An unexpectedly stable dication: Heptacene (a 4n+2 π-electron compound) is so highly reactive that it cannot be isolated. Its dication (a 4n π-electron compound) can be obtained from heptacene dimers/oligomers in sulfuric acid at room temperature and can be kept for more than one year in the absence of oxygen. (See picture for the UV/Vis-NIR spectra of the dications of tetracene to heptacene.)
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Einholz et al. (2013) studied this question.
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