An investigation of the electronic spectral properties of the coloured photoproducts derived from some photochromic naphtho[2,1-b]pyrans

Some chromenes are of current interest as photochromic dyes. The results of PPP molecular orbital calculations, after refinement by parameter optimisation, were found to provide a reasonable account of the experimental λ_max values for the ring-opened photoproducts formed from a range of substituted naphtho[2,1-b]pyrans. The agreement between experimental and calculated values was less good for two compounds, but this may be explained on the basis of steric constraints which are likely to reduce molecular planarity. The electronic structure of one of the photoproducts and the nature of the electronic excitation process are discussed in terms of the calculated π-electron charge densities.