Twisted bimesitylene-based oxadiazoles as novel host materials for phosphorescent OLEDs

Article abstract of DOI:10.1016/j.tet.2012.05.101

The D_2d-symmetric bimesitylene core has been exploited for designing novel host materials required in the construction of phosphorescence-based organic light emitting devices. The oxadiazole- functionalized twisted bimesitylenes are found to exhibit high band gap (triplet energies), and excellent glass transition temperatures and thermal stabilities for ready exploitation as host materials. The electron-transporting ability and respectable luminance efficiencies with triplet dopants (up to 19.0 cd/A) allow oxadiazole-functionalized bimesitylenes as promising materials for phosphorescence-based light emitting devices, namely PHOLEDs.

Full text of DOI:10.1016/j.tet.2012.05.101

Tetrahedron 68 (2012) 7502e7508
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Twisted bimesitylene-based oxadiazoles as novel host materials
for phosphorescent OLEDs
,
b
Parthasarathy Venkatakrishnan a, Palani Natarajan a, Jarugu Narasimha Moorthy a , Zhenghuan Lin ,
*
,
Tahsin J. Chow b
*
a Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
b Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The D2d-symmetric bimesitylene core has been exploited for designing novel host materials required in
the construction of phosphorescence-based organic light emitting devices. The oxadiazole-
functionalized twisted bimesitylenes are found to exhibit high band gap (triplet energies), and excel-
lent glass transition temperatures and thermal stabilities for ready exploitation as host materials. The
electron-transporting ability and respectable luminance efficiencies with triplet dopants (up to 19.0 cd/
A) allow oxadiazole-functionalized bimesitylenes as promising materials for phosphorescence-based
light emitting devices, namely PHOLEDs.
Received 19 March 2012
Received in revised form 22 May 2012
Accepted 24 May 2012
Available online 31 May 2012
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Indeed, oxadiazole units advantageously restrict p-conjugation to
afford materials with deeply lying highest occupied molecular or-
bitals (HOMOs) such that the triplet energies (ETs) are high.15
In continuation of our recent investigations on organic light
emitting materials,16 we envisaged the possibility of functionalizing
twisted bimesitylene core with oxadiazoles to develop host mate-
rials for PHOLEDs, cf. 1 and 2, Fig. 1.
Commercialization of flat-panel displays has gained importance
due to the recent evolution in highly efficient phosphorescence-
based organic light emitting diodes (PHOLEDs);1 the internal
quantum efficiencies of phosphorescent emitters can, in theory,
reach 100% by harvesting both singlet and triplet excitons.2 In re-
cent years, several host emitting materialsdpolymers as well as
small moleculesdhave been synthesized and reported to be effi-
cient for use in PHOLEDs. The prerequisites for application of any
material as a good host are: (i) higher triplet energy than that of the
guest for efficient energy transfer,3 (ii) good thermal and mor-
phological stability, and (iii) appropriate energy level matching
with the adjacent layers for efficient charge injection and re-
combination. Designing materials that satisfy all of the above
conditions is a challenging task and enormous research is still in
progress. The small molecule-based host materials explored for the
purpose of electrophosphorescence include carbazoles,2b,4 silanes,5
phosphine oxides,6 triazoles,7 triazines,8 quinolines,9 triphenyl-
amines,10 transition metal ions,11 etc. While bipolar host systems
incorporating oxadiazole moiety have been reported in the litera-
ture,12 only oxadiazole-based host materials are still rare.13 Oxa-
It was anticipated a priori that (i) orthogonal orientation of the
aryl rings of bimesitylene core would preclude close packing/ag-
gregation in the solid state due to
troduction of the oxadiazole moieties would enhance the electron-
transporting properties across the layer and restrict -conjugation
pep stacking, (ii) the in-
p
to afford materials with wide band-gap energy (Eg) and high triplet
energy (ET), and (iii) the oxadiazoles with bulky substituents at the
termini would permit excellent thermal and morphological sta-
bility of the devices. Herein, we describe synthesis, photophysical,
thermal, and electrophosphorescence properties of novel
bimesitylene-based oxadiazoles 1 and 2. It is shown that the
bimesitylene-based materials based on a rational design and sub-
stitution pattern do indeed serve as efficient electro-
phosphorescent materials.
diazoles
are
excellent
electron-transporting/hole-blocking
2. Results and discussion
materials used in OLEDs due to their electron deficient nature.14
2.1. Synthesis of oxadiazole-functionalized bimesitylenes 1
and 2
* Corresponding authors. Tel.: þ91 512 2597438; fax: þ91 512 2597436 (J.N.M.);
tel.: þ886 2 27898552; fax: þ886 2 27831237 (T.J.C.); e-mail addresses: moorthy@
iitk.ac.in (J.N. Moorthy), tjchow@chem.sinica.edu.tw (T.J. Chow).
