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In this report, several benzoylpyridine-carbazole based fluorescence materials bearing carbazolyl or 4-(t-butyl)carbazolyl groups at the ortho, meta and para carbons of the benzoyl ring, were synthesized and studied for their TADF properties. Some of these molecules show very small ΔEST < 0.05 eV and transient PL characteristics indicate that they are thermally activated delayed fluorescence (TADF) materials. In general, they show low fluorescence efficiencies in solutions, but the efficiencies increase drastically in the thin films with some reaching more than 90%. For examples, o- and m-dicarbazolyl substituted DCBPy (2,5-di(9H-carbazol-9-yl)phenyl)(pyridin-4-yl)methanone) and DTCBPy ((3,5-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)(pyridin-4-yl)methanone) in cyclohexane show fluorescence efficiencies of 14 and 36%, but in the thin films, the values increase to 88.0 and 91.4%, respectively. Based on the TDDFT calculation, the HOMOs of DCBPy and DTCBPy are mainly distributed over the two carbazolyl groups and slightly extended to the phenyl ring. The LUMOs are mostly localized on the BPy core and slightly extended to the phenyl ring. There is a small degree of spatial overlap between the HOMO and LUMO in these two molecules. The OLEDs using DCBPy and DTCBPy as dopants emit blue and green light with EQEs of 24.0 and 27.2%, respectively, and with low efficiency roll-off at practical brightness level. The crystal structure of DTCBPy reveals a substantial interaction between the ortho donor (carbazolyl) and acceptor (4-pyridylcarbonyl) unit. The interaction between donor and acceptor substituents likely plays a key role to achieve very small ΔEST with high photoluminescence. In addition to the above two compounds, we also prepared a series of different benzoylpyridine-carbazole derivatives, the results will also be reported.
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