Carborane-based optoelectronically active organic molecules: Wide band gap host materials for blue phosphorescence

Article abstract of DOI:10.1021/ja3066623

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 (T_g) 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).

Full text of DOI:10.1021/ja3066623

Article
pubs.acs.org/JACS
Carborane-Based Optoelectronically Active Organic Molecules: Wide
Band Gap Host Materials for Blue Phosphorescence
Kyung-Ryang Wee,† Yang-Jin Cho,† Soyeong Jeong,† Soonnam Kwon,*,† Jong-Dae Lee,‡ Il-Hwan Suh,†
and Sang Ook Kang*,†
†Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Sejong 339-700, South Korea
‡Department of Chemistry, Chosun University, Gwangju, South Korea
S
* Supporting Information
ABSTRACT: 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).
hence, it effectively mismatches the electronic π-type overlaps
in the surrounding electro-active organic functional groups.5
However, for the practical application to the vacuum deposition
process, further molecular architecture would be required; it
must bear higher glass transition temperature (Tg) yet allow
reasonable volatility. It is expected that to achieve good thermal
property more functional units should be attached to the silicon
atom to increase molecular weight, but this opposes the
specification for sufficient volatility regarding vacuum evapo-
ration. So, there still remains a challenge to develop an efficient
method from which higher triplet energy can be achieved
maintaining high Tg and at the same time allowing appropriate
volatility.
Taking a lesson from the successful concept of the silicon π-
conjugation breaker, we then extend this strategy to cluster
carborane in hopes that carborane can function not only as a
conjugation breaker but also as an auxiliary for high Tg.
Carboranes are known as inorganic analogues of benzene,6 in
which two carbon and ten boron atoms constitute a 12 vertex
icosahedral structure and three different geometrical isomers
are formed depending on the two carbon atomic positions. Due
to the electron sharing nature of the cluster bonding found in
INTRODUCTION
■
It has been widely accepted that conjugated aromatic
compounds containing an aryl basic unit have provided
controllable optoelectronic properties essential to organic
electronic materials for electroluminescence devices,1a organic
thin film transistors,1b and solar cells.1c Given the wide
spectrum of advantageous optoelectronic properties made out
of the aryl structural motif, there is still a lack of desirable
material specifications for high performance device applications,
particularly in the deep blue phosphorescent organic light-
emitting diodes (PHOLEDs). Most of all, the wide energy gap
is the most demanding material property in the development of
deep blue PHOLEDs.2 For example, several approaches have
been developed in regards to wide band gap materials for blue
phosphorescence. One of the strategies is to use the meta-
positions of benzene as the substitution sites, keeping the
property of organic functional groups abreast of their own, not
altered by benzene resonance.3,4 However, this method only
allowed maximum triplet energy (T1) of 2.75 eV due to
incomplete triplet wave function confinement on either side of
the functional unit. Another method is to utilize the silicon
atom as a conjugation breaker in between the organic
functional groups; it turns out to be the most effective to
confine triplet wave function into the organic functional group
because the silicon atom is larger than the carbon atom and,
Received: July 9, 2012
Published: October 11, 2012
© 2012 American Chemical Society
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carboranes, they are often regarded as electron-withdrawing
and thermally stable functional units. On the basis of these
perceptions, several organic or polymeric materials funtional-
ized by carboranes have been prepared, and advantageous
physical properties such as thermal and oxidative stabilities
were reported.7 However, any systematic attempt to unveil the
true nature of carborane for organic electronic materials has
never been made before in comparison to benzene.
Recently, we elaborated on the unique photophysical
properties of ortho-carborane, when it was functionalized by
an opto-electronically active substituent, carbazolyl phenyl
(CzPh).8a It was noted that ortho-carborane showed enhanced
charge transfer (CT) emission due to the electron-deficient
character of the carborane coupled to juxtaposition of Ph
groups attached to the carbon atoms in the cage.8 On the
contrary, the other isomers did not show CT emission. In this
study, we explore the potential utility of carboranes as active
structural elements for use in optoelectronic materials. To
unveil carboranes’ contribution to physical properties in
optoelectronic materials clearly, iso-structural benzene ana-
logues are referenced together.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out in a dry N2
atmosphere. Solvents were distilled from calcium hydride for
dichloromethane (CH2Cl2), acetonitrile (CH3CN), and dimethylfor-
mamide (DMF) or from sodium/benzophenone for tetrahydrofuran
(THF), ethylene glycol dimethylether (DME), and toluene under a
nitrogen atmosphere and stored over molecular sieves. Other reagents
were obtained commercially and used without further purification.
Glassware, syringes, magnetic stirring bars, and needles were dried in a
convection oven for >4 h. Reactions were monitored with thin layer
chromatography (TLC). Commercial TLC plates (Merck Co.) were
developed, and the spots were identified under UV light at 254 and
365 nm. Column chromatography was done on silica gel 60 G
(particle size 5−40 μm, Merck Co.). All synthesized compounds were
characterized by 1H NMR, 13C NMR, and HRMS (FAB) mass
1
analysis. The H and 13C NMR spectra were recorded on a Varian
Mercury 300 spectrometer operating at 300.1 and 75.4 MHz,
respectively. The elemental analyses (Carlo Erba Instruments
CHNS-O EA 1108 analyzer) and high-resolution tandem mass
spectrometry (Jeol LTD JMS-HX 110/110A) were performed by
the Ochang branch of the Korean Basic Science Institute. All reagents
were purchased and used as received. 9-(4-Bromophenyl)carbazole9
and 4-(carbazol-9-yl) phenylboronic acid10 were prepared using the
methods reported in the literature.
