A pH- and Metal-Actuated Molecular Shuttle in Water.

The structure of the Viologen-Phenylene-Imidazole (VPI) guest, previously shown to be bound by cucurbit[7]uril (CB[7]) with binding modes depending on pH and silver ions, has been extended by adding hydrophobic groups on the two extremities of VPI before investigations of CB[7] binding by NMR, ITC, X-ray diffraction, UV-vis and fluorescence spectroscopies. With an imidazole station extended by a naphthalene group (VPI-N), binding modes of CB[7] are similar to those previously observed. However, with the viologen extended by a tolyl group (T-VPI), CB[7] preferentially sits on station T, shuttling between the T and P stations at acid pH or after Ag+ addition. The CB[7] ⋅ T-VPI complex thus behaves as a metal-actuated thermodynamic stop-and-go molecular shuttle featured by fast and autonomous ring translocation between two stations and a continuum for fractional station occupancy solely and easily controlled by Ag+ concentration.

State of Charge and State of Health Assessment of Viologens in Aqueous-Organic Redox-Flow Electrolytes Using In Situ IR Spectroscopy and Multivariate Curve Resolution.

Aqueous-organic redox flow batteries (RFBs) have gained considerable interest in recent years, given their potential for an economically viable energy storage at large scale. This, however, strongly depends on both the robustness of the underlying electrolyte chemistry against molecular decomposition reactions as well as the device's operation. With regard to this, the presented study focuses on the use of in situ IR spectroscopy in combination with a multivariate curve resolution approach to gain insight into both the molecular structures of the active materials present within the electrolyte as well as crucial electrolyte state parameters, represented by the electrolyte's state of charge (SOC) and state of health (SOH). To demonstrate the general applicability of the approach, methyl viologen (MV) and bis(3-trimethylammonium)propyl viologen (BTMAPV) are chosen, as viologens are frequently used as negolytes in aqueous-organic RFBs. The study's findings highlight the impact of in situ spectroscopy and spectral deconvolution tools on the precision of the obtainable SOC and SOH values. Furthermore, the study indicates the occurrence of multiple viologen dimers, which possibly influence the electrolyte lifetime and charging characteristics.

Paramagnetic relaxation enhancement NMR as a tool to probe guest binding and exchange in metallohosts.

Paramagnetic metallohost systems can bind guest molecules and find application as biomimetic catalysts. Due to the presence of the paramagnetic metal center, rigorous characterization of these systems by NMR spectroscopy can be very difficult. We report here that metallohost-guest systems can be studied by using the paramagnetic relaxation enhancement (PRE) effect. Manganese(III) porphyrin cage compounds are shown through their PRE to thread and bind viologen guests, including a polymeric one. The binding constants and dethreading activation parameters are lower than those of the metal-free porphyrin cage compounds, which is proposed to be a result of charge repulsion of the trivalent metal center and dicationic viologen guest. The threading rate of the manganese(III) porphyrin cage onto the polymer is more than 10 times faster than that of the non-metallated one, which is ascribed to initial binding of the cage to the polymer chain prior to threading, and to an entron effect.

Spontaneous Reduction-Induced Degradation of Viologen Compounds in Water Microdroplets and Its Inhibition by Host-Guest Complexation.

Water serves as an inert environment for the dispersion and application of many kinds of herbicides. Viologen compounds, a type of widely used but highly toxic herbicide, are stable in bulk water, whose half-life can be up to 23 weeks in natural water, imposing a severe health risk to mammals. In this study, we present the striking results of the spontaneous and ultrafast reduction-induced degradation of three viologen compounds in water microdroplets and provide the concentration, time, temperature dependence, mechanism, and scale-up of the reactions. We postulate that the electrons existing at the air-water interface of the microdroplets due to the unique redox potential therein initiate the reduction, from which further degradation occurs. The host-guest complexation between cucurbit[7]uril and viologens only slightly changes the redox potential of viologens in the bulk but completely inhibits the reactions in microdroplets, adding to the uniqueness of the redox potentials at the air-water interfaces of microdroplets. Taken together, microdroplets might have been functioning as naturally occurring ubiquitous tiny electrochemical cells for a plethora of unique redox reactions that were thought to be impossible in the bulk water.

