A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water

Using light energy and O₂ for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O₂ by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)₃]²⁺, generating the highly oxidized chromophore and the powerful reductant methyl‐viologen radical MV^+.. MV^+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O₂ to O₂^.? in an aqueous medium to generate an active iron(III)‐(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.     

Thin-layer spectroelectrochemical kinetic study of viologen cation radicals reacting at hydrogen-evolving gold and nickel electrodes

The kinetics of viologen cation radicals reacting at hydrogen-evolving gold and nickel electrodes in pH 6–8 electrolytes have been investigated. Visible absorption spectroscopy was used to follow the course of the reaction in an optically transparent thin-layer electrochemical cell under quasi-steady-state conditions. The spectroelectrochemical data were analyzed using classical kinetics and yielded zero-order behavior with respect to the viologen cation radical. For methyl viologen cation radical at gold, a formal zero-order rate constant evaluated at zero hydrogen overpotential was found to be 1.0 × 10^?13 mol s^?1 cm^?2. At nickel the comparable rate constant was nearly two orders of magnitude larger than at gold. Increasing pH from 6 to 8 at gold electrodes shifted both the hydrogen evolution and the methyl viologen cation radical reaction 60–70 mV/pH unit in a negative direction. The diquat cation radical behaved in a similar manner. The proposed mechanism involves a fast, non-rate-limiting, chemical reaction between the viologen cation radical and adsorbed hydrogen atom(s). Results are interpreted in terms of previous proposed hydrogen evolution reaction mechanisms.     

Preparation of Polymers with Viologen Moieties and Their Application to the Selective Reduction of Substituted Nitroarenes

Abstract

Polymers with viologen moieties were synthesized by using poly-chlorethyl vinyl ether (PCEVE) as mother supports. These polymers were used as electron-transfer catalysts (ETC) for the reduction of substituted nitroarenes under heterophase conditions (reductant: Na₂S₂O₄ in CH₂CI₂-H₂O). The experimental results show that the substituted nitroarenes were reduced selectively and efficiently to the corresponding aniline derivatives in the presence of viologen polymers. The catalytic active species of viologen were detected by ESR and electrochemical methods. It was found that the viologen cation radical (V⁺) acts as the active species during the viologen-mediated reduction of substituted nitroarenes.     

Radical‐Cation Dimerization Overwhelms Inclusion in [n]Pseudorotaxanes

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril ( CB7 ) in water is a well‐known phenomenon. Herein, two counter‐examples are presented. Two viologen‐containing thread molecules were designed, synthesized, and thoroughly characterized by ¹H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square‐wave voltammetry, and chronocoulometry: BV ⁴⁺, which contains two viologen subunits, and HV ¹²⁺, which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]‐ and [7]pseudorotaxanes that form on complexation with CB7 , that is, BV ⁴⁺?( CB 7)₂ and HV ¹²⁺?( CB 7)₆, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen ( MV ²⁺) and benzyl methyl viologen ( BMV ²⁺), as well as their [2]pseudorotaxane complexes with CB7 ( MV ²⁺? CB7 and BMV ²⁺? CB7 ) were also investigated. As expected, the control pseudorotaxanes remained intact after one‐electron reduction of their viologen‐recognition stations. In contrast, analogous reduction of BV ⁴⁺?( CB 7)₂ and HV ¹²⁺?( CB 7)₆ led to host–guest decomplexation and release of the free threads BV ^{2( . +)} and HV ^{6( . +)}, respectively. ¹H DOSY NMR spectrometric and chronocoulometric measurements showed that BV ^{2( . +)} and HV ^{6( . +)} have larger diffusion coefficients than the corresponding [3]‐ and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical‐cation dimerization. These results demonstrate that radical‐cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches.     

Vibrational and electronic study of the methyl viologen radical cation MV+. in the solid state

A complete vibrational analysis (IR, Raman) of the methyl viologen radical cation MV^+., generated either via thermolysis from the solid MV²⁺(Cl⁻)₂ salt, or via chemical reduction from the dictation intercalated in the layered CdPS₃ compound, is reported fpr different isotopic derivatives. The results are discussed with respect to the conformation and the electronic configuration of the radical species, (MV^+.) and (MV^+.)₂.     

