The Nature of the Primary Photochemical Step in Spiropyrans

The mechanisms of the photocolouring reactions of spiropyrans are considered and the vibrational mechanism for the dissociation of the C₍₂₎–O bond in spiropyrans is analysed taking into account the relative positions of the π,π* and n,π* levels. The nature of the photochemically active state in spiropyrans has been established by examining the electronic nature of the low-lying excited states in organic molecules with heteroatoms. A mechanism is proposed for the photocolouring of spiropyrans, which explains the dependence of the efficiency of the C₍₂₎–O bond dissociation reaction in spiropyrans on their structure. The bibliography includes 63 references.