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

Abstract

Thermally stable photoswitches that are driven with low-energy light are rare, yet crucial for extending the applicability of photoresponsive molecules and materials towards, e.g., living systems. Combined ortho-fluorination and -amination couples high visible-light absorptivity of o-aminoazobenzenes with the extraordinary bistability of o-fluoroazobenzenes. Herein, we report a library of easily accessible o-aminofluoroazobenzenes and establish structure–property relationships regarding spectral qualities, visible light isomerization efficiency and thermal stability of the cis-isomer with respect to the degree of o-substitution and choice of amino substituent. We rationalize the experimental results with quantum chemical calculations, revealing the nature of low-lying excited states and providing insight into thermal isomerization. The synthesized azobenzenes absorb at up to 600 nm and their thermal cis-lifetimes range from milliseconds to months. The most unique example can be driven from trans to cis with any wavelength from UV up to 595 nm, while still exhibiting a thermal cis-lifetime of 81 days.

Solar E→Z photoisomerization of azo switches

“…The n/π mixed character of the πn orbital was reflected by the asymmetrical orbital isosurface around the N atom of dimethylamino group (also see Figure S2) and the atypical pyramidal geometry about the N atom (intermediate between sp 3 and sp 2 hybridization). The πn orbital topography of E-1 was in agreement with previous analyses of ortho-amino azobenzens, [37][38] and the orbital topography of n and π* orbitals of Z-1 was similar with that of other N-heteroaryl azo-switches. 32,[39][40]…”

“…Therefore, our strategy was shifted to the introduction of an intrinsically strong visible-light absorption band. In several endeavors of developing visible-light azoswitches, ortho-amino-substitutions were used to boost the visible-light absorption of E isomers, [34][35][36][37] and an additional π−π* type transition behind this was disclosed very recently. [37][38] Taking the above into account, we examined the effect of ortho-amino-substitution on the spectral property of phenlyazopyrazole, the model heteroaryl azo-switch in this study.…”

“…Moreover, the type of amino groups was expected to affect the strength of πn−π* absorption. [34][35][36][37] Molecule 4 was then synthesized where the dimethylamino was replaced by a pyrrolidino group. As shown in Figure 3c, such a molecular design improved the ε to 8.7×10 3 M -1 cm -1 , resulting in a very strong visible-light absorption band, in agreement with other ortho-pyrrolidino azo molecules.…”

“…As shown in Figure 3c, such a molecular design improved the ε to 8.7×10 3 M -1 cm -1 , resulting in a very strong visible-light absorption band, in agreement with other ortho-pyrrolidino azo molecules. [34][35][36][37] Overwhelmingly strong absorption of E against Z isomers of 2−4 was displayed across most part of the UV-Vis spectral range, and interestingly, their isosbestic points were located near 280 nm, coinciding with the short-wavelength edge of solar spectrum.…”

Azobenzene Photoswitching with Near-Infrared Light Mediated by Molecular Oxygen

“…Unfortunately, the lifetime of the metastable cis isomer is typically short for red-light-absorbing azobenzenes, an undesired feature in most applications. This deficiency can be addressed by synthetic modifications, especially ortho substitution with certain moieties − that stabilize the cis isomer and in some cases separate the low-energy n –π* absorption bands of the isomers, allowing selective trans–cis and cis–trans photoisomerization with visible light. These bands are, however, limited to wavelengths below 600 nm with low molar absorptivity in the red end of the visible spectrum.…”