As can be seen in Chart 1, both benzene and carborane cores
can be functionalized by carbazolyl phenyl (CzPh) units with a
9,9′-(1,1′:2′,1″-Terphenyl-4,4″-diyl)dicarbazole (o-Bz). 1,2-
Dibromobenzene (0.25 mL, 2.06 mmol), 4-(carbazol-9-yl)-
phenylboronic acid (1.3 g, 4.53 mmol), Pd(PPh3)4 (0.095 g, 4 mol
%), and K2CO3 (1.875 g, 29.9 mmol) were charged to a flask. Then
toluene/H2O (33/11 mL) was added to the flask, and the resulting
mixture was stirred under N2 at 110 °C overnight. The organic layer
was extracted with CH2Cl2 (3 × 30 mL) and dried over magnesium
sulfate. The solvent was removed under reduced pressure, and the
residue was purified by silica gel column chromatography using
CH2Cl2/hexane (1:6) as the eluent. o-Bz was obtained as a white
powder (0.78 g, 68%) and further purified by train sublimation in a
62% yield. 1H NMR (CDCl3): δ 8.15 (d, 4H), 7.62 (d, 4H), 7.54−7.42
(m, 8H), 7.42−7.33 (m, 8H), 7.26 (t, 4H). 13C NMR (CDCl3): δ
140.8, 140.6, 140.0, 136.3, 131.5, 130.6, 128.1, 126.6, 126.1, 123.4,
120.4, 120.0, 109.7. HRMS(FAB) calcd for C42H28N2: 560.2252.
Found: 560.2201 [M]+. Anal. Calcd for C42H28N2: C, 89.97; H, 5.03;
N, 5.00. Found: C, 89.93; H, 5.05; N, 4.97.
Chart 1. Three Different Geometrical Isomers
Functionalized by CzPhs at Either the Carborane or
Benzene Framework
9,9′-(1,1′:3′,1″-Terphenyl-4,4″-diyl)dicarbazole (m-Bz). A
procedure analogous to that used for the synthesis of o-Bz was
used, but starting from 1,3-dibromobenzene (0.25 mL, 2.06 mmol)
instead. m-Bz was obtained as a white powder (0.81 g, 70%) and
further purified by train sublimation in a 72% yield. 1H NMR
(CDCl3): δ 8.18 (d, 4H), 8.01 (s, 1H), 7.93 (d, 4H), 7.77−7.60 (m,
7H), 7.53−7.42 (m, 8H), 7.29 (t, 4H). 13C NMR (CDCl3): δ 141.1,
140.9, 140.1, 137.2, 128.8, 127.5, 126.5, 126.2, 126.1, 123.5, 120.5,
120.1, 109.9. HRMS(FAB) calcd for C42H28N2: 560.2252. Found:
560.2292 [M]+. Anal. Calcd for C42H28N2: C, 89.97; H, 5.03; N, 5.00.
Found: C, 89.99; H, 4.98; N, 4.96.
Crystal Structure Determination. Crystals of p-Cb and p-Bz
were obtained in a CH2Cl2/toluene solution, sealed in glass capillaries
under argon, and mounted on the diffractometer. The preliminary
examination and data collection were performed using a Bruker
SMART CCD detector system single-crystal X-ray diffractometer
equipped with a sealed-tube X-ray source (50 kV × 30 mA) using
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The
preliminary unit cell constants were determined using a set of 45
narrow-frame (0.3° in ω) scans. To exclude noise, the double pass
method of scanning was used to exclude noise. The collected frames
were integrated using an orientation matrix determined from the
narrow-frame scans. The SMART software package was used for data
collection, and SAINT was used for frame integration.11 The final cell
constants were determined through global refinement of the xyz
centroids of the reflections harvested from the entire data set.
Structure solution and refinement were carried out using the
SHELXTL-PLUS software package.12
desire of developing potential blue host materials. The resultant
two sets of the corresponding three isomeric structures
comprising ortho-, meta-, and para-substitutions are clearly
seen: The alias for each of those are named to be o-Bz, m-Bz,
and p-Bz and o-Cb, m-Cb, and p-Cb for the benzene system
and carborane system, respectively.
The physical properties, including photophysical properties,
electronic structure, charge mobility, and thermal properties,
have been thoroughly investigated for those six compounds.
Then, deep blue PHOLEDs were fabricated using the
carborane-based materials as hosts in the emission layer
(EML) to examine the feasibility for application to OLEDs,
in which p-Cb exhibited excellent device results. Thus, we offer
a novel synthetic strategy of using para-linked carborane as an
active structural element of material owing to its outstanding
advantages: high triplet energy, high charge mobility, and
formation of glass film with high thermal stability.
