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 not-were 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 (Phi(Sp-->t) and Phi(t-->c) respectively) were obtained. Phi(Sp-->t) is substantial for 1-5 in solvents of low polarity but reduced in polar solvents. For 1-5 a short-lived transient (maximum at 420-440 and 560-590 nm, lifetime less than or equal to 10 mu s) was observed and identified as a triplet state. It is suggested that this is the triplet of the trans isomer in equilibrium with the perpendicular triplet conformation. The nitro group strongly enhances the quantum yield of intersystem crossing for the spiropyran, thereby changing the singlet pathway of photocoloration for 6 or 7 into a triplet pathway for 1-5. The trans --> cis photoisomerization for 1-5, despite the presence of the triplet state, is suggested to occur essentially via the singlet state.