Materials containing 9H-carbazole and 2,5-diphenyoxadiazole functionalities have been widely exploited in organic electronic applications. Only very recently, however, have they been combined to obtain fluorophores exhibiting thermally activated delayed fluorescence (TADF), a process exploited in efficient, third-generation organic light-emitting diodes. A study of donor-acceptor compounds comprised of 9H-carbazole and 2,5-diphenyl-1,3,4-oxadiazole moieties is presented with an emphasis on developing an understanding of how structure affects properties relevant to TADF. An orbital understanding of the lowest energy singlet and triplet excited states is established wherein the singlet state involves a HOMO to LUMO intramolecular charge transfer state and the triplet state is a locally excited state confined largely on the diphenyloxadiazole moiety. The orbital character of the states is confirmed through sensible structural modifications that preferentially affect the energy one of the two states, reducing the energy between them and allowing for the observation of TADF. OLEDs utilizing these fluorophores as the emitter are shown to exhibit external quantum efficiencies well above what is possible if only singlet excitons are being converted to light, confirming the conversion of triplet excitons to photons in the devices. The findings show the limitations of using these materials as blue-emitters in OLEDs and provides strategies for overcoming these restrictions through rational molecular design. Further optimized structures as well as application of the results to other donor-acceptor systems is discussed.
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Excited state design of carbazole-oxadiazole compounds for thermally activated delayed fluorescence
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