Considerable interest is raised by organic materials owing to their exceptional performance in electronic and optoelectronic applications. Among these, electrochromic materials (EC) that can be switched between a distinct color and a bleached state exhibiting high contrast, multicolor and improved long-term stability are attractive in the fabrication of electrochromic devices (ECDs). Ionic materials, in particular, have received persistent attention owing to their tunable optical and electronic properties. 4,4′-Bipyridinium salts, commonly called viologens (V²⁺), are a well-recognized class of electrochromic materials that exhibit three reversible redox states, namely, V²⁺ (dication, pale yellow colored/colorless) ↔ V⁺˙ (radical cation, violet/blue/green) ↔ V⁰ (neutral, colorless). The electrochromic properties of these materials can be modulated by varying the nitrogen substituents on the pyridyl ‘N’; also, besides this, varying the counter ions with specific functionalities has been shown to enhance the electrochromic behavior, such as switching time, cycling stability and device performance. Although ECDs based on viologens are well regarded for their low operational voltages, they exhibit certain disadvantages such as low cycle life and poor efficiency of the device in the long term. Extensive efforts have been made to fine tune the EC properties of viologens, either through alteration or by adding suitable electrochromic counter electrode materials, which includes the incorporation of conducting polymers in the device set-up and/or the addition of complementary redox species. Optimization of the device parameters has shown that the addition of such external agents has a positive effect on the overall device performance. This review describes recent developments in the synthesis of viologen-based electrochromes with co-redox species and their ECD performance.

Organic redox compounds are a fascinating class of active materials used in energy storage applications. The structural diversity as well as ability to be molecularly tailored assists in fine-tuning of their electrochemical properties at the molecular level, which is highly desired for performance improvement. In reality, it is very challenging to achieve the desired physicochemical properties. Viologens represent a unique class of redox-active molecules that undergo two one-electron reductions in steps. Their redox properties have been highly explored in various fields such as electrocatalysis, electrochromism, photochromism, herbicides, and to some extent in energy storage systems. Their redox behavior as well as cationic charges are highly useful for improving device performance in energy storage systems. Different energy systems require different states of viologens, i.e., dicationic, monocation radical, or redox couple. For example, viologen dication shows electrostatic attraction towards polysulfides in Li–S batteries, and similarly, dication radical prevents Li dendrite growth in Li-ion batteries. In the case of redox flow batteries, such as supercapacitors and Li–air batteries, the redox behavior of viologens is useful for improving the performance. Despite these advantages, there are some challenges existing in viologen-based systems that must be conquered at the molecular level. In this regard, a systematic study on structure–activity relationships becomes crucial to address the bottlenecks. This review comprehensively analyses the role of viologens in various energy storage systems and the mode of chemical interactions responsible for enhancing the overall performance of electrochemical storage devices.

Biogenic amines (BA) are biomolecules of low molecular weight with organic basic functionalities (amine group) that are formed by the microbial decarboxylation of amino acids of fermented food/beverages. Hence BAs are an important indicator in estimating the freshness and quality of meat, seafood, and industrial food products with high protein content. The reaction of BAs with nitrites available in certain meat products forms nitrosoamine, a carcinogenic compound. Hence BAs are in general considered to be a food hazard and monitoring the level of BAs in food samples becomes crucial as their high concentrations may lead to health problems. This review offers an overview of the available chemical and electrochemical methods that are typically used for the sensing of BAs in food samples. Certain compounds are known to selectively interact with BAs via chemical or non-covalent interactions and these interactions are often accompanied by fluorescence or visible color changes (sometimes visual detection) that could be monitored/assessed using a fluorescence spectrophotometer or UV-vis spectrophotometer (colorimetric methods). The colorimetric methods are limited by sensitivity and selectivity as they are based on straight-forward chemical reactions. In the case of electrochemical sensing of BAs, mediators are often used which undergo oxidation/reduction to produce intermediates that could interact with BAs accompanied by changes in their electrochemical potential. Overall, this review summarizes the available chemical and electrochemical strategies towards the sensing of BAs with a discussion on further prospects.

Herein, we report the synthesis and application of a Zn-Bp-BTC MOF (Bp – 4,4′-bipyridine; BTC – 1,3,5-benzene tricarboxylic acid; MOF – metal organic framework) as a heterogeneous catalyst for mediating organic reactions. Initially, conditions were optimized for the Knoevenagel condensation reaction using Zn-Bp-BTC as a heterogeneous catalyst by varying the solvent, temperature and catalyst loading. Although the reaction proceeded at room temperature using methanol as the solvent, 60 °C offered the best yield in a shorter duration. Under optimized reaction conditions, a wide range of α,β-unsaturated dicyano compounds were prepared from the corresponding carbonyl precursor and malononitrile, the active methylene counterpart. Systematic investigation was also carried out to assess the role of the ligand and metal salt in the Knoevenagel condensation reaction. It was found that the Zn-Bp-BTC MOF catalyzed the reaction efficiently in comparison to its analogue Zn-BTC MOF and precursor Zn(NO₃)₂·6H₂O. Finally, catalytic recycling and stability studies showed that the catalyst is able to mediate the reaction for up to five consecutive cycles without undergoing any significant chemical or morphological changes. Further, the catalyst was tested for its efficacy in a multi-component reaction (MCR). A MCR with the Zn-Bp-BTC MOF as the catalyst afforded good yields and there was no reaction in the absence of the catalyst. Similarly, the catalyst was tested for its efficiency in benzimidazole synthesis.

The use of ionic liquids (ILs) as a solvent and an electrolyte for electro organic synthesis has been reviewed. To date several ILs exist, however the ILs based on tetraalkylammonium, pyrrolidinium, piperidinium and imidazolium cations with BF₄ ⁻, PF₆ ⁻, and TFSI anions have been widely used and explored the most. Electro organic synthesis in ionic liquid media leading to the synthesis of a wide range of organic compounds has been discussed. Anodic oxidation or cathodic reduction will generate radical cation or anion intermediates, respectively. These radicals can undergo self coupling or coupling with other molecules yielding organic compounds of interest. The cation of the IL is known to stabilize the radical anion extensively. This stabilization effect has a specific impact on the electrochemical CO₂ reduction and coupling to various organics. The relative stability of the intermediates in IL leads to the formation of specific products in higher yields. Electrochemical reduction of imidazolium or thiazolium based ILs generates N-heterocyclic carbenes that have been shown to catalyze a wide range of base or nucleophile catalyzed organic reactions in IL media, an aspect that falls into the category of organocatalysis. Electrochemical fluorination or selective electrochemical fluorination is another fascinating area that delivers selectively fluorinated organic products in Et₃N·nHF or Et₄NF·nHF adducts (IL) via anodic oxidation. Oxidative polymerization in ILs has been explored the most; although morphological changes were observed compared to the conventional methods, polymers were obtained in good yields and in some cases ILs were used as dopants to improve the desired properties.