Substitution-induced band shifts and relative population of cis–trans azobenzene

The structure and absorption properties of a series of azobenzene derivatives are investigated using the density functional theoretical methods. The study showed that π → π* transitions of azobenzene undergo red shift after mono- and di- substitutions at para positions. Considering the fact that o-fluoro and o-methoxy derivatives of azobenzene have long living cis isomers, the effect of ortho substitution on the excitation energies is examined. An investigation on the azo-protonation revealed prominent red shift for n → π* and π → π* transitions bringing both absorptions in visible region. The effect of substitution on the cis to trans thermal relaxation is also analysed.