Modulating the Singlet–Triplet Energy Gap With Regio‐Specific Substitution on Dibenzo <i>[a,c</i> ]phenazine to Realize Red TADF and Their OLEDs

Developing efficient orange-red emitters based on donor-acceptor (D-A) π conjugates with thermally activated delayed fluorescence (TADF) remains a significant challenge. Herein, we designed a series of donor-acceptor emitters (3,6-Az-DBP, 2,7-Az-DBP, and 2-Az-DBP) by incorporating twisted 10,11-dihydro-5H-dibenzo[b,f]azepine (Az) as a donor alongside the rigid and planar dibenzo[a,c]phenazine (DBP) acceptor core molecule at different sites viz., 3,6-, 2,7-, and 2- positions. As a result of site-specific substitution, considerable alteration in electronic properties was observed. For 3,6-Az-DBP, the energy gap between singlet and triplet states (ΔE_ST) was found to be 0.42 eV, which is much higher than that for 2-Az-DBP (0.25 eV) and 2,7-Az-DBP (0.06 eV). Owing to miniscule ΔE_ST, 2-Az-DBP and 2,7-Az-DBP showed orange and red TADF, respectively, in CBP blend. Both these compounds also showed red room temperature phosphorescence in powder samples having lifetimes in milliseconds time regime. Further, organic light emitting diode (OLED) of CBP doped 2-Az-DBP showed a high luminance of 6 × 10³ Cd/m² @ current density of 50 mA/cm²with EQE_max of about 12.3%. These studies provide a promising strategy, site selective substitution of donor on acceptor core, to improve the emission properties for red TADF emitters.