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Absorption, Fluorescence, and Phosphorescence. The
absorption and photoluminescence spectra were recorded on a
Shimadzu UV-3101PC UV−vis−NIR scanning spectrophotometer
and a Varian Cary Eclipse fluorescence spectrophotometer,
respectively. The triplet energies were taken as the maximum of the
first vibronic transition regarding the phosphorescence spectra. The
samples dissolved in 2-methyl tetrahydrofuran (MTHF) were frozen
in liquid nitrogen to 77 K. Phosphorescence was observed by
irradiation of the samples at 309 nm using the third harmonic of a Q-
switched Nd:YAG laser (Continuum, Surelite II, pulse width of 4.5 ns)
combined with a homemade Raman shifter. Temporal profiles were
measured by a photomultiplier (Zolix Instruments Co., CR 131) and a
digital oscilloscope (Tektronix, TDS-784D) equipped with a
monochromator (DongWooOptron, Monora 500i). Reported signals
were obtained as averages of 500 events. The detection window was
set to 15 μs, and the delay was set to 1 ms from the rigging edge of the
laser.
Thermal Property. Thermal property was measured using
differential scanning calorimetry (Perkin-Elmer/Pyris Diamond
DSC). A heating rate of 10 °C/min was used after first melting the
compound, followed by a rapid cooling rate of 40 °C/min to room
temperature.
Cyclic Voltammetry (CV). Cyclic voltammetry experiments were
performed using a BAS 100 electrochemical analyzer. A three-
electrode cell system was used with a glassy carbon, platinum wire, and
Ag/AgNO3 (0.10 M) as the working, counter, and reference
electrodes, respectively. Freshly distilled, degassed CH2Cl2 was used
as the solvent, with 0.1 M tetra-n-butylammonium hexafluorophos-
phate as the supporting electrolyte.
radiometer (Photo Research PR650). Assuming Lambertian emission,
the external quantum efficiency (EQE) was calculated from the
luminance, current density, and electroluminescence spectrum.16
RESULTS
■
Sample Preparation. CzPh-substituted carboranes and
benzenes,8 o-Cb, m-Cb, and p-Cb, and para-benzene, p-Bz,17,18
were prepared using previously reported methods. o-Bz and m-
Bz were synthesized by the Suzuki coupling reaction between
two equivalents of 4-(carbazol-9-yl) phenylboronic acid and
1,2- or 1,3-dibromobenzene, as shown in Scheme S1
(Supporting Information). All products were isolated by flash
column chromatography and further purified by train
sublimation in moderate yield. The formation of all compounds
was confirmed by high-resolution mass spectrometry and
elemental analyses. The compounds showed the expected
signals in their 1H and 13C NMR spectra. Finally, the structural
identities of representative examples p-Cb and p-Bz were
authenticated by means of a single-crystal X-ray crystallography.
Later in the following section, a detailed description of the
molecular geometry governed by the structural motif either by
carborane or benzene is to be made in greater detail.
Photophysical Properties. The absorption (UV) and PL
spectra of the carbazoles studied are shown in Figure 1, and
TOF Transient Photocurrent Measurements for Estimating
Charge Mobility. For TOF drift mobility measurements, the sample
was prepared on 0.7 mm thick glass substrates coated with indium tin
oxide (ITO). Organic film was vacuum deposited onto the patterned
ITO, and a 0.1 μm thick aluminum film was deposited by thermal
evaporation. The thicknesses were confirmed through scanning
electron microscopy (SEM) regarding the cross section of the device
after the mobility measurement, which were 3.1, 3.2, 3.3, 3.5, 3.0, and
3.6 μm for o-Cb, m-Cb, p-Cb, o-Bz, m-Bz, and p-Bz, respectively. The
ITO side was irradiated with pulsed laser light into the organic film for
the generation of photocurrent. The wavelength and pulse width of the
laser (Photon Technology International, GL-3300) were 337 nm and
1 ns, respectively.13 Various bias voltages were applied to the ITO
electrode, and a 100 Ω resistor was connected to the aluminum
electrode. The opposite side of the resistor was connected to the
ground. Also, the photocurrent was measured across the resistor using
an oscilloscope.
Calculation Details. All the calculations were performed on the
platform of the Gaussian 09 package.14 The equilibrium structures of
neutral singlet and neutral triplet states were obtained from the
geometry optimizations performed at the B3LYP/6-31G(D,P) levels
of theory, which were confirmed to be global minima by evaluating
vibrational frequencies. Most of the single-point energies were
calculated at the same level of theory, whereas the calculation of
Kohn−Sham energy levels was refined using the B3LYP/6-311+G-
(D,P) level of theory. The lowest triplet energies were estimated by
calculating ΔSCF, which was obtained by subtracting the SCF energy
of the neutral singlet state from that of the triplet state on the basis of
optimized geometries.15 Zero-point vibrational energy (ZPVE)
corrections were also taken into account.
