Manipulation of Chemistry and Biology with Visible Light Using Tetra-ortho-Substituted Azobenzenes and Azonium Ions

Molecular photoswitches are used for precise and reversible control over the properties and function of chemical, biological and material systems, offering exceptional spatiotemporal control. Their current development focuses on enabling operation with non-damaging and deep tissue penetrating visible/near-IR light. In this context, tetra-orthosubstituted azobenzenes and azonium ions play a leading role, thanks to their unique photophysical properties and easily modifiable structure. However, it is only recently that synthetic approaches to those sterically demanding systems have been established and their structure-photochemistry relations have been understood to provide general rules for their tuning to a given application. In this review, we provide a comprehensive overview of this family of molecular photoswitches, providing an analysis of their photophysical properties, followed by a discussion of the available synthetic methodologies. Finally, we showcase the versatility of tetra-ortho-substituted azobenzenes and azonium ions for enabling light-control in biological and material sciences, providing multiple insights for future applications.