Systematic Studies on the Effect of Fluorine Atoms in Fluorinated Tolanes on Their Photophysical Properties

In this study, we synthesized a series of fluorinated and non-fluorinated tolanes, in which one or more fluorine atoms were systematically introduced into one aromatic ring of a tolane scaffold, and systematically evaluated their photophysical properties. All the tolanes with or without fluorine substituents were found to have poor photoluminescence (PL) in tetrahydrofuran (THF) solutions. On the other hand, in the crystalline state, non-fluorinated and fluorinated tolanes with one or four fluorine atoms were less emissive, whereas fluorinated tolanes with three or five fluorine atoms exhibited high PL efficiencies (Ф(PL)) up to 0.51. X-ray crystallographic analyses of the emissive fluorinated tolanes revealed that the position of the fluorine substituent played a key role in achieving a high Ф(PL). Fluorine substituents at the ortho (2/6) and para (4) positions led to tight and rigid packing due to plural π–π stacking and/or hydrogen bonding interactions, resulting in enhanced Ф(PL) caused by the suppression of non-radiative deactivation. Additionally, fluorinated tolanes with three fluorine atoms exhibited notable aggregation-induced PL emission enhancement in THF/water mixed solvents. This demonstrates that the PL characteristics of small PL materials can be tuned depending on the usage requirements.