THE PRIMARY PHOTOCHEMICAL-REACTION STEP OF UNSUBSTITUTED INDOLINO-SPIROPYRANS

The photochemical ring opening reaction of unsubstituted indolino-spiropyrans in n-pentane, to the corresponding merocyanines, has been followed by transient absorption spectroscopy. The temporal resolution was better than 0.4 ps. During the pump pulse, an ultrafast unstructured absorption covers the entire measurement range 380–680 nm. From it emerges the structured absorption spectrum of a first merocyanine isomer, with typical time constants in the range 0.9 - 1.4 ps. The transient merocyanine spectra are compared to the spectra which are obtained when the spiropyrans are irradiated at low temperature in an argon matrix. In conjunction with semiempirical calculations, the first merocyanine isomer is assigned a trans-trans-cis structure. © 1990 IUPAC