Tetraphenylethene modified perylene bisimide: Effect of the number of substituents on AIE performance

Article abstract of DOI:10.1039/c2cc36060h

Perylene bisimide derivatives substituted with one and two tetraphenylethene moieties at 1 and 1,7-postions show distinct optical properties. The former displays characteristic emission features of perylene bisimides in solution and red emission in the ag

Full text of DOI:10.1039/c2cc36060h

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Tetraphenylethene modified perylene bisimide: effect of the number
of substituents on AIE performancew
Qiuli Zhao,ab Xiao A Zhang,a Qiang Wei,a Jian Wang,a Xiao Yuan Shen,a Anjun Qin,a
Jing Zhi Sun*a and Ben Zhong Tang*ac
Received 21st August 2012, Accepted 9th October 2012
DOI: 10.1039/c2cc36060h
Perylene bisimide derivatives substituted with one and two
tetraphenylethene moieties at 1 and 1,7-postions show distinct
optical properties. The former displays characteristic emission
features of perylene bisimides in solution and red emission in the
aggregate state, while the latter is nonemissive in solution but
highly red-emissive in the aggregate state.
pyrene, triphenylamine, and carbazole by attaching TPE moieties
to the luminorphores.3 For example, the modification of triphenyl-
4
4
4
4
0
0
amine and N ,N ,N ,N -tetraphenylbiphenyl-4,40-diamine with
three and four TPE units resulted in two novel AIE luminogens.
The fluorescence (FL) quantum yields of their films (FF,film) are
91.6% and 100%, while their FL quantum yields in dilute solutions
(FF,solut) are only 0.42% and 0.55%, respectively.3d
These successes triggered our interest in converting larger con-
jugated systems from ACQ to AIE luminophores. Perylene
bisimides (PBIs, Chart 1) are one class of the most investigated
organic dyes that have found promising application as an active
component in organic light harvesting systems, fluorescent emitters
and biosensors.4 PBIs have near-unity fluorescence quantum yield
in dilute solutions, but their aggregates or solids display drastic
fluorescence quenching due to the attractive dipole–dipole
interactions and/or effective intermolecular p–p stacking.5 Thus
the conversion of PBIs from ACQ to AIE materials is of great
significance. Recently, we have shown that the decoration of two
TPE moieties at 1,6- or 1,7-positions on the perylene core can make
the PBI derivatives be AIE active, and the highly efficient red
emission in solid states and the formation of ordered
microstructures have been investigated.6 However, there are still a
few important issues should be figured out. For smaller conjugated
systems, it is enough to convert the emission behavior from ACQ to
AIE by attaching one TPE unit to the core. But for larger
conjugated systems (e.g. PBI and rubrene), is one TPE unit
enough to achieve the same conversion? In addition, the effect of
substitution position is also an important factor relevant to the
conversion process. Moreover, the TPE unit(s) can be directly
attached onto the perylene core via a single bond, or indirectly
via a spacer. It is necessary to evaluate the effect of the linking mode
on the conversion process. Herein, we demonstrate our results by a
comparative investigation on the fluorescence properties of non-,
mono- and di-TPE-substituted PBIs (Chart 1).
Organic luminophores with aggregation-induced emission (AIE)
property have attracted great research efforts, because AIE opens
an avenue to turning away from the aggregation-caused quenching
(ACQ) effect and deriving luminescent materials with high
quantum efficiency in the solid state.1 For example, tetra-
phenylethene (TPE, Chart 1) is a representative AIE-active
luminophore, and TPE-derivatives have been widely used as
fluorescent probes for the detection of ions, biomolecules and
explosives.1,2 More recently, we found that TPE could be used as
a modifier to turn an ACQ luminophore to an AIE one. This
strategy has proved to be workable for a series of classical ACQ
luminophores such as naphthalene, anthracene, phenanthracene,
Chart 1 Molecular structures of the non-, mono- and di-TPE sub-
stituted perylene bisimides.
a MoE Key Laboratory of Macromolecule Synthesis and
Functionalization, Department of Polymer Science and Engineering,
Zhejiang University, Hangzhou 310027, China.
E-mail: sunjz@zju.edu.cn; Fax: +86 571 87953734;
Tel: +86 571 87953734
The synthetic routes to DBuTPEPBI and DBuDTPEPBI are
shown in Scheme S1 (ESIw). The detailed procedures for the
syntheses and the characterization data are described in ESI.w In
dilute dichloromethane (DCM) solution (10ꢀ6 M), the FL spectrum
of the non-TPE substituted PBI (DBuBrPBI) has an emission peak
at 538 nm with a shoulder at around 577 nm (Fig. S3, ESIw). This is
the typical feature of N,N0-substituted PBIs. FF,solut is calculated to
be 95.2%. While its solid film has very weak fluorescence (Fig. 2A
and S4 (ESIw)). The concentration dependent fluorescent behaviors
b College of Materials Science and Engineering, Xi’an University of
Science and Technology, Xi’an, 710054, China
c Department of Chemistry, Institute for Advanced Study, State Key
Laboratory of Molecular NeuroScience, and Division of Biomedical
Engineering, The Hong Kong University of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk; Fax: +852-23581594;
Tel: +852-23587375
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc36060h
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11671–11673 11671

