UV-laser photochemistry: diffusion-controlled trapping of cyclic 1,3-diradicals by molecular oxygen - conformational effects on triplet lifetimes

Abstract

To explore the effects of methyl substitution at radical sites and ring size, 1,3-diradicals were generated from respective azoalkanes and oxygen trapping determined quantitatively. Molecular mechanics calculations suggest that in the lowest energy conformations the interorbital angles are ca. 0/sup 0/, 45/sup 0/, and 60/sup 0/ for the 1,3-diradicals, respectively. Thus, for the planar cyclopenta-1,3-diyl (planarity confirmed be ESR) intersystem crossing spin-orbital coupling should be most difficult, and hence it should have a relatively long triplet lifetime. In contrast, the rigid puckered bicyclo(2.2.1)hepta-2,7-diyl should be the most short-lived species of the three. The flexible twist-boat conformation of the cyclohexa-1,3-diyl possesses intermediate lifetime. Other than such conformational effects, it is difficult to rationalize the lifetimes of these rather similar 1,3-diradicals.