P. Allegrini and V. Malatesta" EniChem Synthesis, 20097 San Donato Milanese ~ Itat./Three spiro[indoline-naphthoxazines] and a spiro[ind~iine-phenanthroxazine], which exhibit photochromic and thermochromic properties, have been investigated. So vent and structure effects on the absorption spectra of the merocyanines produced under UV irradiation and kine:ic parameters for the ring-closure and ring-opening reactions were studied. Positive solvatochromism was fcund, indicating that the opened form is a weakly polar species. Equilibrium constants and rate constants for the forward and back reactions spiroxazine e merocyanine increase with increasing the solvent polarity and with electron-donating groups in the oxazine moiety. The reaction is endothermic by 10-20 kJ mol-' and almost isoentropic. The activation entropy is generally negative, while the activation Gibbs energy is approximately independent of solvent and structure.Photochromism involving changes in the visible absorption spectrum has attracted much attention in the last decades because of the variety of practical applications of photochromic systems. ' The photochromic properties of the spir ocompounds are well known. Spiropyrans have been extensively studied2 and, more recently. spiroxazines hac t' been the subject of many investigation^.^ I s Interest in these compounds is justified by their high durability with respect to photoexci tation.The photochromism of these molecules is due to photocleavage of the spirobond under UV irradiation to give 'in open merocyanine structure (photomerocyanine) which absorbs in the visible region.General agreement can be found in the literature about some aspects of the mechanistic behaviour of these photochromic systems. It is well known, from experiments uirh picosecond time resolution, that the C-0 bond breakage in the excited state occurs on the picosecond timescale.-' Experimental evidence has been reported for the production of several (at least two') merocyanine isomers in a transoid structure.' Thermal bleaching of the coloured form is knoun to be a relatively slow process (rate constant: 0.01 1Cl However, there are some doubts about the nature of the primary photoproduct 'X', whether it is a non-planar cisoid structure,-or a transoid form undergoing very fast thermd equilibration.' Conflicting results have been reported on the quantum efficiency of the photocolouration reaction, which in some cases differs, for the same molecule, by more than 100°/0.4.5.'3 Large discrepancies can also be found in the molar absorption coefficients of the open forms determined by different method^.^.' 3 * 1 J In this paper we report the effect of the solvent and strubture on the thermal equilibrium between coloured and colourless forms, on the kinetics of thermal bleaching of the photomerocyanines and on their absorption properties. These measurements. which give an overall view of the energetics of these systems, can also give mechanistic information about the thermal breaking and reforming of the spirobond. 1 'L.5.13 1. Ex per ...
In this article the analytical integration of kinetic equations describing the dynamic behavior of reversible photoreactions has been undertaken. Photochemical systems giving rise to competing thermal and photochemical steps are kinetically analyzed; the rate law is set up and analytical solutions are obtained under precisely defined boundary conditions. More complicated kinetic systems, where several species interact both thermally and photochemically, are also investigated. The kinetic treatment leads to a system of coupled differential equations which are amenable to analytical solutions, under the appropriate experimental and boundary conditions.The usefulness of the equations developed is illustrated by their application to some spirooxazine and chromene photochromic systems: two examples are described in detail.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.