Multifunctional benzonitrile derivatives with TADF and mechanofluorochromic properties and their application in OLEDs

In this study, we explore the solid-state photophysical properties of four donor-acceptor-donor' (D-A-D') fluorinated benzonitrile compounds, featuring combinations of phenoxazine and carbazole donor units. We have previously shown that these compounds exhibit dual charge transfer thermally activated delayed fluorescence (CT-TADF) emission in solution; here we investigate the steady-state and timeresolved photophysical properties in the solid state. Initial photophysical characterizations were performed in polymethyl methacrylate (PMMA) polymer films, followed by detailed studies of the supramolecular packing in neat films under various preparation conditions, including solvent exposure and thermal treatments. Different polymorphs and amorphous phases were obtained, revealing that the dual TADF emission could be maintained to some extent in solid-state. Mechanical stress and solvent vapour exposure induced mechanochromic luminescence (MCL), with emission changes correlating to the supramolecular structures. OLED devices were also fabricated, demonstrating similar tuneable emission properties based on preparation techniques. This work highlights the potential of this class of materials towards advanced optoelectronic applications with tuneable and stimuli-responsive emission characteristics.