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 ...

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The 1H NMR signal of a proton of a spiropyran photomerocyanine was resolved at low temperature into two signals attributed to the two isomers, TTT and TTC. The equilibrium distribution and free energy di †erence between the TTT and TTC isomers were calculated to be 0.15 and 4.6 ^0.7 kJ mol~1, respectively. By 1H NMR lineshape analysis at various temperatures, an activation barrier for the TTC ] TTT isomerisation of 43.6 ^3 kJ mol~1 was calculated.

Reactions of CMA with Vinyl Ethers. Equimolar amounts of CMA and a vinyl ether (isobutyl, phenyl, and tribromophenyl) were mixed in chloroform and kept at room temperature or at 70 °C for the latter. The occurrence of pyran 14 and 17 was proven by NMR spectroscopy. As shown in our previous paper,16 the proton in the «-position of the ether function was responsible for a doublet of doublets peak pattern in the 5-6-ppm region. AdductlO: NMR 5.4 (dd). Adduct 11: NMR 6.1 (dd). The yields of pyran and copolymer were determined by NMR spectroscopy.

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