Fluorescence enhancements of benzene-cored luminophors by restricted intramolecular rotations: AIE and AIEE effects

Article abstract of DOI:10.1039/b613522f

Photoluminescence of simple arylbenzenes with ready synthetic accessibility is enhanced by two orders of magnitude through aggregate formation; viscosity and temperature effects indicate that the emission enhancement is due to the restriction of their intramolecular rotations in the solid state. The Royal Society of Chemistry.

Full text of DOI:10.1039/b613522f

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Fluorescence enhancements of benzene-cored luminophors by restricted
intramolecular rotations: AIE and AIEE effects{
Qi Zeng,a Zhen Li,*a Yongqiang Dong,b Chong’an Di,c Anjun Qin,b Yuning Hong,b Li Ji,a Zhichao Zhu,a
Cathy K. W. Jim,b Gui Yu,c Qianqian Li,a Zhongan Li,a Yunqi Liu,c Jingui Qin*a and Ben Zhong Tang*b
Received (in Cambridge, UK) 18th September 2006, Accepted 6th November 2006
First published as an Advance Article on the web 21st November 2006
DOI: 10.1039/b613522f
Photoluminescence of simple arylbenzenes with ready synthetic
accessibility is enhanced by two orders of magnitude through
aggregate formation; viscosity and temperature effects indicate
that the emission enhancement is due to the restriction of their
intramolecular rotations in the solid state.
To find technological applications as biological probes, chemical
sensors and organic light-emitting diodes, a luminophor must meet
a prerequisite of exhibiting high fluorescence quantum yield (WF)
in the solid state. The formation of delocalized excitons or
excimers in the solid state, however, often quenches the emissions
of organic luminophors.1–3 The best approach to this notorious
problem is to develop new luminophoric materials whose
aggregates can emit more efficiently than their solutions.4 Along
this line of approach, two novel photoluminescence (PL) processes
have been identified: one is the ‘‘aggregation-induced emission
enhancement (AIEE)’’5 and another is the ‘‘aggregation-induced
emission (AIE)’’.6–8 In the former, light emission of a chromo-
phoric material is enhanced by aggregate formation, while in the
latter, a nonemissive material is induced to emit by aggregation.
Chart 1 Examples of AIE luminophors developed in our laboratories.
rotations (RIR) of the phenyl peripheries against the silacyclo-
pentadiene core in the aggregation state is the main cause for the
AIE effect.6–10 In an effort to verify this hypothesis and to develop
more AIE systems, we have designed a series of propeller-like
molecules consisting of mono- and polyene stators and multiple
aryl rotors (Chart 1, B–G). As expected, all the dyes are AIE-
active, emitting various colours including blue, green and red
efficiently in the aggregation state.9 Among the AIE dyes,
tetraphenylethene (TPE; Chart 1, F) and its derivatives are
particularly noteworthy: they can be synthesized by simple
reactions, emit highly efficiently in the solid state, and work as
excellent biological probes for protein, DNA and RNA detection
and quantitation.9e
An example of AIEE material is
a cyclophane-decorated
poly(p-phenyleneethynylene): it is emissive in the solution but
becomes more emissive when aggregated, with a 3.5-fold increase
in its WF.5a Hexaphenylsilole (Chart 1, structure A), on the other
hand, is an AIE dye. Its solution is nonemissive but its aggregates
are highly emissive: WF of the latter (23%) is two orders of
magnitude higher than that of the former (0.2%).6b
The TPE system is simple, but can we develop even simpler AIE
systems? For example, will the molecules with structures sketched
in Chart 2 be AIE-active? These molecules are mono- (I), di- (II),
and triarylated (III) benzenes with their aryl substituents directly
attached to the benzene core without olefinic double-bond linkers,
which are readily accessible by simple reactions. In this report, we
show that 1–3 are AIE-active while 4–6 are AIEE-active. We prove
that the RIR process is at work in both AIE and AIEE systems.
We first examined the PL behaviours of biphenyl, the simplest
model of I, in solution and aggregation states. As can be seen from
Fig. S1 (ESI{), its PL intensity is greatly enhanced by adding water
into its DMF solution to induce aggregate formation. Encouraged
The novel AIE(E) effects challenge our current understanding of
PL processes, deciphering whose causes and mechanisms may help
spawn new photophysical theories and technological innovations.
