Viologen Radical Research Articles

Driving Quick and Large Amplitude Contraction of Viologen-Incorporated Poly-l-Lysine-Based Hydrogel by Reduction.

A glutaraldehyde-cross-linked poly-l-lysine-based hydrogel with pendant viologens was synthesized with various [viologen unit]/[Lys unit] ratios. The hydrogel with the ratio of 25% was extensively characterized. Characterization of the hydrogel revealed that (i) water content reaches 95.8%, (ii) unbound viologen unit is absent, and (iii) lyophilized gel, showing porous structure, is rewettable. The hydrogel in contact with a gold, glassy carbon, pyrolytic graphite, or ITO electrode was electroactive, showing voltammetric responses involving diffusion process. Occurrence of electron transfer between viologen sites was verified by ESR measurements. The electroreflectance spectrum demonstrated significant dimerization of one-electron-reduced forms, viologen radical cations. Most remarkably, reduction of the hydrogel by dithionite showed a 93% volume decrease, being near to the water content, at 100 s and finally 97% contraction at 380 s. The initial rate of contraction was 0.02 s-1, and the swelling weight ratio was over 17. The contraction rate was much faster than that observed when adding of redox-inactive salts in the outer medium. Recovery by reoxidation was relatively sluggish even when O2-saturated water was used. The quick and large-amplitude contraction with removal of water from the gel body should be the consequence of permeability of reductant, interchain π-stacking of one-electron-reduced forms (viologen monoradical monocation), desolvation of viologen upon reduction, electron hopping between viologen units and its acceleration with contraction, and effective osmotic pressure difference.

Electropolymerization and characterization of carbazole substituted viologen conducting polymers: The effects of electrolytes and potential applications of the polymers

Two electropolymerizable carbazole-functionalized viologen compounds were synthesized and characterized using NMR, ESI-MS, voltammetry, and spectroelectrochemistry. The compounds were electropolymerized onto indium tin oxide electrode and glassy carbon electrode through a radical-induced polymerization triggered by electrochemical oxidation at the carbazole moieties in electrolytes composed of room temperature ionic liquid/organic solvents or NaClO4/acetonitrile. It was found that the properties of the electropolymerized polymers were crucially controlled by the chemical structure of the monomer and the electrolyte compositions. The polymer films showed electrochromic behavior, and the color changed from greenish at the potential where carbazole groups were oxidized to violet or colorless at which reduction of viologen groups occurred; violet or colorless depended on which of the two reductive states of the viologen moieties was reached. The polymer films were capable of enhancing the oxidative current of uric acid and ascorbic acid, respectively, in neutral media, which might be due to the concentrating ability of the viologen moieties towards negatively charged species such as ferricyanide ([Fe(CN)6]3−) and ferrocyanide ([Fe(CN)6]2−).

Three 1D coordination polymers based on bipyridinium carboxylate ligands: Photochromism

With the self-assembly reaction of TM(NO3)2 (TM = Cd, Cu) and two bipyridinium carboxylate ligands, three one-dimensional (1D) double chain-like coordination polymers (CPs) {[Cd0.5(CEbpy) (H2O)]NO3·H2O}n (1), {[Cu0.5(CEbpy) (H2O)]NO3·H2O}n (2), {[Cu(BCEbpy) (H2O)2]NO3·Br·4H2O}n (3), (CEbpy = 1-carboxyethyl-4,4′-bipyridine, BCEbpy = 1,1′-bis(carboxyethyl)-4,4′-bipyridine) have been synthesized and structurally characterized. Compounds 1 and 2 exhibit electron-transfer (ET) photochromism due to the generation of viologen radicals, while the photochromic behavior of compound 3 is a redox process, which is rarely reported in bipyridinium complexes. Furthermore, the photoinduced radicals in 1–3 have long-lived lifetime after irradiation.

