Novel pyrazolyazoindole derivatives as photoswitches: design, synthesis, and photoswitching behavior research combined with theoretical methods

Abstract

In order to exploit novel light-driven molecular switches, a series of pyrazolyazoindole photoswitches was designed and synthesized via modifying the nitrogen atoms of indole, and three typical title compounds were characterized via single crystal X-ray diffraction. Herein, UV-vis and NMR spectroscopy studies were used to confirm that the photostationary state (PSS) proportions of the desired conformations were in the ranges of 59–95% and 74–92% for PSS_trans and PSS_cis, respectively. The thermal half-lives of the photoswitches ranged from a quarter of a day (6 h) to more than a week (231 h) depending on differences in the substituents at the R¹ position. Density functional theory (DFT) calculations were applied to further explain the experimentally observed photochemical properties, such as differences in the spatial structures of the trans and cis isomers of azo switches and the relationships to thermal relaxation in the dark and the photoisomerization yields of pyrazolyazoindoles. Finally, the potential use of pyrazolyazoindole photoswitches for biological applications was verified. A bifunctional model photoswitch (C1) was successfully synthesized and it almost entirely inherited the photoswitching properties of these excellent azo switches.