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Open AccessArticle

Kaxiras’s Porphyrin: DFT Modeling of Redox-Tuned Optical and Electronic Properties in a Theoretically Designed Catechol-Based Bioinspired Platform

Department of Chemical Sciences, University of Naples Federico II, I-80126 Naples, Italy
Author to whom correspondence should be addressed.
Academic Editor: Josep Samitier
Biomimetics 2017, 2(4), 21;
Received: 19 August 2017 / Revised: 24 October 2017 / Accepted: 26 October 2017 / Published: 7 November 2017
(This article belongs to the Special Issue Bioinspired Catechol-Based Systems: Chemistry and Applications)
A detailed computational investigation of the 5,6-dihydroxyindole (DHI)-based porphyrin-type tetramer first described by Kaxiras as a theoretical structural model for eumelanin biopolymers is reported herein, with a view to predicting the technological potential of this unique bioinspired tetracatechol system. All possible tautomers/conformers, as well as alternative protonation states, were explored for the species at various degrees of oxidation and all structures were geometry optimized at the density functional theory (DFT) level. Comparison of energy levels for each oxidized species indicated a marked instability of most oxidation states except the six-electron level, and an unexpected resilience to disproportionation of the one-electron oxidation free radical species. Changes in the highest energy occupied molecular orbital (HOMO)–lowest energy unoccupied molecular orbital (LUMO) gaps with oxidation state and tautomerism were determined along with the main electronic transitions: more or less intense absorption in the visible region is predicted for most oxidized species. Data indicated that the peculiar symmetry of the oxygenation pattern pertaining to the four catechol/quinone/quinone methide moieties, in concert with the NH centers, fine-tunes the optical and electronic properties of the porphyrin system. For several oxidation levels, conjugated systems extending over two or more indole units play a major role in determining the preferred tautomeric state: thus, the highest stability of the six-electron oxidation state reflects porphyrin-type aromaticity. These results provide new clues for the design of innovative bioinspired optoelectronic materials. View Full-Text
Keywords: porphyrin; eumelanin; DFT; ab initio calculation porphyrin; eumelanin; DFT; ab initio calculation
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Crescenzi, O.; D’Ischia, M.; Napolitano, A. Kaxiras’s Porphyrin: DFT Modeling of Redox-Tuned Optical and Electronic Properties in a Theoretically Designed Catechol-Based Bioinspired Platform. Biomimetics 2017, 2, 21.

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