Ultrafast Spectroelectrochemistry of the Catechol/o-Quinone Redox Couple in Aqueous Buffer Solution

Eumelanin is a natural pigment found in many organisms that provides photoprotection from harmful UV radiation. As a redox-active biopolymer, the structure of eumelanin is thought to contain different redox states of quinone, including catechol subunits. To further explore the excited state properties of eumelanin, we have investigated the catechol/o-quinone redox couple by spectroelectrochemical means, in a pH 7.4 aqueous buffered solution, and using a boron doped diamond mesh electrode. At pH 7.4, the two proton, two electron oxidation of catechol is promoted, which facilitates continuous formation of the unstable o-quinone product in solution. Ultrafast transient absorption (femtosecond to nanosecond) measurements of o-quinone species involve initial formation of an excited singlet state followed by triplet state formation within 24 ps. In contrast, catechol in aqueous buffer leads to formation of the semiquinone radical Δt>500 ps. Our results demonstrate the rich photochemistry of the catechol/o-quinone redox couple and provides further insight into the excited state processes of these key building blocks of eumelanin.