We then proceeded to investigate the photophysical properties of each of the emitters in greater detail in solution (Table ). In toluene, the time-resolved PL decays of both of the singly substituted emitters exhibited a single decay process with comparable lifetimes, tp, of 2. We next investigated the photophysical properties in the solid state. In spin-coated 10 wt% doped films using polymethylmethacrylate (PMMA) as a host, the PL is slightly blue-shifted compared to that in toluene at lPL of 441, 462 and 491 nm for 2-PXZ-Nap, 1-PXZ-Nap and 1,4-PXZ-Nap-PXZ, respectively ( Table ). The trend in lPL between the three emitters, however, remained the same as in toluene. The photoluminescence quantum yields, FPL, are very low for the two singly substituted emitters at ca 2% but is notably higher in 1,4-PXZ-Nap-PXZ at 22%, which decreases to 18% upon exposure to air. Time-resolved PL of the films revealed the same pattern as was observed in solution (Figure ), with a rapid and broadly equivalent τp of 2.9 and 3. the change in FPL is close to the measurement error of ±5%. Upon increasing the doping concentration to 20 wt%, the FPL of the 1,4-PXZ-Nap-PXZ film increased to 48%, which is the highest FPL we recorded for this emitter. A full doping screen for the three emitters is available in the supporting information (S17). In the 20 wt% mCP film, the λPL red-shifted by 17 nm to 508 nm (Figure ) compared to the 10 wt% PMMA, owing to increased stabilisation of the 1,4-PXZ-Nap-PXZ excited state in the mCP host. Temperature-dependent time-resolved PL decay measurements of the 20 wt% mCP film are shown in Figure . As the temperature decreases, the emission lifetime of 1,4-PXZ-Nap-PXZ increases, indicating a slower decay process. This is consistent with a thermally activated pathway, which at lower temperatures there is less energy available to thermally upconvert triplets to singlets. In considering 2-PXZ-Nap and 1-PXZ-Nap, we note that: (1) TDA-DFT calculations predicted a similarly small ΔEST for the two singly-substituted emitters as the doubly substituted emitter; (2) the measured FPL of the films was extremely low, even under vacuum;