Heteroaryl azo dyes as molecular photoswitches

We have known of azobenzene for over 150 years, the past 80 of which have seen the study and application of its photochromism. Azobenzene derivatives are now considered archetypical molecular switches, and their stability and reliability make them amenable to many fields of modern chemistry, materials science, biology and photopharmacology. When developing a photoswitch for a given application, a common approach is to tune the properties of an azobenzene. It is also possible to instead use heteroaryl azo dyes — motifs that are less popular even though their diversity offers distinct features. Despite the first discoveries of switching behaviour in heteroaryl azos and azobenzenes being coincident, the former have only recently begun to attract attention. This Review describes how the versatile and multifaceted characteristics of these scaffolds make them serious alternatives to azobenzene derivatives in molecular photoactuation. Heteroaryl azo photoswitches arguably deserve more consideration, and our survey of these systems includes challenges to their successful deployment. When presented with a light stimulus, heteroaryl azo photoswitches undergo molecular motion that can be harnessed for applications in materials science, catalyst design or drug development, among other fields. This Review describes selected subclasses of these versatile chemical motifs, covering their properties and prominent applications.