Self-assembly of tetramers of 5,6-dihydroxyindole explains the primary physical properties of eumelanin: experiment, simulation, and design

Abstract

Eumelanin is a ubiquitous pigment in nature and has many intriguing physicochemical properties, such as broad-band and monotonous absorption spectrum, antioxidant and free radical scavenging behavior, and strong nonradiative relaxation of photoexcited electronic states. These properties are highly related to its structural and mechanical properties and make eumelanin a fascinating candidate for the design of multifunctional nanomaterials. Here we report joint experimental-computational investigation of the structural and mechanical properties of eumelanin assemblies produced from dopamine, revealing that the mass density of dry eumelanin is 1.55 g/cm³ and its Young's modulus is ≈5 GPa. We also find that wet eumelanin has a lower mass density and Young's modulus depending on the water-to-melanin ratio. Most importantly, our data show that eumelanin molecules tend to form secondary structures based on noncovalent π stacking in both dry and wet conditions, with an interlayer distance between eumelanin molecules of 3.3 Å. Corresponding transmission electron microscope images confirm the supramolecular organization predicted in our simulations. Our simulations show that eumelanin is an isotropic material at a larger scale when eumelanin molecules are randomly oriented to form secondary structures. These results are in good agreement with experimental observations, density functional theory calculations, and bridge the gap between earlier experimental and small-scale quantum mechanical studies of eumelanin. We use the knowledge acquired from the simulations to select a partner molecule, a cationic phthalocyanine, allowing us to produce layer-by-layer films containing eumelanin that display an electrical conductivity 5 orders of magnitudes higher than that of pure eumelanin films.