Push-pull azobenzene chromophores with negative halochromism

This work describes the synthesis and properties of monoazobenzene compounds carrying multiple electron-donating and electron-withdrawing substituents. The donors are methoxy and dialkylamine groups. The acceptors are cyano and nitro groups. The position of these groups has a strong influence on the absorption spectrum. When the nitro and cyano groups are located at the ortho and para positions to the azo bond, respectively, then the absorption maximum (λ_max) can be found at 484 nm. However, switching this arrangement leads to a 57 nm red-shift (λ_max = 541 nm). This shift can be enhanced further (λ_max = 584 and 604 nm) by moving one of the methoxy groups from the ortho to the meta position and by encompassing the nitrogen atom in a five-membered ring. Interestingly, under acidic conditions, a reversible blue-shift (negative halochromism) is observed.