Viologen Radical Cation Research Articles

The Unusual Photochromic and Hydrochromic Switching Behavior of Cellulose-Embedded 1,8-Naphthalimide-Viologen Derivatives in the Solid-State.

Stimuli-responsive chromic materials such as photochromics, hydrochromics, thermochromics, and electrochromics have a long history of capturing the attention of scientists due to their potential industrial applications and novelty in popular culture. However, hybrid chromic materials that combine two or more stimuli-triggered color changing properties are not so well known. Herein, we report a design strategy that has led to a series of emissive 1,8-naphthalimide-viologen dyads which exhibit unusual dual photochromic and hydrochromic switching behavior in the solid-state when embedded in a cellulose matrix. This behavior manifests as reversible solid state fluorescence hydrochromism upon changes in atmospheric relative humidity (RH), and reversible solid state photochromism upon generation of a cellulose-stabilized viologen radical cation. In this design strategy, the bipyridinium unit serves as both a water-sensitive receptor for the hydrochromic fluorophore-receptor system, and a photochromic group, capable of eliciting its own visible colorimetric response, generating a fluorescence quenching radical cation with prolonged exposure to ultraviolet (UV) light. These dyes can be inkjet-printed onto cellulose paper or drop-cast as cellulose powder-based films and can be unidirectionally cycled between three different states which can be characteristically visualized under UV light or visible light. The material's photochromism, hydrochromism, and underlying mechanism of action was investigated using computational analysis, dynamic vapor sorption/desorption isotherms, electron paramagnetic resonance spectroscopy, and variable humidity UV-Vis adsorption and fluorescence spectroscopies.

Electron-Triggered Metamorphism in Palladium-Driven Self-Assembled Architectures.

A metal-induced self-assembly strategy is used to promote the π-dimerization of viologen-based radicals at room temperature and in standard concentration ranges. Discrete box-shaped 2:2 (M:L) macrocycles or coordination polymers are formed in solution by self-assembly of a viologen-based ditopic ligand with cis-[Pd(en)(NO3)2], trans-[Pd(CH3CN)2(Cl)2], or [Pd(CH3CN)4(BF4)2]. Changing the redox state of the bipyridium units involved in the tectons, from their dicationic state to their radical cation state, results in a reversible "inflation/deflation" of the discrete 2:2 (M:L) macrocyclic assemblies associated to a large modification in the size of their inner cavity. Viologen-centered electron transfer is also used to trigger a dissociation of the coordination polymers formed with tetrakis(acetonitrile)Pd(II), the driving force of the disassembling process being the formation of discrete box-shaped 2:2 (M:L) assemblies stabilized by π-dimerization of both viologen cation radicals.

Viologen radical stabilization by molecular spectators for aqueous organic redox flow batteries

Aqueous organic redox flow battery (AORFB) using viologens suffers from poor cycling life especially at high concentrations due to the dimerization of viologen radical cations. Here, we report a molecular spectator approach, exploiting α-cyclodextrin (α-CD) with suitable cavity size as the spectator, to weaken the intermolecular interaction of viologen radicals and suppress dimerization in AORFBs. UV–vis spectroscopy reveals that α-CD effectively suppresses the dimerization of viologen radicals. Applying α-CD as the spectator, a symmetric static cell (0.1 M EV-0.01 M α-CD) demonstrated a significantly lower capacity decay rate of 0.075% per day (20 days, 100% capacity utilization) compared to the sole EV electrolyte (0.31% per day). Asymmetric flow cells (1.0 M EV-0.1 M α-CD) coupling with a ferrocene derivative demonstrated stable cycling over 500 cycles during 26 days at 80% capacity utilization. This work provides a facile and effective strategy to stabilize the viologen radicals for AORFBs applications. • A molecular spectator approach was developed to stabilize viologen radical cations. • A lower capacity decay was achieved with spectators (0.075% per day) compared to without (0.31% per day) at 100% capacity utilization. • An asymmetric full flow cell using spectators demonstrated a stable volumetric capacity of 21.4 Ah L –1 negolyte over 26 days.

