We investigated the excited state dynamics, photophysical properties, and exciton utilization mechanism in OLEDs of three pyrazine-based multi-carbazole containing emitters through a combined, detailed theoretical and experimental study. When four carbazoles are connected to the pyrazine core, the emitter 4CzPyz exhibits a moderate ΔEST of 0.23 eV, high ΦPL of 74% and TADF character (τp: 5.9 ns, τd: 150 µs) in PPT film. The device based on 4CzPyz shows sky-blue emission (λEL = 486 nm) with EQEmax of 24.1% thanks to the highly efficient exciton utilization by TADF mechanism. When one carbazole is replaced by an ortho-biphenyl, the ΔEST of 3CzBPz is increased to 0.29 eV, ΦPL decreases to 57%, and the TADF character is lost in PPT film. However, the photophysical properties and DFT calculations support that 3CzBPz can harvest triplet excitons via a higher-energy triplet state channel mechanism. This is reflected also in the OLED performance where the device shows an EQEmax of 9.6% at λEL of 464 nm; indeed, 86% of the excitons are estimated to be converted into light in the OLED based on 3CzBPz. When two orthobiphenyl groups are connected to the pyrazine core, the ΔEST of 2CzBPz is further increased to 0.34 eV and ΦPL is reduced to 45% in PPT film. The calculations and photophysics indicate 2CzBPz to be a deep blue fluorescence emitter, and the OLEDs based on 2CzBPz exhibits an EQEmax 3.2% with λEL of 446 nm.