Tuning of spin-flip efficiency of blue emitting multicarbazolyl-substituted benzonitriles by exploitation of the different additional electron accepting moieties

Abstract

Aiming to improve spin-flip efficiency of emitters exhibiting thermally activated delayed fluorescence (TADF), to five derivatives of tetra-tert-butyl-carbazolyl-substituted benzonotriles the different additional electron accepting moieties were attached, i.e. benzonitrile, benzotriazole, 2-benzotrifluoride, methyl 2-benzoate or methyl 3-methylbenzoate (compounds named as CNCN, CN4T, CNCF3, CNCOA and CNCOAM, respectively). The selection of the additional acceptor moieties provided not only different electron accepting abilities but also enabled different non-covalent intramolecular interactions (e.g., due to C–H···F, O, N hydrogen bonds) which affected emission intensity. Despite the similarity of the molecule structures of the compounds, they demonstrated very different TADF efficiencies with reverse intersystem crossing rates in the wide range from 0.047 × 106 to 2.32 × 106 s−1 observed for their oxygen-free toluene solutions. These solutions exhibited photoluminescence spectra peaking in the range from 456 to 506 nm and photoluminescence quantum yields reaching 82%. The effect of the additional acceptor substituents was on electroluminescent properties of OLEDs based on the designed TADF emitters was observed. External quantum efficiencies of non-doped and doped devices based on the newly synthesized emitters decreased in order of CNCN > CNCOAM > CNCOA > CNCF3 > CN4T. The best blue devices exhibiting electroluminescence peaked at 468 and 457 nm with CIE coordinates of (0.15, 0.17) and (0.15, 0.13), respectively, showed external quantum efficiency of 18.3 and 14.1% which was considerably higher than 11.5% observed for the reference device which was fabricated using a previously published TADF emitter and showed electroluminescence intensity maximum at 480 nm.