Molecular azo–imidazole photoswitches: Property tuning by substitution

Abstract

Azo–imidazole photoswitches based on NH– and N–methylimidazole were designed and prepared via a convenient synthetic route. These azo-compounds were investigated by means of XRD analysis, cyclic voltammetry, thermal analysis, UV–Vis absorption spectroscopy and DFT calculations. The photoswitching was mostly affected by imidazole N-substitution with the half-life of (Z)–isomers found within the range of tens of seconds (NH-imidazoles) vs. tens of hours (N-methylimidazoles). N-Methylimidazole switches can be further tuned by altering electronic effects of the appended substituents (acyl, bromine, trifluoromethyl and pentafluorosulfanyl). The latter group proved to be particularly interesting in stabilizing (Z)–isomer. Its large half-life (∼24 h), high photoconversion (89 %) and diminished nonlinear optical response in contrast to the (E)-isomer (βE/Z = 3.82) point to a very useful strategy in introducing SF5 group in the structure of molecular switches. In summary, this detailed study demonstrates that photoswitching properties of azo-imidazoles can be significantly tuned in a broad extent by (preventing) imidazole tautomerism as well as by altering electronic effects of appended substituents.