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-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.

The fluorescence excitation spectra of free 2,5‐bis(2’‐benzoxazolyl)‐hydroquinone (BBXHQ) cooled in a supersonic jet are examined. By using double‐resonance saturation spectroscopy, we show that adjacent vibronic transitions, leading to either blue or red fluorescence, belong to a single molecular species in the electronic ground state. The molecular geometries of the enol and keto forms which are relevant for intramolecular transfer of a single hydrogen atom in the S₁ state are obtained by the MNDO/H method. The translocation distance is calculated to be 0.43 Å. The complexity of the vibronic spectrum, near the electronic origin for the S₀(¹A_g) → S₁(¹B_u) transition of the enol form at 4178.1 Å, is attributed to vibronic coupling with the nonplanar excited keto form due to H tunneling

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No abstract