Push-pull azobenzene chromophores with negative halochromism

Abstract

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 (λmax) can be found at 484 nm. However, switching this arrangement leads to a 57 nm red-shift (λmax = 541 nm). This shift can be enhanced further (λmax = 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.