Systematically tuning of optoelectronic properties from electron donating to accepting substituents on bicarbazole/cyanobenzene hybrids: Host to dopant materials for phosphorescent and delayed fluorescence OLEDs

Six bicarbazole/cyanobenzene hybrid compounds are designed and synthesized by changing the substituents from electron-donating phenylcarbazole (PCzCNBCz, 1), phenoxy (OPCNBCz, 2) and methyl (MCNBCz, 3), neutral hydrogen (HCNBCz, 4) to electron-accepting trifluomethyl (CF₃CNBCz, 5) and cyano (DCNBCz, 6) moieties, respectively at the same meta-position of cyanobenzene (refer to carbazole). The substituted units are judiciously designed with both conjugated (phenylcarbazole and cyano) and non-conjugated (phenoxy, methyl and trifluomethyl) units. Their HOMO and LUMO energy levels are rationally adjusted from 5.17 to 5.46 eV and 2.13–2.50 eV, respectively. Compared to bare hydrogen-attached 4 with triplet energy (T₁) of 2.7 eV, the conjugated moiety based 1 and 6 lowered T₁ to ≤2.50 eV, while other compounds bearing either non-conjugated donating or accepting units unexpectedly maintain T₁ as high as ∼2.70 eV. Moreover, the singlet-triplet bandgaps (ΔE_ST) can also be tailored from 0.01 to 0.32 eV, therefore, delayed fluorescence characteristics are observed in the electron-accepting trifluomethyl and cyano substituted 5 and 6. The high triplet 2, 3 and 4 are served as host materials for blue phosphorescent OLEDs, with maximum external quantum efficiency (EQE) up to 20.5%, while 5 and 6 are used as sky-blue and greenish-blue emitters in delayed fluorescence OLEDs, with maximum EQE of 4.9 and 10.0%, respectively.