High-Extinction-Coefficient Donor Engineering Enables High-Performance Photothermal Cocrystals

Organic charge-transfer cocrystals have emerged as promising candidates for photothermal conversion, yet current design strategies predominantly focus on intermolecular charge-transfer interactions or nonradiative decay pathways, with less attention paid to the fundamental prerequisite of absorbance. Here, we propose a high-extinction-coefficient donor engineering strategy to boost photothermal conversion. A charge-transfer cocrystal (BNQ) was constructed from DtBuCzB-a donor with a high molar extinction coefficient (5.81 × 104 L mol-1 cm-1)-and the classical acceptor TCNQ. The BNQ cocrystal exhibits a well-ordered 2:1 alternating columnar packing structure with close π-π interactions and C-H···N hydrogen bonds, leading to a narrow optical bandgap of 1.04 eV and broad-spectrum absorption extending to 1198 nm. Under 808 nm laser irradiation, BNQ achieves an outstanding photothermal conversion efficiency of 95.1% and an ultrafast heating rate (130 °C within 10 s), substantially outperforming previously reported TCNQ-based cocrystals. As a practical demonstration, BNQ-incorporated polyurethane foam delivers an average solar-driven water evaporation rate of 1.84 ± 0.03 kg m-2 h-1 under 1-sun illumination with excellent operational stability. This work establishes donor extinction coefficient engineering as a simple yet powerful design strategy for developing next-generation photothermal cocrystals and provides a design parameter for the rational screening of component monomers.