Synthesis of Fluorine-Terminated Phenanthrimidazole under Precise Regulation and Structure–Activity Relationship

Designing hybrid local and charge transfer (HLCT) molecules with an appropriate charge transfer (CT) or local excited (LE) ratio often requires careful consideration. To explore the suitable mixing ratio of CT and LE in the phenanthroimidazole system, in this study, different numbers of fluorine atoms were precisely regulated and introduced onto the terminal benzene ring. A series of D–A type molecules were successfully designed and synthesized using phenanthrimidazole as the donor and varying amounts of fluorobenzene as the acceptor (namely, 0F-mP, 1F-mP, 2F-mP, and 3F-mP). The research results show that the four types of molecules designed demonstrate HLCT characteristics, as confirmed by theoretical calculations and varying the practical experiments. The introduction of fluorine at the C4 position significantly affects the relevant photophysical properties: increasing the photoluminescence quantum yield (PLQY), narrowing the full width at half-maximum (fwhm), and effectively reducing the Stokes shift. The PLQYs of 1F-mP in THF solution and solid states are 50.8 and 50.4%, respectively. However, 1F-mP exhibits a higher PLQY and either unchanged or narrower fwhm in most solvents, which indicated that the appropriate CT/LE ratio regulation could significantly affect the photophysical properties.