Reversible, Red-Shifted Photoisomerization in Protonated Azobenzenes

Abstract

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 <i>Z</i>- to <i>E</i>-conversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the p<i>K</i>_a of the acid is low enough, yielding both the <i>Z</i>- and <i>E</i>-azonium as supported by UV-vis- and ¹H NMR spectroscopy as well as density functional theory calculations. Moreover, the steady-state protonation of <i>para</i>-methoxyazobenzene, specifically, yields photoisomerizable azonium ions in which the direction of switching is essentially reversed, that is, visible light produces the out-of-equilibrium <i>Z</i>-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.