We have been interested in studying structure-property relation-
ship of AIE dyes. During our study on the silole-based AIE
system, we have proposed that the restriction of intramolecular
aHubei Key Laboratory on Organic and Polymeric Optoelectronic
Materials, Department of Chemistry, Wuhan University, Wuhan,
430072, China. E-mail: lizhen@whu.edu.cn; jgqin@whu.edu.cn;
Fax: +86-27-6875-6757; Tel: +86-27-6225-4108
bDepartment of Chemistry, The Hong Kong University of Science &
Technology, Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk; Fax: +852-2358-1594; Tel: +852-2358-7375
cOrganic Solids Laboratories, Institute of Chemistry, The Chinese
Academy of Sciences, Beijing, 100080, China
{ Electronic supplementary information (ESI) available: Preparation and
characterization details for 1–6; change of PL intensity of biphenyl with
water content in water–DMF mixtures; PL spectra of solutions, aggregates
and films of 1–6; temperature effect on PL intensity in THF solutions of 1–
6; dependence of fluorescence quantum yield of 4–6 on water content in
water–acetone mixtures. See DOI: 10.1039/b613522f
Chart 2 Chromophoric molecules comprising a benzene core (black
oval) and one (I), two (II) and three (III) aryl substituents (grey oval).
70 | Chem. Commun., 2007, 70–72
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Table 1 Optical properties of mono-, di- and triarylated benzenes 1–6
b
lem
Emission enhancement
Solnc
Aggtd
Filme
IA/IS
WF,A/WF,S
a
f
g
Dye
lab
1
2
3
4
5
6
a
ndh
ndh
ndh
323
323
323
343
368
365
396
380
394
343i
370i
368i
396j
381j
394j
335
358
359
400
382
401
163i
199i
155i
ndh
ndh
ndh
1.15j
1.43j
1.47j
1.43 (67/47)j
1.37 (41/30)j
1.88 (30/16)j
Absorption maximum (nm). b Emission maximum (nm). c Solution (in
d
acetone). Aggregate. With dye molecules dispersed in a PMMA
e
matrix in y30 wt%. f Ratio of PL peak intensities of aggregate (IA) and
solution (IS). g Ratio of fluorescence quantum yields of aggregate (WF,A
and solution (WF,S), given in the parentheses are the original WF values.
)
h
Not determined. i Aggregate in a water–acetone mixture with 90 vol%
water. j Aggregate in a water–acetone mixture with 30 vol% water.
Chart 3 Mono- (1 and 4), di- (2 and 5), and triarylated (3 and 6)
benzene derivatives studied in this work.
90%, the PL peak intensities were increased by up to y200 times
(Table 1). The similar results were obtained when water was added
into the dye solutions in other organic solvents.
by this result, we designed two groups of arylbenzenes with the
aryl groups being phenyl (1–3) and carbazolyl (4–6; Chart 3).
These molecules were readily prepared by the synthetic routes
shown in Schemes S1 and S2 (ESI{). Their structures were
characterized by spectroscopy methods, from which satisfactory
analysis data were obtained (ESI{). The dyes are soluble in
common organic solvents such as acetone, chloroform and THF
but insoluble in water.
The actual PL intensities of the dye aggregates should be higher
than those shown in Fig. 1, because only have the dye molecules
on the surfaces of the nanoparticles contributed to the PL
spectra.11 We blended the dyes with poly(methyl methacrylate)
(PMMA) and found that the PL peak intensities of the resultant
films were much higher than those of the nanoaggregate
suspensions (Fig. S2, ESI{). The dye molecules may have better
miscibility with PMMA chains, leading to better dispersion of the
dye molecules in the polymer matrix. This should result in the
formation of smaller nanoparticles, hence the observed stronger
emissions. Intriguingly, the lem values of 1–3 in the PMMA films
are generally blue-shifted from those of their aggregates (Table 1),
possibly due to the dye crystallization in the solid films.6e,12
It is known that increasing solvent viscosity and/or decreasing
solution temperature can slow down intramolecular rotations13 or
strengthen the RIR process. We thus checked how variations in
the viscosity and temperature would affect the dye emissions. The
PL intensities of 1–3 are increased by adding glycerol, a viscous
liquid, into their ethanol solutions (Fig. 2(A)). In the mixtures with
glycerol contents of ,50%, the intensity increases are due to a pure
viscosity effect because the dye molecules are soluble in these
mixtures. In the mixtures with glycerol contents of .50%, the
increases are due to the combined effects of viscosity and
Dilute acetone solutions of 1–3 are practically nonluminescent.
