Pyrene-fluorene hybrids containing acetylene linkage as color-tunable emitting materials for organic light-emitting diodes

Article abstract of DOI:10.1021/jo300285v

New blue- to yellow-emitting materials have been developed by incorporating fluorene-based chromophores on pyrene core with acetylene linkage and using multifold palladium-catalyzed cross-coupling reactions. Both mono- and tetrasubstituted derivatives have been synthesized and characterized. The tetrasubstituted derivatives displayed red-shifted emission when compared to the monosubstituted derivative indicative of an extended conjugation in the former. End-capping with a diphenylamine unit further red-shifted the absorption and emission profiles and imparted a weak dipolar character to the molecules. Amine-containing derivatives displayed positive solvatochromism in the fluorescence spectra indicating a more polar excited state due to an efficient charge migration from the diphenylamine donor to the pyrene π-acceptor. All of the derivatives were tested as emitting dopants with host material 4,4′-bis(9H-carbazol-9-yl)biphenyl (CBP) in a multilayered OLED and found to exhibit bright blue or yellow electroluminescence. The device utilizing 1,3,6,8-tetrasubstituted pyrene derivative as a dopant emitter displayed highest maximum luminescence 4630 cd/m² with power efficiency 3.8 lm/W and current efficiency 7.1 cd/A at 100 cd/m² attributable to the proper alignment of energy levels that led to the efficient harvesting of excitons. All of the devices exhibited color purity over a wide range of operating voltages.

Full text of DOI:10.1021/jo300285v

Article
pubs.acs.org/joc
Pyrene-Fluorene Hybrids Containing Acetylene Linkage as Color-
Tunable Emitting Materials for Organic Light-Emitting Diodes
K. R. Justin Thomas,*,† Neha Kapoor,† M. N. K. Prasad Bolisetty,† Jwo-Huei Jou,‡ Yu-Lin Chen,‡
and Yung-Cheng Jou‡
†Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
‡Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
S
* Supporting Information
ABSTRACT: New blue- to yellow-emitting materials have
been developed by incorporating fluorene-based chromo-
phores on pyrene core with acetylene linkage and using
multifold palladium-catalyzed cross-coupling reactions. Both
mono- and tetrasubstituted derivatives have been synthesized
and characterized. The tetrasubstituted derivatives displayed
red-shifted emission when compared to the monosubstituted
derivative indicative of an extended conjugation in the former.
End-capping with a diphenylamine unit further red-shifted the
absorption and emission profiles and imparted a weak dipolar
character to the molecules. Amine-containing derivatives
displayed positive solvatochromism in the fluorescence spectra
indicating a more polar excited state due to an efficient charge migration from the diphenylamine donor to the pyrene π-acceptor.
All of the derivatives were tested as emitting dopants with host material 4,4′-bis(9H-carbazol-9-yl)biphenyl (CBP) in a
multilayered OLED and found to exhibit bright blue or yellow electroluminescence. The device utilizing 1,3,6,8-tetrasubstituted
pyrene derivative as a dopant emitter displayed highest maximum luminescence 4630 cd/m2 with power efficiency 3.8 lm/W and
current efficiency 7.1 cd/A at 100 cd/m2 attributable to the proper alignment of energy levels that led to the efficient harvesting
of excitons. All of the devices exhibited color purity over a wide range of operating voltages.
age associated with pyrene is the facile substitution at the 1, 3,
6, and 8 positions. Similarly, fluorene is flexible toward chemical
modifications at the 2, 7, and 9 positions. It is expected that the
integration of fluorene and pyrene in a molecular structure may
impart favorable optical and charge transport properties desired
for electronic applications. Fluorene-pyrene hybrids16 based on
1,3,6,8-tetrasubstituted pyrene or 2,7-disubstituted fluorene16
have been found to emit intense blue light in OLEDs.
Oligomers and dendrimers featuring pyrene, fluorene, and
carbazole units and acetylene linkages have been reported by
Zhao et al.17 Tetrasubstituted cruciform pyrenes have been
developed and found to be useful in liquid crystals and field
effect transistors.18 Tetrasubstituted pyrene derivatives with
phenyl conjugation and/or acetylene linkers were also
demonstrated as blue emitters in OLEDs, two-photon
absorbers, and materials exhibiting unique charge transfer
dynamics due their X-shaped architecture.19 X-shaped 1,2,4,5-
tetravinyl-benzenes,20 1,4-bis(arylethynyl)-2,5-distyrylben-
zenes,21 and 1,2,4,5-tetrasubstituted(phenylethynyl)benzenes22
have been extensively studied and found to exhibit interesting
photophysical properties. Pyrene cored X-shaped molecules
containing ethynylfluorene π-linkers have been reported as
INTRODUCTION
■
Organic materials possessing extended π-conjugation have
received immense attention in recent years owing to their
unique photophysical and charge transport properties, which
make them potential materials for application in electronic
devices such as organic light-emitting diodes (OLEDs),1
organic photovoltaics (OPV),2 organic thin film transistors
(OTFT),3 etc. They are also attractive due to their promising
two photon absorption4 and nonlinear optical5 characteristics.
Among the π-conjugated molecular materials, those containing
polyaromatic hydrocarbons (PAH) such as fluorene,6 anthra-
cene,7 perylene,8 pyrene,9 pentacene,10 carbazole,11 etc.
tethered with one another by vinyl or acetylene linkages are
attractive due to their flat π-stacking organization in the solid
state. Direct C−C linkage between the PAH has been found to
be detrimental for the extension of conjugation as there is a loss
of coplanarity due to the steric congestion.12 However,
introduction of vinyl or acetylene units as spacers between
the PAH units helped the compounds to acquire planarity,
which is required for efficient intramolecular and intermolecular
interactions and charge transport properties.13
Pyrene-14 and fluorene-based15 functional materials have
been extensively explored as active ingredients for electronic
devices. Fluorene and pyrene derivatives generally displayed
promising optical and electrochemical properties. The advant-
Received: February 10, 2012
Published: March 23, 2012
© 2012 American Chemical Society
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Chart 1. Structures of Pyrene-Fluorene Hybrids (4−9) Containing Acetylene Bridges
Scheme 1. Synthetic Pathway Used for the Preparation of the Compounds 4−9
active constituents for the visual detection of heparin,23 and the
molecules that possessed carbazole end groups displayed
gigantic two-photon absorption cross sections.24 However, no
systematic optical, electrochemical, theoretical, and electro-
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luminescence investigations have been attempted on pyrene-
based materials with ethynylfluorene peripheries.
