NIR-II Photothermal Conversion in Spin Crossover Metal-organic Frameworks

The synthesis of multifunctional materials that exhibit two or more distinct physicochemical properties is critical for the development of advanced smart materials. To date, the research regarding the near-infrared II (NIR-II) photothermal properties of spin crossover (SCO) materials remains scarce. Here, we synthesized an inverse-Hofmann-type cationic framework, [Fe{Ag(CN)₂}(TTF(py)₄)]⁺ (1, TTF(py)₄ = tetra(4-pyridyl)tetrathiafulvalene), which exhibits hysteretic SCO behavior. By introducing p-aminoazobenzene (Azo-NH₂) and pyrene (Pyr) into 1, two solvent-free derivatives, 1@Azo-NH₂ and 1@Pyr, incorporating free radicals were generated. NIR-II photothermal experiments revealed the outstanding photothermal properties of 1@Azo-NH₂ and 1@Pyr under 1064 nm laser irradiation. Solar-driven water evaporation experiment demonstrated that 1@Pyr achieved a water evaporation rate of 0.9197 kg m^-2 h^-1 with an efficiency of 63.9%. This represents the first demonstration of outstanding NIR-II photothermal conversion performance in an inverse-Hofmann-type metal-organic framework incorporating free radicals generated through host-guest interactions. Our findings underscore the critical importance of incorporating NIR absorption units into SCO systems, providing new insights for the development of intelligent multifunctional materials.