However, when large amounts of water were added to their
acetone solutions (with final concentrations kept unchanged at
20 mM), the resultant mixtures gave intense PL spectra (Fig. 1(A)).
Although the mixtures were macroscopically homogeneous,
particle size analysis revealed that nanoscopic aggregates of the
dyes were formed in the mixtures with high water contents.
Evidently, the emissions of the dyes are induced by aggregation
formation, or in other words, 1–3 are AIE-active.
The WF values of 1–3 could not be accurately determined due to
the involved technical difficulty. We thus plotted the changes in
their PL peak intensities vs. water contents in the aqueous mixtures
(Fig. 1(B)). The PL peak intensity remained almost unchanged
when less than y60% water was added to the acetone solutions
but started to swiftly increase afterwards because the solvating
power of the mixture had been worsened to such an extent that the
dyes began to aggregate. When the water fraction was increased to
Fig. 2 (A) Changes in the PL peak intensities (I) of the solutions of 1–3
(20 mM) with the glycerol contents in the ethanol/glycerol mixtures. I0 =
intensity in pure ethanol solution. (B) Temperature effect on the PL peak
intensity of 3. I0 = intensity at room temperature (25 uC).
Fig. 1 (A) PL spectra of the dilute solutions of 3 (20 mM) in water/
acetone mixtures with different water contents. Excitation wavelength:
314 nm. (B) Changes in the PL peak intensities of 1–3 with different water
fractions in the water/acetone mixtures.
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aggregation because the dye molecules become progressively less
soluble in the mixtures with higher glycerol contents. As can be
seen from Fig. 2(B) and Fig. S3(A) (ESI{), decreasing the solution
temperatures leads to enhanced emissions. The viscosity and
temperature effects prove that the RIR process is indeed involved
in the AIE system.
Different from 1–3, dyes 4–6 are emissive in the solution state.
Their aggregates, however, are more emissive (Fig. S4, ESI{). The
dyes are thus AIEE emitters. Their WF values continuously
increase with increasing the water contents in the water/acetone
mixtures and reach maximums at a water content of y30%
(Fig. S5, ESI{). The WF values decrease with further increasing the
water contents because of the poor solubilities of the dyes in the
mixtures with higher water contents, as evidenced by the turbidity
of the mixtures. Again, when the dyes were blended with PMMA,
the resultant films emitted lights in very high intensities (Fig. S4,
ESI{).
The PL intensities of THF solutions of 4–6 were enhanced by
decreasing the solution temperatures (Fig. S3(B), ESI{), suggesting
that the RIR process is also involved in the AIEE systems. It is
interesting to note that their WF values increase in the order of 4 A
5 A 6 (Table 1), while the numbers of their carbazolyl substituents
decrease in the order of 6 A 5 A 4. The WF value of 6 is the
lowest, although it possesses the highest number of carbazolyl
groups. This is easily understandable: the more the carbazolyl
rotors, the higher the probability for intramolecular rotations,
hence the lower the WF value.
Given that a carbozole solution is emissive,13–15 it is no wonder
carbazolylbenzenes 4–6 are emissive in the dilute solutions. In the
PMMA films, the carbazolyl blades may experience p–p stacking
interaction, which red-shifts the dye emissions (Table 1). The p–p
stacking should decrease the PL intensities of the dyes, but the
RIR process in the solid state enhances light emission. The net
outcome of these two antagonistic processes is the observed AIEE
effect, indicating that the RIR process has played a predominant
role in affecting the PL behaviours of the dye aggregates.
In summary, in this work, we have successfully developed a new
series of AIE- (1–3) and AIEE-active (4–6) luminophors with very
simple structures. All these dyes are ‘‘pure’’ aromatic compounds
without olefinic functionality and thus should be very stable. With
the tremendous synthetic flexibility offered by the rich reactions in
aromatic chemistry, it is anticipated that many more AIE(E) dyes
can be created with minimum efforts. Our results have proved that
the RIR process plays a critical role in not only AIE but also
AIEE systems. Further studies on the AIE(E) dyes with visible
emissions are under way in our laboratories.
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(21%)6c.
We acknowledge the financial support from the National
Natural Science Foundation of China (Project Nos. 20402011,
20674059 and 90201002), the National Basic Research ‘‘973’’
Program, the Research Grants Council of Hong Kong (Project
Nos. 602706, 603505 and HKU2/05C), and the Hubei Provincial
Government.
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