Solution processable indoloquinoxaline derivatives containing bulky polyaromatic hydrocarbons: Synthesis, optical spectra, and electroluminescence

Article abstract of DOI:10.1021/jo2004764

New hybrid materials featuring the dipolar fragment 6H-indolo[2,3-b] quinoxaline attached to the bulkier polyaromatic hydrocarbons such as fluoranthene, triphenylene, or polyphenylated benzene have been synthesized by a two-step procedure involving Sonogashira and Diels-Alder reactions. They were characterized by absorption, emission, electrochemical, thermal, and theoretical investigations. The electronic properties of the compounds were dominated by the 6H-indolo[2,3-b]quinoxaline chromophore, and the incorporation of polyaromatic hydrocarbons reduces the chances of nonradiative deactivation processes associated with the excited state and improves the emission properties. The compounds displayed cyan emission with moderate quantum efficiency when excited at the absorption maximum. All of the compounds exhibited an irreversible reduction process corresponding to the addition of electron at the quinoxaline segment. They showed moderate thermal stability and glass transition temperature greater than 100 °C. The presence of rigid 6H-indolo[2,3-b]quinoxaline and bulkier polyaromatic hydrocarbon segments enhances the thermal stability and glass transition temperature significantly. Finally, the dyes were successfully applied as an electron-transporting and emitting layer in multilayered organic light-emitting diodes comprising a N,N′-bis(l-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4, 4′-diamine hole-transporting layer. The cyan emitting devices were characterized by moderate device performance parameters.

Full text of DOI:10.1021/jo2004764

ARTICLE
pubs.acs.org/joc
Solution Processable Indoloquinoxaline Derivatives Containing
Bulky Polyaromatic Hydrocarbons: Synthesis, Optical Spectra,
and Electroluminescence
Payal Tyagi, A. Venkateswararao, and K. R. Justin Thomas*
Organic Materials Lab, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
S Supporting Information
b
ABSTRACT: New hybrid materials featuring the dipolar frag-
ment 6H-indolo[2,3-b]quinoxaline attached to the bulkier
polyaromatic hydrocarbons such as fluoranthene, triphenylene,
or polyphenylated benzene have been synthesized by a two-step
procedure involving Sonogashira and DielsꢀAlder reactions.
They were characterized by absorption, emission, electroche-
mical, thermal, and theoretical investigations. The electronic
properties of the compounds were dominated by the 6H-
indolo[2,3-b]quinoxaline chromophore, and the incorporation
of polyaromatic hydrocarbons reduces the chances of nonra-
diative deactivation processes associated with the excited state and improves the emission properties. The compounds displayed
cyan emission with moderate quantum efficiency when excited at the absorption maximum. All of the compounds exhibited an
irreversible reduction process corresponding to the addition of electron at the quinoxaline segment. They showed moderate thermal
stability and glass transition temperature greater than 100 °C. The presence of rigid 6H-indolo[2,3-b]quinoxaline and bulkier
polyaromatic hydrocarbon segments enhances the thermal stability and glass transition temperature significantly. Finally, the dyes
were successfully applied as an electron-transporting and emitting layer in multilayered organic light-emitting diodes comprising a
N,N0-bis(l-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine hole-transporting layer. The cyan emitting devices were character-
ized by moderate device performance parameters.
’ INTRODUCTION
in the solid state due to their twisted structure.10 Interest in
polyaromatic hydrocarbons containing arylamines stemmed
partially from their bright red-shifted emission and favorable
hole-transport properties when compared to the parent systems.6
Arylamine substituted polyaromatic hydrocarbons have also
been found to suppress the close molecular association and lead
to improved emission and hole-transport properties.10 Polyphe-
nylated phenylene dendrons anchored on low-band gap chro-
mophores such as benzothiadiazole have been developed to
achieve red emission and dual transport properties.11 In addition,
the polyphenylated aromatic cores serve as a platform for the
disposition of electron-donating and -accepting chromophores in
precise locations and pave the way for the tuning of do-
norꢀacceptor interactions. Even though many polyaromatic
hydrocarbons and their derivatives have been exploited for
applications in electro-optics, their heteroaromatic analogues
remain scarce.12
Functional materials derived from polyaromatic hydrocarbons
such as fluorene,1 anthracene,2 pyrene,3 perylene,4 triphenylene5
and fluoranthene6 have been intensively studied in recent years
due to their promising emission thermal and charge transport
properties. They are widely used as emitters in organic light-
emitting diodes, semiconducting constituents in thin-film tran-
sistors, reporters in fluorescent sensors and π-conjugating chro-
mophores in nonlinear optical materials. As a result of their
extended conjugation and rigidity, they display strong emission
with relatively low Stokes shifts. However, because of the planar
structure, they are prone to stronger intermolecular πꢀπ stack-
ing interactions, which often reduce their luminescence and
increase the propensity to form crystals in the solid state.7
Although the stacking interactions are beneficial for charge
transport properties, crystallinity and reduction in emission yield
or unwanted red-shift in emission in the solid state poses a severe
problem in the utilization of polyaromatic hydrocarbon derived
materials in organic light-emitting diodes.8
Earlier, we have reported the synthesis and characterization of
amine substituted indoloquinoxaline derivatives whereby we
observed that the nature of the conjugation connecting the
amine unit with the indoloquinoxaline segment is more impor-
tant than the nature of the aryl substituents on the amino group
One of the answers to the above-mentioned problem is the
introduction of bulkier and nonplanar structural elements in the
polyaromatic hydrocarbon core. For instance, substitution of tert-
butyl groups on anthracene hinders the stacking interactions.9
Similarly, the polyarylated polyaromatic hydrocarbons have also
been demonstrated to resist the close proximity of the molecules
Received: March 4, 2011
Published: May 03, 2011
r
2011 American Chemical Society
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to alter the absorption and emission characteristics.13 The
emission of the amine-substituted derivatives was quenched
due to the presence of strong intramolecular dipolar interactions.
In an attempt to improve the emission properties of the
molecular materials derived from the 6H-indolo[2,3-b]quinoxa-
line fragment, we have attached bulkier and nonplanar multiply
substituted polyaromatic hydrocarbons to the indole nucleus. As
there are no strong electron donors in these structures, they have
exhibited bright luminescence and opened avenues for applica-
tions in electroluminescent devices as emitting and electron-
transporting materials. In this report, we present the synthesis
and characterization of 6H-indolo[2,3-b]quinoxaline-based
M€ullen dendron-like14 molecular materials (Chart 1) and their
function as emitting and electron-transporting components in
double layer organic light-emitting diodes.
Chart 1. Structures of Polyaromatic Hydrocarbons (PAHs)
Containing Indoloquinoxaline Moiety
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The structures of the
compounds synthesized in this study are presented in Chart 1.
