Tetraphenylethenyl-modified perylene bisimide: Aggregation-induced red emission, electrochemical properties and ordered microstructures

Article abstract of DOI:10.1039/c2jm16613e

Perylene bisimides (PBIs) are one class of the most explored organic fluorescent materials due to their high fluorescence quantum efficiency, electron transport behaviour, and ready to form well-tailored supramolecular structures. However, they suffer from heavy aggregation-caused quenching (ACQ) effect which has greatly limited their potential applications. We successfully tackle this problem by chemical modification of the PBI core with two tetraphenylethene (TPE) moieties at the bay positions. This modification resulted in a pronounced fluorescence red-shift (over 120 nm) and rendered the derivatives (1,6-/1,7-DTPEPBI) with evident aggregation-induced emission (AIE) behaviour. Both 1,6-DTPEPBI and 1,7-DTPEPBI emit bright red fluorescence in the solid state. The fluorescence quantum efficiency of the aggregates of 1,7-DTPEPBI (Φ_{F, solid} = 29.7%, formed in a hexane/ dichloromethane mixture, f_h = 90%) is about 424 times higher than that in dichloromethane solution (Φ_{F, solut} = 0.07%). Electrochemical investigation results indicated that 1,7-DTPEPBI sustained the intrinsic n-type semiconductivity of PBI moiety. In addition, morphological inspection demonstrated that 1,7-DTPEPBI molecules easily form well-organized microstructures despite the linkage of the PBI core with bulky TPE moieties.

Full text of DOI:10.1039/c2jm16613e

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PAPER
Tetraphenylethenyl-modified perylene bisimide: aggregation-induced red
emission, electrochemical properties and ordered microstructures†
a
a
a
a
b
c
a
c
Qiuli Zhao, Shuang Zhang, Yi Liu, Ju Mei, Sijie Chen, Ping Lu, Anjun Qin, Yuguang Ma, Jing Zhi Sun
a
*
ab
*
and Ben Zhong Tang
Received 15th December 2011, Accepted 27th January 2012
DOI: 10.1039/c2jm16613e
Perylene bisimides (PBIs) are one class of the most explored organic fluorescent materials due to their
high fluorescence quantum efficiency, electron transport behaviour, and ready to form well-tailored
supramolecular structures. However, they suffer from heavy aggregation-caused quenching (ACQ)
effect which has greatly limited their potential applications. We successfully tackle this problem by
chemical modification of the PBI core with two tetraphenylethene (TPE) moieties at the bay positions.
This modification resulted in a pronounced fluorescence red-shift (over 120 nm) and rendered the
derivatives (1,6-/1,7-DTPEPBI) with evident aggregation-induced emission (AIE) behaviour. Both
1,6-DTPEPBI and 1,7-DTPEPBI emit bright red fluorescence in the solid state. The fluorescence
quantum efficiency of the aggregates of 1,7-DTPEPBI (FF, solid ¼ 29.7%, formed in a hexane/
dichloromethane mixture, fh ¼ 90%) is about 424 times higher than that in dichloromethane solution
(FF, solut ¼ 0.07%). Electrochemical investigation results indicated that 1,7-DTPEPBI sustained the
intrinsic n-type semiconductivity of PBI moiety. In addition, morphological inspection demonstrated
that 1,7-DTPEPBI molecules easily form well-organized microstructures despite the linkage of the PBI
core with bulky TPE moieties.
molecules are prone to form aggregates, which leads to FL
quenching because of the attractive dipole–dipole interactions
and/or effective inter-molecular p–p stacking. This effect is
termed aggregation-caused quenching (ACQ).5 It is of great
significance to make PBI-based dyes efficiently emit in the solid
state. A wise strategy is to modify the PBI-based dyes by
attaching bulky substituents onto the chromophore to restrict
the intermolecular p–p stacking and to hinder the intermolecular
electronic coupling and the resulting PBIs exhibit desirable
quantum efficiency of light emission.6
Introduction
Perylene bisimides (PBIs) are one kind of the most investigated
organic dyes that have found promising applications as active
components in organic light harvesting systems, n-type channel
field effect transistors, fluorescent emitters and biosensors, and
the construction of self-assembling nanostructures.1–4 PBIs
demonstrate near-unity fluorescence (FL) quantum yields in
dilute solutions, but in poor solvents or in solid state, PBI
Tetraphenylethene (TPE) has a bulky size and propeller shape.
