Greener Solvents for the Photoisomerization and Catalytic Heat Release of a Norbornadiene-Based Molecular Solar Thermal Energy Storage System.

Molecular Solar Thermal (MOST) energy storage employs photoswitches to offer a closed-cycle means of capturing, storing, and utilizing solar energy in an emission-free system. In this work, we conduct a systematic solvent screening to elucidate how different media affect the photoisomerization of a norbornadiene-quadricyclane (NBD-QC) photoswitch and the subsequent catalytic activation of the heat-release step for efficient, on-demand energy delivery. For future applications, all system components, including the photoswitch, catalyst, and solvent, should be non-toxic and environmentally benign. Among the greener solvents assessed here, the photoisomerization quantum yield was observed to be 25% points higher and the storage lifetime extended by 15 days in the biodegradable solvent propylene carbonate (Pc) compared with toluene, the most commonly used solvent in MOST systems. Using a custom-built setup, a heat-release temperature increase of 51°C was achieved in Pc, approaching the highest temperature increase reported for MOST systems operating in organic solvents. Pc also displays broad compatibility with a range of catalysts, including those based on first-row transition and noble metals. We thus present a fully integrated MOST/catalyst/solvent system comprising a scalable NBD-QC photoswitch, an active catalyst (including naturally abundant metals), and a low-toxicity, sustainable solvent. Together, these features represent a significant advancement in the feasibility of MOST systems for practical applications.