Dual‐Mode Controlled Self‐Assembly of TiO2 Nanoparticles Through a Cucurbit[8]uril‐Enhanced Radical Cation Dimerization Interaction

The realization of controllable multicomponent self‐assembly through reversible supramolecular interactions is a challenging goal, and is an important strategy for the fabrication of switchable nanomaterials. Herein we show that the self‐assembly of TiO₂ nanoparticles (NP) functionalized with methyl viologen can be controlled both by light irradiation and chemical reduction through cucurbit[8]uril‐enhanced radical cation dimerization interactions. Moreover, the controlled assembly and disassembly of this system are accompanied by switchable photocatalytic activity of the TiO₂ NPs, which shows potential application as a novel smart and recyclable photocatalyst.     

Photoinduced electron transfer catalysis of titania particles modified with a Ru(2,2′-bipyridyl)3 2+-grafted polymer by visible light

 The preparation and photocatalysis of colloidal titania modified with Ru(2,2′-bipyridyl)₃ ²⁺-grafted polyacrylate were investigated. Visible light irradiation of the Ru(2,2′-bipyridyl)₃ ²⁺-tethered titania in pH 7.0 buffer solution gave electron transfer to methyl viologen to form the cation radical via electron migration from the Ru(II) complex to the titania surface. Received: 16 September 1999 Accepted: 24 November 1999     

Halocadmium Clusters with Different Linkage Modes Templated by Methyl Viologen: Structure,Photoluminescence, and Thermochromism

A new halocadmium/methyl viologen hybrid (MV)₄(Cd₂I₆)(Cd₂I₇)₂ (1) (MV²⁺ = methyl viologen) has been structurally determined. In 1, two kinds of dimeric halocadmium clusters constructed from edge/vertex-sharing CdI₄ tetrahedra respectively, i.e., Cd₂I₆ ^2? and Cd₂I₇ ^? dimmers were observed, which were templated by methyl viologen cation (MV²⁺). A highly interesting feature of 1 is the presence of mixed type dimeric halocadmium clusters in one crystal lattice templated by one kind of cation. 1 displays photoluminescences, and its thermochromism behavior is observed, whose mechanism was verified by theoretical calculations.     

Temperature dependence of the absorption spectrum of the methyl viologen cation radical and use of methyl viologen as a scavenger for estimating yields of the primary radicals in the radiolysis of high-temperature water

The absorption spectrum of the methyl viologen cation radical in an aqueous solution was measured as a function of temperature up to 200°C. The absorption coefficients at the two maxima, 605 and 395 nm, were both found to decrease with temperature, though the integral intensity of each of these bands remained almost constant. A pulse-radiolysis experiment, aimed at examining temperature dependence of the yield of hydrated electron, was made by using methyl viologen as a scavenger. The observed yield of the cation radical indicated that the G value of hydrated electron increases by about 28% upon temperature elevation from 20 to 200°C.     

Continuous Photoreduction of Methyl Viologen Using Disubstituted Terthiophenes and EDTA in Aqueous Solution

The continuous photoreduction of methyl viologen by 5,5″-bis(aminomethyl)-2,2′:5′,2″-terthiophene (AT) and 2,2′:5′,2″-terthiophene-5,5″-dicarboxylic acid (CT), using EDTA as a sacrificial electron donor, has been investigated in aqueous solution at various pH. Apparent rates and efficiencies of production of the methyl viologen radical cation (MV⁺) were found to be dependent on the pH, the concentrations of all three components and the intensity of the incident light. The highest conversion efficiency (77%) was shown by the bis(aminoniethyI (-substituted terthiophene AT at pH 7.7. The quantum yield for the formation of MV⁺ under these conditions was 0.24, which is comparable with other common systems     

Spectroelectrochemical and spectrophotochemical properties of N-tetradecyl-N '-ethyl-viologen: Micellar effects

The results of Spectroelectrochemical studies in homogenous solutions have shown that below the cmc value the cation radical of N-tetradecyl-N '-ethyl viologen (TDEV) dimerizes. The TDEV and tetradecyltriethyl-ammonium bromide (TDEA) micelles were found to stabilize the cation radical TDEV^.+ and increase the rate constant for the reaction TDEV+TDEV²⁺ = TDEV^.+ as compared with the results obtained at concentrations below cmc.Based on the spectrophotochemical measurements for TDEV it was found that the quantum yield (Φ) of photoreduction in micellar evironment of TDEA was twice as large as Φ for reactions performed in homogenous solution. Moreover, in micellar solutions photoreduction of TDEV leads to a cation radical of reduced TDEV (TDEV⁺), but in homogenous solution to the dimer of TDEV [TDEV]₂. Therefore, the process of dimerization of TDEV^.+ cation radical is inhibited by micellar catalysis.     

