Electronic Spectra of the Electrogenerated 1,4-Benzoquinone π-Dianion and the Strongly Hydrogen-Bonded Charge-Transfer Complex with Methanol

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Abstract
Electronic spectra of the active π-electronic dianion of 1,4-benzoquinone and its hydrogen-bonded complex with CH3OH, generated at sequential electroreduction steps, have been discussed from the points of view of the observation techniques and n–σ charge-transfer interaction of the dianion with a weak hydrogen donor. An improved method involving rapid circulation of the electrolyzed solution via a photodiode array detector to avoid comproportionation and side reactions was used for successful measurements of the clear spectra resulting from the dianion generation. CNDO/S-CI calculations well reproduced the longest-wavelength band observed for the dianion, assigning to the benzenoid 1B3u←1Ag band under D2h symmetry. The hydrogen-bonding interaction of the 1,4-benzoquinone dianion with CH3OH allows the 1B3u←1Ag band to be blue-shifted. This behavior has been well explained in terms of the strong n–σ type charge-transfer interaction of the dianion with CH3OH. These results have been extensively discussed with regard to coupled electron and proton transfer reactions in the quinone–hydroquinone redox system.