Electron n-doping of a highly electron-deficient chlorinated benzodifurandione-based oligophenylene vinylene polymer using benzyl viologen radical cations

Successful electron-doping of highly electron-deficient chlorinated benzodifurandione-based polyphenylene vinylene using viologen radical cation.

Viologen-cucurbituril host/guest chemistry - redox control of dimerization versus inclusion.

Two calix[4]arene systems, C234+ and C244+ – where 2 corresponds to the number of viologen units and 3–4 corresponds to the number of carbon atoms connecting the viologen units to the macrocyclic core – have been synthesized and led to the formation of [3]pseudorotaxanes when combined with either CB[7] or CB[8]. The [3]pseudorotaxanes spontaneously dissociate upon reduction of the bipyridinium units as the result of intramolecular dimerization of the two face-to-face viologen radical cations. CB[7] and CB[8]-based [2]pseudorotaxanes containing monomeric viologen guest model compounds, MC32+ and MC4+, do not undergo decomplexation and dimerization following electrochemical reduction of their bipyridinium units.

High-performance & thermally stable n-type polymer thermoelectrics based on a benzyl viologen radical cation-doped ladder-type conjugated polymer

Viologen radical cation doped poly(benzimidazobenzophenanthrolinedione) (BBL) has shown enormous potential for high performance and thermally stable n-type polymer thermoelectrics.

Discrete Open-Shell Tris(bipyridinium radical cationic) Inclusion Complexes in the Solid State.

The solid-state properties of organic radicals depend on radical-radical interactions that are influenced by the superstructure of the crystalline phase. Here, we report the synthesis and characterization of a substituted tetracationic cyclophane, cyclobis(paraquat-p-1,4-dimethoxyphenylene), which associates in its bisradical dicationic redox state with the methyl viologen radical cation (MV•+) to give a 1:1 inclusion complex. The (super)structures of the reduced cyclophane and this 1:1 complex in the solid state deviate from the analogous (super)structures observed for the reduced state of cyclobis(paraquat-p-phenylene) and that of its trisradical tricationic complex. Titration experiments reveal that the methoxy substituents on the p-phenylene linkers do not influence binding of the cyclophane toward small neutral guests-such as dimethoxybenzene and tetrathiafulvalene-whereas binding of larger radical cationic guests such as MV•+ by the reduced cyclophane decreases 10-fold. X-ray diffraction analysis reveals that the solid-state superstructure of the 1:1 complex constitutes a discrete entity with weak intermolecular orbital overlap between neighboring complexes. Transient nutation EPR experiments and DFT calculations confirm that the complex has a doublet spin configuration in the ground state as a result of the strong orbital overlap, while the quartet-state spin configuration is higher in energy and inaccessible at ambient temperature. Superconducting quantum interference device (SQUID) measurements reveal that the trisradical tricationic complexes interact antiferromagnetically and form a one-dimensional Heisenberg antiferromagnetic chain along the a-axis of the crystal. These results offer insights into the design and synthesis of organic magnetic materials based on host-guest complexes.

Stabilization of p-GaAs electrode surfaces in organic solvent by bi-phenyl rings for spin dependent electron transfer studies

Biphenyl-4-thiol (B4t) functionalised p-GaAs photocathodes were compared to non-functionalised p-GaAs ones in the presence of methyl viologen cation radical (MV+) as a redox mediator. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies revealed that at bare p-GaAs photocathodes the radical interacts strongly with the electrode surface, displaying unwanted surface states (SS) mediated electron transfer (ET) even in the dark. Functionalising the electrode with B4t passivated the interface against this unwanted process, while ensuring that the conduction band (CB) electrons still facilitated faradaic reduction of MV+ to MV0 with unhindered efficiency.

