From A fluorescent chromophore in solution to an efficient emitter in the solid state

Article abstract of DOI:10.1002/asia.201200489

Polycyclic aromatic hydrocarbons (PAHs) normally exhibit efficient fluorescence in dilute solutions, whereas their emission is significantly quenched in the aggregated state due to the formation of π-aggregates/ excimers. The rigid and planar structure of

Full text of DOI:10.1002/asia.201200489

DOI: 10.1002/asia.201200489
From A Fluorescent Chromophore in Solution to An Efficient Emitter in the
Solid State
Yang Liu,[a, b] Yun Lv,[a] Xiying Zhang,[a] Shuming Chen,[c] Jacky W. Y. Lam,[b] Ping Lu,[b]
Ryan T. K. Kwok,[b] Hoi Sing Kwok,[c] Xutang Tao,[a] and Ben Zhong Tang*[b, d]
Abstract: Polycyclic aromatic hydrocar-
bons (PAHs) normally exhibit efficient
fluorescence in dilute solutions, where-
as their emission is significantly
quenched in the aggregated state due
to the formation of p-aggregates/exci-
mers. The rigid and planar structure of
PAHs plays a positive role in terms of
fluorescence in solution but a negative
one in the aggregated state. To bestow
PAHs a luminescent ability in the solid
state, we constructed a non-coplanar
PAH-substituted ethene. By using the
planar PAH fluoranthene as a building
block, highly efficient solid-state
a
emitter with a fluorescence quantum
efficiency of unity in the aggregated
state was obtained. OLEDs with con-
tain the molecule as an emitter reach
a luminance up to 20520 cdmÀ2 and an
efficiency of 10 cdAÀ1.
Keywords: aggregation-induced
emission · fluoranthene · polycyclic
aromatic hydrocarbons
Introduction
Fluoranthene is a polycyclic aromatic hydrocarbon (PAH)
consisting of a naphthalene and a benzene unit connected
by a five-membered ring (Scheme 1). The chemistry of fluo-
ranthene and studies of the photophysical properties
evolved rapidly after the elucidation of its structure;[1] how-
ever, the interest in fluoranthene faded fast, especially in ap-
plications as a solid-state fluorophore in optoelectronics.[2]
Like other planar PAHs, the utility of fluoranthene and its
derivatives as emitting materials has been greatly restricted
due to aggregation between the planar chromophores. This
kind of aggregation usually leads to the formation of p-ag-
gregates/excimers and causes low solid-state fluorescence
quantum yields.[2] On the other hand, it is well known that
Scheme 1. Chemical structures of fluoranthene, tetraphenylethene (TPE),
and 1,2-bis(8-fluoranthenyl)-1,2-diphenylethene (DFAE).
the rigid and planar chemical structures benefit the fluores-
cent ability of chromophores in solutions. Hence, if the ag-
gregation between molecules of PAH derivatives can be al-
leviated, their emissive nature will be regained in the solid
state. To meet the practical demand for highly efficient
solid-state emitters in the fields of organic optoelectronics
such as organic light-emitting diodes (OLEDs) and organic
laser diodes, much effort has been devoted to eliminate the
notorious effect of aggregation-caused quenching (ACQ) of
PAHs.[3] For fluoranthene in particular, a highly luminescent
solid-state light-emitting molecule, 7,8,10-triphenylfluoran-
thene, has been designed. The steric hindrance of the per-
phenylated PAH was believed to reduce facial contacts that
led to excimer quenching and bathochromic shifts. This de-
[a] Dr. Y. Liu, Y. Lv, X. Zhang, Prof. Dr. X. Tao
State Key Laboratory of Crystal Materials
Shandong University
Jinan 250100 (P. R. China)
[b] Dr. Y. Liu, Dr. J. W. Y. Lam, Dr. P. Lu, R. T. K. Kwok,
Prof. Dr. B. Z. Tang
Department 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
[c] S. Chen, Prof. Dr. H. S. Kwok
Center for Display Research, HKUST
Kowloon, Hong Kong (China)
rivative reached
a
maximum efficiency of 3.02 cdAÀ1
(1.83% external quantum yield) in OLEDs.[4] However, be-
cause every luminophore has an inherent tendency to aggre-
gate in the condensed phase, the resistance against the in-
trinsic aggregation nature is not good enough to make use
of it. From this viewpoint, the characteristics of aggregation-
induced emission (AIE) will serve the role. By introducing
AIE into fluoranthene, we hope to take advantage of the
rigid and planar structure of this compound and turn it into
[d] Prof. Dr. B. Z. Tang
Department of Polymer Science and Engineering
Key Laboratory of Macromolecular synthesis and Functionalization
of the Ministry of Education of China
Zhejiang University
Hangzhou 310027 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200489.
