The tripletlifetimes of hindered 4′-substituted-2,4,6-tri-isopropylbenzophenones decrease (increase) with the increasing electron-donating (withdrawing) ability of the ring substituents; this is the opposite effect to that expected from the rate of the intermolecular hydrogen-abstraction reaction of unhindered benzophenones.
[(4) and (5)] quantum yields indicated that reactions of molecular oxygen with diradicals photogeneratedfrom2,4,6-tri-isopropylbenzophenones (3a–e) were accelerated by the presence of both electron-donating (OMe and Me) and electron-withdrawing (Cl and CF3) substituents, this acceleration probably resulted from the zwitterionic nature of the diradicals.