The role of temperature in the photochromic behaviour

Abstract

This paper reviews a series of photochromic systems belonging to the classes of spirooxazines, chromenes and arylethenes, which have been recently investigated in our laboratory. Temperature variations may have very significant and different effects on the behaviour of photochromic systems, which depend on their type of reversibility (T or P systems). For thermoreversible photochromic molecules, increasing the temperature increases the rate of the thermal bleaching reaction, thus decreasing light-stimulated colouration. For systems being both photochromic and thermochromic, such as spirooxazines and chromenes, a combination of the experimental data gathered from temperature effect studies can bring about a complete description of the kinetics and thermodynamics of the ground state reaction, in terms of activation energies, frequency factors, thermodynamic activation parameters, equilibrium constants of the ground state reaction. Lowering the temperature may either reduce a complex photochemical reaction, which yields several photoproducts, to a simpler one giving a single photoproduct, as observed for some chromenes, or transform a thermo- and photoreversible system into one which is only photoreversible, as found for some dipyrrolylperfluorocyclopentenes: thermal decolouration dominates bleaching at room temperature and above; whilst at low temperature thermostability and photoreversibility are established. For photoreversible systems, such as photochromic arylethenes, the quantum yields for both UV colouration, Φ O→C, and visible decolouration, Φ C→O, were found to depend on the temperature. An in depth understanding of the reaction mechanisms and the design of synthetic strategies for possible practical applications entail a careful analysis of all of these aspects.