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 (<b>1</b>) or in positions 3, 5, 3′, and 5′ of the biphenyl subunit (<b>2</b>), and the fixed spatial arrangement was confirmed by X-ray analysis. The physical properties of CBP derivatives based on parent structure <b>2</b> were further tailored by electron-withdrawing CF₃ groups in positions 3 and 6 (<b>3</b>) or positions 2 and 7 of the carbazole subunits (<b>4</b>) or alternatively by electron-donating CH₃O groups in positions 2 and 7 (<b>5</b>) of the same building blocks. Increased triplet energies (<i>E</i>_T) compared to that of the parent compound CBP were found for all synthesized CBP derivatives <b>1–5</b>. Enhanced glass transition temperatures ranging between 129 and 202 °C further corroborate the application potential of these derivatives for matrix material in blue OLEDs