- Triplet-state lifetimes for anti-1,5-diazabicyclo[3.3.0]octadienediones (anti-9,10-dioxabimanes) decrease substantially on substitution of hydrogen for other substituents at the 3 and 7 positions. The triplet molecules either decay to the original ground state (k TS) or rearrange (k L) to an isomeric lactone, viscosity having marked, but not necessarily parallel, effects on the two channels. The maximum value for k L (10 7 s -1) suggests that ring inversion and photochemistry are associated. Partial rotation of the three-carbon chain (R 1 = H or D) after α-N-CO cleavage of the triplet (if faster than twisting) leads to a diradical which can return to the ground state; rotation of the cleaved chain with twisting leads to the photochemical product via a fully rotated diradical. The lack of isotope effect is consistent with a chemical mechanism for fast triplet decay (via reversible bond cleavage to a partially rotated diradical) for anti-bimanes with R 1 = H or D.