Water‐Soluble Molecular Solar Thermal (MOST) System: Photochemical Switching and Catalytic Back‐Conversion of 2,5‐Norbornadiene‐2,3‐Dicarboxylic Acid

In this study, we conduct a comprehensive analysis of the energy storage and release of water-soluble 2,5-norbornadiene-2,3-dicarboxylic acid (DC-NBD) integrating spectroscopic characterization, pH-dependent speciation, and photochemical response analysis. We evaluate protonation and dimerization equilibria using potentiometric and ¹H-NMR techniques, revealing three well-defined pH intervals that affect the reactivity and stability of the system. The photoinduced conversion of DC-NBD to DC-QC was investigated at different pH conditions, while the catalytic back-conversion of the most stable quadricyclane species (DC-QC²⁻) was evaluated on Au(111) and Pt(111) single-crystal surfaces by time-resolved photochemical infrared reflection absorption spectroscopy (PC-IRRAS) and density functional theory. Our findings demonstrate that photoisomerization and catalytic back-conversion can be efficiently conducted in an aqueous environment, eliminating the need for organic solvents. This study advances the development of water-soluble MOST systems, offering key insights into the molecular design and optimization of sustainable photoactive materials. Future research should focus on enhancing photochemical efficiency, improving long-term stability, searching for more active catalysts and scaling these systems for practical solar energy storage applications.