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 Z - to E -conversion. Here, we show that steady-state protonation of the azo-bridge instead is possible in simple azobenzenes when the p K a of the acid is low enough, yielding both the Z - and E -azonium as supported by UV–vis- and 1 H 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.

Thermal Half-Lives of Azobenzene Derivatives: Virtual Screening Based on Intersystem Crossing Using a Machine Learning Potential

“…Another limitation is that we have not considered azobenzene protonation and azo-hydrazone tautomerism. These effects can be facilitated by substituents such as NH 2 and OH, and by solvation in a protic solvent, weakening the NN double bond and lowering the isomerization barrier. − Protonation from the solvent is not accounted for in a PCM description. Incorporating automated protonation tools into our workflow would be of interest in the future.…”

Supramolecular Complexation of Azobenzene Dyes by Cucurbit[7]uril

“…The cis azo conformation of these compounds exhibits a π → π* transition that is not red-shifted ( i.e. , there are two absorption peaks, π → π* at ∼370 nm with ε ∼ 28,000 M –1 cm –1 and n → π* at ∼460 nm with ε ∼ 5000 M –1 cm –1 ). ,− There is no literature indicating that supramolecular stabilization of an azobenzene dye as a cis azo conformer can induce a large bathochromic and hyperchromic shift in azobenzene absorption . Thus, the only way that a protonated 4-amino- or 4,4′-diaminoazobenzene can exhibit an intense absorption band with peak maximum >500 nm and ε > 40,000 M –1 cm –1 is by forming an azonium tautomer. , Therefore, we infer that the complexation-induced absorption peak at 554 nm is due to CB7 capture and stabilization of mono- or bisprotonated 4,4′-diaminoazobenzene as an azonium tautomer in a trans azo conformation, and the likely structures of these alternative supramolecular complexes are illustrated in Figure .…”

The doorstop proton: acid-controlled photoisomerization in pyridine-based azo dyes

“…35 Kortekaas and coworkers found that protonation of para -methoxyazobenzene resulted in a redshifted absorption spectrum. 34 The unprotonated dye isomerized to the cis -isomer under UV illumination. While small yields of protonated cis p -methoxyazobenzene was observed upon protonation with strong acids, protonation of the azo bond with weaker acids reduced the extent of observable photoisomerization.…”