Photoreduction Of Viologen Research Articles

Reversible Photochromism of 4,4'-Disubstituted 2,2'-Bipyridine in the Presence of SO3.

We report an unusual photochromic behavior of 4,4'-disubstituted-2,2'-bipyridine. It was found that in the presence of a SO3 source and HCl, 2,2'-bipyridine-4,4'-dibutyl ester undergoes a color change from yellow to magenta in solution with maximum absorbance at 545 nm upon irradiation with 395 nm light. The photochromism is thermally reversible in solution. Different from the known bipyridine-based photoswitching pathways, the photo response does not involve any metal which form colored complexes or the formation of colored free radical cations like the photo-reduction of viologens. A combination of experimental and computational analysis was used to probe the mechanism. The results suggest the colored species to be a complex formed between N-oxide of the 2,2'-bipyridine-4,4'-dibutyl ester and SO2; the N-oxide and SO2 are formed from photoactivated oxidation of the bipyridine with SO3 serving as the oxygen source. This complex represents a new addition to the library of photoswitches that is easy to synthesize, reversible in solution, and of high fatigue resistance, making it a promising candidate for applications in photo-switchable materials and SO3 detection. We also demonstrated experimentally similar photochromic behaviors with 2,2'-bipyridine-containing polymers.

Optofluidic Photonic Crystal Fiber Microreactors for In Situ Studies of Carbon Nanodot-Driven Photoreduction.

Performing quantitative in situ spectroscopic analysis on minuscule sample volumes is a common difficulty in photochemistry. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liquid channel and acts as an optofluidic microreactor with a reaction volume of less than 35 nL. The system was used to demonstrate in situ optical detection of photoreduction processes that are key components of many photocatalytic reaction schemes. The photoreduction of viologens (XV2+) to the radical XV•+ in a homogeneous mixture with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradiation cycles, are interpreted with a quantitative kinetic model to determine photoreduction and photobleaching rate constants. The powerful combination of time-resolved, low-volume absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quantitative, and rapid screening platform for novel photocatalysts and flow chemistry in general.

Surface Molecularly Imprinted TiO2 Nanoparticle for Photoreduction of Viologen

ABSTRACTWhen small organic molecules (e.g. amino acids, nucleotides, and so on) are incorporated in metal oxide particles formed from metal alkoxide in the sol-gel process, the organic moieties can be removed readily by solvent washing or by degradative oxidation. Under proper conditions, nano-sized cavities corresponding to the shape of the individual organic moiety are produced within the metal oxide network as molecularly imprinted material such as surface molecularly imprinted TiO2. We tried to fabricate nano-sized particle of TiO2 to increase the effective contact area for the reactants, and further additional function for molecular recognition was intended to introduce through the surface modification of the molecular imprinting method in order to enhance the photoreduction of viologen. In this context, we prepared viologen imprinted TiO2 as an active and stable photocatalyst for photoreduction of viologen as a well-defined and readily identifiable and reversible electron carrier with appropriate redox potential for photoactivated TiO2. The basic concept of molecularly imprinting in this system is as follows. Oxidized viologen is generally twisted and dication molecule. While, reduced viologen is planar and monocation radical. Once TiO2 has such recognition site on its surface, it would be distinguishable through shape or charge recognition or both factors and facilitate the inclusion or exclusion of the target molecule result the enhancement of the reduction. After the preparation of the template, sol-gel imprinting was performed. Measuring its dynamic light scattering monitored changes of the particle sizes and the diameter was stabilized ca 120 nm after 1 week stirring. X-ray diffraction pattern of the TiO2 powder after the dryness without calcination indicates the dominant peak for anatase. Removal of the template was almost 36% after the alkaline rinsing by measuring its UV vis spectra of the washings. A buffered solution of Tris including viologen was irradiated through a xenon lamp under stirring in a quartz cuvett in Ar with or without the TiO2 with cut-off filter at room temperature. The formation of viologen monocation radical whose color is blue was easily recognized only in the reaction with TiO2. The formation of viologen monocation radicals from three types of (methyl, hexyl, and pentacarboxyl) viologens with time in the presence of the imprinted TiO2 and blank one was monitored spectrophotometrically. Blank means that it was fabricated similar way to the imprinted one without the template. Among these results, aliphatic C6 side chains of bipyridinium and terminal carboxyl groups were not effective to enhance. On the other hand, the less bulkiness methyl viologen resulted the highest yield. The most important result is that higher conversion was observed in every case of imprinted TiO2 compared with blank one. Therefore, the key point for recognition is not peripheral part, but bipyridinium part of the viologen.