It has been found that TPE and its derivatives are non-emissive
in solutions but highly emissive by aggregate formation. This
phenomenon is termed ‘‘aggregation-induced emission’’
(AIE).5,7,8 Recently, we have demonstrated that combining TPE
moieties with classical fluorogens such as naphthalene, quinoline,
anthracene, phenanthracene, carbazole, and pyrene can convert
these conventional dyes into efficient solid-state emitters
(Chart 1).9 Based on these achievements, it is a rational
assumption to tackle the notorious ACQ behavior of PBIs by
means of modifying PBI with TPE moieties. But for the expected
PBI-TPE dyads, there still are unknowns and challenges. Firstly,
the fluorogens described in literature are small planar aromatics
(e.g. naphthalene, anthracene, phenanthracene, and pyrene).9
Can it be workable to convert larger planar PBI from a typical
ACQ to an AIE by attaching TPE moieties to the PBI core?
aMOE Key Laboratory of Macromolecular Synthesis and
Functionalization, Institute of Biomedical Macromolecules, Department
of Polymer Science and Engineering, Zhejiang University, Hangzhou,
310027, China. E-mail: sunjz@zju.edu.cn; Fax: 86-571-87953734
bDepartment of Chemistry and State Key Laboratory of Molecular
NeuroScience, The Hong Kong University of Science & Technology,
Clear Water Bay, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.
hk; Fax: + 852-2358-7375
cState Key Laboratory of Supramolecular Structure and Materials, Jilin
University, Changchun, 130012, China
† Electronic Supplementary Information (ESI) available: 1H and 13C
NMR spectra of 1,6-/1,7-DTPEPBI, fluorescence spectra of
1,7-DTPEPBI in different non-solvent/solvent mixtures such as
methanol/DCM and water/THF, fluorescence spectrum of the solid
film of 1,7-DTPEPBI, fluorescence spectra of 1,6-DTPEPBI in
hexane/DCM mixtures with different hexane contents, typical SEM
images of 1,7-DTPEPBI aggregates formed in different conditions,
polarized optical microscopic and confocal fluorescence images of the
microstructures formed by 1,7-DTPEPBI and 1,7-DBrPBI. See DOI:
10.1039/c2jm16613e
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bromination of 3,4,9,10-perylenetetracarboxylic dianhydride
(PTCDA) was according to ref. 10 and a mixture of 1,6-dibromo-
3,4,9,10-perylenetetracarboxylic dianhydride (1,6-DBrPTCDA)
and 1,7-dibromo-3,4,9,10-perylenetetracarboxylic dianhydride
(1,7-DBrPTCDA) was obtained. It was very difficult to separate
the two isomers, thus we proceeded to the imidization reaction
without purification. The reaction afforded a mixture of N,N0-
cyclohexamethyl-1,6-dibromo-3,4,9,10-perylene tetracarboxylic
bisimide (1,6-DBrPBI) and N,N0-cyclohexamethyl-1,7-dibromo-
3,4,9,10-perylene tetracarboxylic bisimide (1,7-DBrPBI). The
intermediate of 4-(1,2,2-triphenyvinyl) phenylboronic acid
(TPVPBC) was prepared according to our previously published
procedures (Scheme S1†).9a Suzuki coupling TPVPBC with
1,6-DBrPBI and 1,7-DBrPBI catalyzed by Pd(PPh3)4 gave rise to
Chart 1 Chemical structures of 1,6-DTPEPBI and 1,7-DTPEPBI, as
well as a series of classical fluorogens modified with tetraphenylethene
(TPE) moiety/moieties. The numbers in bracket are the maximum of the
fluorescence spectrum of the corresponding solid film.
a
mixture of [N,N0-dicyclohexyl-1,6-bis(40-(10,20,20-triphenyl)
vinyl)phenylperylene-3,4:9,10-tetrcarboxylic bisimide] (1,6-
DTPEPBI) and [N,N0-dicyclohexyl-1,7-bis(40-(10,20,20-triphenyl)
vinyl)phenylperylene-3,4:9,10-tetracarboxylic bisimide] (1,7-
DTPEPBI) in high yield. Fortunately, the final products 1,6-
DTPEPBI and 1,7-DTPEPBI can be completely separated by
simple column chromatography.
Secondly, PBI and TPE are n- and p-type organic semi-
conductors, respectively. What is the effect of linking TPE
moieties to PBI on the semiconductive behavior of the derived
dyad? And thirdly, PBIs are prone to self-assemble into ordered
supramolecular structures. It is an important issue to know the
effects of attaching twisted and bulky TPE moieties onto the PBI
core on the assembling behavior of the derived molecules.
Herein, we report the design and synthesis of novel PBI deriva-
tives decorated with TPE moieties (see Chart 1, 1,6-DTPEPBI
and 1,7-DTPEPBI), and demonstrate our efforts to address the
above-mentioned issues by using them as representative
compounds.
