Multi‐azo Photoswitches for Improved Molecular Solar Thermal Energy Storage

Abstract Molecular solar thermal energy storage (MOST) based on photoisomerization represents a novel approach for the capture, conversion and storage of solar energy. Azo photoswitches can store energy by isomerization from their thermodynamically stable E isomers to higher energy metastable Z isomers. The energy density of azo photoswitches is crucial as it directly determines the storage capacity, but it is relatively low (160 J g −1 ). Therefore, enhancing the energy density through molecular structural design represents a central research focus in the field of MOST. A straightforward approach to enhance the energy density is to design multi‐azo photoswitches. This allows multiple azobenzene units to share a common framework while keeping the molecular weight as small as possible. In particular, when two azobenzene units are connected via a phenyl ring in a meta orientation, it facilitates efficient isomerization, thereby maximizing the energy density of the azo photoswitches (392 J g −1 ). This paper provides a brief overview of the development of multi‐azo photoswitches and highlights their outstanding performance as a MOST system. It also offers prospects for their future advancements in the field. We propose that, to further improve the energy density of multi‐azo photoswitches, one approach is to design wide spectrum of light photoisomerization of multi‐azo photoswitches. Additionally, introducing photo‐induced phase changes to multi‐azo photoswitches enables the simultaneous storage of both photon energy and ambient heat.