Synthesis and study of zirconium viologen-phosphonate compounds on a polymer surfactant template and their use in photocatalytic production of hydrogen

This paper describes a novel system, in which colloidal platinum particles and a zirconium viologen-phosphonate material (ZrVP) are both strongly interacting with a polymer surfactant template. A method of sequential treatment and dialysis was developed to grow the metal phosphonate compound on the phosphonate groups of the polymer surfactant template. This polymer surfactant also functions as a protecting agent for platinum colloids in aqueous solution to prevent aggregation and precipitation. Monitoring the growth of ZrVP by electronic spectroscopy shows that the compound grows incrementally on the polymer surfactant template. Transmission electron microscopic study shows a blanket-like morphological feature for these templated materials. Photoreduction of viologen in these compounds leads to the formation of a stable charge separated state. Photoreduced materials are very air-sensitive. Photochemical hydrogen generation shows a 5 times higher yield of H 2 than an analogous microcrystalline sample without a polymer surfactant template.

Photoinduced electron transfer reaction between [Cu(dmp)P 2] +or− (P=PPh 3 or PPh 2( m-C 6H 4SO 3−) and viologen derivatives

The copper(I) complex, [Cu(dmp)P 2] +or− (dmp=2,9-dimethyl-l,10-phenanthroline, P=triphenylphosphine PPh 3 or diphenylphosphinobenzene- m-sulfonate PPh 2( m-C 6H 4SO 3 −)), catalytically photoreduces viologen derivatives, dicationic methylviologen MV 2+ and neutral propylviologen disulfonate PVS 0. The quantum yield Φ(V°) for the viologen photoreduction is discussed on the basis of the lifetime of the photosensitiser, formation of the encounter complex, dissociation of the encounter complex to products, and back electron transfer in the encounter complex. When [Cu(dmp){PPh 2( m-C 6H 4SO 3)} 2] − is used for the photoreduction of MV 2+, the highest quantum yield (Φ(V°)=0.03) is obtained. Kinetic analysis and measurements of the excited state lifetime reveal that this best Φ(V°) value arises from the rapid formation of the encounter complex, owing to the attractive electrostatic interaction between anionic [Cu(dmp){PPh 2( m-C 6H 4SO 3)} 2] − and cationic MV 2+.

Synthesis and photochemical properties of porous zirconium viologen phosphonate compounds

The authors recently reported stable, photoinduced charge separation in layered zirconium phosphonate viologen compounds (Zr(O[sub 3]PCH[sub 2]CH[sub 2](bipyridinium)CH[sub 2]CH[sub 2]PO[sub 3])X[sub 2] (X = Cl, Br, I), ZrPV (X)). Here, they report the synthesis and photochemical properties of porous versions of this material. Porosity can be built into the material by preparing a layered solid from a mixture of the large viologen bisphosphonic acid and a smaller phosphorus acid (i.e., Zr(O[sub 3]P-R)[sub 1](O[sub 3]PCH[sub 2]CH[sub 2](bipyridinium)CH[sub 2]CH[sub 2]PO[sub 3])[sub 0.5]C; R = O[sup [minus]], OH, H, alkyl). These materials were characterized by powder X-ray diffraction and IR and NMR spectroscopies. The resulting structures are microporous and undergo photoreduction of the viologen at a faster rate than in ZrPV (X) compounds. Evidence for the porosity in these mixed materials is seen in the air sensitivity of the photoreduced species. Photoreduction of the viologen groups is observed only under anaerobic conditions, and the photoreduced solid is bleached immediately and quantitatively in the air. In contrast the close-packed analogs (ZrPV(X)) can be photoreduced and are moderately stable in the air. Detailed photophysical and isotopic labeling studies suggest that the mechanism for photoreduction of viologen in these materials is similar to that proposed formore » ZrPV(X). 21 refs., 5 figs.« less

