Entangling imidazolium-based ionic liquids and melanins: A crossover study on chemical vs electronic properties and carrier transport mechanisms

Melanins (mel) are a class of pigments that are nearly ubiquitous and among the oldest compounds found in nature. However, their practical application has been limited due to their low solubility in water and in most organic solvents. Recent research on polydopamine has revealed that ionic liquids (ILs) not only enhance melanin’s dissolution but also fine-tune its redox properties through specific dipolar interactions. In this study, we expand upon previous investigations by customizing new suspensions on a broader range of melanins. Concerning ILs, we focus on imidazolium based ILs, tailoring their chemical properties (pKa) and structural characteristics (cation radius, anion radius).The melanins under consideration include synthetic polymers derived from the oxidative polymerization of 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid counterpart (DHICA), as well as the natural pigment Sepiomelanin (Sepiomel), which is isolated from the ink sack of cuttlefish. Our comparative analysis between neat ILs and ILs-mel responses reveals several key findings: the increase in solubility, the tuning of the redox ability, the decrease of the ionic charge resistance (from MΩ τo kΩ); the partial removal of the electrical polarization effects and the delocalized ionic charge hopping mechanisms in conductivity; the increase of the free ionic diffusion mobility and Debye lengths. Notably, electrical transport is influenced by both the chemical features and the steric hindrance in the ILs as well as the specific type of melanin employed. These factors contribute to the high order correlation between free ionic charge densities and their corresponding Debye lengths. Consequently, our findings suggest that for these complex systems, an improved model extending beyond a simplified electrostatic interaction mechanism is required.