Figure 1. Room-temperature UV/vis absorption and normalized
photoluminescence spectra in the solutions of cyclohexane for benzene
and carborane based CzPhs.
their steady-state photochemical properties are summarized in
Table 1. In particular, there is a stark difference between
Table 1. Optical and Electronic Properties of the
a
Compounds
abs. λmax PL λmax Stokes shift Egopt
E
ELUMO
c
HOMbO
compounds
[nm]
[nm]
[cm−1
]
(eV)
(eV)
(eV)
OLED Devices. The OLED devices were fabricated on glass
substrates precoated with a 150 nm ITO layer having a sheet resistance
of 10 Ω/square. The ITO glass was precleaned using a conventional
solvent cleaning method. The ITO surface was cleaned again with a
UV ozone treatment immediately before depositing the hole
transporting layer (HTL). The organic, LiF, and Al layers were
deposited sequentially onto the substrate without breaking the
vacuum. The current−voltage characteristics of OLEDs were
measured using a source measure unit (Keithley 236). The
electroluminescence spectra, luminance, and commission internatio-
nale de l'elcairage (CIE) coordinates were measured using a spectro-
d
o-Cb
m-Cb
p-Cb
o-Bz
336
338
338
342
342
342
-
-
-
−6.02
−6.05
−6.04
−6.04
−6.04
−6.05
−2.9
346
346
352
352
389
684
684
831
831
3533
3.63
3.63
3.58
3.58
3.43
−2.42
−2.41
−2.46
−2.5
m-Bz
p-Bz
−2.62
a
The HOMO and LUMO levels were determined using the following
equations. EHOMO (eV) = −e(Eonset + 4.8). ELUMO (eV) =
b
c
ox
d
−e(EHOMO − Egopt). ELUMO (eV) = −e(Eonset + 4.8).
red
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Figure 2. Calculated HOMO/LUMO molecular orbitals and comparison with the experimental energy levels: experimental data are shown in blue,
while calculated ones are in black.
carborane and benzene based carbazoles in the PL spectra
depending on the geometries; ortho-substitution at the
carborane (o-Cb) and para-substitution at the benzene (p-
Bz) give rise to effective electronic couplings resulting in a red-
shifted PL spectra where the extent of the shift is much more
profound for o-Cb. Also, all benzene-based carbazoles and o-Cb
exhibit featureless emission spectra indicating that there exist
electronic couplings between CzPh units interconnected by
either benzene or ortho-carborane. By contrast, as can be seen
in Figure 1c and 1e, for m-Cb and p-Cb, notable features are
found in UV and PL spectra, which are essentially the same
with very small Stokes shifts. This indicates that absorption and
emission energy states are originated from the CzPh unit.19
Furthermore, the characteristic CzPh unit’s transverse and
parallel transitions are evidently seen at 324 and 338 nm,
respectively, in the UV spectra of m-Cb and p-Cb. Unlike m-Cb
and p-Cb, o-Cb shows a unique photophysical property at
room temperature; formation of a new CT energy state is
evident in a molecular array of donor−acceptors found in the o-
Cb backbone.8 This new energy state is due to the special
geometry, in which electron donors (CzPhs) are placed on
carborane at ortho-substitution sites. Figure 1a shows a red-
shifted PL emission supporting for the dyad character of o-Cb.
However, o-Bz20 of Figure 1b does not exhibit similar dyad
behavior. This in turn manifests the electron-accepting
capability of carborane with an emphasis on the geometrical
factor; the ortho-substitution creates an energy-accepting state
comprising a bond between two carbon atoms of the o-
carborane. On the basis of the steady-state photophysical
properties given by absorption and emission spectra, it can be
said that meta- and para-carboranes do not allow any electronic
coupling and simply provide the substitution sites; PL spectra
of Figure 1c and 1e show distinct vibronic features of isolated
CzPh for both m-Cb and p-Cb. From the UV and PL spectra of
benzene-based carbazoles (Figure 1b, 1d, and 1f), it is obvious
that the benzene platform contributes greatly in electronic
couplings, and the para-position is most signified (Figure 1f).
Although the meta-position is less affected by electronic
coupling among the benzene series, it still exhibits larger
electronic coupling when compared with those found in m-Cb
and p-Cb. Steady-state photophysical data indicate effective
excited state energy confinement, from which high T1 energies
for m-Cb and p-Cb are expected.
HOMO/LUMO Energy Levels. The highest occupied
molecular orbital (HOMO) energy levels of the compounds
were determined by CV experiments and are listed in Table 1.
