Isomer engineering of dipyrido[3,2-a:3′,4′-c]phenazine-acceptor-based red thermally activated delayed fluorescent emitters

To determine the effect of donor substitution pattern on emission properties, we designed and synthesized two isomeric red thermally activated delayed fluorescent (TADF) emitters, i.e., 11,12-bis(9,9-dimethylacridin-10(9H)-yl)dipyrido[3,2-a:2′,3′-c]phenazine (oDMAC-DPPZ) and 10,13-bis(9,9-dimethylacridin-10(9H)-yl)dipyrido[3,2-a:2′,3′-c]phenazine (pDMAC-DPPZ). Two 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor units were attached at the ortho (11, 12) and para (10, 13) positions of the dipyrido[3,2-a:2′,3′-c]phenazine (DPPZ) acceptor to obtain oDMAC-DPPZ and pDMAC-DPPZ, respectively. Interestingly, the photoluminescence quantum yield, external quantum efficiency, and emission wavelength of the emitters varied depending on the substitution pattern of DMAC on DPPZ. The two emitters exhibited distinct photophysical and organic light-emitting diode (OLED) performances due to structural differences related to steric strain and dihedral angles. It is demonstrated that the ortho (11, 12) donor substitution on DPPZ is more effective in the design of high-performance red TADF OLEDs than the para (10, 13) substitution in terms of singlet-triplet energy gap and up-conversion rate constant. The ortho-substituted TADF emitter demonstrated a high EQE of 13.4% with color coordinates of (0.59, 0.40), whereas the para-substituted isomer displayed an EQE of 4.0% and color coordinates of (0.64, 0.35).