Eumelanin broadband absorption develops from aggregation-modulated chromophore interactions under structural and redox control.
Eumelanins, the chief photoprotective pigments in man and mammals, owe their black color to an unusual broadband absorption spectrum whose origin is still a conundrum. Excitonic effects from the interplay of geometric order and disorder in 5,6-dihydroxyindole (DHI)-based oligomeric/polymeric structures play a central role, however the contributions of structural (scaffold-controlled) and redox (π-electron-controlled) disorder have remained uncharted. Herein, we report an integrated experimental-theoretical entry to eumelanin chromophore dynamics based on poly(vinyl alcohol)-controlled polymerization of a large set of 5,6-dihydroxyindoles and related dimers. The results a) uncover the impact of the structural scaffold on eumelanin optical properties, disproving the widespread assumption of a universal monotonic chromophore; b) delineate eumelanin chromophore buildup as a three-step dynamic process involving the rapid generation of oxidized oligomers, termed melanochromes (phase I), followed by a slow oxidant-independent band broadening (phase II) leading eventually to scattering (phase III); c) point to a slow reorganization-stabilization of melanochromes via intermolecular redox interactions as the main determinant of visible broadband absorption.
Department of Clinical Medicine and Surgery, University of Naples Federico II, I-80131 Naples, Italy.
Raffaella
R
Micillo
Department of Chemical Sciences, University of Naples Federico II, I-80126 Naples, Italy.
Lucia
L
Panzella
Department of Chemical Sciences, University of Naples Federico II, I-80126 Naples, Italy.
Mariagrazia
M
Iacomino
Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area della Ricerca, I-56124 Pisa, Italy.
Giacomo
G
Prampolini
Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area della Ricerca, I-56124 Pisa, Italy.|Dipartimento di Chimica e Chimica Industriale, Università di Pisa, I-56124 Pisa, Italy.
Ivo
I
Cacelli
Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area della Ricerca, I-56124 Pisa, Italy.
Alessandro
A
Ferretti
Department of Chemical Sciences, University of Naples Federico II, I-80126 Naples, Italy.
Orlando
O
Crescenzi
Hair care Products Research Laboratories, Kao Corporation, Tokyo 131-8501, Japan.
Kenzo
K
Koike
Department of Chemical Sciences, University of Naples Federico II, I-80126 Naples, Italy.
Alessandra
A
Napolitano
Department of Chemical Sciences, University of Naples Federico II, I-80126 Naples, Italy.
Marco
M
d'Ischia
D065638:Absorption, Radiation; D005075:Biological Evolution; D019281:Dimerization; D008027:Light; D008543:Melanins / Q000737:chemistry*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011142:Polyvinyl Alcohol / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D013997:Time Factors
D008543:Melanins; C041877:eumelanin; D011142:Polyvinyl Alcohol