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an efficient solid-state emitter. In this report, a non-coplanar
fluoranthene-substituted ethene was designed and synthe-
sized (Scheme 1). This analogue of tetraphenylethene (TPE)
possesses the same AIE property as TPE and does not
suffer from the ACQ effect.
and purity of the product were verified by H and 13C NMR
spectroscopy and high-resolution mass spectrometry. The
purity of the material was also assessed by elemental analy-
sis with satisfactory results.
The molecular conformation of DFAE was analyzed by
density functional theory calculation at the DFT/B3LYP/6-
31G(d) level. The result shows a typical propeller-like non-
planar conformation. The dihedral angles between the two
fluoranthenes and the two phenyl rings are 78.28 and 75.48,
respectively. Such propeller-shaped geometry will hamper
a close intermolecular p–p stacking and facilitate the mole-
cules to adopt a loose packing in the condensed state. How-
ever, the non-coplanar structure does not disrupt the conju-
gation of the whole molecule. Figure 1 shows the molecular
Results and Discussion
In our previous works, we have succeeded in converting the
typical ACQ chromophores triphenylamine and pyrene into
efficient solid-state emitters with AIE characteristics by dec-
orating them with TPE as peripheries.[5] Herein, we report
another facile strategy to generate AIE-active luminogens
from planar PAH chromophores, that is, using them as
building blocks to construct directly the corresponding ana-
logue of TPE. Since the origin of the unusual AIE effect of
TPE has been identified as restriction of rotations of its
multiple benzyl blades in the aggregate,[6] we expected that
their analogues should possess similar AIE features as TPE.
This speculation has been proved in our previous experi-
mental results.[7] By using one or more aromatic substituents
to replace the benzyl ones, the resultant tetrasubstituted
ethenes luminesce indeed efficiently in the solid state. Based
on this idea, an ethene substituted by two fluoranthenes and
two phenyls, 1,2-bis(8-fluoranthenyl)-1,2-diphenylethene
(DFAE) was designed and synthesized. Scheme 2 shows the
Figure 1. Molecular orbital amplitude plots of HOMO and LUMO
energy levels of DFAE.
orbital amplitude plots of HOMO and LUMO energy
levels. It can be seen that all four aryl substituents contrib-
ute to the molecular orbitals. This is different to the dyes
decorated with TPE as peripheries, in which the orbitals are
mainly dominated by the core part. Moreover, the emission
wavelength for that kind of luminogens changed only slight-
ly from that of their core part.[5] In other words, using TPE
as peripheries of PAHs hardly changes their electronic
nature, while building the TPE analogues with PAHs direct-
ly enables uniform electron delocalization along the whole
molecule. The higher extent of conjugation and lower inter-
molecular p–p interactions make the molecule an efficient
solid-state emitter. Figure 2 shows the fluorescent micro-
Scheme 2. Synthetic route to DFAE and its optimized geometry.
synthetic route to DFAE. The procedure is quite facile and
efficient. The fluoranthene was first incorporated into 8-ben-
zoylfluoranthene by AlCl3-catalyzed Friedel–Crafts acyla-
tion. A subsequent Zn/TiCl4-mediated McMurry coupling
reaction produced the target compound in high yield. De-
tails of the synthetic procedures and characterization data
are presented in the Supporting Information. The identity
Abstract in Chinese:
Figure 2. Fluorescence microscopic images of microspheres of DFAE.
