Pyrene-substituted ethenes: Aggregation-enhanced excimer emission and highly efficient electroluminescence

Article abstract of DOI:10.1039/c0jm04449k

Pyrene-substituted ethenes, 1,2,2-tripheny-1-pyrenylethene (TPPyE) and 1,2-diphenyl-1,2-dipyrenylethene (DPDPyE), are synthesized and characterized. Whereas they are weakly emissive in solution they become strong emitters when aggregated in the condensed phase. In contrast to the general observation that excimer formation quenches the light emission of fluorophores, TPPyE and DPDPyE exhibit efficient excimer emissions in the solid state with high fluorescence quantum yields up to 100%. The π-π intermolecular interactions between the pyrene rings, coupled with multiple C-H...π hydrogen bonds, efficiently restrict intramolecular rotations, which block the nonradiative energy decay channel, and hence make the dye molecules highly emissive in the solid state. Non-doped organic light-emitting diodes using TPPyE and DPDPyE as emitters are fabricated, which give green light at low turn-on voltages (down to 3.2 V) with maximum luminance and power, current, and external quantum efficiencies of 49830 cd m^-2, 9.2 lm W^-1, 10.2 cd A^-1 and 3.3%, respectively.

Full text of DOI:10.1039/c0jm04449k

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PAPER
Pyrene-substituted ethenes: aggregation-enhanced excimer emission
and highly efficient electroluminescence†
Zujin Zhao,ab Shuming Chen,c Jacky W. Y. Lam,a Zhiming Wang,d Ping Lu,d Faisal Mahtab,a
a
a
d
c
Herman H. Y. Sung, Ian D. Williams, Yuguang Ma, Hoi Sing Kwok and Ben Zhong Tang
ae
*
Received 20th December 2010, Accepted 15th March 2011
DOI: 10.1039/c0jm04449k
Pyrene-substituted ethenes, 1,2,2-tripheny-1-pyrenylethene (TPPyE) and 1,2-diphenyl-1,2-
dipyrenylethene (DPDPyE), are synthesized and characterized. Whereas they are weakly emissive in
solution they become strong emitters when aggregated in the condensed phase. In contrast to the
general observation that excimer formation quenches the light emission of fluorophores, TPPyE and
DPDPyE exhibit efficient excimer emissions in the solid state with high fluorescence quantum yields up
to 100%. The p–p intermolecular interactions between the pyrene rings, coupled with multiple C–H/p
hydrogen bonds, efficiently restrict intramolecular rotations, which block the nonradiative energy
decay channel, and hence make the dye molecules highly emissive in the solid state. Non-doped organic
light-emitting diodes using TPPyE and DPDPyE as emitters are fabricated, which give green light at
low turn-on voltages (down to 3.2 V) with maximum luminance and power, current, and external
quantum efficiencies of 49830 cd mꢀ2, 9.2 lm Wꢀ1, 10.2 cd Aꢀ1 and 3.3%, respectively.
antagonistic factors. Unfortunately, the negative effect is
dominant in the photoluminescence (PL) process of most
luminophors. This is known as aggregation-caused quenching
(ACQ) of light emission.3
Introduction
Advancement in optoelectronics is directly associated with the
development of new luminescent materials. Whereas many dyes
emit strongly when molecularly dissolved in good solvents, they
become weak fluorophores when aggregated in poor solvents or
fabricated as thin films in the solid state. In the condensed
phase, the dye molecules are located in the immediate vicinity of
each other and experience strong intermolecular interactions,
which promote the formation of species such as excimers and
exciplexes which are detrimental to luminescence. On the other
hand, aggregation enables efficient intermolecular charge
transfer1 and can positively restrict the intramolecular rotation
(IMR) of the fluorophores,2 which blocks the nonradiative
decay channels, and hence enhances their fluorescence efficien-
cies. Whether aggregation quenches or strengthens the light
emission is determined by the competition between these two
We have recently discovered a phenomenon exactly opposite
to the ACQ effect, that is, aggregation-induced emission (AIE):
a series of propeller-shaped non-emissive molecules can be
induced to emit efficiently by aggregate formation.4 In solu-
tion, they are non-emissive because of the active IMR process,
which serves as a relaxation channel for the excited state to
decay. In the aggregate state, they are induced to emit intensely
owing to the restriction of the IMR process5,6 and the
suppression of the intermolecular interactions by their
nonplanar conformations. Among the AIE molecules, tetra-
phenylethene (TPE) enjoys the advantages of facile synthesis
and efficient solid-state emission as well as high thermal
stability. A wide variety of substituents has been attached to its
phenyl blades to endow it with enhanced and/or new electronic
and optical properties. Pyrene is a flat aromatic molecule and
shows interesting fluorescence properties.7,8 It is bulky and
rigid and luminophors containing such a chromophore are
anticipated to show high fluorescence quantum yield (FF) and
thermal stability. In this study, we utilized pyrene as a building
block for the construction of new luminogens. We replaced one
or two phenyl ring(s) of TPE by pyrene ring(s) and investi-
gated the photophysical properties of the resultant luminogens.
We investigated whether pyrene-substituted ethenes, similar to
TPE, are AIE-active and if their aggregates are emissive, and
attempted to clarify the origins for light emission and causes of
the AIE process.
aDepartment of Chemistry, Institute of Molecular Functional Materials,
The Hong Kong University of Science & Technology (HKUST), Clear
Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
bCollege of Material, Chemistry and Chemical Engineering, Hangzhou
Normal University, Hangzhou, 310036, China
cCenter for Display Research, HKUST, Kowloon, Hong Kong, China
dState Key Laboratory of Supramolecular Structure and Materials, Jilin
University, Changchun, 130012, China
eDepartment of Polymer Science and Engineering, Zhejiang University,
Hangzhou, 310027, China
† CCDC reference numbers 755289 (TPPyE) and 755290 (cis-DPDPyE).
