Mes3-DiKTa at RT in mCP has similar behaviour as in zeonex, indicating that intermolecular interactions remain mostly suppressed. The singlet energy (PL onset) is lower compared to zeonex films (2.75 and 2.82 eV), likely because of the difference in environment (Figure ). Both PF and DF emission lifetimes are shorter compared to those in zeonex films, consistent with the smaller experimental ΔEST (230 and 210 meV) and the rigid matrix effect, resulting in a two-fold faster kRISC (Table ). An extra decay component with minor contribution in the DF regime is also observed with a lifetime of 26 μs, likely originating from mixed intermolecular and monomer emission (Figures ). 56 emission that lasts until early µs and most clearly visible in the contour plot of the normalised time-resolved spectra (Figure ). Residual host emission from mCP (<400nm) is also visible in these contour plots during the early PF region, but its contribution is not considered in the time-resolved decay. In the subsequent DF region, the emission becomes narrow again, reflecting a return to pure monomer emission. The intermolecular species contribution dominates at low temperature (where the TADF is completely suppressed) and is coincidentally isoenergetic with the phosphorescence emission. When the room temperature measurement is fitted with exponential lifetimes, the PF has a multi-exponential decay with resolvable components ascribed to both monomer and intermolecular species emission, while the DF lifetime is almost three times shorter compared to zeonex films. As a result, the calculated kRISC becomes over five times faster in mCP (Table ). The effect of intermolecular interactions is even stronger for tBr-DiKTa in mCP (Figure and Figure ). Like dBr-tBu-DiKTa, tBr-DiKTa PF comprises a mixed monomer and intermolecular species emission, with the latter lasting until early μs. The DF lifetime in mCP becomes seven times shorter than in zeonex, and thus the calculated kRISC appears to be over forty times faster (Table ). These big differences in the calculated kRISC for the brominated emitters in the two different hosts at first seem unreasonable, while a second species is clearly involved in the mCP films which complicates the decay kinetics and fitting. The higher emitter concentration in the mCP films also necessarily promotes stronger intermolecular interactions between the guest molecules. In applications, this could either promote a secondary triplet harvesting pathway, or generate a poorly emissive species that quenches the monomer emission. To address the latter, different concentrations of dBr-tBu-DiKTa (0.1, 1, 4 wt%) in the inert zeonex host (a host with no electronic interaction with the guest molecules) were measured. From the steady-state measurements, a bathochromic shift of the PL onset is observed along with a broadening of the FWHM with increasing concentration (Figure ). The red shift is assigned to self-absorption while the increased PL contribution at 480 nm is assigned to aggregate emission. Interestingly, the PL spectrum of the 4 wt% doped film in zeonex is like the one of the 1 wt% doped film in mCP, indicating that the degree of aggregation formation/contribution to the PL spectrum is different in the two hosts. From the time-resolved decays, the monomer prompt lifetime decreases with increasing concentration from 2.1 to 1.5 ns at 0.1 and 4 wt%, respectively, indicating parallel energy transfer from the high-energy monomer to the lowerenergy aggregate state. The aggregate prompt lifetime, identified from the different timeresolved PL spectra, increases both in length and contribution (Figures ). The delayed emission lifetime also appears to decrease with increasing concentration, from 851 to 274 μs for 1 and 4 wt% doped films of dBr-tBu-DiKTa, respectively (Figure ). This decrease in prompt and delayed fluorescence lifetimes indicates that the higher emitter concentration leads to stronger intermolecular interactions between the dBr-tBu-DiKTa molecules, thus generating poorly emissive aggregate species, considering the low transient signal this species produces. This is also confirmed by the decrease of the FPL with increasing dBr-tBu-DiKTa concentration (26 and 20% for 0.1 and 1 wt% doped zeonex films, respectively). Thus, the unshielded brominated DiKTa molecules have dominant intermolecular interactions at high concentrations, which are at least as impactful as any HAE on the overall photophysics. Nevertheless, at 0.1 wt% with minimal aggregation effects, comparing the three emitters in zeonex hosts reveals the impacts of HAEs on improving the SOC and enhancing the ISC/RISC (Table ).