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

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 O2, which lends irreversibility to the redox couples after fewer than 20 s of exposure to air. With appreciation for the O2 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 O2.