Azobispyrazole Family as Photoswitches Combining (Near-)Quantitative Bidirectional Isomerization and Widely Tunable Thermal Half-Lives from Hours to Years

Abstract

Azobenzenes <a>are classical </a>molecular photoswitches that have received widespread application. In recent endeavors of molecular design, replacing one or both phenyl rings by heteroaromatic ones is emerging as a strategy to expand the molecular diversity and to access improved photoswitch properties. However, the currently available heteroaryl azo switches generally show limitations on <i>E </i>⇆ <i>Z</i> photoisomerization yields and/or <i>Z</i>-isomer stability. Here we report a family of azobispyrazoles as new photoswitches, which combine (near-)quantitative bidirectional photoconversions and widely tunable <i>Z</i>-isomer thermal half-lives (<i>t</i><sub>1/2</sub>) from hours to years. A visible-light-activated photoswitch is also obtained. Systematic experimental and theoretical investigations reveal the different geometric and electronic structures of azobispyrazoles from those of phenylazopyrazoles, overcoming the conflict existing in the latter between effective photoconversion and <i>Z</i>-isomer stability. Our work shows the great potential of azobispyrazoles in developing photoresponsive systems and can inspire the rational design of new photoswitches making use of bis-heteroaryl azo architecture.

Design rules for optimization of photophysical and kinetic properties of azoarene photoswitches

“…[9][10][11] Azoarene photoswitches are a general class of azobenzene derivatives that offer considerable structural diversity and undergo light-promoted E➝Z isomerizations when irradiated with ultraviolet to infrared light. 5,[12][13][14] Recent efforts in the field have focused on heteroarenes such as pyrazoles [15][16][17][18][19] and thiophenes, [20][21][22] because of their red-shifted absorption maximum (λ max ) and promising band separation in the E-and Zisomers. There has been a broad effort to red-shift the absorption maximum (λ max ) of the azobenzene E-isomer (λ max = 320 nm) into the visible-light range to minimize photodamage in living organisms.…”

“…25 In 2021, a set of bis-azopyrazoles discovered by Li and co-workers exhibited nearly complete E➝Z photoconversion (94-98%) and a Z-isomer t 1/2 of up to 681 days. 18 Our group recently reported the results of a high-throughput virtual screening (HTVS) where we observed significant differences in the λ max and t 1/2 values for a set of positional isomers of bis-azofurans and bisazothiophenes. 26 We showed that isomeric bis-azothiophenes and bis-diazofurans (2,2 vs. 3,3) increased the λ max of the Eisomers by as much as 77 nm while simultaneously lowering the t 1/2 by six orders of magnitude.…”

“…Past studies on the thermal mechanisms of azoarene photoswitches have determined three possible mechanisms for the Z → E isomerization reaction. These include rotation about the CNNC dihedral, 18,38,39 inversion of the π NN bond, and a hula-twist mechanism where there is a twisting motion about the π-bond, and the aryl groups maintain their relative orientations. 40 Past experimental and computational work on azoarenes shows a preference for the inversion mechanism.…”