At present, commercialized high-resolution OLED displays mainly employ a color filter or microcavity to reach the color purity required by broadcasting standards, which is inevitably accompanied by losses in device efficiency and energy utilization. Therefore, it would be of great utility to devise an emitter showing narrowband emission. Recently, Hatakeyama et al. introduced a subclass of TADF emitters coined multi-resonant TADF (MR-TADF), that show the desired narrowband emission. This is due to their rigid polyaromatic hydrocarbon (PAH) framework, where the rigid molecular skeleton and the short-range CT (SRCT) nature of the emissive singlet excited state enable simultaneously narrowband emission and small DE S1T1 . In this design, the electron densities of the HOMO and LUMO are localized on the adjacent atoms. Typically, boron/nitrogen (B/N) doped PAHs have emerged as the dominant design. For example, Hatakeyama and co-workers designed a B/N-doped MR-TADF compound, n-DABNA, that shows very narrowband emission (FWHM of 18 nm) at a photoluminescence (PL) maximum (l PL ) of 467 nm, with a PL quantum yield (F PL ) of 90%, and where the device's maximum external quantum efficiency (EQE max ) was an impressive 34.4%. A hyperfluorescence (HF) strategy has been extensively employed to mitigate efficiency roll-off in the device, wherein generally a TADF emitter serves as the assistant dopant to efficiently harvest excitons and then these are efficiently transferred to the terminal emitter, a compound showing narrowband emission and with a fast radiative rate constant, k r . Benefitting from these pioneering works, Lee and co-workers reported a hyperfluorescence (HF) device that showed an EQE max of 33.5% and a long operational device lifetime (LT 50 , 50% of initial luminance) of 9500 hours at 100 cd m -2 by employing n-DABNA as the terminal emitter and a triazine derivative PPCzTrz as the assistant dopant; the device with only n-DABNA as the emitter was much less stable (LT 50 = 31 hours). A similarly themed report by Adachi and co-workers documented an HF device using n- DABNA as the terminal emitter with an EQE max of 41% and a LT 50 > 10,000 hours. Besides HF, other strategies have been explored to improve the performance of MR-TADF OLEDs. For instance, Yang and co-workers proposed an effective strategy for mitigating aggregation-caused quenching (ACQ) by introducing sterically bulky groups onto a B/N-doped emitter. Carbonyl (C=O)/N-doped PAHs are a second family of MR-TADF emitters. Initial studies by Yuan et al. and us using DiKTa (Figure ) derivatives documented the potential of this design; however, the devices typically suffered from significant efficiency roll-off (> 30%) at 100 cd/m -2 and low maximum luminance (L max ≤ 13,000 cd/m -2 ). Recently, we demonstrated that by decorating DiKTa with various donors, the OLEDs showed an improved EQE max and reduced efficiency roll-off compared to the device with the parent emitter; however, this improvement in efficiency roll-off was not present beyond 100 cd/m -2 . Similarly, Zhang and co-workers reported three C=O/N-doped PAHs, QAD-Cz, QAD-2Cz, and QAD-mTDPA. Although having very similar molecular structures and near unity F PL , there is a stark dichotomy in the observed efficiency roll-off at 100 cd/m - 2 (QAD-Cz: 73.4%, QAD-2Cz: 12.5%, and QAD-mTDPA: 51.0%). These results provide insight that a high-lying triplet excited state (T n ) is likely responsible for the differences in reverse intersystem crossing rate constants, k RISC , and thus the different efficiency roll-off profiles. Improved MR-TADF design can be realized if we were above to experimentally determine the energy of these close-lying triplet states to the S 1 state. The employment of transient absorption spectroscopy can be used to detect the existence of T n states, but it lacks the capability of quantifying the energy of these states. Very recently, transient electron paramagnetic resonance spectroscopy (EPR) was employed by Evans and co-workers to reveal that multiple triplet excited states aid to mediate delayed fluorescence. In our previous work, we utilized transient EPR to distinguish the T 2 and T 1 states of the anthracene derivative DPAAnCN and revealed that exciton harvesting proceeded by a hybridized local and charge-transfer (HLCT) fluorescence mechanism. Despite these advances, it remains challenging to map out the triplet state landscape.