On the free radical redox chemistry of 5,6-dihydroxyindole

Abstract

The poly-indolequinone polymers such as poly-dopamine and melanin are significant materials for advanced soft materials in coatings and devices. In order to make progress towards a holistic thermodynamic model, one can consider the thermodynamics of the redox chemistry of a key monomer building block: 5,6-dihydroxyindole. The work presented here indicates a new observation, that the quinone methide tautomer plays a key role in semiquinone free radical production. The modelling includes a new set of pH dependent, reduction potential equations to account for tautomerisation. Furthermore, presented are estimations for various thermodynamic variables. The 1 electron potential for the quinone reduction is between 0.14 and 0.17 V (NHE); Semiquinone reduction is between −0.10 to −0.068 (NHE); Tautomarisation constant is estimated to be 0.99 to 3.9; Comproportionation formation constant is estimated to be between 3.8 × 103 and 50 × 103. The work presented here may form the basis for understanding the charge transport properties for poly-indolequinones.