In this report, we present the synthesis and photophysical
and electroluminescent characteristics of the new molecular
materials (Chart 1) based on a pyrene core that is connected to
fluorene chromophores by acetylene units. The optical
properties of the materials are highly dependent on the
number of chromophores present on the pyrene nucleus. The
monosubstituted derivatives display bright blue emission, while
the tetrasubstituted pyrene derivatives exhibit greenish yellow
emission. Compounds (6 and 9) containing triarylamine-
terminated arms displayed red-shifted emission when compared
to the corresponding parent hydrocarbons (4 and 8) in
solution, indicative of an auxochromic effect by the amine unit.
Interestingly, the amine end-capped derivatives 6 and 9
exhibited solvatochromism in the fluorescence suggestive of
photoinduced intramolecular charge transfer from the amine
peripheries to the pyrene core (vide supra).
Figure 1. Absorption spectra of the dyes 4−9 recorded in toluene.
Table 1. Optical Data for Compounds 4−9
RESULT AND DISCUSSION
■
Synthesis. The structures of pyrene-based functional
materials (4−9) developed in this work are depicted in Chart
1. The synthetic route employed for preparing them is shown
in Scheme 1. The synthesis was conveniently accomplished by a
cross-coupling reaction (Sonogashira reaction) of 1-bromopyr-
ene (1) or 1,3,6,8-tetrabromopyrene (2) with 9,9-diethyl-2-
ethynyl-9H-fluorene (3a), 9,9-dibutyl-2,7-diethynyl-9H-fluo-
rene (3b), 9,9-diethyl-7-ethynyl-N,N-diphenyl-9H-fluoren-2-
amine (3c), and 2-ethynyl-9,9-di(octan-3-yl)-9H-fluorene
(3d), respectively, by employing Pd(PPh3)2Cl2/PPh3/CuI
catalytic system. The terminal acetylenes, such as 9,9-diethyl-
2-ethynyl-9H-fluorene (3a), 9,9-dibutyl-2,7-diethynyl-9H-fluo-
rene (3b), and 2-ethynyl-9,9-di(octan-3-yl)-9H-fluorene (3d),
required for the study have been synthesized by a two-step
protocol (not shown in Scheme 1) involving Sonogashira
coupling of 2-methylbut-3-yn-2-ol with the corresponding aryl
bromides and the base-catalyzed cleavage of the functionalized
but-3-yn-2-ols obtained in the first step.25 Compound 7
exhibited poor solubility in common organic solvents, and to
overcome this problem a derivative 8, with long alkyl chain, was
also synthesized. The dyes (4−9) are intensely colored (yellow
or orange) and soluble in a wide variety of solvents including
toluene, dichloromethane, acetonitrile, etc. The amine deriva-
tives (6 and 9) showed better solubility in polar solvents such
as acetonitrile when compared to the rest (4, 5, 7, and 8). All of
the dyes were characterized by spectral methods (NMR and
HRMS), and the data is consistent with the proposed
structures.
Stokes’
a
λem
(film),
nm
c
λmax, nm (εmax, M−1 cm−1
×
λem, nm
ab
shift,
a
,
compound
103)
(ΦF)
cm−1
420
4
286 (34.7), 304 (28.2),
316 (35.3), 322 (33.1),
381 (52.3), 405 (53.9)
412,
436 (0.99)
496
5
6
286 (56.9), 332 (38.5),
414 (97.5), 424 (82.1)
438,
754
486,
505
463 (0.99)
287 (33.5), 304 (34.4),
395 (54.2), 418 (46.8)
449, 475
(sh)
(0.72)
1652
495
7
8
9
285, 321, 374, 466, 486
512,
1045
999
586
587
581
548 (0.64)
287(79.3), 374 (120.1),
464 (70.0), 488 (72.5)
513,
549 (0.67)
310 (90.8), 364 (133.8),
413 (97.2), 476 (75.9),
503 (69.0)
535, 568
(sh)
(0.43)
1189
a
b
Measured for toluene solutions. Relative quantum yield was
obtained by comparing with standards coumarin-1 (0.99 in ethyl
c
acetate) or coumarin-6 (0.75 in ethanol).26 Measured for spin-cast
thin film.
observation. The tetrasubstituted derivatives 7 and 8 showed
two structured absorptions. The longer wavelength peak
occurring at >460 nm is assigned to a π−π* transition
originating from the entire molecule, while the absorption in
the 340−380 nm region may arise from the pyrene/fluorene
localized π−π* transitions (see below for theoretical inter-
pretations). It is interesting to compare the absorption features
of the compounds 7 and 8 with the known tetrasubstituted
derivatives. The absorption maximum of 7 and 8 is 30 nm
bathochromically shifted and displayed significantly higher
molar extinction coefficient when compared to 1,3,6,8-tetrakis-
(phenylethynyl)pyrene.29 Also they exhibited a 92 nm red-shift
compared to 1,3,6,8-tetrakis(9,9-dihexyl-9H-fluoren-2-yl)-
pyrene (400 nm in THF).30 This indicates that the
incorporation of acetylene spacer between the pyrene and
fluorene segments is beneficial for the absorption properties of
these compounds. This may be due to the planarity achieved in
the compounds 7 and 8 due to the presence of an acetylene
linkage that will facilitate the extension the π-conjugation.
Introduction of amine functionality further broadened the
absorption profile for the compounds 6 and 9 due to the
formation of new peaks attributable to the diphenylamine
Optical Properties. The absorption spectra of compounds
4−9, recorded in toluene, are displayed in Figure 1, and the
pertinent data are collected in Table 1. The monosubstituted
derivative 4 showed the shortest wavelength absorption, while
the tetrasubstituted derivative 9 possessed the longest wave-
length peak. The vibronic pattern observed for the absorption
spectra of 4 is similar to pyrene,27 but the peak position is more
red-shifted when compared to pyrene. Moreover, it displayed
longer wavelength absorption with larger extinction coefficient
when compared to 1-(phenylethynyl)pyrene reported by Yang
et al.28 The red-shift is attributed to the extension of
conjugation on replacement of the phenyl group with a
fluorene unit. The pyrene-fluorene-pyrene triad 5 exhibited 15
nm bathochromism and 2-fold increase in the molar extinction
coefficient when compared to 4 in agreement with the above
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localized transitions (<320 nm) and charge transfer transition
from the amine donor to the pyrene acceptor.31 Pyrenylamine
and their derivatives have been demonstrated to display a
charge transfer transition at ∼400 nm.31
The compounds are intensely emitting in the blue to yellow
region on exposure to visible light, and the corresponding
emission spectra recorded for compounds 4−9 in toluene are
displayed in Figure 2. The related data are given in Table 1. All
acetonitrile (ACN). The absorption and emission data
observed for the dyes in selected solvents are listed in Tables
2 and 3. The derivatives 4, 5, and 8 showed negligible changes
in absorption and emission profiles on varying the polarity of
the solvent used for the measurement. However, the amine-
containing derivatives 6 (Figure 3) and 9 showed prominent
positive solvatochromism in the emission spectra only. This
shows that the dyes 6 and 9 are more polarized in the excited
state than in the ground state. It is reasonable to assume that
the pyrene unit acts as a strong π-acceptor in the excited state.
