Synthesis of novel twisted carbazole-quinoxaline derivatives with 1,3,5-benzene core: Bipolar molecules as hosts for phosphorescent OLEDs

Article abstract of DOI:10.1016/j.tetlet.2011.10.074

A series of carbazole/quinoxaline hybrids have been synthesized by classic Ullmann and Pd/Cu-catalyzed Sonogashira coupling reaction. Their photophysical, thermal, and electrochemical properties were investigated. The introduction of electron rich carbazole and electron deficient quinoxaline on to the 1,3,5-benzene center leads to a twisted structure with good glass forming property and imparts a bipolar character. The triplet energies in the range of 2.34-2.53 eV indicate them as potential host materials in phosphorescent OLEDs.

Full text of DOI:10.1016/j.tetlet.2011.10.074

Tetrahedron Letters 52 (2011) 6942–6947
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of novel twisted carbazole–quinoxaline derivatives with
1,3,5-benzene core: bipolar molecules as hosts for phosphorescent OLEDs
M. Ananth Reddy a,b, Anup Thomas a, G. Mallesham a,b, B. Sridhar c, , V. Jayathirtha Rao b,
,
⇑
⇑
K. Bhanuprakash a,
⇑
a Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 607, India
b Organic Chemistry Division (II), Indian Institute of Chemical Technology, Hyderabad 500 607, India
c Laboratory of X-Ray Crystallography, Indian Institute of Chemical Technology, Hyderabad 500 607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of carbazole/quinoxaline hybrids have been synthesized by classic Ullmann and Pd/Cu-catalyzed
Sonogashira coupling reaction. Their photophysical, thermal, and electrochemical properties were inves-
tigated. The introduction of electron rich carbazole and electron deficient quinoxaline on to the 1,3,
5-benzene center leads to twisted structure with good glass forming property and imparts bipolar char-
acter. The triplet energies in the range of 2.34–2.53 eV indicate them as potential host materials in phos-
phorescent OLEDs.
Received 22 August 2011
Revised 12 October 2011
Accepted 13 October 2011
Available online 18 October 2011
Keywords:
Carbazole
Ó 2011 Elsevier Ltd. All rights reserved.
Quinoxaline
Amorphous material
Bipolar nature
PHOLEDs
Phosphorescent OLEDs (PHOLEDs) have gained much attention
leading to a surge in design and synthesis of new materials as
hosts, charge transporting materials, and emitters, presumably
because of the possibility of 100% maximum internal quantum effi-
ciency for phosphorescence based OLEDs.1,2 PHOLEDs are able to
harvest both singlet and triplet excitons for light emission, thus
offer a tremendous possibility of highly efficient OLEDs.2 Triplet
emitters however owing to their relatively long emissive lifetimes,
tend to be less efficient because of concentration quenching and
triplet–triplet (T1–T1) annihilation during device operation.3 An
effective way to resolve this limitation is to dope the triplet emit-
ters into organic host materials.4 The host material in the emitting
layer also serves as a recombination center for holes and electrons
to generate the electronically excited states, followed by excitation
energy transfer from the host to dopant.5 Most of the existing trip-
let host materials are capable of preferentially transporting holes
or electrons.6 Due to the lack of bipolar character of the emitter,
layer recombination occurs at the interface with the charge trans-
port layer.7 One approach to improve the performance of phospho-
rescent OLEDs is to use bipolar host materials.8 Bipolar host
materials contribute to the balanced transport of carriers and help
to increase the probability of carrier recombination.8 Recently
there is a great deal of activity in developing new bipolar host
materials.4,8,9 In addition to the bipolar nature, the host materials
should have amorphous nature for better device stability. Carba-
zole derivatives are widely used as host materials because of their
high triplet energy and good hole-transporting properties.6d,10
A
few examples of generally used carbazole based hosts are 4,40-
N,N0-dicarbazole-biphenyl (CBP),11 and 1,3-di(9H-carbazol-9-
yl)benzene (mCP),6b but are prone to crystallization due to low
glass transition temperatures (CBP = 62 °C, mCP = 60 °C), which is
a major drawback for practical applications.
To our knowledge no study based on carbazole–quinoxaline hy-
brids as bipolar host material has been reported. In this context we
designed bipolar, carbazole/quinoxaline coupled hybrids (Chart 1)
having hole transporting (carbazole) and electron transporting
(quinoxaline) properties.12 The unique shape/geometry of these
carbazole/quinoxaline hybrids further offers better thermal and
film forming properties. The structural design introduced in these
molecules (Chart 1) with triplet excited state can contribute
toward PHOLEDs with good charge balance.
Synthesis of key intermediates (8, 9, 11, 13) are shown in Scheme
1. Classic Ullmann coupling procedure was adopted in making 1, 2, &
3 (Scheme 1). The selectivity in making 1 was achieved by using
>1.5 equiv of 1,3,5-tribromobenzene. 1,4-dibromobenzene was
used in making 3. tert-Butyl bromobenzene was reacted with
TMS–acetylene under Sonogashira conditions to get compound 4
and this was converted readily into 4-tert-butylphenylacetylene
⇑
Corresponding authors. Tel.: +91 40 27191429/1430; fax: +91 40 27160921
(K.B.).
E-mail address: bhanu2505@yahoo.co.in (K. Bhanuprakash).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.074

