Photochromic and thermochromic spiropyrans

Abstract

By means of the SCF MO LCAO method in the π-electronic (PPP) and all-valence (CNDO) approximations the electronic structures and spectral characteristics of the valence isomers of 2H-chromenes, thiochromenes, and 1,2-dihydroquinolines have been calculated. On the basis of a consideration of the electronic configuration of various excited states, an explanation has been given of the mechanism of the photochromism of the chromenes and their analogs. A hypothesis has been put forward concerning the possibility of thermochromism in 1,2-dihydronaphthalene. It has been shown that the diffuseness of the long-wave absorption band of the open forms of the compounds investigated is not connected with the existence of several stereoisomers. The influence of the heteroatom and of benzo-fusion on the long-wave absorption has been analyzed. The nature of the electronic transition responsible for the long-wave absorption of the photo-colored forms has been explained.