Photochemical ring opening in nitrospiropyrans: triplet pathway and the role of singlet molecular oxygen

Spiro[2H-1-benzopyran-2,2′-indoline] compounds without (6, 7) or with a nitro group in the 6-position at the benzopyran portion (1–5), were studied in solution at room temperature. Phosphorescence of singlet molecular oxygen, O2(1Δg), was observed with high quantum yield in those cases where the quantum yields of intersystem crossing into the observable merocyanine triplet state and of photocolouration are also high. The three quantum yields are 0.7–0.9 for 1, 3 and 5 in solvents of low polarity and decrease with increasing polarity. The nitro group changes the singlet pathway of photocolouration exclusively into a triplet pathway in all cases examined. Whereas the spiropyran triplet state is not accessible by ns laser photolysis at room temperature, the observed triplet as precursor of O2(1Δg) is suggested to be the trans-merocyanine conformation in equilibrium with the perpendicular one.