Generally, the full separation of isomers of 1,6- and 1,7-
substituted PBIs is a challenging work. Up to now, there are only
few reports of the isolation and characterization of 1,6-/1,7-
disubstituted PBIs.11 In the present case, the two regioisomers
1,7-DTPEPBI and 1,6-DTPEPBI were easily separated by
conventional column chromatography using DCM/petroleum
ether (2 : 3 by volume) as eluent (for the characterization data see
experimental section and ESI† for details). This can be associated
with the fact that the substitution of bromides at 1,6-/1,7-posi-
tions with TPEs renders 1,7-DTPEPBI and 1,6-DTPEPBI with
enhanced differences in molecular shape and symmetry. The
separation result revealed that the ratio of 1,7-DTPEPBI to 1,6-
DTPEPBI is 4 : 1, indicating that 1,7-DTPEPBI is the dominant
product. Here, we purified 1,7-DTPEPBI from the mixture of the
two regioisomers, and mainly studied the properties of 1,7-
DTPEPBI. Both 1,6-DTPEPBI and 1,7-DTPEPBI have good
solubility in common organic solvents such as tetrahydrofuran
(THF), chloroform, and dichloromethane (DCM), but poor in
hexane, methanol and ethanol. The structures of the intermedi-
ates and products were characterized with multiple spectroscopic
techniques and satisfactory data were obtained (experimental
section and ESI†).
Results and discussion
Synthesis of 1,6-DTPEPBI and 1,7-DTPEPBI
The synthetic route to 1,6-/1,7-DTPEPBI is shown in Scheme 1.
The detailed procedures for the syntheses of the intermediates
and final products are described in the experimental section and
Electronic Supplementary Information (ESI).† In brief, the
Fluorescent behavior of 1,6-DTPEPBI and 1,7-DTPEPBI
PBI derivatives usually emit intense fluorescence in solution, but
become faintly or even non-emissive in aggregated or solid
states.3,4,12 This was confirmed again by the emission behavior of
1,6-DBrPBI and 1,7-DBrPBI in water/THF mixtures with
different water fractions (fw). The fluorescence spectrum of 1,7-
DBrPBI in THF solution shows a maximum at 545 nm and
a pronounced shoulder around 580 nm. With addition of water,
the emission intensity decreases. When fw < 50%, the change in
emission intensity is rather small; when 50% # fw < 80%, the
emission intensity decreases drastically; and when fw $ 80%,
the emission spectra are nearly parallel to the abscissa (Fig. 1B).
The photographs displaying the emission change are shown in
Fig. 1C. The decrease of emission intensity with the increasing of
Scheme 1 Synthetic route to 1,6-DTPEPBI and 1,7-DTPEPBI.
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a hydrophilic and a hydrophobic solvent, respectively (Fig. 2 and
S3, S4†). However, when enough non-solvent (hexane, methanol
or water) was added into the solution, the resultant mixture
emitted evidently enhanced fluorescence. We investigated the
fluorescence behavior of 1,7-DTPEPBI in three different non-
solvent/solvent systems, which were hexane/DCM (hydrophobic/
hydrophobic), methanol/DCM (hydrophilic/hydrophobic) and
water/THF (hydrophilic/hydrophilic). In hexane/DCM, the
fluorescence spectra of 1,7-DTPEPBI in mixed solvents are
nearly parallel to the abscissa as fh < 40% (fh, the percentage
volume fraction of hexane in the hexane/DCM mixture).
However, the emission is gradually enhanced when fh > 40%
(Fig. 2A and B). When I/I0 ꢀ 1 is calculated, it can be figured out
that the emission intensity at fh ¼ 90% is over 530 times that at
fh ¼ 0; where I0 and I are the fluorescence intensities recorded for
fh ¼ 0 and another hexane percentage, respectively. Since hexane
is a non-solvent of 1,7-DTPEPBI, containing enough hexane in
the mixture would make the molecules insoluble and aggregates
should form in the mixture. According to the well-accepted
mechanism of the AIE phenomenon, in solution, the active
intramolecular rotations of the multiple phenyl rotors in TPE
around the PBI stators consume the excited energy and quench
the fluorescence; while in aggregates, molecular stacking restricts
the rotation of the phenyl rotors and induces the emission of the
fluorescence.
