Photoswitchable and long-lived seven-membered cyclic singlet diradicals for the bioorthogonal photoclick reaction.

Annularly 1,3-localized singlet diradicals are energetic and homolytic intermediates, but commonly too short-lived for widespread utilization. Herein, we describe a direct observation of a long-lived and seven-membered singlet diradical, oxepine-3,6-dione-2,7-diyl (OXPID), spectroscopic experiments and also theoretical evidence from computational studies, which is generated photo-induced ring-expansion of 2,3-diaryl-1,4-naphthoquinone epoxide (DNQO). The photo-generated OXPID reverts to the thermally stable σ-bonded DNQO with in the μs level, thus constituting a novel class of T-type molecular photoswitches with high light-energy conversion efficiency ( = 7.