Chromenopyrazole-Based Bipolar Blue Host Materials for Highly Efficient Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

New electron-acceptor cores are necessary for developing highly efficient bipolar hosts, particularly for blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Herein, chromenopyrazole (CP) was used for the first time as an electron-acceptor core to design and synthesize two novel blue bipolar hosts, viz., 8-(9H-carbazol-9-yl)-3-methyl-1-phenylchromeno4,3-cpyrazol-4(1H)-one (CzCP) and 8-(9H-3,9′-bicarbazol-9-yl)-3-methyl-1-phenylchromeno4,3-cpyrazol-4(1H)-one (2CzCP). The influence of donor strength on the photophysical, electrochemical, and electroluminescent performances was systematically investigated. CzCP and 2CzCP both maintain high triplet energy (∼3.0 eV), appropriate highest occupied and lowest unoccupied energy levels (HOMO/LUMO), and bipolar nature. Consequently, OLEDs containing CzCP as a host in the emissive layer exhibited state-of-the-art performance with external quantum efficiency of 27.9% and CIE color coordinates of (0.15, 0.21), thus achieving excellent performance among all reported blue host materials in TADF-OLEDs. This work highlights the importance of the CP unit in developing new host materials and paves the way for the realization of high-efficiency blue TADF-OLEDs.