Photochromism of naphthoxazine-spiro-indolines

Naphthoxazine-spiro-indolines (NOSIs) can exist in a colourless and a coloured form. These compounds can interconvert between these two forms either photochemically or thermally and are resilient to degradation even after repeated cycles of colouration and bleaching. Absorption spectra of both the stable colourless form and the energetically less stable coloured form have been measured. Several NOSI compounds have been shown to photoconvert to the coloured form with an efficiency of between 0.06–0.74, depending upon the compound conditions under which the conversion is brought about. The factors which have been varied in this work are: the basicity of the 6'-substituent; the size of the N-alkyl group within these compounds; and the nature solvent or matrix in which the reaction occurs. Electron donation from the 6'-group has been shown to increase the quantum yield of the forward reaction whereas solvent polarity increases have been shown to reduce the quantum yield. [Continues.]