The photophysical properties of the materials in solid-state were first studied in the host matrix mCP (Figure ). This host was chosen as it has a relatively high triplet energy (2.9 eV), which makes it suitable for both green and blue devices. The FPL of t BuPh-DMAC-TRZ appears to decrease slightly at concentrations above 10 wt% (Table ). Thus, 10 wt% loading was chosen for all the emitters in mCP. The trend in the solid-state PL spectra of the six emitters at 10 wt% in mCP (Figure ) matches that observed in toluene (Figure ). Interestingly, the lPL of DMAC-TRZ, CNPh-DMAC-TRZ and CF3Ph-DMAC-TRZ are almost isoenergetic in mCP (Table ). This is attributed to the stronger intermolecular interactions taking place in the films. The TRPL behaviour in the mCP films is also comparable to that observed in toluene; notably, there is a small degree of delayed emission observed for CNPh-DMAC-TRZ (Error! Reference source not found.S38). The presence of electron-donating substituents on the donor leads to a longer-lived prompt fluorescence, while the opposite trend is the observed for the delayed fluorescence lifetimes.