Modulation of Photoluminescence and Solar Thermal Energy Storage in Norbornadiene–Quadricyclane Dimers

The norbornadiene/quadricyclane (NBD/QC) photoswitch pair is a promising system for molecular solar thermal (MOST) energy storage. Multichromophoric systems with two or more photoswitches can offer red-shifted absorption, higher energy densities, and additional functionality. Here, a series of ortho- and para-substituted NBD dimers bearing methoxy, hexoxy (for solubility), and cyano groups were synthesised and evaluated for their MOST properties. Compared to monomers, the dimers display red-shifted absorption and improved solar spectrum match, with onsets between 448-488 nm, owing to their donor-acceptor design and extended conjugation. A key finding is the tunable relationship between molecular structure, photoluminescence and photoisomerisation: para-dimers exhibit efficient fluorescence, whilst ortho-dimers are superior photoswitches with quantum yields of isomerisation, Φ_i, up to 63%. Solvent choice further modulates behaviour; Φ_i is higher in acetonitrile, whereas fluorescence is more efficient in toluene. This interplay allows tailoring for specific functions. The best-performing photoswitches were studied in a liquid-chip device, achieving a record solar conversion efficiency of 2.9%. Catalytic back-conversion using cobalt phthalocyanine on carbon and macroscopic heat release experiments at 0.1 M yielded a 5.78 °C temperature increase. This first experimental macroscopic heat release of a dimeric system provides important insights into design challenges and opportunities for advancing multichromophoric systems towards MOST applications.