M. Ananth Reddy et al. / Tetrahedron Letters 52 (2011) 6942–6947
6943
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
C1Q2
C1Q2-T
C2Q1
CPQ
Chart 1. Structures of the benzene centered carbazole–quinoxaline hybrid compounds.
N
1
2
Br
Br
Br
N
i
+
N
H
Br
Br
N
Br
i
Br
N
+
Br
Br
N
H
3
iii
ii
H
TMS
Br
TMS-Acetylene
5
4
1-bromo-4-tert-butylbenzene
N
N
N
ii
R
R
iv
iv
iv
+
Br
Br
R
O
O
O
8 : R = H
9 : R = tert-butyl
O
6 : R = H
R
R
R = H ( Phenyl acetylene)
7
: R = tert-butyl
R = tert-butyl (5)
N
N
O
N
N
N
N
+
ii
11
Br
O
10
O
ii
N
Br
N
N
+
O
13
12
Scheme 1. Synthetic routes adopted in making key intermediates. Reagents and conditions: (i) CuI, K2CO3, 1,10-phenanthroline, DMF, reflux; (ii) Pd(PPh3)2Cl2, CuI, TEA,
Toluene, 80 °C; (iii) K2CO3, MeOH, CH2Cl2, rt; (iv) KMnO4, NaHCO3, MgSO4ꢀ7H2O, Acetone, 4 h, rt .

6944
M. Ananth Reddy et al. / Tetrahedron Letters 52 (2011) 6942–6947
O
N
N
N
N
N
O
O
O
O
O
O
O
O
O
O
O
9
11
13
8
v
v
v
v
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
C1Q2
C1Q2-T
C2Q1
CPQ
Scheme 2. Synthesis of C1Q2, C1Q2-T, C2Q1 and CPQ using key intermediates. Reagents and conditions: 1,2-phenylenediamine, CHCl3, PTSA, reflux.
5.13 Sonogashira coupling condition developed in making com-
pounds 6, 7, 10, and 12 (Scheme 1). Compounds 6, 7, 10, and 12 were
subjected to triple bond oxidation leading to diketones (11 and 13)
and tetraketones (8 and 9) as the main intermediates for the final
compounds.14 Scheme 2 illustrates the synthesis of final carba-
zole/quinoxaline coupled hybrid compounds. The diketones 11
and 13 and teraketones 8 and 9 were cyclocondensed with o-phen-
ylenediamine to get final carbazole–quinoxaline bipolar benzene
centered hybrid compounds. The compounds are characterized by
spectroscopic analysis and additionally X-ray crystal structures for
C1Q2 and C1Q2-T are also solved to understand the molecular inter-
actions in the solid state (Fig. 1).
Figure 1. ORTEP diagrams of C1Q2 and C1Q2-T.

