The fluorescence properties and aggregation behavior of tetraphenylethene-perylenebisimide dyads

Article abstract of DOI:10.1039/c4tc02550d

Two tetraphenylethene (TPE) modified perylenebisimides (PBIs) were synthesized through linking the TPE moieties to the PBI core at the imide positions. Theoretical calculations predict that, in both of the mono-TPE and di-TPE substituted derivatives (i.e.

Full text of DOI:10.1039/c4tc02550d

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DOI: 10.1039/C4TC02550D
ARTICLE TYPE
Fluorescence Property and Aggregation Behavior of Tetraphenylethene-
Perylenebisimide Dyads
a
a
a
a
a,b
a,*
a,b,c,*
Yi Jia Wang, Zeyu Li, Jiaqi Tong, Xiao Yuan Shen, Anjun Qin , Jing Zhi Sun, Ben Zhong Tang
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Two tetraphenylethene (TPE) modified perylenebisimides (PBIs) were synthesized through linking the
TPE moieties to PBI core at the imide positions. Theoretical calculations predict that, in both of the
mono-TPE and di-TPE substituted derivatives (i. e. TPE-N-PBI and DTPE-N-PBI), the TPE and PBI units
are orthogonal to each other, thus have no electronic conjugation. Thus the two compounds are TPE-PBI
dyads rather than TPE-PBI conjugates. This property is supported by absorption and emission spectral
features. In solution, aggregate and solid film, the fluorescence from both of TPE and PBI subunits are
evidently quenched. The underlying mechanism is the photo-induced charge transfer between the electron
donor TPE and electron acceptor PBI subunits. In high polar tetrahydrofuran/water mixture with large
fraction of water, TPE-N-PBI can form H-aggregate. In low polar hydrophobic dichloromethane/hexane
mixtures, due to the bulky size and rigidity of the TPE subunit, the dyad molecules cannot take a parallel
alignment to form classical J- or H- aggregates, but have to exist a large offset angle. Consequently, X-
aggregate is formed, which were confirmed by the absorption features, morphological observations and
comparative investigation on reference compound.
1
0
1
5
We associated the emission enhancement of the TPE-PBI dyads
in solid state with the TPE units. TPE is a unique species of
luminogen possessing aggregation-induced emission (AIE)
instinct. AIE is the phenomenological description of a special
emission property, for which the dyes are weakly or non-emissive
1
. Introduction
2
2
3
3
4
4
0
5
0
5
0
5
Perylenebisimide (PBI), a kind of organic dye with high optical
and chemical stability, has attracted great research interest in the
fields of n-type channel of organic field effect transistors,
5
5
6
6
0
5
0
5
[
1]
[9]
in dilute solution but highly emissive in aggregate/solid states.
Direct attachment of TPE units at 1,6- or 1,7-positions of PBI
[
2]
[3]
synthetic light harvesting systems and bio-/chemo-sensors.
Different structures and functions have been achieved by
chemical modification with proper substitution at the imide
positions and/or bay area of the PBI core. For example,
modification at the imide positions by attaching perfluorocarbon
chains afforded the PBI derivatives with high field effect charge
mobility and stability, and the fabricated FETs could work in
ambient condition. By direct linking to conjugated units such as
thiophenes and benzothiazoles at the bay positions, PBI-based
dyes or conjugated polymers were derived, and the resultants
showed improved performance in organic photovoltaic cells.
Highly fluorescent liquid crystals were obtained by attaching
mesogens and bulky substituents at the imide and bay positions of
PBI core, respectively. The high emission efficiency of these
liquid crystals benefited from the formation of J-aggregates in the
solid state.
In addition to J-aggregation, the PBI-based dyads derived from
chemical modification with tetraphenylethene (TPE) units at bay
area of the PBI core demonstrated high fluorescence efficiency in
core could turn the emission behavior of the PBI dyes from ACQ
[7a,c]
to AIE.
Linking TPE units to the PBI core via oxygen atoms
[7b]
at 1,6- or 1,7-positions showed similar effect.
