Norbornadienes for molecular solar thermal energy storage

Sammanfattning: Over the last decade, there has been great scientific progress in terms of harvesting solar energy but the area of storing energy is still facing challenges. One approach to store solar energy is via photoinduced isomerization of chemical bonds, referred to as molecular solar thermal energy storage (MOST). Norbornadiene is a promising candidate molecule for this purpose, since upon irradiation isomerization occurs to form the metastable quadricyclane. The reaction is reversible and back conversion can be triggered to release the energy as heat. In this work, a new synthetic approach was developed to obtain 2-bromo-3-chloronorbornadiene that serves as an important starting material for further functionalization of norbornadienes. Through the developed procedure, a series of norbornadienes, decorated with electron donating and accepting substituents were synthesized. Photophysical characterization revealed that norbornadienes with cyano acceptor groups and ethynyl-substituted aromatic donor groups show a good solar spectrum match and high energy storage densities (296 – 545 kJ/kg). The obtained quantum yields for the photoisomerization processes were between 28 – 58% and the half-lives of the corresponding quadricyclanes in the range of 5 – 22 hours for the compounds with an ethynyl linker, and 55 days for the quadricyclane with similar substituents but without the ethynyl linker. The synthesized compounds illustrate the challenge in optimizing all parameters for an efficient MOST system in a single molecule system. For a deeper understanding of the relation between the structure and the properties, more variations of norbornadienes have to be synthesized and evaluated in future research.