M. Ananth Reddy et al. / Tetrahedron Letters 52 (2011) 6942–6947
6945
Fluorescence
on emission than on absorption band, indicates a small dipole
moment of the ground state relative to the dipole of excited
state.16 This suggests intramolecular charge transfer occurring
from the less polar ground state to the more polar excited state.
The incredibly large stokes shift of >100 nm in polar solvents like
acetonitrile and DMF clearly shows the stabilization of the intra-
molecular CT excited state. The relative fluorescence quantum
yields (Uf) of these compounds were measured in dilute toluene
solution using 9,10-diphenylanthracene (Uf ꢁ0.9) as standard.17a
The low quantum yields of fluorescence for these derivatives are
also explained by this characteristic ICT state.16
Phosphorescence
Absorption
C1Q2
demo
demo
demo
demo
demo
440 nm
demo
493 nm
290 nm
520 nm
340 nm
demo
demo
demo
demo
C1Q2-T
290 nm
demo
demo
demo
demo
437 nm
demo
500 nm
531 nm
344 nm
demo
demo
demo
demo
demo
The basic requirement for any material to be applicable as an effi-
cient host material, the triplet energy (ET) of host should be higher
than that of guest to prevent reverse energy transfer from the guest
back to host and confine triplet excitons on guest molecules.18 Phos-
phorescent spectrum is red shifted by 60 nm compared to the fluo-
rescence spectrum. The triplet energy (ET), determined from the
highest-energy vibronic sub-band of the phosphorescence spec-
tra17b at 77 K in ethyl acetate matrix, follow the order of C2Q1
(2.53 eV) > C1Q2 (2.51 eV) > C1Q2-T (2.48 eV) > CPQ (2.34 eV). The
experimentally observed ET and DFT B3LYP calculated adiabatic sin-
glet–triplet gaps are in good agreement (Table 1).19 The ET values of
these compound are lower than the independent carbazole
(ET = 3.02 eV).20 The ET values of C2Q1 and C1Q2 are close to gener-
ally used host material CBP (2.56 eV),10a implying that they may act
as appropriate host materials for green emitter iridium(III) fac-
tris(2-phenylpyridine) [Ir(ppy)3, ET = 2.42 eV] and the red-emitter
C2Q1
44d6emno m
demo
demo
demo
demo
490 nm
291 nm
522 nm
demo
demo
demo
335 nm
demo
demo
CPQ
43d4emno m
demo
291 nmdemo
demo
demo
demo
524 nm
345 nm
demo
demo
demo
demo
200
250
300
350
400
450
500
550
600
Wavelength/ nm
Figure 2. UV–vis absorption, fluorescence and phosphorescence of C1Q2, C1Q2-T,
C2Q1 and CPQ. 10ꢂ5 M toluene solutions at rt for fluorescence; 10ꢂ5 M Ethyl acetate
solutions at 77 K for phosphorescence.
bis(2,4-diphenylquinolyl-N,C2)
iridium
(acetylacetonate)
[(ppq)2Ir(acac), ET = 2.01 eV].18b
Absorption, fluorescence and phosphorescence spectra were re-
corded for all the compounds and are shown in Figure 2 and sum-
marized in Table 1. Similar UV–Vis absorption spectra were
obtained for these compounds with two main absorption peaks,
a carbazole centered n–p⁄ transitions at ꢁ290 nm and less intense
absorption bands in the region ꢁ335–345 nm could be assigned to
The thermal stabilities of the new compounds were investi-
gated by thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) (Table 1 and Fig. 4). Their decomposition
temperatures (5% weight loss) range from 420 to 470 °C. A key