For crystallographic data in CIF or other electronic format see DOI:
10.1039/c0jm04449k
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Results and discussion
Scheme 1 illustrates the synthetic routes to the pyrene-substituted
ethenes. Detailed procedures and characterization data are given
in the Experimental section. Single crystals of TPPyE were grown
from a hexane–dichloromethane solution and analyzed by X-ray
diffraction crystallography. Crystals of both cis- and trans-
isomers of DPDPyE were obtained by a very slow evaporation of
a chloroform solution. Fortunately we were able of isolate
crystals of cis-DPDPyE from the mixture. The crystal structures
and B3LYP/6-31G* calculated molecular orbital amplitude plots
of HOMO and LUMO levels of TPPyE and cis-DPDPyE are
shown in Fig. 1. The electron clouds in both HOMO and LUMO
levels of TPPyE and cis-DPDPyE are mainly located on the
pyrene ring(s), revealing that this chromophoric unit predomi-
nantly controls the absorption and emission of the molecules.
The thermal properties of TPPyE and DPDPyE were examined
by differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA) measurements. The DSC thermograms of
TPPꢁyE and DPDPyE revealed melting temperatures at 203 and
279 C, respectively. No glass-transition temperature, however,
was detected. As shown in Fig. 2A, both molecules show high
thermal stability and decompose at high temperatures (269 ꢁC
for TPPyE and 370 ꢁC for DPDPyE). These values are much
higher than that for TPE (224 ꢁC)5c owing to the high rigidity and
bulkiness of the pyrene ring.
Fig. 1 ORTEP drawings and B3LYP/6-31G* calculated molecular
orbital amplitude plots of HOMO and LUMO levels of TPPyE and cis-
DPDPyE.
present in one molecule of DPDPyE, which encourages the
formation of J-aggregates through intermolecular p–p stacking.
For a 10 mM THF solution, the FF value of TPPyE is 2.8%, of
which about 65% is contributed from the excimer. For the same
solvent and concentration, DPDPyE has a higher FF value
(9.8%), but is still much lower than that of the pyrene monomer
(32% in ꢂ50 mM cyclohexane)9 and most pyrene-substituted
luminophors.11 Such phenomena have been observed in
conventional planar luminophors with rotatable phenyl rings
and are caused by the active IMR process, which promotes the
nonradiative decay of the excited states.12
The UV-vis spectrum of TPPyE resembles that of DPDPyE
and shows an absorption maximum at ꢂ350 nm (Fig. 2B). The
molar absorptivity of DPDPyE at 353 nm (1.9 ꢃ 104 Mꢀ1 cmꢀ1) is
about two times higher than that of TPPyE, but is much lower
than the value exhibited by pyrene at 335 nm (5.4 ꢃ 104 Mꢀ1
cmꢀ1).9 The PL spectrum of TPPyE in dilute THF solution (10ꢀ8
M) displays a sharp peak at 388 nm which well extends to 570 nm
(Fig. 3A). When the solution concentration is increased to 10ꢀ7
M, a new peak is observed at 483 nm. Whereas the former peak
(388 nm) is assigned to the monomer emission of the pyrene
moiety7,8 the latter one (483 nm) may be associated with the
emission of pyrene excimers. To prove this, we prepared more
concentrated solutions, as excimer emission is known to be
concentration dependent. With a progressive increase in the
solution concentration, the emission at 483 nm becomes domi-
nant, and at 10ꢀ3 M, only emission at the longer wavelength is
observed, demonstrating that it is truly originated from pyrene
excimers.8,10 The same phenomenon is also observed in the
spectrum of DPDPyE but at the same concentration the excimer
emission in DPDPyE is much stronger. The tendency for excimer
formation in DPDPyE is so high that even at a concentration as
low as 10ꢀ8 M, its PL spectrum still exhibits excimer emission at
523 nm (Fig. 3B). The excimer emission of DPDPyE is red-
shifted relative to that of TPPyE because two pyrene rings are
We further studied their optical properties in the crystal and
amorphous states. Upon photoexcitation, crystals of TPPyE and
cis-DPDPyE emit at 481 and 486 nm, respectively (Table 1). The
PL of the amorphous film of TPPyE is found at 484 nm, which is
close to those in concentrated solution and the crystal state
(Fig. 4A), suggesting that they originate from the same emitting
species with similar molecular interactions. On the other hand,
the PL of an amorphous film of DPDPyE is located at 503 nm,
which is 17 nm red-shifted from the spectrum of the crystals. The
unusual blue shift observed in the crystalline phase may be
attributable to conformation twisting in the crystal packing
process, during which the DPDPyE molecules may conforma-
tionally adjust themselves by twisting their aromatic rings to fit
into the crystalline lattice. Without such restraint, the molecules
Fig. 2 (A) TGA thermograms for TPPyE and DPDPyE recorded under
nitrogen at a heating rate of 10 ꢁC minꢀ1. (B) Absorption spectra of
TPPyE and DPDPyE in THF solutions (10 mM).
Scheme 1 Synthetic routes to TPPyE and DPDPyE. PTSA ¼ p-tolue-
nesulfonic acid.
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Fig. 4 (A) PL spectra of thin films of TPPyE and DPDPyE and (B) EL
spectra of TPPyE and DPDPyE in multilayer devices with configurations
of ITO|NPB (60 nm)|emitter (20 nm)|TPBi (30 nm)|LiF (1 nm)|Al
(100 nm) (device I) and ITO|NPB (60 nm)|emitter (20 nm)|TPBi (10 nm)|
Alq3 (30 nm)|LiF (1 nm)|Al (100 nm) (device II).
emission maximum. In contrast to their weak emission in dilute
solutions, the amorphous films of TPPyE and DPDPyE emit
more efficiently. Their FF values reach 61 and 100%, respectively,
and are much higher than those of pyrene film (44%) and TPE
(49.2%)5c measured under the same conditions. This demon-
strates a novel phenomenon of aggregation-enhanced excimer
emission (AEEE) and suggests that replacement of the phenyl
ring(s) of TPE by pyrene ring(s) has generated new luminogens
with more efficient solid-state emissions.
