Visible Light Switching of <i>Ortho</i>-Functionalized Azo Phthalimides with Tunable <i>Z</i>-Isomer Stability

Herein, a novel class of azo photoswitches based on a phthalimide with an <i>ortho</i> 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 <i>Z</i> isomer, ranging from seconds to days. They differ drastically in the half-life of <i>Z</i>-isomer stability, ranging from several seconds (<i>N</i>-methylpyrrole) to days (<i>N</i>-methylimidazole). The <i>ortho</i>-cyanophenyl azo dye shows an n-π* band separation of 22 nm between the <i>E</i> and <i>Z</i> isomers, enabling on-off switching by green (530 nm) and blue (430 nm) light. X-ray structural analysis reveals that the <i>E</i> isomer adopts a planar geometry. The <i>Z</i> isomers exhibit a T-shaped configuration for dyes with heteroaryl segments, while aryl <i>Z</i> isomers adopt a twisted conformation. Ground-state density functional theory calculations suggest a different mechanism for <i>Z</i>/<i>E</i> thermal relaxation depending on the heteroaryl or aryl substituents, explaining the varying stability of the <i>Z</i> isomers. Van't Hoff analysis of the thermal <i>Z/E</i> isomerization provided enthalpic and Gibbs free energy changes, supporting the substituent-dependent trends in <i>Z</i>-isomer stability.