Unprecedented stacking of MV2+ dications and MV˙+ radical cations in the mixed-valence viologen salt (MV)2(BF4)3 (MV = methylviologen)

Synthesis of anionic alkaline earth metal methanesulfonates bearing photo-responsive methylviologen

Anionic coordination frameworks stabilized by methylviologen (MV2+; 1,1'-dimethyl 4,4'-bipyridinium) as a photochromic module are a distinct class of functional materials. Herein, we report the first examples of alkaline earth metal methanesulfonates [MV{Mg(OSO2Me)3(H2O)2}2] (1), [MV{Ca(OSO2Me)3(H2O)2}2] (2), [2{Mg(OSO2Me)2(H2O)4}MV·2OSO2Me] (3), and [Me4-pip{Ca2(OSO2Me)6(H2O)2}]n (4) incorporating methylviologen or tetra-N-methylpiperazinium (Me4-pip) as a charge-balancing cation. X-ray crystallographic studies of 1-3 emphasize the role of isolated ion-pairs for the construction of hydrogen-bonded supramolecular assemblies and the ability of methanesulfonates to donate electrons (centroid⋯O = 3.09-3.15 Å) to promote photoinduced one-electron reduction of MV2+ to an MV˙+ radical cation in the solid state. The inorganic framework in 4 adopts a layered motif with an inter-lamellar spacing of 11.8 Å, providing spatial expanse to accommodate the organic cations.

Harnessing Radicals: Advances in Self-Assembly and Molecular Machinery

Radicals, with their unpaired electrons, exhibit unique chemical and physical properties that have long intrigued chemists. Despite early skepticism about their stability, the discovery of persistent radicals has opened new possibilities for molecular interactions. This review examines the mechanisms and applications of radically driven self-assembly, focusing on key motifs such as naphthalene diimides, tetrathiafulvalenes, and viologens, which serve as models for radical assembly. The potential of radical interactions in the development of artificial molecular machines (AMMs) are also discussed. These AMMs, powered by radical-radical interactions, represent significant advancements in non-equilibrium chemistry, mimicking the functionalities of biological systems. From molecular switches to ratchets and pumps, the versatility and unique properties of radically powered AMMs are highlighted. Additionally, the applications of radical assembly in materials science are explored, particularly in creating smart materials with redox-responsive properties. The review concludes by comparing AMMs to biological molecular machines, offering insights into future directions. This overview underscores the impact of radical chemistry on molecular assembly and its promising applications in both synthetic and biological systems.

Resonance Raman spectra and excited state properties of methyl viologen and its radical cation from time-dependent density functional theory

Time-dependent density functional theory (TDDFT) was applied to gain insights into the electronic and vibrational spectroscopic properties of an important electron transport mediator, methyl viologen (MV2+ ). An organic dication, MV2+ has numerous applications in electrochemistry that include energy conversion and storage, environmental remediation, and chemical sensing and electrosynthesis. MV2+ is easily reduced by a single electron transfer to form a radical cation species (MV•+ ), which has an intense UV-visible absorption near 600 nm. The redox properties of the MV2+ /MV•+ couple and light-sensitivity of MV•+ have made the system appealing for photo-electrochemical energy conversion (e.g., solar hydrogen generation from water) and the study of photo-induced charge transfer processes through electronic absorption and resonance Raman spectroscopic measurements. The reported work applies leading TDDFT approaches to investigate the electronic and vibrational spectroscopic properties of MV2+ and MV•+ . Using a conventional hybrid exchange functional (B3-LYP) and a long-range corrected hybrid exchange functional (ωB97X-D3), including with a conductor-like polarizable continuum model to account for solvation, the electronic absorption and resonance Raman spectra predicted are in good agreement with experiment. Also analyzed are the charge transfer character and natural transition orbitals derived from the TDDFT vertical excitations calculated. The findings and models developed further the understanding of the electronic properties of viologens and related organic redox mediators important in renewable energy applications and serve as a reference for guiding the interpretation of electronic absorption and Raman spectra of the ions.