To further confirm the AEEE feature of TPPyE and DPDPyE,
the PL behaviors of their aggregates in poor solvents were
investigated. When a large amount of water is added into the
THF solutions, their emissions are strengthened. At 90% water
content, intense excimer emission was observed in the spectrum
of TPPyE at 485 nm. The emission in a 99.5% aqueous mixture is
so strong that the monomer emission at ꢂ388 nm is hardly dis-
cerned (Fig. 3C). The excimer emission of DPDPyE also becomes
stronger in THF–water mixtures with higher water contents
(Fig. 3D). Since TPPyE and DPDPyE are not soluble in water,
their molecules must be aggregated in solvent mixtures with large
amounts of water. The solutions are, however, homogeneous
with no precipitates, suggesting that the aggregates are nano-
dimensional. The discernible vibronic structure in the excimer
emission band of DPDPyE (Fig. 3D) is indicative of the
formation of extremely structured aggregates as ascertained from
the vibronic progression of the C]C stretching mode.13 The
electron diffraction (ED) patterns of the aggregates of TPPyE
and DPDPyE formed in 90% aqueous mixtures display many
diffraction spots (Fig. 3E and 3F), demonstrating that they are
crystalline in nature.
Fig. 3 Normalized PL spectra of (A) TPPyE and (B) DPDPyE in THF
solutions with different concentrations. PL spectra of (C) TPPyE and (D)
DPDPyE in THF–water mixtures (1 mM) with different water contents.
Insets in panels (C) and (D): photographs of TPPyE and DPDPyE in
THF and in THF–water (99.5% water content) taken under UV illumi-
nation. Excitation wavelength: 350 nm. ED patterns of crystalline
aggregates of (E) TPPyE and (F) DPDPyE formed in THF–water with
90% water content.
Table 1 Optical properties of TPPyE and DPDPyE in solution (Soln)a
and crystalline (Cryst) and amorphousb (Film) states
lem/nm
FF (%)
labs/nm
Soln
Soln
Cryst
Film
Solnc
Filmd
TPPyE
DPDPyE
353
352
(388) 483
(391) 523
481
486e
484
503
2.8
9.8
61
100
a
b
In THF solution (10 mM). Film spin-coated on quartz plate.
Quantum yields (FF) determined in THF solutions using 9,10-
c
diphenylanthracene (FF
Quantum yield of the amorphous film measured by an integrating
¼
90% in cyclohexane) as standard.
Several papers have reported that the pyrene excimers in the
crystal state can emit more efficiently than their monomeric
species in solution.14 Concentration quenching, however, is still
a general phenomenon observed in pyrene and its derivatives.7,8
Although scientists have tried hard to hamper intermolecular
interactions, the emission efficiencies of most pyrene-containing
luminophors are reduced in solid films and the crystal state.11 We
thus investigated the crystal structures to try and rationalize why
an opposite effect is observed in TPPyE and DPDPyE. As shown
in Fig. 5, the pyrene rings of two adjacent TPPyE molecules are
stacked in a parallel fashion and about half of their surfaces
(ꢂ7 carbon atoms) are overlapped (Fig. 5C). The distance
d
sphere. e Crystals of cis-DPDPyE.
in the amorphous state may assume a more planar conformation,
which enables better p–p stacking interactions, and hence results
in a red-shifted emission. In concentrated solution, extensive
excimers may be readily formed because the molecules can adjust
their conformations and locations with little constraint, in order
to maximize the intermolecular interactions or overlapping of
pyrene rings. This may account for the further red-shift in the
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emission quenching of the luminogens.4f,15 In addition to p–p
stacking, multiple C–H/p hydrogen bonds with distances of
ꢀ
2.970 and 3.086 A are formed between the hydrogen atoms of the
phenyl rings in one TPPyE molecule and the p cloud of the
pyrene ring in another molecule. C–H/p hydrogen bonds with
ꢀ
a distance of 2.835 A are also observed between the hydrogen
atoms of the pyrene ring in one cis-DPDPyE molecule and the p
cloud of the pyrene ring in another molecule. These weak but
attractive forces of multiple C–H/p hydrogen bonds plus p–p
interactions help rigidify the molecular conformation and lock
the molecular rotations.2,6 As a result, the excited energy
consumed by the IMR process is greatly reduced, enabling
TPPyE and DPDPyE to emit efficiently in the solid state.
The efficient PL of TPPyE and DPDPyE in the aggregate state
prompted us to investigate their electroluminescence (EL).
