From tetraphenylethene to tetranaphthylethene: Structural evolution in AIE luminogen continues

Article abstract of DOI:10.1039/c3cc00010a

Replacement of phenyl ring(s) in tetraphenylethene by naphthalene ring(s) generates a series of new luminogens with aggregation-induced emission (AIE) characteristics, demonstrating that bulky naphthalene rings can serve as a rotor to construct AIE luminogens.

Full text of DOI:10.1039/c3cc00010a

Chemical Communications
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Volume 49 | Number 25 | 28 March 2013 | Pages 2465–2580
ISSN 1359-7345
COMMUNICATION
Zujin Zhao, Ben Zhong Tang et al.
From tetraphenylethene to tetranaphthylethene: structural evolution in AIE luminogen continues

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From tetraphenylethene to tetranaphthylethene:
structural evolution in AIE luminogen continues†
Jian Zhou,a Zhengfeng Chang,a Yibin Jiang,b Bairong He,a Man Du,a Ping Lu,c
Yuning Hong,d Hoi Sing Kwok,b Anjun Qin,e Huayu Qiu,a Zujin Zhao*a and
Ben Zhong Tang*d
Cite this: Chem. Commun., 2013,
49, 2491
Received 1st January 2013,
Accepted 28th January 2013
DOI: 10.1039/c3cc00010a
www.rsc.org/chemcomm
Replacement of phenyl ring(s) in tetraphenylethene by naphthalene the connection patterns between rotors and stators. When the
ring(s) generates a series of new luminogens with aggregation-induced aromatic rotors become coplanar with the stators, both segments
emission (AIE) characteristics, demonstrating that bulky naphthalene are conjugated effectively. Such planar p-conjugated luminogens
rings can serve as a rotor to construct AIE luminogens.
show moderate emission efficiency due to the good conjugation
and synergistic effect between the rotors and stators, even if the
Aggregation-induced emission (AIE),1 a novel phenomenon opposite motion of the aromatic rotors indeed quenches the light emission
to commonly observed aggregation-caused quenching (ACQ), is to a certain extent.8 This may be one of the reasons why the AIE
attracting intense research interest due to its significant academic effect is not observed for most p-conjugated planar chromophores.
value and diverse promising applications in fluorescent sensors,2 To gain a deeper insight into the creation of AIE luminogens, we
bioprobes,3 optoelectronic devices,4 mechanochromic smart cast our attention on tetraphenylethene (TPE),4a,6d,9 a splendid AIE
materials,5 etc. Great endeavors have been devoted to funda- luminogen with a twisted, propeller-like conformation. We won-
mental understanding of the mechanism behind the AIE dered whether AIE luminogens are accessible when bulky aromatic
phenomenon, both to solve ACQ and to develop efficient rotors take the place of phenyl rings. Herein, a series of tailored
solid-state light emitters. Numerous evidences from theoretical naphthalene-substituted ethenes (Chart 1) were prepared and their
calculations and experimental results have rationalized that photoluminescence (PL) and electroluminescence (EL) properties
restriction of intramolecular rotation (IMR) is the main cause compared with those of the TPE parent.
for the AIE effect.6,7 The rotation of aromatic rotors, that
The naphthalene-substituted ethenes were synthesized
annihilate the excitons in a nonradiative fashion in the solution via homo- or cross-couplings of diphenyl ketone, 2-naphthyl
state, is restricted in the aggregated state and thus the radiative phenyl ketone and 2,20-dinaphthyl ketone in the presence of
decay of the excited state becomes dominant, rendering the TiCl4 and zinc dust (Scheme S1†). The detailed procedures and
luminogens emissive. Therefore, aromatic rotors are important characterization data are given in ESI†. All the luminogens are
in the design of AIE luminogens. The mostly used rotors are soluble in common organic solvents such as dichloromethane
phenyl rings, while other bulky planar aromatic rings are rarely and tetrahydrofuran (THF) but are insoluble in water. Theoret-
adopted as rotors in AIE luminogens. The difficulty may lie in ical calculations disclose that they possess a highly twisted
a College of Material, Chemistry and Chemical Engineering, Hangzhou Normal
University, Hangzhou 310036, China. E-mail: zujinzhao@gmail.com
b Center for Display Research, The Hong Kong University of Science & Technology,
Kowloon, Hong Kong, China
c State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
Changchun 130012, China
d Department of Chemistry, Institute of Molecular Functional Materials,
The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon,
Hong Kong, China. E-mail: tangbenz@ust.hk
e Department of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, China
† Electronic supplementary information (ESI) available: Experimental, optimized
