Magnetic-induced isomerization of paramagnetic bridged azobenzene derivatives with conjugated alkyl chains

Abstract

Inducing a reversible structural transformation in organic photochromophores under the effect of a magnetic field is challenging owing to their poor magnetic properties. Compared with common azobenzene materials, bridged azobenzene materials exhibit a considerable potential for rapid trans-cis isomerization induced by an external magnetic field because of the restricted torsion of N=N bonds during the transformation. Herein, we designed and synthesized pentenyl-grafted bridged azobenzene (BA-X5), hexenyl-grafted bridged azobenzene (BA-X6), and pentynyl-grafted bridged azobenzene (BA-Q5). Density functional theory calculations indicate that the activation energy for the trans-cis transition of BA-X5 and BA-X6 is ∼18.0 kcal/mol, which is 8.2% lower than that of BA-Q5 (19.6 kcal/mol). The results obtained using EPR and a superconducting quantum interference device demonstrate that during the isomerization process, a net spin reduction of bridged azobenzene occurred because of the aggregation of the electron cloud toward the C–N bond, leading to a reduction in the paramagnetism of the materials. BA-X5 and BA-X6 exhibit a clear and rapid magnetically induced trans-cis isomerization with short half-lives, which are 10.4% and 16.9%, respectively, lower than those obtained under dark conditions. In contrast, the isomerization of BA-Q5 under the effect of the same magnetic field does not change. Magnetically induced isomerization might be attributed to the combined effect of the magnetothermal effect, changes in the net spin density of the electron cloud, and regularity of molecular arrangement under the effect of the magnetic field. These results provide a basis for exploring the design and research of magnetically controlled azobenzene derivatives.

Keywords

bridged azobenzene / heat release / isomerization / magnetic field induction

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