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, ortho-, meta-, and para-carboranes. These materials consist of two carbazolyl phenyl (CzPh)
groups as photoactive units on each side of the carborane carbons
to be bis­[4-(N-carbazolyl)­phenyl]­carboranes, o-Cb, m-Cb,
and p-Cb. To elaborate on the role of
the carboranes, comparative analogous benzene series (o-Bz, m-Bz, and p-Bz) were prepared, and their photophysical
properties were compared to show that advantageous photophysical properties
were originated from the carborane structures: high triplet energy.
Unlike m-Bz and p-Bz, carborane-based m-Cb and p-Cb 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
(Tg) of 132 and 164 °C for m-Cb and p-Cb, respectively. Albeit p-Cb exhibited
slightly lower hole mobility when compared to p-Bz, 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 m-Cb and p-Cb. Low-temperature PL spectra indeed provided unequivocal
data with higher triplet energy (T1) of
3.1 eV for both m-Cb and p-Cb. p-Cb 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).