We developed a novel and powerful photothermal switch based on the reversible cyclization between 1H-2benzo [c]oxocins and dihydro-4Hcyclobuta [c]isochromenes. Ring-closure is triggered by light and ring-opening by heat, providing a convenient approach to realize unidirectional switching. The photothermal switch is efficient in both directions, Table . Scope of dihydronapthalene formation from 1H-2-benzo[c]oxocins a exhibiting outstanding conformational flexibility and high thermal stability in both isomeric states, coupled with sizable quantum yields for the photoreactions. The photoswitching behaviour is independent from the solvent polarity and easy to adjust by variation of structural substituents. Visible-light photoactivation and facile functionalization of these new photoswitches are promising features for a broad range of applications, ranging from energy storage to smart materials. The proposed pathways of the thermal conversion are supported by DFT calculations and confirmed with experimental observations. While the light-induced ringclosure adheres to the Woodward-Hoffmann rules, the ring-opening reversion unexpectedly produces the anti-Woodward-Hoffmann product. This can be explained by ring-opening to an o-QDM intermediate that preferentially ring-closes to the desired 1H-2benzo [c]oxocin for kinetic reasons. Next to the formation of dihydro-4H-cyclobuta [c]isochromenes, we also disclosed other transformations of 1H-2-benzo [c]oxocins: In the presence of air and tetraphenylporphyrin (TPP) as a photosensitizer photochemical activation of 1H-2benzo [c]oxocins leads to formation of dihydro-1Hepidioxybenzo [c]oxocines via [4+2] cycloaddition of singlet oxygen to the diene moiety of the 1H-2benzo [c]oxocin substrates. Heating the 1H-2benzo [c]oxocins at elevated temperatures with longer reaction time results in formation of dihydronaphthalenes via o-QDM intermediates. These reactions also proceed with good chemoselectivities, thus providing new synthetic protocols for substructures that are found in several bioactive molecules but are difficult to prepare otherwise.