Polypeptide organic radical batteries.

In only a few decades, lithium-ion batteries have revolutionized technologies, enabling the proliferation of portable devices and electric vehicles1, with substantial benefits for society. However, the rapid growth in technology has highlighted the ethical and environmental challenges of mining lithium, cobalt and other mineral ore resources, and the issues associated with the safe usage and non-hazardous disposal of batteries2. Only a small fraction of lithium-ion batteries are recycled, further exacerbating global material supply of strategic elements3-5. A potential alternative is to use organic-based redox-active materials6-8 to develop rechargeable batteries that originate from ethically sourced, sustainable materials and enable on-demand deconstruction and reconstruction. Making such batteries is challenging because the active materials must be stable during operation but degradable at end of life. Further, the degradation products should be either environmentally benign or recyclable for reconstruction into a new battery. Here we demonstrate a metal-free, polypeptide-based battery, in which viologens and nitroxide radicals are incorporated as redox-active groups along polypeptide backbones to function as anode and cathode materials, respectively. These redox-active polypeptides perform as active materials that are stable during battery operation and subsequently degrade on demand in acidic conditions to generate amino acids, other building blocks and degradation products. Such a polypeptide-based battery is a first step to addressing the need for alternative chemistries for green and sustainable batteries in a future circular economy.

An anthracene extended viologen-incorporated ionic porous organic polymer for efficient aerobic photocatalysis and antibacterial activity.

A new conjugated ionic porous organic polymer (AN-POP), incorporated with anthracene-extended viologen, has been rationally designed and prepared to explore its dual functions in photocatalytic oxidation and bacterial killing. Compared with its anthracene-free counterpart (BD-POP), AN-POP showed a superior photocatalytic oxidation performance and antibacterial activity demonstrating the critical role of an anthracene-extended viologen structure.

Viologen Tweezers to Probe the Force of Individual Donor-Acceptor π-Interactions.

Donor-acceptor (DA) π-interactions are weak attractive forces that are exploited widely in molecular and supramolecular chemistry. They have been characterized extensively by ensemble techniques, providing values for their energies that are useful for the design of soft materials. For implementation of motions or operations based on these DA π-interactions in wholly synthetic molecular machines, the mechanical strength and force associated with their out-of-equilibrium performance are the key parameters, in addition to their energies obtained at thermodynamic equilibrium. In this context, we have used single-molecule force spectroscopy as a nonequilibrium technique to determine the mechanical strength of individual DA π-interactions in solution. We designed and synthesized a molecular tweezer that is able to encapsulate π-donors and also demonstrated a precise opening extension. The mechanical breaking of the noncovalent interactions between viologen units-π-acceptors commonly employed in mechanically interlocked molecules-and several π-donors afforded a characteristic force-distance signature, revealing the opening of individual viologen tweezers with an unambiguous extension. Single-tweezer host-exchange experiments performed in situ demonstrated the sensitivity of the technique. This simple design could be exploited in quantifying the force of a large range of weak noncovalent bonding interactions as well as the potential work that molecular machines can generate at the single-molecule level.

A viologen-based multifunctional Eu-MOF: photo/electro-modulated chromism and luminescence.

A novel viologen-based multifunctional Eu-MOF was obtained by integrating a luminescent component Eu(NO3)3·6H2O and a viologen-functionalized ligand. The Eu-MOF not only exhibited reversible photochromic and electrochromic properties, but also displayed photoluminescent and electroluminochromic properties.

Helquats as Promoters of the Povarov Reaction: Synthesis of 1,2,3,4-Tetrahydroquinoline Scaffolds Catalyzed by Helicene-Viologen Hybrids.

Helquats (HQs) are structurally linked to helicenes and viologens, and they represent an attractive field of research in chemistry and medicinal chemistry. In the present work they were used as catalysts for the synthesis of 1,2,3,4-tetrahydroquinolines in good yields by the Povarov reaction. The substrate scope and the capability of different helquats to promote Povarov reactions are demonstrated. Studies to elucidate mechanistic details revealed that helquats act as single-electron transfer oxidants through a cation-radical mechanism. The screening of the catalytic activity of HQs confirmed that an active HQ must have a LUMO energy below -8.67 eV and the standard redox potential higher (less negative) than -1.2 V vs. the ferrocene/ferrocenium redox couple.