Fig. 1 (A) Absorption spectra of 1,7-DBrPBI in H2O/THF mixtures
with different water fractions (fW, by volume%). (B) Fluorescence spectra
of 1,7-DBrPBI in H2O/THF mixtures with different fW values. Excitation
wavelength: 485 nm. (C) Images showing the fluorescence of 1,7-DBrPBI
in H2O/THF mixtures with different fW values taken under UV illumi-
nation. Excitation wavelength: 365 nm. [1,7-DBrPBI] ¼ 10ꢀ5 M.
fw can be ascribed to the formation of aggregates in the mixtures
of water/THF, which is due to the p–p electronic coupling
between perylene chromophores. Absorption spectra in Fig. 1A
give convincing evidence. The absorption bands at 520 and 485
nm are assigned to the 0–0 and 0–1 electronic transitions of the
PBI chromophore. They decreased gradually with the increasing
of fw. Moreover, the relative intensity of 0–0 and 0–1 bands
reverses as fw $ 60%. According to literature, such a feature
change indicates strong molecular aggregation induced by p–p
stacking.4,12 Furthermore, the absorption data in Fig. 1A also
indicate the precipitation upon addition of water since the overall
absorption band is decreasing and could account for the decrease
in fluorescence as shown for 1,7-DBrPBI in the same solvent
mixture. Based on the analyses of the absorption and emission
spectra, 1,7-DBrPBI is assigned to an ACQ molecule.
Similar fluorescent behavior was observed for methanol/DCM
and water/THF systems. In methanol/DCM mixtures, the fluo-
rescence intensity of the mixtures was greatly enhanced when
fm $ 70% (fm, the percentage volume fraction of methanol in
methanol/DCM). However, when fm was over 80%, the fluores-
cence intensity decreased gradually (Fig. S3†). This phenomenon
can be explained as follows. At high fm value, 1,7-DTPEPBI
molecules easily form larger aggregates, which precipitate
quickly. Meanwhile, the formation of larger aggregates decreases
the number of the emissive species in the mixture. The tendency
to form large aggregates can be associated with the hydrophobic
nature of 1,7-DTPEPBI molecules, which allows them to isolate
when hydrophilic methanol is added into the mixture. In water/
THF (hydrophilic/hydrophilic) mixtures, the precipitate can be
quickly observed when fw is only 40%. This made the fluores-
cence enhancement be lower than that observed in hexane/DCM
and methanol/DCM systems The fluorescence intensity only
showed very a limited increase when a large amount of water was
introduced into the water/THF mixture (Fig. S4†). In this case,
the large particles precipitated during the measurement, only
those smaller ones in the suspension contributed to the emission.
Such an observation suggests that the hydrophobic 1,7-
DTPEPBI molecules had formed large aggregates, because both
water and THF are hydrophilic solvents.
In contrast, 1,7-DTPEPBI is non-emissive when dissolved in
good solvents, such as THF and dichloromethane (DCM),
In the mixed solvents such as water/THF or methanol/DCM,
when enough non-solvents (fw $ 40% or fm $ 70%) were added,
rather broad and red-shifted absorption bands were observed
(Fig. S4A and S5†). Such feature changes are typical character-
istics of J-aggregation.3a,4,12,13 Moreover, the relative intensity of
0–0 and 0–1 bands reverses. These spectral changes indicated
that 1,7-DTPEPBI molecules are arranged in slipped face-to-face
fashion to form J-aggregates.12,13
Fig. 2 (A) Fluorescence spectra of 1,7-DTPEPBI in hexane/DCM
(dichloromethane) mixtures with different hexane fractions (fh, by
volume%). Excitation wavelength: 452 nm. (B) Quantum yield [estimated
by using Rhodamine B as standard (FF ¼ 70% in ethanol)] of 1,7-
DTPEPBI in hexane/DCM mixtures with different fh values. Inset:
fluorescence images of 1,7-DTPEPBI in hexane/DCM mixtures with fh
values of 0 and 90%, respectively. The photographs were taken under UV
illumination. Excitation wavelength: 365 nm. [1,7-DTPEPBI] ¼ 10ꢀ5 M.
The fluorescence quantum efficiencies (FF) of 1,7-DTPEPBI
was evaluated by using Rhodamine B as standard. In dilute
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DCM solution, the FF, solut of 1,7-DTPEPBI is only 0.07%, while
in solid film, the FF, solid becomes 6.3%, indicating an amplifi-
cation factor (FF, solid/FF, solut) of 90. It is noticeable that the
FF, solid value of 1,7-DTPEPBI in solid film is close to the best
data up to now obtained by introducing bulky substituents onto
the fluorogens to prevent the molecules from strong intermo-
lecular p–p interaction.4,6 In a hexane/DCM mixture with fh ¼
90%, FF increases up to 29.7% and an amplification factor of
424.3 (Fig. 2B). As displayed in Fig. S6,† 1,6-DTPEPBI
demonstrates identical fluorescent behavior in hexane/DCM
mixtures. When fh is higher than 50%, the fluorescence begins to
elevate; when fh is 90%, FF increases to 22.8%, indicating an
amplification factor of 326. Qualitatively, in methanol/DCM and
water/THF mixed solvents, 1,7-DTPEPBI demonstrated fluo-
rescent behavior similar to that in hexane/DCM mixtures
(Fig. S3 and S4†). Quantitatively, the fluorescence amplification
is not as high as that observed in the hexane/DCM system.
