Structural disorder as a key to photoprotection in eumelanin multimers

Abstract

Eumelanin, a ubiquitous pigment in animals, is known for its ability to protect against UV-induced damage by efficiently dissipating energy as heat. Despite its importance, the mechanisms underlying eumelanin's broadband absorption and ultrafast energy relaxation remain unclear, primarily due to the inherent structural complexity of the pigment. To address this, we employed model eumelanin multimers, including a monomer (DMICE), dimer (DMICE-D), and trimer (DMICE-T), and investigated their optical properties in both solution and aggregated thin films. Our results reveal that increasing multimer size and aggregation significantly broaden the absorption spectrum, a phenomenon attributed to amplified excitonic interactions. The non-radiative decay processes, governed by internal conversion and intersystem crossing, become increasingly efficient as the multimer lengthens. In thin films, the dimer and trimer exhibit ultrafast excited state relaxation (<30 picoseconds), driven predominantly by internal conversion, which closely parallels eumelanin's characteristic ultrafast energy dissipation. By quantifying the exciton interactions within the multimers, we uncover the interplay of coulombic and charge-transfer couplings in modulating the observed optical behaviour. This work provides insights into how structural organization and excitonic interactions contribute to eumelanin's unique photophysical properties and its photoprotective role, thereby advancing development of eumelanin-inspired biomimetic materials.