Thermotropic Liquid-Crystalline and Light-Emitting Properties of Bis(4-aalkoxyphenyl) Viologen Bis(triflimide) Salts.

A series of bis(4-alkoxyphenyl) viologen bis(triflimide) salts with alkoxy chains of different lengths were synthesized by the metathesis reaction of respective bis(4-alkoxyphenyl) viologen dichloride salts, which were in turn prepared from the reaction of Zincke salt with the corresponding 4-n-alkoxyanilines, with lithium triflimide in methanol. Their chemical structures were characterized by 1H and 13C nuclear magnetic resonance spectra and elemental analysis. Their thermotropic liquid-crystalline (LC) properties were examined by differential scanning calorimetry, polarizing optical microscopy, and variable temperature X-ray diffraction. Salts with short length alkoxy chains had crystal-to-liquid transitions. Salts of intermediate length alkoxy chains showed both crystal-to-smectic A (SmA) transitions, Tms, and SmA-to-isotropic transitions, Tis. Those with longer length of alkoxy chains had relatively low Tms at which they formed the SmA phases that persisted up to the decomposition at high temperatures. As expected, all of them had excellent thermal stabilities in the temperature range of 330-370 °C. Their light-emitting properties in methanol were also included.

Poly(selenoviologen)-Assembled Upconversion Nanoparticles for Low-Power Single-NIR Light-Triggered Synergistic Photodynamic and Photothermal Antibacterial Therapy.

The development of a highly effective photosensitizer (PS) that can be activated with a low-power single light is a pressing issue. Herein, we report a PS for synergistic photodynamic and photothermal therapy constructed through self-assembly of poly(selenoviologen) on the surface of core-shell NaYF4:Yb/Tm@NaYF4 upconversion nanoparticles. The hybrid UCNPs/PSeV PS showed strong ROS generation ability and high photothermal conversion efficiency (∼52.5%) under the mildest reported-to-date irradiation conditions (λ = 980 nm, 150 mW/cm2, 4 min), leading to a high efficiency in killing methicillin-resistant Staphylococcus aureus (MRSA) both in vitro and in vivo. Remarkably, after intravenous injection, the reported PS accumulated preferentially in deep MRSA-infected tissues and achieved an excellent therapeutic index. This PS design realizes a low-power single-NIR light-triggered synergistic phototherapy and provides a simple and versatile strategy to develop safe clinically translatable agents for efficient treatment of deep tissue bacterial inflammations.

Active Anion Delivery by Self-Propelled Microswimmers.

Self-propelled micro- and nanomachines are at the forefront of materials research, branching into applications in biomedical science and environmental remediation. Cationic frameworks enabling the collection and delivery of anionic species (A-) are highly required, due to the large variety of life-threatening pollutants, such as radioactive technetium and carcinogenic chromium, and medicines, such as dexamethasone derivatives with negative charges. However, such autonomous moving carriers for active transport of the anions have been barely discussed. A polymeric viologen (PV++)-consisting of electroactive bicationic subunits-is utilized in a tubular autonomous microswimmer to selectively deliver A- of different sizes and charge densities. The cargo loading is based on a facile anion exchange mechanism. The packed crystal structure of PV++ allows removal of an exceptionally high quantity of anions per one microswimmer (2.55 × 10-13 mol anions per microswimmer), a critical factor often neglected regarding the real-world application of microswimmers. Notably, there was virtually no leakage of anions during the delivery process or upon keeping the loaded microswimmers under ambient conditions for at least 4 months. Multiple release mechanisms, compatible with different environments, including electrochemical, photochemical, and a metathesis reaction, with high efficiencies up to 98% are introduced. Such functional autonomous micromachines provide great promise for the next generation of functional materials for biomedical and environmental applications.

Near-Infrared Optical Nanosensors for Continuous Detection of Glucose.

