Regulation of the Light Absorption and Photothermal Performance of Melanin-Like Polymers

ConspectusMelanin-like polymers have attracted significant attention for their excellent light absorption and photothermal conversion properties. Unlike sequence-controlled biomacromolecules such as proteins and DNA, natural melanin and its analogues derive their broadband light absorption and photothermal performance from heterogeneous polymerization of 5,6-dihydroxyindole (DHI), indolequinone (IQ), and uncycled dopamine derivatives, as well as simultaneous progressive assembly between different monomeric species or oligomers. A key challenge in this field lies in establishing their structure-property relationships as well as precisely regulating the light absorption and photothermal performance of these bioinspired polymers to meet specific application requirements.Our research has revealed that nonradiative decay dominates their photothermal behavior, with absorbed optical energy converting to thermal vibrations within 1 ns, while regulating the light harvesting ability depends on molecular control over conjugation length, bandgap, and charge-transfer pathways through deliberate chemical design. By harnessing supramolecular assembly (hydrogen bonding, π-π interaction, cation-π interaction, etc.) and chemical reactions (metal-catechol coordination, Schiff base reaction, "click" chemistry, etc.), we have developed various emerging strategies, enabling customization of the light absorption regulation of melanin-like polymers. By leveraging covalent chemical toolboxes, electron donor-acceptor (D-A) pairs are constructed within the microstructures of melanin-like polymers, through nitroxide radicals (2,2,6,6-tetramethylpiperidinyl-1-oxide, TEMPO), nitrogen-containing heterocycles (e.g., hexachlorocyclotriphosphazene (HCCP), cyanogen chloride (CC), and trichloroisocyanuric acid (TCCA)), and mercaptotetrazole (MT) derivatives, narrowing the energy bandgap and enhancing the light absorption spectra in the visible and near-infrared regions. Doping various metal ions into the melanin-like polymers though metal-catechol coordination, d-d transition, and ligand-to-metal charge transfer (LMCT) amplifies the light-to-heat conversion performance. Notably, involving condensation polymerization using aldehyde linkers during the polymerization process of melanin-like polymers can not only avoid side reactions and achieve well-defined structure but also result in numerous D-A pairs for light harvesting improvement.Fabricating melanin-like polymers into fibers, thin films, capsules and shells, nanoparticles, and bulk materials (e.g., gels and elastomers) can profoundly optimize both light scattering and heat localization simultaneously. These strategies coupled with computational modeling and machine learning have also provided valuable insights into the structure-function relationships of melanin-like polymers, accelerating the precise design of materials for biomedical, energy, and environmental applications. This Account highlights our contributions to decode the chemistry of regulating the light absorption ability and photothermal performance of melanin-like polymers, offering a roadmap to bridge fundamental insights into practical photothermal technologies.