Scheme 1 depicts the synthetic route for preparation of 1
and
2 based on Pd(0)-catalyzed Suzuki coupling reaction
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.05.101

P. Venkatakrishnan et al. / Tetrahedron 68 (2012) 7502e7508
7503
Me
Me
Me
Me
Me
N
N
Me
N
N
O
O
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
O
O
Me
Me
Me
N
2
N
1
Me
N
N
Me
Fig. 1. Novel host materials 1 and 2 based on bimesitylenes.
between 3,30-diiodobimesityl16a and 4-formylphenylboronic
acid.17 The dialdehyde thus obtained16c was subjected to oxi-
dation using oxone18 to afford the corresponding 3,30-bis(4-
carboxyphenyl)bimesityl. Reactions of the latter with thionyl
chloride followed by treatment with suitably-functionalized
2.2. Photophysical properties
The UVevis absorption and photoluminescence spectra of 1 and
2 in chloroform solution at room temperature are shown in Fig. 2
and the results are summarized in Table 1. The oxadiazole 1 is
found to exhibit absorption maximum (lmax) at 293 nm, while 2
shows at 298 nm. In a similar manner, the room temperature
fluorescence spectrum of 2 displays emission maximum around
363 nm, which is red-shifted by 8 nm from that of 1. The fluores-
cence lifetimes of 1 and 2 were determined to be 1.06 and 0.93 ns,
tetrazoles led to oxadiazole-functionalized bimesitylenes
1
and 2 in excellent isolated yields (82e87%). Both compounds 1
and 2 were thoroughly characterized by 1H, 13C NMR spectros-
copy, mass spectrometry, and elemental analysis, cf.
Experimental section.
Me
Me
Me
N
N
O
Me
Me
Me
Me
Me
Me
O
Me
Me
Me
1
N
N
i) SOCl2, 90 oC, 3 h
ii) 3, pyridine, 110 oC, 6 h
82%
OHC
HOOC
Me
N
N
N
Me
Me
Me
Me
3
oxone
N
H
Me Me
Me
Me
Me
Me
Me
Me
Me
DMF, rt, 3 h
96%
Me
Me
N
N
N
Me
4
CHO
COOH
N
H
Me
Me
i) SOCl2, 90 oC, 3 h
Me
ii) 4, pyridine, 110 oC, 6 h
N
87%
Me
N
O
Me
Me
Me
Me
Me
Me
Me
O
N
2
Me
N
Me
Scheme 1. Synthesis of the bimesitylene-based oxadiazoles 1 and 2.

7504
P. Venkatakrishnan et al. / Tetrahedron 68 (2012) 7502e7508
1
2
1
2
260
280
300
320
340
360
300
350
400
450
500
wavelength (nm)
wavelength (nm)
Fig. 2. UVevis absorption (left) and photoluminescence (right) spectra for the oxadiazole-functionalized bimesitylenes 1 and 2 in chloroform.
respectively. A marginal bathochromic shift observed in both ab-
sorption and emission spectra of 2 may be attributed to the pres-
ence of the additional mesitylene unit in 2, which presumably
subtracting the optical band gap energy (Eg) from the corre-
sponding HOMO energy (Table 1); Eg was calculated in each case
from the tail-end of the absorption spectrum.
imparts weak
well known that the oxadiazole restricts
p
-communication due to the restricted rotation. It is
-conjugation beyond the
The wide band-gap energy (singlet energy gap) of ca. 3.71 eV is
expected for the oxadiazoles 1 and 2 due to their large triplet energy
gap.19a Indeed, suchmaterials with widebandgaps can inprinciple be
used as hole-blocking/electron-transporting materials.14
p
ring even if the rings were to be coplanar.15 Similarity in both shape
and pattern of the absorption as well as emission spectra of 1 and 2
attests to similar electronic behavior of the chromophoric moieties
present in the two molecules.