Considering that the chemical modification at imide positions
is a widely used strategy to alter the solubility, molecular packing,
self-assembling fashions, and photo/electronic properties, it is of
scientific interest to examine the effects of the modification of
PBI core with TPE units at the imide positions on the self-
assembling behaviours and photophysical properties. To this end,
we synthesized two TPE-PBI dyads, i.e. TPE-N-PBI and DTPE-
N-PBI. As shown in Scheme 1, TPE-N-PBI is composed of one
TPE and one PBI units; while DTPE-N-PBI contains two TPE
and one PBI units. The two building blocks are directly linked by
using the N-atoms as joints.
[
4]
[
5]
[
6]
2
. Experimental section
2.1 Materials and reagents.
[
7]
solid states. This observation is quite distinct from conventional 70 3,4,9,10-Perylenetetracarboxylic dianhydride was purchased from
PBI-based dyes, which are highly fluorescent in dilute solution
but weakly emissive in aggregated states. This phenomenon
refers to as aggregation-caused quenching (ACQ), which is due to
Accelerating Scientific and Industrial Development thereby
Serving Humanity. 2-Ethylhexylamine and 4-aminobenzophen-
one were purchased from Alfa Aesar and used as received
without further purification. Other reagents including potassium
[8]
the strong intermolecular interactions in molecular aggregates.
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Journal of Materials Chemistry C
Page 2 of 10
hydroxide, n-propanol, zinc acetate, imidazole, quinoline, benzo-
2.4. Theoretical Calculation.
phenone, zinc dust, titanium tetrachloride, pyridine, potassium
carbonate, magnesium sulfate, ethyl acetate and dichloromethane
All calculations on the considered molecules were performed
View Article Online
by using the Gaussian 09 program package. B3lYP/6-31g (d) was
DOI: 10.1039/C4TC02550D
(DCM) were purchased from Sinopharm Chemical Reagent Co.,
employed to do the single point energy calculation based on the
5
0
5
0
5
0
5
0
5
Ltd. Tetrahydrofuran (THF) was distilled under normal pressure 55 crystal structure. The relative energies of the HOMO and LUMO
from sodium benzophenone ketyl under nitrogen immediately
prior to use.
levels were obtained from the computed results.
The quenching factor Φ is calculated by using the equation of:
q
2
.2 Instrumentations.
q
Φ = (Φsolut - Φaggre)/Φsolut
Where Φsolut and Φaggre are the emission quantum efficiency of the
1
13
H and C NMR spectra were measured on a Mercury plus 300
1
1
2
2
3
3
4
4
MHz NMR spectrometer in CDCl or d-DMSO with tetramethyl- 60 dye in dilute solution and in aggregation state, respectively.
3
silane (TMS; δ = 0 ppm) as internal standard. FT-IR spectra were
recorded on a Bruker Vector 22 spectrometer as thin films on
KBr pellets. High-resolution mass spectra (HRMS) were taken on
a GCT premier CAB048 mass spectrometer operating in a
MALDI-TOF mode. UV absorption spectra were taken on a
Varian CARY 100 Bio spectrophotometer. Fluorescence (FL)
spectra were recorded on a spectrofluorophotometer (RF−5301PC,
SHIMADZU, Japan). Scanning electron microscope (SEM)
images were taken on a JSM-5510 (JEOL, Japan) scanning
electron microscope. Fluorescent images were taken with a Zeiss
Axiovert 200 inverted microscope equipped with a 100 × oil
immersion objective with a numerical aperture of 1.4 and an Ebq
3
. Results and discussion
3.1. Aggregation models and their absorption and emission
features
A PBI core can be viewed as an elliptical (discotic) chromophore.
For this kind of chromophores, as illustrated in Fig. 1, their
aggregation modes can be largely classified into four normal
types. Based on this model, Types I and II exemplify two well-
known close-packing modes, i.e. H- and J-aggregates, which can
be determined when the angle θ between the chromophore’s
plane and the packing direction is more or less than a theoretical
6
7
7
8
8
5
0
5
0
5
o
[11]
value of 54.7 , respectively.