Highly stable viologens-based electrochromic devices with low operational voltages utilizing polymeric ionic liquids

In this work, we proposed an effective route of introducing poly(ionic liquid) (PIL) gel as electrolyte to suppress dimer formation of viologen radical cation during switching in electrochromic devices (ECDs). The ECDs were fabricated based on ion gels consisting of various viologens, PIL and ferrocene (Fc). We found that incorporation of PIL contributed to the suppression of dimer production. The suppression of dimer formation can provide ECDs with improved colouration efficiencies, faster switching times, longer cycle lives, and potentially reduced costs. The results showed that an excellent cyclic stability over 96% of initial transmittance change after 4000 cycles of switching.

Benzyl viologen radical cation: an effective n-dopant for poly(perylenediimide-bithiophene)

Poly(perylenediimide-bithiophene) can be efficiently n-doped by benzyl viologen radical cation and its thermoelectric performance was measured for the first time.

The benzyl viologen radical cation: an effective n-dopant for poly(naphthalenediimide-bithiophene)

n-Doping of poly(naphthalenediimide-bithiophene) was demonstrated using benzyl viologen radical cation.

Functionalized GaAs Photocathodes for Spin-Dependent Charge Transfer Studies

Chiral-Induced Spin Selectivity (CISS) is a well-documented phenomenon occurring at electrochemical interfaces, where chiral functional groups can hinder the transfer of electrons with a specific spin state, while allowing the electrons with an opposite spin state to be transferred to the electroactive species in the solution.1,2 Spin dependent electron transfer has also been observed at p-GaAs/electrolyte interface without chiral functionalities, relying on the spin selectivity between polarised electrons in the conduction band (CB) of the p-GaAs and spin-oriented radical species in the solution.3,4 Here we report spin-dependent charge transfer in electrochemistry, where the spin selectivity is achieved by manipulating the electron spins of a radical redox mediator in the sample solution using microwaves (MW) under resonance conditions. This work has required us to characterise the photocathodic behaviour of p-GaAs in contact with methyl viologen radical cation (MV+•) in organic solvents under very low light intensities, a domain not traditionally covered in semiconductor electrochemistry. We have also contrasted the behaviour of the bare p-GaAs to biphenyl- and peptide-functionalised surfaces, showing that the parasitic valence band (VB) and surface state (SS) mediated processes can be completely avoided. In the future, we hope that these functionalised surfaces allow us to increase the spin-dependent signal intensity, as the probability of electron’s spin state relaxing due to parasitic processes can be minimised. Figure 1

Cyclotris(paraquat-p-phenylenes).

Reported here is the synthesis, solid-state characterization, and redox properties of new triangular, threefold symmetric, viologen-containing macrocycles. Cyclotris(paraquat-p-phenylene) (CTPQT6+ ) and cyclotris(paraquat-p-1,4-dimethoxyphenylene) (MCTPQT6+ ) were prepared and their X-ray single-crystal (super)structures reveal intricate three-dimensional packing. MCTPQT6+ results in nanometer-sized channels, in contrast with its parent counterpart CTPQT6+ which crystallizes as a couple of polymorphs in the form of intercalated assemblies. In the solid state, MCTPQT3(.+) exhibits stacks between the 1,4-dimethoxyphenylene and bipyridinium radical cations, providing new opportunities for the manipulation and control of the recognition motif associated with viologen radical cations. These redox-active cyclophanes demonstrate that geometry-matching and weak intermolecular interactions are of paramount importance in dictating the formation of their intricate solid-state superstructures.

Cyclotris(paraquat‐p‐phenylenes)

AbstractReported here is the synthesis, solid‐state characterization, and redox properties of new triangular, threefold symmetric, viologen‐containing macrocycles. Cyclotris(paraquat‐p‐phenylene) (CTPQT6+) and cyclotris(paraquat‐p‐1,4‐dimethoxyphenylene) (MCTPQT6+) were prepared and their X‐ray single‐crystal (super)structures reveal intricate three‐dimensional packing. MCTPQT6+ results in nanometer‐sized channels, in contrast with its parent counterpart CTPQT6+ which crystallizes as a couple of polymorphs in the form of intercalated assemblies. In the solid state, MCTPQT3(.+) exhibits stacks between the 1,4‐dimethoxyphenylene and bipyridinium radical cations, providing new opportunities for the manipulation and control of the recognition motif associated with viologen radical cations. These redox‐active cyclophanes demonstrate that geometry‐matching and weak intermolecular interactions are of paramount importance in dictating the formation of their intricate solid‐state superstructures.