2.3. Thermal properties
As mentioned earlier, for a material to be a good host, it is
necessary that it exhibits higher triplet energy than that of the
phosphorescent emitter (dopant). This criterion benefits us with
high efficiencies by arresting the back energy transfer from dopant
to the host.3 To gauge the triplet energies (ET), the phosphorescence
The thermal and morphological stabilities of 1 and 2 were ex-
amined by thermogravimetric and differential scanning calorimet-
ric analyses, respectively. The bimesitylene-based oxadiazoles
reveal excellent thermal stabilities as indicated by their high de-
composition temperatures (Td); Tds measured at 5% weight loss for 1
and 2 are 431 and 449 ꢀC, respectively (Fig. 3). Compound 1
exhibited glass transition temperature (Tg) around 173 ꢀC and an
endothermic melting temperature (Tm) at 302 ꢀC, where as 2 dis-
played Tg around 182 ꢀC with Tm at 316 ꢀC (Table 1, Fig. 3). It is
noteworthy that the Tgs for 1 and 2 are much higher than the popular
and most often employed host materials, i.e., m-CP (Tg¼60 ꢀC) and
CBP (Tg¼62 ꢀC).19b Also, it is important to mention that the glass
transition temperatures of 1 and 2 are higher than those of the most
popular oxadiazole-based electron-transporting materials, such as,
spectra for
1
and
2
were recorded at 77 K in 2-
methyltetrahydrofuran. The spectra revealed a three-band pattern
that is common to both (Table 1, see Supplementary data for de-
tails). It was on the basis of their longer lifetimes (2.45
ms for 2; 0.71
(49%) and 4.53 (51%) s for 1) when compared to the fluorescence
m
lifetimes (singlet) that the emission was attributed to that ema-
nating from triplet states. The triplet energies for 1 and 2 were
derived from the 0e0 transitions (highest energy vibronic peak) at
ca. 462 nm, which corresponds to 2.68 eV. This value is compara-
tively higher than that of CBP19 (4,40-bis(N-carbazolyl)biphenyl,
ET¼2.56 eV), a popular host material used for PHOLEDs, which at-
tests to the utility of 1 and 2 as an appropriate host material for red
and green phosphorescent emitters. Additionally, the ET of 1/2 is
2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD,
Tg¼60 ꢀC),1b,14c 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phe-
nylene (OXD-7, Tg¼77 ꢀC),1b,14c,e and are higher than the recently
reported non-bipolar host-emitters that contain oxadiazoles (BOBP,
Tg¼45 ꢀC; TPOs and PPOs, Tgs up to 121 ꢀC).13 The excellent thermal
and morphological stability of our oxadiazoles may be attributed to
the orthogonal arrangement of the rigid bimesitylene core. The at-
tachmentof additional mesitylene ring in the case of 2 contributes to
5a,19a,c
sufficiently larger than the phosphorescent emitter Ir(PPy)3
(tris(2-phenylpyridine)iridium(III), ET¼2.42 eV) for the triplet ex-
citons to be confined. The HOMO energy levels for 1 (5.86 eV) and 2
(6.06 eV) were determined from the ultraviolet photoelectron
spectroscopy and the LUMO energy level was deduced by
Table 1
Optical and thermal properties of the bimesitylene-based oxadiazoles 1 and 2
Compound
labs (nm)
lem (nm)
lem (nm)
Tg,Tm,Tc (ꢀC)
HOMOe (eV)
LUMOf (eV)
Eg (eV)
ET (eV)
a
a
b
c
d
Td (ꢀC)
g
h
1
2
294
297
356
363
462, 497, 522
463, 497, 525
173, 302, 211
431
449
5.86
6.06
2.15
2.36
3.71
3.70
2.68
2.68
182, 316, di
a
Measured in chloroform solutions at room temperature.
Measured in 2-methyltetrahydrofuran at 77 K.
Determined from differential scanning calorimetric analyses performed under nitrogen at a heating rate of 10 ꢀC/min; Tgdglass transition temperature, Tmdmelting
b
c
temperature, and Tcdcrystallization temperature.
d
Determined from thermogravimetric analyses performed under nitrogen at a heating rate of 10 ꢀC/min; Tdddecomposition temperature.
Derived from UVPES.
Calculated by subtracting the optical band gap energy (Eg) from the corresponding HOMO energy.
Calculated from the tail-end of the absorption spectrum.
Derived from the 0e0 transitions of the phosphorescence spectrum.