BACKGROUND: Continuous glucose monitors (CGMs) enable people with diabetes to proactively manage their blood glucose and reduce the risk of hypoglycemia. Commercially available CGMs utilize percutaneous electrodes that, after days to weeks of implantation, are subjected to the foreign body response that severely reduces sensor accuracy. The previous work demonstrated the use of hydrogels containing a glucose-responsive viologen that quenches a nearby fluorophore. Here, we investigate the immobilization of this sensing motif onto a nanoparticle surface and optimize local surface concentrations for optical glucose sensing. METHODS: A viologen quencher-fluorescent dye system was incorporated into poly(2-hydroethyl methacrylate) hydrogels in varying quantities to assess the effect of quencher-fluorophore concentration on glucose responsiveness. The sensing motif was then immobilized onto silica nanoparticles by carbodiimide chemistry. Nanosensors with a range of dye and quencher concentrations were challenged for glucose responsiveness to determine the optimal sensor formulation. RESULTS: When incorporated into a hydrogel, high concentrations of viologen quencher and fluorophore were required to permit electron transfer between the two components and yield a detectable glucose response. Immobilization of this glucose-responsive system onto a silica nanoparticle facilitated this electron transfer to yield detectable responses at even low concentrations. Increasing quencher concentration on the nanoparticle, relative to the fluorophore, resulted in the greatest apparent glucose response. CONCLUSION: The nanoparticle sensors demonstrated excellent glucose response in the physiological range and are a promising tool for real-time glucose tracking.

Mono- and Di-Quaternized 4,4'-Bipyridine Derivatives as Key Building Blocks for Medium- and Environment-Responsive Compounds and Materials.

Mono- and di-quaternized 4,4'-bipyridine derivatives constitute a family of heterocyclic compounds, which in recent years have been employed in numerous applications. These applications correspond to various disciplines of research and technology. In their majority, two key features of these 4,4'-bipyridine-based derivatives are exploited: their redox activity and their electrochromic aptitude. Contemporary materials and compounds encompassing these skeletons as building blocks are often characterized as multifunctional, as their presence often gives rise to interesting phenomena, e.g., various types of chromism. This research trend is acknowledged, and, in this review article, recent examples of multifunctional chromic materials/compounds of this class are presented. Emphasis is placed on solvent-/medium- and environment-responsive 4,4'-bipyridine derivatives. Two important classes of 4,4'-bipyridine-based products with solvatochromic and/or environment-responsive character are reviewed: viologens (i.e., N,N'-disubstituted derivatives) and monoquats (i.e., monosubstituted 4,4'-bipyridine derivatives). The multifunctional nature of these derivatives is analyzed and structure-property relations are discussed in connection to the role of these derivatives in various novel applications.

Visible-light driven hydrogen production using chlorophyll derivatives conjugated with a viologen moiety in the presence of platinum nanoparticles.

The utilization of the light-harvesting and electron-transferring function of chlorophylls (Chls) has received attention for visible-light driven hydrogen production. In this work, a series of Chl derivatives based on pyropheophorbide-a (Pyro-a) conjugated with a viologen moiety, including a Pyro-a methyl ester directly bonded with the viologen at the 3-position 1, its 31-methylene analog 2 and Pyro-a connected with the viologen in the 17-substituent 3, were synthesized from chemical modification of naturally occurring Chl-a and characterized in terms of their photochemical and photophysical properties. As the photoexcited singlet state of the Pyro-a moiety was strongly quenched by the viologen moiety in a molecule, the effective photoinduced intramolecular electron transfer from Pyro-a to the bonded viologen moiety occurred. Moreover, these molecules were applied as a photosensitizer in the system for visible-light driven hydrogen production with platinum nanoparticles via intramolecular reduction of the bonded viologen moiety. Efficient photoreduction of external methyl viologen and successive hydrogen production on platinum nanoparticles were achieved using the synthetic conjugate of Pyro-a with the viologen moiety as a photosensitizer. In particular, effective visible-light driven hydrogen production was accomplished using 3 and platinum nanoparticles via the reduction of external methyl viologen.

pH/Redox-Controlled Interaction between Lipid Membranes and Peptide Derivatives with a "Helmet".

