Singlet versus triplet photoproceses in indodicarbocyanine dyes and spiropyran-derived merocyanines

The photophysical and photochemical properties of the indodicarbocyanine 1 and three derivatives (2-4, the substituents in the meso-position of the polymethine chain being R: Br, CN and 1,3,3-trimethylindoline-2-ethenyl, respectively), as well as of three spiropyrans containing a nitro group in 6-position at the benzopyran portion (Sp-n, n=5-7), were studied by time resolved spectroscopy. The quantum yield of trans-->cis photoisomerization is substantial for 2 and small for 1 and 3; the lifetime of the observed cis form is in the sub-ms range at room temperature. The triplet state plays a role only for 2. The Irans-isomers of 1-4 and 5-7 (merocyanine form) exhibit weak and strong fluorescence in ethanol at 24 degrees C and -196 degrees C, respectively. An intermediate, which is ascribed to a photoisomer with cis structure, was observed for 5-7 at room temperature upon excitation of either the spiropyran or the trans-isomer. The activation energies of the thermal cis-->trans isomerization were determined for 1-7. The quantum yield of the spiropyran-->trans-merocyanine photocoloration is substantial in solvents of low polarity but reduced in polar solvents. The nitro group strongly enhances the quantum yield of intersystem crossing for the spiropyran, leading to a triplet pathway for photocoloration. The trans-->cis photoisomerization, however, is suggested to occur essentially via the singlet state. (C) 1997 Elsevier Science S.A.