No Tc observed.
e
f
g
h
i

P. Venkatakrishnan et al. / Tetrahedron 68 (2012) 7502e7508
7505
1
2
: Td = 449 oC
100
80
60
40
20
0
2
Tm = 302 °C
1
: Td = 431 oC
Tm = 316 °C
Tg = 173 °C
Tg = 182 °C
100 200 300 400 500 600 700 800
temperature (°C)
100 150 200 250 300 350 400
temperature (°C)
Fig. 3. Thermogravimetric (left) and differential scanning calorimetric (right) scans for oxadiazole-functionalized bimesitylenes 1 and 2.
increase in the molecular size and rigidity of the system leading to
better thermal properties. Such thermally/morphologically stable
compounds permit fabrication of devices via vacuum sublimation
without any decomposition.
This device A exhibited luminance and external quantum efficien-
cies of 4.6 cd/A and 1.4%, respectively. The poor efficiencies could be
reasoned from the very proximate HOMO levels of Alq3 (HOMO-
¼6.0 eV, LUMO¼3.3 eV; ET¼2.0 eV)19a,20 and 1 (5.86 eV). Conse-
quently, one may presume distribution of holes in both emitting
(EM) and ET layers such that recombination of holes and electrons
may occur in the two zones leading to an overall reduction in the
electrophosphorescence efficiency (hex). Hence, it was deemed
important to introduce a hole-blocking layer in combination with
Alq3 or to use ET materials with much higher LUMOs and higher
triplet energies for effective confinement of holes and electrons as
well as triplet excitons in the EM layer. As a next step, a simplified
device configuration B was adopted, wherein the ET layer was to-
tally eliminated from the device structure with an aim to probe the
electron-transporting ability of the oxadiazole-based material in
2.4. Electroluminescence properties
The ability of oxadiazoles 1 and 2 to function in PHOLEDs as host
materials was tested by fabricating devices with following configura-
ꢀ
ꢀ
ꢀ
tions: (A) ITO/NPB (400 A)/Ir(PPy)3:1 (6%, 400 A)/Alq3 (400 A)/LiF
ꢀ
ꢀ
ꢀ
ꢀ
(10 A)/Al (1500 A), (B) ITO/NPB (400 A)/Ir(PPy)3:1 (6%, 400 A)/LiF
ꢀ
ꢀ
ꢀ
ꢀ
(10 A)/Al (1500 A), (C) ITO/NPB (400 A)/Ir(PPy)3:1 (6%, 400 A)/TPBI
ꢀ
ꢀ
ꢀ
ꢀ
(400 A)/LiF (10 A)/Al (1500 A), and (D) ITO/NPB (400 A)/Ir(PPy)3:2 (6%,
ꢀ
ꢀ
ꢀ
ꢀ
400 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A), where ITO (indium tin ox-
ide) was the anode, NPB (N,N0-bisnaphthalen-1-yl)-N,N0-bis(phenyl)-
benzidine) served as the hole-transporting layer, Ir(PPy)3 acts as
a dopant, and 1 or 2 as the host material, TPBI (2,20,200-(1,3,5-
benzenetriyl)-tris(1-phenyl-1-H-benzimidazole) or Alq3 (tris(8-
hydroxyquinolinato)aluminum) as an electron-transporting (ET)
layer, and LiF/Al as the composite cathode. The electro-
phosphorescence spectrum and the HOMOeLUMO energy level dia-
ꢀ
PHOLED device structure. The device B (ITO/NPB (400 A)/Ir(PPy)3:1
ꢀ
ꢀ
ꢀ
(6%, 400 A)/LiF (10 A)/Al (1500 A)) that does not contain any
electron-transporting layer displayed better performance than the
previous Alq3-based device A with luminance and power efficien-
cies of 9.4 cd/A and 3.3 lm/W, respectively. In other words, the
device B showed almost a two-fold increase in the device perfor-
mance compared to that of device A. It is well known that oxa-
diazole derivatives act as good electron-transporting materials.14cee
One would expect that removal of the electron-transporting layer
would result in electron injection difficult. However, the dual
naturedelectron-transport as well as hostingdof the oxadiazole-
based molecular systems seemingly allow facile electron injection
into the EM layer leading to EL emission as a consequence of im-
mediate recombination with well-confined holes in EM layer to
account for the observed two-fold enhancement of the efficiencies.
gram for the devices
C and D are provided in Fig. 4. The
currentevoltageeluminance characteristics (IeVeL) and current effi-
ciency, power efficiencyand quantum efficiency versus current density
plots of the TPBI-based devices A and D are shown in Fig. 5. The device
characteristics for all the PHOLED devices are compiled in Table 2.
First of all, the oxadiazole
1 was tested for electro-
phosphorescence using Alq3dthe most common electron-
transporting materialdas the ET layer (device A: ITO/NPB
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(400 A)/Ir(PPy)3:1 (6%, 400 A)/Alq3 (400 A)/LiF (10 A)/Al (1500 A)).
