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.
Highlights
The deposition of black insoluble polymers by oxidative polymerization of 5,6-dihydroxyindole (DHI) is a chemically complex, yet still poorly understood process that mimics the biosynthesis of eumelanins[1] in man and mammals and that provides a most useful model system to inquire into the structure and physicochemical properties of these functional pigments[1, 2]
The data show a marked decrease in transmittance of DHI thin films after a period of 4 weeks
Complete conversion of DHI to eumelanin-type materials was supported by the substantial superposition of the transmittance curves of the 0.2 μm and 1 μm samples produced by spontaneous solid state polymerization (SSSP) with those of matched AISSP-generated 0.2 μm eumelanin films and 1 μm eumelanin, respectively
Summary
The deposition of black insoluble polymers by oxidative polymerization of 5,6-dihydroxyindole (DHI) is a chemically complex, yet still poorly understood process that mimics the biosynthesis of eumelanins[1] in man and mammals and that provides a most useful model system to inquire into the structure and physicochemical properties of these functional pigments[1, 2]. An expedient protocol to overcome solubility issues in eumelanin film deposition was reported[8] which involved spin coating deposition of homogeneous thin films of freely soluble colorless DHI followed by exposure to gaseous ammonia in the presence of air (ammonia-induced solid state polymerization, AISSP). Under AISSP conditions, DHI undergoes quantitative polymerization to eumelanin in about 24 h without significant structural degradation and/or morphological alteration
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