theory,
According to exciton-coupling
[
12]
the absorption maximum splits into a strong blue-
100 Isolated electronic ballast for mercury vapor compressed-arc
shifted peak and a relative weak red-shifted peak, and both peaks
have reduced absorbance coefficient. While the fluorescence
spectrum exhibits pronounced emission quenching and red-
shifted peak. Type II or J-aggregate exhibits evidently red-shifted
absorption spectrum and sharpened absorption peak. Meanwhile,
its emission spectrum exhibits enhanced intensity and red-shifted
peak. Type III is a special kind of J-aggregate, where the adjacent
chromophores arrange with an offset angle (ϕ) between the long
molecular axis. In this case, one dimensional column aggregation
is expected to be formed. Type III aggregate displays the typical
absorption and emission features of J-aggregate. Type IV is
referred as to X-aggregate, where the angle ϕ is large enough to
make transition dipole of the adjacent chromophores almost
lamps. The fluorescence quantum efficiency (Φ ) of the samples
F
F
were estimated using fluorescein in ethanol (Φ = 70%) as
standard. The absorbance of the solutions was kept between 0.04
and 0.06 to avoid the internal filter effect. Φ of the solid films
F
were measured using integrating-sphere photometer.
2.3 Materials synthesis.
The synthetic routes towards the target compounds of M1, TPE-
N-PBI and DTPE-N-PBI are shown in Scheme 1 and the detailed
experimental procedures are described in the Electronic
Supplementary Information (ESI). The reference compound M1
was obtained by condensation reaction of the semi-hydrolyzed
3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and 2-
2
ethylhexylamine in H O-PrOH mixture at room temperature for 4
h and at 90 °C for another 2 h. The mono amino-functionalized
[
13]
orthogonal to each other. In this case, intermolecular electronic
coupling is too weak to be obviously monitored. Consequently,
both the absorption and emission spectra of the type IV
aggregates display similar features to that of the isolated
tetraphenylethene (M2) was synthesized according to the
[
10]
procedures reported elsewhere.
By using zinc acetate as
catalyst, condensation of M1 and M2 in imidazole solvent at
o
90 chromophore.
elevated temperature (180 C) afforded TPE-N-PBI in a yield of
2
4.1%. Reaction of PTCDA and M2 at higher temperature (215
o
2
C) in quinoline with the presence of ZnAc resulted in DTPE-N-
PBI. The yield was 44.2%. All of the compounds have been
characterized with multiple spectroscopic technologies including
1
13
H NMR, C NMR, FTIR, HRMS and the analytical data are
satisfactory (see Figures S1 to S9 and the text in ESI for details).
O
O
O
O
O
O
O
O
O
O
NH
2
O
O
O
O
N
O
Figure 1. Illustration of aggregation modes of discotic chromophores.
KOH
HAc
OH
OK
There are three key geometric parameters, d = the distance between two
adjacent chromophores, θ = the tilted angle between the packing
direction and the plane of the chromophore, and ϕ = the offset angle
between the long molecular axes of the adjacent chromophores. The
Cartesian coordinate system is shown to help understanding.
H2O, PrOH
O
PTCDA
1
M1
9
5
ZnAc
2
ZnAc
imidazole
2
quinoline
2
15
o
C
180 C
o
NH
2
NH
2
M2
M2
O
N
O
O
N
O
O
N
O
O
N
O
Organic chromophores can also form random aggregation, in
which the chromophores are disorderly packed together. In this
00 case, no long-range order can be achieved, but short-range order
may exist in a certain degree. Thus, broadened and weakened
absorption and emission features could be expected. There is also
1
DTPE-N-PBI
TPE-N-PBI
Scheme 1. Synthetic routes to tetraphenylethene-perylene bisimide dyads
TPE-N-PBI and DTPE-N-PBI and the reference compound M1.
5
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Page 3 of 10
Journal of Materials Chemistry C
a rare case that was observed for the PBI derivative with
PBI-dyes with substituents at bay positions. Due to steric
perfluorophenyl modifiers at the imide positions. The electron- 50 repulsion, the bulky substituents at bay positions lead to core-
View Article Online
DOI: 10.1039/C4TC02550D
substitutions only at the imide positions, the line-shape of the
spectra for the bay-substituted dyes are broader and display less
vibronic structures due to the loss of planarity of the perylene
withdrawing perfluorophenyl groups localized between the
twisted PBI-dyes. In comparison with the spectra of PBIs with
[
14]
perylene cores thus isolated the π–π stacking.
