Solid State Photochemistry of Isocoumarins and Isothiocoumarins

1H-2-Benzothiopyran-1-ones (isothiocoumarins) and 1H-2-benzopyran-1-ones (isocoumarins) behave dissimilarly on solid state irradiation at room temperature as the former photocyclodimerize efficiently while the latter only undergo slow photodecomposition. Both 4H-naphtho[2,1-c]pyran-4-one and thiopyran-4-one turn out to be photostable under these same reaction conditions.