Thiazolylazopyrazoles as Nonsymmetric Bis‐Heteroaryl Azo Switches: High‐Yield Visible‐Light Photoisomerization and Increased Z ‐Isomer Stability by o ‐Carbonylation

Abstract Following the progress on mono‐heteroaryl azo switches (Het‐N=N‐Ph), a few bis‐heteroaryl azo switches (Het‐N=N‐Het) have been studied recently, whereas the nonsymmetric bis‐heteroaryl ones (Het 1 ‐N=N‐Het 2 ) that can combine the respective merits of each heterocycle, have received little attention. Here we report thiazolylazopyrazoles as nonsymmetric bis‐heteroaryl azo switches that combine the visible‐light switching character of the thiazole ring and the ease of o ‐substitution of the pyrazole ring. Thiazolylazopyrazoles can achieve (near‐)quantitative visible‐light isomerization in both directions and long Z ‐isomer thermal half‐lives of several days. In contrast to the drastically destabilizing effect of o ‐methylation, o ‐carbonylation of the pyrazole ring can remarkably stabilize Z isomers by inducing attractive intramolecular interactions (dispersion, C−H⋅⋅⋅N bond, and lone‐pair⋅⋅⋅π interaction). Our work highlights the importance of the rational combination of two heterocycles and suitable structural substitution in developing bis‐heteroaryl azo switches.