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

Abstract

The photophysical and photochemical properties of the indodicarbocyanine 1 and three derivatives ( 2–4, the substituents in the mesoposition 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 trans-isomers of 1–4 and 5–7 (merocyanine form) exhibit weak and strong fluorescence in ethanol at 24°C and −196°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.