Rapidly Generated, Ultra‐Stable, and Switchable Photoinduced Radicals: A Solid‐State Photochromic Paradigm for Reusable Paper Light‐Writing

Although photochromic molecules have attracted widespread interest in various fields, solid-state photochromism remains a formidable challenge, owing to the substantial conformational constraints that hinder traditional molecular photoisomerization processes. Benefiting from the significant color change upon radical generation, chemical systems enabling a photoinduced radical (PIR) behavior through photoinduced electron transfer (PET) could be ideal candidates for solid-state photochromism within minimized need of conformational freedom. However, the transient nature of radicals causes a dilemma in this scheme. Herein, we present a general crystal engineering strategy for rapidly generated (7-s irradiation to saturation) and ultra-stable (lasting 12 weeks) PIRs in the solid state, based on the anti-parallel alignment of para-hydroxyphenyl groups of persulfurated arenes to form a strong non-covalent network for efficient PET and radical stabilization. Using this strategy, a PIR platform was constructed, with a superior photochromic behavior remaining in different solid forms (even in the fully-ground sample) due to their transcendent crystallization ability. On this basis, our compounds can be further processed into reusable papers for light-writing, accompanied by water fumigation for modulating the reversible process. This work provides new insights into addressing solid-state photochromism and can inspire a wide range of optical material design from the switchable radical perspective.