Synthesis, self-assembly, and photomechanical actuator performance of a sequence-defined polyviologen crosslinker

ABSTRACT Although it is well known that viologen radical cations can self-assemble into stacks or complexes on account of radical-radical pairing interactions, it has only recently been demonstrated that reduction of main-chain polyviologens integrated into hydrogel networks can trigger actuation. In these earlier examples, hydrogels comprising oligoethylene glycol-based polyviologens and poly(ethylene glycol) were functionalized with terminal azide groups to prepare ‘click’-based gels. Here, we report a new structural design for the functional polyviologen that consists of main-chain viologen subunits separated by hexamethylene groups instead of glycols and is capped at each end with styrene groups. Activation of this viologen-based macrocrosslinker was achieved using chemical- and photoreduction methods and its ability to undergo intramolecular chain-folding was monitored by absorption spectroscopy. Acrylate-based organogels and hydrogels were also prepared and a comparison was carried out to assess the actuator performance in each gel in terms of the rate of contraction and changes in stiffness.

Effect of trifluoromethyl substituents in benzyl-based viologen on the electrochromic performance: Optical contrast and stability

Viologen (V) based molecules are classic and prestigious electrochromic (EC) materials which have dramatic optical contrast and tunable coloring states in solution-type electrochromic devices (ECDs). However, the stability of viologen based ECDs was significantly reduced by the dimer formation, and the resultant aggregation of viologens, which may occur once viologen dication (V2+) was reduced to viologen radical cation (V+•). To diminish this phenomenon, two novel symmetric benzyl-based viologens including 1,1′-bis(3,5-bis(trifluoromethyl)-benzyl)-4,4′-bipyridine-1,1′-diium (DTFMBzV) and 1,1′-bis(4-(trifluoromethyl)benzyl)-4,4′-bipyridine-1,1′-diium (TFMBzV) with various numbers of trifluoromethyl (CF3) substituents were synthesized and studied in this work. Substituent of CF3 in benzyl-based viologens not only enhance its optical property by increasing the π‒conjugation of bipyridine, but also avoid the aggregation by repulsing the viologen with fluorine atoms. The electrochromic performance of ECDs consisted of benzyl viologen (BzV/Fc ECD) and benzyl-based viologens with various numbers of CF3 substituents (DTFMBzV/Fc ECD and TFMBzV/Fc ECD) incorporated with ferrocene (Fc) were studied. According to the result of UV–vis absorbance spectra, the absorbances of benzyl viologen-based ECDs were strengthened at 399 and 605 nm by increasing the number of CF3 substituents. The DTFMBzV/Fc ECD gives a high transmittance change (ΔT) of 63.5% at 605 nm initially with a short response time (<3 s) by biasing the cell potential from 0 to 1.1 V. As for the stability, the DTFMBzV/Fc ECD exhibits an extremely high retention (97% of its initial ΔT) after switching for 10,000 cycles, which implied the dimerization of viologen was diminished by introducing the CF3 substituents with stronger electrostatic repulsion. As the result, DTFMBzV/Fc ECD exhibited the best electrochromic performance in terms of optical contrast and long-term stability. This study provides a simple yet effective strategy to tailing the electrochromic properties of viologen molecules for further application in electrochromism.

Reversible Hydrogel Photopatterning: Spatial and Temporal Control over Gel Mechanical Properties Using Visible Light Photoredox Catalysis.

