Reversible photocyclization of imidazoleisoindole-based stilbene derivative apparently over non-triene form

Diarylethene is one of the most extensively studied photochromic compounds due to its remarkable properties. Recently, nitrogen-based diarylethenes 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 C=C-C=C-C=N form as a minor mesomeric bipolar form. Crystal structural analysis supported major contribution of the former from. Surprisingly, the molecule exhibited not only E–Z photoisomerization but also the photocyclization with photochemical quantum yields over 0.3 for cyclization reaction. These values are comparable to the usual diarylethenes. The symmetry of the HOMO and LUMO orbitals around the reactive center closely resembled to that of typical diarylethenes 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 diarylethenes, uncovering the importance of the symmetric feature of LUMO and HOMO around the reactive center for the photocyclization.