Crystal-to-crystal solar energy storage in a hydrogen bonded dianthracene lattice retrocyclized by heat or e-beam

Solid-state (SS) systems for molecular solar thermal (MOST) energy storage are desirable for their simplicity and higher energy storage density compared to solution-based MOST systems, but it is difficult to design systems that achieve both high conversion to the metastable photoproduct and undergo thermally-driven release of the stored heat without melting. Anthracene (An) photocycloaddition to dianthracene (diAn) can proceed to high conversion in the solid state, but few crystalline Ans are appropriately aligned for dimerization, and even fewer diAns thermally retrocyclize without melting. By combining 9-pyridyl anthracene and 9-anthracenecarboxylic acid, we form hydrogen-bonded (H-bonded) and π-stacked chains that undergo high-conversion single-crystal-to-single-crystal photocycloaddition under simulated sunlight. Disordered packing in the lattice produces a mixture of the pyridyl-carboxylate "heterodimer," the pyridyl homodimer, and the carboxylic acid homodimer. Calorimetry reveals that the H-bonded diAn product has a large separation between its thermal retrocyclization and melting temperatures, allowing the material to be cycled in the solid state. The H-bonded diAn lattice is also shown to undergo crystal-to-crystal retrocyclization under e-beam exposure. This work presents an easily-assembled all-SS crystal-to-crystal cyclable MOST system and demonstrates that H-bonding can be leveraged to increase the gap between thermal retrocyclization and melting.