Photoswitches are light-responsive molecules that have the ability to store isomerization energy upon irradiation and to release thermal energy when triggered by various stimuli. Azobenzenes that photoswitch between <i>trans</i> and <i>cis</i> forms are able to store a substantial amount of photoisomerization energy, however their isomerization in the solid state has been difficult to achieve because of significant steric hindrance caused by close packing. To address this problem, we designed a set of asymmetric azobispyrazoles that have shown promise in the development of condensed-phase photoswitchable compounds. The loose packing of the photochromes in the crystalline phase suggests that the intermolecular interactions are weaker and the conformational freedom of molecules is larger in the case of azobispyrazoles. Photoliquefaction experiments demonstrate that this set of azobispyrazoles undergoes facile photoswitching in the solid state, also displaying penetration depths of incident light exceeding 1000 microns. This allows for the photoactivation of a much larger quantity of material in the solid state compared to azobenzene. These results encourage a change in direction toward azoheteroarenes for further studies on condensed-phase photoswitches as well as applications in thermal energy storage.

Novel asymmetric azobispyrazoles with high energy densities and facile condensed-phase photoisomerization for thermal energy storage applications

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ACS Fall 2022

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