Compared with the fluorescent behavior of other AIE active
materials reported in literature,7–9 we can conclude that both
1,6-DTPEPBI and 1,7-DTPEPBI are typical AIE active
molecules.
Fig. 3 (A) Molecular orbital amplitude plots of HOMO and LUMO
energy levels of 1,7-DTPEPBI. (B) and (C) show the optimal structures of
1,7-DBrPBI and 1,7-DTPEPBI calculated by the semi-empirical AM1
method. The white, grey, blue, red, and scarlet balls represent H, C, N, O,
and Br atoms, respectively. The H atoms are omitted for clarity.
which are nearly the same as reported in literature,4e and these
results suggest that 1,7-DTPEPBI has a more planar configura-
tion than 1,7-DBrPBI. This result may partially explain the
observed large red-shift florescence of 1,7-DTPEPBI relative to
1,7-DBrPBI.
In comparison of the fluorescence spectra of 1,7-DTPEPBI
with those of 1,7-DBrPBI, another significant difference can be
recognized. That is, the aggregates of 1,7-DTPEPBI formed in
hexane/DCM, methanol/DCM and water/THF emit red fluo-
rescence, and the emission peaks appear at 638, 668 and 693 nm,
respectively. Meanwhile, the maximum of the fluorescence
spectra for the solid film of 1,7-DTPEPBI also appears at 668 nm
(Fig. S7†). As a comparison, the emission maximum for the
powder of the 1,7-DBrPBI appears at 607 nm (Fig. S8†). The
experiment data indicate that evident red-shifts of 31, 61 and 86
nm have been achieved in different aggregate states. This spectral
shift means the modification of PBI with TPE moieties has
transferred the emission from green to red. This is different from
our previous observations for the decoration of naphthalene,
anthracene, and pyrene with TPE moieties, in which only a small
red-shift of emission spectra has been achieved. Even attachment
of four TPE moieties to the pyrene core can only result in
a fluorescent change from purple and blue (400 nm for pyrene
monomer and 468 nm for excimer) to blue-greenish (483 nm)
(see Chart 1).
Since the electronic structure of 1,7-DTPEPBI is calculated to
be dominated by the PBI core, it is expected that the n-type
semiconductor behaviour will remain unchanged by the attach-
ment of TPE moieties onto the 1,6-/1,7-positions of a PBI
moiety. This has been confirmed by experiment results of cyclic-
voltammetry measurements. In DCM solution, 1,7-DBrPBI
shows two sets of single redox waves and the related first redox
wave E1/2 and DEP values are 589 and 76 mV, respectively. 1,7-
DTPEPBI shows similar redox behavior, its E1/2 and DEP values
are 715 and 207 mV, respectively (Fig. 4). These results indicate
that 1,7-DTPEPBI has the same electrochemical behavior as PBI
and can be assigned to an n-type organic semiconductor.
Ordered microstructures
Owing to the strong p–p interaction between perylene cores, PBI
derivatives are prone to assemble into ordered one-dimensional
(1D) nanostructures such as wires, rods and belts. However,
some investigation results demonstrated that the PBI derivatives
with bulky substituents at the bay area (1,6-/1,7-positions) were
Electronic structure
To get better understanding of the mechanism of the pronounced
fluorescence red-shift, the electronic structure of 1,7-DTPEPBI is
estimated by theoretical simulation and their molecular struc-
tures were optimized by the semi-empirical AM1 method. The
calculation results reveal that the HOMO and LUMO of 1,7-
DTPEPBI are localized predominately on the molecular skeleton
of the central PBI moiety, just like its precursor 1,7-DBrPBI
(Fig. 3A and S9†), while the TPE moieties have hardly contrib-
uted to the molecular orbital of 1,7-DTPEPBI. Meanwhile, the
phenyls in the TPE units are arranged in a propeller-like shape
(Fig. 3C). As a result, the remarkable red-shift fluorescence
cannot be fully ascribed to the extended conjugation of PBI with
TPE moieties. Based on the theoretical calculation, the twist
angles of the two naphthalenoid moieties of 1,7-DBrPBI and 1,7-
DTPEPBI are 24.10ꢁ and 12.05ꢁ (Fig. 3B and C), respectively,
Fig. 4 Cyclic voltammograms of 1,7-DBrPBI (A) and 1,7-DTPEPBI (B)
in dichloromethane containing 0.1 M Bu4NPF6.
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difficult to form well-defined nanostructures due to the distorting
p–p stacking.14 TPE is a bulky substituent and has a propeller-
like shape, thus the attachment of TPEs at the bay area of a PBI
core may heavily distort the p–p stacking between the PBI units.
