In this paper, two new pyridazine based donor-acceptor type materials, i.e., 3CzPyaPy: 9,9'-(3-(6-(9H-carbazol-9-yl)pyridazin-3-yl)pyridine-2,6-diyl)bis(9H-carbazole) and 4CzPyPyaPy: 3,6-bis(2,6-di(9H-carbazol-9-yl)pyridin-3-yl)pyridazine, were synthesized with high yields. These two materials exhibited strong absorption/emission with high molar extinction coefficients and moderate photoluminescence quantum yield. The glass transition temperature of 3CzPyaPy was detected to be as high as 131 °C, showing its high thermal stability. Although the absorption energies and oxidation/reduction behaviors of the two materials were similar, the emission from 4CzPyPyaPy with longer effective-conjugation length presented hypsochromic shift both in films and in dilute solutions, contradicting to the common sense. The single crystal structure study disclosed their different space stretching and packing: 3CzPyaPy was twisted in larger angles and adopted dimerlike packing, while 4CzPyPyaPy showed smaller torsion angles and exhibited slipped herringbone packing. The dimerlike packing in 3CzPyaPy is responsible for its bathochromic shift of emission in solid state, while its unsymmetrical molecular structure accounts for that in solution. We believe that the unsymmetrical molecular structure of 3CzPyaPy is partially responsible for its high thermal-stability and also responsible for its HOMO dispersion which renders it slightly more difficult to oxidize. 3CzPyaPy was proved to be a bipolar-transport material and when served as a phosphor host, a green phosphorescent device achieved maximum efficiencies of 54.0 cd A, 42.4 lm W, and 17.7%, which are among the best with nonoptimized device structure, demonstrating its great potential for optoelectronic application. Furthermore, the new synthesized pyridazine derivatives and the corresponding structural and molecular-packing influences on material properties give a new insight into molecule tailoring.

A bipolar host material, 2,4,6-tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ), was synthesized according to reported method. Due to the higher triplet energy compared to green and red phosphors, TCPZ is suitable to host them in phosphorescent organic light-emitting diodes (PhOLEDs). Although the triplet of TCPZ is slightly lower than a common blue phosphor, good blue PhOLEDs using TCPZ as the host were successfully demonstrated in this work. By low temperature emission measurement, it was found that the energy splitting between the singlet and triplet of TCPZ is as small as 0.24 eV. Therefore, thermal activated energy transfer from triplet to singlet in the host TCPZ is expected to occur, which can be afterwards efficiently transferred to the blue phosphor, hence enabling it to host blue phosphor. As a result, TCPZ can be used as host for phosphors in panchromatic range. Additionally, single-carrier devices clearly prove its good bipolar transport feature, beneficial to device performance. By using TCPZ as a host, high performance deep-red, green and blue PhOLEDs have been achieved, with maximum efficiencies of 9.3 cd•A 1 (13.2%), 81.3 cd•A 1 (23.1%) and 17.03 cd•A 1 (10.4%), respectively.

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