The smaller ΔEST for 4CzPyz makes it possible to utilize triplet excitons in OLEDs via TADF; indeed, the device exhibited a maximum external quantum efficiency (EQEmax) of 24.1% and sky-blue emission with λEL of 486 nm. The close-to-resonant T3/S1 states in 3CzBPz allow the RISC process occurring via the higher-energy Tn channels. The OLED with this compound as the emitter shows pure blue emission with λEL of 464 nm and an an EQEmax 9.6%. However, the too large ΔEST for 2CzBPz precludes either of these two exciton-harvesting mechanisms from being operational and the OLED based on 2CzBPz exhibits an EQEmax of only 3.2% but the device is deep blue with λEL of 446 nm.