Photochromism, thermochromism and solvatochromism of some spiro[indolinoxazine]-photomerocyanine systems: effects of structure and solvent

Abstract

Three spiro[indoline-naphthoxazines] and a spiro[indoline-phenanthroxazine], which exhibit photochromic and thermochromic properties, have been investigated. Solvent and structure effects on the absorption spectra of the merocyanines produced under UV irradiation and kinetic parameters for the ring-closure and ring-opening reactions were studied. Positive solvatochromism was found, indicating that the opened form is a weakly polar species. Equilibrium constants and rate constants for the forward and back reactions spiroxazine ⇄ 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–1 and almost isoentropic. The activation entropy is generally negative, while the activation Gibbs energy is approximately independent of solvent and structure.