2.15
2.3
1,0
0,8
0,6
0,4
0,2
0,0
device C (
1
)
2.7
pn754
2.36
2.9
device D (
2
)
4.1
NPB Ir(PPy)3
TPBI
LiF/Al
ITO
4.8
5.4
5.4
5.86 (1)
6.06 (2)
6.2
500
600
Wavelength (nm)
700
Fig. 4. The normalized electrophosphorescence spectrum (left) and the HOMO and LUMO energy level (eV) diagram for the devices C and D. In the emitting layer, solid line
represents 1, dashed line 2, and the dotted line Ir(PPy)3.

7506
P. Venkatakrishnan et al. / Tetrahedron 68 (2012) 7502e7508
250
200
150
100
50
120
V-I
V-I
V-L
V-L
100
80
60
40
20
0
10
10
10
10
10
10
10
10
0
0
2
4
6
8
10 12 14 16 18 20 22
Voltage (V)
0
2
4
6
8
10 12 14 16 18 20 22
Voltage (V)
20
18
16
14
12
10
8
15
luminous efficiency (cd/A)
power efficiency (lm/W)
external quantum efficiency (%)
luminous efficiency (cd/A)
power efficiency (lm/W)
external quantum efficiency (%)
10
5
6
4
2
0
0
0
50
100
150
200
0
20
40
60
80
100
Current Density (mA/cm )
Current Density (mA/cm2)
Fig. 5. The IeVeL characteristics (top), and luminous, power, external efficiencies versus current density (bottom) for the TPBI-devices of 1 (left) and 2 (right).
It must be emphasized that the materials that perform dual roles of
electron transporting and hosting are in high demand, as they
simplify the device construction. Although ETL materials have been
tested as host-emitters in PHOLEDs, study on their dual nature is
relatively less common.13,19a,21 Due to the above-mentioned rea-
sons, TPBI was introduced in place of Alq3 as an electron-trans-
porting material/hole-blocking material between the cathode and
the emitting layer, as in the configuration of device C (ITO/NPB
(400 A)/Ir(PPy)3:1 (6%, 400 A)/TPBI (400 A)/LiF (10 A)/Al (1500 A)).
As expected, 1 revealed maximum luminance efficiency of 18.6 cd/
A, maximum power efficiency of 9.3 lm/W and maximum external
quantum efficiency of 5.4% demonstrating superior performance
under device C conditions. It is noteworthy that at a current density
of 20 mA/cm2 (13.8 V) the device exhibited a brightness of 2130 cd/
m2 with a luminance efficiency of 10.6 cd/A. Such an enhancement
in the EL efficiencies of device C can be readily pointed to facile
electron injection into the EM layer (barrier¼0.55 eV) from the TPBI
layer; with Alq3, the electron-injection barrier is estimated to be
1.15 eV. The use of TPBI in place of Alq3 reduces the electron-
injection barrier considerably (0.6 eV). The advantage associated
with TPBI is that it can be used both as an electron-transporting
material as well as a hole-blocking material (HOMO¼6.2 eV;
LUMO¼2.7 eV).22 Evidently, qualities of TPBI that include higher
lying LUMOs, wide band-gap energy and higher triplet energy
(ET¼2.74 eV)23 contribute to the enhancement of the device C. Thus,
the performance of the TPBI-based device C, in general, was found
to be superior when compared to the Alq3-based device A and the
device lacking ETL, i.e., B. Therefore, compound 2 was fabricated
with a similar configuration as that of device C. The resultant device
D for oxadiazole 2 exhibited respectable EL performance with
maximum luminance and external quantum efficiencies 15.1 cd/A
and 4.7%, respectively; the power efficiency for this device was
found to be 7.9 lm/W. At a current density of 20 mA/cm2, the lu-
minance and external quantum efficiencies were found to be 7.0 cd/
A and 2.1%, respectively. Despite the fact that Ir(PPy)3 (HOMO-
¼5.4 eV; LUMO¼2.9 eV)19c possesses greater depth of 0.66 eV for
trapping holes in the host 2 (than in 1, 0.46 eV), the better EL device
performance of device C than device D is probably due to the higher
LUMO level of 1 for efficient electron trapping. In other words, the
HOMO and LUMO levels of the host oxadiazole 1 that lie 0.46 eV
below and 0.75 eV above those for Ir(PPy)3, respectively, are crucial
for trapping holes and electrons to form excitons effectively in the
dopant sites. The dramatic increase in the turn-on voltage in device
D could be attributed to the difficulty in charge injection due to the
presence of bulky mesitylene units at the termini in the case of 2.
