The chemical and spectroscopic characterization of 5,6-indolequinones and their semiquinones, keytransient intermediates in the oxidative conversion of 5,6-dihydroxyindoles to eumelanin biopolymers, isa most challenging task. In the present paper, we report the characterization of a novel, relatively long-lived 5,6-indolequinone along with its semiquinone using an integrated chemical, pulse radiolytic, andcomputational approach. The quinone was obtained by oxidation of 5,6-dihydroxy-3-iodoindole (
1a) with
o-chloranil in cold ethyl acetate or aqueous buffer: it displayed electronic absorption bands around 400and 600 nm, was reduced to
1a with Na
2S
2O
4, and reacted with
o-phenylenediamine to give small amountsof 3-iodo-1
H-pyrrolo[2,3-
b]phenazine (
2). The semiquinone exhibited absorption maxima at 380 nm (sh)and 520 nm and was detected as the initial species produced by pulse radiolytic oxidation of
1a at pH7.0. DFT investigations indicated the 6-phenoxyl radical and the
N-protonated radical anion as the moststable tautomers for the neutral and anion forms of the semiquinone, respectively. Calculated absorptionspectra in water gave bands at 350 (sh) and 500 nm for the neutral form and at 310 and 360 (sh) nm forthe anion. Disproportionation of the semiquinone with fast second-order kinetics (2
k = 1.1 × 10
10 M
-1s
-1) gave a chromophore with absorption bands resembling those of chemically generated
1a quinone.Computational analysis predicted
1a quinone to exist in vacuo as the quinone-methide tautomer, displayinglow energy transitions at 380 and 710 nm, and in water as the
o-quinone, with calculated absorptionbands around 400 and 820 nm. A strong participation of a p orbital on the iodine atom in the 360-380nm electronic transitions of the
o-quinone and quinone-methide was highlighted. The satisfactoryagreement between computational and experimental electronic absorption data would suggest partitioningof
1a quinone between the
o-quinone and quinone-methide tautomers depending on the medium.