All compounds exhibited quasi-reversible oxidation peaks. The
HOMO energy levels were estimated from the first oxidation
onset potential, exhibiting approximately −6.05 0.03 eV. The
lowest unoccupied molecular orbital (LUMO) energy level of
o-Cb was determined from the first reduction onset potential,
which is −2.90 eV. The LUMO energy levels of the other
compounds were estimated from the optical band gaps and the
HOMO energy levels because no cathodic reduction processes
were recorded except for o-Cb. Most of the LUMO energy
levels show similar values in the range of −2.45
0.05 eV
except for o-Cb and p-Bz whose LUMOs are significantly lower
than those of the rest shown to be −2.90 and −2.62 eV,
respectively. Again, a markedly lowered LUMO clearly explains
the electron-accepting capability observed in o-Cb, which is the
unique characteristic of ortho-carborane.8
Figure 2 illustrates the comparison between the experimental
and calculated energy levels, and the calculated physical
properties are listed in Table 2. The calculated HOMO levels
Table 2. DFT Calculation Results of HOMO/LUMO Energy
Levels, Triplet Energies, and Permanent Dipole Moments
a
a
HOMO
(eV)
LUMO
(eV)
dipole Moment
(D)
compounds
T1−S0 (eV)
b
o-Cb
m-Cb
p-Cb
o-Bz
−5.96
−5.84
−5.79
−5.65
−5.70
−5.65
−1.97
−1.55
−1.63
−1.50
−1.64
−1.82
2.46 (2.34)
3.16 (2.98)
3.16 (2.97)
2.80 (2.68)
2.84 (2.72)
2.57 (2.53)
4.11
1.64
0.00
2.62
1.71
0.04
b
b
b
b
b
m-Bz
p-Bz
a
Carried out at the B3LYP/6-311+G(D,P) level of theory. The others
b
were calculated at the B3LYP/6-31G(D,P) level of theory. ZPVE
corrected results.
are in the range of −5.86
compounds and −5.68
0.1 eV for the carborane-based
0.03 eV for the benzene-based
compounds, which are higher than the experimental values of
−6.0 eV. The HOMO wave functions are delocalized over two
CzPhs for all of the compounds with only one exception for o-
Cb. The molecular orbital distributions of LUMOs are located
over central carborane cages and benzenes. The calculated
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energy levels of LUMOs show higher values than the
experimental ones; however, the values are consistently shifted
by +0.91 0.12 eV for the six compounds, which is consistent
with the reports by Su et al.4c This correspondence supports
the appropriateness of the calculation. The LUMO levels of the
compounds show values comparable to normal carbazole-based
materials in the range of −2.50 eV, except for those of o-Cb and
p-Bz.
The origin of deep LUMO level of o-Cb (−2.92 eV) is
related to the electronic coupling between CzPh and the
carborane cage.8 The relatively deep LUMO level of p-Bz
(−2.62 eV) is comparable to the other electron-transporting
materials in the range of −2.7 eV.4c This is mainly caused by
central triphenyl chromophores, from which electron mobility
is also likely to be originated. This will be contemplated in the
following mobility description section.
o-Bz and m-Bz of 2.74 eV, finally reaching the highest value of
3.05 eV for m-Cb and p-Cb. Also, these observed values are
closely matched to those calculated (see the values shown in
Table 2). Of special importance is a difference in the behavior
of triplet wave functions that reside in the molecular arrays
formed by benzene and carborane based CzPhs, as illustrated in
Figure 3a.
It is well-known that to acquire high triplet energy the triplet
wave functions should be localized.4a This criterion is well
applied to the compounds of this study in that, except for o-Cb,
the carborane-based CzPhs, m-Cb and p-Cb, localize their
triplet wave functions to one end of the carbazolyl unit. This
again explains the lower triplet energy of o-Cb in the carborane
series and accounts for the anomalous photochemical behavior
of o-Cb. On the other hand, the benzene-based CzPhs, o-Bz,
m-Bz, and p-Bz, either locate their triplet wave functions to the
central benzenes for o-Bz or spread them over to the biphenyl
for the m-Bz or triphenyl for the p-Bz group rather than
localize to one end of the carbazolyl unit. This spread of triplet
wave functions makes the benzene systems fail to bear a higher
T1 energy. However, the triplet wave functions of meta- and
para-substituted carboranes are highly localized to a carbazolyl
end-on group bearing a high T1 of 3.05 eV, as shown in Figure
3a. Also, m-Cb and p-Cb show a well-developed T1 state with
the same vibronic coupling pertinent to CzPh,9 which is
obviously shown in Figure 3b.
Triplet Energies. The properties of triplet excited states for
the compounds were estimated using low-temperature (77 K)
time-resolved PL experiments in a dilute solution of 2-methyl-
tetrahydrofuran (MTHF), as shown in Figure 3. After 1 ms
Thermal Properties. The thermal properties of the
compounds were determined by differential scanning calorim-
etry (DSC) analysis as listed in Table 3. A heating rate of 10 °C
min−1 was used after the first melting, which was followed by
rapid cooling to room temperature. As shown in Figure S3
(Supporting Information), the first heating cycle revealed a Tm,
and in subsequent heating cycles, weak endothermic transitions
associated with Tg were found. As a result, all of the carborane-
based compounds showed high Tg above 130 °C. Moreover,
the Tg value of the derivatives increased when changing the
position of the substitution groups at the carborane cluster,
exhibiting o-Cb (130 °C) < m-Cb (132 °C) < p-Cb (164 °C),
as shown in Table 3. In particular, the linear shape of p-Cb
demonstrates a relatively high glass transition temperature, and
no distinct crystallization temperature was observed in the
range between 30 and 350 °C, which demonstrates stable
glasses of this material in its film state.
Figure 3. (a) Spin density distributions of carborane and benzene
based CzPhs in their first triplet excited states calculated at the
UB3LYP/6-31G(D,P) level of theory. Phosphorescence (1 ms delay)
spectra of (b) the carborane and (c) the benzene based CzPhs in
MTHF at 77 K.
