Cross-conjugation controls the stabilities and photophysical properties of heteroazoarene photoswitches.

Azoarene photoswitches are versatile molecules that interconvert from their -isomer to their -isomer with light. Azobenzene is a prototypical photoswitch but its derivatives can be poorly suited for applications such as photopharmacology due to undesired photochemical reactions promoted by ultraviolet light and the relatively short half-life () of the -isomer (2 days). Experimental and computational studies suggest that these properties ( of the isomer and of the -isomer) are inversely related. We identified isomeric azobisthiophenes and azobisfurans from a high-throughput screening study of 1540 azoarenes as photoswitch candidates with improved and values relative to azobenzene. We used density functional theory to predict the activation free energies and vertical excitation energies of the - and -isomers of 2,2- and 3,3-substituted azobisthiophenes and azobisfurans. The half-lives depend on whether the heterocycles are π-conjugated or cross-conjugated with the diazo π-bond. The 2,2-substituted azoarenes both have values on the scale of 1 hour, while the 3,3-analogues have computed half-lives of 40 and 230 years (thiophene and furan, respectively). The 2,2-substituted heteroazoarenes have significantly higher absorptions than their 3,3-substituted analogues: 76 nm for azofuran and 77 nm for azothiophene.