Multilayer non-doped organic light-emitting diodes (OLEDs)
with configurations of ITO|NPB (60 nm)|TPPyE or DPDPyE (20
nm)|TPBi (30 nm)|LiF (1 nm)|Al (100 nm) (device I) and ITO|
NPB (60 nm)|TPPyE or DPDPyE (20 nm)|TPBi (10 nm)|Alq3 (30
nm)|LiF (1 nm)|Al (100 nm) (device II) were fabricated. In these
EL devices, TPPyE and DPDPyE act as light emitters, N,N-bis
(1-naphthyl)-N,N-diphenylbenzidine (NPB) functions as a hole-
transport material and 2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-
1-H-benzimidazole) (TPBi) and/or tris(8-hydroxyquinolinolato)
aluminium (Alq3) serve as hole-block and/or electron-transport
materials, respectively. The device performances are summarized
in Table 2. The EL spectra of TPPyE and DPDPyE show peaks
at 504–520 nm, which are slightly red-shifted from the PL of their
amorphous films due to the microcavity effect (Fig. 4B). In
device I, TPPyE and DPDPyE turn-on at 4.6 and 5.3 V, exhib-
iting maximum luminance of 15450 and 45550 cd mꢀ2 and
current efficiency of 3.7 and 9.1 cd Aꢀ1, respectively (Fig. 6A and
6B). The maximum external quantum efficiency (2.9%) attained
by the device of DPDPyE is more than two times higher than the
value achieved in TPPyE, presumably due to its higher solid-state
FF value and enhanced carrier mobility arising from the p–p
stacking interactions between the pyrene rings. Even better EL
performances are observed in device II. The DPDPyE device
starts to emit at a low voltage of 3.2 V and radiates brilliantly
with maximum luminance of 49830 cd cmꢀ2 (Fig. 6C) and power,
current and external quantum efficiencies of 9.2 lm Wꢀ1, 10.2 cd
Aꢀ1 and 3.3%, respectively (Fig. 6D). Similar large enhancement
in the device performance is also observed in TPPyE, thanks to
the additional Alq3 layer, which improves the electron trans-
portation in the EL device.
Fig. 5 (A) and (D) C–H/p hydrogen bonds and p–p interactions with
ꢀ
indicated distances (A) between the adjacent molecules of TPPyE and cis-
DPDPyE in the crystal state. (B) and (E) Side and (C), (F) and (G) top
views of adjacent molecules of TPPyE and cis-DPDPyE along the plane
of pyrene stacking.
ꢀ
between two pyrene planes is 3.483 A, which is shorter than the
ꢀ
typical distance for p–p interaction (3.5 A). Similar packing with
ꢀ
a interplanar distance of 3.402 A is also observed in the crystals
of cis-DPDPyE. This provides evidence that the PL of TPPyE
and cis-DPDPyE in the crystal state arises from the pyrene
excimers (Fig. 5F). The second pyrene ring of cis-DPDPyE is also
located parallel to the pyrene blade of its neighboring molecule
ꢀ
with a spacing of 3.367 A (Fig. 5D). Although the extent of
overlapping is not so large, it helps to hinder the free rotation of
the aromatic rings (Fig. 5G). Moreover, since both pyrene rings
in one cis-DPDPyE molecule can form p–p stacking, the lumi-
nogen can self-assemble into a supermolecular structure (Fig. 5E)
resembling that of a J-aggregate.4f,15 Such behavior is also
possible in concentrated solution, and it may be even easier for
the molecules to form short dimer or trimer chains in a similar
stacking pattern via maximized p–p intermolecular interactions.
This may account for the red-shifted emission of the dye mole-
cule in the solution state. However, such head-to-tail packing is
not observed in TPPyE because there is only one pyrene ring in
the molecule (Fig. 5B). The absence of such a linkage may lead to
similar emission behaviors in the solution, amorphous and
crystal states. Nevertheless, the crystal structure analysis has
excluded the possibility of H-aggregate formation that leads to
So far, there are several reports on high-efficiency green
OLEDs fabricated using a doping method. For example, phos-
phorescent green OLEDs with a standard bottom-emitting
structure can attain current efficiency over 60 cd Aꢀ1. For those
with advanced structures utilizing a host-dopant emitting
layer such as CBP : Ir(ppy)3, the power, current and external
quantum efficiencies can reach 194 cd Aꢀ1, 210 lm Wꢀ1 and
56.9%, respectively.16 For fluorescent green OLEDs based on
Alq3 : C545T, a high current efficiency of 24 cd Aꢀ1 can be
achieved due to triplet–triplet annihilation.17 However, the
fabrication of these devices requires a strict control on the
dopant concentration. If the concentration is too high, the energy
transfer efficiency will be limited by concentration quenching and
phase separation. The complicated fabrication process of the
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Table 2 EL performances of TPPyE and DPDPyE.a
Device
lEL/nm
Von/V
Lmax/cd mꢀ2
hP,max/lm Wꢀ1
hC,max/cd Aꢀ1
hext,max (%)
TPPyE
I
I
II
II
508
520
504
516
4.6
5.3
4.6
3.2
15450
45550
25470
49830
2.1
4.1
2.7
9.2
3.7
9.1
4.0
10.2
1.4
2.9
2.0
3.3
DPDPyE
TPPyE
DPDPyE
a
Abbreviations: Von ¼ turn-on voltage at 1 cd mꢀ2, Lmax ¼ maximum luminance, hP,max, hC,max and hext,max ¼ maximum power, current and external
quantum efficiencies, respectively.
synthesized by simple synthetic procedures, enabling them to find
practical applications in electronics.
Conclusions
In summary, two pyrene-substituted ethenes are synthesized and
their photophysical properties in solution and aggregate states
are investigated. Both luminogens are weakly emissive in the
solutions but become strong emitters when aggregated in the
condensed phase. In contrast to the general observation that
excimer formation quenches the light emissions of luminophors,
both TPPyE and DPDPyE exhibit efficient excimer emissions in
the solid state with high FF values up to 100%. The p–p inter-
molecular interactions between the pyrene rings and the multiple
C–H/p hydrogen bonds have efficiently restricted the IMR
process of the molecules, which block the nonradiative energy
decay channels, and thus enable them to emit intensely in the
solid state. Multilayer devices using TPPyE and DPDPyE as host
emitters are fabricated, which exhibit outstanding performances.
The DPDPyE-based devices show low turn-on voltage (down to
3.2 V) with high luminance (up to 49830 cd mꢀ2) and power,
current and external quantum efficiencies (up to 9.2 lm Wꢀ1, 10.2
cd Aꢀ1 and 3.3%, respectively), making the present luminogens
promising materials for the construction of efficient non-doped
Fig. 6 Change in luminance (A) and current density (C) with the applied
voltage and current efficiency vs. current density curves in devices I (B)
and II (D).
doped OLEDs makes them unsuitable for rapid and mass
green OLEDs.
production. Highly efficient green fluorescent emitters for non-
doped OLEDs are in great demand for commercialization but
Experimental
unfortunately there are only few routes available in the literature
that present the fabrication of efficient non-doped green OLEDs.
