Photochromic materials, classified as organic or inorganic, have existed since the end of the XIX century. Photochromic materials change their optical properties when they are exposed to visible or UV light and reverse properties in the darkness. Basically, the phenomenon consists of a reversible change of isolated chemical species between two states with different absorption spectra. A widely known inorganic photochromic material is one used in photochromic lenses, which use metallic halides, such as: AgBr or AgCl. When UV light illuminates these glasses, the metallic halide crystals dissociate into metallic silver and halide. This effect causes absorption in the visible range. When removed from the light source, the metallic halide thermally recombines and recovers its original color and glass transmission. The typical optic response for photochromic glasses in colored and discolored states takes 3-4 minutes at room temperature, with an approximate typical optical response of T = 85–50%. This study presents hybrid materials consisting of titanium oxides obtained by the sol-gel process in a polymeric matrix, the combination of which constitutes a system with color centers that can absorb in a wide range of the visible spectrum, resulting in a brown tonality. Color changes are caused by the energy at which, electron-hole pairs are generated in the system. It is assumed that this propitiates a valence change of titanium oxide/hydroxide in an OH radical environment inside of a surrounding polymeric matrix.
There are many varieties of photochromic materials with quite different physicochemical principles. Organic photochromic materials have excellent color and tonality, but due to the great need to improve their characteristic absorption in the visible spectrum, which can be a basis for effective coloring, the molecular structure must often contain a delocalized π-electron.
Molecules associated with this type of electronic structure, polycyclic aromatic hydrocarbons, pigments, and heterocyclic quinoline azo- dyes, are often carcinogenic, penetrating skin and having a high toxicity risk (Beamson ET AL., 1992). Major problems faced by photochromic compounds include stability loss when repeatedly exposed to intermittent or continuous radiation in the air. This leads to decay in a few days, with a consequent lower response to light (Li-Qiong ET AL., 1994). To address these problems, additives or other polymeric materials are added to increase resistance. These are not always compatible, due to photochromic material-matrix interactions. Their photochromic response can be strongly modified by the presence of polar groups, as these may induce complexation, protonation, stiffness and steric hindrance in the matrix (Gonc-alves et al., 2010).