Computational Exploration of Molecular Solar Thermal Energy Storage via Substituted Anthracenes [4 + 4] Photodimerizations and Thermal Retro-Cycloadditions in Solution and Crystalline States

Innovative molecular solar thermal (MOST) energy storage systems based on the [4 + 4] photodimerization and exothermic thermal retro-cycloaddition of substituted anthracenes have been developed by the Han group. We present new experimental results and comprehensive computational investigations elucidating the mechanisms underlying these MOST systems. Density functional theory (DFT) and spin-flip time-dependent DFT (SF-TDDFT) calculations are employed to examine the photochemical [4 + 4] dimerization and the stepwise diradical thermal retro-cycloaddition responsible for heat release. The onset temperature for retro-cycloaddition is found to relate to the computed activation barrier. A linear relationship between the calculated reaction exothermicity and the experimental activation temperature provides a principle that is useful for the design of anthracene-based MOST energy storage systems. QM/MM calculations on reactions in the crystalline state reveal how crystal forces influence the mechanism and rate of a solid-state retro-cycloaddition. The quantitative relationships among these energetic quantities lead to new designs of promising MOST molecules.