Densely Charged Dodecacationic [3]- and Tetracosacationic Radial [5]Catenanes

Summary Here, we report the efficient syntheses of a dodecacationic [3]catenane, which consists of three mechanically interlocked 4+ charged rings despite their mutual Coulombic repulsion, and a tetracosacationic radial [5]catenane, which consists of four 4+ charged rings mechanically interlocked around an 8+ charged ring and bears up to 24 positive charges in its co-constitution. The solid-state structures, determined by single-crystal X-ray crystallography, show that the two mechanical bonds in the [3]catenane result in the nano-confinement of 12 positive charges within a volume of less than 1.65 nm3, corresponding to 7.3 positive charges per nm3, whereas the radial [5]catenane has a charge density of 6.0 positive charges per nm3 and a molecular length of up to 3.7 nm. The template-directed strategy for constructing mechanically interlocked molecules, consisting of multiply charged rings, represents a departure in chemistry tantamount to producing a blueprint for exploring a novel class of organic molecules that boast large numbers and high densities of like charges.

Redox‐Triggered Folding of Self‐Assembled Coordination Polymers incorporating Viologen Units

We report the study of stimuli-responsive ZnII and FeII coordination polymers (MC34+ or MC24+ with M=Fe2+ or Zn2+ ). These soluble metallopolymers were formed spontaneously by reaction of an organic ligand (C34+ or C24+ ) with one molar equivalent of metal ions. The C34+ and C24+ ligands incorporate two chelating terpyridine groups bridged by a redox responsive hinge featuring two viologen units (viologen=N,N'-dialkyl-4,4'-bipyridinium) linked either with propyl (C34+ ) or ethyl (C24+ ) chains. The viologen units in the polymer chains were reduced (1 e- per viologen group) either by bulk electrolysis or by visible-light irradiation carried out in the presence of a photosensitizer. The 1 e- reduction of the viologen units in the MC24+ polymers induced a slight decrease in the viscosity of the solutions due to a modification of the overall charge carried by the metallopolymers. In strong contrast, reduction of coordination polymers involving propyl linkers (MC34+ ) led to a remarkable increase (≈+400 %) in observed viscosity. This reversible effect was attributed to a folding of the polymer chains triggered by π-dimerization of the photo-generated viologen cation radicals.

Mechanism of Efficient Viologen Radical Generation by Ultrafast Electron Transfer from CdS Quantum Dots

Artificial photosynthetic systems consisted of nanocrystal light absorbers, molecular redox mediators, and catalysts are one of the most promising and flexible approaches for solar fuel generation because their constituents can be independently tuned. In this work, we investigate the photoreduction of three viologen derivatives, one of the most widely investigated molecular redox mediators, of different redox potentials, 7,8-dihydro-6H-dipyrido[1,2-a:2′,1′-c][1,4]diazepinediium (PDQ2+), methyl viologen (MV2+), and benzyl viologen (BV2+), using CdS quantum dots (QDs) as the light absorber and mercaptopropionic acid as a sacrificial electron donor in aqueous (pH = 7) solution. Under continuous 405 nm light-emitting diode illumination, the steady-state radical generation quantum yield (QY) follows the order of PDQ•+ (15.99%) > MV•+ (12.61%) > BV•+ (6.56%). Transient absorption spectroscopy studies show that while the rates of initial electron transfer (ET) from the excited QD conduction band to the mediators...

Acetocatechol functionalized viologen as polyfunctional material that responds to anion, cation and reductant in aqueous and organic solvents

