FPL: 35% τd: 500 µs (1 wt% in PMMA); ΔEST: 0.249 eV) revealed that TpCz possesses a larger oscillator strength leading to a higher FPL, but at the expense of a lower triplet energy, stronger charge transfer character and larger ∆EST. A blue vacuum-deposited OLED using TmCz as the emitter showed an EQEmax of 9.5% at CIE coordinates of (0.16, 0.23) and λEL at 475 nm, demonstrating that TmCz emits via TADF in the device.