Photoisomerization of 2-Arylazoimidazoles under Visible Light: Identifying a Predictive Tool to Anticipate and Tune Likely Photoswitching Performance and Cis Half-Life
Anirban Dolai, Sk Majid Box, Supriya Bhunia, Satyajit Bera, Arpan Das, Subhas Samanta
Abstract
Azopyrazoles are an emerging class of photoswitches, whereas analogous azoimidazole-based switches are unable to draw much attention because of their short cis half-lives, poor cis–trans photoreversion yields, and toxic ultraviolet (UV) light-assisted isomerization. A series of 24 various aryl-substituted N -methyl-2-arylazoimidazoles were synthesized, and their photoswitching performances and cis–trans isomerization kinetics were thoroughly investigated experimentally and theoretically. Para -π-donor-substituted azoimidazoles with highly twisted T-shaped cis conformations showed nearly complete bidirectional photoswitching, whereas di- o -substituted switches exhibited very long cis half-lives (days–years) with nearly ideal T-shaped conformations. This study demonstrates how the electron density in the aryl ring affects cis half-life and cis–trans photoreversion via twisting of the NNAr dihedral angle that can be used as a predictive metric for envisaging and tuning the likely switching performance and half-life of any given 2-arylazoimidazole. By applying this tool, two better-performing azoimidazole photoswitches were engineered. All switches permitted irradiation by violet (400–405 nm) and orange (>585 nm) light for forward and reverse isomerization, respectively, and displayed comparatively high quantum yields and impressive resistance to photobleaching.