molecular structures and calculated HOMO and LUMO energy levels, TGA and
DSC curves, absorption spectra, VT 1H NMR spectra of TNE, PL spectra and
Chart 1 Molecular structures of naphthalene-substituted ethenes.
Chem. Commun., 2013, 49, 2491--2493 2491
characteristic curves of EL device, see DOI: 10.1039/c3cc00010a
c
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Table 1 Optical and thermal properties of naphthalene-substituted ethenesa
l
em/nm
FF (%)
l
Soln
abs/nm
Tg/Td
(1C)
Solna Filmb Solnc Filmd aAIE
e
TPE
NTPE
DNDPE 333
TNPE
TNE
299
324
—
392
393
393
394
475
487
491
493
494
0.24
0.81
1.09
1.64
1.66
49.2
45.0
30.1
37.4
22.4
205
56
28
23
13
—/224
53/236
56/270
95/314
—/348
341
349
a
In THF solution (10 mM). b Film drop-casted on quartz plate. c Determined
in THF solution using 9,10-diphenylanthracene (FF = 90% in cyclo-
d
Fig. 1 (A) PL spectra of TNE in THF–water mixtures with different water fraction
(fw). (B) Plots of (I/I0) ꢀ 1 vs. fw, where I0 is the PL intensity in pure THF solution.
Inset: photos of TNE in THF–water mixtures (fw = 0 and 90%) under UV lamp
illumination.
hexane) as standard. Determined in amorphous film by integrating
e
sphere. aAIE = FF,A/FF,S
.
conformation (Fig. S1†), which leads to little orbital overlapping
and poor electronic communication between the aromatic rings.
The thermal stability of the luminogens is increased on replacement
of phenyl by naphthyl, as indicated by enhanced decomposition
temperatures (Td) from TPE (224 1C) to TNE (348 1C) (Fig. S2†); TNPE
shows a highest glass-transition temperature (Tg) of 95 1C (Table 1).
The absorption spectra of the luminogens in THF solutions are
red-shifted with increasing naphthalene ring substitution (Fig. S3†),
which is in good agreement with the calculation results of progres-
sively narrowed energy bandgaps (Fig. S4†). Replacement of a phenyl
with a naphthalene ring results in a bathochromic shift of B9 nm in
the absorption spectra of naphthalene-substituted ethenes. TNE
shows its absorption maximum at 349 nm, bathochromically shifted
by 50 nm in comparison with that of TPE. Whereas TPE emits
almost no light in the solution state, weak fluorescence peaks at
392–394 nm are recorded from solutions of naphthalene-substituted
ethenes. The fluorescence quantum yields in solutions (FF,S) of these
luminogens are in the range of 0.81–1.66%, which are enhanced
slightly with increase of naphthalene rings. The FF,S values are
higher than that of TPE (0.24%) but much lower than that of
naphthalene itself (23%),10 which is consistent with our previous
findings.11 The IMR process of naphthalene ring(s) against the olefin
stator that is active in the solution state can consume the excited-
state energy of the luminogens, rendering the luminogens weakly
fluorescent. Meanwhile, the IMR process also undermines the
electronic communication between the naphthalene and/or phenyl
rings and thus results in an ineffective conjugation. Therefore, the
short-wavelength emission should be due to the naphthalene
moieties of limited extension rather than the whole molecule. On
the other hand, since naphthalene is more bulky than phenyl, the
stiffness of the luminogen is reinforced slightly with increase of
naphthalene substitution. In addition, the naphthalene ring is
more fluorescent than the phenyl ring.10 Both effects work
collectively making the naphthalene-substituted ethenes fluoresce
more efficiently than TPE in solution.
To check whether the naphthalene-substituted ethenes are
AIE active water was added into their THF solutions and the PL
spectra in THF–water mixtures recorded. Fig. 1 illustrates the PL
spectra of TNE as an example. When the water fraction (fw, vol%) in
the THF–water mixture is low (fw r 60%), the PL emission remains
very weak and varies only slightly. When a large amount of water is
added (fw Z 70%), a long-wavelength emission with a peak at
492 nm appears with greatly enhanced intensity. Since TNE is
insoluble in water, increase of water content causes TNE molecules
to aggregate. The aggregate formation deactivates the IMR process
and thus populates the radiative decay of the excitons, making
the luminogen fluoresce strongly. Meanwhile, the conjugation is
improved by restriction of the IMR process, and thereby the long-
wavelength emission originating from the whole molecule becomes
dominant. Similar emission behaviors are observed for NTPE,
DNDPE and TNPE in THF–water mixtures (Fig. S6†), verifying that
these luminogens are AIE-active.
The naphthalene-substituted ethenes are also emissive in
the solid state. Their films show intense PL emissions with
peaks in the range of 487–494 nm (Fig. 2A). The fluorescence
quantum yields in the aggregate state (FF,A) of TPNE, DNDPE,
