Phosphonate substituted 4,4′-bis(N-carbazolyl)biphenyl with dominant electron injection/transport ability for tuning the single-layer device performance of self-host phosphorescent dendrimer

A novel phosphonate substituted 4,4'-bis(N-carbazolyl)biphenyl (CBP), namely PCBP, has been designed and successfully synthesized by an indirect palladium catalyzed Suzuki-Miyaura reaction. X-Ray crystallography analysis from a PCBP single crystal demonstrates that there is a hydrogen bond interaction between the two adjacent molecules due to the presence of phosphonate, which promotes their one-dimensional line arrangement along the c-axis. Compared with the prototype CBP (-5.55 eV), in addition, the highest occupied molecular orbital (HOMO) level of PCBP is reduced to -6.00 eV, leading to a large hole injection barrier. On the other hand, the introduction of phosphonate substitutes can endow PCBP with excellent electron injection/transport ability. As a result, PCBP shows an electron-dominated behaviour observed in single carrier devices, which is different from the hole-dominated one for CBP. Such a transition is then used to tune the single-layer device performance of a self-host phosphorescent dendrimer, and the peak luminous efficiency significantly increases from 1.7 cd A(-1) of CBP to 31.4 cd A(-1) of PCBP.