Recently, Lee and co-workers18 demonstrated a non-doped green
OLED constructed from a dually functional material, phenyl-9-
[8-(7,10-diphenylfluoranthenyl)]phenylcarbazole, that displays
impressive high current efficiency of 10.1 cd Aꢀ1 and power
efficiency of 12.1 lm Wꢀ1. Wong and co-workers19 reported an
optimal device fabricated using 9,9-diarylfluorene-terminated
2,1,3-benzothiadiazole as green emitter and two triaryldiamines
as hole transporters, which exhibits a maximum current effi-
Materials and instrumentation
THF was distilled from sodium benzophenone ketyl under dry
nitrogen immediately prior to use. All other chemicals and
reagents were purchased from Aldrich and used as received
without further purification. 1H and 13C NMR spectra were
measured on a Bruker AV 300 spectrometer in deuterated chlo-
roform using tetramethylsilane (TMS; d ¼ 0) as internal refer-
ence. UV spectra were measured on a Milton Roy Spectronic
3000 Array spectrophotometer. PL spectra were recorded on
a Perkin-Elmer LS 55 spectrofluorometer. High-resolution mass
spectra (HRMS) were recorded on a GCT premier CAB048 mass
spectrometer operating in MALDI-TOF mode. Elemental
analysis was performed on an Elementary Vario EL analyzer.
Single-crystal X-ray diffraction intensity data were collected on
ciency of 12.9 cd Aꢀ1 and power efficiency of 11 lm Wꢀ1
.
Although the device configuration is yet to be optimized, the
performances of the devices based on our new molecules are
outstanding compared with those of current non-doped green
OLEDs. The EL efficiencies of DPDPyE (10.2 cd Aꢀ1 and 9.2 lm
Wꢀ1) are much better than those of Alq3,12c,20 a widely investi-
gated green emitter. The values are also superior to those of
commercial pyrene-based light-emitting materials,11a,b clearly
demonstrating the high potential of the present molecules as
solid emitters for the construction of efficient EL devices. An
additional merit of our luminophors is that they can be readily
a
Bruker–Nonices Smart Apex CCD diffractometer with
graphite-monochromated Mo-Ka radiation. Processing of the
intensity data was carried out using the SAINT and SADABS
routines, and the structure and refinement were conducted using
the SHELTL suite of X-ray programs (version 6.10). TGA
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analysis was carried ꢁon a TA TGA Q5000 under dry nitrogen at
a heating rate of 10 C minꢀ1. Thermal transitions were investi-
gated by a TA DSC Q1000 under dry nitrogen at a heating rate of
10 ꢁC minꢀ1. The ground-state geometries were optimized using
the density functional with B3LYP hybrid functional at the basis
set level of 6-31G*. All the calculations were performed using the
Gaussian 03 package. ED patterns were obtained using a JOEL
2010 transmission electron microscope at an accelerating voltage
of 200 kV. Samples were prepared by drop-casting dilute
suspensions onto a copper 400-mesh carrier grid covered with
carbon-coated formvar film and dried in open air at room
temperature.
Crystal data for cis-DPDPyE. C46H28$CHCl3, Mr ¼ 700.05,
ꢁ
ꢀ
triclinic, P1, a ¼ 8.7695(6), b ¼ 13.3414(11), c ¼ 15.9829(11) A,
ꢁ
3
ꢀ
a ¼ 77.119(8), b ¼ 89.166(6), g ¼ 71.187(6) , V ¼ 1722.2(2) A , Z
¼ 2, Dc ¼ 1.350 g cmꢀ3, m ¼ 2.667 mmꢀ1 (Mo-Ka, l ¼ 1.54178), F
(000) ¼ 724, T ¼ 173(2) K, 2qmax ¼ 66.5ꢁ (95.1%), 8722 measured
reflections, 5520 independent reflections (Rint ¼ 0.0649), GOF on
F2 ¼ 1.076, R1 ¼ 0.0875, wR2 ¼ 0.2066 (all data), Drmax,min
¼
ꢀ3
ꢀ
0.515 and ꢀ0.356 e A
.
Synthesis
Pyrenophenone (2). To a solution of pyrene (2 g, 10 mmol) and
aluminium(III) chloride (1.6 g, 12 mmol) in dry dichloromethane
(100 mL) was added dropwise benzoyl chloride (1.4 g, 10 mmol)
at 0 ꢁC. The resultant black solution was stirred overnight at
room temperature. The solution was poured into water and
extracted with dichloromethane. The organic layer was washed
with saturated brine solution and dried over anhydrous magne-
sium sulfate. After filtration and solvent evaporation, the residue
was purified by silica-gel column chromatography using
Device fabrication
The devices were fabricated on 80 nm-ITO coated glass with
a sheet resistance of 25 U ,ꢀ1. Prior to loading into the
pretreatment chamber, the ITO-coated glass was soaked in
ultrasonic detergent for 30 min, followed by spraying with de-
ionized water for 10 min, soaking in ultrasonic de-ionized water
for 30 min, and oven-baking for 1 h. The cleaned samples were
treated by perfluoromethane plasma with a power of 100 W, gas
flow of 50 sccm, and pressure of 0.2 Torr for 10 s in the
pretreatment chamber. The samples were transferred to the
organic chamber with a base pressure of 7 ꢃ 10ꢀ7 Torr for
the deposition of NPB, emitter, TPBi, and/or Alq3, which served
as hole-transport, light-emitting, hole-block, and/or electron-
transport layers, respectively. The samples were then transferred
to the metal chamber for cathode deposition to lithium fluoride
(LiF) capped with aluminium (Al). The light-emitting area was 4
mm2. The current density–voltage characteristics of the devices
n-hexane–dichloromethane as eluent. Orange solid
2 was
1
obtained in 80% yield (2.4 g). H NMR (300 MHz, CDCl3),
d (TMS, ppm): 8.36 (d, 1H, J ¼ 9.27 Hz), 8.27–8.17 (m, 4H),
8.13–8.03 (m, 4H), 7.92–7.89 (m, 2H), 7.65–7.60 (m, 1H), 7.51–
7.46 (m, 2H). 13C NMR (75 MHz, CDCl3), d (TMS, ppm): 199.1,
139.4, 133.8, 133.77, 133.74, 131.8, 131.33, 131.28, 130.4, 129.8,
129.6, 129.1, 127.9, 127.6, 127.1, 126.7, 126.6, 125.5, 125.4, 124.4.
