Photoswitches with different numbers of azo chromophores for molecular solar thermal storage

We investigate three azo-chromophore-containing photoswitches ( 1 , 2 and 3 ) for molecular solar thermal storage (MOST) based on reversible Z - E isomerization. 1 , 2 and 3 are photoswitchable compounds that contain one, two and three azo chromophores, respectively. In solution, 1 , 2 and 3 were charged via UV-light-induced E -to- Z isomerization. Among these three compounds, 2 exhibited an energy density as high as 272 ± 1.8 J g −1 , which showed the best energy storage performance. This result originated from the low molecular weight, a high degree of photoisomerization, and moderate steric hindrance of 2 , which demonstrated the advantages of the meta -bisazobenzene structure for MOST. In addition, we studied the performances of these photoswitches in the solvent-free state. Only 1 showed photoinduced reversible solid-to-liquid transitions, which enabled the charging of 1 in a solvent-free state. The stored energy density for 1 in a solvent-free state was 237 ± 1.5 J g −1 . By contrast, 2 and 3 could not be charged in the solvent-free state due to the lack of solid-state photoisomerization. Our findings provide a better understanding of the structure-performance relationship for azobenzenebased MOST and pave the way for the development of high-density solar thermal fuels. Photoswitches with different numbers of azo chromophores are designed for molecular solar thermal storage. The relationship between the molecular structures and the energy storage performance is investigated.