Electrochemical and spectroscopic properties of a series of tert-butyl-substituted para-extended quinones

Six extended para-quinones 1-6 with sterically hindered keto groups have been characterized by UV-VIS, H-1 NMR and C-13 NMR spectroscopy, Their electrochemical properties were investigated in pyridine solution using cyclic voltammetry, differential pulse voltammetry, chronoamperometry and controlled-potential electrolysis, All species exhibit two successive one-electron reductions leading to the dianions via the monoanions; the dianions can be reoxidized to the quinones, An EE-type mechanism for 1-6 was verified by computer simulation; the standard rate constants (k(s1) and k(s2)) of the heterogeneous charge-transfer are in the region of 6.5-12.5 x 10(-3) cm s(-1). The first reduction peak potentials show a good linear relationship with the calculated LUMO energy levels, The radical anions, prepared electrochemically in the first reduction step, were persistent for several hours in the absence of air, They were also characterized by UV-VIS, EPR and ENDOR spectroscopy revealing that the odd electron is delocalized over the whole pi-system.