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 E-conversion. 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 ¹H 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 out-of-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.

中文翻译：

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质子化偶氮苯中的可逆红移光异构化

偶氮苯是研究最好的分子光开关之一，在寻找用于扩展应用的红移光响应材料方面发挥着关键作用。目前，大多数方法处理芳族取代模式以实现可见光应用，有时与质子化配对以产生氮鎓物质的红移吸收。适当的取代模式对于稳定后一种方法至关重要，因为已知常规酸会诱导快速的Z到E转换。在这里，我们表明当酸的 p K _a足够低时，偶氮桥的稳态质子化反而在简单的偶氮苯中是可能的，产生Z - 和E^{由紫外可见和1} H NMR 光谱以及密度泛函理论计算支持的 -azonium 。此外，对甲氧基偶氮苯的稳态质子化，特别是产生了光致异构化的氮鎓离子，其中转换的方向基本上是相反的，也就是说，可见光产生了不平衡的Z-偶氮鎓。尽管目前的条件使可见光光开关不适合体内和材料应用，但对简单偶氮苯的理解为进一步研究这种已经广泛研究的光开关铺平了道路。