A fast photochromic molecule that colors only under UV light.

We demonstrate that photochromism based on light-driven reversible bond cleavage can enable rapid coloration upon UV light irradiation and successive fast thermal bleaching within tens of milliseconds at room temperature. We have succeeded in developing a bisimidazole derivative with a paracyclophane moiety, pseudogem-bisDPI[2.2]PC, that changes from colorless to blue when irradiated with UV light. The incident light homolytically cleaves the C-N bond that links the two imidazole rings, generating a pair of imidazolyl radicals. A paracyclophane moiety connects the two radicals, thereby preventing them from diffusing away from one another, and the radicals quickly bind with each other to flip back to the original bisimidazole. Photochromic materials showing such intense photocoloration and fast thermal bleaching performance could be promising materials for possible fast light modulator applications.