Current research focuses on ortho-substituted azobenzenes to reach thermally stable Z-isomers or an absorption in the visible range of light. Switching occurs with wavelengths as long as 720 nm, and the PSS can be very selective (PSSZ 91% and PSS E 86% for tetra-ortho-fluorinated azobenzene). In addition, they show some of the highest thermal stabilities in the Z-form (a half-life of 92 h at 60 °C in acetonitrile has been reported for the tetra-ortho-fluorinated azobenzene). 15a