Case Study on Exciton-Vibrational Coupling in Molecular Bifluorene Crystals: The Role of Torsional Disorder

The strong correlation between dipole–dipole-induced excitonic coupling and molecular vibrations plays a key role in determining optical and excitation transport properties. Here, we study the impact of torsional vibrational freedom in molecular crystals on exciton-vibronic coupling. Single crystals of 1,4-Bis(9,9-dimethyl-9H-fluoren-2-yl)benzene exhibit substantial vibronic coupling to a low-frequency (79 cm–1) torsional mode of the central phenyl unit, which was directly observed by femtosecond transient absorption spectroscopy. Thermally induced dynamic torsional disorder may play a critical role in exciton decoherence, ultimately a monomer-like radiative rate in crystals at room temperature. Conversely, 1,2-bis(9,9-dimethylfluorene-2-yl)ethylene exhibits weaker coupling to local intramolecular vibrational modes, indicated by low reorganizational energy. Despite similar molecular packing, molecules with restricted torsional freedom exhibited weak-to-intermediate J-type excitonic coupling and super-radiance at low temperatures. An exciton bandwidth of 1435 cm–1 was experimentally estimated by accounting for polarizability effects in guest–host crystals. These findings highlight the impact of molecular design on excitonic interactions and provide insights into strategies for optimizing exciton transport in molecular solids.