Accessing Highly Efficient Photothermal Conversion with Stable Open‐Shell Aromatic Nitric Acid Radicals

Abstract It is very challenging to prepare stable radicals as they are usually thermodynamically or kinetically unstable in air. Herein, a series of star‐shaped aromatic nitric acid radicals were prepared via facile demethylation and consequent oxidation. As phenol radicals without steric hindrance group protection, they exhibit high electrochemical and thermal stability due to their rich resonance structures including closed‐shell nitro‐like and open‐shell nitroxide structure with unpaired electrons delocalized in conjugated backbones. Among them, TPA‐TPA‐O 6 powder exhibited extremely wide absorption from 300 to 2000 nm covering the whole solar spectral irradiance, high photothermal conversion efficiency, and negligible photobleaching effect in seawater desalination. Under the irradiation of one sunlight, the water evaporation efficiency of TPA‐TPA‐O 6 is recorded to be as high as 89.41 % and the water evaporation rate is 1.293 kg m −2 h −1 , which represents the top performance in pure organic small molecule photothermal materials.