Otherwise, it is instructive to note that the performances of devices
C and D are improved by 3e4 times when compared with those for
the device A. The device efficiencies for 1 and 2 are indeed very
comparable to other non-bipolar oxadiazole-based host-emitters,
for example, BPBO,13a and TPOs and PPOs.13b Based on the color
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Table 2
Device characteristics of PHOLEDs based on oxadiazoles
EL
max
Devicea
l
ðnmÞ Von
hlc
hp
hex
Lmax
FWHMg CIEh x, y
b
d
e
f
A
B
C
508
512
512
512
4.7
4.5
4.5
5.8
4.6 1.4 1.4
9.4 3.3 2.9
18.6 9.3 5.4
15.1 7.9 4.7
3326 56
4493 64
4397 72
3254 64
0.26, 0.62
0.28, 0.63
0.30, 0.62
0.29, 0.62
D
a
ꢀ
The configurations of four devices are: (A) ITO/NPB (400 A)/Ir(PPy)3:1 (6%,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
400 A)/Alq3 (400 A)/LiF (10 A)/Al (1500 A), (B) ITO/NPB (400 A)/Ir(PPy)3:1 (6%,
400 A)/LiF (10 A)/Al (1500 A), (C) ITO/NPB (400 A)/Ir(PPy)3:1 (6%, 400 A)/TPBI
(400 A)/LiF (10 A)/Al (1500 A), and (D) ITO/NPB (400 A)/Ir(PPy)3:2 (6%, 400 A)/TPBI
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(400 A)/LiF (10 A)/Al (1500 A).
b
Turn-on voltage (V).
Maximum luminance efficiency (cd/A).
Maximum power efficiency (lm/W).
c
d
e
Maximum external quantum efficiency (%).
f
Maximum luminance achieved (cd/m2).
g
Full-width at half-maximum from the EL spectrum.
1931 CIE chromaticity coordinates.
h

P. Venkatakrishnan et al. / Tetrahedron 68 (2012) 7502e7508
7507
chromaticity diagram, the CIE coordinates of all these devices in-
dicate a yellowish-green electrophosphorescence emission. Clearly,
more experimentation entailing doping concentration and hole-
transport as well as electron-transport materials with better
matching of HOMO and LUMOs facilitating triplet-exciton con-
finement should lead to better device performance characteristics,
and we are currently exploring these possibilities. Although pres-
ently observed results with minimum device experimentation are
not readily comparable to some of the best results with oxadiazoles,
they nonetheless emphasize the utility of bimesityl-based oxadia-
zoles as a new class of electron-transport-type host materials.
added NaN3 (2.2 g, 33.9 mmol) and NH4Cl (1.8 g, 33.9 mmol) and
the reaction mixture was heated at 120 ꢀC. The turbid solution
became clear in 5 h. The heating was continued for further 18 h, the
reaction mixture was cooled and poured into ice. Resulting solid
was filtered and washed with CHCl3 to obtain 4 as a pure colorless
solid in 86% yield, mp 271e274 ꢀC: IR (KBr, cmꢁ1) 3003, 2948, 2860,
1635, 1452, 837; 1H NMR (CDCl3, 500 MHz)
d 1.97 (s, 6H), 2.33 (s,
3H), 6.95 (s, 2H), 7.27 (d, J¼8.25 Hz, 2H), 7.72 (d, J¼8.25 Hz, 4H); 13C
NMR (CDCl3, 125 MHz)
d 20.8, 21.2, 110.7, 119.2, 121.5, 128.5, 130.4,
132.4, 137.7, 146.5, 155.4.
