On the photo and back isomerization of phenylazothiazoles anchored onto peptoids : one step further toward visible-light photoswitches for solar energy harnessing

Abstract

The development of efficient azobenzene-based chromophores for MOlecular Solar Thermal (MOST) energy storage requires improved visible-light absorption and enhanced Z-isomer proportion as well as lifetime. In this work, we investigated two complementary strategies: substitution of one phenyl ring with a thiazole unit to red shift the absorption and grafting the resulting phenylazothiazole (PAT) moiety onto a peptoid backbone to increase lifetime through structural cooperativity. By varying the position of the PAT unit along the peptoid sequence (N-terminal, central, and C-terminal), we assess the impact of molecular architecture on photo and back isomerization. All PAT-peptoids absorb at the edge of the visible region (375–390 nm). Grafting onto the peptoid scaffold enhances the thermal stability of the Z-isomer compared to non-grafted analogues, with C-terminal derivatives exhibiting the longest half-lives. Kinetic analyses reveal that the thermal back-isomerization pathway depends on the chromophore position and local environment. Additionally, solvent polarity and proticity markedly influence both the rate and mechanism of back isomerization. Overall, this study highlights the decisive role of chromophore positioning, sequence design, and solvent environment in tuning the performance of PAT-based MOST systems, providing key guidelines for the rational design of advanced solar thermal energy storage materials.