Synthesis and characterization of novel phosphorescent host materials based on triphenylpyridine derivatives

Three novel phosphorescent host materials, 4 ',4'-(4-phenylpyridine-2,6-diyl)bis(N,N-diphenyl-[1,1 '-biphenyl]-4-amine) (P1), 2,6-bis(4 '-(naphthalen-1-yl)-[1,1 '-biphenyl]-4-yl)-4-phenylpyridine (P2), and 2,6-bis(3 ',5 '-dimethyl-[1,1 '-biphenyl]-4-yl)-4-phenylpyridine (P3), based on triphenylpyridine with a symmetrical molecular conformation were designed and synthesized from 2,6-bis(4-bromophenyl)-4-phenylpyridine (3) reacting with [4-(diphenylamino)phenyl]boronic acid (A1), [4-(naphthalen-1-yl)phenyl]boronic acid (A2), and (3,5-dimethylphenyl)boronic acid (A3) through Suzuki coupling reaction, respectively. The compounds obtained were characterized by elemental analysis, H-1 NMR, and C-13 NMR spectroscopy. The thermal properties of these materials were studied by thermogravimetric analysis and differential scanning calorimetry measurements, which showed that they exhibited excellent thermal stability with glass transition temperatures over 94 degrees C. Their photophysical properties were also investigated, and the results indicated that all compounds possessed considerably high triplet energy level greater than 2.6 eV; they also emitted blue fluorescence in different solvents. Finally, the results of the density function theory calculations revealed that the compound P1 showed the well-separated frontier molecular orbital, which indicated that it possessed bipolar carrier transport ability for holes and electrons, whereas the compound P2 and P3 exhibited greater overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.