Both viologen and catechol have been studied extensively. However, the stability of catechol-Fe(III) without additional oxidant is still not well understood. In this paper, we introduced acetocatechol into viologen to investigate its interactions with anion, cation, and reductant, as well as the stability of its Fe(III) complexes. This acetocatechol functionalized viologen, 1,1′-bis(2-(3,4-dihydroxyphenyl)-2-oxoethyl)- [4,4′-bipyridine]-1,1′-diium chloride (H6V·Cl2) exists in central symmetric ketone cation form in solid state. Viologen cation increased the acidity of the aceto group and deprotonated the enolic proton to form monodeprotonated enolic H5V+ in the presence of anion/base, which had the deepest color in organic solvents. The absorbance maximum of H5V+ increased with the decrease of solvent polarity. It also interacted with B4O72− and MoO42− by forming catechol ester in DMSO solution. The catechol moiety can coordinate to metal ion, especially Fe(III), in both aqueous and DMSO solution. In particular, it coordinated to Fe(III) much more readily in aqueous solution than in DMSO. Green monocatecholato Fe(III) and red-brown bis-catecholato Fe(III) complex also formed in aqueous solution. The monocatecholato Fe(III) complex first-order dimerized in aqueous solution at room temperature but underwent second-order decomposition to Fe(II) complex at 60 °C. The biscatecholato Fe(III) complex also transferred to other Fe(III) complexes at first- and second-order at room-temperature and 60 °C respectively. The t1/2 varied from several hours at room-temperature and several minutes at 60 °C at 10−4 M concentration. The interactions of Fe(III) in DMSO is much more complex than that of acetocatecholate without viologen. Fe(III) can also be reduced to free viologen radicals in the presence of sodium Na2S2O4, but not N2H4. In conclusion, this polyfunctional compound responds to anion via aceto and catechol, metal ion via catechol hydroxyl, while reductant via viologen.

Visible-Light Photoinduced Electron Transfer Promoted by Cucurbit[8]uril-Enhanced Charge Transfer Interaction: Toward Improved Activity of Photocatalysis

Visible-light photoinduced electron transfer (Vis-PET) is highly important for optoelectronic devices and photoredox catalysis. Herein, we propose a supramolecular strategy to promote the Vis-PET process by charge transfer (CT) interactions. As a proof of concept, a molecular system containing 1,5-alkoxy-substituted naphthalene and viologen moieties was designed to form a CT complex in water. The HOMO/LUMO energy gap was lowered by CT interaction, which turned on the Vis-PET process to generate viologen radical cations. Moreover, when CT interaction was enhanced by cucurbit[8]uril, the efficiency of the Vis-PET process was further promoted and the required irradiation wavelength could be further red-shifted by 100 nm. The Vis-PET system exhibited an improved activity of photocatalysis, as supported by the fast photoreduction of Cytochrome c. This study represents a facile supramolecular way to fabricate radicals with maintained activity under mild conditions, which holds potential to enrich the scope of visible-light photoredox catalysis by the rational utilization of CT systems.

Interrogation of a PS1-Based Photocathode by Means of Scanning Photoelectrochemical Microscopy.

In the development of photosystem-based energy conversion devices, the in-depth understanding of electron transfer processes involved in photocurrent generation and possible charge recombination is essential as a basis for the development of photo-bioelectrochemical architectures with increased efficiency. The evaluation of a bio-photocathode based on photosystem 1 (PS1) integrated within a redox hydrogel by means of scanning photoelectrochemical microscopy (SPECM) is reported. The redox polymer acts as a conducting matrix for the transfer of electrons from the electrode surface to the photo-oxidized P700 centers within PS1, while methyl viologen is used as charge carrier for the collection of electrons at the reduced FB site of PS1. The analysis of the modified surfaces by SPECM enables the evaluation of electron-transfer processes by simultaneously monitoring photocurrent generation at the bio-photoelectrode and the associated generation of reduced charge carriers. The possibility to visualize charge recombination processes is illustrated by using two different electrode materials, namely Au and p-doped Si, exhibiting substantially different electron transfer kinetics for the reoxidation of the methyl viologen radical cation used as freely diffusing charge carrier. In the case of p-doped Si, a slower recombination kinetics allows visualization of methyl viologen radical cation concentration profiles from SPECM approach curves.

