Aggregation-induced emission enhancement of aryl-substituted pyrrole derivatives

Article abstract of DOI:10.1021/jp108254g

The relationship between the structures and light emission properties of five aryl-substituted pyrrole derivatives was studied during aggregation in THF-water mixtures. Only pentaphenylpyrrole clearly shows, however, an aggregation-induced emission enhanc

Full text of DOI:10.1021/jp108254g

J. Phys. Chem. B 2010, 114, 16731–16736
16731
Aggregation-Induced Emission Enhancement of Aryl-Substituted Pyrrole Derivatives
Xiao Feng,†,‡ Bin Tong,*,† Jinbo Shen,† Jianbing Shi,† Tianyu Han,† Long Chen,† Junge Zhi,§
Ping Lu,| Yuguang Ma,| and Yuping Dong*,†
College of Materials Science and Engineering, Beijing Institute of Technology, 5 South Zhongguancun Street,
Beijing, 100081, China, Department of Materials Molecular Science, Institute for Molecular 45 Science,
National Institutes of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan, College of
Science, Beijing Institute of Technology, Beijing 100081, China, and Key Laboratory for Supramolecular
Structure and Materials of Ministry of Education, Jilin UniVersity, Changchun, 130012, China
ReceiVed: August 30, 2010; ReVised Manuscript ReceiVed: NoVember 7, 2010
The relationship between the structures and light emission properties of five aryl-substituted pyrrole derivatives
was studied during aggregation in THF-water mixtures. Only pentaphenylpyrrole clearly shows, however,
an aggregation-induced emission enhancement (AIEE) phenomenon. On comparison of the optical properties
and single-crystal structures of these pyrrole derivatives, it is suggested that the more twisted configuration
which prevented parallel orientation of conjugated chromophores combined with the restricted intramolecular
rotation (RIR) effect was the main cause of the AIEE phenomenon.
1. Introduction
Such compounds were regarded as competitive candidates
for practical use as highly emissive materials, and they seem
Development of luminescent materials is a hot topic of current
interest because of their applications to the development of
fluorescence sensing and light-emitting diode fabrication.1
Although many luminophores are highly luminescent in dilute
solutions, their light emissions are often quenched in the solid
state due to aggregation of their chromophoric units in the
condensed phase.2 This aggregation-caused emission quenching
(ACQ) is a notorious problem that has seriously obstructed
advancement in the development of efficient organic light-
emitting diodes (OLEDs), chemosensors, biosensors, etc. It is
thus highly desirable to develop “unconventional” luminophoric
systems that can overcome this obstacle.3-5
especially ideal for the nondoped red14 or blue15 organic light-
emitting diodes (OLED). Moreover, some functional groups
introduced into AIE molecules will favor new development of
chemo- or biosensors for detecting metal cations, biomolecules,
organic vapors, chiral molecules, and explosives.16
The novel AIE(E) effects challenge our current understanding
of photoluminescence (PL) processes; investigating the causes
and mechanisms may help develop new photophysical theories
and technological innovations. The ACQ process may generally
be attributed to a nonradiative deactivation process, such as
excitonic coupling, excimer formation, and excitation energy
migration to the impurity traps.17 What then is the exact cause
for the “abnormal” AIE phenomenon? Several models, such as
restricted intramolecular rotations (RIR),18 excimer formation,19
fluorescent organic nanoparticles (FON) formation,20 and con-
figuration changed,21 have previously been proposed for en-
hanced emission in the solid state, compared with lower PL
quantum yield or PL quenching in solution. However, the effect
of molecular structure and packing arrangement on the AIE(E)
processes has rarely been investigated, although the structure-
property relationship is of great value in terms of gaining new
insights into AIE(E) mechanisms and guiding further research
efforts in the development of new AIE(E) luminogens.
Development of luminogens whose films emit more efficiently
than their solutions has aroused much interest in recent years.
Tang’s group in 2001 first reported that the luminescence of
silole molecules is stronger in the aggregate state than that in
the solution state.6-9 A variety of luminogens, including
distyrylbenzene, fluorene, pentacene, and pyrene derivatives,10-13
were successively proved to have the same properties. These
molecules can be generally categorized into two groups. In the
first group, the luminogenic molecules are nonemissive when
dissolved in good solvents but become highly luminescent when
aggregated in the solid state, thus behaving exactly opposite to
the conventional ACQ luminophores. Since the emission is
induced by aggregation, Tang coined “aggregation-induced
emission” (AIE) for this unusual phenomenon.6 In the second
group, the luminogens are luminescent in solution and become
more emissive in the aggregate state. Because the light emission
is enhanced by aggregate formation, the effect is referred to as
“aggregation-induced emission enhancement” (AIEE).7
Herein, a series of aryl-substituted pyrrole derivatives, 1,2,5-
triphenylpyrrole (TriPP), 1,2,3,5-tetraphenylpyrrole (TetraPP),
1,2,3,4,5-pentaphenylpyrrole (PentaPP), 2,5-di(anthracen-9-yl)-
1-phenylpyrrole (DiAnPP), and 2,5-di(phenanthren-9-yl)-1-
phenylpyrrole (DiPhenPP), was designed and easily synthesized.
In these pyrrole derivatives, the structure of PentaPP is very
similar to that of silole. We show that the molecular structure,
conformational twisting, structural rigidification, and morpho-
logical packing play important roles in the photophysical
processes of aryl-substituted pyrrole derivatives, which provide
us a convenient way to have a better understanding on the
influence of the aryl-substituted numbers and structures to the
AIE(E) phenomenon.
* To whom correspondence should be addressed: Phone: +86-10-6894-
8982. Fax: +86-10-6894-8982. E-mail: tongbin@bit.edu.cn (B.T.) and
chdongyp@bit.edu.cn (Y.P.D.).
† College of Materials Science and Engineering, Beijing Institute of
Technology.
‡ National Institutes of Natural Sciences.
§ College of Science, Beijing Institute of Technology.
| Jilin University.
10.1021/jp108254g  2010 American Chemical Society
Published on Web 11/24/2010