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of mono-TPE substituted PBI (DBuTPEPBI) are shown in Fig. 1,
the FL spectrum recorded for its dilute DCM solution (10ꢀ6 M)
shows a peak at 537 nm with a shoulder at around 574 nm, which is
similar to that of its precursor DBuBrPBI. When concentration is
increased to 10ꢀ5 M, the shoulder becomes more pronounced. And
as it is increased to 10ꢀ4 M, the emission peak shifts to 590 nm. For
its powder sample, the emission peak red-shifts further to 682 nm.
The FF,solut of DBuTPEPBI in DCM is measured to be 2.2%. The
solid sample emits bright red light (Fig. 2B). Noticeably, the di-TPE
substituted PBI (DBuDTPEPBI) shows quite distinct fluorescent
behavior from both DBuBrPBI and DBuTPEPBI. In DCM
solutions with different concentrations (10ꢀ6–10ꢀ4 M), only FL
signals in the noise level were detected. FF,solut is merely 0.07%. For
DBuDTPEPBI cast film, the emission is still efficient (Fig. 2C) and
the emission peak appears at 706 nm (Fig. S5, ESIw).
The comparison of the three PBI derivatives discloses significant
information. Firstly, they take more and more planar configuration
in an order of DBuBrPBI, DBuTPEPBI and DBuDTPEPBI. For
DBuBrPBI, the perylene core takes the most distorted configu-
ration due to the steric repulsion between the Br and H atoms on
positions 1 and 12 (Fig. 2A). Although TPE is a conjugated moiety,
the optimized molecular geography indicates that the phenyl groups
at 1 or 1,7-positions are perpendicular to the perylene core (Fig. 2B
and C). Meanwhile, the C atom is smaller than Br in size. As a
result, the perylene core has a more planar configuration. The
corresponding dihedral angles decrease from 21.091 for DBuBrPBI
to 18.641 for DBuTPEPBI and 12.871 for DBuDTPEPBI. As a
result, there is only partial electronic conjugation between the
TPE moieties and the PBI core (Fig. S6, ESIw). This is also
reflected by the absorption spectra of DBuBrPBI, DBuTPEPBI
and DBuDTPEPBI, which show absorption peaks at 522, 533
and 567 nm (Fig. S7–S9, ESIw), respectively.
Fig. 2 (A), (B) and (C) show the optimal structures of DBuBrPBI
DBuTPEPBI, and DBuDTPEPBI calculated by the semi-empirical
AM1 method. The photographs display the solution cast films of
DBuBrPBI (A), DBuTPEPBI (B) and DBuDTPEPBI (C) taken under
day light (left column) and 365 nm UV light (right column).
intramolecular rotation mechanism for the AIE phenomenon,1
for molecule DBuDTPEPBI, the rotations of phenyl groups in
two TPE moieties dissipate the excitation energy thereby rendering
it with weak emission. For molecule DBuTPEPBI, which has only
one TPE moiety attaching onto the PBI core, the number of freely
rotary phenyl groups is the half of that in DBuDTPEPBI. Thus the
intramolecular rotations cannot completely dissipate the excitation
energy. As a result, DBuTPEPBI shows subdued but characteristic
emission of the PBI moiety in solution.
The third is that the emission of their solid films. DBuBrPBI
emits very weak red light (Fig. 2A). Compared with the strong
emission of its DCM solution, the drastic FL quenching of
DBuBrPBI is due to the effective intermolecular p–p stacking.7
However, the cast films of DBuTPEPBI and DBuDTPEPBI emit
effective red light upon illumination of UV light (see photographs
in Fig. 2B and C). Based on the RIR mechanism,1 the rotations of
the phenyls around the perylene core are restricted in the solid
state, and the channel of consuming the excitation energy is
switched out. Meanwhile, the bulky TPE moieties impair the
intermolecular p–p stacking. Thus, the increased FL intensity in
solid films over that in solutions was observed.
The second significant information is the evolution of the FL
quantum yield of their aggregates. In dilute DCM solution,
FF,solut for DBuBrPBI and DBuDTPEPBI is 95.2% and
0.07%, respectively, which are the highest and the lowest one
among the three compounds. As revealed by the optimized
molecular geometry, the phenyl rings of the TPE moieties are
in a propeller shape. In good solvents, they undergo active
intramolecular rotations and consume the excitation energy,
which leads to the vanishing of the fluorescence. Similar pheno-
menon has been observed in other TPE modified conventional
planar luminophores.3 It is interesting that DBuTPEPBI has a
The solid state fluorescent behaviors indicate that precursors and
the TPE-modified PBIs are typical ACQ and AIE molecules,
respectively. Using a common method,2,3,6 we investigated the
AIE behaviours of DBuTPEPBI and DBuDTPEPBI in hexane/
DCM mixtures with different volume percentage of hexane (fh). As
shown in Fig. 3A, when fh r 60%, the FL spectra of DBuTPEPBI
display the PBI’s characteristic green band peaked at 537 nm with a
shoulder at 574 nm. When fh increases from 60% to 80%, the
intensity of the red emission band peaked at 638 nm is enhanced. At
fh = 90%, the red band becomes dominant; while the green band
still exists. The green and red bands are assigned to the free and
aggregated DBuTPEPBI molecules, respectively. In the whole
process, the absorption features have little changes with the varia-
tion in fh values (Fig. S8, ESIw).
FF,solut of 2.2% in DCM solution, which is much lower than that
of its precursor but much higher than that of the disubstituted
counterpart DBuDTPEPBI. According to the restricted
The FL spectra of the disubstituted counterpart of DBuDT-
PEPBI demonstrate different features from that of DBuTPEPBI
in hexane/DCM mixtures. When fh r 40%, no peak can be
detected. However, a peak at 645 nm can be observed when fh is
increased. This deep-red band is assigned to the emission from
Fig. 1 Normalized fluorescence (FL) spectra of DBuTPEPBI cast film
and in DCM solution with different concentrations (lex = 479 nm).
c
11672 Chem. Commun., 2012, 48, 11671–11673
This journal is The Royal Society of Chemistry 2012