(Invited) Spectroelectrochemical Evidence of an Electron Transfer through Viologen Moiety from an ITO Electrode to a Molecular Catalyst for Hydrogen Evolution Reaction Confined within a Viologen Monolayer

To realize highly efficient and selective chemical and electrochemical processes, catalyst is essential. Although precious metals are often key components of many catalysts, they are expensive and of limited resources. Many efforts have been made to develop non-precious metal catalyst and maximize the utilization of these elements. 100% atom efficiency is expected for molecular and single atom catalysts but both have difficulties in practical use. It is difficult to separate molecular catalysts from reactants and products after the reaction as they are usually used as homogeneous catalyst in solution. Preparation of single atom catalysts is difficult and they easily aggregate if they are not prepared on proper supports. We have recently proposed that these problems can be overcome by confining a metal complex on and within molecular layers so that “confined molecular catalyst” is formed and reported that metal complexes confined within viologen molecular layer, which is constructed on Si(111) electrode, indeed act as electrocatalysts for both hydrogen evolution [1-4] and CO2 reduction [4, 5] reactions, demonstrating the effectiveness of the “confined molecular catalyst”. Although we suggested that viologen moiety of the molecular layer and a Pt complex act as an electron relay and a catalyst, respectively, no direct evidence for electron to be transferred via viologen moiety has been provided. If electron is transferred via viologen moiety, viologen radical cation (V+•), which is formed by one electron reduction of viologen dication (V2+), should be detected as an intermediate state upon potential step to negative potential where hydrogen evolution reaction (HER) takes place. Here we carried out spectroelectrochemical measurements at a viologen monolayer constructed on indium tin oxide (ITO) with and without a Pt complex. Two redox peaks corresponding to viologen dication/radical cation (V2+/V+•) and viologen radical cation/neutral form (V+•/V0) are observed in cyclic voltammogram (CV) of viologen monolayer modified ITO electrode without Pt complex. Stable spectra corresponding to radical cation (V+•) and neutral form (V0) of viologen are obtained at potentials between V2+/V+• and V+•/V0 redox peaks and more negative than V+•/V0 redox peak, respectively. On the other hand, at viologen monolayer modified ITO electrode with Pt complex, no redox peaks but large current due to HER is observed in CV and UV/visible spectra obtained during the potential scan show no absorption peaks corresponding to V+• or V0. Time-resolved spectroelectrochemical measurements show, however, that V+• is formed upon the potentials step to the potentials more negative than V2+/ V+• redox potential and disappeared within ca. 1 ms, confirming that electron is transferred from ITO electrode to proton via viologen moiety and Pt complex.

Size-Matched Radical Multivalency.

Persistent π-radicals such as MV+• (MV refers to methyl viologen, i.e., N,N'-dimethyl-4,4'-bipyridinum) engage in weak radical-radical interactions. This phenomenon has been utilized recently in supramolecular chemistry with the discovery that MV+• and [cyclobis(paraquat-p-phenylene)]2(+•) (CBPQT2(+•)) form a strong 1:1 host-guest complex [CBPQT⊂MV]3(+•). In this full paper, we describe the extension of radical-pairing-based molecular recognition to a larger, square-shaped diradical host, [cyclobis(paraquat-4,4'-biphenylene)]2(+•) (MS2(+•)). This molecular square was evaluated for its ability to bind an isomeric series of possible diradical cyclophane guests, which consist of two radical viologen units that are linked by two ortho-, meta-, or para-xylylene bridges to provide different spacing between the planar radicals. UV-Vis-NIR measurements reveal that only the m-xylylene-linked isomer (m-CBPQT2(+•)) binds strongly inside of MS2(+•), resulting in the formation of a tetraradical complex [MS⊂m-CBPQT]4(+•). Titration experiments and variable temperature UV-Vis-NIR and EPR spectroscopic data indicate that, relative to the smaller trisradical complex [CBPQT⊂MV]3(+•), the new host-guest complex forms with a more favorable enthalpy change that is offset by a greater entropic penalty. As a result, the association constant (Ka = (1.12 ± 0.08) × 105 M-1) for [MS⊂m-CBPQT]4(+•) is similar to that previously determined for [CBPQT⊂MV]3(+•). The (super)structures of MS2(+•), m-CBPQT2(+•), and [MS⊂m-CBPQT]4(+•) were examined by single-crystal X-ray diffraction measurements and density functional theory calculations. The solid-state and computational structural analyses reveal that m-CBPQT2(+•) is ideally sized to bind inside of MS2(+•). The solid-state superstructures also indicate that localized radical-radical interactions in m-CBPQT2(+•) and [MS⊂m-CBPQT]4(+•) disrupt the extended radical-pairing interactions that are common in crystals of other viologen radical cations. Lastly, the formation of [MS⊂m-CBPQT]4(+•) was probed by cyclic voltammetry, demonstrating that the radical states of the cyclophanes are stabilized by the radical-pairing interactions.

