However, the brightness of this film was very low and there is significant background scattering from the emission source present in the spectrum, so it is difficult to ascertain the true intensity and nature of this 'delayed' fluorescence. The strong sensitivity of the phosphorescence to the presence of oxygen is conserved in the 0.1 wt% doped films; for example, 6,10NICz shows no phosphorescence in air (Figure ). Upon increasing the doping concentration to 10 wt%, there is a red-shift in the PL We summarize the photophysics of the doped films in PMMA as follows: At 10 wt%, the PL is broadened, red-shifted, and less well resolved compared to lower doping concentrations or solution measurements in toluene due to the influence of intermolecular emission from aggregates. All five of the emitters are biluminescent, with simultaneous emission from the S1 and T1 excited states, in the form of fluorescence/phosphorescence for the singly substituted emitters (4NICz, 5NICz, 6NICz, and 7NICz) and MR-TADF/phosphorescence for 6,10NICz. The PL spectra at 1 wt% and 0.1 wt% are equivalent and results from emission from monomolecular species, resulting in narrower, blueshifted and better resolved spectra. The biluminescence behaviour is preserved in the 10 wt% doped films except that MR-TADF has been switched on in 6NICz in addition to 6,10NICz. Finally, the proportion of RTP (relative to fluorescence) shows non-linear dependence on the doping concentration, while the oxygen sensitivity of the RTP is greatest at 1 wt%, of the concentrations tested here.