physical parameter which indicates the stability of the amorphous
state of a given material is the glass transition temperature (Tg).
1,4-Substituted, CPQ derivative has the lowest glass transition
(Tg = 81 °C), whereas the 1,3,5- trisubstituted, meta linked deriva-
tives C1Q2, C1Q2-T, C2Q1 exhibit high Tg values of 138, 132, and
134 °C ,respectively, and are much higher than the most commonly
used host materials namely (CBP, 62 °C) and (mCP, 60 °C)10b indi-
cating that these are preferred.
p–
p⁄ transitions from electron rich carbazole to electron accepting
quinoxaline moiety.15,9f All the compounds emit in blue region
with the emission peaks in the range of 434–440 nm in toluene
and 427–450 nm in the solid state. The similarity of emission in
solution and solid suggests weak interactions in the solid state.
C1Q2 and C1Q2-T have optical bandgap values of 3.33 and
3.35 eV, respectively (Table 1). CPQ has the lowest bandgap of
The electrochemical behavior of the four new derivatives C1Q2,
C1Q2-T, C2Q1, and CPQ is probed by cyclic voltammetry (CV) with
standard calomel electrode as reference in CH2Cl2 solvent. All of
them showed reversible/quasireversible oxidations and reversible
reductions. An additional peak observed for the oxidation scan of
C1Q2, C1Q2-T, and CPQ at ꢁ1.1 V is attributed to electropolymer-
3.24 eV, which is attributed to better
p-conjugation of the para-
linked over the meta substituted derivatives. The influence of sol-
vent polarity on absorption wavelengths was negligible (no red
shifts), but prominent bathochromic shifts were observed for the
emission bands for all the compounds. Figure 3 shows the effect
of solvent on fluorescence. The influence of solvents polarity, more
Table 1
Photophysical, thermal and electrochemical data of C1Q2, C1Q2-T, C2Q1 and CPQ
Compds.
Photo physical
Thermal
Electrochemical
HOMOf,g (eV)
a
a,b
c
e
Eogpt (eV)
d
Eo1 xd (V)
Er1ed (V)
kabs (nm)
kem (nm)
Uf
ET (eV)
Td/Tg (°C)
LUMOg,h (eV)
HLGi
2
2
C1Q2
C1Q2-T
C2Q1
CPQ
340, 290
344, 290
335, 291
345, 291
440(446)
437(451)
446(443)
434(427)
0.27
0.35
0.19
0.32
3.33
3.35
3.42
3.24
425/138
470/132
420/134
395/81
1.37
1.43
1.08, 1.39
1.37
ꢂ1.62
ꢂ1.68
ꢂ1.57
ꢂ1.64
ꢂ5.77 (ꢂ5.62)
ꢂ5.83 (ꢂ5.62)
ꢂ5.48 (ꢂ5.75)
ꢂ5.77 (ꢂ5.69)
ꢂ2.78 (ꢂ2.46)
ꢂ2.72 (ꢂ2.39)
ꢂ2.83 (ꢂ2.53)
ꢂ2.76 (ꢂ2.42)
2.99 (3.16)
3.11 (3.24)
2.65 (3.22)
3.01 (3.27)
2.48 (2.47)
2.53 (2.49)
2.34 (2.42)
a
b
c
Recorded in 10ꢂ5 M Solutions of toluene at rt.
Values in parentheses correspond to fluorescence in solid state.
Quantum yield (Uf) measured in dilute toluene relative to that of 9,10-diphenyl anthracene(0.9 in cyclohexane). Excitation wavelength = 340 nm.
Optical band gap estimated from the absorption threshold.
d
e
Triplet energy obtained from the highest energy transition of the phosphorescence spectrum. Spectra recorded in ethyl acetate at 77 K. The values in parentheses are
theoretically estimated using B3LYP/6-31g(d,p).
f
Obtained by adding 4.4 to the E1/2 oxidation potentials.
The B3LYP/6-311+G(d,p)//6-31G(d,p) obtained values are given in parenthesis.
Obtained by adding 4.4 to the E1/2 reduction potentials.
HOMO-LUMO gap (HLG), theoretical obtained values are in parentheses.
g
h
i