16732 J. Phys. Chem. B, Vol. 114, No. 50, 2010
Feng et al.
SCHEME 1: Synthetic Routes of Aryl-Substituted Pyrrole Derivatives
(a) Aniline, 100 °C; (b) NBS, DMF, (1) 0 °C and (2) room temperature; (c) Phenylboronic acid, Pd(PPh3)4, toluene/K2CO3 aqueous solution,
110 °C.
2. Results and Discussion
Synthesis. Scheme 1 shows the synthesis routes and molec-
ular structures of the aryl-substituted pyrrole derivatives, TriPP,
TetraPP, PentaPP, DiPhenPP, and DiAnPP. Aryl-substituted
pyrrole derivatives can be efficiently synthesized in less than
four steps from the starting material aryl acetylene. The oxidative
coupling reaction gives 1,4-bisaryl butadiyne in good yield.22
TriPP, DiPhenPP, and DiAnPP were synthesized by reaction
of 1,4-bisaryl butadiyne with aniline employing copper chlo-
ride.23 Then TetraPP and PentaPP were readily obtained by
Suzuki coupling reaction of phenylboronic acid with aryl
bromide pyrroles which were synthesized by N-bromosuccin-
imide (NBS) bromination of TriPP.24 The reaction products were
characterized spectroscopically and crystallographically, from
which satisfactory analysis data corresponding its expected
structure were obtained.
Figure 1. UV spectra of PentaPP in THF-water mixtures with
different volume fractions of water. PentaPP concentration: 5 × 10-5
mol/L.
Aggregation-Induced Emission Enhancement. The photo-
physical properties of PentaPP during aggregation were first
studied due to its similarity structure of silole. The UV-vis
absorption spectra and PL spectra of PentaPP were measured
in THF/water mixtures with different volume fractions of water,
and the final concentrations were kept constant at 1 × 10-5
mol/L according to the previous methods.25
respectively. Thus, the absorption spectra of PentaPP in the
aqueous THF mixtures with 70% and 80% water contents are
revealed in the light-scattering tails in the long wavelength
region. However, the size of PentaPP aggregates (20-30 nm
diameter) obtained in THF-water mixtures with 90% volume
fractions of water is much smaller than that in 70% and 80%
water content. It caused the decreasing of light-scattering tails
in the long wavelength region observed in the absorption
spectrum for 90% water content solvent (Figure 1).
Upon photoexcitation, the dilute THF solution of PentaPP
shows a PL spectrum with an emission peak at 386 nm (Figure
2). When water is continually added into the THF solution of
PentaPP while keeping the luminogen concentration unchanged
at 1 × 10-5 mol/L, the PL intensity of PentaPP is slowly
increased when the water content in the aqueous THF mixtures
is “low” (e60%) but greatly increased when the water content
is “high” (>60%) (Figure 2). Since water is a poor solvent for
PentaPP, the molecules of PentaPP must have aggregated in
the aqueous THF mixtures with high water contents, in
agreement with observation of the light-scattering tails in the
absorption spectra discussed above (cf., Figure 1). Evidently,
the emission of PentaPP is spectacularly boosted by aggregation;
in other words, PentaPP is AIEE active. Compared with the
As can be seen from Figure 1, when the water content in the
aqueous THF mixture is e60 vol %, the absorbance of PentaPP
is almost decreased. At 60% water content, the absorption peak
of PentaPP is bathochromically shifted with a big decrease in
intensity. At high water contents (g90%), the absorption peaks
of PentaPP are shifted back to the position of its THF solution
with moderate decreases in intensity. The absorption spectra of
PentaPP in the aqueous THF mixtures with g70% water
contents contain light-scattering tails in the long wavelength
region, suggesting that the molecules of PentaPP have clustered
into nanoaggregates in the poor solvents.25,26 The morphologies
of PentaPP aggregates obtained in water/THF solution with
different fraction volume of water were observed by FE-SEM
measurements (Figure S1a-c, Supporting Information). The
experimental results show that the sizes of most PentaPP
aggregates obtained in THF-water mixtures with 70% and 80%
volume fractions of water are about 140-300 nm × 80-100
nm and 200-400 nm × 50-100 nm with an irregular rectangle,