More polarized excited state of the molecules will be stabilized
by polar solvents thus will cause a red-shift in emission.32
The solvatochromism data of the dyes 6 and 9 were analyzed
by Lippert−Mataga plot33 and Stokes shift versus ET(30)
parameter. The dye 6 exhibited linear correlations (Figure 4) in
the above-mentioned analyses, but the dye 9 showed notable
deviation (Figure 5) for the data observed for dichloromethane
solution from linearity in the Lippert−Mataga and ET(30)
parameter correlation plots. This distortion is due to the
unusual red-shift in the absorption and emission spectra
observed for the dye in dichloromethane solution. Similar
behavior has been earlier observed for organic dyes and
attributed to the instant stabilization due to a fast rearrange-
ment of polarizable electrons during excitation.34
The emission spectra were also measured for spin-cast thin
films of the dyes. The peak values are presented in Table 1.
Compounds 4, 5, and 8 exhibited red-shifted emission in thin
film when compared to those observed for toluene solutions.
Moreover, these derivatives showed a broad single peak in thin
film without vibronic features. This clearly indicates that the
red-shifted broad peak in thin film is due to molecular
aggregation or excimer formation in the excited state.35
However, the diphenylamine end-capped derivatives 6 and 9
showed no significant change in emission wavelength when
recorded in thin film state. This suggests that the trigonal amine
functionality effectively suppresses the formation of aggregates
for 6 and 9 in the solid state.36
Electrochemical Characteristics. The electrochemistry of
the compounds was studied by cyclic voltammetric measure-
ments using a three electrode assembly comprising a glassy
carbon working, platinum auxiliary, and nonaqueous Ag/
AgNO3 reference electrodes. The redox potentials were
calibrated by using ferrocene as an internal standard. The
data are listed in Table 4, and the cyclic voltammograms
measured for the selected dyes (6, 8, and 9) are presented in
Figure 6. From Table 4 and Figure 6, it can be observed that all
of the compounds display an oxidation wave arising from the
conjugation backbone. This oxidation is irreversible in the
molecules 4 and 5 but appeared as a quasi-reversible process in
the rest of the dyes (6, 8, and 9). The electron-richness in these
molecules is significantly increased on introduction of amino
groups in 6 and 9 and extension of conjugation in 8 and 9,
which led to the improved stability of the cation radicals formed
by the removal of electron. The tetrasubstituted derivative 8
displayed an additional quasi-reversible oxidation process,
which further supports the enhancement in oxidation
propensity due to multiple substitutions on pyrene. The
amine-substituted derivatives 6 and 9 exhibited a facile
oxidation at the lower potentials attributed to the removal of
electron from the amine unit. The oxidation due to the
conjugation segment is cathodically shifted in 9 due to the
presence of four amine units, which increases the electron
density in the conjugation pathway.
Figure 2. Emission spectra of the dyes 4−9 recorded in toluene.
of the compounds showed vibronic transitions in the emission
spectra recorded in toluene attesting the rigidity of the
structure. The emission profile of 4 is red-shifted when
compared to that observed for pyrene and 1-(phenylethynyl)-
pyrene (392 nm),30 suggestive of involvement of an extended
π-system in the electronic excitation. The emission wavelengths
of the compounds are in the order 4 < 5 < 6 < 7 ≈ 8 < 9. This
is largely in keeping with the trend in the conjugation length
present in the molecules. Though the amine-containing
derivatives 6 and 9 have displayed absorption peaks similar to
their parent hydrocarbons 4 and 7, they exhibited significantly
red-shifted emission peaks. This may be a result of a polarized
excited state that can undergo dipolar relaxation before
emission could occur.1c,32 The larger Stokes shifts observed
for the tetrasubstituted derivative (8 and 9) and the amine-
containing compound (6) point to the existence of non-
radiative relaxation pathways in these molecules. The
fluorescence quantum yield measured by comparing with the
standards indicates that these materials are promising emitters.
The tetrasubstituted derivatives (7 and 8) and the amine-
containing compounds (6 and 9) show relatively less quantum
yield when compared to the monosubstituted analogues 4 and
5. It is probable that the photoexcitation of above the
derivatives leads to intramolecular charge transfer at the excited
state (ICT). ICT normally reduces the quantum yield due to
more pronounced relaxation and nonradiative decay in the
excited state.19e
The solvatochromism study was performed for all of the dyes
in different solvents to evaluate the effect of solvent polarity on
the optical properties of the molecules. The absorption and
emission spectra of 4−9 was recorded in the solvent of different
polarity such as cyclohexane (CH), toluene (TOL), chloroform
(CHCl3), ethyl acetate (EA), tetrahydrofuran (THF), dichloro-
methane (DCM), N,N-dimethylformamide (DMF), and
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The energies of frontier molecular orbitals (HOMO and
LUMO) derived from the electrochemical and optical data are
also listed in Table 4. Incorporation of triarylamine
functionality raises the HOMO, while the tetrasubstitution is
beneficial for stabilizing the LUMO. Combination of these two
factors results in a relatively low band gap emitter 9. The orbital
energies observed for these materials, particularly the
derivatives 6, 8, and 9 are more appropriate for facile charge
injection at the electrode/molecular layer interfaces.
Theoretical Investigations. To gain further information
about the electronic structure of the compounds, we have
performed theoretical calculations using density functional
theory37 on approximated model compounds. To save the
computational time the lengthy alkyl chains were removed in
the model structures (M4−M9). It is opined that removal of
alkyl groups from the molecules is not going to affect the
parameters relevant for the discussion here. The isosurface
plots of the frontier molecular orbitals of the selected model
compounds of 6, 8, and 9 are shown in Figure 7, and the
vertical transitions of all model compounds are listed in Table
5.
