Conformational Planarization versus Singlet Fission: Distinct Excited‐State Dynamics of Cyclooctatetraene‐Fused Acene Dimers

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited‐state dynamics in solution. While the anthracene dimer showed a fast V‐shaped‐to‐planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems. FLAPjacks: Distinct excited‐state dynamics were observed in a set of COT‐fused flapping acene dimers (FLAP), depending on the acene length. While the anthracene dimer showed a fast conformational planarization in S1, the tetracene dimer and the pentacene dimer underwent singlet fission (SF) in a fast and reversible manner and a fast and quantitative manner, respectively, providing new aspects of ultrafast dynamics of the FLAP molecules.