Synthesis of Conformationally Flexible <i>π</i>-Expanded Cyclooctatetraenes

The author has proposed and developed FLAP (Flexible and Aromatic Photofunctional Systems), a unique hybrid molecular system with flexible and rigid moieties, that can induce a dynamic motion to express meaningful macroscopic events. The essence of the molecular design is to change the electronic structure as well as the conformation of the molecule by hybridizing flexible and rigid parts based on deep consideration of molecular orbitals. Specifically, we have proposed a molecular design with a flexible 8π ring structure at the center and fused rigid π-conjugated skeletons on both sides. The structure of the flexible moiety of FLAP is not necessarily limited to an eight-membered ring, and countless structures are possible for FLAP by combining flexible and rigid moieties. Although it is interesting to study to explore functional FLAPs from the chemical space, this paper only focuses on the FLAP skeleton with cyclooctatetraene (COT) at the center as a typical 8-membered ring of the 8π system. Synthetic trials and errors and development history of FLAPs are described.