Flexible Organic Radical Cocrystal With 94% Photothermal Conversion Efficiency

Flexible photothermal materials made of particulate carbon, metal, polymer, or semiconductors often suffer from interfacial incompatibility, leading to cracking and delamination over prolonged use. These limitations make it difficult for flexible composite materials to simultaneously meet the requirements of long-term interfacial stability and high photothermal performance. Here we circumvented these persistent challenges by using flexible organic crystals, where the absorber is a structurally homogeneous radical cocrystal and strong light absorption is accomplished by charge transfer (CT) between two molecular components. We cocrystallized electron donor perylene (PE) and acceptor naphthalene diimide (NDI) to prepare mechanically flexible, centimeter-size cocrystals (PE-NDI), which demonstrate persistent radical characteristics with a spin coherence time of 2.1 µs. Prominent donor-acceptor interaction (-87.7 kJ mol^-1) facilitates strong light absorption from 200 to 780 nm, while hydrogen bonds are thought to account for the reversible elastic bending. Excitation at 685 nm yields an extraordinarily high photothermal conversion efficiency of 94%. Integration of PE-NDI in a thermoelectric generator enabled direct solar energy harvesting via a photo-thermo-electric conversion sequence, demonstrating the potential of flexible cocrystals for renewable energy harvesting. This work highlights the untapped potential of mechanically compliant organic crystals as flexible, single-component, lightweight photothermal materials.