Photoinduced Charge Transfer Studies in Bolaamphiphile-Gramicidin-Porphyrin Membranes

ABSTRACTSynthetic ether-linked bolaform amphiphile vesicles and a novel gramicidin-porphyrin “diad” have been prepared. Photoinduced electron transfer properties of the diad were compared in bilayer (dihexadecylphosphate, DHP) and monolayer (2,2′-0-didecyl-1,1′-0-eicosamethylene-racdiglycero- 3,3′-diphosphoric acid, PS20) membrane vesicles with dithiothreitol sacrificial donor and methyl viologen electron acceptor present on both vesicle membrane surfaces. Although the rates of methyl viologen photoreduction varied depending on the mode of diad orientation within DHP bilayer membranes, photoreduction rates were not orientation-dependent in bolaform membrane vesicles containing the gramicidinporphyrin diad. The relevance of these results on vectorial electron transfer processes in closed membrane systems is briefly discussed.

Unusual photoreduction of viologen by dimethyl sulfoxide on anew polymer having direct pendants of viologen and benzophenone structures

A new viologen copolymer with direct pendants of viologen and benzophenone structures was effectively photoreduced by dimethyl sulfoxide for the first time derived by the cooperative functional group interactions.

The sites of electron donation of Photosystem I to methyl viologen

Effectiveness of methyl viologen as an electron acceptor was studied by oxygen polarography and millisecond time range flash photolysis spectroscopy at room temperature on three types of chloroplast preparation deficient in NADP + photoreduction activity. HgCl 2-treated chloroplasts which had completely lost Fe-S center B and also NADP + photoreduction activity were still partially active in methyl viologen photoreduction. Compared with untreated chloroplasts, V max of the oxygen uptake in this preparation was almost halved and its apparent K m for methyl viologen was about 10 times greater. Photosystem I particles extracted with digitonin from the treated chloroplasts showed, in the absence of methyl viologen, a flash-induced absorption transient at 430 nm whose magnitude and decaying time were very similar to those of the particles extracted from untreated chloroplasts. However, the former required a concentration of methyl viologen for stabilization of P-700 + more than 10 times higher than control particles. The shape of the difference spectrum of the faster decaying component in the presence of methyl viologen was similar to P-430. Our conclusions are: (1) this spectral component represents the redox of Fe-S center A, and chloroplasts can transfer electrons from center X to center A even when their center B is destroyed; (2) center B is the main site of electron donation to methyl viologen, and center A can donate electrons to methyl viologen although with a lower affinity. Chloroplasts anaerobically photoinactivated under strongly reducing conditions in which electron transport between A 0 and center X was impaired showed very low oxygen uptake activity which was almost insensitive to methyl viologen. Dependence of oxygen uptake on methyl viologen concentration by aerobically photoinactivated chloroplasts in which three Fe-S centers were partially destroyed somewhat resembled that of HgCl 2-treated chloroplasts.

The photoreduction of 8‐substituted 5‐deazaflavins and the 5‐deazaflavin catalyzed photoreduction of methyl viologen

AbstractProlonged illumination of 8‐X‐5‐deazaflavins (X = C1, N(CH3)2, NH2, p‐NH2‐C6H4) in the presence of an electron donor leads to the formation of a 5,5′‐dimer and/or a 6,7‐dihydro compound. The course and rate of these photoreductions were established and discussed in terms of electronic and steric effects, exerted by the substituent at position 8 and the electron donor. Pseudo first‐order kinetics were found to apply to the photoreduction of 8‐X‐5‐deazaflavins (X = Cl, NH2, p‐NH2‐C6H4) while the rate of the photoreduction of 8‐X‐5‐deazaflavin (X = N(CH3)2) appeared to contain an autocatalytic element. The catalytic effect of 8‐X‐5‐deazaflavins in the photoreduction of methyl viologen by EDTA was investigated. The substituent effect on the rate of the 8‐X‐5‐deazaflavin mediated photoreduction of methyl viologen by EDTA was found to be comparable with that on the photoreduction rate of 8‐X‐5‐deazaflavin in the presence of EDTA with the exception of 8‐X‐5‐deazaflavin (X = N(CH3)2), which showed a remarkable relative enhancement of the reactivity towards methyl viologen photoreduction.

