Carborane-Based Optoelectronically Active Organic Molecules: Wide Band Gap Host Materials for Blue Phosphorescence

Carborane-based host materials were prepared to fabricate deep blue phosphorescence organic light-emitting diodes (PHOLEDs), which constituted three distinctive geometrical structures stemming from the corresponding three different isomeric forms of carboranes, namely, <i>ortho</i>-, <i>meta</i>-, and <i>para</i>-carboranes. These materials consist of two carbazolyl phenyl (CzPh) groups as photoactive units on each side of the carborane carbons to be bis­[4-(<i>N</i>-carbazolyl)­phenyl]­carboranes, <b><i>o</i>-Cb</b>, <b><i>m</i>-Cb</b>, and <b><i>p</i>-Cb</b>. To elaborate on the role of the carboranes, comparative analogous benzene series (<b><i>o</i>-Bz</b>, <b><i>m</i>-Bz</b>, and <b><i>p</i>-Bz</b>) were prepared, and their photophysical properties were compared to show that advantageous photophysical properties were originated from the carborane structures: high triplet energy. Unlike <b><i>m</i>-Bz</b> and <b><i>p</i>-Bz</b>, carborane-based <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b> showed an unconjugated nature between two CzPh units, which is essential for the blue phosphorescent materials. Also, the carborane hosts showed high glass transition temperatures (<i>T</i>_g) of 132 and 164 °C for <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>, respectively. Albeit <b><i>p</i>-Cb</b> exhibited slightly lower hole mobility when compared to <b><i>p</i>-Bz</b>, it still lies at the high end hole mobility with a value of 1.1 × 10^–3 cm²/(V s) at an electric field of 5 × 10⁵ V/cm. Density functional theory (DFT) calculations revealed that triplet wave functions were effectively confined and mostly located at either side of the carbazolyl units for <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>. Low-temperature PL spectra indeed provided unequivocal data with higher triplet energy (<i>T</i>₁) of 3.1 eV for both <b><i>m</i>-Cb</b> and <b><i>p</i>-Cb</b>. <b><i>p</i>-Cb</b> was successfully used as a host in deep blue PHOLEDs to provide a high external quantum efficiency of 15.3% and commission internationale de l’elcairage (CIE) coordinates of (0.15, 0.24)