AN INVESTIGATION OF THE ELECTRONIC SPECTRAL PROPERTIES OF THE COLOREDPHOTOPRODUCTS DERIVED FROM SOME PHOTOCHROMIC NAPHTHO[2,1-B]PYRANS

Some chromenes are of current interest as photochromic dyes. The resul ts of PPP molecular orbital calculations, after refinement by paramete r optimisation, were found to provide a reasonable account of the expe rimental lambda(max) values for the ring-opened photoproducts formed f rom a range of substituted naphtho[2,1-b]pyrans. The agreement between experimental and calculated values was less good for two compounds, b ut 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 proce ss are discussed in terms of the calculated pi-electron charge densiti es. (C) 1997 Elsevier Science Ltd.