Photoprocesses in spiropyran-derived merocyanines: singlet versus triplet pathway

The properties of two indoline spirobenzopyrans, one (1) containing a 6-nitro and an 8-methoxy group in the benzopyran portion and the other no nitro group (2), were studied by nanosecond laser photolysis in solution at room temperature. A transient, which is ascribed to a cis photoisomer, was observed on excitation of either the spiropyran or the trans isomer of compound 1 (λexc = 353 and 530 nm respectively). The activation parameters for thermal cis → trans isomerization were measured, and the ground state properties of the spiropyran - trans-merocyanine couple are described. The quantum yields of the spiropyran → merocyanine photocoloration and trans → cis photoisomerization were obtained in solvents of different polarity. A short-lived transient (⩽ 10 μs) of compound 1 is identified as a triplet state of the trans isomer. The effects of excitation wavelength on the photophysical and photochemical properties of both compounds are discussed and a new mechanism is suggested.