Fatigue-Resistant Dithienylarene Photoswitches with Acid-Regulated Thermal Ring Opening.

Dithienylarene photoswitches are an emerging class of diarylethenes, in which photochemical ring closure is accompanied by pronounced changes in aromaticity. This distinctive feature enables new opportunities for applications, including molecular solar thermal energy (MOST) storage, a promising approach within renewable energy technologies. Molecular switches used as MOST systems are designed to absorb solar energy, store it in chemical bonds, and release it as heat. However, fast and efficient on-demand release of energy poses a significant challenge. Herein, we present the synthesis of a pyridine-appended, biphenylene-bridged dithienylarene switch, BPPyr, and demonstrate that its protonation state exerts a major influence on the kinetics of its thermal ring opening. Specifically, protonation of the pyridine units ([BPPyr-H2]2+ ) drastically reduces the thermal half-life of the ring-closed isomer from 2.9 h to 15 s in EtOH at 25 °C but, crucially, without significantly deteriorating the fatigue resistance upon repeated isomerization cycles. The parent BPPyr form with its original properties could be recovered via deprotonation; furthermore, N-methylation of BPPyr produced a water-soluble, ionic compound ([BPPyr-Me2]2+ ) switchable by visible light. Leveraging its unique properties also in a confined environment possibly akin to that of a future device, [BPPyr-Me2]2+ was incorporated in a gelatin matrix to form a sunlight-responsive hydrogel. Complementing the experimental results, quantum chemical calculations were performed to explain the mechanism by which protonation/methylation of BPPyr lowers the free-energy barrier for thermal ring opening. Altogether, this work advances the understanding of structure-property relationships for diarylethenes and provides a novel design for on-demand energy release by MOST systems.