Consequently, it seems that ordered nano/microstructures
cannot be expected. To our surprise, however, we have found
that 1,7-DTPEPBI can easily form micro-fibers in the mixtures of
hexane/DCM, methanol/DCM and water/THF. The typical
morphologies are displayed by the SEM images in Fig. 5 and
S10–S12.† For example, in the mixture of methanol/DCM with
fm ¼ 70%, 1,7-DTPEPBI molecules assembled into fibers with
lengths of several tens of microns and diameters of 100–300
nanometres (Fig. 5A and the inset). Changing of the fm value
resulted in fibrils with similar morphologies (Fig. S10†). In the
mixed solvent of water/THF, 1,7-DTPEPBI molecules could also
assemble into micro-scale fibrils at different fw values; an
apparent difference can be found that the size of the micro-fibers
is smaller than that observed in the mixture of methanol/DCM at
the same volume fraction of non-solvent (Fig. 5B, the inset and
S11†). These observations imply that 1,7-DTPEPBI keeps the
intrinsic behaviour to form 1D microstructures. Thus it is
reasonable to conceive that better organized 1D microstructures
may be generated by adjusting the solvent or/and experiment
condition. Indeed, we obtained well-ordered fibrils in other non-
solvent/solvent mixtures (Fig. 5C, 5D and S11, S12†). As dis-
played in Fig. 5C, in the methanol/THF mixture with 90%
methanol (by volume), micro-tapes with length of 50–200
micrometres and width about 100 micrometres have been
formed. The inset demonstrates that microstructures have
smooth surfaces and regular edges, indicating better organized
1D microstructures have formed. In Fig. 5D, we can find that
fibrils with length of hundreds of micrometres with different
diameters have been generated in the methanol/dioxane mixture
with 80% methanol.
When slowly evaporating THF from the water/THF mixture,
the fw value increases gradually and 1,7-DTPEPBI molecules can
assemble into higher ordered microstructures. As shown by
Fig. 5E and the inset, fibrils with lengths of hundreds of micro-
metres and diameters of 100–200 nanometres are derived from
the water/THF mixture at fm ¼ 40%. In fact, needle- and prism-
like microstructures of 1,7-DTPEPBI can be obtained by slowly
evaporating the solvent from its THF or DCM solution
(Fig. 5F). The polarized optical microscopy (POM) images
demonstrate evident anisotropic reflections, indicating that the
fibrils and prism-like microstructures are crystals (Fig. 6A
and B). Moreover, thanks to the AIE behaviour of 1,7-
DTPEPBI, the microcrystals and micro-fibrils emit intense red
fluorescence upon excitation with green light, as shown by the
confocal fluorescence images in Fig. 6C, D and S13.† In contrast,
the POM observation for a powder sample of DBrPBI only
showed weak orange fluorescence during the confocal imaging
measurement (Fig. S14A and S14B†). In addition, both of the
Fig. 5 (A)–(F): SEM images of the morphologies of the aggregates
formed by 1,7-DTPEPBI molecules in different conditions. (A) In
methanol/DCM mixture with a methanol fraction (fm, by volume) of
70%. (B) In water/THF mixture with a water fraction (fw, by volume) of
60%. (C) In methanol/THF mixture with a methanol fraction (fm, by
volume) of 90%. (D) In methanol/dioxane mixture with a methanol
fraction (fm, by volume) of 80%. (E) Fibrils formed by slowly evaporation
of solvent from water/THF mixture with an fw of 40%. (F) Rod- and
prism-like microstructures formed by slow evaporation of 1,7-DTPEPBI
THF solution. The scale bar shown in all of the insets is 1 mm. In all
experiments, the concentration of 1,7-DTPEPBI is 10ꢀ5 M.
Fig.
6 Polarized optical microscopic images of the microcrystals
obtained from a methanol/DCM mixture (A), the fibrils formed by 1,7-
DTPEPBI molecules in a water/THF mixture with a fW of 40% (B). (C)
and (D) are the confocal fluorescence images corresponding to the
microstructures shown in (A) and (B), respectively. Images showing the
waveguide properties of the microcrystals (E) and fibrils (F), some
brighter spots are indicated by arrows. Excitation wavelength is 488 nm.
Amplification factor for all images (A), (B), (C) and (D) is 40.
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prism-like microcrystals and fibrils display brighter red light
spots at the tips in comparison with the emission from the side
(Fig. 6E, F, S15 and S16†). Meanwhile, the fluorescence contrast
between the tips and the middle exhibits a dependence on the
specific ratio of diameter and length of the microstructures;
a larger specific ratio corresponds to better performance
(Fig. S15A to S15D†). Besides, the waveguide performance relies
on the surface properties of the microcrystals, and the micro-
crystals with higher surface quality exhibit better guiding
behaviour (Fig. S16†). Moreover, the waveguide performance
also depends on the morphology of the microcrystals. The nee-
dle-like crystals with sharp ends or the powder samples did not
exhibit waveguide behaviour due to the non-uniform
morphology (Fig. S17†) which dissipates the absorbed optical
energy. These observations demonstrated primary characteristics
of waveguides, implying DTPEPBIs could be potentially used as
molecular materials for optical devices.15,16
immediately prior to use. All other solvents were of analytical
grade and were purified using standard methods.
