A tetra-ortho-Chlorinated Azobenzene Molecule for Visible-Light Photon Energy Conversion and Storage

The development of photoactive molecules for photothermal energy storage is a focus of research in solar energy utilization technology. Azobenzene photoswitch has emerged as a promising candidate for solar energy conversion and storage due to its unique photoisomerization characteristics. Nonetheless, a majority of azobenzene-based molecular photothermal systems have a significant drawback: they depend on ultraviolet light for <i>E</i>-to-<i>Z</i> isomerization to store photon energy rather than visible light, which seriously hinders the development of azobenzene photoswitch in practical solar energy utilization applications. In this study, an azobenzene photothermal molecule that can effectively store visible-light photon energy was design and synthesized, which includes a <i>tetra</i>-<i>ortho</i>-chlorinated azo structure as the "head" part and an alkyl chain at <i>para</i>-position as the "tail" part. The ultraviolet-visible and ¹H NMR spectrum indicated that the obtained <i>tetra</i>-<i>ortho</i>-chlorinated azobenzene photothermal molecule could effectively absorb and store photon energy under 550 nm irradiation and release the stored energy upon 430 nm light irradiation. The storage energy density of the charged azobenzene photothermal molecule was determined to be 13.50 kJ/mol through differential scanning calorimetry and 28.21 kJ/mol via density functional theory theoretical calculations. This discrepancy was ascribed to the 64% Z-isomer yield harvesting during the charging process. Furthermore, the obtained <i>tetra</i>-<i>ortho</i>-chlorinated azobenzene exhibited long-term energy storage (approximately 11 days of half-life) and cyclic stability (100 cycles). Notably, the <i>E</i>-isomer of <i>tetra</i>-<i>ortho</i>-chlorinated azobenzene exhibited a high degree of supercooling, which may be advantageous for use in extremely low-temperature environments.