4.2.2. Preparation of 3,30-bis(4-carboxylphenyl)bimesityl. To a solu-
tion of 3,30-bis(4-formylphenyl)bimesityl16c (3.0 g, 6.7 mmol) in
30 mL of DMF and 15 mL of CH2Cl2 and oxone (KHSO5) (8.3 g,
13.4 mmol) was added. The reaction mixture was stirred at room
temperature for 3 h, solvent removed under reduced pressure,
acidified with 10% HCl, and extracted with ethyl acetate. The
combined extracts were washed with water, dried over Na2SO4, and
concentrated. The pure 3,30-bis(4-carboxylphenyl)bimesityl was
obtained as a colorless solid after recrystallization from 20:80 ethyl
acetate and petroleum ether mixture, yield 96%, mp 278e282 ꢀC: IR
(KBr, cmꢁ1) 3003, 2918, 2820, 1706; 1H NMR (DMSO-d6, 500 MHz)
3. Conclusions
It is shown that bimesityl core can be exploited to develop host
materials for phosphorescent light emitting diodes. The twofold
oxadiazole-functionalized bimesitylenes 1 and 2 are shownto display
significantly high glass transition temperatures, thermal stabilities
and permit vacuum sublimation as a means for the construction of
devices. The functional (electron-transporting as well as hosting)
behavior of these materials has been demonstrated by fabricating
yellowish-green-emitting electrophosphorescent devices that ex-
hibit luminance efficiencies as high as 18.6 cd/A. Clearly, the bimesityl
scaffold allows rational design and functionalization to produce ma-
terials of varying properties for application in OLEDs, and we are
endeavoring to develop new materials based on this design.
d
1.49 (s, 6H),1.85 (s, 6H),1.97 (s, 6H), 7.08 (s, 2H), 7.21e7.25 (m, 4H),
7.95e7.99 (m, 4H); 13C NMR (DMSO-d6, 125 MHz)
d
18.2, 20.2, 20.9,
129.6, 129.8, 130.1, 132.1, 132.8, 133.8, 134.7, 138.0, 139.3, 146.3,
167.8; FABMS Mþ 478.21.
4. Experimental section
4.2.3. Preparation of oxadiazole (1). 3,30-Bis(4-carboxylphenyl)
bimesityl (1.5 g, 3.2 mmol) was reacted with an excess amount of
SOCl2 (ca. 5 mL). The unreacted thionyl chloride was first removed
and the volatiles were removed in vacuo at 50 ꢀC. The resulting acid
chloride was taken to the next step immediately without further
purification. To 3,30-bis(4-chlorocarbonylphenyl)bimesityl obtained
above was added p-tert-butylphenyltetrazole (3) (1.9 g, 9.4 mmol) in
pyridine (15 mL) drop-wise under constant stirring. The whole of the
solution was heated at 110 ꢀC for a period of 6 h under a nitrogen gas
atmosphere; during the stirring, a white precipitate gradually
appeared. The reaction mixture was subjected to silica gel column
chromatography using 10% ethyl acetate in CHCl3 as an eluent to af-
ford compound 1 as a colorless solid in 82% yield, mp 302e305 ꢀC: IR
(KBr, cmꢁ1) 2955, 2859, 1602, 1551, 1483, 1449; 1H NMR (CDCl3,
4.1. Materials and characterization
All palladium-mediated cross-coupling reactions were performed
under a nitrogen gas atmosphere in oven-dried pressure tubes. An-
hydrous tetrahydrofuran(THF)andtoluenewerefreshlydistilledover
sodium prior to use. All the reactions were monitored by analytical
thin layer chromatography (TLC) using commercial aluminum sheets
pre-coated with silica gel. Chromatography was conducted on silica
gel (Acme, Mumbai, 60e120 mesh). All the commercial chemicals
were used as received. The starting materials, viz. bimesityl, diiodo-
bimesityl, and 3,30-bis(4-formylphenyl)bimesityl were synthesized
according to the earlier reported procedures established in our lab-
oratories;16 p-tert-butylphenyltetrazole 324 and p-mesitylbenzoni-
trile25dthe precursor for 4dwere synthesized according to the
literature known protocol.24,25 The detailed synthetic procedures for
preparing 1 and 2 are given below (cf. Scheme 1). 1H and 13C NMR
spectra were recorded on Bruker (500 MHz) spectrometer in CDCl3 as
500 MHz)
2H), 7.33e7.38 (m, 4H), 7.56 (d, J¼8.25 Hz, 4H), 8.08 (d, J¼8.25 Hz,
4H), 8.18e8.22 (m, 4H); 13C NMR (CDCl3, 125 MHz)
18.0, 20.1, 20.9,
d 1.37 (s, 18H), 1.65 (s, 6H), 1.97 (s, 6H), 2.06 (s, 6H), 7.09 (s,
d
31.2, 35.2, 121.2, 122.4, 126.2, 126.9, 127.2, 129.2, 130.4, 133.2, 134.1,
135.1, 138.2, 138.9, 145.8, 155.5; HRMS (ESI-MS) m/z calcd for
C54H54N4O2 790.4325, found 791.4324 (Mþ1). Anal. Calcd for
C54H54N4O2: C, 81.99; H, 6.88; N, 7.08. Found: C, 81.42; H, 6.78; N, 7.04.
