was designed and synthesized,in which thefluorescence was enhanced by intro-ducing Me-β-CD to weaken the interactions between viologen molecules.Then a viologen-based host-guest supramolecule of 1^(2+)@Me-β-CD was obtained by elec-trostatic interactions.Photo-/chemo-responded guest 1^(2+)supplies 1^(2+)@Me-β-CD,a green and dark purple caused by intramolecular and intermolecular ET.Further-more,1^(2+)@Me-β-CD displays an additional thermal responsive purple color.The triple chromic behaviors all exhibit excellent reversibility and cycling stability.As expected,1^(2+)@Me-β-CD exhibits strong photoluminescence(PL)in solid-liquid dual states,presenting an improved quantum yield(Φ)from 1^(2+)(Φ_(s)=0.37%,Φ_(1)= 16.74%)to=1^(2+)@Me-β-CD(Φ_(s)= 10.45%,Φ_(1)= 25.86%),and thefluores-=cence intensity can be dynamically modulated by light,heat,and acid/base vapors.The multi-responsive chromism and tunablefluorescence of 1^(2+)@Me-β-CD allow for potential applications in information security and smart windows.

Steric hindrance shielding viologen against alkali attack in realizing ultrastable aqueous flow batteries

Viologens known as a kind of promising negolyte materials for aqueous organic redox flow batteries,face a critical stability challenge due to the S_N2 nucleophilic attack by hydroxide ions(OH-)during the battery cycling.In this work,a N-cyclic quaternary ammonium-grafted viologen molecule,viz.1,1'-bis(4,4'-dime thylpiperidiniumyl)-4,4'-bipyridinium tetrachloride((DBPPy)Cl_(4)),is developed by the molecular engineering strategy.The obtained(DBPPy)Cl_(4) molecule shows a decent solubility of 1.84 M and a redox potential of-0.52 V vs.Ag/AgCl,Experimental and theoretical results reveal that the grafted N-cyclic quaternary ammonium groups act as the steric hindrance to prevent nucleophilic attack by OH~-,increasing the alkali resistance of the electroactive molecule.The symmetrical battery with 0.50 M(DBPPy)Cl4shows negligible decay during the 13-day cycling test.As demonstration,the flow battery utilizing 1.0 M(DBPPy)Cl_(4) as the negolyte and 1-(1-oxyl-2,2',6,6'-tetramethylpiperidin-4-yl)-1'-(3-(trimethylammonio)propyl)-4,4'-bipyridinium trichloride as the posolyte exhibits a high capacity retention rate of 99.99%per cycle at 60 mA cm^(-2).

Dynamic color-tunable ultra-long room temperature phosphorescence polymers with photo-chromism and water-stimuli response for multilevel anti-counterfeiting

Developing dynamic color-tunable ultra-long room temperature phosphorescence(URTP)polymers with afterglow of over 1 s,photo-chromism,and multi-stimuli response for practical anti-counterfeiting and information security applications is attractive but very challenging.Herein,by doping multicolor phosphorescence pyri-dinium bromide L block or viologen-based photo-chromic V block into polyvinyl alcohol matrixes,the water-stimuli-responsive color-tunable URTP polymerfilms with afterglow of up to 8 s and the reversible viologen-based photochromic polymerfilms have been developed.More significantly,a series of dynamic color-tunable URTP polymerfilms with ultra-long afterglow of over 6 s,photo-chromism,and water-stimuli response have been successfully exploited by integrating L and V blocks into one polymer system.Mechanistic investigations have revealed that their photo-chromism mainly comes from the photo-generated viologen free radicals.Furthermore,their dynamic multilevel anti-counterfeiting applications have been demonstrated.These results pave the way to develop smarter multifunctional URTP materials for anti-counterfeiting and optical sensing.