This kind of
5
aggregation can be ruled out here because the TPE groups cannot
play the role as the perfluorophenyl groups do. All of the above-
mentioned aggregates have their own spectral and morphological 55 core and the lower molecular symmetry caused by the bay
[
6a, 15, 16]
features. Our discussion will rely on the knowledge of the
correlations between the aggregation types and the corresponding
spectral and morphological features.
substituents.
Recently, we found that the substitutions of
TPE units at bay (1,6- or 1,7-) positions of PBI core resulted in
the broadening of the absorption feature and the symmetrically
forbidden S0–S2 transition band was evidently enhanced due to
the core-twisting induced symmetry broken.
The fluorescence spectrum of TPE-N-PBI in DCM solution is
shown in Fig. 2B, which is approximately the mirror image of its
S0–S1 absorption bands. The emission peak appears at 539 nm
with a pronounced shoulder at 575 nm. In dilute DCM solution,
M1 shows similar emission spectrum, with a peak and a shoulder
at 535 and 572 nm, respectively (Fig. 2B). Such spectral features
are typical for PBI derivatives with modification at the imide
positions. Despite for the similarity of spectral features, the
1
0
3.2. Electronic interaction between PBI and TPE units in
6
6
7
0
5
0
isolated molecules
There are voluminous literature relating to substituted PBIs, and
the substituents attached to the imide positions are mainly alkyls
or alkyl/alkyloxyl modified phenyls, which have been used to
improve the solubility and to tune the molecular packing. In
single molecular level, these substitutions only have trivial
influence on the electronic structure of the PBI core. Here, TPE is
a conjugated unit and it has unique AIE property. TPE-N-PBI and
DTPE-N-PBI can be considered as novel TPE-PBI dyads. It is
important to examine whether the attachment of TPE at the imide
positions of the PBI core has any electronic interactions with PBI
core or not.
1
5
2
0
emission efficiency (Φ ) of TPE-N-PBI and M1 in DCM solution
is dramatically different. When illuminated with UV light, TPE-
N-PBI and M1 emit dim and bright yellow-green light,
respectively. By using fluorescein in ethanol (Φ = 70%) as the
F
F
A)
B)
calibration, the Φ
calculated to be 0.32% and 57.86% respectively, possessing a 180
75 times difference (ref. Fig. 4).
In accordance with our previous explanations to the
fluorescence quenching of TPE-substituted PBIs at bay areas, the
drastically reduced ΦF of TPE-N-PBI can be tentatively
associated with the intramolecular rotations of the phenyl groups
of the TPE-unit, which largely exhaust the energy of the excited
state. Another possible mechanism for the fluorescence
quenching of TPE-N-PBI in dilute DCM solution is the process of
energy transfer from TPE to PBI, which is frequently observed in
the dyads constructed by two units having well-matched
absorption and emission bands. As revealed by the absorption
features, TPE and PBI units are individual parts and have only
trivial electronic interaction in ground state. In photo-excited
state, TPE emits greenish-blue fluorescence with emission peak
at around 470-480 nm. This band falls into the S0–S1 transition
absorption band of PBI, thus the fluorescent energy transfer from
F
of TPE-N-PBI and M1 in DCM solution is
5
21nm
527nm
535nm
539nm
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
M1
M1
TPE-N-PBI
DTPE-N-PBI
TPE-N-PBI
DTPE-N-PBI
8
8
9
0
5
0
300
400
500
600
500
550
600
Wavelength (nm)
650
700
Wavelength (nm)
2
5
Figure 2. Absorption and fluorescence (FL) spectra of TPE-modified
perylenebisimde dyads (TPE-N-PBI and DTPE-N-PBI) and the reference
compound M1. Solvent: dichloromethane (DCM); Concentration of
-5
solutes: 10 M; Excitation wavelength: 480 nm.
To eliminate the latent interaction between aromatic solvent
(e.g. toluene) and PBI/TPE unit, a hydrophobic (dichloromethane,
DCM) and a hydrophilic (tetrahydrofuran, THF) solvents were
used in the experiments. The UV-vis absorption spectra of TPE-
N-PBI and DTPE-N-PBI dyes in their dilute DCM (10 M)
solution were measured and the results are displayed in Fig. 2.
