We next investigated the photophysical properties of these compounds in a nonpolar host, 4,4'-N,N'-dicarbazolebiphenyl (CBP), since this host has a sufficiently high triplet energy (T1 = 2.60 eV) to confine the excitons to the emitter molecules. The PL spectra and ΦPL of the emitters at doping concentrations ranging from 1-10 wt% were first investigated (Figure and Table ). A progressive red-shifting of the PL was observed as the doping concentration increased, indicating greater aggregate formation. A significant drop in ΦPL values was also observed upon increasing the doping concentration (Table ). DMACPyBP, PXZPyBP and DPACzPyBP are 3.4 × 10 7 , 5.4 × 10 7 , and 5.0 × 10 7 s -1 , respectively, while the reverse intersystem crossing rate constants (kRISC) are 0.5 × 10 5 and 0.3 × 10 5 s -1 for DMACPyBP and DPACzPyBP, which are much faster than for PXZPyBP (kRISC = 0.04 × 10 5 s -1 ). This is due, in part, to the larger ΔEST and smaller FPL for PXZPyBP. A summary of all relevant rate constants is provided in Table . Temperature-dependent TRPL decays of the 1 wt% doped films in CBP under vacuum showed enhanced delayed emission with increasing temperature, confirming TADF behavior in these compounds (Figure and).