Effect of ortho-biphenyl substitution on the excited state dynamics of a multi-carbazole TADF molecule

Abstract

We report a new thermally activated delayed fluorescence (TADF) molecule developed by the ortho-biphenyl substitution of a multi-carbazole TADF molecule. The new TADF molecule, 4mCzBN-BP, is composed of four dimethylcarbazole donors, a benzonitrile acceptor core and an ortho-biphenyl triplet scaffold. 4mCzBN-BP exhibits a bluish-green emission with a peak wavelength at 491 nm and a very high photoluminescence quantum yield (PLQY) of 95% in an mCBP host. To investigate the effect of the ortho-biphenyl substitution on the excited-state dynamics and photophysical properties of the TADF molecule, we performed time dependent density functional theory (TD-DFT) calculations of 4mCzBN-BP and two multi-carbazole TADF molecules with similar molecular structures. A local excited triplet state ((LE)-L-3) with an energy (2.81 eV) close to the lowest singlet charge-transfer state ((CT)-C-1) can be formed at the biphenyl of 4mCzBN-BP. However, due to the planarization of the biphenyl at the triplet state and a large steric hindrance caused by adjacent carbazole donors, (LE)-L-3 at the biphenyl can only be formed by a subset of possible conformations of 4mCzBN-BP molecules in the solid mCBP host, leading to a multiexponential decay of the delayed fluorescence. Color coordinates of (0.196, 0.452), a high external quantum efficiency of 23.7% and a long operational lifetime (LT50) of 750 hours were achieved with the organic light emitting diodes based on 4mCzBN-BP.