1,3,4-Oxadiazole-based Deep Blue Thermally Activated Delayed Fluorescence Emitters for Organic Light Emitting Diodes

A series of four 1,3,4-oxadiazole-based thermally activated delayed fluorescence (TADF) derivatives are reported as emitters for organic light emitting diodes (OLEDs). As a function of the nature of the substituent on the weak 1,3,4-oxadiazole acceptor, their emission color could be tuned from sky-blue to blue. The highly twisted conformation between carbazoles and oxadiazoles results in effective separation of the highest occupied and the lowest unoccupied molecular orbitals resulting in a small singlet-triplet splitting. The corresponding singlet-triplet energy gaps (ΔE_ST) range from 0.22 to 0.28 eV resulting in an efficient reverse intersystem crossing (RISC) process and moderate to high photoluminescence quantum yields (φ_PL), ranging from 35 to 70% in a DPEPO matrix. OLEDs based on i-2CzdOXD4CF₃Ph achieve maximum external quantum efficiency (EQE_max) of up to 12.3% with a sky-blue emission at CIE of (0.18, 0.28), while the device based on i-2CzdOXDMe shows blue emission at CIE of (0.17, 0.17) with a maximum EQE of 11.8%.