The synthetic routes used for the preparation of the new
materials are shown in Scheme 1. These compounds were easily
obtained by the DielsꢀAlder reaction between the suitably
substituted cyclopentadienones and indoloquinoxaline based
acetylenes followed by decarbonylation. The acetylenes required
for the synthesis were obtained by the Sonogashira coupling
Scheme 1. Synthetic Route to the Compounds
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Figure 1. Absorption spectra of the compounds recorded in
dichloromethane.
reaction15 of 9-bromo-6-butyl-6H-indolo[2,3-b]quinoxaline (1b)
with a corresponding terminal acetylene. All target compounds (3,
4, and 5) were characterized by NMR spectroscopy, mass spectro-
metry, and elemental analysis. The compounds are yellow in color
and reasonably soluble in solvents such as dichloromethane,
tetrahydrofuran, toluene, etc. and insoluble in alcohols. The dilute
solutions of the dyes appear yellow and fluoresce in the cyan
region. The analytically pure samples were used for the electro-
optical investigations.
Photophysical Properties. The photophysical behavior of
the compounds has been examined by measuring absorption and
fluorescence spectra in five different solvents, viz., cyclohexane
(ch), toluene (tol), tetrahydrofuran (thf), dichloromethane
(dcm) and acetonitrile (acn). The pertinent data are compiled
in Table 1 and Table S1 (Supporting Information). The absorp-
tion spectra recorded for the dyes in dichloromethane are
displayed in Figure 1. For comparison, the absorption spectrum
of the parent compound, 6-butyl-6H-indolo[3,2-b]quinoxaline
(1a) is also shown.
All of the compounds exhibit a well-resolved absorption
profile in the range of 250ꢀ460 nm. Invariably all of the
compounds show four absorption envelopes at ∼270, ∼340,
∼360 and ∼410 nm, respectively. Since these peaks are present
in the parent compound, 6-butyl-6H-indolo[3,2-b]quinoxaline
(1a), they are assigned to the indoloquinoxaline localized
electronic excitations.13 Among these four transitions, the three
higher energy absorptions are probably originating from the
πꢀπ* and nꢀπ* transitions, while the lower energy band is
attributed to the charge transfer transition between the indole
and quinoxaline segments. The mediocre intensity observed for
the charge transfer transition indicates a poor orbital overlap.
Such a weak charge transfer transitions are also reported for fused
donorꢀacceptor compounds such as thienopyrazines.16
Additionally, all of the compounds exhibit an intense absorp-
tion peak at 286ꢀ300 nm due to the aryl core of the M€ullen
dendron-like structure. The intensity of this band increases in the
order benzene < fluoranthene < triphenylene. The increase in
extinction coefficient for this band is probably attributed to the
progressive extension in conjugation on moving from benzene to
triphenylene. Extended delocalization of π-electrons over a large
area of a molecule is expected to broaden the absorption band
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Figure 4. Fluorescence spectra of the compounds recorded for the spin-
coated films.
Figure 2. Fluorescence spectra of compounds 3, 4, and 5 recorded in
cyclohexane.
Figure 5. Emission spectra of the compound 5b recorded for the spin-
coated film and at various concentrations in toluene.
Figure 3. Emission spectra of 4a recorded in different solvents.
and increase optical density.17 For the triphenylene based dyes 5a
and 5b, the absorption peaks appear at shorter wavelength (295
and 286 nm) when compared to those containing fluoranthene
cores (4a and 4b) (300 and 289 nm). From this it is clearly
evident that the fluoranthene segment has a slightly lower πꢀπ*
transition energy than the triphenylene analogue.18
A bright bluish green emission is observed for all of the
derivatives in cyclohexane (Figure 2). This underwent a pro-
nounced bathchromic shift on increasing the polarity of the
solvent as illustrated for compound 4a in Figure 3. The most red-
shifted emission for the each dye was observed for acetonitrile
solutions with substantial reduction in the fluorescence quantum
efficiency. These solvent-dependent emission charecteristics are
more likely to stem from the difference in the dielectric constant
of the medium, which would result in a more efficient solvation in
the excited state.19 Also, the excited state appears to be more
polar than the ground state in view of the large Stokes shifts
observed for these compounds (Table 1). A reasonably large
Stokes shift is also desirable for the application in electrolumi-
nescent devices as it ensures the reduction of self-absorption.
As solid films all of the compounds emit with appreciable
intensity (Figure 4), and the emission maxima for the films are
located between those observed for toluene and dichloro-
methane solutions with the exception of compound 5b. For
instance, 5a displays a blue-shifted emission profile in solid film
(485 nm) when compared to that observed for dichloromethane
solution (499 nm). This suggests that the dielectric constant for
the solid films of the compounds lies close to that in tetrahy-
drofuran and less than that in dichloromethane.20 However, for
the solid film of 5b two bands were observed with a green
emission band centered at 494 nm and a yellow emission band
peaking at 559 nm. Since the higher energy band is similar to the
emission band noticed in dilute dichloromethane solution, it can
be assigned to the monomer emission of compound 5b. To
identify the nature of the dual emission observed for compound
5b, the emission spectra of 5b in toluene with different concen-
trations were measured by exciting at 360 nm. The spectra
recorded in different concentrations are shown in Figure 5 along
with the photoluminescence spectrum of the solvent cast thin
film. On increasing the concentration, the intensity of the
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Figure 6. Frontier molecular orbitals of the dyes computed by using TDDFT at the B3LYP level.
emission band at 494 nm decreases and a new band at 559 nm
gradually appears. The higher wavelength emission band starts
appearing at a concentration of 1 ꢁ 10ꢀ3 M. At higher concen-
trations, probably the molecular aggregation takes place, leading
to excimer formation. It is possible that the excimers emit in the
red-shifted wavelength when compared to the monomer.21 It is
interesting to observe aggregation resulting from πꢀπ interac-
tions despite the presence of twisted aryl substituents. The πꢀπ
stacking interactions between the triphenylene moieties leading
to columnar structures have been well documented in the
literature, and this behavior was found to be responsible for their
special liquid-crystalline properties.22 It is to be noted here that
aggregation-induced emission, once considered as detrimental
for electroluminescence, has been exploited for applications such
as chemosensors and favorable charge transporting as long as
there is no aggregation-induced quenching.23
To understand the electronic structure of the compounds,
TDDFT calculations were performed on 3bꢀ5b using the
Gaussian 09 package.24 Figure 6 shows the theoretically com-
puted molecular orbitals in the ground states for compounds 3b,
4b and 5b. In all of the compounds the HOMO and LUMO are
mainly constituted by the indoloquinoxaline segment. This
clearly demonstrates that the HOMO to LUMO transition
occurring in these compounds at higher wavelength with low
optical density is originating from the indoloquinoxaline unit.