How to reduce the cytotoxicity of antitumor peptides to normal cells remains an ongoing challenge. Here, we designed a pH/redox-responsive supramolecular structure (Pep-V⊂P-PEG) composed of a peptide modified with viologen (Pep-V) and polyethylene glycol bearing pillar[5]arene (P-PEG) as a "helmet". By shielding the hydrophobic moiety of the peptide derivative with pillar[5]arene via host-guest interactions, its disruption on normal cells can be effectively reduced. At acidic pH, the supramolecular structure can selectively adsorb onto negatively charged lipid membranes because of electrostatic interactions. Owing to redox responsiveness of the viologen group, Pep-V could be separated from P-PEG after the addition of reductants and inserted into lipid bilayers, which leads to membrane disruption. Cargo leakage of liposome models was investigated to understand Pep-V⊂P-PEG-induced liposomal membrane disruption under different pH values and redox conditions. Results showed that Pep-V⊂P-PEG caused almost no cargo leakage from (1,2-dipalmitoyl-sn-glycerol-3-phosphocholine) liposomes at pH 7.4 but significant leakage from negatively charged (1,2-dipalmitoyl-sn-glycerol-3-phospho-(1-rac-glyerol)) liposomes at pH 5.0 under a reducing environment. Pep-V⊂P-PEG displayed low destructive effects on mimic normal cells and significant disruption to mimic tumor cells when exposed to a reducing environment that is expected to be a potential antitumor agent.

Optimizing electron transfer from CdSe QDs to hydrogenase for photocatalytic H2 production.

A series of viologen related redox mediators of varying reduction potential has been characterized and their utility as electron shuttles between CdSe quantum dots and hydrogenase enzyme has been demonstrated. Tuning the mediator LUMO energy optimizes the performance of this hybrid photocatalytic system by balancing electron transfer rates of the shuttle.

Polyviologen as Electron Transport Material in Photosystem I-Based Biophotovoltaic Cells.

The photosynthetic protein complex, photosystem I (PSI), can be photoexcited with a quantum efficiency approaching unity and can be integrated into solar energy conversion devices as the photoactive electrode. The incorporation of PSI into conducting polymer frameworks allows for improved conductivity and orientational control in the photoactive layer. Polyviologens are a unique class of organic polycationic polymers that can rapidly accept electrons from a primary donor such as photoexcited PSI and subsequently can donate them to a secondary acceptor. Monomeric viologens, such as methyl viologen, have been widely used as diffusible mediators in wet PSI-based photoelectrochemical cells on the basis of their suitable redox potentials for accepting electrons. Polyviologens possess similar electronic properties to their monomers with the added advantage that they can shuttle electrons in the solid state. Depositing polyviologen directly onto a film of PSI protein results in significant photocurrent enhancement, which confirms its role as an electron-transport material. The polymer film not only improves the photocurrent by aiding the electron transfer but also helps preserve the protein film underneath. The composite polymer-PSI assembly enhances the charge-shuttling processes from individual protein molecules within the PSI multilayer, greatly reducing charge-transfer resistances. The resulting PSI-based solid-state platform demonstrates a much higher photocurrent than the corresponding photoelectrochemical cell built using a similar architecture.

A pH-driven ring translocation switch against cancer cells.

A molecular switch built with cucurbit[7]uril and a 3-station viologen-phenylene-imidazole compound exhibited pH actuated ring translocation with high fatigue resistance (up to 102 cycles). The switch movement was harnessed toward selectively masking the toxicity of the viologen fragment at neutral pH near non-cancerous cells, while exposing it at acid pH near cancer cells.

Fluorescent Probes for Sugar Detection.

Herein, a new class of polymerizable boronic acid (BA) monomers are presented, which are used to generate soft hydrogels capable of accurate determination of saccharide concentration. By exploiting the interaction of these cationic BAs with an anionic fluorophore, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (pyranine), a two-component sugar-sensing system was realized. In the presence of such cationic BAs ( o-BA, m-BA, and p-BA), the fluorescence of pyranine becomes quenched because of the formation of a nonfluorescent BA-fluorophore complex. Upon addition of saccharides, formation of a cyclic boronate ester results in dissociation of the nonfluorescent complex and recovery of the pyranine fluorescence. The response of this system was examined in solution with common monosaccharides, such as glucose, fructose, and galactose. Subsequent polymerization of the BA monomers yielded cross-linked hydrogels which showed similar reversible recovery of fluorescence in the presence of glucose.