There is a growing interest in being able to control the mechanical properties of hydrogels for applications in materials, medicine, and biology. Primarily, changes in the hydrogel's physical properties, i.e., stiffness, toughness, etc., are achieved by modulating the network cross-linking chemistry. Common cross-linking strategies rely on (i) irreversible network bond degradation and reformation in response to an external stimulus, (ii) using dynamic covalent chemistry, or (iii) isomerization of integrated functional groups (e.g., azobenzene or spiropyran). Many of these strategies are executed using ultraviolet or visible light since the incident photons serve as an external stimulus that affords spatial and temporal control over the mechanical adaptation process. Here, we describe a different type of hydrogel cross-linking strategy that uses a redox-responsive cross-linker, incorporated in poly(hydroxyethyl acrylate)-based hydrogels at three different weight percent loadings, which consists of two viologen subunits tethered by hexaethylene glycol and capped with styrene groups at each terminus. These dicationic viologen subunits (V2+) can be reduced to their monoradical cations (V•+) through a photoinduced electron transfer (PET) process using a visible light-absorbing photocatalyst (tris(bipyridine)ruthenium(II) dichloride) embedded in the hydrogel, resulting in the intramolecular stacking of viologen radical cations, through radical-radical pairing interactions, while losing two positive charges and the corresponding counteranions from the hydrogel. It is shown how this concerted process ultimately leads to collapse of the hydrogel network and significantly (p < 0.05) increases (by nearly a factor of 2) the soft material's stiffness, tensile strength, and percent elongation at break, all of which is easily reversed via oxidation of the viologen subunits and swelling in water. Application of this reversible PET process was demonstrated by photopatterning the same hydrogel multiple times, where the pattern was "erased" each time by turning off the blue light (∼450 nm) source and allowing for oxidation and reswelling in between patterning steps. The areas of the hydrogel that were masked exhibited lower (by 1-2 kPa) shear storage moduli (G') than the areas that were irradiated for 1.5 h. Moreover, because the viologen subunits in the functional cross-linker are electrochromic, it is possible to visualize the regions of the hydrogel that undergo changes in mechanical properties. This visualization process was illustrated by photopatterning a larger hydrogel (∼9.5 cm on its longest side) with a photomask in the design of an array of stars.

Photo/redox-responsive 2D-Supramolecular assembly involving Cucurbit[8]uril and a star-shaped porphyrin tecton

Abstrac The present paper reports on the formation and on the electrochemical/spectroscopic characterization of inclusion complexes formed in aqueous media between cucurbit[7 or 8]urils cavitands (CB[7], CB[ 8 ]) and a rigid four-pointed star-shaped viologen-appended porphyrin tecton. The formation of discrete 4:1 pseudo-rotaxane-like caviplexes, involving threading of CB[n] rings on the rigid viologen-based star's branches has been demonstrated by nuclear magnetic resonance and mass spectrometry measurements. Then, the photo- and redox-triggered formation of 2D supramolecular assemblies involving CB[8]s and the four electron reduced tectons as key building elements, has been established on the ground of in-depth electrochemical and spectroscopic analyses supported by quantum calculations. The CB[8]-promoted intermolecular π-dimerization of the viologen cation radicals introduced at the meso positions of the porphyrin plateform has been brought to light through the diagnostic signatures of the 1:2 host-guest ternary caviplexes formed between viologen and CB[8] and by spectroscopic data collected after electrochemical, chemical or photochemical reduction of the viologen-based tectons. The CB[8] hosts not only proved useful to promote the redox-triggered formation of supramolecular assemblies, it was also found to prevent the chemical reduction of the porphyrin ring in aqueous media and its subsequent conversion into phlorin products.

Efficient and Selective Electrochemically Driven Enzyme-Catalyzed Reduction of Carbon Dioxide to Formate using Formate Dehydrogenase and an Artificial Cofactor.