The LUMO of the compounds are invariably constituted by
the pyrene unit in all of the compounds, while the HOMO is
mainly on the pyrene unit and extended to the ethynylfluorene
segment in the parent hydrocarbons 4, 5, and 8 or extended up
to the amine nitrogen in compounds 6 and 9. It is interesting
note that the HOMO is spread over the diphenylamine
fragment also in 6, but the phenyl groups in the amine do not
contribute for the construction of HOMO in 9. The
bathochromic shift in the absorption realized for the
tetrasubstituted derivatives (8 and 9) on comparison to the
monosubstituted derivatives (4 and 6) is easily understood on
comparing the HOMO and LUMO of the respective molecules.
The contribution of all four arms in the electronic
delocalization strongly suggests that these molecules cannot
be treated simply as cross-conjugated molecules and the π-
acceptor ability of pyrene is enhanced on tetrasubstitution. The
involvement of amine nitrogen in the HOMO for the
compounds 6 and 9 further explains the slight red-shift
observed for them on comparison to the parent hydrocarbons 4
and 8. In general the HOMO to LUMO excitation in the
compounds can be treated as a π−π* transition with the amine
units in the compounds 6 and 9 rendering auxochromic effect.
The trends observed in the peak positions and molar extinction
coefficients of the compounds match well with the theoretical
predictions (Tables 1 and 5). The absorption maxima observed
for the compounds closely resemble to those computed using
MPW1K model.38 Thus, the computations based on this model
can be utilized to predict functional properties of molecular
materials at the design level and avoid the wastage of expensive
chemicals on synthetic trials.
Thermal Properties. The thermal properties of com-
pounds 4−9 were studied by thermogravimetric analysis (Table
4). All of the derivatives (4−9) exhibited excellent thermal
stability, and the decomposition temperatures are in the range
of 419−535 °C. The temperature corresponding to 5% weight
loss for the dyes is in the range 394−467 °C. Among the dyes,
5 and 8 exhibited lower decomposition temperatures
attributable to the fragile alkyl chains from in them. It is
interesting to note that the dye 8 displayed improved thermal
stability when compared to 1,3,6,8-tetrakis(9,9-dihexyl-9H-
fluoren-2-yl)pyrene (377 °C).29 It appears that the acetylene
linkage present in 8 is acting as spacer and relieves the steric
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Table 3. Emission Data for Compounds 4−9 Recorded in Different Solvents
λem, nm
Stokes shift, cm−1
solvent
4
5
6
8
9
4
5
6
8
9
cyclohexane
toluene
407, 432
412, 436
413, 436
406, 431
410, 434
412, 435
412, 435
408, 430
433, 457
438, 463
439, 464
433, 459
437, 462
439, 465
441, 465
433, 457
436, 460
449
507, 542
513, 549
514, 550
509, 545
513, 549
514, 551
518, 554
524, 558
535, 568
545
368
420
539
369
485
542
481
553
829
754
974
772
978
806
743
715
988
1652
3696
3923
4133
5010
6632
6617
852
999
911
845
832
870
856
916
1189
1532
1572
1647
2205
1810
chloroform
ethyl acetate
THF
486
490
545
498
552
dichloromethane
DMF
516
567
565
557
acetonitrile
557
Figure 3. Emission spectra of 6 recorded in selected solvents of
different polarity.
strain between the pyrene and fluorene units and contributes to
the hike in stability. Highest thermal stability observed for the
dyes 6 and 9 is attributed to the triarylamine chromophore in
these molecules. Enhancement of thermal stability in molecular
materials due to triarylamine chromophores has been well
documented in the literature. Additionally, the role of fluorene
unit in thermal stability is also evident on comparing 6 with the
known derivative N,N-diphenyl-4-(pyren-1-yl)aniline (416
°C).33
Electroluminescent Properties. Two types of electro-
luminescent devices were fabricated using the newly developed
materials [ITO/PEDOT:PSS/4, 5, 6, 8 or 9/TPBi/LiF/Al
(device I) and ITO/PEDOT:PSS/CBP:(4, 5, 6, 8 or 9)/TPBi/
LiF/Al (device II)], where ITO acts as anode, PEDOT:PSS as
hole-injection layer, LiF as electron-injection layer, and Al as
cathode. Compounds 4−9 were used as hole transporting and
emitting material in device I, while host material 4,4′-bis(9H-
carbazol-9-yl)biphenyl (CBP) doped with compounds 4−9
(10%) acts as ambipolar transporting and emitting layer in
Device II. Figure 8 displays the I-V-L characteristics observed
for devices I and II. The pertinent electroluminescent data for
the dyes are listed in Table 6.
Devices II showed better performance statistics when
compared to the corresponding devices I. From this it is
clear that the materials are not effectively functioning as charge
transporting emitting layer. The reason for the failure of devices
I is mainly attributed to the lack of an amorphous state for the
compounds, thus leading to poor quality films. The higher
current density observed for devices I when compared to
devices II also suggests that the charges are leaking at the
Figure 4. (a) Lippert−Mataga plot and (b) Stokes shift vs ET(30) plot
for the dye 6.
respective electrodes without recombining inside the molecular
layer. The success of devices II is assigned to the favorable
energy levels (Figure 9) of the compounds, which is suitable for
the trapping of the excitons generated in the CBP layer. Among
the dyes, the best performance was registered for 5 and 8 in
terms brightness and efficiency for the device type II. These
devices excel in performance compared to those reported based
on 1,3,6,8-tetrakis(9,9-dihexyl-9H-fluoren-2-yl)pyrene.29
Figure 10 shows the EL spectra recorded for the devices II
fabricated using dyes 4−9 at 100 cd/m2 brightness. EL spectra
of device II with compounds 4−9 were also recorded at the
luminance of 1000 cd/m2 and current density of 50 mA/m2,
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Figure 6. Cyclic voltammograms recorded for the dyes 6, 8, and 9.
region. However, it is not the case for the present devices, i.e.,
the electroluminescence spectral peak remains constant over a
wide luminance range (Table 7), which is of particular
importance for the commercial full-color applications.
CONCLUSIONS
■
In summary we have developed pyrene-based molecular
emitters featuring multiple fluorenylethnyl arms as potential
candidates for organic light-emitting diodes. The compounds
were synthesized by multifold palladium-catalyzed cross-
coupling reactions and isolated in moderate to good yields.
