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

Abstract

Molecular photoswitches enable reversible external control of biological systems,n anomachines,a nd smart materials.T heir development is driven by the need for low energy (green-red-NIR) light switching, to allow non-invasive operation with deep tissue penetration. The lackofclear design principles for the adaptation and optimization of such systems limits further applications.Here we provide adesign rulebook for tetra-ortho-chloroazobenzenes,a ne merging class of visible-light-responsive photochromes,byelucidating the role that substituents play in defining their key characteristics:a bsorption spectra, band overlap,p hotoswitching efficiencies,a nd half-lives of the unstable cis isomers.This is achieved through joint photochemical and theoretical analyses of arepresentative library of molecules featuring substituents of varying electronic nature.Aset of guidelines is presented that enables tuning of properties to the desired application through informed photochrome engineering.

Towards low-energy-light-driven bistable photoswitches: ortho-fluoroaminoazobenzenes

“…ortho-Substitution with suitable moieties, in turn, decouples the n-π* absorption bands of the cis and trans isomers, red-shifting it for the trans-form while simultaneously prolonging the cislifetime. 31,32 However, while ortho-methoxylated 33 , -fluorinated 31,34 and chlorinated [35][36][37] compounds enable efficient switching between two essentially bistable forms with visible light, high irradiation intensities or extremely long illumination periods (hours for ortho-chlorinated compounds 36,37 ) are required for efficient switching because of the low molar absorptivity of the n-π* band. This is a drawback particularly when the photoswitch is illuminated through tissue or other lightscattering or -absorbing material.…”

Azobenzene/Tetraethyl Ammonium Photochromic Potassium Channel Blockers: Scope and Limitations for Design of Para-Substituted Derivatives with Specific Absorption Band Maxima and Thermal Isomerization Rate

“…However, if there is a necessity to go beside the above-described limits, a modification of the present scaffold in its azobenzene part is apparently the most practical way to do it. Several modifications can be considered: extension of the -electron system by introducing additional benzene rings or other -conjugated parts; switching to tetra-ortho-substituted azobenzenes [ 41 , 42 , 43 ], bridged azobenzenes (dihydrobenzodiazocines) [ 44 ], azo heteroarene scaffolds [ 45 ], or even to use other known classes of photoswitches.…”

Towards low-energy-light-driven bistable photoswitches: ortho-fluoroaminoazobenzenes

“…ortho-Substitution with suitable moieties, in turn, decouples the n-π* absorption bands of the cis and trans isomers, red-shifting it for the trans-form while simultaneously prolonging the cislifetime [31,32]. However, while ortho-methoxylated [33], fluorinated [31,[34][35][36] and chlorinated [37][38][39] compounds enable efficient switching between two essentially bistable forms with visible light, high irradiation intensities or long illumination periods (hours for ortho-chlorinated compounds [38,39]) are required for efficient switching because of the low molar absorptivity of the n-π* band. This is a drawback particularly when the photoswitch is illuminated through tissue or other light-scattering or -absorbing material.…”