Construction of Highly Efficient Carbazol-9-yl-Substituted Benzimidazole Bipolar Hosts for Blue Phosphorescent Light-Emitting Diodes: Isomer and Device Performance Relationships

Four isomers of carbazol-9-yl-substituted 1,2-diphenylbenzimidazole at 7, 6, 5, and 4 positions, named as <b>1-CbzBiz</b>, <b>2-CbzBiz</b>, <b>3-CbzBiz</b>, and <b>4-CbzBiz</b>, respectively, have been synthesized. Instead of having an identical molecular weight, the <b>CbzBiz</b>’s have their glass transition temperatures (<i>T</i>_g) spanning a large range from 53 to 90 °C. Their <i>T</i>_g and melting point (<i>T</i>_m) basically obey the Boyer–Kauzmann rule (<i>T</i>_g = <i>g</i>·<i>T</i>_m with <i>g</i> ≈ 0.7) on the absolute temperature scale (in kelvins). However, while <b>1-CbzBiz</b> and <b>4-CbzBiz</b> demonstrate a small <i>g</i> value of 0.66, which is significantly smaller than other common organic glass molecules, <b>3-CbzBiz</b> shows an unexpectedly high <i>g</i> value of 0.73, implying higher intermolecular interactions in the glass. These <b>CbzBiz</b>’s are suitable hosts for bis­[2-(4,6-difluorophenyl)­pyridinato-<i>C</i>²,<i>N</i>]­(picolinato)­iridium­(III) (FIrpic) in phosphorescence organic light-emitting diodes (PhOLEDs). The optimized PhOLED of indium tin oxide/4,4′-cyclohexylidenebis­[<i>N</i>,<i>N</i>-bis­(4-methylphenyl)­benzenamine]/<b>4Cbz</b>/<b>4-CbzBiz</b>-FIrpic­(15%)/diphenylbis­(4-(pyridin-3-yl)­phenyl)­silane/LiF–Al shows a maximum brightness of 18 760 cd/m², a current efficiency of 64.1 cd/A, and the external quantum efficiency of 30.9%.