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.

Molecular photoswitches in aqueous environments

“…157,158,165 Moreover, the photophysical properties are influenced by the presence of the hetero atoms, rendering azoswitches based on pyridine, [166][167][168][169][170] indole, [171][172][173][174] indazole, 175 purine, 176,177 pyrimidine, 178,179 pyrrole, 165 imidazole, 165,[180][181][182] pyrazole, 46,165,[183][184][185][186][187][188] oxazole, 189 thiazole, 180 thiophene, [190][191][192] etc., generally responsive to light of longer wavelength. In contrast, in classical azobenzenes the UV-Vis absorption spectrum can be tuned by introducing, for instance, (push-pull) substituents in the para position 156,193,194 or tetraortho substituents [195][196][197][198][199][200][201]…”

“…As this is damaging for tissue and has limited penetration depth, researchers strive to find better solutions in the longterm moving to photoactuators responsive to visible and NIR. 195,215,537,759,760 A great challenge in this context is to redesign photoswitches or modify them in a suitable manner while retaining the necessary water-solubility as showcased in the following example.…”

Active Ester Functionalized Azobenzenes as Versatile Building Blocks

“…In addition, a separation of the absorption spectra of both isomers is important to avoid mixtures of isomers. Starting from 2009, ortho -substituted azobenzenes [ 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 ] and ethylene-bridged azobenzenes, the so-called diazocines [ 51 , 52 ], have been discovered to be promising candidates to overcome the two limitations of a typical azobenzene. Depending on the substituents in ortho -position, not only the absorption is shifted towards longer wavelength but also the thermal half-life may be drastically extended due to steric and electronic interactions [ 48 ].…”

Azoacetylenes for the Synthesis of Arylazotriazole Photoswitches

“…2,6-Disubstituted arylazoacetylenes ( 2d – 2g ) were prepared (74–89%) because of the beneficial photophysical properties of the corresponding arylazobenzenes. 36 − 38 …”