Engineering the Melanin Building Block DHICA into a Fe³⁺ Coordination Coating for Solar Evaporation

Natural eumelanin exhibits exceptional photoprotective and light-management functions, largely attributed to its fundamental building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA). However, the disordered polymeric structure of eumelanin has hindered the establishment of clear structure-property relationships. While DHICA is a pivotal precursor in eumelanin biosynthesis with superior energy dissipation capacity and metal-binding capability, its potential in constructing bioinspired photothermal systems remains underexplored. Herein, we propose a well-defined Fe³⁺-DHICA coordination complex as a model system to modulate the light-harvesting properties via ligand-to-metal charge transfer (LMCT) strategy. This molecular-level engineering significantly narrows the energy bandgap and extends absorption into the near-infrared (NIR) region. Through rational complexation with Fe³⁺, we construct stable Fe³⁺-DHICA networks that exhibit broad and intense LMCT-mediated absorption spanning the UV to NIR regions. This coordination not only suppresses undesired decarboxylation but also promotes efficient nonradiative relaxation for heat generation. The optimized complex demonstrates exceptional solar-driven water evaporation performance when coated on a cellulose foam-based evaporator and enables efficient solar-driven water desalination with excellent evaporation rates and cycling stability. This work offers a deeper understanding of metallo-melanin photophysics and provides a versatile strategy for designing high-performance solar-thermal materials based on eumelanin precursors.