We next assessed the photophysical properties of these emitters in an OLED-relevant host 1,3-bis(N-carbazolyl)benzene (mCP), as this host matrix has a sufficiently high triplet energy (T1 = 2.91 eV) to confine the excitons onto the emitter. Figure shows the PL spectra of the 1 and 10 wt% doped films in mCP and the neat film. The emission spectra gradually red-shifted by up to 10 nm as the doping concertation increasing from 1 wt% to 10 wt%, while the red- shift was more pronounced at over 50 nm in neat films due to strong intermolecular interactions. The optimized doping concentration for 2GCzBP, 2DPACzBP, 2FBP2GCz and 2FBP2DPACz was determined to be 10 wt% as a function of FPL, and the corresponding ΦPL values under a N2 atmosphere are 77, 28, 68, and 44%, respectively, which decrease to 68, 27, 53, and 41%, respectively, under air. The FPL values of all four compounds increased compared to those in toluene due to a combination of suppressed non-radiative decay in the solid host and efficient Förster resonance energy transfer (FRET) from host to dopant emitters. The PL spectra of the 10 wt% doped films in mCP, are all unstructured, with λPL at 570, 640, 560, and 615 nm for 2GCzBP, 2DPACzBP, 2FBP2GCz and 2FBP2DPACz, respectively (Figure , Table ), values that are slightly blue-shifted compared with those in toluene [2GCzBP (585 nm), 2DPACzBP (690 nm), 2FBP2GCz (565 nm) and 2FBP2DPACz (675 nm), Figure ].