However, in the case of 4b and 5b the HOMO is spread over the
fluoranthene and triphenylene segments, respectively, which
explains the perturbations seen in the higher wavelength absorp-
tion for these compounds. The energies of the HOMO and
LUMO levels and the ground state dipole moment computed for
the dyes (3bꢀ5b) are also collected in Table 2. The frontier
orbital energies remain the same for all of the compounds, which
is consistent with the values deduced from the electrochemical
results (vide supra). This shows that the frontier orbitals are
derived from the same chromophore in all three compounds.
Vertical transitions for compounds 3bꢀ5b were calculated by
employing the TDDFT method. The transition energies, oscil-
lator strengths, and assignments for the most relevant singlet
excited states in the each molecule are listed in Tables 2. The
theoretically computed gas phase low energy vertical transitions
match well with the values realized for the compounds in
dichloromethane solutions. As the compounds have not shown
any solvatochromism in the ground state (see Supporting
Information for absorption spectra recorded in toluene
solutions), it is likely that the solvents play a less significant role
in the absorption profiles of the present compounds. The striking
resemblance of the experimental optical parameters to the
theoretically computed values using TDDFT and B3LYP/
6-31(d,p) is interesting to note here as the donorꢀacceptor
compounds are generally believed to show deviations from
TDDFT predictions at the B3LYP level. The next higher energy
transitions occurring in the range 330ꢀ360 nm are composed of
several electronic excitations such as HOMO-1 to LUMO,
HOMO-1 to LUMOþ1, and HOMO-3 to LUMO.
Electrochemical Properties. The redox propensity of the
materials is a crucial factor that affects the performance of the
OLEDs fabricated using them, as it will exert a direct impact on
the charge injection and mobility in the molecular layer.25 We
have examined the redox behavior of the compounds by the
cyclic voltammetric and differential pulse voltammetric methods
in dichloromethane solutions. The potentials are reported re-
lative to the internal standard, ferrocene. All of the molecules
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Table 2. Computed Vertical Excitation Energies, Dipole Moments, and Frontier Orbital Energies for Dyes 3bꢀ5b
compd
λmax, nm
f
assignment
μg, D
HOMO, eV
LUMO, eV
3b
407.2
336.3
328.4
299.3
294.6
408.7
386.0
362.7
0.025
0.065
0.242
0.297
0.124
0.039
0.047
0.194
HOMO f LUMO (97%)
HOMO-1 f LUMO (95%)
HOMO-3 f LUMO (83%)
HOMO f LUMOþ1 (88%)
HOMO f LUMOþ2 (82%)
HOMO f LUMO (93%)
HOMO f LUMOþ1 (78%)
HOMO-1 f LUMO (47%)
HOMO-1 f LUMOþ1 (29%)
HOMO-3 f LUMO (83%)
HOMO f LUMO (95%)
HOMO-4 f LUMO (48%)
HOMO-3 f LUMO (37%)
2.18
ꢀ5.42
ꢀ1.80
4b
5b
2.20
2.52
ꢀ5.39
ꢀ5.41
ꢀ1.81
ꢀ1.83
328.5
409.5
330.0
0.256
0.026
0.345
Table 3. Electrochemical and Thermal Data of the Compounds
compd
Ered, mVa,b
LUMO, eVc
HOMO, eVd
E0ꢀ0, eVe
Tg, °Cf
Tonset, °Cg
Td, °Ch
Tc, °Ci
Tm,°Cj
3a
4a
5a
3b
4b
5b
ꢀ2110 (i)
ꢀ2100 (i)
ꢀ2100 (i)
ꢀ1990 (i)
ꢀ2120 (i)
ꢀ2130 (i)
2.69
2.70
2.70
2.81
2.68
2.67
5.36
5.37
5.37
5.48
5.35
5.34
2.67
2.67
2.67
2.67
2.67
2.67
103
116
400
363
450
414
460
465
491
494
492
491
483
496
303
359
319
232
207
135
156
a Measured in dichloromethane. b Obtained from differential pulse voltammograms. Potentials reported are referenced to the ferrocene internal
standard. c Derived from reduction potential using ELUMO = 4.8þEred. d Deduced using the formula EHOMO = ELUMO þ E0ꢀ0. e Calculated from the
optical edge measured for dichloromethane solution. Glass transition temperature obtained from differential scanning calorimetry. g Temperature
f
corresponding to 5% weight loss. h Decomposition temperature corresponding to the peak in the differential thermogravimetric traces. i Crystallization
temperature. j Melting temperature.
underwent an irreversible reduction originating from the qui-
noxaline segment.17 The reduction potential is more negative
than those observed for the simple quinoxaline derivatives,21
which indicates the electron-donating role of the indole unit. No
oxidation wave was noticed within the observable potential
window. LUMO values were calculated from the reduction
potentials by comparing with the ionization potential of ferro-
cene (LUMO = 4.8 ꢀ Ered; where Ered is the reduction potential
of the material with reference to ferrocene) and the HOMO
values subsequently deduced from the band gap obtained from
the optical edge. The data are listed in Table 3. The orbital energy
values are favorable for charge injection and transport in a
multilayered OLED configuration. The similar HOMO and
LUMO levels observed for the dyes are in keeping with the
expectation as they possess similar structural elements, and the
peripheral M€ullen dendrons are not expected to play a pro-
nounced electronic role. The HOMO and LUMO energy levels
observed for the dyes are comparable to the commonly used
electron-transport materials such as tris(8-quinolinolate)Al(III)
(Alq3)26 (ꢀ6.0 eV, ꢀ3.0 eV) and 1,3,5-tris(N-phenyl-benzimi-
dizol-2-yl)benzene (TPBI)27 (ꢀ6.2 eV, ꢀ2.7 eV).
