Reversible, Red-Shifted Photoisomerization in Protonated Azobenzenes

Azobenzenes are among the best-studied molecular photoswitches and play a key role in the search for red-shifted photoresponsive materials for extended applications. Currently, most approaches deal with aromatic substitution patterns to achieve visible light application, on occasion paired with protonation to yield red-shifted absorption of the azonium species. Appropriate substitution patterns are essential to stabilize the latter approach, as conventional acids are known to induce a fast Z- to Econversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the pK(a) of the acid is low enough, yielding both the Z- and E-azonium as supported by UV-vis- and H-1 NMR spectroscopy as well as density functional theory calculations. Moreover, the steady-state protonation of para-methoxyazobenzene, specifically, yields photoisomerizable azonium ions in which the direction of switching is essentially reversed, that is, visible light produces the outof-equilibrium Z-azonium. Although the current conditions render the visible light photoswitch unsuitable for in vivo and material application, the demonstrated understanding of simple azobenzenes paves the way for a great range of further work on this already widely studied photoswitch.

Automated summary

Azobenzenes are well-studied molecular photoswitches and are important in the search for red-shifted photoresponsive materials. The current approaches mostly rely on aromatic substitution patterns to achieve visible light application, sometimes combined with protonation to produce red-shifted azonium species. This study demonstrates that steady-state protonation of the azo-bridge is possible in simple azobenzenes, leading to the formation of both Z- and E-azonium species. Protonation of para-methoxyazobenzene specifically yields photoisomerizable azonium ions with reversed switching direction under visible light irradiation.