As a result, 4mCzBN-BP exhibited faster kRISC of 2.28×10 6 s -1 in toluene, compared to 1.26×10 6 s -1 for the reference emitter 4mCzBN. Inspired by this work, here we modified the reference emitter 4CzPyz by replacing one or two of the carbazole donors with ortho-substituted biphenyl groups and obtained two blue/deep blue emitters, 3CzBPz and 2CzBPz. The three compounds exhibit distinct photophysics and exciton harvesting mechanisms in OLEDs. The S1 energies of 3CzBPz and 2CzBPz are destabilized to 2.94 eV and 3.00 eV, respectively, compared to 2.88 eV for 4CzPyz in 10 wt% PPT films, while the ΔEST values progressively increase from 0.23 eV (4CzPyz) to 0.29 eV (3CzBPz) and 0.34 eV (2CzBPz), respectively.