compounds 3bꢀ5b containing an extra phenyl group. This
means that the phenyl substituted compounds 3bꢀ5b resist
decomposition. Irrespective of the variation in the onset tem-
peratures, the more or less comparable decomposition tempera-
tures observed for the dyes indicate that the thermal robustness
of the compounds primarily arises from the indoloquinoxaline
segment. M€ullen dendrons containing pyrenylamine substitu-
ents and related derivatives have been demonstrated to function
as hole-transporting and emitting materials in organic light-
emitting diodes possessing relatively low thermal decomposition
temperatures (296ꢀ456)10 when compared to the materials
studied in this work. Insertion of indolo[3,2-b]quinoxaline as
core highly benefits the thermal decomposition temperature of
these materials and raises them to 480ꢀ496 °C. It may be
attributed to the planar nature of the indolo[3,2-b]quinoxaline
moiety, which provides additional thermal stability. The glass-
forming capability of the dyes was examined by differential
scanning calorimetry. Compounds 3a, 3b, 4a, and 5b exhibited
glass transition temperatures (Tg) in the range 103ꢀ156 °C. These
values are significantly larger than those reported earlier for
triarylamines-substituted indoloquinoxalines13 and that of com-
monly used hole-transporting agents such as 1,4-bis(phenyl-m-
tolylamino)biphenyl (TPD, Tg = 60 °C) and 4,40-bis(1-naphthyl-
phenylamino)biphenyl (NPB, Tg = 95 °C).28 The pronounced
stability of the glassy state in these molecules is attributable to the
presence of indoloquinoxaline and other polyaromatic hydrocar-
bons such as fluoranthene and triphenylene. The additional phenyl
Thermal Properties. Thermal stability of the materials was
investigated by thermogravimetric analyses (TGA). All of the
materials showed very good thermal stability. The onset decom-
position temperatures of the compounds lie in the range from
363 to 465 °C (Table 3). In general, the onset temperatures of
compounds 3aꢀ5a are smaller than those of the analogous
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Figure 7. EL characters of the device with the structure ITO/NPB/3bꢀ5b/Al (device I): (a) current densityꢀvoltage (IꢀV); (b) luminanceꢀvoltage
(LꢀV) curves.
Figure 8. EL characters of device with the structure ITO/NPB/3bꢀ5b/TPBI/Al (device II): (a) current densityꢀvoltage (IꢀV); (b) luminanceꢀ
voltage (LꢀV) curves.
Table 4. EL Parameters for the Devices Fabricated Using Compounds 3bꢀ5b
compd
Von, Va
λem, nm
CIE (x, y)
fwhm, nmb
Lmax, cd/m2c
L, cd/m2d
ηt, %e
ηp, lm/Wf
ηc, cd/Ag
3b
Device I
Device II
Device I
Device II
Device I
Device II
14.8
19.1
15.2
11.8
20.0
10.9
498
496
504
496
516
454
0.21, 0.46
0.19, 0.45
0.23, 0.51
0.20, 0.39
0.23, 0.62
0.20, 0.24
80
78
2280
2890
3910
2950
740
832
1256
1329
750
0.315
0.476
0.455
0.321
0.075
0.198
0.178
0.208
0.276
0.200
0.040
0.090
0.836
1.257
1.333
0.751
0.253
0.306
4b
5b
82
86
76
253
138
940
304
a Turn-on voltage. b Full width at half-maximum. c Maximum luminance. d Luminance at 100 mA/cm2. e External quantum efficiency. f Power efficiency at
100 mA/cm2. g Current efficiency at 100 mA/cm2.
group present in 3b is also helpful to increase Tg when compared to
the analogue 3a.
Electroluminescent Characteristics. To evaluate the perfor-
mance of novel 6H-indolo[3,2-b]quinoxaline based M€ullen
dendrons as a constituent in organic light-emitting diodes we
have first attempted devices having ITO/3bꢀ5b/Al where the
compounds served as emissive and dual charge transporting
materials. However, no significant light emission was observed
even at higher operating voltages. This is quite expected as the
compounds lack hole-transporting segments such as amines.
So we have turned our attention to use them as emitting/
electron transporting materials. Thus, we have fabricated two
types of devices with the structures ITO/NPB/3bꢀ5b/Al
(device I) and ITO/NPB/3bꢀ5b/TPBI/Al (device II) where
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NPB is N,N0-diphenyl-N,N0-bis(1-naphthylphenyl)-1,10-biphe-
nyl-4,40-diamine serving as the hole-transporting layer and TPBI
is 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene acting as an
electron-transporting material. The compounds were spin-
coated on the NPB layer, and a TPBI layer was deposited on
the top of it to achieve the device II. In the device II, the 6H-
indolo[3,2-b]quinoxaline based M€ullen dendrons act as emissive
layer, whereas in device I, they act as electron-transporting layer
as well as emitting layer.
The current densityꢀvoltage (IꢀV) and luminanceꢀcurrent
density (LꢀI) characteristics of the OLEDs fabricated using
these dyes are shown in Figure 7 and 8 respectively. The device
performance and EL emission characteristics are summarized in
Table 4. In the device configuration I, the lower current density
and larger turn-on voltage was observed for the diode fabricated
using 5b while for configuration II these parameters were realized
for 3b. It appears that 5b lacks electron-transporting ability and
inclusion of electron-transporting layer derived from TPBI
improves the current flow in the device. On the contrary for
3b addition of TPBI layer probably inhibits the electron injection
into the molecular layer and reduces the current density. For 4b
addition of TPBI layer is beneficial for the electroluminescence
performance. Some of these observations may be rationalized by
looking at the energy alignments for the device configurations
shown in Figure 9. Holes leaking to 4b/5b is less favorable when
compared to 3b due to higher energy barrier at the 4b,5b/TPBI
interface. Similarly a barrier of 0.1 eV is there for the injection of
electrons into the 3b layer from TPBI. In the case of 5b despite an
improvement in the current density and turn-on voltage with
additional TPBI layer, the luminance drastically decreased due to
the exciplex formation between TPBI and 5b and the confine-
ment of charge recombination of zone comprising of TPBI layer
(see below).
The emission in both the devices originates from the indolo-
quinoxaline derivatives to give bluish-green or green color
depending on the dye used (Figure 10). As can be seen in
Figure 10, λmax (∼490ꢀ520 nm) of the EL spectra is about
10ꢀ20 nm red-shifted when compared to that of the PL
spectrum of the solid film (shown in Figure 4). The EL spectra
(Figure 10a) for the devices made from compound 5b containing
TPBI (device II), are characteristic of contamination of emission
from TPBI or exciplex indicating the formation of excitons at the
interface of TPBI/5b layers. On the contrary the device where
compound 5b is acting as an electron transport layer and emitter
(device I), the emission peak is centered at 520 nm (Figure 10b).
This may be due to the excimer formation of 5b.29 The
compound 5b is already demonstrated to form excimers in
toluene solutions and spin-coated thin films (vide supra).