Aggregation-Induced Emission Enhancement
J. Phys. Chem. B, Vol. 114, No. 50, 2010 16733
Figure 2. PL spectra of PentaPP in THF-water mixtures. The inset
plot indicates the relationship between PL intensities of PentaPP at
maximum intensity and water volume fraction in THF-water mixtures.
PentaPP concentration: 1 × 10-5 mol/L. Excitation wavelength: 310
nm.
Figure 3. Dependence of the PL intensity of PentaPP to aggregating
time in THF-water mixture (20:80 v/v). PentaPP concentration: 1 ×
10-5 mol/L. Excitation wavelength: 310 nm.
PL spectrum of silole, a moderate blue shift (ca. 15 nm) in the
emission peak was observed when the water fraction is increased
from 60% to 80%. However, the PL intensity is decreased and
a red-shift in the emission peak is from 372 to 378 nm when
the water fraction is from 80% to 90%. Careful inspection of
the PL spectra of PentaPP in the aqueous mixtures reveals an
increase/decrease in the intensity ratio and a blue/red shift in
the emission peak, while increasing the water fraction from 60%
to 90% agreed well with the absorbance spectra shown in Figure
1. This is probably due to the change in the packing mode of
the PentaPP molecules in the aggregates. In the mixture with
the “low” water content, the dye molecules may steadily
assemble in an ordered fashion to form more emissive, bluer
crystalline aggregates. In the mixture with the “high” water
content, however, the dye molecules may quickly agglomerate
in a random way to form less emissive, redder, and smaller
particles (Figure S1a-c, Supporting Information).
To further investigate the process of AIEE phenomenon, we
studied the relationship between the PL intensity of PentaPP
and the aggregating time in the THF/water mixture with 80%
water content. Water was injected to the THF solution of
PentaPP with vigorous stirring at room temperature. PL detection
was done after stirring for 2 min. The spectra show that the
rate of the fluorescence enhancement is first order with respect
to the time prolonged, and at the same time the maximum peak
gradually blue shifts from 381 to 374 nm (Figure 3). It is
assumed that, initially, the mechanical shear stress leads to the
raising of valid collision chance; thus, a potion of the dye
molecules clustered together to form tiny particles. The portion
of the dye molecules remaining in the solvent mixture then
gradually deposits onto the initially formed particles in a way
similar to recrystallization. Thus, the size of PentaPP aggregates
and the arranging degree between PentaPP molecules in the
aggregates can be increased with the increasing of the aggregat-
ing time, which are more propitious to enhance restricted
intramolecular rotations effect. This is the reason why the peak
is blue shifted and enhanced.
porting Information). In the crystal, PentaPP adopts a highly
twisted conformation with torsion angles between the pyrrole
group and the neighboring phenyl groups of 40.09°, 53.23°,
56.18°, 48.04°, and 60.07°, respectively (Figure 4a). As shown
in Figure 4b, the molecules of PentaPP are packed into
molecular columns that are perpendicular to the plane of the
central pyrrole rings. The distance between two molecules within
one column for PentaPP is 5.14 Å, which is too large to form
the π-π interaction. As shown in Figure 4c, there is no face-
to-face π-π stacking but edge-to-face interactions such as
aromatic CH · · ·π hydrogen bonding in the crystal structure.9,11,27
The delocalized system of sp2-hybridized covalent bonds can
act as an acceptor group, and hydrogen atoms serve as proton
donors for the formation of aromatic CH · · ·π hydrogen bonds.
The difference in the torsion angles is caused by the CH · · ·π
interaction, which in turn stabilizes the twisted conformation
of the dye molecule, which helps to hinder rotation of the σ
bond between the phenyl rings and the pyrrole group. This
structural rigidification may have made the crystals stronger
emitters.7-9
The fluorescence quenching of PentaPP in solution might be
understood as dominant nonradiative decay. In dilute solution,
twisting of the σ bond between phenyl and the pyrrole group
might facilitate approach between the excited and the ground
states of PentaPP and, thus, efficient rapid radiationless decay
occurrence. Once the water content is above 70%, however,
the degree of internal conversion is insignificant because the
rigid environment (C-H· · ·π bond formed) restricts intramo-
lecular rotations of PentaPP, resulting in the observed enhanced
emission. Furthermore, crystallographic analysis indicates that
a long molecular distance (∼5.14 Å) reduces the distance-
dependent intermolecular quenching effects to produce intense
fluorescence in the aggregation state. Therefore, the enhanced
emission of PentaPP is attributed to the synergistic effect of
restricted intramolecular rotations with a twisted geometry
configuration.
Mechanism for AIEE. The crystal structures of the fluoro-
phores in the aggregation state are the most direct evidence to
help us to explain the mechanism of AIEE. Single crystals of
PentaPP were grown through its slow crystallization in a THF/
water mixture. Crystals of high quality were used for the XRD
analysis. The crystal structure of PentaPP belongs to the
monoclinic system with space group P2(1)/c (Table S1, Sup-
On the basis of B3LYP/6-31G* calculation, Liu et al. found
that the filled π orbitals (or HOMOs) and the unfilled orbitals
(or LUMOs) are mainly dominated by orbitals originating from
the silole ring and two phenyl groups at the 2,5-positions in all
cases, while the LUMOs have significant orbital density at two
exocyclic σ bonds on the ring silicon, implying that σ*-π*
conjugation plays an important role.8a The HOMO and LUMO

