3-Pyridylazoindole derivatives as photoswitches: Synthesis, photoswitching behavior research, and applied to “on–off” fluorescence sensor for Cu2+ combined with theoretical calculations

Abstract

In order to exploit the compounds with excellent photoswitching properties, a series of 3-pyridylazoindole derivatives were designed and synthesized. The molecular structures were characterized via1H and 13C NMR, the structure of typical compound A6 was also confirmed via single crystal X-ray diffraction. This series of 3-pyridylazoindole derivatives had outstanding characteristics compared with traditional azo photoswitches, such as the red-shifted of maximum absorption wavelength, UV/vis light isomerization in both directions (PSScis: 45–78%; PSStrans: 47–98%) can be achieved easily and broad range of trans-isomer thermal half-lives from minutes to days (0.44–38.4 h). The relationship between structure and properties verified that the photoswiching behavior of azo photoswitches was closely related to the conjugation groups and electronic effects modifying groups. Besides, the frontier molecular orbital and differences of spatial structure calculated by density functional theory (DFT) further explained the performance characteristics of photoswitches. What more, the compound A7 exhibited excellent aggregation-induced emission (AIE) property and could also be used as selective “on–off” fluorescence sensor for Cu2+, which expanded the photochemical properties of 3-pyridylazoindole derivatives.