Among the compounds (3bꢀ5b), that containing the fluor-
anthene skeleton, 4b, displays larger current density at higher
voltages. The turn-on voltage of compound 4b in device II was
11.8 V and it exhibited bluish-green EL (CIE: 0.20, 0.39) with a
maximum brightness of 2950 cd/m2 at 16.5 V and a maximum
efficiency of 0.32%. A device II of fluoranthene derivative, 4b
showed better characteristics with the turn-on voltage of 15.2 V, a
maximum brightness of 3910 cd/m2 at 18.7 V, and a maximum
efficiency of 0.46%. However, 3b as emitting material displayed a
bluish-green electroluminescence(CIE: 0.19, 0.45) of 2890 cd/m2
and maximum external quantum efficiency of 0.48%. On the
other hand, the device fabricated using a layer of 3b as electron
transporting and emitting material has shown inferior currentꢀ
voltage-luminance characteristics (maximum luminescence
2280 cd/m2 and maximum external quantum efficiency of
0.32%). In short, inclusion of an electron-transport layer of TPBI
benefits the device performance for 3b however decrements the
luminance characteristics for 4b. Thicker layer of 4b probably
reduces the chances of electrons leaking into the NPB layer by
retarding their movement, while the addition of facile electron-
transport TPBI enhances the possibility of electron-leaking into
the NPB and spreading the recombination zone inside the NPB
Figure 9. Orbital energy alignments in the molecular materials used in
this study.
Figure 10. EL spectra observed for (a) device II and (b) device I using compounds 3bꢀ5b as emitters/electron transporters.
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ARTICLE
layer. Thus for 4b in device II residual emission from NPB is also
observed (see Figure 10a).
The precursors 2,3,4,5-diphenylcyclopenta-2,4-dienone,10 7,9-di-
phenyl-8H-cyclopenta[a]acenaphthylen-8-one,30 1,3-diphenyl-2H-cyclo-
penta[l]phenanthren-2-one31 and 9-bromo-6-butyl-6H-indolo[3,2-b]-
quinoxaline13 were prepared by following literature procedures.
6-Butyl-9-ethynyl-6H-indolo[3,2-b]quinoxaline (2b). 9-Bromo-6-
butyl-6H-indolo[3,2-b]quinoxaline (1b, 1.06 g, 3.0 mmol), Pd-
(PPh3)2Cl2 (0.035 g, 0.03 mmol), CuI (0.005 g, 0.09 mmol), PPh3
(0.026 g, 0.03 mmol), 2-methylbut-3-yn-2-ol (0.3 g, 3.6 mmol), and
triethylamine (50 mL) were charged sequentially in a two-neck flask
under nitrogen atmosphere and heated to reflux for 12 h. The volatiles
were removed under vacuum, and the resulting solid was extracted into
diethyl ether. The organic extract was washed with brine solution, dried
over anhydrous Na2SO4, and evaporated to leave a pale green solid. It
was further purified by column chromatography on silica gel using
dichloromethane as eluant to obtain 4-(6-butyl-6H-indolo[3,2-b]-
quinoxalin-9-yl)-2-methylbut-3-yn-2-ol (2a) as a yellow solid 0.95 g
(89%). 1H NMR (500 MHz, CDCl3) δ 0.98 (t, J = 7.3 Hz, 3 H), 1.42
(sext, J = 7.5 Hz, 3 H), 1.67 (s, 6 H), 1.92 (quin, J = 7.5 Hz, 2 H), 2.33 (s,
1 H), 4.47 (t, J = 7.3 Hz, 2 H), 7.38 (d, J = 8.5 Hz, 2 H), 7.67ꢀ7.69 (m, 2
H), 7.76 (td, J = 8.0 Hz, 1.25 Hz, 1 H), 8.13 (dd, J = 6.0 Hz, 1.0 Hz 1 H),
8.29 (dd, J = 9.5 Hz, 1.0 Hz, 1 H), 8.54 (d, J = 1.0 Hz, 1 H).
It was quantitatively converted into the 6-butyl-9-ethynyl-6H-indolo-
[3,2-b]quinoxaline (2b) by treatment with refluxing solution of potas-
sium hydroxide in toluene at 110 °C temperature for 6 h. Reaction
mixture was extracted with diethyl ether and dried over Na2SO4. Yellow
solid; yield 0.85 g (95%); 1H NMR (500 MHz, CDCl3) δ 0.99 (t, J = 7.5
Hz, 3 H), 1.46 (sext, J = 7.5 Hz, 3 H), 1.67 (s, 6 H), 1.92 (quin, J = 7.5 Hz,
2 H), 3.11 (s, 1 H), 4.49 (t, J = 7.5 Hz, 2 H), 7.41 (d, J = 8.5 Hz, 1 H),
7.66ꢀ7.69 (m, 1 H), 7.76 (td, J = 8.0 Hz, 1.25 Hz, 1 H), 8.13 (dd, J = 6.0
Hz, 1.5 Hz, 1 H), 8.14 (dd, J = 8.5 Hz, 1.5 Hz, 1 H), 8.30 (dd, J = 8.5 Hz,
1.3 Hz, 1 H), 8.63 (d, J = 2.0 Hz, 1 H); 13C NMR (125 MHz, CDCl3) δ
13.7, 20.3, 30.6, 41.4, 83.7, 100.0, 109.5, 114.4, 119.5, 126.2, 126.6, 127.9,
129.0, 129.4, 134.5, 139.2, 139.4, 140.7, 144.1, 145.8; HRMS calcd for
C20H17N3 m/z 299.1422, found 299.1429.
6-Butyl-9-(2-phenylethynyl)-6H-indolo[3,2-b]quinoxaline (2c). 9-
Bromo-6-butyl-6H-indolo[3,2-b]quinoxaline (1b) (1.06 g, 3.0 mmol),
Pd(PPh3)2Cl2 (0.035 g, 0.03 mmol), CuI (0.005 g, 0.09 mmol), PPh3
(0.026 g, 0.03 mmol), phenylacetylene (0.34 g, 6.0 mmol), and
diisopropylamine (40 mL) were charged sequentially into a two-neck
flask under nitrogen atmosphere and heated to reflux for 24 h. After the
stipulated time, the volatiles were removed under vacuum, and the
resulting solid was extracted into ethyl acetate. The organic extract was
washed with brine solution, dried over anhydrous Na2SO4, and evapo-
rated to leave a yellow solid. It was further purified by column
chromatography using CH2Cl2 and n-hexane (2:3) mixture. Yellow
solid; yield 1.06 g (94%); 1H NMR (500 MHz, CDCl3) δ 0.99 (t, J = 7.3
Hz, 3 H), 1.45 (sext, J = 7.8 Hz, 2 H), 1.94 (quin, J = 7.5 Hz, 2 H), 4.51(t,
J = 7.5 Hz, 2 H), 7.35ꢀ7.40 (m, 3 H), 7.43 (d, J = 8.5 Hz, 1 H),
7.57ꢀ7.59 (m, 2 H), 7.69 (td, J = 5.8 Hz, 1 Hz, 1 H), 7.77 (td, J = 5.8 Hz,
1.0 Hz, 1 H), 7.85 (dd, J = 8.5 Hz, 1.5 Hz, 1 H), 8.15 (d, J = 8.5 Hz, 1 H),
8.31 (d, J = 8.0 Hz, 1 H), 8.68 (s, 1 H); 13C NMR (125 MHz, CDCl3) δ
13.7, 20.2, 30.4, 41.3, 88.6, 89.4, 109.5, 110.9, 113.4, 115.5, 119.5, 121.0,
123.4, 126.0, 126.0, 126.1, 127.8, 128.0, 128.3, 128.8, 129.0, 129.4, 131.4,
133.2, 133.9, 138.6, 139.3, 139.3, 140.6, 142.8; HRMS calcd for
C26H21N3 m/z 375.1735, found 375.1722.