a solvent. Chemical shifts are reported in d-scale downfield from
tetramethylsilane. The FAB-mass spectra were recorded on Jeol SX-
102/mass system using argon/xenon as the FAB gas in a linear mode
with m-nitrobenzyl alcohol as a matrix. The high-resolution ESI-mass
spectra were recorded on Waters Q-TOF mass spectrometer. The el-
emental analyses were performed on a Thermoquest CE (EA1110)
elemental analyzer. TGA and DSC analyses were carried out using
aPerkineElmerinstrument(DSC-7)at10 ꢀC/minunderanitrogengas
atmosphere. UVevis absorption spectra were recorded using a Jasco-
550 spectrophotometer. Photoluminescence spectra were recorded
with Spex Fluolog-2 spectrofluorimeter. The UVevis absorption,
photoluminescence, and phosphorescence spectra were recorded
using the spectral grade solvents. The ionization potentials were
calculated by photoelectron spectroscopy (Riken Keiki, AC2) with UV
intensity of 50 nW.
4.2.4. Preparation of oxadiazole (2). This compound was synthe-
sized using p-mesitylphenyltetrazole following the procedure used
above for the preparation of oxadiazole,1, 87% yield, mp 315e320 ꢀC:
IR (KBr, cmꢁ1) 3028, 2963, 2867, 1617, 1494; 1H NMR (CDCl3,
500 MHz)
12H), 7.09 (s, 6H), 7.34e7.38 (m, 8H), 8.04 (d, J¼8.25 Hz, 4H),
8.18e8.22 (m, 4H); 13C NMR (CDCl3, 125 MHz)
17.9, 20.1, 20.9, 21.3,
d 1.65 (s, 6H), 1.74 (s, 6H), 1.97 (s, 6H), 2.06 (s, 6H), 2.45 (s,
d
121.2, 122.4, 126.2, 126.9, 127.1, 127.2, 129.3, 130.0, 130.1, 130.4, 133.0,
133.9,135.0,135.2,138.1,138.9,148.9,155.5; HRMS (ESI-MS) m/z calcd
for C64H58N4O2 914.46333, found 915.4642 (Mþ1). Anal. Calcd for
C64H58N4O2:C, 83.99;H, 6.39; N, 6.12. Found: C, 84.40;H, 6.39;N, 6.18.
4.3. Optical measurements
4.2. Synthetic procedure
UVevis absorption spectra were recorded using ca. 10ꢁ5 M so-
lutions in chloroform. Fluorescence measurements were performed
on dilute solutions (ca. 10ꢁ6 M) in chloroform and emission
4.2.1. Preparation of p-mesitylphenyltetrazole (4). To a solution of p-
mesitylbenzonitrile25 (5.0 g, 22.6 mmol) in 20.0 mL of DMF were

7508
P. Venkatakrishnan et al. / Tetrahedron 68 (2012) 7502e7508
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bimesitylene-based oxadiazoles 1 and 2 as host emitting materials.
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U per square were cleaned
sequentially in an ultrasonicator using acetone, detergent solution,
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ꢀ
range 0.1e0.5 A/s. The evaporation rate and thickness of the organic
layers were monitored by a quartz oscillator. After the vacuum
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rapid analyzer system.
4.6. Electrophosphorescence measurements
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Current voltage and light intensity measurements were done on
a Keithley 2400 Source meter and a Newport 1835C Optical meter
equippedwithaNewport818-STsiliconphotodiode, respectively. The
device was placed close to the photodiode such that all the forward
light entered the photodiode. The effective size of the emitting diode
was 4.0 mm2, which is significantly smaller than the active area of the
photodiode detector, a condition known as ‘under-filling’, satisfying
the measurement protocol.26 The EL spectra were recorded using
Hitachi F-4500 spectrofluorimeter by blocking the incident radiation.
The Commission Internationale de l’Eclairage (CIE) coordinates of the
devices were measured by a PR650 spectroscan spectrometer.
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Acknowledgements
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J.N.M. is thankful to the Department of Science and Technology
(DST), India for generous financial support. P.N. is grateful to UGC
for a senior research fellowship. We are thankful to Prof. Shih-
Sheng Sun for his generous help with the phosphorescence spectra.
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Supplementary data
1H and 13C NMR scans and mass spectra for the intermediates
and final compounds 1 and 2. Supplementary data associated with
this article can be found in the online version, at doi:10.1016/
j.tet.2012.05.101.
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