Conformational Effect of Polymorphic Terfluorene on Photophysics, Crystal Morphologies, and Lasing Behaviors

Molecular conformation is an important factor in flexible organic molecules and deeply influences their physical and chemical properties. Here, a model fluorene trimer, 2,2′:7′,2″-ter(9,9-dimethylfluorene) (TDMeF), was designed and synthesized to investigate the effect of conformational diversity of oligofluorene on the morphologies of microcrystals and photophysical properties. Single crystal X-ray diffraction analysis indicates that TDMeF has four polymorphs with different molecular conformations, and crystalline polymorphism is first observed in oligofluorenes. Moreover, the slight change in molecular conformation leads to form different crystal morphologies, namely, ribbon and rodlike microcrystals, in virtue of different intermolecular interactions. Finally, although both microcrystals display deep blue lasing behaviors, the rodlike microcrystal shows a threshold of 114 W/cm², which is twice times lower than that of the ribbon-like one due to the effect of molecular orientation and optical microcavity.