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 onesystem 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.

DFT investigations of phenyldithiafulvene dimers at different oxidation states

“…For example, TTF radical cations have been reported to form MV dimers with neutral TTFs through weak interactions. [20][21][22] Considering that TTF and DTF are p-analogues each other, the formation of MV dimers of Ar-DTF is likely to occur as well (see Fig. 1B).…”