Photochromic coatings derived by sol-gel processing from epoxysilane, bisphenol-a, 1-methylimidazole and spirooxazine

Abstract

As photochromic organic dyes in solution suffer from heavy aggregation and poor thermal and phtochemical stability, solid matrices are preferred as their hosts. The low-temperature wet-chemical characteristics of the sol-gel process, fortunately, offer vast possibilities for the incorporation of organic dyes into solid matrices. In this work we present our experimental results on photochromic coatings cotaining 1,3,3-trimethyl-spiro[indoline-2,3'-[3H]naphth[2,1-b][1,4]oxazine] (SO) and derived from 3-glycidyloxypropytrimehtoxysilane (GPTMS), bisphenol-A (BPA) and 1-methylimidazole (MI) by sol-gel processing. The experimental results of the investigation of the photochromic properties as a function of matrix composition and additives include four main aspects in terms of the photochromic intensity (DeltaA0) at the photostationary state under UV irradiation and the half-life time (t0.5) of the coloured form after cessation of the UV irradiation: (1) heat treatment temperature for densification of the coatings has a great effect on DeltaAk0. (2) The BPA content shows a significant effect on both DeltaA0 and t0.5. WHile there exists an optimal content of BPA for DeltaA0, there is a steadily increase in t.05 with increasing BPA content. (3) The most encouraging result is that we are now able to increase the dye concentration in our samples to a similar concentration as is used in dye-polymer systems but with better photochromic intensity (4) The effect of additives, e.g. fluorosilnae (FAS), is also attractive to further work, because the addition of FAS not only leads to a further increase in DeltaA0 but also greatly enhances the photostability.