Similarly, the emitters Pery-2PXZ and Pery-2PTZ, reported by Li et al., contain a perylene core substituted with either phenoxazine or phenothiazine as donor groups, respectively. Each of these four emitters has a large ΔEST of > 0.6 eV (in DPAAnCN and CzAnCN this ΔEST is reported from TD-DFT, while in Pery-2PXZ and Pery-2PTZ it is calculated by the authors from the difference in energy of the TD-DFT-calculated T1 energy and the measured onset of fluorescence ), which is too large for efficient TADF. Instead, these emitters operate via alternative triplet harvesting mechanisms. In the case of DPAAnCN and CzAnCN, the exciton harvesting mechanism was hypothesised to employ a 'hot-exciton' channel, while the perylene derivatives Pery-2PXZ and Pery-2PTZ were shown to operate via triplet-triplet-annihilation (TTA). Devices based on these hot-exciton emitters achieved appreciable EQEmax of 6.0% and 4.0% for the OLEDs with DPAAnCN and CzAnCN, respectively, 23 while no devices were fabricated based on the TTA emitters. Finally, a family of pyrene derived emitters reported by Park et al., exemplified by 3Me-1Bu-TPPDA, which possesses a very large ΔEST of 0.84 eV and was reported to be fluorescent. However, it should be noted that an OLED based on this emitter achieved an EQEmax of 9.3%, in excess of the 5% limit for fluorescent devices. This strongly suggests that a triplet-harvesting mechanism is operational in the device.