Photoinduced hydrogen evolution in micellar system

Photoreduction of viologens by the irradiation of the system containing NADPH, zinc mesotetraphenylporphyrintrisulfonate (Zn-TPPS 3 3−), viologen and colloidal platinum has been investigated in the presence of surfactant micelles. In the presence of either cationic micelles or anionic micelles, a remarkable increase in the accumulation of the reduced form of viologen was observed. The existence of the micelles depressed both the quenching rate of the photoexcited Zn-TPPS 3 3− by viologen and the back reaction rate, recombination rate of the oxidized Zn-TPPS 3 3− and reduced viologen. Compared with both reactions, it was clarified that the recombination rate strongly influenced the viologen reduction rate. The effect of the micelles was explained by the electrostatic effect among the charges of the micellar surface, Zn-TPPS 3 3− and viologen. By the addition of colloidal platinum to the system, photoinduced hydrogen evolution was also studied.

The site and mechanism of duroquinol oxidation by the cytochrome b6- f complex in Synechococcus sp.

Duroquinol (tetramethyl- p-hydroquinone) served as an excellent electron donor to Photosystem I and promoted methyl viologen photoreduction in 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea-poisoned cells of the thermophilic cyanobacterium Synechococcus sp. at a rate comparable to or, more likely, higher than that of the photoreduction with water as electron donor. The restored activity was inhibited by 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB). The inhibition is competitive and partially reversed by increasing duroquinol concentration. Spectrophotometric experiments with flashes demonstrated that duroquinol accelerates the reduction of P-700, cytochrome c-533 and cytochrome f in the DBMIB-poisoned cells similarly and in a way just opposite to the inhibitory action of DBMIB: the electron donor specifically increased the fraction of the three electron carriers that was reduced with the 2 ms half-time. The results indicate that duroquinol does not serve as a reductant of the plastoquinone pool, nor of P-700 and cytochrome c-533, but directly donates its electrons to the cytochrome b 6- f complex by binding to the plastoquinone-binding site and that DBMIB inhibits the quinol oxidation by binding competitively to the same site.

PHOTOREDUCTION OF VIOLOGENS WITH NADPH AS REVERSIBLE ELECTRON DONOR

Abstract— The photoreduction of viologens with NADPH as an electron donor was investigated. As the photosensitizer in that reaction, ZnTPP33‐ and HmP were applied. HmP dissolved by CTAB or Triton X‐100 was a more active photosensitizer than ZnTPPS33‐. The reduction rate of viologen under steady state irradiation depends remarkably on the type of surfactant used for dissolving HmP. The effect of the surface charge of micelle was examined by laser flash photolysis.

Photoinduced electron transfer between Zn(TPPS)33- and viologens

The photoreduction of viologens, such as methylviologen (MV/sup 2 +/) and zwitterionic viologenpropanesulfonate (PVS), with Zn(TPPS)/sub 3//sup 3 -/ were investigated. By laser flash photolysis, the quenching rate constant, the back-electron-transfer rate constant, and the charge separation efficiencies were determined. Complex formation between Zn(TPPS)/sub 3//sup 3 -/ and viologen was observed, and the association constants were determined by laser flash photolysis. 17 references, 11 figures, 2 tables.