For comparison, also shown in Fig. 2 is the UV-visible spectrum
3
0
-5
3
4
4
5
0
5
-5
of M1 in DCM solution (10 M). All three perylene-based dyes
display very similar absorption features: the absorption bands
cover between 400 and 550 nm, which are assigned to the S0–S1
transition of the PBI core with absorption maximum (λabs) and
series of well-resolved vibronic structures that can be ascribed to
the breathing vibration of the perylene skeleton. For both TPE-
N-PBI and DTPE-N-PBI, the λmax appears at 527 nm, while for
M1 it appears at 521 nm. These molecules showed very similar
absorption features in dilute THF solution (Fig. 4A and 5A). The
spectral features observed above indicate that the TPE-
substitution really has little influence on the electronic structure
of the PBI core.
[
15]
Figure 3. Diagram of the front orbitals energy level of TPE-N-PBI,
DTPE-N-PBI and M1. HOMO and LUMO stand for the highest occupied
molecular orbital and the lowest unoccupied molecular orbital.
The above results are quite different from that observed for the
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

Journal of Materials Chemistry C
Page 4 of 10
TPE to PBI units is theoretically allowable. In fact, however, in
donor at the imide position has no interference with the PBI core.
dilute solution, the intramolecular rotations in TPE unit are active, 60 The CV data of DTPE-N-PBI are unavailable due to its poor
View Article Online
DOI: 10.1039/C4TC02550D
thereby TPE unit is non-emissive. Consequently, the energy
transfer process is an unimportant factor to the fluorescence
quenching of TPE-N-PBI dyad. Moreover, when the TPE-N-PBI
in dilute DCM solution was directly photo-excited by 480 nm
light source, the solution was still faintly emissive. Therefore, the
fluorescence quenching phenomenon cannot be addressed by the
energy transfer mechanism.
solubility in DCM and other organic solvents. The CV data
suggest that, at least for the TPE-N-PBI dyad, the electron
transfer process is one of the reasonable mechanisms of the
observed fluorescence quenching in DCM solution.
All of the results, including the absorption and emission
spectra, CV test, and theoretical calculation indicate that TPE and
PBI units are individual components in the molecules of TPE-N-
PBI and DTPE-N-PBI, although the TPE units are directly linked
to PBI core at the imide positions. At ground state, there is no
5
0
5
0
5
0
5
0
5
0
5
65
1
1
2
2
3
3
4
4
5
5
Considering that PBI unit is a well-known electron acceptor (A)
and TPE unit is an electron donor (D), electron transfer from TPE
to PBI units at photo-excited state is a reasonable mechanism for 70 electronic coupling between TPE and PBI units, but at excited
the fluorescence quenching of TPE-N-PBI in dilute DCM
solution. To verify this mechanism, we carried out theoretical
calculations to TPE-N-PBI as well as its reference compounds
DTPE-N-PBI and M1, and the data are shown in Fig. 3. It is
state the electron transfer from TPE to PBI unit is an
energetically favourable process. Both active intramolecular
rotation and electron transfer are the possible mechanisms to
explain the drastic fluorescence quenching of TPE-N-PBI. More
found in the geometry of TPE-N-PBI that the plane of the phenyl 75 proofs are wanted to draw the final conclusion.
group on TPE unit, which is directly linked to the PBI core via
3
.3. Absorption and emission of the dyads in DCM/hexane
the N-atom, is nearly perpendicular to the plane of PBI core. As
to the molecular orbital, TPE units have no electronic coupling
with the PBI core. These results provide theoretical support to the
experimental observations in absorption spectra.
and THF/water mixtures
In view of molecular orbital, TPE-N-PBI molecule takes the
HOMO and LUMO orbitals from the HOMO of TPE and LUMO
of PBI units, respectively. For DTPE-N-PBI, the HOMO is
composed of the two degenerate HOMOs (on the same energy
level) of the two TPE units. Identically to TPE-N-PBI, the LUMO
of DTPE-N-PBI is the LUMO of PBI unit. The direct S0–S1
transition from the HOMO of TPE to the LUMO of PBI is a
forbidden process in quantum mechanism, although calculation
results assigned the HOMO and LUMO for TPE-N-PBI and
DTPE-N-PBI. The vertical transitions from their own HOMOs to
LUMOs are permitted processes. In fact, from 200 to 700 nm,
PBI and TPE show their corresponding absorption bands with
absorption maximum at 315 and 527 nm, respectively (Fig. 2A).
