and prolonged the heating time to 72 hours for the thermal conversion of dihydro-4H-cyclobuta [c]isochromene 1b (Figure ). To our surprise, almost full conversion to dihydronaphthalene 1d was observed, demonstrating that formation of 1d from 1a upon prolonged heating is the predominant fatigue pathway of this photoswitching system. This observation is also consistent with the assumption based on previous work that dihydronaphthalenes are the thermodynamically controlled products of the cobalt-catalyzed ring-closure reaction. We attempted to find conditions that facilitate the reversible photothermal switching between alkyl substituted 1H-2-benzo [c]oxocin 13a and dihydro-4Hcyclobuta [c]isochromene 13b (Figure ). However, much to our surprise dihydro-4H-cyclobuta [c]isochromene 13b-featuring an aliphatic substituted at the 4-membered ring instead of an aromatic substituent-did not convert back to 13a at 110C, with higher temperatures affording slow decomposition to unknown products. Remarkably, 13a could convert smoothly to 1d at 120C in 72 h, demonstrating that the formation pathway of 13b from 13a is inaccessible thermally. These experimental results suggest that phenyl substituents at the 4-membered ring fragment of dihydro-4H-cyclobuta [c]isochromenes serve to lower the energy barrier of the ring-opening process when compared to aliphatic substituents. Previously, we reported DFT studies that support the pathway to dihydronaphthalene formation shares the same o-QDM intermediate as accessible pathways leading to the [Co(TPP)]-catalyzed 1H-2-benzo [c]oxocin formation, with the latter being the kinetically controlled product. Specifically, the chemoselectivity for 1H-2benzo [c]oxocin formation over (the more thermodynamically-stable) dihydronaphthalene was demonstrated to be determined by energy barrier differences of the cyclization process from the o-QDM intermediates. Coupling these DFT studies with our observations of dihydronaphthalene (1d) formation from dihydro-4H-cyclobuta [c]isochromene (1b) as the predominant origin of switching fatigue during cycling, we anticipated that formation of the anti-Woodward-Hoffmann product 1a-upon heating 1b at lower temperatures-might also proceed via an o-QDM intermediate, again with formation of 1a over 1b being a kinetically controlled process.