A high-capacity viologen-based anolyte for high energy density neutral pH aqueous redox-flow batteries

Redox-flow batteries(RFBs)are a promising energy storage technology with remarkable scalability and safety for storing vast amounts of renewable energy and mitigating outputfluctuations of renewable power grids.We demonstrate a neutral pH aqueous RFB using a custom-designed 1’,1’’’,1’’’’’-(benzene-1,3,5-triyltris(methylene))tris(1-(3-(trimethyl ammonio)propyl)-[4’’,4’’’-bipyridine]-1,1’-diium)nonachloride(BTTMPB)as a 3 e-storage anolyte.The custom design with the high polarization in charge density has led to the excellent water solubility of 4.0 M in H_(2)O(321.6 A h L^(-1))and 2.4 M in 2.0 M NaCl(192.9 A h L^(-1)).The density functional theory(DFT)calculations and electrochemical experiments have shown 3 e-storage response of BTTMPB with a diffusion coefficient of 3.1×10^(-6)cm^(2) s^(-1)and rate con-stant of 1.6×10^(-2)cm s^(-1) for thefirst reduction process.The synthesized anolyte was paired with(Ferrocenylmethyl)trimethylammonium chloride(FcNCl)as catholyte enabling a 0.92 V aqueous RFB with 125.9 W h L^(-1)theoretical energy density.The aqueous RFB has an excellent cycling performance from 10-30 m A cm^(-2),energy efficiency up to 80%,capacity retention of 99.96%per cycle at 20 m A cm^(-2),and a high demonstrated energy density of 29.1 W h L^(-1).

The Synthesis of 1-Ethyl-1'-(4-vinylbenzyl)-4,4'-bipyridinium Chloride and Iodide and its Electrochromic Property

Abstract： A new compound 1-ethyl-1＇-（4-vinylbenzyl）-4, 4＇-bipyridinium chloride and iodide has been synthesized. The cyclic voltammogram and impedance spectra indicated that a layer of viologen＇s electrochromic （EC） film could be deposited on conductive ITO glass working electrode With polyelectrolyte as ionic conduction layer, solid EC devices based on this compound have been assembled and their thickness was about 2.35 mm. When different voltages were added, they showed blue or violet red color. After optimization, its response time was less than 50 ms, the number of redox circulation was over 107 and the color of coloration states could be kept for 3 days. This kind of EC device can meet the demand of electronic ink.

A New Iodiplumbate-based Hybird Constructed from Asymmetric Viologen and Polyiodides：Structure,Properties and Photocatalytic Activity for the Degradation of Organic Dye

A new iodiplumbate/organic hybrid constructed from asymmetric viologen and polyiodides,（PBPY-H2）2[PbI4（I3）2]（1, PBPY = N-（propionate）-4,4？-bipyridinium）, has been synthesized via solvothermal reaction and structurally determined by X-ray diffraction method. Compound 1 crystallizes in monoclinic system, space group P21/c with Mr = 1936.72, a = 11.622（2）, b = 14.839（3）, c = 13.372（2）A^°, β = 109.447（3）°, V = 2174.6（7）A^°^3, Z = 2, Dc = 2.958 g/cm^3, F（000） = 1712, μ（Mo Kα） = 11.011 mm^–1, the final R = 0.0389 and wR = 0.0854 for 3866 observed reflections with I 〉 2σ（I）. [Pb I4（I3）2]^4-mononuclear cluster of 1 features a seldom example of coordinated I-3 donors for the lead center. Intermolecular hydrogen bonds between [Pb I4＋4（I3）2]^4-clusters and（PBPY-H）2^4＋ dimmers contribute to the formation of a 2-D layer. Its absorption spectrum was investigated, and lower energy band gap of 1.42 eV was explained by DFT calculation. Interestingly, 1 exhibits photocatalytic activity for the degradation of rhodamine B.

Design, Synthesis, Crystal Structure and Photophysical Properties of New Oxadiazole Extended Viologen Fluorophore

Viologens (N,N'-dimethyl-4,4-bipyridinium) are advanced functional materials, found important applications in electrochromic devices, molecular machines, organic batteries, and carbohydrate oxidation catalysts in alkaline fuel. In this article, we investigated the design, synthesis and photophysical properties of N,N'-dimethyl-2,5-Bis(pyridinium)oxadiazole 4 and its precursor 2,5- Bis(pyridine)oxadiazole 2. The crystal structure and photophysical properties of viologen 4 and precursor 2 have been determined. The viologen molecule 4 crystallized in monoclinic form, space group P21/n with four molecules in unit cell. Precursor molecule 2 also crystalized in monoclinic form, space group C2/c with four molecules in unit cell. From X-rd data, we found three cations in the molecular structure of viologen molecule 4, which is unusual in viologens. In the three-dimensional molecular packing diagram of molecule 4, the three cations and iodate anions are stabilized by C···C, C···I, N···I, N···H, H···I, N—H···I and C—H···I. The dihedral angle between planes having oxadiazole and two benzene rings are 5°and 8°, suggesting the molecule 4 is a slightly strained one. The molecular structure of precursor molecule 2 stabilized by C···C and N···H short contacts between the molecules. The molecule 4 displayed strong absorbance at 315 nm and emissions between 390 - 410 nm.

