General Principles for the Design of Visible‐Light‐Responsive Photoswitches: Tetra‐ortho‐Chloro‐Azobenzenes

Molecular photoswitches enable reversible external control of biological systems, nanomachines, and smart materials. Their development is driven by the need for low energy (green‐red‐NIR) light switching, to allow non‐invasive operation with deep tissue penetration. The lack of clear design principles for the adaptation and optimization of such systems limits further applications. Here we provide a design rulebook for tetra‐ortho‐chloroazobenzenes, an emerging class of visible‐light‐responsive photochromes, by elucidating the role that substituents play in defining their key characteristics: absorption spectra, band overlap, photoswitching efficiencies, and half‐lives of the unstable cis isomers. This is achieved through joint photochemical and theoretical analyses of a representative library of molecules featuring substituents of varying electronic nature. A set of guidelines is presented that enables tuning of properties to the desired application through informed photochrome engineering. Molecular photoswitches that respond to visible light irradiation enable external, non‐invasive control over biological systems, materials, and molecular machines. Here we present a systematic spectroscopic and theoretical investigation into the photochemistry of tetra‐ortho‐chloro‐azobenzenes and outline the design principles that allow the optimization of those emerging photochromes on a molecular level.