Electronic spectra of the electrogenerated 1,4-benzoquinone π-dianion and the strongly hydrogen-bonded charge-transfer complex with methanol

Electronic spectra of the active pi-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-sigma 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 B-1(3u)<--(1)A(g), band under D-2h symmetry. The hydrogen-bonding interaction of the 1,4-benzoquinone dianion with CH3OH allows the B-1(3u)<--(1)A(g) band to be blue-shifted. This behavior has been well explained in terms of the strong n-sigma 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.