Dynamics of the spiropyran-merocyanine conversion in solution

The photoinduced processes leading to the isomerization and complexation which proceed spontaneous aggregation are investigated for 1-(β-methacryloxyethyl)-3,3-dimethyl-6′-nitrospiro(indoline-2, 2′-[2H]benzopyran) (SP) by picosecond and nanosecond transient absorption spectroscopy in toluene and acetonitrile. By monitoring the light (wavelength and intensity) and concentration dependences of the species produced upon photoexcitation of the ring-closed from (SP-A) a mechanism was developed which accounts for the formation of colored species in the picosecond to microsecond time domain. In toluene and methylcyclohexane colored species are formed by two routes: isomerization of a species X, in which the spiro C-O bond is broken (but the nonplanar geometry of the parent is retained) to a ring opened form B and secondly by a bimolecular reaction of the ππ* triplet of SP-A with a ground-state molecule to form a complex AB. In acetonitrile radiationless deactivation of X is favored and only one colored form AB is produced.