Modern Spin on the Electrochemical Persistence of Heteroatom-Bridged Triphenylmethyl-Type Radicals

Abstract

Herein we present a clarification of the ambiguous persistence of the 10-methyl-9-phenylacridanyl, 9-phenylxanthenyl, and 9-phenylthioxanthenyl radicals in electrochemical experiments. Each of these radicals has separately been the subject of conflicting literature results for decades with publications claiming both their chemical inertness and propensity to dimerize. We assert that each radical is persistent at conventional electrochemical time scales up to several minutes based on reversible redox couples and cyclic voltammogram simulations of the radicals and their respective cations. All three radicals are rapidly consumed by aerial O, which lends irreversibility to the redox couples after fewer than 20 s of exposure to air. With appreciation for the O sensitivity of these radicals, their electrochemically generated UV-visible absorption spectra have been acquired and matched to predictions made by TD-DFT calculations. Further, we propose that previous claims to have electrochemically measured radical-radical dimerizations have only observed reaction of these radicals with dissolved O.

Tailoring Valence Tautomerism by Using Redox Potentials: Studies on Ferrocene‐Based Triarylmethylium Dyes with Electron‐Poor Fluorenylium and Thioxanthylium Acceptors

“…Some decomposition is even noted on storing solid samples under inert atmosphere at −20 °C for several days. This instability is likely due to an inherent reactivity of their diradical valence tautomers (vide infra) towards oxygen,[ 21 , 35 ] or to hydrogen or chlorine atom abstraction from the solvent. In order to obtain reproducible results, it was therefore mandatory to prepare samples of all complexes freshly in a nitrogen‐filled glovebox before every measurement for the entirety of all further studies.…”

4-Ferrocenylphenyl-Substituted Tritylium Dyes with Open and Interlinked C+Ar2Entities: Redox Behavior, Electrochromism, and a Quantitative Study of the Dimerization of Their Neutral Radicals

“…Trityl radicals with exclusively para substituted phenyl rings do not tend to dimerize according to the literature . Likewise, 9-phenylthioxanthyl radicals do normally not show any sign of dimerization, at least on an electrochemical time scale of seconds to minutes . Some other literature-known trityl-type radicals were, however, found to dimerize reversibly and to provide authenticated hexaphenylethanes.…”

N‐Substituted Acridinium as a Multi‐Responsive Recognition Unit in Supramolecular Chemistry

“…Spectroelectrochemistry shows stability of the acridine radical for days if kept under argon. Further reduction leads to an irreversible second reduction wave ( E 2 =−1.48 V vs SCE) [16a] resulting in a carbanionic species able to deprotonate solvent molecules [16c] . This observation was corroborated by a drop in the anodic current of the re‐oxidation peak.…”