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M. Ananth Reddy et al. / Tetrahedron Letters 52 (2011) 6942–6947
Hexane
Toluene
CHCl3
Hexane
Toluene
CHCl3
1.0
0.8
0.6
0.4
0.2
0.0
1.0
C1Q2-T
C1Q2
THF
DMF
ACN
THF
DMF
ACN
0.8
0.6
0.4
0.2
0.0
350
400
450
500
550
600
650
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Hexane
Toluene
CHCl3
1.0
0.8
0.6
0.4
0.2
0.0
CPQ
1.0
0.8
0.6
0.4
0.2
0.0
C2Q1
Hexane
Toluene
CHCl3
THF
DMF
ACN
THF
DMF
ACN
350
400
450
500
550
600
650
700
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 3. Solvent polarity effect on the fluorescence of C1Q2, C1Q2-T, C2Q1 and CPQ at 10ꢂ5 M solutions.
ization at the 3rd and 6th positions of carbazole.9f The formation of
stable radical cation and radical anion suggests bipolar nature of all
the synthesized molecules. Table 1 summarizes the HOMO and
LUMO levels determined from the E1/2 oxidation and reduction po-
tential values.21 The HOMO levels varied from ꢂ5.48 to ꢂ5.83 eV
and the LUMO values were in the range ꢂ2.72 to ꢂ2.83 eV for
the four derivatives. C1Q2, C1Q2-T, and CPQ have similar HOMO
energy levels (ꢁ5.8 eV), while C2Q1 has higher value at ꢂ5.48 eV.
The HOMO and LUMO levels determined at B3LYP/6-311+G(d,p)//
B3LYP/6-31G(d,p) level are also shown in the same table. A good
correlation with the experimental data is seen. The distribution
of HOMO and LUMO electron densities obtained by DFT calcula-
tions is shown in Supplementary data. From the density distribu-
tion it is clear that for all the molecules HOMO is mainly located
on the carbazole group while LUMO on qunioxaline groups. It is
interesting to note that for C2Q1 the distribution of HOMO is only
on one carbazole group. C2Q1 undergoes two quasireversible oxi-
dations (E1/2 = 1.08 and 1.39 V) which could be understood in
terms of the electron density distribution of HOMO-1 (Supplemen-
tary data).
In conclusion, we have described a facile and efficient method
for the synthesis of benzene centered novel carbazole–quinoxaline
hybrid derivatives. The new compounds exhibit excellent thermal
and morphological stabilities owing to the twisted geometry of the
molecules. The optical studies reveal the interaction of the donor
carbazole with the electron deficient quinoxaline in the molecule.
The bipolar nature of these compounds along with high thermal
stability (Tg ꢁ138 °C) and favourable electrochemical properties
makes them promising for their potential use as host materials
in red and green phosphorescent based OLEDs.
Acknowledgments
This work has been carried out under NWP-0025 of CSIR-New
Delhi. M.A.R. and G.M. thank UGC for the fellowship, A.T. thanks
CSIR for the fellowship.
Figure 4. DSC thermograms for C1Q2, C1Q2-T, C2Q1 and CPQ, measured at 10 °C/
min under N2 atm.

M. Ananth Reddy et al. / Tetrahedron Letters 52 (2011) 6942–6947
6947
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Supplementary data
Supplementary data (The synthesis, 1H and 13C NMR spectra,
molecular orbital pictures, crystal data and cyclic voltammograms
(CV) of compounds C1Q2, C1Q2-T, C2Q1, CPQ) associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.10.074.
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