4-Pyridyl-9,9′-spirobifluorenes as Host Materials for Green and Sky-Blue Phosphorescent OLEDs

We report herein new pyridine-substituted spirobifluorene (SBF) dyes, i.e., 4-(9,9'-spirobi[fluoren]-4-yl)pyridine (<bold>4-4Py-SBF</bold>), 3-(9,9'-spirobi[fluoren]-4-yl)pyridine (<bold>4-3Py-SBF</bold>), and 2-(9,9'-spirobi[fluoren]-4-yl)pyridine (<bold>4-2Py-SBF</bold>), built on the association of the 4-substituted spirobifluorenyl core and various regioisomers of pyridine. These organic semiconductors possess high triplet energy levels (E-T around 2.7 eV) in accordance with their use as hosts for green and sky-blue phosphorescent organic light-emitting diodes (PhOLEDs). These dyes have been synthesized from the 4-bromo-spirobifluorene (<bold>4-Br-SBF</bold>) platform, obtained from a new and efficient synthetic approach using the promising building block 4-bromofluorenone as a key intermediate. Synthesis, structural, thermal, electrochemical, and photophysical properties of the three dyes have been investigated in detail and compared to other model compounds, namely, 4-phenyl-SBF (<bold>4-Ph-SBF</bold>), 2-phenyl-SBF (<bold>2-Ph-SBF</bold>), and 2,4-pyridyl-SBF (<bold>2-4Py-SBF)</bold>, in order to precisely study the influence of (i) the pyridine unit, (ii) the position of the nitrogen atom within the pyridyl core (in position 4, 3, or 2), and (iii) the substitution at the C4 position of SBF. This rational structure-properties relationship study sheds light on the effect of the substitution in position 4 of the SBF core and may pave the way to the development of such materials in electronics. Finally, the high performance of green PhOLEDs, ca. 63 cd/A, and of sky-blue PhOLED, ca.16 cd/A, clearly evidence the potential of these new SBF derivatives as hosts for phosphorescent dopants.