For comparison, we also examined benzene-based com-
pounds and found relatively low glass transition temperatures
in which o-Bz, m-Bz, and p-Bz show Tg values of 106, 121, and
150 °C, respectively. From this result, the restricted motion20
and higher molecular mass nature21 of the carborane cluster are
thought to contribute a great deal to the increased Tg values.
Granting that higher Tg hosts can contribute device stability for
PHOLEDs, and the carborane hosts developed in this study, m-
Cb or p-Cb, can shed new light on device stability and
lifetime.22
Crystal Packing from X-ray Crystallography. A crystal
structure could provide important information on the thermal
property of the solid state as well as on charge mobility. The
crystals of p-Cb and p-Bz were grown from a CH2Cl2/hexane
mixed solution. Their crystal data and collection parameters are
summarized in Tables S1 and S2 (Supporting Information).
Although the photophysical characteristics of p-Bz have been
reported as a bipolar host,17,18 its structure has never been
established. For this reason, this work includes a structural
study of p-Bz along with that of p-Cb for comparison. p-Cb and
from excitation, most of the fluorescent emissions decayed, and
phosphorescent emissions enhanced. The highest energy
phosphorescence peak is taken as triplet energy, and the
corresponding values are listed in Table 3. T1 energies of o-Cb
and p-Bz are in the lower range of 2.5 eV, followed by those of
Table 3. Experimentally Determined Physical Properties of
the Compounds
electron
T
mobility
hole mobility thermal properties
b c
1 a
compounds (eV)
b
(cm2/(V s))
(cm2/(V s))
Tg/Tc/Tm (°C)
o-Cb
m-Cb
p-Cb
o-Bz
2.12
3.05
3.05
2.74
2.74
2.47
-
1.97 × 10−4
6.47 × 10−4
1.10 × 10−3
6.75 × 10−4
8.35 × 10−4
1.30 × 10−3
130/−/297
132/197/286
164/−/>400
106/−/251
121/−/256
150/−/325
-
-
-
m-Bz
p-Bz
-
2.5 × 10−4
a
b
In MTHF (77 K). Under electric field of 5 × 105 V/cm.
c
Determined by differential scanning calorimetry (DSC).
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p-Bz crystallize in the Pbca and P21/n space group, respectively
(Figure 4). With regards to the crystal structure of p-Cb, a half
Figure 4. Molecular packings of (a) p-Cb and (b) p-Bz with unit cell
by the Mercury program. Hydrogen atoms are omitted for clarity.
molecule belongs to an asymmetric unit, and a molecule is
completed by another half molecule translated by an inversion
symmetry located at the center of the carborane cage
[symmetry code: 1 − x, 1 − y, 1 − z]. Similarly a complete
molecule of p-Bz is accomplished through an inversion center
located at the centroid of the benzene ring Ccent···Ccent
[symmetry code: −x, −y, −z]. Thus, for each structure, there
are two molecules in a unit cell.
Figure 5. Molecular packings of (a) p-Cb in the b-axis and (b) p-Bz in
the a-axis in a single unit cell by the Mercury program. Hydrogen
atoms are omitted for clarity.
pack into a zig-zag structure in the a-axis. The adjacent
carbazole rings are situated in an overlapping mode along the b-
axis, and the overlapping area is approximately one benzene
ring. As shown in Figure 5b, however, p-Bz packs into a column
structure along the a-axis and adopts face-to-face packing. All of
the adjacent p-Bz slightly slips along a direction. Although there
seems to be no clear measure to estimate the intermolecular
interactions for p-Cb and p-Bz, based on the interatomic
distances between the molecular layers and the orientations in
the molecular packings governed by their morphologies, the
higher thermal stability of the thin film of p-Cb is expected.
Charge-Transporting Properties. The charge-transport-
ing properties of the compounds were systematically
investigated using TOF transient photocurrent measurements.
Figure 6 shows the electric field dependence of the hole and
Molecular packings of p-Cb and p-Bz are shown in Figure 4a
and 4b, respectively. The aromatic groups and carbazole units
of the structure have a dominating influence upon the overall
crystal packing. The crystal structures for both p-Cb and p-Bz
are similar to each other in terms of the overall morphology
caused by the inversion symmetry existing at the center of each
molecule, whereas the packing motif of p-Cb is quite different
from that of p-Bz. The undulating topology of the former is
highlighted in Figure 4a. The carbazole units of adjacent rows
partially overlap between each phenyl ring of carbazole units.
However, p-Bz packs into two different column structures in
the ab and bc planes, respectively, so that the column structures
join in a crossing manner with each other at roughly 90° when
viewed along the c-axis (see Figure 4b and 5b). The N···N
distances of p-Cb and p-Bz between the overlapped carbazole
units are 4.03 and 5.46 Å, respectively. The dihedral angle
between the carbazole/phenylene linker of p-Cb is 16.42°, and
the dihedral angles between the carbazole/phenylene linker and
the phenylene linker/central phenyl unit of p-Bz are calculated
as 54.08 (6) and 30.6 (2)°, respectively. The shortest C···C
distance between the adjacent carbazole units of p-Cb is 3.36 Å
(C17···C14i [symmetry code: (i) −x + 0.5, y + 0.5, z]), whereas
that of p-Bz is 3.43 Å (C4···C8i [symmetry code: (i) x + 1, y,
z]), taking into account the van der Waals radii of C being 1.70
Å.23
As shown in Figure 5a, the carborane cluster induces
favorable packings in the solid-state structure. p-Cb molecules
Figure 6. Charge mobility of solid state thin films of (a) carborane and
(b) benzene based CzPhs as a function of square root electric field.
