Molecular modelling aided design and synthesis of photochromic dyes containing a permanent chromophore
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Photochromic dyes are a very important and relatively novel class of dyes. The usual, though not exclusive, behaviour of these dyes is to show a reversible colour change from colourless to coloured when exposed to UV light. Among the photochromic dye classes, spirooxazines and naphthopyrans were selected for investigation. An attempt was made to construct molecules with a permanent chromophore (azo) in spirooxazines as well as naphthopyrans separately, with a view to providing a colour change from one colour to another. Three different isomers of dihydroxynaphthalene were used as one group of starting materials for the synthesis of spirooxazines with the introduction of the azo (hydrazone) chromophore by coupling. Other starting materials used were anthraquinones, naphthoquinones and pyrazolones. A range of molecular modelling techniques (molecular mechanics, MM2 and quantum mechanics, AM1) using the CAChe system, were applied to predict optimized geometrical conformations and energies of the ring-closed form and ringopened merocyanine forms of all the dyes. PPP-MO calculations were also carried out to predict the potential colour of the dyes. The dyes were characterized using DSC, FTIR, NMR, UV-Visible spectroscopy and elemental analysis. The photochromic properties of one of the azospirooxazines was subjected to a detailed study under different experimental conditions, and showed a unique slow colour change from orange to grey.