scopic images of the microspheres of DFAE. Perhaps also
due to its propeller-shaped structure, DFAE is morphologi-
cally quite stable. Hence, many efforts to grow crystals
failed; instead we always obtained microspheres. The micro-
spheres shown in the images were obtained from a solution
of DFAE in dichloromethane/ethanol after slow evaporation
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From A Fluorescent Chromophore in Solution to An Efficient Emitter in the Solid State
gen molecules begin to aggregate. From the solution in pure
THF to the aggregate suspension in 5% aqueous THF (fw =
95%), the fluorescence intensity of DFAE at 526 nm in-
creases approximately 25-fold. The change from the none-
missive to the emissive state of DFAE with increasing fw,
which demonstrates that DFAE is AIE-active, can clearly
be observed visually (Figure 3E).
The fluorescence quantum yields of DFAE in the solution
and in the solid state were quantitatively measured by a ref-
erence and absolute method, respectively. The obtained re-
sults further validated its AIE activity. The quantum yield of
DFAE in THF is as low as 0.68%, while that of its solid
thin film reaches 100%. Accordingly, DFAE behaves just
like its analogue TPE, displaying enhanced emission in the
solid state. Intramolecular rotations along the single bonds
between the central ethene and the peripheral fluoranthenyl
or phenyl rings are likely responsible for its AIE behavior.
That is, the aromatic rings undergo an active intramolecular
rotation process in solution at the excited states, which non-
radiatively annihilates their energy and renders the mole-
cules non-luminescent, while the rotation process is impeded
in the aggregated state, thus blocking non-radiative decay
channels.[9] Moreover, the propeller-shaped structure pre-
vents the luminogens from packing in a close p–p stacking
mode. The emission of DFAE exhibited a bathochromic
shift of about 70 nm as compared to that of fluoranthene,
thereby suggesting electron delocalization along the entire
molecule. Thus, the restriction of intramolecular rotations
and the lack of intermolecular interactions account for the
restraint of the solid-state emission quenching. Upon over-
coming the ACQ effect, the large conjugation and rigid
structure of fluoranthene facilitate its luminescent ability.
Next, the thermal stability of DFAE was investigated by
thermogravimetric (TGA) and differential scanning calorim-
etry (DSC) analyses. As shown in Figure 4, DFAE had a deg-
radation temperature (Td) of 3668C. The DSC measurement
on the second heating cycle showed a glass transition tem-
Figure 3. Photoluminescence spectra of A) fluoranthene and C) DFAE in
THF/water mixtures with different water fractions (fw). Plots of (I/I0)
values as a function of solvent composition of B) fluoranthene and
D) DFAE. E) Fluorescent images of DFAE in THF/water mixtures with
different water fractions.
of solvent. Under UV irradiation, the spheres emit a strong
greenish yellow light.
Figure 3 depicts the change in fluorescence of DFAE
from the solution state to the aggregated state in aqueous
suspension. For comparison, the fluorescence of fluoran-
thene was recorded under the same conditions. In pure THF
solution, fluoranthene displays a nice fluorescence with
a quantum yield (FF,s) of about 30%. Upon the addition of
the non-solvent water, its fluorescence intensity increased
slightly initially, which is attributed to the increase in solvent
polarity.[8] When the water fraction (fw) becomes high (>
80%), the dissolved fluoranthene starts to aggregate, which
is accompanied by a decrease in fluorescence. At a fw of
95%, the fluorescence intensity is reduced to about 60% of
its original value. Thus, fluoranthene is a typical ACQ fluo-
rophore. In sharp contrast, the propeller-shaped tetrasubsti-
tuted ethene DFAE is nearly non-emissive when dissolved
in a good solvent (THF, chloroform, dichloromethene, etc.).
As shown in Figure 3C, very weak photoluminescence (PL)
signals are recorded in a THF/water mixtures of fw <60%,
in which the DFAE molecules are genuinely dissolved. The
increase in polarity of the solvent does not promote the
fluorescence as in the case of fluoranthene. The emission in-
tensity starts to increase upon increasing the fw to values
larger than 60%; in such mixtures the solvating power of
the mixture is worsened to such an extent that the lumino-
Figure 4. TGA and DSC thermograms of DFAE recorded under N2 at-
mosphere at a heating rate of 20 (TGA) and 10 (DSC)8CminÀ1
.