Increasing levels of carbon dioxide in the atmosphere and the growing need for energy necessitate a shift toward reliance on renewable energy sources and the utilization of carbon dioxide. Thus, producing carbonaceous fuel by the electrochemical reduction of carbon dioxide has been very appealing. We have focused on addressing the principal challenges of poor selectivity and poor energy efficiency in the electrochemical reduction of carbon dioxide. We have demonstrated here a viable pathway for the efficient and continuous electrochemical reduction of CO2 to formate using the metal-independent enzyme type of formate dehydrogenase (FDH) derived from C andida boidinii yeast. This type of FDH is attractive because it is commercially produced. In natural metabolic processes, this type of metal-independent FDH oxidizes formate to carbon dioxide using NAD+ as a cofactor. We show that FDH can catalyze the reverse process to generate formate when the natural cofactor NADH is replaced with an artificial cofactor, the methyl viologen radical cation. The methyl viologen radical cation is generated in situ, electrochemically. Our approach relies on the special properties of methyl viologen as a "unidirectional" redox cofactor for the conversion of CO2 to formate. Methyl viologen (in the oxidized form) does not catalyze formate oxidation, while the methyl viologen radical cation is an effective cofactor for the reduction of carbon dioxide. Thus, although the thermodynamic driving force is favorable for the oxidized form of methyl viologen to oxidize formate to carbon dioxide, the kinetic factors are not favorable. Only the reverse reaction of carbon dioxide reduction to formate is kinetically viable with the cofactor, methyl viologen radical cation. Binding free energy calculated from atomistic molecular dynamics (MD) simulations consolidate our understanding of these special binding properties of the methyl viologen radical cation and its ability to facilitate the two-electron reduction of carbon dioxide to formate in metal-independent FDH. By carrying out the reactions in a novel three-compartment cell, we have demonstrated the continuous production of formate at high energy efficiency and yield. This cell configuration uses judiciously selected ion-exchange membranes to separate the reaction compartments to preserve the yields of the methyl viologen radical cation and formate. By the electroregeneration of the methyl viologen radical cation at -0.44 V versus the normal hydrogen electrode, we could produce formate at 20 mV negative to the reversible electrode potential for carbon dioxide reduction to formate. Our results are in sharp contrast to the large overpotentials of -800 to -1000 mV required on metal catalysts, vindicating the selectivity and kinetic facility provided by FDH. Formate yields as high as 97% ± 1% could be realized by avoiding the adventitious reoxidation of the methyl viologen radical cation by molecular oxygen. We anticipate that the insights from the electrochemical studies and the MD simulations to be useful in redesigning the metal-independent FDH and alternate artificial cofactors to achieve even higher rates of conversion.

Revisiting aqueous redox process of alkyl-linked bis-viologen: Evaluation of redox potential inversion

Redox reactions of highly water-soluble bis-viologens were examined at Au electrodes using ultra-micro electrode steady state voltammetry and electroreflectance methods in addition to conventional voltammetry. A bulk UV–vis absorption spectral approach was also used. Alkane-1,n-diyl bis-viologens appeared as a class of prototypical electroactive molecule possessing two equivalent redox centers; they undergo two-consecutive one-electron transfer processes. When intramolecular dimerization takes place immediately after accepting the second electron to produce a form having two viologen radical cation (V+) sites in one molecule, the second reduction potential may be more positive than the first one. This redox potential inversion is caused by strong tendency to form a stable intramolecular π−π stacking between the two V+ sites. The redox potential inversion of a highly water-soluble bis-viologen with a butane-1,4-diyl linkage was quantitatively obtained to be 116 mV (E1 – E2 = −116 mV). In contrast, the bis-viologen with an ethane-1,2-diyl linkage exhibited no dimerization below 0.2 mM, and its second redox potential was found to at 63 mV more negative than the first (E1 – E2 = 63 mV), highlighting repulsive interaction. The value of E1 – E2 = −116 mV for the bis-viologen with a butane-1,4-diyl linkage corresponds to the attractive interaction energy in the redox process of 5.9 kBT of room temperature; this value is apparently the same as the dimerization Gibbs free energy of methyl viologen in water.