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Figure 7. Device characteristics of the PHOLEDs using carborane-based compounds as hosts in the devices with the device structures of ITO/NPB
(40 nm)/TAPC (15 nm)/Host (10 nm)/Host: FIr6 10 wt % (35 nm)/SiTAZ (40 nm)/LiF (0.5 nm)/Al (60 nm). (a) Current−voltage and
luminance−voltage curves. Inset: Schematic energy levels of the materials used in the devices. (b) Electroluminescence spectra measured during
device operation with a constant current of 1 mA/cm2. Inset: Current efficiency−luminance curves.
electron mobility for the compounds. All of the mobility data
could be well fitted with the Poole−Frenkel function of μ = μo
exp(E1/2/σ), of which the results are summarized in the
Supporting Information. Electron mobility was observed only
for p-Bz, and the value is much inferior to that of hole for the
same compound. This is a commonly observed phenomenon
for benzene-based materials when compared to heteroaromatic-
based materials regarding electron-transporting properties.4c
Regardless of the benzene or carborane platform, hole
mobility appears to be governed by the general geometric rule,
in which meta- and para-geometries showed enhanced mobility
over the ortho geometry, and among the meta- and para-
geometries, the para-isomer proceeds to the meta-isomer in a
small degree: p-Cb and p-Bz exhibited relatively higher hole
mobility among the series. Inferior hole mobility can be related
to the geometrical hindrance imposed by two facing bulky π-
groups, which would then confuse the intermolecular
interactions within the solid-state organization. This confusion
would be the greatest in the ortho-isomer and alleviated for
meta-isomers. With the same argument, a higher degree of
symmetry imposed on the para-geometry can help to increase
intermolecular electronic interactions enhancing the charge
mobility.24 The solid-state packing structure for p-Bz and p-Cb
shown in Figures 4 and 5 provides grounds for such
speculation. Taking into account the geometrical preference
of para-isomers, p-Cb and p-Bz mark the best hole mobility of
1.10 × 10−3 and 1.30 × 10−3 cm2/(V s), respectively, under an
electric field of 5 × 105 cm/V, which is a bias condition similar
to that applied for the real device operation.
Device Characteristics. From the structure−property
relationships, m-Cb and p-Cb are expected to be promising
host materials for deep blue PHOLEDs because of their high
hole mobility, high thermal stability, and high triplet energy,
whereas o-Cb is expected to exhibit poor performance if it is
used as a host for a deep blue guest due to its low triplet energy
of 2.36 eV and relatively lower hole mobility compared to the
other two. To examine the feasibility of application for
carborane-based materials regarding deep blue PHOLEDs, a
series of iridium(III)[bis(4,6-difluorophenyl)pyridinato-N,C2′]-
tetrakis(1-pyrazolyl)borate (FIr6) based PHOLEDs were
fabricated with the following device structure: ITO/NPB (40
nm)/TAPC (15 nm)/Host (10 nm)/Host: FIr6 (35 nm, 10 wt
%)/SiTAZ (40 nm)/LiF (0.5 nm)/Al (60 nm). Three different
host materials were used in three different devices, device A, B,
and C, where the Hosts are o-Cb, m-Cb, and p-Cb,
respectively. A thin interlayer of 1-bis[4-[N,N-di(4-tolyl)-
amino]phenyl]-cyclohexane (TAPC) was inserted between
the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine
(NPB) and EML as a triplet exciton blocking layer since
TAPC has a high triplet energy (T1 ∼ 2.9 eV). As a deep blue
emitter, FIr6 was chosen because it has high PL quantum yield
and is commercially available. The electron-transporting layer
(ETL) material, bis(4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-
phenyl)-dimethylsilane (SiTAZ), was used for its high triplet
energy (2.84 eV), deep HOMO level (6.45 eV), and high
LUMO level (2.5 eV).16b Figure 7a shows both the current
density−voltage and luminance−voltage characteristics. As
anticipated from the hole mobility results, the devices using
m-Cb and p-Cb (device B and C) as hosts show higher current
density than device A.
Figure 7b illustrates the current efficiency−luminance curves
and normalized EL spectra at a current density of 1 mA/cm2.