Compound 3a. A mixture of 2,3,4,5-tetraphenylcyclopenta-2,4-dienone
(0.58 g, 1.5 mmol), 2b (0.50g, 1.65mmol) anddiphenylether(5mL) were
heated under nitrogen atmosphere at 200 °C for 24 h during which the dark
purple color vanished and an orange color developed. After cooling, hexane
was added to precipitate the product formed. The yellow color precipitates
formed was filtered and dried under vacuum. Further purification of
the crude product was performed by column chromatography on silica
gel and using hexaneꢀdichloromethane mixture (1:1) as eluant. Yellow
solid; yield 92%; mp 400 °C (dec); 1H NMR (500 MHz, CDCl3) δ 1.29
’ CONCLUSIONS
In summary, we have demonstrated that M€ullen dendron-like
chromophores attached to an indoloquinoxaline core is an
effective structural skeleton for improving thermal properties.
They were easily synthesized by performing a DielsꢀAlder
reaction between the acetylene derivatives and appropriate
cyclopentadienones. We have investigated their electrochemical,
photophysical, and electroluminescence properties. All of the
novel compounds display good quantum yields in solutions and
also show remarkable fluorescence in the solid state. They
displayed irreversible electrochemical reduction, demonstrating
their electron-accepting nature. In accordance with these results
they reasonably functioned as emitting material or as emitting
and electron-transporting material in multilayered organic light-
emitting diodes. Two types of multilayered device were fabri-
cated, using these materials as the electron-transporting and/or
emitting layers. Both the devices exhibited moderate perfor-
mance. An OLED device fabricated using a layer of 4b as
electron-transporting and emitting material displayed a max-
imum luminescence of 3910 cd/m2 and maximum external
quantum efficiency of 0.46%, whereas the device fabricated using
a layer of 4b as emitting material along with a TPBI electron-
transporting layer slightly reduced the device performance para-
meters (maximum luminescence 2950 cd/m2 and maximum
external quantum efficiency of 0.32%). Incorporation of an
additional layer of TPBI is detrimental to the device function,
which probably indicates the retardation of electron injection
into the molecular layer of 4b from the TPBI layer. These results
reveal that the newly developed materials are good electron-
transporting as well as emitting materials suitable for double layer
organic light-emitting diodes.
’ EXPERIMENTAL SECTION
General. 1H and 13C NMR spectra were recorded on a spectrometer
operating at 500 MHz. Mass spectra were collected on an ESI TOF high-
resolution mass spectrometer. Electronic absorption spectra were
measured on a spectrophotometer using dichloromethane or toluene
solutions. Emission spectra were recorded using a spectrofluorimeter
operating at room temperature (∼30 °C). Emission quantum yields
were obtained by using coumarin-6 (ΦF = 0.78 in ethanol) as reference.
Cyclic voltammetric experiments were performed using an electroche-
mical analyzer with a conventional three-electrode configuration con-
sisting of a glassy carbon working electrode, platinum auxiliary electrode,
and a nonaqueous Ag/AgNO3 reference electrode. The E1/2 values were
determined as 1/2(Ep þ Ep ), where Ep and Ep are the anodic and
a
c
a
c
cathodic peak potentials, respectively. All potentials reported are
referenced to ferrocene, which served as the internal standard. The
solvent in all experiments was CH2Cl2, and the supporting electrolyte
was 0.1 M tetrabutylammonium hexafluorophosphate. DSC measure-
ments were carried out on a differential scanning calorimeter at a heating
rate of 10 °C/min and a cooling rate of 30 °C/min under nitrogen
atmosphere. Melting temperatures of the compounds were obtained
from the DSC measurements. The decomposition temperatures are
reported as the melting points for the dyes that did not exhibit a melting
exotherm in the DSC trace. TGA measurements were performed on a
thermogravimetric analyzer at a heating rate of 10 °C/min under a flow
of air.
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ARTICLE
(t, J = 7.5 Hz, 3 H), 1.71ꢀ1.76 (m, 2 H), 2.2ꢀ2.23 (m, 2 H), 4.75 (t, J =
7.25 Hz, 2 H), 7.16ꢀ7.28 (m, 12 H), 7.45ꢀ7.53 (m, 7 H), 7.58 (s, 1 H),
7.25 (dd, J = 1.0 Hz, 8.5 Hz, 2 H), 7.98 (t, J = 7.0 Hz, 1 H), 8.05ꢀ8.08 (m, 3
H), 8.44 (d, J = 8.5 Hz, 1 H), 8.59 (d, J = 8.5 Hz, 1 H), 8.85 (s, 1 H); 13C
NMR (125 MHz, CDCl3) δ 13.8, 20.3, 29.7, 30.6, 41.3, 123.7, 125.4, 125.7,
125.7, 125.8, 126.3, 126.7, 127.0, 127.1, 127.6, 129.4, 129.9, 131.5, 131.6,
131.8, 132.0, 133.3, 134.7, 139.3, 139.3, 139.3, 140.0, 140.1, 140.2, 140.2,
140.4, 140.6, 141.1, 141.7, 141.9, 143.1, 145.9; HRMS calcd for C48H38N3
[M þ H] m/z 656.3066, found 656.3099. Anal. Calcd for C48H37N3: C,
87.91; H, 5.69; N, 6.41. Found: C, 87.80; H, 5.76; N, 6.37.
Compound 4a. Synthesized from 7,9-diphenyl-8H-cyclopenta-
[a]acenaphthylen-8-one and 2b by following the procedure described
above for 3a: yellow solid; yield 95%; mp 303 °C; 1H NMR (500 MHz,
CDCl3) δ 0.95ꢀ0.99 (m, 3 H), 1.26ꢀ1.4 (m, 2 H), 1.56ꢀ1.57 (m, 2 H),
4.47 (t, J = 7.25 Hz, 2 H), 6.71ꢀ6.72 (m, 1 H), 7.21ꢀ7.22 (m, 1 H),
7.31ꢀ7.59 (m, 14 H), 7.66 (t, J = 8.0 Hz, 1 H), 7.73ꢀ7.77 (m, 5 H), 7.76
(d, J = 8.5 Hz, 1 H), 8.28 (d, J = 9.0 Hz, 1 H), 8.51 (s, 1 H). 13C NMR
(125 MHz, CDCl3) δ 13.8, 20.3, 29.7, 30.6, 41.3, 108.4, 119.2, 123.0,
123.4, 123.7, 125.8, 126.7, 126.7, 127.4, 127.6, 127.7, 127.8, 127.9, 128.7,
128.7, 129.2, 129.4, 130.5, 131.7, 133.2, 134.0, 135.8, 136.0, 136.3, 136.7,
138.2, 138.3, 139.3, 139.4, 140.2, 140.3, 140.6, 140.8, 143.1, 145.9.
