The photochemical reactivities of 1, 1-diarylspiropentanes 1a–c have been investigated under various photolysis conditions. Upon direct photolysis or acetone-sensitized photolysis, 1 underwent skeletal rearrangement to afford methylidenecyclobutanes 2, 3 and 4. A mechanism involving diradical intermediates has been proposed. Also studied were the photochemical and thermal reactions of the electron donor–acceptor (EDA) complexes of ...