title: Anomalous evolution of broadband optical absorption reveals dynamic solid state reorganization during eumelanin build-up in thin films.
abstract: The origin of eumelanin optical properties remains a formidable conundrum preventing a detailed understanding of the complex photo-protective role of these widespread natural pigments and the rational design of innovative bioinspired materials for optoelectronic applications. Here we report the unusual kinetic and thickness-dependent evolution of the optical properties of black eumelanin polymers generated by spontaneous aerial polymerization of 5,6-dihydroxyindole (DHI) thin films (0.1-1 μm), consistent with peculiar solid state reorganization mechanisms governing broadband absorption. The complete reversal of eumelanin UV-visible transmittance spectrum curvature on passing from 0.2 to 0.5 μm thick films, the marked increase in visible extinction coefficients with increasing film thickness and the higher UV extinction coefficients in slowly vs. rapidly generated polymers concur to support distinct dynamic regimes of solid-state molecular reorganization at the nanoscale level and to do affect the development of broadband visible absorption. Solid state control of molecular reorganization disclosed herein may delineate new rational strategies for tuning optical properties in eumelanin thin films for optoelectronic applications.
authors.affiliation: Department of Physics "E. Pancini", University of Naples "Federico II", Via Cintia, I-80126, Napoli, Italy. bonavolo@na.infn.it.|INFN, Sezione di Napoli, Via Cintia, 80126, Napoli, Italy. bonavolo@na.infn.it.
authors.forename: Carmela
authors.initials: C
authors.lastname: Bonavolontà
authors.affiliation: Department of Physics "E. Pancini", University of Naples "Federico II", Via Cintia, I-80126, Napoli, Italy.|CNR-SPIN U.O.S. di Napoli, Via Cintia, 80126, Napoli, Italy.
authors.forename: Corrado de
authors.identifier: http://orcid.org/0000-0001-8791-1226
authors.initials: C
authors.lastname: Lisio
authors.affiliation: Department of Chemical Sciences, University of Naples "Federico II", Via Cintia 4, I-80126, Napoli, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: d'Ischia
authors.affiliation: Department of Physics "E. Pancini", University of Naples "Federico II", Via Cintia, I-80126, Napoli, Italy.
authors.forename: Pasqualino
authors.initials: P
authors.lastname: Maddalena
authors.affiliation: Department of Chemical Sciences, University of Naples "Federico II", Via Cintia 4, I-80126, Napoli, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Manini
authors.affiliation: INFN, Sezione di Napoli, Via Cintia, 80126, Napoli, Italy. alessandro.pezzella@unina.it.|Department of Chemical Sciences, University of Naples "Federico II", Via Cintia 4, I-80126, Napoli, Italy. alessandro.pezzella@unina.it.|Institute for Polymers, Composites and Biomaterials (IPCB), CNR, Via Campi Flegrei 34, 80078, Pozzuoli (Na), Italy. alessandro.pezzella@unina.it.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Pezzella
authors.affiliation: INFN, Sezione di Napoli, Via Cintia, 80126, Napoli, Italy.|CNR-SPIN U.O.S. di Napoli, Via Cintia, 80126, Napoli, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Valentino