They were found to exhibit interesting photophysical properties
that are highly dependent on the conjugation length and the
nature of the terminal functional groups. Thus, the tetrasub-
stituted pyrene derivatives displayed red-shifted absorption and
emission characteristics when compared to the monosubsti-
tuted pyrene derivatives due to the elongation of conjugation.
The choice of acetylene linkage between the pyrene and
fluorene chromophores has also been found to benefit the
optical, electrochemical, and thermal properties of the
compounds when compared to the derivatives in which
fluorene and pyrene units are directly connected, which leads
to sterically distorted and comparatively less conjugated
molecules.29 Incorporation of diphenylamine terminal groups
red-shifts the absorption/emission profile and enhances the
thermal stability. Compounds 4−9 were used as efficient
dopant in multilayered organic light-emitting diodes with CBP
as host. Device based on monosubstituted pyrene derivatives
(4−6) showed bright blue emission, while the tetrasubstituted
pyrene derivatives 8 and 9 exhibited yellow emission. The
device based on dopant 8 displayed the highest maximum
luminescence 4630 cd/m2 with power efficiency 3.8 lm/W and
current efficiency 7.1 cd/A at 100 cd/m2, which is significantly
better than the previously reported tetrasubstituted pyrene
derivatives.13a,29
Figure 5. (a) Lippert−Mataga plot and (b) Stokes shift vs ET(30) plot
for the dye 9.
Table 4. Thermal and Electrochemical Data for Compounds
4−9
Tonset
a
,
Td,
°C
Tm,
°C
Eox (ΔE ),
b p
HOMO, LUMO, E0−0,
c d e
dye
°C
mV
eV
eV
eV
4
5
6
435
409
421
460
426
473
150 752
264 752
5.55
5.55
5.20
2.50
2.69
2.44
3.05
2.86
2.76
210 396 (65),
792 (66)
8
9
394
467
419
535
110 660 (56),
920 (66)
5.46
5.21
3.00
2.87
2.46
2.34
274 408 (71),
748 (61)
a
b
Temperature corresponding to 5% weight loss. Measured for 0.2
mM dichloromethane solutions and the potentials are quoted with
c
d
reference to ferrocene internal standard. HOMO = 4.8 + Eox. LUMO
= HOMO − E0−0. Optical band gap obtained from the intersection of
normalized absorption and emission spectra (optical edge).
e
and the details are provided in Table 6. The fwhm of the EL
spectra remained unchanged over a wide luminance range and
current density (see Table 6). It suggests that the hole and
electron recombination is well confined within the CBP
molecular layer that carries the emitter molecules. It is a
common phenomenon that the EL spectrum of blue-emitting
OLEDs shifts under different electric fields. This is mainly
attributed to the difference in the charge carrier mobility in the
employed organic layers, resulting in the shift of the emission
EXPERIMENTAL SECTION
■
General Methods. All commercial chemicals were used as
received. Column chromatography was performed by using silica gel
(100−200 mesh) as stationary phase. All solvents used in synthesis
and spectroscopic measurements were distilled over appropriate drying
and/or degassing reagents. 1H and 13C NMR spectra were recorded on
a FT-NMR spectrometer operating at 500 and 125 MHz, respectively,
in CDCl3. Me4Si (0.00 ppm) or residual signals for CHCl3 (1H NMR,
3927
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Article
Figure 7. Computed isosurface plots of frontier molecular orbitals (HOMO and LUMO) for the model compounds of 6, 8, and 9.
a
Table 5. Computed Vertical Transitions and Their Oscillator Strengths and Configurations
B3LYP
MPW1K
compd λmax, nm
f
configuration
λmax, nm
f
configuration
HOMO→LUMO (95%)
M4
420.2
319.5
1.3408
0.3473
HOMO→LUMO (98%)
HOMO→LUMO+1 (51%);
HOMO-1→LUMO (37%)
378.9
294.2
1.4747
0.4324
HOMO-1→LUMO (52%); HOMO→LUMO+1 (32%)
HOMO→LUMO+2 (32%); HOMO→LUMO+4 (26%)
HOMO-2→LUMO (21%)
256.6
412.5
362.8
287.7
285.4
275.2
0.1227
3.0971
0.1124
0.1146
0.1227
0.2748
M5
477.0
376.8
333.0
2.4884
0.4324
0.1610
HOMO→LUMO (98%)
HOMO-1→LUMO+1 (89%)
HOMO→LUMO+2 (44%);
HOMO-2→LUMO (42%)
HOMO→LUMO (82%); HOMO-1→LUMO+1(11%)
HOMO-1→LUMO (51%); HOMO→LUMO+1 (43%)
HOMO-2→LUMO (52%); HOMO −1→LUMO+1 (15%)
HOMO→LUMO+2 (56%); HOMO-1→LUMO+1 (15%)
HOMO-2→LUMO+1 (33%); HOMO-1→LUMO+2 (25%)
HOMO-5→LUMO (10%)
M6
465.4
395.6
348.8
1.2439
0.5507
0.1371
HOMO→LUMO (98%)
HOMO-1→LUMO (89%)
HOMO→LUMO+2 (26%);
HOMO-2→LUMO (18%);
HOMO-3→LUMO (15%);
HOMO→LUMO+1 (15%);
HOMO-1→LUMO+1(13%)
HOMO-1→LUMO+1 (35%);
HOMO-2→LUMO (32%);
HOMO-3→LUMO (14%)
HOMO→LUMO+4 (88%)
HOMO →LUMO (99%)
HOMO-1→LUMO (80%);
HOMO→LUMO+1 (16%)
HOMO→LUMO+1 (78%);
HOMO-1→LUMO (18%)
HOMO→LUMO (98%)
HOMO-1→LUMO (97%)
HOMO-2→LUMO (97%)
393.8
299.1
1.9664
0.2226
HOMO→LUMO (82%)
HOMO→LUMO+1 (52%); HOMO→LUMO+2 (11%)
HOMO-1→LUMO (11%)
277.9
274.2
0.1961
0.2332
HOMO-2→LUMO (45%); HOMO-1→LUMO+1 (28%)
HOMO→LUMO+4 (64%); HOMO-1→LUMO+4 (29%)
313.0
0.1263
312.1
567.7
452.7
0.2024
1.6860
0.3131
M8
M9
495.3
377.4
357.2
1.9341
0.5914
2.9266
HOMO→LUMO (95%)
HOMO-1→LUMO (68%); HOMO-5→LUMO (10%)
HOMO→LUMO+1 (67%); HOMO-1→LUMO (15%)
415.1
2.2161
604.9
534.2
532.7
1.9035
0.4966
0.3402
508.6
404.1
2.3823
2.3112
HOMO→LUMO (91%)
HOMO-1→LUMO (64%); HOMO-5→LUMO (13%)
HOMO-2→LUMO (10%)
a
Orbital contributions below 10% are omitted.
