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Stable open-shell aromatic nitric acid radicals: accessing highly efficient photothermal conversion with restricted radiative decay

Most of the open-shell radicals are usually thermodynamically or kinetically unstable in air due to their incompletely satisfied valency. As phenol radical without steric hindrance group protection, aromatic nitric acid radical exhibits high electrochemical and thermal stability due to its rich resonance structures including closed-shell nitro-like and open-shell nitroxide structure with unpaired electrons delocalized in conjugated backbones. Herein, a series of star-shaped aromatic nitric acid radical materials were prepared via facile demethylation and consequent oxidation of their phenolic hydroxyl precursors in air. Interestingly, they exhibit extremely high spin concentration and highly enhanced nonradiative decay, which make them exhibit great potential for photothermal conversion. Among them, TPA-TPA-O6 exhibits high photothermal conversion efficiency and negligible photobleaching effect in seawater desalination. Under irradiation of one sunlight, the water evaporation efficiency of TPA-TPA-O6 is recorded as high as 89.41% and the water evaporation rate is 1.293 kg/m2 h, which represent as the top performance in pure organic small molecule photothermal materials.