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.