Synthesis, Stabilities, and Redox Behavior of Di(1-azulenyl)(6-azulenyl)methylium Hexafluorophosphates. Generation of a Donor−Acceptor-Substituted Neutral Radical by Azulenes

Several di(1-azulenyl)(6-azulenyl)methanes and 1,3-bis[(1-azulenyl)(6-azulenyl)methyl]azulenes
were prepared by the condensation reaction of azulenes with diethyl 6-formylazulene-1,3-dicarboxylate under acidic conditions. The products were converted into di(1-azulenyl)(6-azulenyl)methylium hexafluorophosphates and azulene-1,3-diylbis[(1-azulenyl)(6-azulenyl)methylium] bis(hexafluorophosphate)s via hydride abstraction reaction with DDQ following the exchange of
counterions. These mono- and dications exhibited high stability with large pKR+ values (5.6−10.1),
despite the capto-dative substitution of azulenes. The electrochemical reduction of the monocations
upon cyclic voltammetry (CV) exhibited a reversible two-step, one-electron reduction wave with a
small difference between the first reduction potential (E1red) and the second one (E2red), which
exhibited the generation of highly amphoteric neutral radicals in solution. The electrochemical
reduction of dications showed voltammograms, which were characterized by subsequent two single-electron waves and a two-electron transfer upon CV attributable to the formation of a radical cation,
a diradical (or twitter ionic structure), and a dianionic species, respectively. Formation of a persistent
neutral radical from a monocation was revealed by ESR and UV−vis spectroscopies and theoretical
calculations. The ESR spectra of the neutral radical gave two hyperfine coupling constants:  aH =
0.083 (6H) and 0.166 mT (9H) (g = 2.0024), indicating that an unpaired electron delocalizes over
all three of the azulene rings. The stable monoanion, which shows the localization of the charge on
the 6-azulenyl substituent, was also successfully generated from the di(1-azulenyl)(6-azulenyl)methane derivative.