Photo/Electrochromic Dual Responsive Behavior of a Cage-like Zr(IV)-Viologen Metal-Organic Polyhedron (MOP)

Stable stimulus-responsive materials are highly desirable due to their widespread potential applications and growing demand in recent decades. Despite the fact that viologen derivatives have long been known as excellent photochromic and electrochromic materials, the development of stable viologen-based multifunctional smart materials with short coloration times remains an exciting topic. To obtain photochromic and electrochromic dual responsive materials, embedding the viologen ligand into a robust metal oxide cluster to increase its stability and sensitivity is an effective strategy. Herein, a viologen-based metal-organic polyhedron (MOP) {[Zr₆L₃(μ₃-O)₂(μ₂-OH)₆Cp₆]·8Cl·CH₃OH·DMF} [Zr-MOP-1; H₂L·2Cl = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium dichloride, and Cp = η⁵-C₅H₅] was successfully prepared and characterized. It consists of trinuclear Zr-oxygen secondary building units and exhibits reversible photochromic and electrochromic dual responsive behaviors. As expected, the designed robust viologen-based nanocage with a V₂E₃ (V = vertex, and E = edge) topology can maintain its stability and rapid photo/electrochromic behaviors with an obvious reversible change in color from purple (brown) to green, mainly due to the enclosed cluster structure and the abundant free viologen radicals that originate from the effective Cl → N and O → N electron transfers. Spectroelectrochemistry and theoretical calculations of this Zr-MOP were also performed to verify the chromic mechanism.

Exploring Curious Covalent Bonding: Raman Identification and Thermodynamics of Perpendicular and Parallel Pancake Bonding (Pimers) of Ethyl Viologen Radical Cation Dimers

Viologen radical cations can dimerize in solutions, and the resulting "pimers" were predicted to assemble into parallel and perpendicular conformers by density functional theory (DFT) calculations. Using resonance Raman, we could identify both perpendicular and parallel forms of ethyl viologen dimers. The distinction between the two forms was accomplished by studying the formation of a host-guest complex with γ-cyclodextrin. The dimer's perpendicular form was excluded due to the host cavity size, and γ-cyclodextrin addition caused a decrease in peak intensities at 1171, 1511, and 1602 cm^-1 that could be assigned to the perpendicular form. DFT modeling of the vibrational spectra under preresonance conditions allowed us to assign the remaining vibrational modes for the parallel and perpendicular forms. Using variable-temperature UV-vis, the bond dissociation energy (ΔH) for this pancake-bonded dimer was measured as 13.1 ± 0.2 kcal/mol. This type of covalent pancake bonding is a challenge to properly describe using DFT methods. Previously, B97D was found to best describe the ΔG of this dimerization (Angew. Chem. 2017, 129, 9563-9567), but this method underestimates the ΔH by 6 kcal/mol. Of the 11 functionals tested, we found that B3LYP with Grimme's D3 dispersion effect can best reproduce the ΔH. Energy decomposition analysis of the bonding energy showed that solvation effects were the most important contributor-polar solvents are needed to overcome the Coulomb repulsion between the two positively charged monomers. Dispersion effects are second in importance and appear larger than the favorable orbital interaction obtained by singly occupied molecular orbital (SOMO)-SOMO orbital overlap. This study brings forth important insights into the curious cases of covalent bonding between two π-delocalized radicals.

Broadband Photoresponsive Bismuth Halide Hybrid Semiconductors Built with π-Stacked Photoactive Polycyclic Viologen

Photoresponse ranges of commercially prevailing photoelectric semiconductors, typically Si and InGaAs, are far from fully covering the whole solar spectrum (∼295-2500 nm), resulting in insufficient solar energy conversion or narrow wave bands for photoelectric detection. Recent studies have shown that infinite π-aggregation of viologen radicals can provide semiconductors with a photoelectric response range covering the solar spectrum. However, controlled assembly of an infinite π-aggregate is still a great challenge in material design. Through directional self-assembly of electron-transfer photoactive polycyclic ligands, two crystalline inorganic-organic hybrid photochromic viologen-based bismuth halide semiconductors, ((Me)₃pytpy)[BiCl₆]·2H₂O [1; (Me)₃pytpy = N,N',N″-trimethyl-2,4,6-tris(4-pyridyl)pyridine] and ((Me)₃pytpy)[Bi₂Cl₉]·H₂O (2), have been synthesized. They represent the first series of pytpy-based photochromic compounds. After photoinduced coloration, the conductivities of both 1 and 2 increased. The radical products have electron absorption bands in the range of 200-1600 nm, exceeding that of Si. Both the conductivity and the photocurrent intensity of 2 are stronger than those of 1, due to better planarity, tighter π-stacking, and higher degrees of overlap of ((Me)₃pytpy)³⁺ cations. This study not only provides a new design idea for synthesizing radical-based multispectral photoelectric semiconductors but also enriches the family of electron-transfer photochromic compounds.