Synthesis of Diazacrown Ethers with Chromophores and Their Photoinduced Charge‐Separation with Methyl Viologen

Two 4,13‐diaza‐18‐crown‐6 ethers with either two pyrenyl or two carbazolyl groups were synthesized. The two crown ethers can form complexes with methyl viologen in methanol solution. Photoirradiation of the complexes resulted in the electron transfer from the excited states of the chromophores to methyl viologen as demonstrated by the quenching of the chromophore fluorescence and the detection of the absorption spectrum of the generated viologen radical cation. The back electron transfer in these systems was inhibited by the electrostatic repulsion between the positively charged viologen radical cation and the generated chromophore radical cation. Long‐lived charge separation states (up to tens of min) were observed.     

Photo-induced electron transfer study of rhenium(I) bipyridyl complexes with covalently linked phenothiazine donor through different bridge

A novel rhenium(I) bipyridyl complex 1a, [(4,4’-di-COOEt-bpy)Re(CO)₃(py-NHCO-PTZ)PF₆] and a model 1b, [(4,4’-di-COOEt-bpy)Re(CO)₃(py-PTZ)PF₆] (bpy is 2, 2’-bipyridine, py-NHCO-PTZ is phenothiazine-(10-carbonyl amide) pyridine and py-PTZ is 10-(4-picolyl) phenothiazine) were synthesized. Their photo-induced electron transfer (ET) reaction with electron acceptor methyl viologen (MV²⁺) in acetonitrile was studied by nanosecond laser flash photolysis at room temperature. Photoexcitation of 1 in the presence of MV²⁺ led to ET from the Re moiety to MV²⁺ generating Re(II) and methyl viologen radical (MV^·+). Then Re(II) was reduced either by the charge recombination with MV^·+ or by intramolecular ET from the attached PTZ, regenerating the photosensitizer Re(I) and forming the PTZ radical at 510 nm. In the case of 1b, the absorption for PTZ radical can be observed distinctly accompanied intermolecular ET, whereas not much difference at 510 nm can be detected for 1a on the time scale of the experiments. This demonstrates that the linking bridge plays a key role on the intramolecular ET in complex 1.     

Flavin-mediated photoinactivation of spinach leaf nitrate reductase involving superoxide radical and activating effect of hydrogen peroxide

Irradiation with white light of spinach leaf nitrate reductase (NR) in the presence of flavin mononucleotide (FMN) and ethylenediaminetetraacetic acid (EDTA) resulted in a gradual loss of the enzyme activity, measured with reduced methyl viologen as electron donor. Inactivation of NR was dependent on oxygen and was prevented by superoxide dismutase. On the contrary, the presence of catalase markedly enhanced the rate of inactivation. Nitrate showed a protective effect. NR previously inactivated by irradiation in the presence of FMN and EDTA was greatly reactivated by a short preincubation of the inactive enzyme with either ferricyanide or H₂O₂. These results suggest that spinach leaf NR is reversibly inactivated by photogenerated O₂ , while H₂O₂ has an activating effect.     

Application of immobilized hydrogenase to h2 storage in concentrated solutions of methyl viologen

It has been found that immobilized cells ofC. pasteurianum possessing hydrogenase activity efficiently catalyze reversible reduction of concentrated (up to 0.5M) solutions of methyl viologen with H₂. A 0.5M aqueous solution of methyl viologen dissolves 240 times as much H₂ as pure water under the same pressure of hydrogen. The experimentally obtained levels of methyl viologen reduction and H₂ evolution are in satisfactory agreement with theoretical calculations. The potential of concentrated solutions of methyl viologen containing immobilized hydrogenase as a H₂ storage medium is discussed.     

A Smart Photochromic Semiconductor: Breaking the Intrinsic Positive Relation Between Conductance and Temperature

Breaking the intrinsic rule of semiconductors that conductivity increases with increase of temperature and realizing a dramatic dropping of conductivity at high temperature may arouse new intriguing applications, such as circuit overload or over‐temperature protecting. This goal has now been achieved through T‐type electron‐transfer photochromism of one organic semiconductor assembled by intermolecular cation???π interactions. Conductivity of the viologen‐based model semiconductor (H₂bipy)(Hox)₂ (H₂bipy=4,4′‐bipyridin‐1,1′‐dium; ox=oxalate) increased by 2 orders of magnitude after photoinduced electron transfer (a record for photoswitchable organic semiconductors) and generation of radical cation???π interactions, and fell by approximately 81 % at 100 °C through reverse electron transfer and degeneration of the radical cation???π interactions. The model semiconductor has at least two different electron transfer pathways in the decoloration process.     