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Ben Zhong Tang et al.
FULL PAPER
perature (Tg) at 1378C, which is quite a high value com-
pared to most of the electroluminescence materials. A good
thermal and morphological stability is essential to improve
the performance and lifetime of optoelectronic devices.
The high thermal stability and efficient solid-state emis-
sion of DFAE prompted us to study its electrolumi-
nescence (EL) properties. The device architecture
ness of 20520 cdmÀ2. At a practical brightness of 500 cdmÀ2
(with an applied voltage of 8 V and a current density of
5.3 mA/cm2), the device can still maintain a high efficiency
(9.4 cdAÀ1). For the fluorescence OLEDs, the EL perfor-
mance of DFAE is really a good result.[10]
Table 1. Electroluminescence performances of a device based on DFAE.
was designed according to the energy level of the
material, and the electrochemical property of
DFAE was studied by cyclic voltammetry. The
HOMO energy level determined from the onset of
the oxidation is 5.5 eV (relative to the vacuum
energy level). The LUMO energy level deduced
from the difference between the HOMO level and
the optical band gap is located at 2.8 eV. Thus, in
a device structure as shown in Figure 5A, the lumi-
V
on [V]
L [cdmÀ2
]
hC [cdAÀ1
]
hP [lmWÀ1
]
hext [%]
lem [nm]
Maximum Values
3.8
20520
500
10 (at 6.6 V) 5.7 (at 4.6) 3.1 (at 6.6 V) 548
Values at 500 cdmÀ2
8
9.4
3.7
2.9
548
nogen should work well under a balanced charge transport.
Figure 5B–E show the EL performances of the device
based on DFAE. The device emits yellow light with an EL
maximum (lEL) of 548 nm, which is quite close to that of the
PL in thin film. The device based on this AIE luminogen ex-
hibits good EL performances. The maximum current effi-
ciency (hC), power efficiency (hP), and external quantum ef-
ficiency (hext) attained are 10 cdAÀ1, 5.7 LmWÀ1, and 3.1%,
respectively (Table 1). The device reaches a maximal bright-
Conclusions
In summary, a new facile strategy of generating highly solid-
state emitters from planar PAH chromophores was devel-
oped in this work. By using the planar fluoranthene as
a building block for a tetrasubstituted ethene, the resulting
DFAE is endowed with an AIE effect. Efficient solid-state
fluorescence (FF =100%) was achieved. DFAE works well
as a solid emitter in OLEDs, with a maximum luminance of
up to 20520 cdmÀ2 and an efficiency of 10 cdAÀ1. Thus, fluo-
ranthene, a PAH with an intrinsic ACQ effect, was success-
fully incorporated into an AIE luminogen. We are hopeful
to create more efficient solid emitters with other PAHs by
using this strategy.
Acknowledgements
This project was partially supported by the Research Grants Council of
Hong Kong (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. thanks the
support from Cao Guangbiao Foundation of Zhejiang University. Y. Liu
thanks the support from the Independent Innovation Foundation of
Shandong University (Grant No. 2011TB020).
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Received: June 2, 2012
Published online: && &&, 0000
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Solid-State Fluorescence
Switching roles: As a polycyclic aro-
matic hydrocarbon, fluoranthene effi-
ciently emits in dilute solution but dis-
plays quenched fluorescence in the
aggregated state due to its rigid and
planar structure. By incorporating it
into a tetrasubstituted ethene, the
resulting compound, 1,2-bis(8-fluoran-
thenyl)-1,2-diphenylethene (DFAE), is
endowed with an aggregation-induced
emission effect. Efficient solid-state
fluorescence (FF =100%) was ach-
ieved.
Yang Liu, Yun Lv, Xiying Zhang,
Shuming Chen, Jacky W. Y. Lam,
Ping Lu, Ryan T. K. Kwok,
Hoi Sing Kwok, Xutang Tao,
Ben Zhong Tang*
&&&& — &&&&
From A Fluorescent Chromophore in
Solution to An Efficient Emitter in the
Solid State
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Chem. Asian J. 0000, 00, 0 – 0
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