Maximum current efficiencies of device A and B are 17.5 and
14.3 cd/A, respectively. These values are quite low compared to
the recent reports of deep blue devices.16 The low efficiency of
device A has been expected because of its low triplet energy and
CT-type excited states of o-Cb, which is also observed in the
EL spectrum as broad emission ranging from 500 to 750 nm.8
However, the low efficiency of device B denies easy
interpretation considering the high triplet energy and wide
band gap of m-Cb. An abnormal additional peak near 580 nm
appears in the EL spectrum of device B, which is different from
the FIr6 emission. This peak can be assigned as an electromer
emission from the TAPC layer.25a This observation implies that
parts of the electrons passed through EML without
recombination with the holes and reached the TAPC layer,
forming an electromer with holes inside the TAPC layer.25b
Late activation of device A and B (5.5 and 6.3 V) compared to
that of device C (4.7 V) can also be attributed to the above
inefficient recombination of holes and electrons inside EML.
One of the possible reasons for the inefficient recombination of
electrons and holes inside EML with host m-Cb or o-Cb can be
attributed to the interface effects such as slight vacuum level
shifts between organic layers, which is in a close relationship
with the dipole moment of a molecule.26,27 Looking at the
calculation results in Table 2, dipole moments of o-Cb and m-
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Cb are considerably high compared to that of p-Cb, which is
almost zero.
host materials for deep blue PHOLEDs developed in our group
as well as others are limited only to Si-templates or phosphine
oxides,15 the carborane templates developed in this study
provide new insight into designing materials for PHOLEDs.
Most of all, the representative example given by the p-Cb
exhibits the most favorable material specifications: a high T1,
high Tg, and good hole mobility.
On the contrary, high current efficiency of 25.3 cd/A (EQE
of 15.3%) is achieved for device C, which is comparable to the
best results of the FIr6-based monopolar EML devices
reported.16 No additional emission other than FIr6 is seen in
the EL spectrum, indicating efficient energy transfer from host
to guest and efficient triplet energy confinement into EML. As a
result, CIE coordinates of (0.15, 0.24) are obtained at a current
density of 1 mA/cm2. In the case of device B, due to electromer
emission, CIE coordinates are shifted to (0.17, 0.25). CIE
coordinates of device A exhibit (0.43, 0.41), which show a
yellowish white color owing to the long wavelength emission of
CT excited states.8
CONCLUSIONS
■
Three isomeric carborane-based materials (o-Cb, m-Cb, and p-
Cb) as well as three respective benzene analogues (o-Bz, m-Bz,
and p-Bz) were synthesized, and their photophysical properties
were systematically compared with each other to reveal that the
carborane linkers, m- and p-carboranes, were effective in
localizing triplet energy at one end of the CzPh unit reaching
their T1 values of 3.1 eV. Spin density distribution in the first
triplet exited state corroborated well to this higher T1. As we
anticipated, a desired higher Tg of 164 °C was recorded for p-
Cb. Comparable hole mobility equivalent to p-Bz merits the
use of inorganic templates, carboranes. In the case of p-Cb,
high triplet energy, high Tg, and high charge mobility were
accomplished, which have been regarded as conflicting targets
to achieve. Moreover, p-Cb showed excellent device perform-
ance when it was used as a host in EML for deep blue
PHOLEDs, exhibiting a high external quantum efficiency of
15.3% and CIE coordinates of (0.15, 0.24), which are
comparable to the best results with the simplest device
structure. Considering the physical property of p-Cb and its
possibility toward the application regarding real electronic
devices, the para-linkage of functional chromophores to a
carborane cage is a promising novel strategy for the synthesis of
electronic materials.
DISCUSSION
■
It is now clear that carborane platforms provide the best
synergistic molecular arrays in regards to the blue materials for
OLEDs, and among the candidates, m-Cb and p-Cb are shown
to be the most promising. So far, m-Cb and p-Cb confine
triplet wave functions effectively to a carbazolyl unit giving rise
to the high T1 which is characteristic of the isolated CzPh
chromophore. Moreover, high hole mobility was observed for
p-Cb, which is comparable to that of p-Bz. In the case of p-Bz,
high hole mobility can be attributed to the extended
conjugation due to delocalization of the HOMO wave function
and the extensive intermolecular overlap of wave functions.28
However, this in turn resulted in lower triplet energy due to
conjugation. On the contrary, p-Cb not only shows
unperturbed T1 but also exhibits comparable hole mobility. A
symmetric molecular structural motif as well as para-
substitution is expected to be the prime reason for the
enhanced mobility. From the solid state packing analysis, p-Cb
provided the grounds for such surmises.
ASSOCIATED CONTENT
■
Above all, those carborane-based organic materials are quite
different from their organic congeners, benzenes, and are even
superior to those developed from Si-based organic materials,
which disconnect effectively conjugation of the chromophores
bridged by the Si atom. As shown in Figure 8, despite the
successful energy confinement by the silicon atom, the
corresponding silicon template lacks thermal stability, showing
a relatively low Tg value of 99 °C. p-Cb’s high Tg value has
reached the highest among the phosphorescence host materials
developed thus far. Considering the fact that the wide band gap
S
* Supporting Information
DSC thermograms and complete listing of the crystal data for
p-Bz, including complete drawings with labeling schemes, bond
lengths, and angles. Raw data of TOF measurements for charge
mobility are also provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
andrew2@korea.ac.kr; sangok@korea.ac.kr
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the Basic Science Research Program
(No. 2012-0006248) and WCU program (No. R-31-2009-000-
10083-0) through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology.
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