ABSTRACT
The parent molecule of aromatic azo compounds is azobenzene, whose UV/VIS spectroscopic features and photochemistry are basically determined by the azo group -N = N-in conjugation with two phenyl substituents. The most important applications of azobenzene derivatives (azobenzenes) are their use as dyestuffs. Azo dyes absorb visible light and generally show remarkable light-fastness, which is due to an effective deactivation mechanism of the excited-state molecules that includes photoisomerization. Zollinger
has discussed this aspect from the viewpoint of dyestuff chemistry. The double bond between the two nitrogen atoms gives rise to
E
-or
Z
-configuration of the two substituents. In the following, the more general terms “
E
” and “
Z
” will be used, although the terms “
trans
” and “
cis
” are still in use and unequivocally distinguish the two geometric isomers of azobenzenes. This chapter describes the photochromism of azobenzenes; that is, the interconversion of both isomers by light; see Figure 89.1. The thermal
Z
→
E
isomerization reaction is also included because it is more or less effective in photochemical reaction systems of azobenzenes.