The experimentally recorded absorption maxima are identical to
that of TPE and PBI units, indicating that TPE and PBI separately
play their absorption role but without electronic cross-talking. As
the TPE unit is photo-excited, the electron on the LUMO of the
TPE unit can transfer to the LUMO of PBI unit due to the energy
level alignment. Based on the same principle, as the PBI unit is
photo-excited, the electron on the HOMO of the TPE unit can
transfer to the unoccupied HOMO of PBI unit (or hole on the
HOMO of PBI transfer to the HOMO of TPE unit).
We explored further proofs by examining the electronic
structure of TPE-N-PBI and M1. Cyclic voltammetry (CV)
technique was used to determine the energy level of the
molecules. It can be found that TPE-N-PBI shows a couple of
reversible oxidation/reduction peaks under the negative bias. The
reduction potential of TPE-N-PBI is approximate to most of the
N-term substituted PBIs. This observation implies that the TPE
substitution at the imide position leads little variation to the
oxidation/reduction behaviors of PBI unit. By comparing the CV
curve of TPE-N-PBI with that of M1, an excess oxidation peak an
irreversible oxidation peak at around +0.85 V under the positive
bias is observed. This signal is obviously associated with the TPE
unit. Its appearance at positive bias indicates the electron donor
character of TPE unit. The CV data reveal that linking a TPE
8
0
Figure 4. (A) Absorption and (B) FL spectra of M1 in DCM/hexane
mixtures with different hexane fractions (f ). (C) The changes of the FL
H
efficiency (Φ
F
) and peak wavelength (λem) with f
H
. (D) A typical image of
of 90%.
the microstructure of M1 in the DCM/hexane mixture with f
H
As discussed in Section 3.1, different aggregation states have
different spectral features, thus the aggregation behaviors of the
PBI-TPE dyads can be investigated by monitoring the changes of
the UV-vis spectral features in solution at different concentrations.
Herein, we investigate the aggregation behaviors of TPE-N-PBI
and DTPE-N-PBI dyads in the mixtures of two types of
solvent/non-solvent mixtures. DCM and THF were used as
solvents while hexane and water as non-solvents, which represent
a hydrophobic and a hydrophilic system, respectively. At low
concentration, aggregates can form when the fraction of non-
solvent is high enough due to poor solubility of the dyes in the
solvent and non-solvent mixture. This method is frequently used
in monitoring the aggregate formation and fluorescence turn-on
processes of AIE active molecules. For M1, in DCM/hexane
system, the absorption spectra keep unchanged in line-shape
8
9
9
5
0
5
when hexane fraction (f ) changes from 0 to 70%. But at higher
H
4
| Journal Name, [year], [vol], 00–00
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Page 5 of 10
Journal of Materials Chemistry C
H
shifted in comparison with the absorption peak in lower f .
Meanwhile, the main blue-shifted absorption peak appears at
View Article Online
around 470 nm. Thirdly, the spectra greatly lost the fine structure
DOI: 10.1039/C4TC02550D
3
3
4
4
5
0
5
0
5
0
at higher f values. The absorption spectra of the aggregated dyes
H
agreed very well to those documents for the perylene bisimide
pigments with alkyl substitutions at N-terms. For example, Graser
and Hädicke reported similar absorption spectra of PBIs in the
[
18]
crystalline state.
These observations indicate the existence of
strong intermolecular π−π interactions, and the formation of H-
aggregate is expected. The observation of ordered micro-fibers is
considered as a proof (Fig. 5D).
