Design, Synthesis and Evaluation of Photoswitches for Molecular Solar Thermal Energy Storage Systems

Sammanfattning: To meet the future energy demands and handle climate changes, new sustainable energy sources must be developed. Over the last decades, great scientific progress on harvesting solar energy has been made, but storing the energy is still a challenge. One way to store solar energy is in a compound that absorbs solar energy while being converted to a metastable isomer through a photoisomerization process, a technique referred to as molecular solar thermal energy storage (MOST). In this work, the norbornadiene/quadricyclane system, a promising candidate for MOST, was evaluated and new synthetic methods were developed to obtain a series of novel norbornadiene derivatives in an efficient way. Photophysical characterization of the series revealed a better solar spectrum match and high energy storage densities (114–124 kJ/mol) compared to previous series. Quantum yields for the photoisomerization processes were between 28–58% and the half-lives of the photoisomers in the range of hours to days. Solvent effects on the norbornadiene/quadricyclane system were studied, showing a pronounced effect on both the half-lives of the photoisomers and on the photoisomerization process when going from polar to non-polar solvents. Additionally, bicyclooctadiene derivatives were synthesized and for the first time evaluated as MOST candidates. The series exhibited very high storage densities (143–153 kJ/mol) compared to the corresponding norbornadiene derivatives (52–63 kJ/mol) and could switch back and forth for 645 cycles without significant degradation. The absorption profile and half-lives of the photoisomers need further improvement for MOST applications, but the molecular engineering concepts presented here can be used to develop future MOST systems based on the bicyclooctadiene/tetracyclooctane system. Altogether, this work illustrates the importance of detailed molecular design and the importance of the local environment of the photoswitches for obtaining desired MOST properties.