We designed and synthesized two novel thermally activated delayed fluorescence emitters,3-CzAIAd and 3-DPFCzAIAd,and performed a comparative analysis with their isomeric counterparts,4-CzAIAd and 4-DPFCz-AIAd.The substitution position of the carbazole unit plays a critical role in modulating the photophysical properties of the emitters and the device performance of organic light-emitting diodes.Emitters with carbazole units substituted at the 3-position of phthalimide group exhibited higher triplet energy levels,smaller singlet-triplet energy gaps,lower photoluminescence quantum yields,and more red-shifted emission spectra.Devices incorporating 3-DPFCzAIAd achieved a maximum external quantum efficiency of 20.8%,a peak current efficiency of 57.1 cd·A⁻¹,and an opera-tional half-lifetime of 260 h under an initial luminescence of 500 cd·m⁻2.Conversely,while devices based on 4-DPF-CzAIAd exhibited a higher peak EQE of 28.2%,they experienced a shorter operational lifetime of 178 h under an ini-tial luminescence of 500 cd·m⁻2 and more pronounced efficiency roll-off of up to 77%.
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