Merocyanine–spiropyran relaxation processes

In the paper mechanisms of merocyanine–spiropyran transformation were investigated. It was shown that in dark conditions for such transformation it is necessary to overcome the potential barrier with height of ~0.6 eV. A barrierless transformation into the spiropyran molecule is possible due to the rotation of the phenyl moiety of the merocyanine molecule by 90∘ relative to the plane of the molecule. It was proved that a 90∘ orientation of moieties in the merocyanine molecule is achievable only in a photochemical way because in S1 state of the molecule energy of the potential surface is lowered by rotation the phenyl moiety relative to its plane