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.