Isomeric thermally activated delayed fluorescence emitters based on indolo[2,3-b]acridine electron-donor: a compromising optimization for efficient orange-red organic light-emitting diodes

Energy differences (ΔE _STs) between the lowest singlet and triplet excited states as well as the oscillator strengths (fs) are two contradictory factors controlling performance of thermally activated delayed fluorescence (TADF) emitters. To figure out their optimum balance, we developed two orange-red isomeric TADF emitters 12-(13,13-dimethyl-5-phenyl-5,13-dihydro-11H-indolo[2,3-b]acridin-11-yl)dibenzo[a,c]dipyrido[3,2-h:2',3'-j]phenazine (IDAC-BPPZ) and 12-(13,13-dimethyl-11-phenyl-11,13-dihydro-5H-indolo[2,3-b]acridin-5-yl)dibenzo[a,c]dipyrido[3,2-h:2',3'-j]phenazine (ACID-BPPZ), which are constructed with the same electron-donor (D) and the same electron-acceptor (A) cores via different connecting sites. ACID-BPPZ has a weaker D-A conjugation and realizes an extremely small ΔE_ST of 0.02 eV, but its f is also very small (0.001). These factors are better compromised in IDAC-BPPZ which has a more conjugated D-A connections. This successfully realizes a much higher f of 0.068 and a reasonably small ΔE_ST of 0.07 eV. As a result, the orange-red OLEDs using IDAC-BPPZ as emitter achieve a maximum external quantum efficiency 18.3% comparing to that of 14.7% in the ACID-BPPZ-based device. The results suggest that a balance between ΔE_ST and f is important for developing high performance TADF emitters.