Photochromism. II. Photochemistry of Salicylideneaniline

Abstract

The processes following electronic excitation of salicylideneaniline include the photochemical formation of a colored orthoquinonelike molecule, phosphorescence, and several modes of radiationless decay back to the ground state. These pathways are examined in detail to learn the steric requirements, relative yields, temperature effects and the electronic states involved for each. It is shown that the photochemistry occurs from the lowest singlet (nπ^*) level, and that phosphorescence (τ=80 μsec in EPA at 77°K) is in competition with it. An independent parallel radiationless process appears to lead from the lowest ππ^* singlet to the lowest ππ^* triplet. Evidence for the latter state is a metastable absorption at 5000 Å, τ=330 μsec in EPA at 77°K. The steric inhibition of the photochemistry was investigated by using a mixed crystal system, salicylideneaniline in dibenzyl.