Wavelength and solvent independent photochemistry: the electrocyclic ring-closure of indolylfulgides

A wavelength and solvent dependent study of a photochromic indolylfulgide is presented. The ring-closure reaction is characterized using stationary and time-resolved spectroscopy with femtosecond time resolution. After excitation into the first excited singlet state (S₁) the photoprocesses proceed on ultrafast timescales (0.3–0.45 ps) in both polar and non-polar solvents. Excitation into higher electronic states results in similar reaction kinetics as found for S₁ excitation. A simple kinetic scheme can be established for the photoprocesses under all different experimental conditions: as expected from organic textbooks neither the solvent surroundings nor the excitation wavelength strongly alter the reaction scheme. The experimental study demonstrates that the ring-closure reaction of photochromic indolylfulgides can be considered as a very robust photoprocess: this fact may lead to a great variety of different applications where the reaction dynamics of the molecular switch are not disturbed by any surrounding effects.