16734 J. Phys. Chem. B, Vol. 114, No. 50, 2010
Feng et al.
Figure 4. (a) Molecule structure of PentaPP. (b) Stacking image of PentaPP. The hydrogen atoms have been omitted for clarity. (c) The schematic
intermolecular interactions in the crystal of PentaPP. The interaction distance of C-H· · ·π center is 2.86 Å (carbon, gray; hydrogen, white; nitrogen,
yellow).
To gain insight into the relationship between structures and
AIEE phenomenon, we further checked the PL spectra of other
Figure 5. Molecular orbital amplitude plots of HOMO and LUMO
energy levels of PentaPP calculated using the B3LYP/6-31+G** basis
set.
of PentaPP were calculated by using the B3LYP/6-31+G**
method (Figure 5). The results revealed that the HOMO of
PentaPP is similar to that of 1,1,2,3,4,5-hexaphenylsilole
(HPS),8a,g which is located on the core ring (pyrrole or silole)
as well as on the phenyl rings. The LUMO wave function of
PentaPP is, however, found to be localized on both the pyrrole
ring and the three phenyl rings at the 1,2,5-positions with a
staggered distribution, which is much more complicated than
that of HPS. It may cause the decrease of the fluorescence in
the solid state, leading to weaker AIEE response compared to
HPS.
Figure 6. Relationship between PL intensity of aryl-substituted pyrrole
derivatives and different water volume fraction in THF-water mixtures.

