Multistate Azobenzene-Norbornadiene Photoswitches for Molecular Solar Thermal Energy Storage

The combination of two photochromic molecules into a multimodal photoswitch offers the potential to generate new properties related to molecular solar thermal (MOST) systems. A system can only be considered suitable for MOST applications if it meets key criteria, including a high quantum yield and an extended half-life of the high-energy isomer. Achieving this combination remains a significant challenge. In this study, we present the coupling of an azobenzene unit (AZO) with a norbornadiene system (NBD) via ester functions to develop new bi- and trimodal AZO-NBD photoswitches that exhibit synergistic effects. Photochemical studies reveal that both the NBD and AZO components of the hybrid are switchable. In one case, all isomer types could be selectively produced. Furthermore, each metastable unit can be switched back independently to its corresponding thermodynamically stable form. The quantum yields and half-lives for the NBD and AZO components of the hybrid system could be determined separately. The integration of AZOs has demonstrated the potential to extend the half-lives of NBDs to up to 122 days in certain cases, as confirmed through comparative analysis with reference systems. The enhanced rate of back-conversion was further facilitated by the presence of TFA as a catalyst.