Role of electron withdrawing moieties in phenoxazine–oxadiazole-based donor–acceptor compounds towards enriching TADF emission

Abstract

Herein, we reported the effect of electron withdrawing units, such as trifluoromethyl (CF₃) and cyanide (CN), substitution on a thermally activated delayed fluorescent (TADF) molecule (10-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-10H-phenoxazine (PXZ-OXD)). The addition of electron withdrawing units has increased the acceptor strength and reduced the interaction between PXZ and OXD, thus reducing the gap between the singlet and triplet states (ΔE_ST) of excitons. To understand the variation in the acceptor strength of PXZ-OXD as a function of its molecular structure and density of states, density functional theory (DFT) and time-dependent DFT (TDDFT) were conducted. Transition density matrix investigations revealed that the addition of −CF₃ and –CN to PXZ-OXD triggers CT excitons, while inducing lower ΔE_ST values. Particularly, the lowest ΔE_ST (0.05 eV) with a notable red shift in the emission spectrum was observed with 4′-CF₃PXZOXD. The delayed component lifetime of PXZOXD is found to be reduced after the substitution of −CNwhm and −CF₃. Despite the weak charge transfer transitions, the substitution of conjugative –CN group is found to be beneficial in improving the HOMO-LUMO overlap with a moderate decrease in reverse intersystem crossing (K_RISC), which attained enhancement in the photoluminescent quantum yield. Additionally, the substitution of the above electron withdrawing units on PXZ-OXD yielded a red shift in the electroluminescence spectrum. Furthermore, the external quantum efficiency (at 100 cd/m², i.e., EQE₁₀₀) of the 4′-CNPXZOXD-based organic light-emitting diode is found to improve by 21.13 % against the PXZOXD (16.9 %).