Aggregation-Induced Emission Enhancement
J. Phys. Chem. B, Vol. 114, No. 50, 2010 16735
after 70% water fraction. To explain the reason, the single-
crystal structure of TriPP was emphatically studied. The single-
crystal structure of TriPP belongs to the monoclinic system with
a space group of C2, which is different from that of PentaPP.
The analysis revealed that the CH· · ·π bond also plays an
important role in stabilizing the crystal structure instead of π-π
interactions in the crystal structure of TriPP (Figure 7).
We compared the UV spectra of these compounds (Figure
8). The π-π* transition of isolated phenyl groups in TetraPP
and PentaPP occurs at 258 nm, whereas in TriPP it is hard to
find the absorbance of isolated phenyl groups. The bathochromic
shift of absorption bands to 303 nm in TriPP is due to the
conjugation of the phenyl groups and pyrrole group, which can
be explained in terms of interaction or delocalization of the π
and π* orbital of each chromophore to produce a new orbital
in which the highest π orbital and the lowest π* orbital are
closer in energy. It indicates that the lowest energy, most stable
conformation of TriPP in solution is coplanar with extension
of π-delocalization. However, it is hard to find face-to-face π-π
stacking in the single-crystal X-ray diffraction of TriPP, which
means that the conformation of the pyrrole system in the single
crystal favored the nonplanar structure instead of the planar
structure in the solution state. Thus, we consider that aggrega-
tion, which happened while adding water into THF solutions,
is the process under kinetic control; π-π stacking, which is
the nonradiative de-excitation pathway, happened during col-
lisions of the chromophores. It is not similar to the conformation
of TriPP in the single crystal we obtained, which is under
thermodynamic control.
Figure 7. Molecule structure of TriPP. The interaction distance of
C-H· · ·π center is 2.86 Å (carbon, gray; hydrogen, white; nitrogen,
yellow).
Furthermore, anthracenyl and phenanthrenyl groups with a
bigger conjugated structure and steric hindrance were used to
substitute phenyl groups in TriPP and induced to the pyrrole
system, respectively. The bigger conjugated substitutes did
restrict rotation of the single bond between the pyrrole group
and the neighboring aryl groups in DiAnPP and DiPhenPP as
we expected (the torsion angles between the pyrrole group and
the neighboring aryl groups in the single crystal reach almost
80° (Figure 8a and 8b); the UV spectra also indicate that in the
solution state the structures of these two compounds are isolated
(Figure 7)), which made the conjugation in these two compounds
totally destroyed. However, the changing trend of PL intensities
of the two compounds with the water fraction remained almost
the same as TriPP, which are not AIEE properties (Figure 3).
We found that the distance between the π-π stacking of the
Figure 8. UV spectra of aryl-substituted pyrrole derivatives in THF
solution.
aryl-substituted pyrrole derivatives containing different substi-
tuted groups numbers and structures.
As shown in Figure 6, PL intensities of TriPP and TetraPP
remained unchanged when the water fraction in THF-water
mixtures is gradually increased to 60% and begin to decrease
Figure 9. Molecule structure of (a) DiAnPP and (b) DiPhenPP (carbon, gray; hydrogen, white; nitrogen, yellow. Hydrogen atoms of DiAnPP and
DiPhenPP have been omitted for clarity).

16736 J. Phys. Chem. B, Vol. 114, No. 50, 2010
Feng et al.
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3. Conclusions
In summary, we successfully synthesized a series of aryl-
substituted pyrrole derivatives and investigated the mechanism
for the AIEE effect. On consideration of the difference in the
chromophore structures and their steric hindrance, it is concluded
that the RIR process combines with the huge steric hindrance
to void the intermolecular π-π interactions during the aggrega-
tion state, which might play a crucial role in overcoming
fluorescence quenching in the solid state, proving the value of
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Acknowledgment. The authors are grateful to the National
Natural Scientific Foundation of China (Grant No. 20634020)
and Basic research foundation of the Beijing Institute of
Technology for financial support of this work.
Supporting Information Available: General procedure for
the synthesis of five aryl-substituted pyrroles; analytical data
of NMR, elemental analysis, mass spectroscopy, and single-
crystal structure. This material is available free of charge via
the Internet at http://pubs.acs.org.
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