Triarylverdazyl radicals as promising redox-active components of rechargeable organic batteries
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Triarylverdazyl radicals as promising redox-active components of rechargeable organic batteries* S. G. Kostryukov, O. Yu. Chernyaeva, B. S. Tanaseichuk, A. Sh. Kozlov, M. K. Pryanichnikova, and A. A. Burtasov N. P. Ogarev Mordovia State University, 68 ul. Bolshevistskaya, 430005 Saransk, Russian Federation. Fax: +7 (834) 224 2444. E-mail: [email protected] A novel design of electroactive components of rechargeable organic batteries based on stable verdazyl radicals bearing various substituents is proposed. 3-Positioned aromatic substituents at the verdazyl moiety affect the reduction potentials and almost do not affect the oxidation potential, while 1-positioned aromatic substituents aff ect contrariwise the oxidation potential of this radical without any influence on the reduction potential. The acquired electrochemical data allowed us to reveal the structure—potential relationship for the cathodic and anodic processes, which provided the design of triarylverdazyl radicals possessing recordbreaking parameters of the "electrochemical gap". Key words: verdazyl radicals, cyclic voltammetry, electron-donating substituent, electronwithdrawing substituent, electrode potential, electrochemical gap.
Verdazyl (tetrazinyl) radicals first reported in 1964 by Kuhn and Trischman1 are "electron amphoteric", i.e. capable of both oxidizing and reducing2 to give stable cations and anions (Scheme 1). Scheme 1
Red is reduction, and Ox is oxidation.
Verdazyls are the only family of neutral radicals, whose stability approaches to that of the known nitroxyl radicals. The favorable properties of verdazyls include tolerance to dimerization, as well as resistance to air and moisture.3 * Based on the materials of the XXI Mendeleev Congress on General and Applied Chemistry (September 9—13, 2019, St. Petersburg, Russia).
Until quite recently, mostly magnetic4—7 and complexing properties8—10 of verdazyls were considered. The redox properties of these radicals remain much less explored. In particular, electrochemical investigations of a verdazyl series aimed to find a correlation between their redox properties and structure11 have revealed such a correlation between the distribution of electron spins and redox properties. The boundary orbitals that are involved in the redox processes are localized in different parts of the molecule: the HOMO of nitrogen atom at the position 1 makes the largest contribution to the oxidation process, while a carbon atom at the position 3 makes the most significant contribution to the reduction process proceeding via LUMO. As a material for the storage of charge, 3-phenyl-1,5di(p-tolyl)verdazyl has been applied since it can be reversibly oxidized and reduced in single-electron processes12 and employed it in a symmetric all-organic non-aqueous redox battery of the flow-type. The opportunity to utilize verdazyl radicals as materials for the electricity storage has been also mentioned in the review.13 Results and Discussion The unique structure of verdazyl radicals allows one to finely tune the redox propertie
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