Spiropyrans: structural features and photochemical properties

The structure of photochromic systems based on different classes of spiropyrans, which are promising materials for the optical recording of information and control of luminous fluxes is examined. The structural factors which affect the photochemical activity of spiropyrans and the stability of the coloured forms derived from them have been established. It is shown that the photochemical properties of spiropyrans occurs to a considerable extent because even in the ground state the C_spiro–O bond being cleaved on photoexcitation is weakened and elongated through specific n–σ* orbital interactions. The effect of the nature, electronic state, and relative orientation of the heteroatoms in the spiro centre on the efficiency of n–σ* interactions is analysed. The formation of different strong intermolecular associates and solvates has been found in the crystalline structure of the coloured forms of spiropyrans, which to a considerable degree determines the stabilisation of the complex resonance structure of these compounds.
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