Solid-state photon energy storage via reversible [2+2] cycloaddition of donor-acceptor styrylpyrylium system

Donor-acceptor styrylpyrylium derivatives are reported to absorb visible light to undergo intermolecular [2+2] cycloaddition to form cyclobutane structures in crystal, storing up to 42 kJ/mol of energy in strained chemical bonds with half-lives as long as 32 years. The reversion is triggered either by UV irradiation or thermal activation to release the energy, and the solid-state energy storage-release process is repeated without decomposition for over 10 cycles. By varying functional groups on the donor moiety and counter-anions, we fine-tuned the 3D packing in crystals and electrostatic interactions between the cationic molecules and anions, which primarily determine the energy storage capacity of the compounds. This quantitative and mechanistic study on the energy storage capability of styrylpyryliums, supported by an in-depth crystal structure analysis as well as optical and thermal property measurements, will shed light on the development of new molecular solar thermal energy storage materials with highly desired properties beyond conventional photoswitches.