Instrumentations
1H and 13C NMR spectra were measured on a Bruker AV 400 or
500 spectrometer in CDCl3. UV spectra were measured on
a Milton Roy Spectronic 3000 Array spectrophotometer. Fluo-
rescence spectra were recorded on a Perkin-Elmer LS 55 spec-
trofluorometer. FF values were estimated using Rhodamine B in
ethanol (FF ¼ 70%) as standard. The absorbance of the solutions
was kept between 0.04 and 0.06 to avoid the internal filter effect.
Elemental analysis was performed on a ThermoFinnigan Flash
EA1112. Scanning electron microscope (SEM) images were
taken on a JSM-5510 scanning electron microscopy. Fluores-
cence micrographs were taken on an inverted fluorescence
microscope (Nikon Eclipse TE2000-U). CHI-600 Electro-
chemical Workstation using 0.1 M n-Bu4NPF6 in dichloro-
methane as supporting electrolyte. The working and counter
electrodes were platinum wires, and the reference electrode was
Hg/HgCl2. The polarized optical images were observed with an
Olympus BX 60 polarized optical microscope (POM).
Concluding remarks
In summary, two novel isomeric PBI derivatives, i.e. 1,6-/1,7-
DTPEPBI, have been synthesized, clearly separated and char-
acterized. Both 1,6- and 1,7-DTPEPBI demonstrate evident AIE
behaviour. For 1,7-DTPEPBI, the fluorescence quantum effi-
ciency of the solid film (FF,solid ¼ 6.3%) is 90 times that of the
dilute solution (FF,solut ¼ 0.07%). The successful transition of the
PBIs from ACQ to AIE behaviour, together with our previous
achievements on other fluorogens such as pyrene, anthracene,
and naphthalene, indicates that decoration of ACQ fluorogens
with TPE moieties is an effective strategy to construct novel AIE
active materials. In the present work, linking two TPE moieties
to the 1,7-positions of the PBI core results in little change of the
electronic structure, thus 1,7-DTPEPBI exhibits electrochemical
behaviour of a typical n-type organic semiconductor. On the
other hand, such a decoration results in an evident red-shift of
fluorescence spectrum. For the solids of 1,7-DTPEPBI formed in
water/THF mixed solutions, the spectral shifts are both around
120 nm (from 546 to 668 nm), indicating that the two PBI
derivatives are red-emission materials. The evident red-shift is
partially ascribed to the planar configuration of the perylene
core. It is an interesting observation that the attachment of bulky
and twisted TPE moieties onto the PBI core has not interrupted
the formation of ordered assemblies and well-defined 1D
microstructures (fibers, wires and rods) have been obtained in
different non-solvent/solvent (water/THF, methanol/DCM,
hexane/DCM, methanol/dioxane) mixtures. These microstruc-
tures emit bright red fluorescence and demonstrate waveguide
activity. Based on the advantages arising from the combination
of classical PBI fluorogens and AIE active TPE moieties, we
expect the present strategy could provide a generic route towards
novel and advanced fluorescent materials and these materials
may find versatile applications in high-tech fields.
Synthesis of 1,6-dibromo-3,4:9,10-tetracarboxylic perylene
dianhydride and 1,7-dibromo-3,4:9,10-tetracarboxylic perylene
dianhydride
In a 250 mL three-necked round-bottom flask, perylene-3,4:9,10-
tetracarboxylic dianhydride (4.0 g, 10.3 mmol) was added, and
100 mL concentrated sulfuric acid (98%) was added, then the
mixture were stirred for18 h. Iodine (0.1 g,ꢁ0.4 mmol) was added
to the mixture for an additional 5 h at 55 C. Bromine (0.6 mL,
11.5 mmol) was added dropwise to the mixture over 0.5 h, and
stirred for 24 h at 85 ꢁC. After cooling to 40 ꢁC, the excess
bromine was eliminated by N2. Then the mixture was poured into
ice water, the precipitate was separated by filtration, and gave
compound 2 (5.4 g, 96%). The crude product was not further
purified for its poor solubility.