HRMS (MALDI-TOF): m/z 306.2141 (M+, calc. 306.1045).
Anal. Calc, for C23H14O: C, 90.17; H, 4.61; O, 5.22. Found: C,
90.12; H, 4.48; O, 5.52%; mp 118 ꢁC.
were measured by
a HP4145B semiconductor parameter
1,2,2-Triphenyl-1-pyrenylethene (TPPyE). To a solution of
diphenylmethane (1 g, 6 mmol) in dry THF (30 mL) was added
dropwise a 1.6 M solution of n-butyllithium in hexane (3.7 mL, 6
mmol) at 0 ꢁC under nitrogen. After stirring for 1 h at 0 ꢁC, the
resultant orange–red solution was transferred slowly to a solu-
analyzer. The forward direction photons emitted from the
devices were detected by a calibrated UDT PIN-25D silicon
photodiode. The luminance and external quantum efficiencies of
the devices were inferred from the photocurrent of the photo-
diode. The electroluminescence spectra were obtained by
a PR650 spectrophotometer. All measurements were carried out
under air at room temperature without device encapsulation.
ꢁ
tion of 2 (1.5 g, 5 mmol) in THF (20 mL) at 0 C. The reaction
mixture was allowed to warm to room temperature and stirred
for 6 h. The reaction was quenched with aqueous ammonium
chloride solution. The organic layer was extracted with
dichloromethane and the combined organic layers were washed
with saturated brine solution and dried over anhydrous magne-
sium sulfate. After filtration and solvent evaporation, the resul-
tant crude alcohol with excess diphenylmethane was dissolved in
about 50 mL of toluene. A catalytic amount of p-toluenesulfonic
acid (0.25 g, 1.3 mmol) was then added. After heating under
reflux for 6 h, the mixture was cooled to room temperature,
washed with saturated brine solution and water, and dried over
anhydrous magnesium sulfate. After filtration and solvent
evaporation, the residue was purified by silica-gel column chro-
matography using n-hexane–dichloromethane as eluent. Pale
Preparation of nanoaggregates
Stock THF solutions of TPPyE and DPDPyE with a concentra-
tion of 10ꢀ5 M were prepared. Aliquots of the stock solution were
transferred to 10 mL volumetric flasks. After appropriate
amounts of THF were added, water was added dropwise under
vigorous stirring to furnish 10ꢀ6 M solutions with different water
contents (0–99.5 vol%). The PL measurements of the resultant
solutions were then performed immediately.
X-Ray crystallography
1
ꢁ
Crystal data for TPPyE. C36H24, Mr ¼ 456.55, triclinic, P1,
yellow solid of TPPyE was obtained in 72% yield (1.6 g). H
ꢀ
a ¼ 9.5346(6), b ¼ 9.5932(6), c ¼ 13.9476(9) A, a ¼ 96.674(5),
NMR (300 MHz, CDCl3), d (TMS, ppm): 8.29 (d, 1H, J ¼ 9.3
Hz), 8.15–8.08 (m, 2H), 8.03–7.91 (m, 5H), 7.82 (d, 1H, J ¼ 7.8
Hz), 7.24–7.20 (m, 5H), 7.06–9.67 (m, 7H), 6.83–6.80 (m, 3H).
13C NMR (75 MHz, CDCl3), d (TMS, ppm): 144.3, 144.23,
144.19, 144.0, 140.1, 139.7, 139.3, 132.2, 131.9, 131.6, 131.4,
131.1, 130.5, 130.9, 128.6, 128.4, 128.1, 128.0, 127.7, 127.5, 127.1,
126.5, 126.2, 125.6, 125.5, 125.2. HRMS (MALDI-TOF): m/z
ꢁ
3
ꢀ
b ¼ 105.479(6), g ¼ 94.841(5) , V ¼ 1212.24(13) A , Z ¼ 2,
Dc ¼ 1.251 g cmꢀ3, m ¼ 0.537 mmꢀ1 (Mo-Ka, l ¼ 1.54178), F
(000) ¼ 480, T ¼ 173(2) K, 2qmax ¼ 66.5ꢁ (95.1%), 6354 measured
reflections, 4112 independent reflections (Rint ¼ 0.0277), GOF on
F2 ¼ 1.012, R1 ¼ 0.0476, wR2 ¼ 0.0994 (all data), Drmax,min
¼
ꢀ3
ꢀ
0.148 and ꢀ0.183 e A
.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 7210–7216 | 7215

View Article Online
456.2043 (M+, calc. 456.1878). Anal. Calc. for C36H24: C, 94.70;
T. M. Swager, J. Am. Chem. Soc., 2000, 122, 8565; (j)
T. P. I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-Lieker and
J. Salbeck, Chem. Rev., 2007, 107, 1011.
ꢁ
H, 5.30. Found: C, 94.58; H, 5.51%; mp 203 C.
