Cisoid-cis intermediate plays a crucial role in decolouration rate in photochromic reaction of 8H-pyranoquinazolines and 3H-naphthopyrans

The photochromic reaction of 8H-pyranoquinazoline shows intriguing aspects in comparison to 3H-naphthopyrans. These come from a stabilized excited singlet state for 8 H-pyranoquinazoline, which has a longer lifetime and is strongly sensitive to the solvent. The polar solvent acetonitrile reduces the quantum yield of coloured isomer formation (transoid-cis species). Colour fading related to the decay of transoid-cis population is faster than that for 3H-naphthopyran family and occurs within few seconds time-window in solution at room temperature. Theoretical calculations show the crucial role in fading process played by the potential energy landscape, with the cisoid-cis intermediate located in a shallow high-energy minimum.