Considering aggregation can induce conformational changes, we next investigated the photophysics of the compounds at a higher concentration in PMMA films (10 wt%) to investigate the influence of intermolecular interactions on the phosphorescence behavior. Compared to the 1 wt% doped films of PXZ-Nap and PTZ-Nap in PMMA, a spectral red-shift of the steady-state PL occurs (Figures and). For PTZ-Nap, the CT emission at around 505 nm is enhanced, associated with a 13.06 ns lifetime, compared with the LE emission at 400 nm (tPL = 5.02 ns) (Figure , Table ). The 10 wt% film of PXZ-Nap in PMMA exhibits similar RTP and LTP behavior to the 1 wt% doped film in PMMA, but with a shorter associated RTP lifetime (tPh = 42.1 ms) (Figures and). Due to the similarity in RTP spectra with those of the 1 wt% doped films, it can be concluded that there is no additional contribution to the emission from aggregates. The distinct afterglows of the 10 wt% doped PMMA films of PXZ-Nap at 298 K and 77 K also indicate emissions from two different T 1 states (Figure ). Similarly, temperature-dependent, and timegated-dependent PL spectra also reveal the dual phosphorescence from T 1 H and T 1 L (Figures ). At room temperature, the 10 wt% PTZ-Nap doped PMMA film is more emissive than the 1 wt% PTZ-Nap doped PMMA film, which is reflected also in the much longer RTP lifetime (Figure ) and the increased F PL of the RTP (2.1% vs. 0.3%, Table ). The LTP recorded at 77 K is more structured and is 33 nm red-shifted compared with the RTP (Figure ). The temperaturedependent delayed emission decay at 500 nm shows two distinct regimes. There is a shorter component that is thermally activated and that we assign to TADF (Figure ), which likely results from RISC from T 1 H to S 1 as these two states are nearly degenerate, while the other longlived component (up to sub-second) originates from phosphorescence. Compared with PXZ-Nap, the improved FPL (Table ) of PTZ-Nap is ascribed to the suppression of nonradiative decay processes. Based on these photophysical results, the intermolecular interactions present at higher doping concentrations are responsible for the modulation of the T 1 H and T 1 L emissions by influencing the T 1 geometry. We next investigated how the conformational dynamics affects the T 1 energy. A relaxed potential energy surface scan modelling for T 1 conformations was conducted to rationalize the observed dual phosphorescence mechanism (Figure ). The dihedral angle between the donor and Nap was progressively modulated. From Figure , the interconversion barrier (transition state, TS) between the two conformers is sufficiently small and thus the population of the T 1 H conformer dominates. Thus, under photoexcitation, the populated singlet excitons intersystem cross and eventually relax to the lowest energy T 1 L state (Figure ). At ambient temperature, the T 1 L excitons can be thermally activated to access the T 1 H and from this state RTP is observed. This process is suppressed at low temperature, resulting in the observed T 1 L phosphorescence. Finally, benefitting from the significant difference in the spectral afterglow response to temperature of PXZ-Nap doped in PMMA at 1 wt% and 10 wt%, these films were envisioned to act as temperature sensors. As shown in Figure , the afterglow gradually varies from blue to green over the temperature range of 77-295 K. Considering most COVID-19 vaccines must be stored below room temperature to be stable (Pfizer-BioNTech COVID-19: £ -70 °C; Moderna COVID-19 vaccine: ≤ -20 °C; CoronaVac vaccine-SARS-CoV-2: 2-8 °C), we demonstrate how our RTP system could be employed as a cost-effective temperature sensor for monitoring the environment temperature in real time during the ultracold chain logistics and storage (Figure ).