δ = 7.26; 13C NMR, δ = 77.36 ppm) or DMSO (1H NMR, δ = 2.5
ppm; 13C NMR δ = 40.45 ppm) served as internal standard. IR spectra
were measured on a FT-IR spectrometer. The electronic absorption
spectra were obtained with a UV−vis spectrophotometer. High
resolution mass spectra were measured in an electron spray ionization
(ESI) mass spectrometer operating in positive ion mode. Emission
spectra were recorded spectrofluorimeter by using freshly prepared
dilute solutions. The fluorescence quantum yields (ΦF) were
determined using the formula Φs = (Φr × Ar × Is × ηs2)/(As × Ir ×
ηr2) where A is the absorbance at the excitation wavelength, I is the
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Article
Figure 9. Energy level alignment in the materials used for the
fabrication of devices.
Figure 10. EL spectra measured at100 cd/m2 for the devices II using
10% dyes 4−9 with CBP host.
a
c
Ep and Ep are the anodic and cathodic peak potentials, respectively.
The potential are quoted against ferrocene internal standard. The
solvent in all experiments was dichloromethane and the supporting
electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate.
Thermogravimetric analyses (TGA) were performed under nitrogen
atmosphere at heating rate of 10 °C/min.
1-Bromopyrene (1) and 1,3,6,8-tetrabromopyrene (2) were made
by reported procedure.39 Precursor acetylenes 3a, 3b, and 3d were
synthesized by known literature procedure.40
Synthesis of 9,9-Diethyl-7-ethynyl-N,N-diphenyl-9H-fluoren-
2-amine (3c). A mixture of 7-bromo-9,9-diethyl-N,N-diphenyl-9H-
fluoren-2-amine (5.0 g, 10.68 mmol), 2-methylbut-3-yn-2-ol (1.07 g,
12.8 mmol), Pd(PPh3)2Cl2 (75 mg, 0.11 mmol), PPh3 (56 mg, 0.21
mmol), and CuI (21 mg, 0.11 mmol) were mixed in triethylamine
(100 mL) under nitrogen atmosphere. The resulting mixture was
stirred and heated at 100 °C for 24 h. After completion of the reaction,
Figure 8. I-V-L characteristics observed for the devices (a) I and (b)
II.
integrated area under the fluorescence curve, and η is the refraction
index. Subscripts r and s refer to the reference and to the sample of
unknown quantum yield, respectively. Coumarin-6 in ethanol (ΦF =
0.78) and coumarin-1 in ethyl acetate (ΦF = 0.99) were taken as the
reference.24 Cyclic voltammetry experiments were performed with a
electrochemical analyzer. All measurements were carried out at room
temperature with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum wire
auxiliary, and a nonaqueous acetonitrile Ag/AgNO3 reference
a
c
electrode. The E1/2 values were determined as (Ep + Ep )/2, where
Table 6. Electroluminescence Characteristics of Devices I and II Based on Compounds 4−9
at 100/1000 cd/m2
material
driving voltage (V) at 10 cd/m2 power efficiency (lm/W) current efficiency (cd/A) EQE (%) max luminance (cd/m2)
CIE
4
6.1
6.0
6.3
6.1
5.9
5.0
4.3
5.2
4.8
5.7
65
111
(0.19, 0.23)
(0.20, 0.34)
(0.20, 0.34)
(0.48, 0.43)
(0.50, 0.46)
(0.16, 0.12)
(0.16, 0.21)
(0.17, 0.29)
(0.52, 0.47)
(0.48, 0.51)
5
0.03/0.09
6
99
8
0.09/0.21
0.09/0.04
1.1/0.5
3.6/1.9
2.3/1.2
3.8/1.9
3.1/1.2
282
9
143
4 + CBP
5 + CBP
6 + CBP
8 + CBP
9 + CBP
2.2/1.3
6.0/4.4
4.7/3.3
7.1/4.8
7.3/3.7
2.6/1.2
4.5/3.3
2.6/1.9
3.2/2.1
2.3/11
1340
3440
3310
4630
2650
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Article
1
2195 (νCC); H NMR (CDCl3, 500 MHz) δ 8.74 (d, J = 9.0 Hz, 1
H), 8.25−8.20 (m, 4 H), 8.15 (d, J = 8.0 Hz, 1 H), 8.11−8.03 (m, 3
H), 7.71−7.65 (m, 3 H), 7.61 (d, J = 8.5 Hz, 1 H), 7.30−7.27 (m, 4
H), 7.17−7.14 (m, 5 H), 7.09−7.03 (m, 3 H), 2.05 (m, 2 H), 1.96 (m,
2 H), 0.44 (t, J = 7.5 Hz, 6 H); 13C NMR (CDCl3, 125 MHz) δ 151.8,
150.1, 148.0, 147.8, 141.9, 135.9, 131.9, 131.3, 131.18, 131.16, 131.0,
129.6, 129.3, 128.3, 128.1, 127.3, 126.3, 126.0, 125.7, 125.63, 125.57,
124.64, 124.60, 124.4, 124.2, 124.1, 123.6, 122.9, 122.8, 120.9, 120.8,
119.2, 119.1, 118.2, 96.5, 88.7, 56.3, 32.8, 8.7, 8.6; HRMS calcd for
C47H35N m/z 613.2770, found 613.2769. Anal. Calcd for C47H35N: C,
91.97; H, 5.75; N, 2.28. Found: C, 91.68; H, 5.62; N, 2.14.
