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

Azobenzenes are one of the most attractive class of molecular photoswitches. 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 E ⇆ Z photoisomerization yields and/or Z-isomer stability. Here we report a family of azobispyrazoles as new photoswitches, which combine (near-)quantitative bidirectional photoconversions and widely tunable Z-isomer thermal half-lives (t_1/2) 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 Z-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.