Host–guest cooperative bridged bicyclopolyynic (BBP) open-molecular cages with optical-switching properties†

The in situ experimental characterization of highly reactive cyclo[18]carbon using STM-AFM at 5 K has opened a new avenue in the field of carbon chemistry. Owing to its instability, C₁₈ is recognized as a precursor for the synthesis of novel carbon-based structures. Inspired by the polyynic structure of C₁₈, herein, bridged bicyclic molecular cages are rationally designed. Based on state-of-the-art electronic structure methods, the structure, stability, and electronic and photophysical properties of the cages are predicted. The results reveal that the polyynic cages are stable structures that enable host–guest interactions. Further, the open-caged architecture is flexible enough to facilitate reversible switching between endohedral and exohedral configurations. These systems can be regarded as optical switches for promising applications in next-generation functional optical devices that can be operated in the visible range. The report elucidates that the sizeable cage acts as a scavenger and shows a propensity to encapsulate Li and Na with an exclusive endohedral stability. The report reveals that the complexes of the cage with alkali metal atoms exist as charge-separated states (CSSs) in their low-lying energy states. Moreover, the work sheds light on the lower electronic energy levels of alkali metal complexes in a CSS compared with non-CSS based on the contribution of interaction energy components. It is worth mentioning that the properties of complexes can be remarkably modulated by varying the nature and size of the guest/cage, thus opening the opportunity for further modification. It is certain that the work will lay a theoretical foundation and receive widespread attention in both theoretical as well as experimental research.