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.