Preparation and characterisation of thienonaphthoquinones and their radical ions

Three naphtho[2,3-c]thiophene-4,9-diones (thienonaphthoquinones) have been prepared and their redox properties studied by chemical, electrochemical and EPR spectroscopic methods. All three quinones were readily reduced electrochemically to the corresponding radical anions and subsequently to dianions. The cyclic voltammograms showed evidence of ion pairing of the dianions. All three quinones were also electrochemically oxidised to radical cations. The structures and thermodynamic properties of the quinones, the radical cations, radical anions and dianions were studied by the semi-empirical AM1 method. The experimental redox potentials were found to correlate with the computed energies of the appropriate frontier MOs. The electrochemically generated radical anions were observed by EPR spectroscopy and shown to decay with second-order kinetics. The rate constants increased as the electron-releasing character of the substituents in the benzene ring increased. The only EPR spectroscopically detectable radical cation was that from the 6,7-dimethoxynaphtho[2,3-c]thiophene-4,9-dione. Attempts by electrochemical and chemical methods to polymerise the thienonaphthoquinones were unsuccessful.