Revealing couplings among chromophores in melanin through femtosecond laser spectroscopy*

Unlike the many natural pigments that are small molecules, the melanin pigments found in nearly all organisms occur as nanoscale granules. These natural, organic nanomaterials have an attractive palette of properties that include broadband absorption, intrinsic electronic and ionic conductivity, metal ion chelation, redox activity, paramagnetism, and a rich radical chemistry. Common properties are found in many natural and lab-made carbon-rich nanomaterials for reasons that are obscure. Efforts to develop multi-functional materials inspired by melanin for applications in bioelectronics, catalysis, and energy conversion and storage have been hampered by a lack of knowledge of the atomic level structures present inside a melanin granule. There is uncertainty both about the covalent units present and the supramolecular interactions that hold them together, impeding the discovery of structure-function-property relationships.

My research group uses femtosecond laser spectroscopy to study excited state deactivation in synthetic melanin polymers and molecular mimics. An emerging theme is the role that hydrogen bonds play in creating redox-active networks of chromophores that foster the reversible transfer of electrons, protons, and hydrogen atoms within and between fragments. Even small numbers of covalent oligomers that differ in their heavy-atom connectivity can support many chemically distinct species due to disorder in the positions of protons. Transient spectral hole burning experiments reveal this disorder, while spectral hole filling experiments provide evidence of the interactions among melanin’s chromophores. Our top-down studies are augmented by bottom-up investigations of the optoelectronic properties of structurally well-defined monomeric and oligomeric eumelanin molecules. These model compounds are protected against polymerization, but can be reversibly oxidized and reduced, allowing optical studies of oxidized building blocks of eumelanin for the first time. A stable indole quinone has been shown to mimic several eumelanin properties. In addition, the optical properties of these molecular building blocks can be tuned through hydrogen bonding and charge transfer interactions in non-covalent complexes formed with small molecules.