Does thiol-functionalized viologen monolayer memorize anion present when forming the monolayer?

We investigated the effect of the presence of an additive anion while forming a self-assembled monolayer of a thiol-functionalized redox active species upon the behavior in aqueous solutions. A bromide salt was added in an acetonitrile solution of a thiol-functionalized viologen (viologen-thiol: N-pentyl-N′-(11-mercapto)undecyl-4,4′-bipyridinium bishexafluorophosphate) in which the self-assembled monolayer was formed on a polycrystalline Au electrode. We examined the structure and electrochemical behavior of the resulting monolayer-modified electrode in three different aqueous electrolyte solutions. The viologen-thiol monolayer prepared in the presence of Br⁻ exhibited obviously different behavior in both KBr and KF electrolyte solutions from that prepared in the absence of Br⁻. On the other hand, the difference was minor in KPF₆ solution. The difference of the electrochemical behavior was represented by the coverage of the viologen-thiol and the formal potential of viologen dication/radical cation redox couple. The memory that the viologen-thiol monolayer was formed in the presence of Br⁻ was retained in KF and KBr solutions, though it was erased in the electrolyte solution containing PF₆⁻, which is a softer anion than Br⁻. However, no definitive difference of the film structure was deduced from the electroreflectance study in regard to the monomer content and the average orientation of the viologen moiety.     

Photo-Activating Biomimetic Polyoxomolybdate for Boosting Oxygen Evolution in Neutral Electrolytes

Inspired by the metal-oxo cluster structural feature and charge separation behaviour of the oxygen evolving center (OEC) in photosystem II (PS-II) under photoirradiation, a new crystalline photochromic polyoxomolybdate, MV₂[β-Mo₈O₂₆] ( 1 , MV=methyl viologen cation), is designed as a biomimetic oxygen evolution reaction (OER) catalyst in neutral electrolytes. After photoinduced electron transfer (PIET) with colour change from colourless to grey, it remains in an ultra-stable charge-separated state over a year under ambient conditions. The observed overpotential at 10 mA ⋅ cm⁻² and Tafel slope decrease by 49 mV and 62.8 mV ⋅ dec⁻¹ after coloration, respectively. The outstanding OER performance of the coloured state in neutral electrolytes even outperforms the commercial RuO₂ benchmark. Experimental and theoretical studies show that oxygen holes within polyanions after irradiation serve as sites for enhancing direct O−O coupling, thus effectively promoting OER. This is the first successful application of electron-transfer photochromism to realize OER activity gain.     

KINETICS AND MECHANISM OF PHOTO-INDUCED METHYL VIOLOGEN REDUCTION WITH ZINC-MESO-TETRAPHENYLPORPHYRINTRISULFONIC ACID AND HYDROGEN EVOLUTION FROM WATER HYDROGENASE

Abstract— The formation of the cation radical of methyl viologen was established when an aqueous solution containing zinc-meso-tetraphenylporphyrintrisulfonic acid, methyl viologen and mercaptoethanol was irradiated by visible light. The reduction of methyl viologen has been studied kinetically and the reduction rate expressed as follows

where [Zn]₀ is the total amount of zinc compounds in the system.
On the basis of the rate expression, the reaction mechanism is discussed.
On the addition of hydrogenase to the above photoirradiation system, hydrogen evolution was observed by both the irradiation of sunlight and a tungsten lamp.     

Preparation of polyacrylate–polysiloxane core–shell latex particles

Crosslinked polymer seed latexes of butyl acrylate, methyl methacrylate and methacrylic acid were synthesized with ethylene glycol dimethacrylate as a crosslinking agent in a first step. Three different processes of seeded emulsion polymerization were used to prepare an outlayer of polysiloxane on the above seed latex particles: (A) direct anionic polymerization of D₄ (octamethyl tetracyclosiloxane) catalyzed by potassium hydroxide; (B) direct cationic polymerization of D₄ onto the seed catalyzed by dodecylbenzene sulfonic acid; (C) a vinyl-containing polysiloxane prepared by copolymerization of D₄ and vinyl septamethyl tetracyclosiloxane was added before the D₄ cationic polymerization. Characterization by transmission electron microscopy showed that only process C provided satisfactory results. Film hardness was measured, and the latex film from process C demonstrated the lowest hardness of all the films. The mechanism of polymerization is discussed.