Donor‐Strength‐Driven Evolution From Charge‐Transfer to Polymethine Excited States in CurcBF 2 Derivatives

ABSTRACT Thermally activated delayed fluorescence (TADF) has emerged as a powerful strategy to replace heavy‐metal phosphorescent emitters in organic light‐emitting diodes (OLEDs), yet achieving highly efficient deep‐red and near‐infrared (NIR) emission remains challenging. Donor–acceptor–donor curcuminoid borondifluoride (CurcBF 2 ) dyes have recently demonstrated promising NIR OLED emission through vibration‐ and spin‐orbit‐coupling‐assisted TADF. Here, a series of CurcBF 2 derivatives bearing donor groups of varying strengths is synthesized and systematically investigated by steady‐state and time‐resolved spectroscopy in solution and thin films. The results reveal that the lowest singlet excited‐state character is strongly governed by donor strength. Weak to moderately electron‐donating substituents promote charge‐transfer (CT) emission and TADF exciplex formation in thin films, whereas a stronger donor, such as TPA, enables TADF directly from the CurcBF 2 molecule. Further increasing donor strength induces a structural transformation toward a zwitterionic polymethine, resulting in predominantly local excited‐state (LE) emission and suppressed TADF. These findings demonstrate that efficient TADF in CurcBF 2 systems requires a predominantly CT singlet excited state and highlight donor oxidation potential as a key parameter for controlling excited‐state character. This work establishes molecular design principles for high‐performance deep‐red and NIR TADF emitters.