SELECTIVE REDUCTION OF NITROARENE MEDIATED BY AN INSOLUBLE RESIN WITH VIOLOGEN STRUCTURE AS ELECTRON-TRANSFER CATALYST

Abstract: : The substituted nitroarenes were reduced effectively to the corresponding aniline deriva tives by an insoluble resin with viologen structure as an electron-transfer catalyst(ETC) in dichloromethane-resin-water -three-phase system.

SYNTHESIS AND PHOTOCHROMISM OF ACRYLIC ESTER COPOLYMERS BEARING PENDANT VIOLOGEN GROUPS

A series of acrylic ester copolymers with viologen group as pendant were synthesizedthrough the reaction of MMA-EBA (Ethenyl bromoacetate) copolymer with 4-(4'-pyridyl)-N-alkyl (or arylalkyl) pyridiniums. These viologen copolymers can exchange their anionwith NH_4PF_6 in methanol to improve their solubility in organic solvents such as DMFand acetone. Compared with the corresponding low molecular viologens, these viologencopolymers have relatively lower color development rate under UV light and fade fasterin air due to less affinity to photo-reductant and deficiency of association between theircation radicals. Their photofatigue resistant ability is also slightly better than that of lowmolecular vinlogens.

Synthesis,Structure and Photoluminescence of (CH_3-4,4'-H_2bipy)-(HgCl_4) with CH_3-4,4'-H_2bipy Generated in Situ

A novel viologen（4,4＇bipyridinium）-based complex （CH3-4,4＇-H2bipy）（HgCl4） （1）,in which the CH3-4,4＇-H2bipy （MQ2＋） was generated in situ,has been synthesized via hydrothermal reaction and structurally characterized. Complex 1 crystallizes in the space group P21/c of monoclinic system with four formula units in a cell：a = 8.1848（6）,b = 21.809（2）,c = 9.0285（6） ,β = 107.377（1）°,V = 1538.0（2） 3,C11H11Cl4HgN2,Mr = 513.61,Dc = 2.218 g/cm3,S = 1.009,μ（MoKα） = 10.685 mm-1,F（000） = 956,R = 0.0360 and wR = 0.0812. The crystal structure analysis reveals that the title complex features an isolated structure based on a CH3-4,4＇-H2bipy moiety and a mercury atom terminally bound by four chlorine atoms. Photoluminescence investigation reveals a strong emission in blue region,which may originate from π→π＊ charge-transfer interaction of the CH3-4,4＇-H2bipy moiety.

Two electron utilization of methyl viologen anolyte in nonaqueous organic redox flow battery

Methyl viologen （MV） as a bench-mark anolyte material has been frequently applied in aqueous organic redox flow batteries （AORFBs） towards large-scale renewable energy storage. However, only the first re- duction of MV was utilized in aqueous electrolytes because of the insoluble MV0generated from the second reduction of MV. Herein, we report that methyl viologen with bis（trifluoromethane）sulfonamide counter anion, MVTFSI, can achieve two reversible reductions in a nonaqueous supporting elec- trolyte. Paired with （Ferrocenylmethyl）trimethylammonium bis（trifluoromethanesulfonyl）imide, FcNFFSI, as catholyte, the MVTFS/FcNTFSI nonaqueous organic redox flow battery （NOARFB） can take advantage of either one electron or two electron storage of the methyl viologen moiety and provide theoretical energy density of 24.9Wh/L and a cell voltage of up to 1.5V. Using a highly conductive LiTFSI/CH_3CN supporting electrolyte and a porous Daramic separator, the NOARFB displayed excellent cycling performance, includ- ing up to a 68.3g energy efficiency at 40 mA/cm2, and more than 88g total capacity retention after 100 cycles.