Reversible dissociation of singly-bonded (C60−)2 dimers in (MV˙+)2(C60−)2·solvent salt containing paramagnetic methyl viologen MV˙+ radical cations

Interaction of MV⁰ with C₆₀ produces salt (MV˙⁺)₂(C₆₀⁻)₂⋅solvent (1) containing singly-bonded (C₆₀⁻)₂ dimers dissociated at 235–375 K. There is exchange interaction between MV˙⁺ and C₆₀˙⁻ in the monomeric phase which is switched off in the dimeric phase.

Serendipitous isolation of a triazinone-based air stable organic radical: synthesis, crystal structure, and computation

Here we report the synthesis, isolation, and characterization of a dication salt, namely 4,6-bis(4,4′-bipyridinium)-1,3,5-triazin-2-one {1²⁺(PF₆)₂²⁻·2H₂O1(PF6)2·2H2O}, and its radical cation salt, namely 4,6-bis(4,4′-bipyridinium)-1,3,5-triazin-2-one (1+˙PF₆⁻1˙PF6).

A macrocyclic receptor containing two viologen species connected by conjugated terphenyl groups

A macrocyclic receptor molecule containing two viologen species connected by conjugated terphenyl groups has been designed and synthesised. The single-crystal X-ray structure shows that the two viologen residues have a transannular NN separation of ca. 7.4 Å. Thus, the internal cavity dimensions are suitable for the inclusion of π-electron-rich species. The macrocycle is redox active, and can accept electrons from suitable donor species including triethylamine, resulting in a dramatic colour change from pale yellow to dark green as a consequence of the formation of a paramagnetic bis(radical cationic) species. Cyclic voltammetry shows that the macrocycle can undergo two sequential and reversible reduction processes (E1/2 = -0.65 and -0.97 V vs. Fc/Fc+). DFT and TD-DFT studies accurately replicate the structure of the tetracationic macrocycle and the electronic absorption spectra of the three major redox states of the system. These calculations also showed that during electrochemical reduction, the unpaired electron density of the radical cations remained relatively localised within the heterocyclic rings. The ability of the macrocycle to form supramolecular complexes was confirmed by the formation of a pseudorotaxane with a guest molecule containing a π-electron-rich 1,5-dihydroxynaphthalene derivative. Threading and dethreading of the pseudorotaxane was fast on the NMR timescale, and the complex exhibited an association constant of 150 M-1 (±30 M-1) as calculated from 1H NMR titration studies.

Efficient Ultraviolet Light Detector Based on a Crystalline Viologen-Based Metal-Organic Framework with Rapid Visible Color Change under Irradiation

A convenient colorimetric molecular system constructed by the zinc viologen-carboxylate framework is developed for naked eye detection of instantaneous UV exposure levels. Only narrow-band absorption in UV regions and a unique interpenetrated structure of its colorless crystal enable the system to give a fast response toward UV irradiance with intensity as low as 0.001 mw/cm².

