Azothiophenes as Visible-Light Photoswitches via n-π∗ Excitations

Selective excitation of the n-π* bands of the E and Z isomers in azoswitches holds great promise for visible-light photoisomerization. However, it has been demonstrated only in a few ortho-substituted azobenzenes and its potential remains largely underexplored. Here we report a novel heteroaryl azo scaffold-azothiophenes-in which the thiophene-2-ester serves as a privileged unit that promotes the separation of n-π* bands. Suitable ortho-electron-withdrawing substituents on the azothiophene scaffold give rise to well-separated n-π* bands of the two isomers in the visible-light region. Therefore, the E→Z and Z→E isomerizations can be selectively induced by green and blue light, respectively. They can achieve impressive photochromic characteristics, including large absorption separation, long switching wavelength (up to 549 nm), high photoconversion yields, prolonged half-lives, and excellent reductant stability. The fundamental design principles established in this work offer new insights for further exploration and advancement of visible-light azoswitches via n-π* excitations.