New 4,4'-Bis(9-carbazolyl)-Biphenyl Derivatives with Locked Carbazole-Biphenyl Junctions: High-Triplet State Energy Materials

We synthesized a series of 4,4'-bis(9-carbazolyl)-biphenyl (CBP) derivatives, using methyl groups as spatially demanding groups, locking the angle between the carbazole subunit and the biphenyl backbone as potential matrix material for blue organic light-emitting diodes (OLEDs). The locked rotation was achieved by four methyl groups either in positions 1 and 8 of the carbazole subunit (1) or in positions 3, 5, 3', and 5' of the biphenyl subunit (2), and the fixed spatial arrangement was confirmed by X-ray analysis. The physical properties of CBP derivatives based on parent structure 2 were further tailored by electron-withdrawing CF3 groups in positions 3 and 6 (3) or positions 2 and 7 of the carbazole subunits (4) or alternatively by electron-donating CH3O groups in positions 2 and 7 (5) of the same building blocks. Increased triplet energies (ET) compared to that of the parent compound CBP were found for all synthesized CBP derivatives 1-5. Enhanced glass transition temperatures ranging between 129 and 202 °C further corroborate the application potential of these derivatives for matrix material in blue OLEDs.