Monoaryl-Substituted Norbornadiene Photoswitches as Molecular Solar Thermal Energy Storage Compounds: Synthesis and Investigation

In the search for improved photochromic systems that may be used for the efficient reversible conversion of light into chemical energy, eight monoaryl-substituted norbornadienes are presented, which carry naphthyl, anthracenyl, or donor-acceptor-phenyl substituents to establish an extended π system. The substrates were synthesized by Suzuki-Miyaura coupling reactions, and their absorption properties, the key parameters of the photochromic equilibrium, and the energy storage density of the quadricyclane products were examined. It was observed that these compounds have favorable properties for chemical energy storage. Specifically, 2-(1-naphthyl)norbornadiene showed a pronounced red shift of the absorption, a long half-life (35 d) and a relatively high energy storage density (361 KJ/Kg) of the respective quadricyclane. Therefore, the so far neglected monoaryl-norbornadienes represent a useful and complementary class of compounds that should be considered in the development of efficient molecular solar thermal energy storage (MOST) compounds, which can reversibly convert sunlight into chemical energy.