Modulating Supramolecular Topology via Hydrogen Bonding in a Eumelanin Monomer

Eumelanin is a bio-pigment with remarkable photoprotective properties, whose function is tied to its complex supramolecular organization. Although hydrogen bonding and π-π interactions are recognized as key forces in shaping its architecture, their individual contributions remain poorly defined. Among its molecular precursors, the canonical monomer 5,6-dihydroxyindole (DHI) is notable for forming extensive hydrogen-bonded networks that stabilize supramolecular assemblies. To clarify how hydrogen bonding influences supramolecular topology in eumelanin precursors, we investigate selectively methylated derivatives of DHI, examining how subtle modifications to hydrogen-bonding motifs alter crystal packing, intermolecular arrangements, and optical properties. Single-crystal structures reveal that O-methylation (DMI) changes hydrogen-bonding dimensionality yet preserves helical assemblies, whereas full O- and N-methylation (DMI-Me) suppresses hydrogen-bonding, giving rise to zigzag arrangements dominated by van der Waals forces. Solid- and solution-state NMR measurements independently confirm the differences in the hydrogen-bonded assemblies. Complementary electronic spectroscopy shows that noncovalent architectures of the eumelanin derivatives exhibit pronounced excitonic coupling between neighboring chromophores. Electronic structure calculations support these observations, demonstrating that coupling originates primarily from Coulombic interactions, with minimal contributions from charge transfer. By linking molecular substitution to packing motifs and excitonic interactions, this work establishes hydrogen bonding as a design element for directing supramolecular order and emergent optoelectronic behavior in eumelanin-inspired materials.