Photoprocesses in Spiropyran-Derived Merocyanines

The photophysical and photochemical properties of a series of spiro[2H-1-benzopyran-2,2‘-indoline] compounds (Sp-n)five of them containing a nitro group in the 6-position of the benzopyran portion (n = 1−5) while two (n = 6, 7) do notwere studied by nanosecond laser photolysis in solution. An intermediate, which is attributed to a photoisomer with cis structure, was observed for 1−5 upon excitation of either the spiropyran form or the trans isomer. The ground state properties of the spiropyran/trans isomer couple are characterized in a variety of solvents, and the activation parameters (e.g., activation energies of 38 and 49 kJ mol-1 for 1 and 5 in acetone, respectively) of the thermal cis → trans isomerization were determined. Relative quantum yields of the spiropyran → trans-merocyanine photocoloration and trans → cis photoisomerization (ΦSp→t and Φt→c, respectively) were obtained. ΦSp→t is substantial for 1−5 in solvents of low polarity but reduced in polar solvents. For 1−5 a short-lived transient (ma...