Synthesis of 1,3,6,8-Tetrakis((9,9-diethyl-9H-fluoren-2-yl)-
ethynyl)pyrene (7). Compound 7 was prepared from 2 and 3a by
following a procedure similar to that described above for 4. Orange
solid. Yield 10.3 g (87%); mp >360 °C; IR (KBr, cm−1) 2195 (νCC);
1H NMR (CDCl3, 500 MHz) δ 8.88 (s, 4 H), 8.55 (s, 2 H), 7.81−7.71
Table 7. Emission and Electroluminescence Spectral Data of
Compounds 4−9
a
b
λem (nm)
λEL (nm)
at 50
thin
film
at max
at 100
at 1000
d
mA/
c
d
d
m2
compound
dcm
412,
luminance
cd/m2
cd/m2
4
496
480
424, 448 424, 448 424, 448
435
439,
465
516
514,
551
567
5
486,
505
480
452, 476 452, 476 452, 476
6
8
495
587
484
588
468
596, 528 588, 528 588, 528
(sh) (sh) (sh)
556, 592 556, 592 556, 592
(sh) (sh) (sh)
468
468
9
581
576
a
b
(m, 16 H), 7.39−7.38 (m, 12 H), 2.18−2.08 (m, 16 H), 0.40 (t, J = 7.0
Hz, 24 H); 13C NMR (CDCl3, 125 MHz) δ 149.3, 149.2, 141.3, 139.8,
130.7, 129.9, 126.7, 126.03, 125.96, 125.2, 122.0, 120.4, 119.1, 118.8,
118.2, 99.0, 96.3, 86.9, 55.3, 45.2, 31.8, 7.5; HRMS calcd for C92H74
m/z 1178.5791, found 1178.5786. Anal. Calcd for C92H74: C, 93.68; H,
6.32. Found: C, 93.34; H, 6.08.
Emission wavelength. Electroluminescence wavelength for OLED
c
d
device. Wavelength for device I (without CBP). Wavelength for
device II (doping with CBP).
the mixture was poured into water and extracted with ethyl acetate.
The organic extract was washed with brine solution and dried over
Na2SO4. Finally, the solvent was removed under vacuum to yield a
yellow residue, which was purified by column chromatography as a
yellow liquid (5.2 g, 55%) that further undergo cleavage reaction with
KOH and toluene to produce the acetylene 3c. Yellow solid. Yield 3.58
(71%); 1H NMR (CDCl3, 500 MHz) δ 7.57−7.55 (m, 2 H), 7.46 (dd,
J = 6.5, 1.5 Hz, 1 H), 7.42 (d, J = 1.0 Hz, 1 H), 7.28−7.25 (m, 4 H),
7.13−7.11 (m, 4 H), 7.09 (d, J = 2.0 Hz, 1 H), 7.05−7.01 (m, 3 H),
3.12 (s, 1 H), 1.95−1.87 (m, 4 H), 0.35 (t, J = 7.5 Hz, 6 H); 13C NMR
(CDCl3, 125 MHz) δ 151.7, 149.9, 147.9, 147.8, 142.2, 135.7, 131.3,
126.5, 123.5, 122.7, 120.8, 119.3, 119.0, 118.9, 84.8, 56.1, 32.6, 8.5.
Synthesis of 1-((9,9-Diethyl-9H-fluoren-2-yl)ethynyl)pyrene
(4). A mixture of 1-bromopyrene (2.82 g, 10 mmol), 9,9-diethyl-2-
ethynyl-9H-fluorene (3a) (2.95 g, 12 mmol), Pd(PPh3)2Cl2 (70 mg,
0.1 mmol), PPh3 (52 mg, 0.2 mmol), and CuI (20 mg, 0.1 mmol) were
mixed in triethylamine (100 mL) under nitrogen atmosphere. The
resulting mixture was stirred and heated at 100 °C for 24 h. After
completion of the reaction, the mixture was poured into water and
extracted with ethyl acetate. The organic extract was washed with brine
solution and dried over Na2SO4. Finally, the solvent was removed
under vacuum to yield a yellow residue, which was purified by column
chromatography. Yellow solid. Yield 2.2 g (48%); mp 150−152 °C; IR
(KBr, cm−1) 2191 (νCC); 1H NMR (CDCl3, 500 MHz) δ 8.75 (d, J =
9.0 Hz, 1 H), 8.26−8.21 (m, 4 H), 8.17−8.03 (m, 4 H), 7.79−7.70 (m,
4 H), 7.39−7.36 (m, 3 H), 2.12 (q, J = 7.5 Hz, 4 H), 0.39 (t, J = 7.5
Hz, 6 H); 13C NMR (CDCl3, 125 MHz) δ 150.3, 150.2, 142.1, 140.9,
131.9, 131.3, 131.21, 131.17, 130.9, 129.7, 128.3, 128.1, 127.6, 127.3,
127.0, 126.3, 126.1, 125.7, 125.63, 125.57, 124.62, 124.59, 124.4,
123.0, 121.8, 120.1, 119.8, 118.1, 96.3, 88.7, 32.8, 8.6; HRMS calcd for
C35H26 m/z 446.2035, found 446.2034. Anal. Calcd for C35H26: C,
94.13; H, 5.87. Found: C, 93.95; H, 5.69.
Synthesis of 1,3,6,8-Tetrakis((9,9-di(octan-3-yl)-9H-fluoren-
2-yl)ethynyl)pyrene (8). Compound 8 was prepared from 2 and 3d
by following a procedure similar to that described above for 4. Red
solid. Yield: 10.3 g (80%); mp: 110−112 °C; IR (KBr, cm−1) 2195
1
(νCC); H NMR (CDCl3, 500 MHz): δ = 8.87 (s, 4 H), 8.55−8.57
(m, 2 H), 7.70−7.80 (m, 16 H), 7.43−7.47 (m, 4 H), 7.31−7.39 (m, 8
H), 2.09−2.10 (m, 16 H), 1.02−0.82 (m, 78 H), 0.80−0.72 (m, 10 H),
0.56−0.65 (m, 32 H); 13C NMR (CDCl3, 125 MHz): δ = 150.89,
150.85, 150.8, 142.1, 140.61, 140.59, 133.8, 131.8, 130.9, 130.80,
130.75, 129.1, 128.3, 127.4, 127.3, 127.2, 127.1, 127.0, 126.9, 124.4,
124.20, 124.17, 121.0, 120.93, 120.88, 120.1, 119.8, 119.3, 97.4, 87.7,
55.0, 44.8, 44.5, 44.5, 34.7, 33.8, 33.73, 33.66, 33.6, 32.0, 29.73, 29.70,
29.4, 28.2, 27.08, 27.06, 22.8, 22.7, 14.21, 14.15, 14.0, 10.43, 10.41,
10.4. HRMS calcd for C140H170 m/z 1851.3303, found 1851.3294.
Anal. Calcd for C140H170: C, 90.75; H, 9.25. Found: C, 90.59; H, 9.12.
