Visible Light Switching of Ortho-Functionalized Azo Phthalimides with Tunable Z-Isomer Stability

Herein, a novel class of azo photoswitches based on a phthalimide with an ortho azo bond to the imide ring is presented, exhibiting reversible isomerization under a broad range of visible light irradiation from 405 to 530 nm. Structural variations with heteroaryl or aryl segments attached to the 3-phthalylazo unit exhibit distinct spectral features, such as red-shifted absorption, well-separated absorption bands, and tunable stability of the metastable Z isomer, ranging from seconds to days. They differ drastically in the half-life of Z-isomer stability, ranging from several seconds (N-methylpyrrole) to days (N-methylimidazole). The ortho-cyanophenyl azo dye shows an n-π* band separation of 22 nm between the E and Z isomers, enabling on-off switching by green (530 nm) and blue (430 nm) light. X-ray structural analysis reveals that the E isomer adopts a planar geometry. The Z isomers exhibit a T-shaped configuration for dyes with heteroaryl segments, while aryl Z isomers adopt a twisted conformation. Ground-state density functional theory calculations suggest a different mechanism for Z/E thermal relaxation depending on the heteroaryl or aryl substituents, explaining the varying stability of the Z isomers. Van't Hoff analysis of the thermal Z/E isomerization provided enthalpic and Gibbs free energy changes, supporting the substituent-dependent trends in Z-isomer stability.