Influence of Substitution on the Interplay of Photochemical Switching and Phase Transitions in Azobenzene-Based Molecular Materials.

Understanding and translating molecular-scale switching of azobenzenes into a macroscopic, functional response in the solid state remains a challenge. Herein, the photochemical and thermal properties of different substituted p-Ohexyl-azobenzenes were systematically investigated to establish structure-property relationships to optimize the performance of azobenzene (AB) based materials for energy storage applications. The influence of various alkyl substituents in different positions on the AB was screened to address the effect of increasing bulkiness on the multidimensional parameters required for efficient energy storage. For example, substituents in the meta- and para-positions increased the isomerization enthalpy (∆Hiso.) due to attractive intermolecular interactions; 2,6-F- and 2,6-Me-substitution induces an increased half-life, however, it also a reduced isomerization enthalpy. Substitution in ortho- and meta-positions leads to a suppressed crystallization process and ineffective energy release. Interestingly, it was demonstrated that even sterically demanding para-substituents exhibit sharp crystallization behavior. Additionally, photoisomerization efficiency in the condensed state was investigated, which revealed a strong relation with the melting points (Tmelt.) of the solid (E)-isomers. These systematic findings connecting photochemistry, thermal properties, and phase change parameters will be a solid foundation for the rational development of new AB-based materials also beyond energy storage applications.