To obtain further mechanistic information on the ringopening process, we set the reaction temperature at 120C 46 y 70% a Photoisomerization quantum yield of ring-closure (λ = 365 nm). b ε at λonset is 1% of ε at λmax. Based on first-order rate constants at 298 K calculated using the Eyring equation. Conversion and the ratio of compounds were determined by integration of the 1 H-NMR signals in the presence of dimethyl sulfone as internal standard. Figure . Dihydro-4H-cyclobuta [c]isochromene 13b does not thermally relax to 1H-2-benzo [c]oxocin 13a, but 13a does contract to dihydronaphalene 13d upon heating to 120C.