For TPE-N-PBI in DCM/hexane mixtures, the absorption
spectra are identical in line-shape and the absorption peak has no
evident red- or blue-shift before fH increases up to 90%. In
general, the strong π–π stacking is the driving force of assembling
process. The absence of the spectral signs of obvious blue- or red-
shifted peaks and decreased coefficient in the absorption spectra
implies that no strong π–π interaction exists between TPE-N-PBI
molecules in the high fH mixtures. Thus aggregation types I, II
and III can be excluded from the possible aggregation states.
Type IV is a possible aggregation state. Up to now, we can only
draw a primary conclusion that that the modification with TPE at
the imide positions of the PBI core has significant impacts on the
aggregation behaviors of the TPE-modified PBI dyes.
Figure 5. (A) Absorption and (B) FL spectra of M1 in THF/water
mixtures with different water fractions (f ). (C) The changes Φ
with f . (D) A typical image of the microstructure formed by M1 in the
THF/water mixture with f of 90%.
W
F
and λem
W
5
W
H
f values (80 ∼ 90%), the absorption feature undergoes dramatic
changes. There appears a new peak with λmax at around 575 nm,
which is prominently red-shifted in comparison with the
absorption peak in lower f . Meanwhile, the apparent absorption
H
1
1
2
0
5
0
coefficient decreases evidently. These two signs indicate the
strong electronic coupling between the perylene cores. Based on
the understanding of the aggregation behavior of PBI-derivatives,
the prominent red-shift is assigned to the formation of type I or
[
15-17]
face-to-face stacking aggregate.
Similar behaviors have been observed for M1 in THF/water
mixtures, but the details have some differences (Fig. 5). Firstly,
the threshold of dramatic absorption feature variation appears at
the water fraction (f ) of 60 %, which is lower than that observed
W
H
for M1 in DCM/hexane mixture (f = 70 %, Fig. 4). Secondly, at
higher fW values (80% ∼ 90%), there appears a pronounced
shoulder peaked at around 555 nm, which is prominently red-
Figure 7. (A) Absorption and (B) FL spectra of TPE-N-PBI in THF/water
mixtures with different f
D) A typical image of the microstructure formed by TPE-N-PBI
molecules in the THF/water mixture with f of 90%.
W
values. (C) The changes of Φ
F
w
and λem with f .
(
5
5
W
In THF/water mixtures, the variation of the absorption features
with f (Fig. 7A) looks like that observed for M1 in THF/water
w
mixtures (Fig. 5A). The maximal absorption peak shows evident
blue-shift (523 nm to 470 nm) and a red-shifted (552 nm) when
fW is higher than 60%. As discussed above, such spectral line-
shape indicates the presence of strong inter-chromophore π–π
stacking and H-aggregate or Type-I aggregation is suggested to
form. Up to now, we can only draw a primary conclusion that that
the modification with TPE at the imide positions of the PBI core
has significant impacts on the aggregation behaviors of the TPE-
modified PBI dyes.
6
0
Figure 6. (A) Absorption and (B) FL spectra of TPE-N-PBI in
6
5
DCM/hexane mixtures with different f
and λem with f . (D) A typical image of the microstructure formed by
TPE-N-PBI molecules in the DCM/hexane mixture with f of 90%.
H F
values. (C) The changes of Φ
2
5
H
To get further understanding of the emission and aggregation
H
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

Journal of Materials Chemistry C
Page 6 of 10
properties of TPE-N-PBI dyad, we measured the fluorescence
spectra of TPE-N-PBI and its reference M1 in DCM/hexane and 60 luminogen in THF/water solvent system. The Φ
THF/water mixtures with different f and f values. As shown in be as high as 99.73%.
and 90%, respectively, indicating that M1 is a typical ACQ
q
is calculated to
View Article Online
H
W
DOI: 10.1039/C4TC02550D
Fig. 4B, with the addition of hexane, the fluorescence spectra of
M1 show a monotonous and short blue-shift. When f changes
from 0 to 60%, emission maximum (λem) undergoes blue-shift
from 535 to 528 nm (Δλem = 7 nm) and Φ increases from 57.86% 65 to be 0.32% and 2.65%, respectively. But in the hydrophobic
to 67.89% (ΔΦ = 10.03%). At higher f of 80% and 90%, Φ DCM/hexane mixture, the Φ increases with the addition of nono-
polar hexane component; while in hydrophilic THF/water
mixture, the Φ decreases with the addition of high polar water
component. The oppositely shifted λem can be also found in these
face aggregate (Type-I) through strong π–π stacking, which 70 two systems. The changes of Φ are rationally associated with the
When we compare the emission behaviours of TPE-N-PBI in
DCM/hexane and THF/water mixtures, it can be found that the
dyad is weakly fluorescent in both systems, and Φ is measured
F
5
0
5
0
5
0
5
0
5
0
5
H
F
F
H
F
F
decreases quickly from 51.78% to 36.91%, respectively.
