The electron inductive effect of CF₃ on penta-carbazole containing blue emitters: Trade-off between color purity and luminescent efficiency in TADF OLEDs

A blue thermally activated delayed fluorescence (TADF) emitter 4'-(trifluoromethyl)-[1,1'-biphenyl]-2,3,4,5,6-pentayl-pentakis(9H-carbazole) 5CzDPhCF₃ is designed by involving the non-conjugated CF₃ moiety as electron-withdrawing unit, which dominates the acceptor properties by the electron inductive effect instead of other widely reported conjugated effects in TADF materials. Inserting a simple phenyl bridge into the light blue TADF emitter of (6-(trifluoromethyl)benzene-1,2,3,4,5-pentayl)pentakis(9H-carbazole) (5CzPhCF₃) to dilute the electron-withdrawing capability and reduce D-A interactions, significantly hypsochromicly shifted emission by 48 and 64 nm is observed in 5CzDPhCF₃ with deep blue emission peak at 435 and 431 nm in CH₂Cl₂ and film, respectively. 5CzDPhCF₃ exhibits transient lifetime of 6.9 ns and 0.14 μs, comparative HOMO of ∼ -5.55 eV, whereas higher LUMO of −2.25 eV than −2.56 eV of the control 5CzPhCF₃. The triplet energy and singlet-triplet energy gap (ΔE_ST) for 5CzDPhCF₃ is 2.95 and 0.23 eV respectively, higher than 2.80 and 0.02 eV of 5CzPhCF₃. And the photoluminescence quantum yield measured for 5CzDPhCF₃ and 5CzPhCF₃ is 22/27% and 24/43% in toluene at ambient/N₂ atmosphere, respectively. Due to the larger ΔE_ST for less efficient reverse intersystem crossing in 5CzDPhCF₃ for TADF emission, a trade-off between the color purity and electroluminescent (EL) efficiency can be found. A maximum external quantum efficiency of 2.0 and 9.5% is achieved in the vacuum-deposited TADF devices based on 5CzDPhCF₃ and 5CzPhCF₃ emitter, respectively. And the phenyl-bridged 5CzDPhCF₃ containing device displays pure deep blue EL emission with satisfactory CIE of (0.157, 0.066), while 5CzPhCF₃ only shows light blue emission with CIE of (0.175, 0.327). To the best of our knowledge, the CIE of 5CzDPhCF₃ is among the only few examples on deep blue TADF emission with CIE(y) values < 0.07.