Stabilizing benzyl viologen radical cation by cucurbit[7]uril rotaxanation

A dumbbell-like molecule 3 consisting of one benzyl viologen unit and two end stoppers was synthesized and transformed into a [2]rotaxane R3 by slipping a CB[7] ring over the isophthalic acid stopper onto the viologen group. The 1H NMR, ESR and UV–Vis measurements indicated that R3 kept stable under the chemical reduction with excess Na2S2O4 while 3 would decompose. The CV curve of R3 showed a more negative reduction potential (ΔE = −0.30 V) when compared with 3, resulting from the CB[7] encapsulation. R3 underwent reversible electrochemical redox processes while the redox peaks of 3 disappeared gradually after thirty electrochemical redox cycles. Such a stabilization strategy by CB[7] rotaxanation provides potential applications in viologen-based electrochromic display devices.

Well‐defined polymers containing high density of pendant viologen groups

ABSTRACTAtom transfer radical polymerization (ATRP) of a viologen‐containing methacrylate, 1‐propyl‐1′‐[2‐(methacryloyloxy)ethyl]‐4,4′‐bipyridinium dihexafluorophosphate, is reported. To achieve good polymerization control, it was essential to use the viologen‐based monomer with a hexafluorophosphate instead of halide counterion, and 2,2′‐bipyridine as the ligand for the Cu‐based ATRP catalyst. The solubility of produced cationic polymers could be tuned by anion metathesis: the polymers with hexafluorophosphate counterions were soluble in organic solvents (e.g., acetone, DMF), and those with chloride counterions were water‐soluble. In aqueous solutions, the polymers (chloride salts) formed large aggregates, the sizes of which ranged from about 200 to about 400 nm (based on dynamic light scattering measurements) depending on the molecular weight. Upon addition of electrolytes (e.g., NaCl), the aggregates underwent dissociation. The apparent diffusion coefficients of the aggregates existing in aqueous solutions and the products of their electrolyte‐induced dissociation were measured by diffusion‐ordered NMR spectroscopy. The association–dissociation processes were also studied by fluorescence spectroscopy: the aqueous polymer solutions, which were originally fluorescent (λem = 402 nm at λex = 350 nm), lost their fluorescence in the presence of NaCl. The addition of small amounts of the viologen‐containing polyelectrolytes to solutions of inorganic salts (NaCl) altered the crystal morphology of the salts due to interaction of the multiple charged pendant groups with small ions. In the presence of reducing agents, the pendant viologen groups were converted to viologen radical‐cations, which are prone to dimerize reversibly in aqueous solutions. Indeed, marked dimerization of viologen radical cations (with absorbance maxima at 520 and 870 nm) was observed in relatively dilute aqueous solutions (4 mg mL−1) upon addition of reducing agents (hydrazine). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 55, 1173–1182

The Reactivity of Selenite toward Methyl Viologen in Mildly Acidic Aqueous Solutions

