Reversible Photocyclization of Imidazoleisoindole-Based Stilbene Derivative Apparently over NonTriene Form

Diarylethene is one of the most extensively studied photochromic compounds due to its remarkable properties. Recently, nitrogen-based diarylethenes (DAEs) have been reported to exhibit unusual photochromic behavior. In this study, we synthesized an imidazoleisoindole-based stilbene analogue, which involves a neutral C═C─C═C─C─N framework, along with a C═C─C═C─C═N form as a minor mesomeric form. Crystal structural analysis supported the major contribution of the former form. Surprisingly, the molecule exhibited not only E-Z photoisomerization but also the photocyclization reaction with a quantum yield of 0.3. The orbital symmetry of the HOMO and LUMO around the reactive center closely resembled to that of typical DAEs and satisfied the Woodward-Hoffmann rules. Imidazolium analogue is also studied for comparison. The imidazoleisoindole-based stilbene analogue represents a novel and unusual example of photocyclization in DAEs, uncovering the importance of the symmetric feature of LUMO and HOMO around the reactive center for the photocyclization.