HRMS calcld for C46H34N 3 [M þ H] m/z 628.2753, found 628.2833.
Anal. Calcd for C46H33N3: C, 88.01; H, 5.30; N, 6.69. Found: C, 87.88;
H, 5.38; N, 6.61.
(d, J = 8.0 Hz, 1 H); 13C NMR (125 MHz, CDCl3) δ 13,7, 20.1, 39.5,
40.2, 107.8, 118.2, 123.2, 123.3, 125.2, 125.4, 125.6, 126.6, 126.86,
126.89, 126.91, 127.7, 128.1, 128.3, 128.4, 128.5, 129.2, 129.7, 129.8,
130.0, 130.2, 130.4, 131.1, 131.6, 132.9, 133.3, 134.2, 136.6, 136.6, 137.3,
137.7, 139.1, 139.9, 140.2, 140.3, 140.5, 141.2, 142.4; FABMS m/z 704
[M þ H]; HRMS calcd for C52H38N3 [M þ H] m/z 704.3066, found
704.3081. Anal. Calcd for C52H37N3: C, 88.73; H, 5.30; N, 5.97. Found:
C, 88.81; H, 5.24; N, 5.89.
Compound 5b. Synthesized from 1,3-diphenyl-2H-cyclopenta-
[l]phenanthren-2-one and 2c by following the procedure described
above for 3a: yellow solid; yield 60%; mp 465 °C (dec); 1H NMR (500
MHz, CDCl3) δ 0.94 (t, J = 7.5 Hz, 3 H), 1.31 (sext, J = 7.5 Hz, 2 H),
1.84 (quin, J = 7.5 Hz, 2 H), 4.36 (t, J = 7.25 Hz, 2 H), 6.71ꢀ6.83 (m,
5 H), 6.98ꢀ7.10 (m, 14 H), 7.42 (q, J = 8.25 Hz, 1 H), 7.60ꢀ7.64 (m,
3 H), 7.70 (td, J = 7.5 Hz, 1.5 Hz, 1 H), 8.08 (dd, J = 8.25 Hz, 1.03 Hz,
1 H), 8.20 (dd, J = 8.5 Hz, 1.0 Hz 1 H), 8.44 (d, J = 8.0 Hz, 1 H); 13C
NMR (125 MHz, CDCl3) δ 13.8, 20.2, 30.4, 41.2, 107.9, 118.3, 123.2,
125.2, 125.4, 125.6, 126.1, 126.2, 126.4, 126.5, 126.9, 127.9, 128.0, 128.3,
128.4, 129.3, 130.0, 130.1, 130.8, 130.9, 131.3, 131.36, 131.39, 131.6,
131.9, 132.0, 132.1, 132.2, 132.4, 133.3, 134.4, 137.2, 137.6, 139.1, 140.0,
140.30, 140.34, 140.5, 140.9, 142.4; FABMS m/z 730 [M þ H]; HRMS
calcd for C54H40N3 [M þ H] m/z 730.3222, found 730.3243. Anal.
Calcd for C54H39N3: C, 88.86; H, 5.39; N, 5.76. Found: C, 88.75; H,
5.47; N, 5.81.
Light-Emitting Diode Fabrication and Characterization.
Prepatterned ITO substrates with an effective individual device area of
3.14 mm2 were cleaned via repeated ultrasonic washing with detergent,
deionized water, ethanol and finally oxygen plasma treatment. A layer of
N,N0-bis(l-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine (NPB)
with a thickness of 40 nm was vacuum-deposited on the precleaned ITO
glass substrates as a hole-transport layer and then baked at 100 °C in air
for 1 h. Then, the indoloquinoxaline derivative (3b, 4b, or 5b) was
dissolved in dichlorobenzene (concentration 10 mg mLꢀ1) and filtered
with a 0.2 mm filter. A thin film of the desired material (3b/4b/5b) was
coated at a spin rate of 1500 rpm. The film thickness of this layer was
around 40 nm, measured by a surface profilometer Dektak 3 (Veeco/
Sloan Instrument Inc.) for device I and 10 nm for device II. Finally, a
layer of LiF/Al (1 nm/120 nm) was thermally evaporated as a cathode in
a vacuum chamber (under a pressure of less than 2.5 ꢁ 10ꢀ5 Torr). For
device II, a layer (with a thickness of 30 nm) of electron-transporting
1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) was deposited
under vacuum before the deposition of LiF/Al layer. IꢀV curves were
measured on a Keithley 2400 Source Meter in ambient environment,
and the light intensity was measured with a Newport 1835 Optical
Meter.
Computational Methods. The ground state geometry of the
compounds at the gas phase were optimized using the density functional
theory method with the B3LYP functional32 in conjugation with the
basis set 6-31G(d,p) as implemented in the Gaussian 09 package. The
default options for the self-consistent field (SCF) convergence and
threshold limits in the optimization were used. The optimized structures
did not show imaginary frequency vibrations. The electronic transitions
were calculated using the time-dependent DFT (B3LYP) theory and the
6-31G (d,p) basis set. Even though the time-dependent DFT method
less accurately describes the states with charge-transfer nature, the
qualitative trends in the TDDFT results can still offer correct physical
insights. At least 10 excited states were calculated for each molecule.
