Thermal and photochemical interconversion of spiropyrans and merocyanines. A semi-empirical all-valence-electron study

The interconversion spiropyrans ⇌ merocyanines, which is responsible for their photochromic properties, has been investigated by studying four model molecules: pyran, aminopyran, chromene and nitrochromene. Excited-state energies were calculated for closed and open forms using CNDO/S and INDO/S programs to establish the nature of the lowest excited states. Potential-energy curves for the reaction in the ground state were computed using the MNDO method with geometry optimization. Excited-state energy curves were computed using INDO/2. The results obtained allow the mechanism of the photoreaction to be elucidated.