Gilding the Lily: Studies on the Effects of Successive Loading of Light Energy‐Storing Molecules with Norbornadiene Units

Abstract

The photochromic norbornadiene‐quadricyclane cycle has emerged as a promising basis for molecular solar thermal (MOST) energy storage. In this context, it has been estab­lished that multifunctional photoswitches with several norborna­diene units at one central aromatic core have favorable photochemical properties and storage capacities. Nevertheless, a maximum number of three norbornadiene units has been combined in one molecule, so far. In this work, scope and limits of this approach should be further assessed. Thus, exemplary polyfunctionalized derivatives have been synthesized by Suzuki‐Miyaura reactions, and their suitability for MOST applications was investigated. These compounds were converted into the correspon­ding quadricyclanes by direct UV irradiation (λex = 315 nm) with moderate quantum yields, whereas the reaction is more efficient upon irradiation with λex = 520 nm in the presence of photosensitizers. But these molecules showed low stabilities and short half‐lifes of 1 to 2 days. Moreover, the photo­chemical reaction is hampered by the close vicinity of photoactive units. Therefore, it appears that in such assemblies, a maximum number of three norbornadiene units is the limit to keep the optimal balance of photochemical properties required for MOST applications.