Compound 5a. Synthesized from 1,3-diphenyl-2H-cyclopenta-
[l]phenanthren-2-one and 2b by following the procedure described
above for 3a: yellow solid; yield 72%; mp 359 °C; 1H NMR (500 MHz,
CDCl3) δ 0.98 (t, J = 7.25 Hz, 3 H), 1.25 (sext, J = 7.5 Hz, 2 H), 1.93 (q,
J = 7.5 Hz, 2 H), 4.46 (t, J = 7.25 Hz, 2 H), 7.01ꢀ7.02 (m, 2 H),
7.08ꢀ7.12 (m, 2 H), 7.18ꢀ7.19 (m, 5 H), 7.38ꢀ7.56 (m, 6 H),
7.52ꢀ7.56 (m, 3 H), 7.65 (t, J = 8.5 Hz, 1 H), 7.65ꢀ7.78 (m, 2 H),
7.84 (s, 1 H), 8.13 (d, J = 8.5 Hz, 1 H), 8.26 (d, J = 8.5 Hz, 1 H), 8.45 (d,
J = 8.5 Hz, 2 H), 8.52 (s, 1 H); 13C NMR (125 MHz, CDCl3) δ 145.9,
144.4, 143.0, 142.2, 140.6, 140.1, 139.7, 139.3, 138.4, 136.6, 134.6, 133.2,
133.0, 132.3, 132.0, 131.7, 131.4, 130.8, 130.2, 130.1, 130.0, 129.8, 129.7,
129.6, 129.4, 129.1, 128.7, 128.6, 127.8, 127.2, 126.7, 126.4, 125.9, 125.6,
125.3, 123.5, 123.3, 123.2, 119.4, 118.9, 108.5, 41.3, 31.0, 30.6, 20.3,
13.8; HRMS calcld for C48H36N3 [M þ H] m/z 654.2909, found
654.2914. Anal. Calcd for C48H35N3: C, 88.18; H, 5.40; N, 6.43. Found:
C, 88.22; H, 5.34; N, 6.36.
Compound 3b. Synthesized from 2,3,4,5-tetraphenylcyclopenta-2,4-
dienone and 2c by following the procedure described above for 3a:
1
yellow solid; yield 1.0 g (91%); mp 319 °C; H NMR (500 MHz,
CDCl3) δ 0.91 (t, J = 7.5 Hz, 3 H), 1.29 (m, 2 H), 1.66 (m, 2 H), 4.30 (t,
J = 7.5 Hz, 2 H), 6.70ꢀ6.73 (m, 2 H), 6.76ꢀ6.93 (m, 23 H), 6.98 (d, J =
8.5 Hz, 1 H), 7.19 (dd, J = 8.5 Hz, 1.5 Hz 1 H), 7.61 (td, J = 8.5 Hz, 1.5
Hz, 1 H) 7.69 (td, J = 8.5 Hz, 1.5 Hz, 1 H), 7.96 (dd, J = 1.5 Hz, 1 H),
8.05 (dd, J = 8.0 Hz, 1.25 Hz, 1 H), 8.20 (dd, J = 8.0 Hz, 1.25 Hz, 1 H).
13C NMR (125 MHz, CDCl3) δ 13.8, 20.2, 30.4, 41.2, 107.8, 118.2,
125.1, 125.2, 125.4, 125.5, 126.5, 126.6, 126.7, 126.8, 127.7, 128.4, 129.2,
131.3, 131.4, 131.5, 131.7, 133.7, 134.3, 139.1, 140.35, 140.39, 140.43,
140.46, 140.59, 140.64, 140.8, 142.3, 145.8. FABMS m/z 732 [M þ H];
HRMS calcd for C54H42N3 [M þ H] m/z 732.3379, found 732.3392.
Anal. Calcd for C54H41N3: C, 88.61; H, 5.65; N, 5.74. Found: C, 88.72;
H, 5.54; N, 5.78.
Compound 4b. Synthesized from 7,9-diphenyl-8H-cyclopenta-
[a]acenaphthylen-8-one and 2c by following the procedure described
1
above for 3a: greenish yellow solid; yield 92%; mp 460 °C (dec); H
NMR (500 MHz, CDCl3) δ 0.90 (t, J = 7.5 Hz, 3 H), 1.26ꢀ1.33 (m,
2 H), 1.80 (quin, J = 7.0 Hz, 2 H), 4.30 (t, J = 7.0 Hz, 2 H), 6.60 (d, J =
7.0 Hz, 1 H), 6.64 (d, J = 7.0 Hz, 1 H), 6.80 (t, J = 7.0 Hz, 1 H), 6.84 (t,
J = 7.0 Hz, 1 H), 6.96 (d, J = 7.5 Hz, 1 H), 7.01 (d, J = 8.0 Hz, 2 H),
7.17 (t, J = 7.5 Hz, 1 H), 7.28ꢀ7.44 (m, 9 H), 7.43 (d, J = 7.5 Hz, 1 H),
7.61 (t, J = 7.5 Hz, 1 H), 7.63ꢀ7.73 (m, 3 H), 8.05ꢀ8.07 (m, 2 H), 8.24
’ ASSOCIATED CONTENT
S
Supporting Information. Absorption and emission spec-
b
tra, 1H and 13C NMR of the newly synthesized compounds, and
Cartesian coordinates of the optimized structures. This material is
available free of charge via the Internet at http://pubs.acs.org.
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ARTICLE
’ AUTHOR INFORMATION
Huang, T.-H.; Lin, J. T.; Tao, Y.-T.; Chuen, C.-H. Chem. Mater. 2002,
14, 3860.
(10) Thomas, K. R. J.; Velusamy, M.; Lin, J. T.; Chuen, C. H.; Tao,
Y. T. J. Mater. Chem. 2005, 15, 4453.
(11) Thomas, K. R. J.; Velusamy, M.; Lin, J. T.; Sun, S. S.; Tao, Y. T.;
Chuen, C. H. Chem. Commun. 2004, 2328.
Corresponding Author
*Phone: þ91-1332-285376. FAX: þ91-1332-286202. E-mail:
krjt8fcy@iitr.ernet.in.
(12) (a) Gregg, D. J.; Ollagnier, C. M. A.; Fittchet, C. M.; Draper,
C. M. Chem.—Eur. J. 2006, 12, 3043. (b) Gao, P.; Cho, D.; Yang, X. Y.;
Enkelmann, V.; Baumgarten, M.; Mullen, K. Chem.—Eur. J. 2010,
16, 5119.
(13) Thomas, K. R. J.; Tyagi, P. J. Org. Chem. 2010, 75, 8100.
(14) (a) Li, C.; Liu, M.; Pschirer, N. G.; Baumgarten, M.; M€ullen, K.
Chem. Rev. 2010, 110, 6817. (b) Schmaltz, B.; Weil, T.; M€ullen, K. Adv.
Mater. 2009, 21, 1067.
’ ACKNOWLEDGMENT
We thank the Department of Science and Technology (DST),
New Delhi, India for financial support (Grant No. SR/S1/
OC-11/2007) and Prof. J. T. Lin, Institute of Chemistry, Academia
Sinica, Taipei, Taiwan for his assistance in the fabrication and
characterization of electroluminescent devices. Prof. R. Barthwal of
the Department of Biotechnology is thanked for allowing us to use
the NMR facility in the Institute Instrumentation Centre.
(15) (a) Chinchilla, R.; Nꢀajera, C. Chem. Rev. 2007, 107, 874. (b)
Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron. Lett. 1975,
16, 4467.
(16) Thomas, K. R. J.; Lin, J. T.; Tao, Y.-T.; Chuen, C.-H. Adv. Mater.
2002, 14, 822.
’ REFERENCES
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