Preparation and Intramolecular Electron Transfer of A Covalently Linked Zinc Porphyrin-Methyl Viologen (MV^(2+)) Complex

Porphyrins and methyl viologen(MV;) are widely used as a photosensitizer and an electron acceptor respectively in photoinduced electron transfer systems. Literatures covered numerous examples of photochemical studies of porphyrins covalently linked to various electron acceptor units, such as viologen, which functioned as both a photosensitizer and an electron acceptor in the same molecule. However, the length of the linked chain between porphyrin and viologen is very important. To improve the rate of photoin-

A New Iodocuprate/methyl Viologen-based Hybird：Structure,Properties and Photocatalytic Activity for the Degradation of Organic Dye

A new copper iodide/organic hybrid incorporated with methyl viologen, [（MV）（Cu2I4）·H2O]n（1, MV = methyl viologen）, has been solvothermally synthesized and structurally determined by X-ray diffraction method. 1 crystallizes in tetragonal system, space group P42212 with Mr = 838.95, a = 12.4721（18）, c = 6.5892（13A, V = 1025.0（3） A3, Z = 2, Dc = 2.7125 g/cm3, F（000） = 756, μ（Mo Kα） = 8.097 mm–1, the final R = 0.0273 and wR = 0.0665 for 1062 observed reflections with I 〉 2σ（I）. In 1, the（Cu2I4）n2n- chain was constructed from edge-sharing of distorted Cu I4 tetrahedra. Its absorption spectrum and photoluminescence were investigated, and energy band gap of 1.76 e V indicates its semiconductor nature. Interestingly, 1 exhibits photocatalytic activity for the degradation of rhodamine B.

CATALYTIC DEBROMINATION OF DIPHENYL BROMOMETHANE UNDER HETEROPHASE CONDITIONS USING POLYMERS WITH VIOLOGEN STRUCTURE

The debromination of diphenyl bromomethane (Ph_2CHBr) using polymers with viologen structure as electron--transfer catalyst (ETC) afforded tetraphenylethane in good yields under het- erophase conditions.

EPR STUDIES ON THE DEBROMINATION OF DIPHENYL BROMOMETHANE IN THE PRESENCE OF VIOLOGEN DERIVATIVES AS ELECTRON-TRANSFER CATALYSTS (ETC)

The active intermediate'carbon radical' was detected by the spin—trapping reagent (POBN) during the debromination of Ph_2CHBr in the presence of viologen derivatives as electron—transfer catalysts(ETC).

Structures,Properties and Theoretical Studies on Two Viologen/Iodoargentate Supramolecular Hybrids

Two iodoargentate hybrids templated by viologen or viologen-like cations, i.e. [MV（Ag2I4）]n （1） and [MBYP（AgI2）]n （2）, （MV^2＋ = methyl viologen, MBYP^＋ = 1-methyl-2,2＇- bipyridine） have been synthesized in polar organic solvents. （Ag2I4）n^2- and （AgI2）n^2- chains in both compounds were constructed from edge-sharing of normal and distorted AgI4 tetrahedra, and a 2-D supramolecular layer based on C–H…I hydrogen bond was observed in 1. The band gaps of 1.54 and 2.67 eV indicated their semiconductor nature. Their band gap difference was led by different hydrogen bond donors in MV^2＋ and MBYP^＋, which result in more diffused p-π＊ antibonding orbitals in MV^2＋.

A new class of crystalline X-ray induced photochromic materials assembled from anion-directed folding of a flexible cation

Electron-deficient viologens are widely used as ligands or structure-directing agents(SDAs)to synthesize crystalline X-ray induced photochromic materials.Here,a new rational strategy of anion-directed fold-ing a flexible cation(H_(2)imb)^(2+)((H_(2)imb)^(2+)=di-protonated 2,3-bis(imidazolin-2-yl)-2,3-dimethylbutane)has been developed.Electron-donating Cl−and(ZnCl4)2−are used to direct folding a flexible electron-deficient(H_(2)imb)^(2+)cation.Three complexes(H_(2)imb)(NO_(3))2(1),(H_(2)imb)Cl2·H_(2)O(2),and(H_(2)imb)ZnCl4(3)have been synthesized in which(H_(2)imb)^(2+)crystallize in an anti-conformation,88.8°-gauche,and 51.8°-gauche,respectively.In contrary to X-ray silent complex 1,X-ray induced photochromism has been achieved in both complex 2 and 3.An intermolecular charge-transfer mechanism has been elucidated and the anion directed folding of(H_(2)imb)^(2+)has been validated to be critical to yield colored long-lived charge-separated states.