The chemical and electrochemical reactivity of selenite, SeO32−, toward methyl viologen, MV2+, has been examined in mildly acidic (pH 4) aqueous electrolytes. Solutions containing these two species heated at 60°C for ca. 2 hours, were found to yield both 1-methyl-[4,4’-bipyridine]-1-ium chloride and elemental Se as the main reaction products, as evidenced from information derived from mass spectrometric and spectroscopic techniques. Cyclic voltammetric measurements recorded in acetate buffer (pH 4) with a glassy carbon, GC, electrode bearing a self-assembled monolayer, SAM, of N-ethyl-N’-octadecyl-4,4’-bipyridinium (EOB), which had been exposed to a 100 mM SeO32− solution in aqueous acetate buffer (pH 4) for over 200 min and then rinsed with ultrapure water, displayed a significantly smaller redox peak ascribed to the reduction of the viologen moiety in the EOB SAM. This observation is consistent with the loss of the redox active species within the layer and supports the view that SeO32− can cleave the N-C bond in EOB leading to the release of the viologen group into the solution. Cyclic voltammetric experiments involving a freshly prepared EOB SAM on GC in 1 mM SeO32− in the same buffer yielded a large reduction current with an onset potential virtually identical to that associated with the reduction of the EOB SAM in SeO32−-free solutions. On this basis, the extraordinary shift in the onset of SeO32− reduction induced by the presence of MV2+ in the same electrolyte reported by Koshikumo et al. [Electrochemistry (Tokyo, Jpn.), 81(5), 350 (2013)], may be explained by invoking formation of MV2+ aggregates on the GC surface in amounts too small to be detected by conventional voltammetric techniques.

Organic Ion-Pair Charge-Transfer (IPCT) Nanoparticles: Synthesis and Photoinduced Electrochromism.

We report fabrication of photoinduced-electrochromic (or photochromic) organic nanoparticles that can be regarded as a new class of photoswitchable nanomaterials. Ion association between dicationic benzyl viologen (BzV2+) and tetraphenylborate anion (TPB-) in water produces organic ion-pair charge-transfer-type (IPCT) nanoparticles with a mean diameter of 15-20 nm. Electrostatic binding of BzV2+ with TPB- results in the prominent appearance of a new broad absorption band at 400-550 nm, which can be assign to the CT transition between these species. The IPCT nanoparticles show an interesting photoinduced electrochromic behavior caused by the electron transfer (ET) from TPB- to BzV2+, giving an intense blue-color dispersion. The feed charge stoichiometry (= [TPB-]/(2 × [BzV2+])) does not influence the kinetics of the photoinduced electrochromism, but it remarkably controls the reverse ET reaction (or retention time) in the nanoparticles; in the case of neutral stoichiometry, a very long-lived ET state can be observed, which is mainly due to decomposition of the oxidized TPB (sacrificial donor) and the resultant absence of the counterpart of benzyl viologen radical cations (BzV+•) for back ET reaction. On the other hand, the excess presence of TPB- does not cause the decomposition of oxidized TPB in the nanoparticle, making the reverse ET reaction faster, but a moderately long-lived ET state is still observable, which is probably originated from efficient electron diffusion or transport between molecules inside the nanoparticle.

A Viologen-Perylenediimide Conjugate as an Efficient Base Sensor with Solvatochromic Property.

A viologen-perylenediimide conjugate, denoted PDEV, is prepared for efficient base sensing. The conjugate shows solvatochromic behavior as well. The base sensitivity of viologen is purposefully coupled with the emission property of perylenediimide (PDI) to lower the detection limit. PDEV shows base-sensing ability at the ppb level, which is at least three orders of magnitude lower than those of previously reported sensors. The probe is sensitive toward solvent polarity and generates different shades of colors according to the polarity of the medium (solvent). The photophysical properties show a linear correlation with the solvent polarity, and this makes it an efficient solvatochromic agent. On the other hand, the generation of viologen radical cations by bases affects the aggregation and consequently the absorption and emission behavior of the PDI core. The effect of bases can also be visualized, because the probe generates different colors in the presence of bases, both under normal and under UV light. Organic amines can be detected even in the crystalline state, since the dark red color of the PDEV crystals changes to purple in a reversible fashion on exposure to amine vapors. An easy and practical paper-based tool created by using the probe can efficiently be used to detect solvent polarity and presence of bases optically.

