The photochemistry of azo dyes. Photoisomerisation versus photoreduction from 4-diethylaminoazobenzene and 4-diethylamino-4′-methoxyazobenzene

Abstract

4-Diethylaminoazobenzene (2) and 4-diethylamino-4′-methoxyazobenzene (3) undergo geometrical isomerization with a quantum yield of almost unity by irradiation in the visible region, and, with lower efficiency, by irradiation at a shorter wavelength or sensitization both with high and low energy hydrocarbons. These dyes are also photoreduced, this time however only by irradiation at λ⩽ 313 nm and with low quantum yield (10^–4–4 × 10^–2). Photoreduction also takes place by energy transfer from high energy sensitizers as well as by radical initiation by ketones. It is concluded that the lowest singlet and triplet states of these azobenzene derivatives, which are reasonable models for commercially useful monoazo-dyes, are only capable of geometrical isomerization, whereas hydrogen abstraction takes place from the high-lying triplet state of both the trans- and cis-configuration of these dyes.