Synthesis of 7,7′,7″,7‴-(Pyrene-1,3,6,8-tetrayltetrakis-
(ethyne-2,1-diyl))tetrakis(9,9-diethyl-N,N-diphenyl-9H-fluoren-
2-amine) (9). Compound 9 was prepared from 2 and 3c by following
a procedure similar to that described above for 4. Orange solid. Yield
1
14.4 g (78%); mp 274−276 °C; IR (KBr, cm−1) 2192 (νCC); H
NMR (CDCl3, 500 MHz) δ 8.85 (s, 4 H), 8.51 (s, 2 H), 7.73−7.65
(m, 11 H), 7.61 (d, J = 8.5 Hz, 4 H), 7.28−7.29 (m, 12 H), 7.13−7.16
(m, 22 H), 7.00−7.09 (m, 15 H), 2.09−2.05 (m, 8 H), 1.94−1.98 (m,
8 H), 0.42−0.45 (m, 24 H); 13C NMR (CDCl3, 125 MHz) δ 151.8,
150.1, 147.9, 147.8, 142.2, 135.8, 133.7, 131.7, 131.1, 129.2, 126.9,
126.0, 124.3, 123.9, 123.5, 122.7, 120.8, 120.5, 119.22, 119.17, 119.0,
97.5, 87.9, 56.3, 32.7, 29.7, 8.6. HRMS calcd for C140H112N4 [M + 2H]
1848.8887, found 1848.8871. Anal. Calcd for C140H110N4: C, 90.97; H,
6.00; N, 3.03. Found: C, 90.78; H, 5.77; N, 2.86.
OLED Fabrication and Performance Evaluation. The OLED
devices for the dyes 4−9 were fabricated on a precleaned glass
substrate containing a 125 nm layer indium tin oxide as anode, 35 nm
poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate) (PE-
DOT:PSS) as hole-injection layer (HIL), emissive layer (EML), 32
nm 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) as electron
transporting layer (ETL), a 0.7 nm LiF electron injection layer (EIL),
and a 150 nm Al layer as cathode. The aqueous solution of
PEDOT:PSS was spin coated at 4000 rpm for 20 s to form a 40 nm
HIL layer. The dyes 4−9 doped in 4,4′-bis(9H-carbazol-9-yl)biphenyl
(CBP) were deposited by spin-coating at 2500 rpm for 20 s and served
as emissive layer. Subsequently, lithium fluoride and aluminum
cathode were thermally evaporated at 1.0 × 10−5 Torr.
Computational Methods. All computations were performed with
the Gaussian 09 program package.41 The ground-state geometries were
fully optimized without any symmetry constrains at the DFT level with
Becke’s three parameters hybrid functional and Lee, Yang and Parr’s
correlational functional B3LYP42 and MPW1K38 using the 6-31G*
basis set on all atoms. Vibrational analyses on the optimized structures
Synthesis of 1,1′-((9,9-Dibutyl-9H-fluorene-2,7-diyl)bis-
(ethyne-2,1-diyl))dipyrene (5). Compound 5 was prepared from
1 and 3b by following a procedure similar to that described above for
4. Yellow solid. Yield 2.6 g (35%); mp 264−266 °C; IR (KBr, cm−1)
1
2187 (νCC); H NMR (CDCl3, 500 MHz) δ 8.76 (d, J = 9.0 Hz, 2
H), 8.29−8.22 (m, 8 H), 8.18 (d, J = 8.0 Hz, 2 H), 8.13−8.04 (m, 6
H), 7.81 (d, J = 8.0 Hz, 2 H), 7.77 (dd, J = 1.5, 6.5 Hz, 2 H), 7.73 (s, 2
H), 2.15−2.12 (m, 4 H), 1.21−1.17 (m, 4 H), 0.89−0.69 (m, 10 H);
13C NMR (CDCl3, 125 MHz) δ 151.4, 140.9, 131.9, 131.33, 131.29,
131.2, 131.0, 129.7, 128.4, 128.2, 127.3, 126.3, 126.0, 125.7, 125.6,
124.64, 124.60, 124.4, 122.3, 120.2, 118.0, 96.3, 89.2, 55.4, 40.4, 29.7,
26.0, 23.2, 14.0; HRMS calcd for C57H42 m/z 726.3287, found
726.3283. Anal. Calcd for C57H42: C, 94.18; H, 5.82. Found: C, 94.01;
H, 5.70.
Synthesis of 9,9-Diethyl-N,N-diphenyl-7-(pyren-1-ylethyn-
yl)-9H-fluoren-2-amine (6). Compound 6 was prepared from 1
and 3c by following a procedure similar to that described above for 4.
Yellow solid. Yield 3.5 g (57%); mp 210−212 °C; IR (KBr, cm−1)
3930
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Article
were performed to confirm the structure. The excitation energies and
oscillator strengths for the lowest 10 singlet−singlet transitions at the
optimized geometry in the ground state were obtained by TD-DFT
calculations using the same basis set as for the ground state using the
same theory level.
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ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C NMR spectra of the newly synthesized compounds,
optical spectra of the compounds recorded in different solvents,
and Cartesian coordinates of the theoretically modeled
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Fax: +91-1332-286202. Tel: +91-1332-285376. E-
mail:krjt8fcy@iitr.ernet.in.
■
Notes
(7) (a) Wang, J.; Wan, W.; Jiang, H.; Gao, Y.; Jiang, X.; Lin, H.;
Zhao, W.; Hao, J. Org. Lett. 2010, 12, 3874−3877. (b) Wee, K. R.;
Han, W. S.; Kim, J. E.; Kim, A. L.; Kwon, S.; Kang, S. O. J. Mater.
Chem. 2011, 21, 1115−1123. (c) Zhu, M. R.; Wang, Q. A.; Gu, Y.;
Cao, X. S.; Zhong, C.; Ma, D. G.; Qin, J. G.; Yang, C. L. J. Mater.
Chem. 2011, 21, 6409−6415. (d) Huang, J. H.; Su, J. H.; Li, X.; Lam,
M. K.; Fung, K. M.; Fan, H. H.; Cheah, K. W.; Chen, C. H.; Tian, H. J.
Mater. Chem. 2011, 21, 2957−2964. (e) Cho, I.; Kim, S. H.; Kim, J. H.;
Park, S.; Park, S. Y. J. Mater. Chem. 2012, 22, 123−129.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.R.J.T. is thankful to Council of Scientific and Industrial
Research, New Delhi for financial support (01(1799)/11/
EMR-II). The Advanced Instrumentation Research Facility at
Jawaharlal Nehru University at New Delhi is acknowledged for
providing access to ESI mass spectrometer.
(8) (a) Wurthner, F.; Stolte, M. Chem. Commun. 2011, 47, 5109−
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