Switchable Conformational Isomerization of an Overcrowded Tristricyclic Aromatic Ene.

We prepared a new overcrowded tristricyclic aromatic ene (TAE) and investigated its external stimuli-responsive behavior for the switching between a closed-shell folded form and an open-shell twisted form. Upon photoirradiation, the folded form transforms into the biradical twisted form, whereas by keeping the twisted form in the dark, the reverse reaction gradually occurs at room temperature. This switchable conformational change is analyzed by means of UV-vis and electron spin resonance spectroscopies, cyclic voltammetry, density functional theory calculations, and kinetic studies.