Temperature dependence of the optical and kinetic properties of photochromic spirooxazine derivatives in sol‐gel thin films

The photochromic molecules (1,3,3,5,6-pentamethyl-spiro[indoline -2-3′-[quinolino]oxazine]) were embedded in a phenyl functionalized silica layer prepared by the sol-gel method. The transparent film acquires a profound blue coloration upon irradiation with ultraviolet light (λ=365 nm) showing an absorption peak at 620 nm. The material recovers the colourless state by thermal bleaching or by irradiation with light at the wavelength of the absorption peak (in the visible). The kinetics of the photochromic transitions depends on the chemical environment of the molecule in the matrix (i.e. polarity), the irradiation intensity and the temperature. A particular set of conditions will determine the equilibrium balance, and hence, the optical constants for the specific steady state. The temperature dependence of the optical properties of photochromic thin films was studied using variable angle spectroscopic ellipsometry. The complex refractive index was analyzed from –30 °C to +70 °C, studying the correlation of the results with the change in the kinetics constants of the material. The main conclusion is that at –30 °C no thermal bleaching is observed, while at +70 °C the thermal kinetic constant is high enough to prevent the coloration of the film. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)