Fabrication of a Targeted Drug Delivery System from a Pillar[5]arene‐Based Supramolecular Diblock Copolymeric Amphiphile for Effective Cancer Therapy

Effective cancer therapy will profit from the development of sophisticated drug delivery systems with stimuli‐responsive properties that are capable of delivering therapeutic doses to the active sites, while minimizing the accumulation of highly toxic drugs in off‐target sites. Herein, the fabrication of a pillararene‐based amphiphilic supramolecular diblock polymer (P5‐PEG‐Biotin⊃PCL‐C2V) based on the host–guest recognition between a water‐soluble pillar[5]arene and a viologen salt is reported. P5‐PEG‐Biotin⊃PCL‐C2V self‐assembles into polymersomes, which are utilized as drug delivery vehicles for doxorubicin hydrochloride (DOX). Decorated by the biotin groups, these smart nanocarriers deliver the anticancer drug preferentially to biotin receptor over‐expressing cancer cells. After internalization by the cells, the viologen group is reduced into the cationic radical state by the intracellular reductase NAD(P)H, leading to the release of the loaded DOX by the disassembly of the polymersomes. More importantly, DOX‐loaded polymersomes maintain the therapeutic efficacy towards cancerous HeLa cells, while exhibiting relatively low cytotoxicity towards normal HEK293 cells. In vivo studies reveal that the DOX‐loaded supramolecular polymersomes prolong the circulation time in the bloodstream, promote the antitumor efficacy and reduce the systematic toxicity of the drug through flexible and modular supramolecular strategy.

Modulation of Cu(2-x)S Nanocrystal Plasmon Resonance through Reversible Photoinduced Electron Transfer.

Copper sulfide (Cu(2-x)S) nanocrystals with nonstoichiometric composition exhibit plasmon resonance in the near-infrared region. Compositional changes and varying electron density markedly affect the position and intensity of the plasmon resonance. We report a photochemically induced phenomenon of modulating the plasmon resonance in a controlled fashion. As photogenerated reduced methyl viologen radicals transfer electrons to Cu(2-x)S in inert solutions, we observe a decrease in localized surface plasmon resonance (LSPR) absorbance at 1160 nm. Upon exposure to air, the plasmon resonance band recovers as stored electrons are scavenged away by oxygen. This cycle of electron charge and discharge of Cu(2-x)S nanocrystals is reversible and can be repeated through photoirradiation in N2 saturated solution followed by exposure of the suspension to air. The spectroscopic studies that provide mechanistic insights into the reversible charging and discharging of plasmonic Cu(2-x)S are discussed.

Enhanced Surface Electron Transfer with the Aid of Methyl Viologen on the Co3O4-g-C3N4 Photocatalyst

Due to the unique character of methyl viologen (MV 2+ ), namely, the electron oscillation that may occur in the process of MV 2+ reduction and oxidation, by means of MV 2+ addition, reacting system inert-atmosphere and electron donor addition, based on the peculiar contribution of MV 2+ with an electron oscillation characters, the stabilities and hydrogen evolution of the Co 3 O 4 -g-C 3 N 4 photocatalyst were greatly improved. Charge separation and electron transfer were more efficient under the condition of MV 2+ addition, as supported by fluorescence studies. The photoreduction of MV 2+ by Eosin Y (EY 2− ) can form a persistent methyl viologen radical cation (MV •+ ). MV 2+ , as an excellent electron relay, greatly facilitated electron transfer from the excited state 3 (EY 2− )* to the conduction band of Co 3 O 4 -g-C 3 N 4 . Accordingly, the excited state 3 (EY 2− )* was oxidized by MV 2+ ; this process effectively promoted the circulation regeneration of EY 2− , suppressed the generation of unstable intermediate EY •3− , and reduced the decomposition of EY.

Well-defined polymers containing a single mid-chain viologen group: synthesis, environment-sensitive fluorescence, and redox activity

A difunctional viologen-based alkyl halide initiator was employed in the atom transfer radical polymerization of methyl methacrylate, which afforded well-defined polymers with a single mid-chain viologen functionality. The materials were fluorescent and also served as redox catalysts.