Photochromism. I. The Spectroscopy and Energy Levels of Salicylideneaniline

Abstract

As a prerequisite to understanding the photochemistry of salicylideneaniline, its energy levels and their characteristics are deduced from absorption and emission spectra, and by comparison with related molecules. Theoretical calculations are used to help to establish relations between stilbene, benzylideneaniline, azobenzene, and salicylideneaniline (SA). It is concluded that all the ππ^* bands observed in SA below 6 eV have analogs in the allowed or forbidden transitions of stilbene. From a study of solvent effects, it is concluded that the internal hydrogen bond of SA causes large changes of band intensity, but relatively small energy changes. The luminescence of SA appears to come from the second triplet rather than the first. This could be understood if the two lowest triplets have quite different geometries. The problem of the triplets is discussed further in the second part of this series.