Dual Charge Transfer Generated from Stable Mixed‐Valence Radical Crystals for Boosting Solar‐to‐Thermal Conversion

Abstract Realizing dual charge transfer (CT) based on stable organic radicals in one system is a long‐sought goal, however, remains challenging. In this work, a stable mixed‐valence radical crystal is designed via a surfactant‐assisted method, namely TTF‐(TTF+•)2‐RC (where TTF = tetrathiafulvalene), containing dual CT interactions. The solubilization of surfactants enables successful co‐crystallization of mixed‐valence TTF molecules with different polarity in aqueous solutions. Short intermolecular distances between adjacent TTF moieties within TTF‐(TTF+•)2‐RC facilitate both inter‐valence CT (IVCT) between neutral TTF and TTF+•, and inter‐radical CT (IRCT) between two TTF+• in radical π‐dimer, which are confirmed by single‐crystal X‐ray diffraction, solid‐state absorption, electron spin resonance measurements, and DFT calculations. Moreover, TTF‐(TTF+•)2‐RC reveals an open‐shell singlet diradical ground state with the antiferromagnetic coupling of 2J = −657 cm−1 and an unprecedented temperature‐dependent magnetic property, manifesting the main monoradical characters of IVCT at 113–203 K while the spin‐spin interactions in radical dimers of IRCT are predominant at 263–353 K. Notably, dual CT characters endow TTF‐(TTF+•)2‐RC with strong light absorption over the full solar spectrum and outstanding stability. As a result, TTF‐(TTF+•)2‐RC exhibits significantly enhanced photothermal property, an increase of 46.6 °C within 180 s upon one‐sun illumination.