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(lem,solid = 706 nm, lem,aggr = 645 nm, FF,aggr = 18.9%).
The robusticity of the PBI core is also embodied by the
observations that both DBuTPEPBI and DBuDTPEPBI can
form ordered microstructures as revealed by the SEM images of
the aggregates. The effects of the linking mode and substitution
positions on the perylene core on the AIE performance are
underway in our laboratory. These results will provide a useful
criterion for deriving AIE molecules by rational modification
of classical luminogens and construction of highly efficient
luminescent solid materials.
This work was supported by the National Science Foundation of
China and Zhejiang Province (51273175, 50573086 and Z4110056);
the key project of the Ministry of Science and Technology of China
(2009CB623605), the Research Grants Council of Hong Kong
(603509, HKUST2/CRF/10, and 604711), the University Grants
Committee of Hong Kong (AoE/P-03/08).
Notes and references
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Fig. 3 FL spectra of DBuTPEPBI (A) and DBuDTPEPBI (C) in
hexane/DCM mixtures with different fh. Variation in quantum yield of
DBuTPEPBI (B) and DBuDTPEPBI (D) with fh in hexane/DCM
mixtures (lex = 479 and 453 nm, respectively). The insets of B and D
show the photographs of the FL images (taken under 365 nm UV
light) for the corresponding mixtures. Concentration: 10 mM.
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molecular aggregates. For DBuDTPEPBI, only this deep red
band can be detected in hexane/DCM mixtures. At fh = 90%,
the quantum yields of the aggregates (FF,aggr) of DBuTPEPBI
and DBuDTPEPBI are 9.0% and 18.9%, respectively. In aggre-
gates, the active intramolecular rotations of multiple phenyl rotors
of TPE are restricted, which renders their aggregates with strong
luminescence. Since hexane is non-solvent for both DBuTPEPBI
and DBuDTPEPBI, the molecules must have aggregated when a
large amount of hexane was added. The morphological observa-
tions provide direct evidence for the formation of aggregates. As
shown in Fig. S10 and S11 (ESIw), the SEM images display that
both DBuDTPEPBI and DBuTPEPBI can assemble into ordered
microstructures such as fibers and belts with hundreds of microns
in length and several microns in diameter, just as PBI dyes
reported in the literature.4,7
In summary, the comparative and controlled investigations
on the fluorescent behaviors of non-, mono- and di-TPE
substituted PBI derivatives have demonstrated the feasibility
to convert the fluorescence performance of PBIs from ACQ
to AIE. Unlike those small conjugated systems such as
naphthalene, anthracene and so on, whose FL behavior can
be completely converted from ACQ to AIE by modification
with a single TPE moiety, the modification of the larger PBI core
with one TPE moiety can only partially alter its FL property. The
mono-TPE substituted PBI (DBuTPEPBI) in dilute solution shows
the characteristic FL features with decreased FF,solut (2.2%). Its
aggregates formed in hexane/DCM mixture (fh = 90%) and the
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J. Z. Sun and B. Z. Tang, J. Mater. Chem., 2012, 22, 7387.
powder sample emit red fluorescence (lem,solid = 682 nm, lem,aggr
=
7 (a) F. Wurthner, Chem. Commun., 2004, 1564; (b) F. Wurthner,
¨
50, 3376; (c) L. Zang, Y. Che and J. S. Moore, Acc. Chem. Res.,
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¨
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638 nm, FF,aggr = 9.0%). The di-TPE substituted PBI (DBuDT-
PEPBI) in dilute solution has very low FF,solut (0.07%), but its
aggregates formed in hexane/DCM mixture (fh = 90%) and
the powder sample emit efficient red to near infrared FL
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11671–11673 11673