Fast dark cis-trans isomerization of azopyridine derivatives in comparison to their azobenzene analogues: Experimental and computational study

Thermal cis-trans isomerization of azobenzene and azopyridine derivatives was studied. The impact of the structural features of azo derivatives on its isomerization kinetics being important for molecular switching materials was investigated experimentally and theoretically based on the density-functional theory (DFT). Eleven azo-dyes in which the phenyl rings were substituted in the para-position to NN– linkage were prepared. Additionally, their thermal properties were evaluated by thermogravimetric analysis and differential scanning calorimetry. DFT calculations were also used for optimization ground and transition state geometries, the density of states, electronic structures of studied azo-dyes. The kinetics of the dark-return cis to trans-form of azo-compounds was investigated after UV-irradiation. It was found that attaching hydroxyalkoxy-substituents to the azo-dyes structure retarding the photoisomerization process in comparison to the hydroxyl group. Moreover, dark-return cis to trans-isomer was ca. twice times faster for azopyridine derivatives than for their azobenzene analogues. The calculated rate constants were in qualitative agreement with the experimental values.