D-A-D-type bipolar host materials with room temperature phosphorescence for high-efficiency green phosphorescent organic light-emitting diodes

Bipolar host materials with donor (D)-acceptor (A) structures play an important role in high-efficiency phosphorescent organic light emitting diodes (PhOLEDs) owing to the excellent carrier transport capability, which can be optimized by a tight and regular packing mode in the molecular aggregation. Notably, this arrangement is also beneficial for enhancing intersystem crossing channels and reducing the non-radiative transition for realizing room temperature phosphorescence (RTP). On this basis, RTP materials with a rigid packing mode as the host could be utilized to prepare high-quality devices. Herein, we design and synthesize two bipolar D-A-D molecules constructed by utilizing pyrimidine as an acceptor unit, and dibenzothiophene and dibenzofuran as the donor units, named MDBT and MDBF, respectively, exhibiting considerable RTP performance. In particular, MDBT possesses a longer phosphorescence lifetime (25.1 ms) than MDBF (4.9 ms). From the single crystal analysis, MDBT has a relatively planar configuration through the intramolecular S-N bond. Meanwhile, it is arranged in a compact mode due to the presence of S-S, C-H⋯S and S-π bonds, which suppress the molecular vibration and rotation. Moreover, the rigid structure of MDBT is also certified to be conducive to the formation of a nanoscale high-quality film and charge transport. Significantly, the MDBT-based device doped with 8 wt% Ir(ppy)₂(acac) exhibits a very low turn-on voltage (2.7 V) and a high electroluminescence efficiency (85.00 cd A^-1, EQE = 23.03%). Even at the practical luminance of 5000 cd m^-2, the efficiencies remain high (81.09 cd A^-1, EQE = 21.97%), making MDBT a promising candidate as the host material for green PhOLEDs.