€
5 (a) Z. Li, Y. Dong, B. Mi, Y. Tang, M. Haussler, H. Tong, P. Dong,
1,2-Diphenyl-1,2-dipyrenylethene (DPDPyE). To a solution of
2 (1.5 g, 5 mmol) and zinc dust (0.65 g, 10 mmol) in 50 mL dry
THF was added dropwise titanium(IV) chloride (0.95 g, 5 mmol)
under nitrogen at ꢀ78 ꢁC. After stirring for 20 min, the reaction
mixture was warmed to room temperature and then heated to
reflux for 12 h. The reaction mixture was then cooled to room
temperature and poured into water. The organic layer was
extracted with dichloromethane and the combined organic layers
were washed with saturated brine solution and water and dried
over anhydrous magnesium sulfate. After filtration and solvent
evaporation, the residue was purified by silica-gel column chro-
matography using n-hexane–dichloromethane as eluent. Pale
yellow solid of DPDPyE was obtained in 56% yield (0.81 g). 1H
NMR (300 MHz, CDCl3), d (TMS, ppm): 8.48–8.40 (m, 2H),
8.20–7.95 (m, 16H), 7.01–6.96 (m, 4H), 6.83–6.74 (m, 6H).
HRMS (MALDI-TOF): m/z 580.4069 (M+, calc. 580.2129).
Anal. Calc. forꢁC46H28: C, 95.14; H, 4.86. Found: C, 94.87; H,
4.96%; mp 279 C.
J. W. Y. Lam, Y. Ren, H. H. Y. Sun, K. Wong, P. Gao,
I. D. Williams, H. S. Kwok and B. Z. Tang, J. Phys. Chem. B,
2005, 109, 10061; (b) G. Yu, S. Yin, Y. Liu, J. Chen, X. Xu,
X. Sun, D. Ma, X. Zhan, Q. Peng, Z. Shuai, B. Z. Tang, D. Zhu,
W. Fang and Y. Luo, J. Am. Chem. Soc., 2005, 127, 6335; (c)
Z. Zhao, S. Chen, J. W. Y. Lam, C. K. W. Jim, C. Y. K. Chan,
Z. Wang, P. Lu, H. S. Kwok, Y. Ma and B. Z. Tang, J. Phys.
Chem. C, 2010, 114, 7963.
6 (a) Z. Zhao, Z. Wang, P. Lu, C. Y. K. Chan, D. Liu, J. W. Y. Lam,
H. H. Y. Sung, I. D. Williams, Y. Ma and B. Z. Tang, Angew. Chem.,
Int. Ed., 2009, 48, 7608; (b) H. Tong, Y. Dong, Y. Hong,
M. Haeussler, J. W. Y. Lam, H. H. Y. Sung, X. Yu, J. Sun,
I. D. Williams, H. S. Kwok and B. Z. Tang, J. Phys. Chem. C,
2007, 111, 2287; (c) Z. Zhao, P. Lu, J. W. Y. Lam, Z. Wang,
C. Y. K. Chan, H. H. Y. Sung, I. D. Williams, Y. Ma and
B. Z. Tang, Chem. Sci., 2011, 2, 672.
7 (a) F. M. Winnik, Chem. Rev., 1993, 93, 587; (b) J. B. Birks,
Photophysics of Aromatic Molecules, Wiley-Interscience, London,
1970; (c) J. R. Lakowicz, Principles of Fluorescence Spectroscopy,
Plenum Press, New York, 1999.
8 (a) S. H. Lee, S. H. Kim, S. K. Kim, J. H. Jung and J. S. Kim, J. Org.
Chem., 2005, 70, 9288; (b) S. Nishizawa, Y. Kato and N. Teramae, J.
Am. Chem. Soc., 1999, 121, 9463; (c) T.-C. Chou, C.-L. Hwa, J.-
J. Lin, K.-C. Liao and J.-C. Tseng, J. Org. Chem., 2005, 70, 9717; (d)
I. Suzuki, M. Ui and A. Yamauchi, J. Am. Chem. Soc., 2006, 128, 4498.
9 I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic
Molecules, Academic Press, New York and London, 1971.
10 J. A. Mikroyannidis, L. Fenenko and C. Adachi, J. Phys. Chem. B,
2006, 110, 20317.
Acknowledgements
This project was partially supported by grants from the Research
GrantsCouncilofHongKong(603509, 601608, CUHK2/CRF/08,
and HKUST2/CRF/10), the Innovation and Technology
Commission (ITP/008/09NP and ITS/168/09), the University
Grants Committee of Hong Kong (AoE/P-03/08), and the
National Science Foundation of China (20974028). B. Z. T.
acknowledges support from the Cao Gaungbiao Foundation of
Zhejiang University. Z. J. Z. acknowledges financial support from
theInitialFundingofHangzhouNormalUniversity(HSQK0085).
11 (a) K.-C. Wu, P.-J. Ku, C.-S. Lin, H.-T. Shih, F.-I. Wu, M.-J. Huang,
J.-J. Lin, I.-C. Chen and C.-H. Cheng, Adv. Funct. Mater., 2008, 18,
67; (b) S. T. Tao, Z. K. Peng, X. H. Zhang, P. F. Wang, C.-S. Lee and
S.-T. Lee, Adv. Funct. Mater., 2005, 15, 1716; (c) C. Tang, F. Liu, Y.-
J. Xia, L.-H. Xie, A. Wei, S.-B. Li, Q.-L. Fan and W. Huang, J.
Mater. Chem., 2006, 16, 4074; (d) M. Y. Lo, C. Zhen, M. Lauters,
G. E. Jabbour and A. Sellinger, J. Am. Chem. Soc., 2007, 129, 5808;
(e) Z. Zhao, J.-H. Li, P. Lu and Y. Yang, Adv. Funct. Mater., 2007,
17, 2203.
