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

Abstract

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 (ΔEST) 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 ΔEST 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.