A few representative examples of successful TADF/fluorescence emitter combinations are described as follows: F1 is one of the best fluorescence emitters used for the development of narrow-emitting HF-OLED (Fig. ). The emitter F1/MR-T1 exhibits a strong absorption band at 457 nm and a sharp fluorescence band at 468 nm, with full width at half maximum (FWHM) of about 14 nm in toluene. This is related to the combined effect of its polycyclic structure and the multiple resonance (MR) effect of the boron and nitrogen atoms. In addition, F1 exhibits a PL of 74% in toluene. In fact, F1 and its derivatives are also TADF active and recently been employed in many HF-OLED as terminal emitters with MR-TADF characteristics (discussed later).