12 (a) Y. Dong, J. W. Y. Lam, A. Qin, J. Liu, Z. Li, B. Z. Tang, J. Sun
and H. S. Kwok, Appl. Phys. Lett., 2007, 91, 011111; (b) Z. Zhao,
S. Chen, X. Shen, F. Mahtab, Y. Yu, P. Lu, J. W. Y. Lam,
H. S. Kwok and B. Z. Tang, Chem. Commun., 2010, 46, 686; (c)
Z. Zhao, S. Chen, J. W. Y. Lam, P. Lu, Y. Zhong, K. S. Wong,
H. S. Kwok and B. Z. Tang, Chem. Commun., 2010, 46, 2221; (d)
W. Yuan, P. Lu, S. Chen, J. W. Y. Lam, Z. Wang, Y. Liu,
H. S. Kwok, Y. Ma and B. Z. Tang, Adv. Mater., 2010, 22, 2159;
(e) Z. Zhao, J. W. Y. Lam and B. Z. Tang, Curr. Org. Chem., 2010,
14, 2109.
References
€
€
1 (a) M. D. Watson, F. Jackel, N. Severin, J. P. Rabe and K. Mullen, J.
Am. Chem. Soc., 2004, 126, 1402; (b) H. Zhang, Y. Wang, K. Shao,
Y. Lin, S. Chen, W. Qiu, X. Sun, T. Qi, Y. Ma, G. Yu, Z. Su and
D. Zhu, Chem. Commun., 2006, 755; (c) W.-L. Jia, T. McCormick,
Q.-D. Lin, H. Fukutani, M. Motala, R.-Y. Wang, Y. Tao and
S. Wang, J. Mater. Chem., 2004, 14, 3344; (d) J. E. Anthony, Chem.
Rev., 2006, 106, 5028.
ꢂ
13 M. Knaapila, R. C. Evans, V. M. Garamus, L. Almasy,
N. K. Szekely, A. Gutacker, U. Scherf and H. D. Burrows,
ꢂ
2 (a) R. Nandy, M. Subramoni, B. Varghese and S. Sankararaman, J.
Org.
Langmuir, 2010, 26, 15634.
Chem.,
2007,
72,
938;
(b)
S.
Sankararaman,
14 (a) J. B. Birks, A. A. Kazzaz and T. A. King, Proc. R. Soc. London,
Ser. A, 1966, 291, 556; (b) T. Seko, K. Ogura, Y. Kawakami,
H. Sugino, H. Toyotama and J. Tanaka, Chem. Phys. Lett., 1998,
291, 438; (c) R. Katoh, K. Suzuki, A. Furube, M. Kotani and
K. Tokumaru, J. Phys. Chem. C, 2009, 113, 2961.
G. Venkataramana and B. Varghese, J. Org. Chem., 2008, 73, 2404.
3 (a) A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz and
A. B. Holmes, Chem. Rev., 2009, 109, 897; (b) J. Liu, J. W. Y. Lam
and B. Z. Tang, Chem. Rev., 2009, 109, 5799.
4 (a) J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740; (b) M. Shimizu, H. Tatsumi, K. Mochida,
K. Shimono and T. Hiyama, Chem.–Asian J., 2009, 4, 1289; (c)
A. Iida and S. Yamaguchi, Chem. Commun., 2009, 3002; (d)
M. Shimizu, Y. Takeda, M. Higashi and T. Hiyama, Angew. Chem.,
Int. Ed., 2009, 48, 3653; (e) M. Levitus, K. Schmieder, H. Ricks,
K. D. Shimizu, U. H. F. Bunz and M. A. Garcia-Garibay, J. Am.
Chem. Soc., 2001, 123, 4259; (f) B.-K. An, S.-K. Kwon, S.-D. Jung
and S. Y. Park, J. Am. Chem. Soc., 2002, 124, 14410; (g) C. Belton,
D. F. O’Brien, W. J. Blau, A. J. Cadby, P. A. Lane,
15 (a) S. Zhang, Y. Guo, L. Wang, Q. Li, K. Zheng, X. Zhan, Y. Liu,
R. Liu and L.-J. Wan, J. Phys. Chem. C, 2009, 113, 16232; (b)
€
S. Ghosh, X.-Q. Li, V. Stepanenko and F. Wurthner, Chem.–Eur.
J., 2008, 14, 11343.
16 A. Mikami and T. Koyanagi, Dig. Tech. Pap. - Soc. Inf. Disp. Int.
Symp., 2009, 40(1), 907.
17 J. Kidoa and Y. Iizumi, Appl. Phys. Lett., 1998, 73, 2721.
18 (a) Q.-X. Tong, S.-L. Lai, M.-Y. Chan, Y.-C. Zhou, H.-L. Kwong,
C.-S. Lee and S.-T. Lee, Chem. Phys. Lett., 2008, 455, 79; (b) Q.-
X. Tong, S.-L. Lai, M.-Y. Chan, K.-H. Lai, J.-X. Tang, H.-
L. Kwong, C.-S. Lee and S.-T. Lee, Appl. Phys. Lett., 2007, 91,
153504.
€
D. D. C. Bradley, H. J. Byrne, R. Stockmann and H.-H. Horhold,
Appl. Phys. Lett., 2001, 78, 1059; (h) L. Antolini, E. Tedesco,
G. Barbarella, L. Favaretto, G. Sotgiu, M. Zambianchi,
D. Casarini, G. Gigli and R. Cingolani, J. Am. Chem. Soc., 2000,
122, 9006; (i) R. Deans, J. Kim, M. R. Machacek and
19 S.-Y. Ku, L.-C. Chi, W.-Y. Hung, S.-W. Yang, T.-C. Tsai, K.-
T. Wong, Y.-H. Chen and C.-I. Wu, J. Mater. Chem., 2009, 19, 773.
20 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
7216 | J. Mater. Chem., 2011, 21, 7210–7216
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