Second-order nonlinear optical switching with a record-high contrast for a photochromic and thermochromic bistable crystal

Nonlinear optical (NLO) switchable materials are important for photonic and optoelectronic technologies. One important issue for NLO photoswitching, the most studied physical switching approach, is how to improve the switching contrast of second harmonic generation (SHG) in crystals, because the known values are generally below 3 times. Thermoswitching, as another approach, has shown impressive high SHG-switching contrasts (4-∞ times), but the fast decay of thermally induced states demands constant heat sources to maintain specific SHG intensities. We have synthesized a photochromic and thermochromic bistable acentric compound, β-[(MQ)ZnCl3] (MQ+ = N-methyl-4,4'-bipyridinium), which represents the first crystalline compound with both photo- and heat-induced SHG-switching behavior and the first example of a thermoswitchable NLO crystal that can maintain its expected second-order NLO intensity without any heat source. The SHG-switching contrast can reach about 8 times after laser irradiation or 2 times after thermal annealing. The former value is the highest recorded for photoswitchable NLO crystals. This work also indicates that higher SHG-switching contrasts may be obtained through increasing electron-transfer efficiency, variation of permanent dipole moment, and self-absorption.

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.

Synthesis of Dimethyl-Substituted Polyviologen and Control of Charge Transport in Electrodes for High-Resolution Electrochromic Displays

Electrochromic (EC) polymers such as polyviologens have been attracting considerable attention as wet-processable electrodes for EC displays, thanks to their brilliant color change accompanied with reversible redox reactions. To establish wider usage, achieving multicolor and high-resolution characteristics is indispensable. In this paper, we demonstrated that the introduction of substituents such as methyl groups into bipyridine units changed the stereostructure of the cation radicals, and thus shifted the color (e.g., ordinary purple to blue). Also, by relaxing excessive π-stacking between the viologen moieties, the response rate was improved by a factor of more than 10. The controlled charge transport throughout the polyviologen layer gave rise to the fabrication of EC displays which are potentially suitable for the thin film transistor (TFT) substrate as the counter electrodes with submillimeter pixels. The findings can be versatilely used for the new design of polyviologens with enhanced electrochemical properties and high-resolution, multicolor EC displays.

A fascinating multifaceted redox-active chelating ligand: introducing the N,N'-dimethyl-3,3'-biquinoxalinium "methylbiquinoxen" platform

N,N′-Dimethyl-3,3′-biquinoxalinium, “Methylbiquinoxen”: a stable redox-active chelating ligand with easily accessible functional states.

To intimately combine a chelating ligand function with the numerous properties of a viologen-like redox-active centre would offer a rare possibility to design controllable multi-redox states, whose properties arise from strongly correlated phenomena between the organic ligand as well as with any metalloid coordinated centres. Such a concept previously proved to be feasible, however is not widely applicable owing to challenges in terms of synthesis, isolation, and aerial sensitivity of both the ligand and its metal complexes. Here we report the first stable example of such a redox-active molecule, N,N′-dimethyl-3,3′-biquinoxalinium^2+/˙^+/0 “methylbiquinoxen, MBqn^2+/˙^+/0”, which shows a rich redox chemistry and chelates a metal ion in the case of the metal complex [CdCl₂(MBqn⁰)]. This goes beyond what is possible to achieve using viologens, which are limited by not providing chelation as well as having no accessible biradicaloid state, corresponding to the neutral direduced MBqn⁰ open-shell behaviour we observe here.

A viologen-functionalized chiral Eu-MOF as a platform for multifunctional switchable material

A novel crystalline Europium-organic framework (Eu-MOF) incorporates chirality and viologen, thereby combining photochromism, photo-modulated luminescence, photoswitchable NLO and piezoelectric properties.

Viologen cyclophanes: redox controlled host–guest interactions

Bis(viologen) cyclophanes with alkyl linkers demonstrate redox-dependent host–guest properties, including the formation of a crystalline mixed valence species.

Approaching the limit of CuII/CuI mixed valency in a CuIBr2–N-methylquinoxalinium hybrid compound

We present a novel 1D hybrid salt (MQ)[CuBr₂]_∞ (MQ = N-methylquinoxalinium) containing less than 0.1% Cu^II and yet shows properties consistent with a mixed valent structure.

A biferrocenium salt containing paramagnetic tetracyanoquinodimethane hexamers: charge disproportionation via donor-acceptor interactions

[Dineopentyl-biferrocene]2[Cl1TCNQ]7, which has an unprecedented high donor-acceptor ratio of 2 : 7, contains a linear paramagnetic hexamer of Cl1TCNQ. Both the donor and acceptor molecules exhibit charge disproportionation in the crystal through mutual electrostatic interactions.