π-Electron Manipulation of the 5,6-Dihydroxyindole/Quinone System by 3-Alkynylation: Mild Acid-Mediated Entry to (Cross)-Conjugated Scaffolds and Paradigms for Medium-Tunable Chromophores

5,6-Dihydroxyindole-based systems engender increasing interest for the design and implementation of new functional aromatic scaffolds and eumelanin-like materials with tailored absorption and electronic properties. However, studies aimed at elucidating the influence of external pi-conjugating groups on the redox properties and acid-induced reactivity of these highly oxidizable indolic platforms are lacking. We report herein the synthesis (as acetyl derivatives) and chemical/quantum chemical characterization of the first pi-extended 5,6-dihydroxyindole derivatives, 3-ethynyl-5,6-dihydroxyindole (1) and 3,3'-(1,2-ethynediyl)bis-5,6-dihydroxyindole (2), in order to understand whether and how beta extension of the enamine-like pyrrole sector affects the absorption properties, redox behavior, and protonation equilibria at both the o-diphenol and quinone levels. Oxidation of 1 and 2 proceeded smoothly to generate dark insoluble materials with eumelanin-like UV properties. On exposure to phosphate buffer at pH 3, 1 was rapidly converted to 3-acetyl-5,6-dihydroxyindole (5) and, in the presence of 5,6-dihydroxyindole, to the cross-conjugated 3,3'-ethenylidenebis-5,6-dihydroxyindole (6). DFT calculations on 1 and 2 and their quinones in their pristine states and after protonation provided a mechanistic frame to rationalize the unusual acid-mediated chemistry of 1 and disclosed 2-quinone as the prototype of a novel class of medium-dependent chromophores. The ethynyl(ene) structural motif is thus proposed as the key to new tunable pi-electron extended 5,6-dihydroxyindole/5,6-indolequinone paradigms for the rational design of alkyne-containing hybrid eumelanin-type polymers.