Photochemical reactions of aromatic compounds XLV: controlling factors for reactivities and mechanisms in photoelectron transfer reactions of some selected diarylcyclobutanes with electron acceptors

We have attempted to explore mechanistic aspects of the photosensitized ring-cleavage reactions of cis-1,2-diphenylcyclobutane (1), cis-transoid-cis-cyclobutal1,2-a:4,3-a′] diindene (2) and r-1,c-2-dimethyl-t-3,t-4-di(4-methoxyphenyl)cyclobutane (3) by electron acceptors (A) in acetonitrile. The experimental results demonstrate that the ring cleavage of 1 and 2 occurs as a consequence of the rapid geminate recombination of ion-radical pairs occurring at a rate of well over 10⁹ s⁻¹ without ionic dissociation. In the case of 3, however, the photoreactions proceed by way of a chain-reaction mechanism involving the free cation radical of 3 which undergoes ring cleavage at much less than 10⁷ s⁻¹. The rapid ring cleavage of 1⁺ and 2⁺ is attributed to significant perturbations of the cyclobutane ring by the population of positive charge on the orbital array of the two π-electron systems and the cyclobutane-ring σ framework because of strong through-bond couplings. It is presumed that the cyclobutane ring of 3⁺ is much less distorted since the positive charge is mostly localized on the aryl group. The rapid geminate recombination of the A⁻⁻−1⁺ and A⁻⁻−2⁺ pairs is discussed in terms of a very efficient transition from the “distorted” and “ring-opened” minima of the A⁻⁻−D⁺ surface to the A–D surface. In the case of 3, this mechanism cannot be expected to operate in the geminate recombination.