Tuning the optoelectronic properties of carbazole/ oxadiazole hybrids through linkage modes: Hosts for highly efficient green electrophosphorescence

A series of bipolar transport host materials: 2,5-bis(2-(9H-carbazol-9-yl) phenyl)-1, 3, 4-oxadiazole (o-CzOXD) (1), 2,5-bis(4-(9H-carbazo(-9-yI)phenyl)-l, 3, 4-oxadiazole (p-CzOXD) (2), 2,5-bis(3-(9H-carbazoI-9-yl)phenyI)-l, 3, 4-oxadiazole (m-CzOXD) (3) and 2-p-(9H-carbazol-9-yl)phenyI)-5-(4-(9H-carbazoI- 9-yllphenyl)-1, 3, 4-oxadiazole (op-CzOXD) (4) are synthesized through simple arp,atoc nucleophtic substitution reactions. The incorporation of the oxadiazole moiety greatly improves their morphological stability, with T_d and T_g in the range of 428-4640C and 97-133 °C, respectively. The ortho and meta positions of the 2, 5- dipheny intramolecular charge transfer and a higher triplet energy compared to the paraposition linked analogue (2). The four compounds exhibit similar LUMO levels (2.55-2.59eV) to other oxadiazole derivatives, whereas the HOMO levas vary in a range from 5.55 eV to 5.69eV, depending on the linkage modes. DFT. calculation results indiote that 1, 3, and 4 have almost complete separation of their HOMO and LUMO levels at the hole- and electron-transporting moieties, while 2 exhibits only partial separation of the HOMO and LUMO levels possibly due to intramolecular charge transfer. Phosphorescent organic light-emitting devices fabricated using 1-4 as hosts and a green emitter, lr(ppy)3 or (ppy)₂ lr(acac), as the guest exhibit good to excellent performance Devices hosted by o-CzOXD (1) achieve maximum current efficiencies (η_c) as high as 77.9 cd A_-1 for (ppy)2lr(acac). The excellent device performance may be attributed to the well-matched energy levels between the host and hole-transport layers, the high triplet energy of the host and the complete spatial separation of HOMO and LUMO energy levels.