Molecular Configuration Fixation with C–H···F Hydrogen Bonding for Thermally Activated Delayed Fluorescence Acceleration

The Bigger PictureOrganic light-emitting diodes (OLEDs) have been commercially applied in flat-panel displays because of their high efficiency, low cost, and flexibility. Generally, triplet excitons are typically non-emissive, lower in energy, and longer lived than singlet ones, and could not be effectively used in traditional fluorescent OLEDs. An alternative emission mechanism named thermally activated delayed fluorescence (TADF) has been extensively illustrated and can realize theoretically 100% internal quantum efficiency without involving noble-metal complexes. Nevertheless, the increase of reverse intersystem crossing efficiency might lead to the decrease of singlet radiation, thus greatly affect the optoelectronic performance of OLED devices. Here, we demonstrated a highly twisted multi-carbazolyl compound with both intra- and inter-molecular hydrogen bonds to fix the excited-state configurations, aiming to establish multiple triplet-to-singlet conversion channels for high-efficiency TADF OLEDs.Highlights•Intramolecular hydrogen bonds fix excited-state configurations•Multiple triplet-to-singlet conversion channels accelerate TADF•High RISC rate constant ~106 s−1 and photoluminescence quantum efficiency of 94%•Achieving 20% efficiency in both unipolar and ambipolar hostsSummaryThe conflict between improving reverse intersystem crossing (RISC) and singlet radiation is one of the key fundamental issues for thermally activated delayed fluorescence (TADF) materials. Here, we demonstrate that the excited-state structural relaxation can be effectively suppressed through fixing the molecular configuration with intramolecular C–H···F hydrogen bonds. The resulted TADF emitter 7CzFDCF3DPh reveals the nearly identical ground state, the first singlet (S1) and triplet (T1) excited-state configurations. Therefore, the singlet-triplet energy gap is reduced to <0.06 eV, establishing the multiple RISC channels through either electronic or vibrational couplings. 7CzFDCF3DPh reveals a state-of-the-art RISC rate constant (kRISC) of ~106 s−1, accompanied by the doubled RISC efficiency (φRISC) as high as 97%. The dramatically accelerated TADF renders the photoluminescence and electroluminescence quantum efficiencies more than 90% and 20% for 7CzFDCF3DPh. These results manifest the significance of the configuration fixation for improving TADF performance.Graphical abstract