Acridin-9(10H)-one-based blue thermally activated delayed fluorescence materials: improvement of color purity and efficiency stability

A series of donor-acceptor-donor (D-A-D) type blue thermally activated delayed fluorescence (TADF) emitters, namely, 2,7-DtBuCz-AD, 3,6-DtBuCz-AD, 3,6-DMAC-AD, and 3,6-DMAC-AD-CF3, were developed with highly rigid acridin-9(10H)-one (i.e. acridone [AD]) as acceptor. The regioisomeric effect study revealed that the attachment of donors at 3,6-sites of AD ring dramatically enhanced TADF ratio in comparison with the 2,7-site isomer. On the one hand, by varying donors from dimethylacridine (DMAC) to tert-butylcarbazole (tBuCz) at 3,6-sites of AD ring, the emission color purity of blue TADF emitters was improved from sky blue to deep blue. On the other hand, by introducing trifluoromethyl (CF3) onto 9-site phenyl ring of 3,6-DtBuCz-AD, the efficiency stability of the sky blue emission for 3,6-DMAC-AD-CF3 was remarkably improved. The deep blue organic light-emitting diode (OLED) of 3,6-DtBuCz-AD exhibited a maximum external quantum efficiency (EQEmax) of 17.88% with CIE coordinates of (0.15, 0.08), which is among the best performances ever reported for deep blue TADF-OLEDs. The sky-blue OLED of 3,6-DMAC-AD realized an EQEmax of 23.16%. And with the incorporation of CF3, the sky blue device of 3,6-DMAC-AD-CF3 exhibited extremely low efficiency loss of only 5.1% at the high brightness of 1,000 cd/m2.