Tuning the Excited State Character of Amine/Carbonyl Thermally Activated Delayed Fluorescence Emitters with Ring Fusion

Multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters exhibit short-range charge transfer (SRCT) and have narrow emission, whereas donor-acceptor TADF emitters show long-range charge transfer (LRCT) and rapid reverse intersystem crossing (RISC). While some work has been done to create intermediate-type materials with narrow emission and rapid RISC simultaneously, no studies of structurally comparable donor-acceptor versus MR-TADF systems have been described. Here we report two TADF emitters, Acr-DiKTa and Iso-DiDiKTa, to examine the effect of ring fusion on LRCT and SRCT character. Acr-DiKTa appends an acridone moiety to quinolino[3,2,1-de]acridine-5,9-dione to create a donor-acceptor-type structure, while a carbonyl bridge fuses these moieties in Iso-DiDiKTa to give an MR-TADF structure. Acr-DiKTa emits blue-green in toluene with a full width at half maximum (FWHM) of 38 nm whereas Iso-DiDiKTa emits green-yellow with a FWHM of 34 nm. Calculated orbital distributions show SRCT in the first excited state for both compounds. Both compounds are TADF active in doped films, and derived rate constants show that RISC is four times more rapid in Acr-DiKTa. This study shows that while ring fusion enhances color purity, donor-acceptor structures can achieve comparable FWHMs and rapid RISC through dominant SRCT supported by LRCT for best performance in TADF materials.