TNPE and TNE are 45.0, 30.1, 37.4 and 22.4%, respectively,
which are much higher than those in solutions, but are lower
than that of TPE (49.2%). The AIE effect is weakened as phenyl
is replaced by naphthalene, as evidenced by the decreased aAIE
values from 205 of TPE to 13 of TNE (Table 1). The planar
naphthalene ring is about twice the size of phenyl and this makes
it much easier to form p-stacking between naphthalene rings.
Variable-temperature (VT) 1H NMR study reveals that the
sharp NMR resonance peaks of TNE become broad and struc-
tureless when the temperature is lowered (Fig. S5†), due to the
restriction of the IMR process that causes fast conformational
exchanges, and hence sharp NMR resonance peaks. The changes
1
in H NMR spectral profiles of TNE are more significant than
those of TPE at low temperatures,12 demonstrating that the
motions of the bulky naphthalene rotors are constrained more
readily than for phenyl.
Fig. 2 (A) PL and (B) EL spectra of thin neat films of naphthalene-substituted
ethenes.
c
2492 Chem. Commun., 2013, 49, 2491--2493
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Table 2 EL properties of naphthalene-substituted ethenesa
(973 Program, 2013CB834702), the Program for Changjiang
Scholars and Innovative Research Teams in Chinese Universities
(IRT 1231) and the Project of Zhejiang Key Scientific and Tech-
nological Innovation Team (2010R50017). B. Z. Tang thanks the
support of the Guangdong Innovative Research Team Program of
China (20110C0105067115).
Lmax
/
ZP,max
/
ZC,max
/
Zext,max
(%)
EL/nm
Von/V
cd mꢀ2
lm Wꢀ1
cd Aꢀ1
TPEb
445
464
488
492
520
2.9
5.6
5.6
4.2
5.2
1800
4130
5900
12300
8840
0.35
0.5
0.7
2.0
1.6
0.45
1.0
1.4
3.0
2.9
0.4
0.6
0.7
1.2
1.0
NTPE
DNDPE
TNPE
TNE
Notes and references
a
Abbreviations: Von = turn-on voltage at 1 cd mꢀ2, Lmax = maximum
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luminance, ZP,max, ZC,max and Zext,max = maximum power, current and
external quantum efficiencies, respectively. Ref. 9b.
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The naphthalene rings of adjacent TNE molecules can partially
overlap each other in the solid state, leading to strong inter-
molecular interactions and decreased emission efficiency.
To evaluate the EL property of naphthalene-substituted ethenes,
multilayer OLEDs with a configuration of ITO|NPB (60 nm)|emitter
(20 nm)|TPBI (40 nm)|LiF (1 nm)|Al (100 nm) were fabricated,
where the new luminogens functioned as emitters, and N,N-bis(1-
naphthyl)-N,N-diphenylbenzidine (NPB) and 2,20,200-(1,3,5-benzine-
triyl)tris(1-phenyl-1-H-benzimidazole) (TPBI) served as hole- and
electron-transporting layers, repectively. The device performances
are summarized in Table 2. The EL emissions of the luminogens
move to longer wavelength with increase of naphthalene rings
(Fig. 2B) and the emission color become much redder than that of
TPE (445 nm). NTPE shows EL emission at 464 nm, which is blue-
shifted by 23 nm than that of its amorphous film, probably due to
the crystalline nature of the vacuum-deposited film in the device.
The EL emissions of DNDPE (488 nm) and TNPE (492 nm) are
close to the PL emissions of their films, while that of TNE is
bathochromically shifted to 520 nm due to the microcavity effect.
The device performances of naphthalene-substituted ethenes are
superior to that of TPE (Fig. S7†). The device of TNPE shows the
best EL performances. It is turned on at 4.2 V and radiates
strongly with a maximum luminance of 12 300 cd cmꢀ2 at 15 V.
The maximum current and external quantum efficiencies attained
by the device are 3.0 cd Aꢀ1 and 1.2%, respectively, which are
improved remarkably in comparison with those obtained from
the device of TPE (1800 cd mꢀ2, 0.45 cd Aꢀ1 and 0.4%),9b probably
due to the better carrier transport relative to TPE.
In summary, a series of AIE-active naphthalene-substituted
ethenes are synthesized and characterized. They are weakly
fluorescent in the solution state because the rotation of naphtha-
lene rotors against the olefin stator quenches the light emission.
However, they are induced to fluoresce efficiently in the aggregate
state, where the nonradiative decay pathway is blocked. Although
the AIE effect is weakened with the accumulation of naphthalene
rotors, the luminogens are compensated with improved thermal
and EL properties. The results obtained in this work demonstrate
that bulky aromatic rings can be used as rotors in the construction
of AIE luminogens with tailored propeller-like architecture.
We acknowledge the financial support from the National
Natural Science Foundation of China (51273053, 21104012
and 21074028), the National Basic Research Program of China
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 2491--2493 2493