Photocyclization of Fluorinated Acetophenones Unlocks an Efficient Way to Solar Energy Storage.

The ability to store and release energy efficiently is crucial for advancing sustainable energy technologies, and light-driven molecular isomerization presents a promising solution. However, a persistent challenge in this field is achieving both high stability of the energy-storing photoisomer and establishing efficient catalysis for back-isomerization, a critical process for releasing the stored energy as heat. In this work, we introduce a conceptually new molecular system designed for long-term energy storage, which is based on the reversible isomerization of ortho-methylacetophenone ⇄ benzocyclobutenol. Key to the success of this system is the strategic placement of a trifluoromethyl group, which enhances the overall performance by preventing unwanted side reactions during photochemical cyclization and by increasing the stability of the benzocyclobutenol moiety. Back isomerization is established using simple organic bases as catalysts, taking advantage of significant rate differences between normal and anionic electrocyclic ring-openings. This approach allows for controlled and predictable heat release under ambient conditions, positioning this molecular pair as a promising candidate for practical energy storage solutions.