We then investigated the photophysics of PXZ-Nap and PTZ-Nap as 1 wt% doped films in mCP, a suitably high triplet energy host matrix. The photophysical data acquired under vacuum of PXZ-Nap and PTZ-Nap are summarized in Table . The steady-state PL behavior of PXZ-Nap and PTZ-Nap in mCP is similar to that observed in PMMA (Figure ). Notably, the lowtemperature PL is red-shifted compared to the room temperature PL, which we contend is due to the dominant T 1 L emission at 77 K as conformational dynamics are expected to be essentially arrested at this temperature. The RTP spectrum of PXZ-Nap, collected with a time-gated window of 10-100 ms, is red-shifted (l em = 490 nm) and the t ph is shortened to 10.04 ms compared to that in 1 wt% doped PMMA (Figure ). A much more structured LTP emission at 77 K is observed (Figure ) compared to that of PXZ-Nap in PMMA (Figure ). The enhanced LE character (~400 nm) results from host-guest interactions between PXZ-Nap and mCP. For PTZ-Nap, a new high-energy RTP emission band at around 425-500 nm can be observed, with an associated t ph of 20.76 ms (Figure ). At 77 K, this emissive triplet state is suppressed compared to the main emission band; however, the lifetime at 475 nm significantly increases to 1.5 s (Figure ). The F PL values of PXZ-Nap and PTZ-Nap are improved to 10.8% and 7.7% in air compared to those in 1 wt% doped PMMA (1.3% and 5.3%, Tables ), respectively, values that are enhanced to 26.9% and 15.9% under vacuum, respectively (Table ). Therefore, the F PL associated with just the RTP (F RTP ) of PXZ-Nap and PTZ-Nap in mCP are no smaller than 16.1% and 8.2%, respectively. Compared with the F PL values of PXZ-Nap and PTZ-Nap in doped PMMA films, those in doped mCP films are enhanced 80.5 and 27.3 times, respectively (Figure ). This enhancement benefits from the Dexter energy transfer process occurring between the T 1 states of mCP and the guest emitters. These results demonstrate that different host-guest interactions can regulate triplet excited dynamics. To exclude that the triplet emission at around 525 nm originates from an aggregate, the prompt and delayed emission spectra were investigated in dilute 2-MeTHF glass (1´10 -6 M) at 77 K (Figure ). Notably, the structured phosphorescence in 2-MeTHF at 77 K for both compounds are at about the same energy, and also of similar energy to those measured in PMMA and mCP, which demonstrates that the phosphorescence from T 1 L does not originate from an aggregate. The DE ST values of PXZ-Nap and PTZ-Nap between S 1 and T 1 L in 2-MeTHF glass are 0.40 eV and 0.43 eV, respectively. Despite the difference between the measured DE ST in 2-MeTHF compared with the predicted value (Figure ), we can clearly see that in doped PMMA and mCP films DE ST between S 1 and T 1 H is almost degenerate, it is these values that coincide with the theoretical predictions.