Novel Effect of Man‐Made Molecular Assemblies on Photoinduced Charge Separation. 4. Synthetic Bilayer Membrane of Amphipathic Viologen as an Efficient Electron Collector in Photosensitized Redox Reactions

AbstractAddition of synthetic bilayer membrane formed by a new type of amphipathic viologen containing two hexadecyl groups to aqueous solution of zwitterionic propylviologen sulfonate (PS2 – V2+) remarkably increased the yield of viologen photoreduction as sensitized by tris(2,2'‐bipyridine)‐ruthenium complex, Ru(bpy), in the presence of triethanolamine. Zwitterionic viologen was confirmed to shuttle as an efficient electron mediator between bulk aqueous solution and the interface of the bilayer membrane with a viologen head group. The quantum yield of viologen cation radicals was close to the net cage‐escape yield of photogenerated Ru(bpy) ‐ PS2‐V+ ion pair. The electrostatic field of positively charged interface of bilayer membrane was concluded to attract PS2‐Vc which transports electron to viologen head groups of bilayer membranes. Remarkable (500‐fold) suppression of the reverse electron transfer reaction was attained by the addition of viologen bilayer membrane. It was attributed to Coulombic repulsion between Ru(bpy) and positively charged interface of bilayer membranes. Relevance of the present system to chemical conversion of solar energy is discussed.

COOPERATIVE SENSITIZATION IN PHOTOREDUCTION OF VIOLOGENS BY METAL COMPLEXES AND CHARGE-TRANSFER COMPLEXES ON AMPHIPATHIC VIOLOGEN MICELLES

Abstract Amphipathic viologens having alkyl chain with a naphthyloxy group formed micelles and showed charge-transfer (CT) absorption. The quantum yield of photoreduction of viologens sensitized by metal complex in these micelles was about twice of the sum of those sensitized by metal complex in micelle without CT and by CT-complex alone. These results strongly indicated the cooperative action between CT- and metal complexes.

ChemInform Abstract: Photoreduction of Viologens and Hydrogen Evolution with Hydrogenase.

AbstractDie Photoreduktion von Ethyl‐, Propyl‐ und Butyl‐Viologen wurde in wäßriger Lösung in Gegenwart von Zn‐tetraphenylporphyrintrisulfonsäure als Photosensitizer und 2‐Mercaptoethanol als Elektronendonator durchgeführt.

Photoreduction of viologens and hydrogen evolution with hydrogenase

Ethyl-, propyl-, and n-butyl-viologens are found to be electron carries for photoinduced hydrogen production and also can be cofactors of hydrogenase. As these viologens are photoexcited by visible light irradiation or sunlight, simpler systems without another photosensitizer can be developed for the photochemical utilization of solar energy.

Triplet-mediated photoreduction of viologens by aromatic hydrocarbons. A laser flash-photolysis study

Using laser flash photolysis the electron-transfer (e.t.) reactions involving a number of aromatic hydrocarbon triplets (donors) and several bipyridinium and pyridinium salts (acceptors) have been examined in terms of redox quenching rate constants (ke.t.), e.t.-derived photoproduct yields (η) and rate constants (kb) for back e.t. reactions among the photoproducts. The observed values for ke.t., η and kb with paraquat as the quencher in a 1:9 water + acetonitrile mixture are typically in the ranges (5–10)× 109 dm3 mol–1 s–1, 0.7–1.0 and (2–4)× 109 dm3 mol–1 s–1, respectively. In both methanol and 1:9 water + acetonitrile the transient absorption spectra of primary photoproducts show the formation of radical cations of aromatic hydrocarbons in addition to the one-electron-reduction products of viologens. For free-energy changes (ΔG) of triplet-mediated electron transfer ranging from 17 to –96 kJ mol–1, plots of log (ke.t.) against redox potentials of viologen acceptors or triplet hydrocarbon donors are found to bend at small |ΔG| values with slopes estimated at 8–10 V–1 in the bending regions; there is no indication of the bending of the plots at very large, negative ΔG values (up to –96 kJ mol–1).

Photoreduction of viologens by visible light in the presence of electron donor and hydrogen evolution with hydrogenase

Photoreduction of viologens by visible light in the presence of electron donor and hydrogen evolution with hydrogenase