Synthesis of N,N0-dicyclohexyl-1,6/1,7-dibromo-3,4:9,10-
tetracarboxylic perylene bisimide (1,6-/1,7-DBrPBI)
In a 250 mL two-necked round-bottom flask, the mixture of 1,7-
and 1,6-dibromo-3,4:9,10-tetracarboxylic perylene dianhydride
(2.9 g, 5.2 mmol), and 90 mL NMP were added, and the mixture
were placed in a sonicator for 0.5 h. Then cyclohexylamine (2.4
mL, 20.8 mmol) and 10 mL acetic acid were added to the reaction
mixture. The mixture was stirred for 24 h at 85 ꢁC under N2
atmosphere. After cooling to room temperature, ice water was
added to the flask; then the mixture was poured into 600 mL
water. The resulting precipitate was separated by filtration. The
crude product was purified by a silica gel column using chloro-
form/petroleum ether (3 : 2 by volume) as eluent. A red solid was
1
Experimental
obtained in 62% yield (2.3 g). H NMR (400 MHz, CDCl3, d):
9.45 (d, 2H), 8.85 (s, 2H), 8.65 (d, 2H), 5.00 (m, 2H), 2.54 (m,
4H), 1.93 (m, 4H), 1.76 (m, 6H), 1.47 (m, 4H), 1.35 (m, 2H); 13
Chemicals and materials
C
All the chemicals used were purchased from Acros or Alfa, unless
specifically stated. Bromine was purchased from Alladin. THF
was distilled under nitrogen from sodium benzophenone ketyl
NMR (300 MHz, CDCl3, d): 163.5, 162.9, 138.1, 132.9, 132.8,
130.1, 129.4, 128.6, 127.2, 123.8, 123.5, 120.9, 54.5, 29.3, 26.7,
25.6.
7392 | J. Mater. Chem., 2012, 22, 7387–7394
This journal is ª The Royal Society of Chemistry 2012

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Synthesis of N,N0-dicyclohexyl-1,6-/-1,7-di(4-(1,2,2-triphenyl)
vinyl)phenyl-3,4:9,10-tetracarboxylic perylene bisimide (1,6-/
1,7-DTPEPBI)
Notes and references
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In a 250 mL two-necked round-bottom flask, the mixture of 1,6-
DBrPBI and 1,7-DBrPBI (0.5 g, 0.7 mmol), TPVPBC (0.79 g, 2.1
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pure 1,7-DTPEPBI.
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5.01–5.06 (m, 2H), 2.53–2.61 (m, 4H), 1.91–1.93 (m, 4H), 1.75–
1.77 (m, 6H), 1.49–1.52 (m, 4H), 1.37–1.42 (m, 2H); 13C NMR
(500 MHz, CDCl3, d): 163.9, 163.7, 144.5, 143.6, 143.4, 143.3,
142.0, 140.8, 140.1, 135.0, 134.5, 133.0, 132.2, 131.3, 129.9,
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122.7, 122.3, 53.9, 29.1, 26.5, 25.4. Anal. Calcd for
C88H66N2O4$2H2O: C 84.45, H 5.64, N 2.24; found: C 84.36, H
5.79, N 2.18.
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Characterization data of 1,6-DTPEPBI. A dark red solid was
obtained in 12% yield (0.098 g). 1H NMR (500 MHz, CDCl3, d):
8.50 (s, 2H), 8.17 (d, 2H), 7.78 (d, 2H), 7.06–7.31 (m, 38H), 5.02–
5.06 (m, 2H), 2.53–2.61 (m, 4H), 1.89–1.94 (m, 4H), 1.76 (m, 6H),
1.45–1.51 (m, 4H), 1.34–4.38 (m, 2H); 13C NMR (500 MHz,
CDCl3, d): 164.2, 164.1, 144.6, 143.8, 143.7, 142.2, 140.8, 140.3,
135.3, 134.3, 133.4, 132.9, 131.6, 129.9, 129.6, 128.4, 128.3, 128.1,
127.9, 127.2, 126.9, 122.8, 122.7, 54.3, 54.1, 29.9, 29.4, 26.8, 25.7.
Anal. Calcd for C88H66N2O4$2H2O: C 84.45, H 5.64, N 2.24;
found: C 84.26, H 5.91, N 1.97.
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Acknowledgements
This work was supported from the National Science Founda-
tion of China (21074113); the key project of the Ministry of
Sci. & Technol. of China (2009CB623605), the Science Foun-
dation of Zhejiang Province (Z4110056), the Research Grants
Council of Hong Kong (603509, HKUST2/CRF/10, and
604711), the SRFI grant of HKUST (SRFI11SC03PG), the
NSFC/RGC grant (N_HKUST620/11), the University Grants
Committee of Hong Kong (AoE/P-03/08). B.Z.T. thanks the
support from the Cao Guangbiao Foundation of Zhejiang
University.
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7394 | J. Mater. Chem., 2012, 22, 7387–7394
This journal is ª The Royal Society of Chemistry 2012