Transition Metal‐Regulated Ultraviolet–Visible–Near‐Infrared Absorption and Photothermal Conversion of Protocatechuic Acid/Ethylene Glycol‐Based Low‐Melting Mixture Solvents

Low‐melting mixture solvents (LoMMSs) have attracted considerable attention as versatile functional liquid systems, and regulating their optical absorption and photothermal conversion through transition‐metal coordination provides a promising strategy for developing efficient and tunable liquid photothermal materials. In this work, a series of metal‐based LoMMSs were prepared by incorporating Fe 3+ , Cu 2+ , and Co 2+ into a protocatechuic acid/ethylene glycol (PCA/EG) eutectic system. FT‐IR and 1 H NMR analyses confirmed the formation of metal‐functionalized LoMMSs through synergistic hydrogen bonding and metal–oxygen coordination. UV–Vis–NIR spectroscopy revealed that metal coordination broadened the optical absorption from the ultraviolet to the visible–NIR region, which is associated with ligand‐to‐metal charge transfer (LMCT), d–d electronic transitions, and enhanced electron delocalization. Under 808 nm laser irradiation, the photothermal performance strongly depended on the metal species. Fe‐LoMMS exhibited the highest equilibrium temperature (78.7 °C), whereas Cu‐LoMMS achieved the highest photothermal conversion efficiency (46.75%) together with excellent cycling stability. These findings demonstrate that rational selection of transition‐metal ions provides an effective strategy for tailoring the optical absorption and photothermal properties of LoMMSs, offering new opportunities for the development of multifunctional photothermal materials.