The Role of Torsional Dynamics on Hole and Exciton Stabilization in pi-Stacked Assemblies: Design of Rigid Torsionomers of a Cofacial Bifluorene

Exciton and charge delocalization across -stacked assemblies is of importance in biological systems and functional polymeric materials. To examine the requirements for exciton and hole stabilization, cofacial bifluorene (F2) torsionomers were designed, synthesized, and characterized: unhindered (model) (Me)F2, sterically hindered (tBu)F2, and cyclophane-like (C)F2, where fluorenes are locked in a perfect sandwich orientation via two methylene linkers. This set of bichromophores with varied torsional rigidity and orbital overlap shows that exciton stabilization requires a perfect sandwich-like arrangement, as seen by strong excimeric-like emission only in (C)F2 and (Me)F2. In contrast, hole delocalization is less geometrically restrictive and occurs even in sterically hindered (tBu)F2, as judged by 160 mV hole stabilization and a near-IR band in the spectrum of its cation radical. These findings underscore the diverse requirements for charge and energy delocalization across -stacked assemblies.