Correlating these data with the absorption spectral data, it is
found that, at f of 80% and 90%, M1 molecules form face-to-
H
1
1
2
2
3
3
4
4
5
5
F
F
induced the fluorescence quenching. In solid film, M1 has a low
variation of the environmental polarity and the shift of λem can be
Φ = 0.9%. Consequently, M1 is a typical fluorescent dye with
F
explained by the solvatochromic effect.
aggregation-caused quenching (ACQ) property and the quenching
q
factor (Φ ) is calculated to be 36.3%. The observations of the
spectral blue-shift and the small intensity enhancement in f
H
range from 0 to 60% can be ascribed to the hydrophobic effect of
the mixture solvent. With the increase of f , the dye molecules in
H
the solvent localize in lower polar environment. According to the
[
19]
solvatochromic mechanism,
a polar chromophore exhibits
shorter emission wavelength and higher emission efficiency in
more hydrophobic solvents. M1 is a low polar chromophore
because of its symmetric conjugated skeleton.
The f related fluorescence spectra of TPE-N-PBI are shown in
H
Fig. 6B. The λem shows monotonous blue-shift from 539 to 529
nm (Δλem = 10 nm) when f increases from 0 to 90%. At the same
H
time, the fluorescence intensity gradually increases, and Φ
F
increases from 0.32% to 0.55%, equivalent to 72% emission
enhancement. We ascribe the spectral blue-shift and the intensity
enhancement to the weak solvatochromic effect in the mixture
solvent, as described for the reference compound. A significant
difference from the reference compound M1 is that the emission
intensity of TPE-N-PBI continues increasing when fH increases
from 80% to 90%; whereas M1 begins to display its ACQ
behaviour.
Figure 8. Absorption (A) and FL (B) spectra of DTPE-N-PBI in
7
5
DCM/hexane mixtures with different f
with f . (D) A typical image of the microstructure formed by DTPE-N-
PBI in the DCM/hexane mixture with f of 90%.
H F
. (C) The changes of Φ and λem
H
H
The emission behaviors of TPE-N-PBI in DCM/hexane
mixtures with high fH values imply that there is no strong
electronic coupling or π–π interaction between the adjacent PBI
The solubility of DTPE-N-PBI in THF is lower than 5 mg/L (<
10 μM), which is much less than in DCM. It is insoluble in water.
cores. A simple linear extrapolation of the absorption and 80 As a result, we can only investigate its absorption and emission
emission behaviours of the reference compound M1 can lead to a
conclusion that no aggregates of TPE-N-PBI molecules have been
behaviours in DCM/hexane mixtures. As shown in Fig. 8A, the
absorption spectra of this dyad show very small changes with the
formed in the DCM/hexane mixtures with high f values. But this
is not true because TPE-N-PBI has lower solubility in the
DCM/hexane mixtures with high fH values than M1. It is 85 PBI, but the enhancement of Φ
increase of f
emission behaviors of DTPE-N-PBI are also similar to TPE-N-
shows a little difference. As
of DTPE-N-PBI dyad increases from
.39% to 1.48% when f changes from 0 to 90%. Meanwhile, λ
blue-shifts from 539 to 523 nm (Δλem = -16 nm). The higher
fluorescence efficiency and larger intensity enhancement can be
associated with the double TPE units in the DTPE-N-PBI dyad.
The two TPE units strengthen the intramolecular D-A interaction,
thereby lead to stronger solvatochromic effect. The spectral
features of DTPE-N-PBI suggest that X-aggregate is a possible
aggregation mode.
H
from 0 to 90%, which is similar to TPE-N-PBI. The
H
F
acceptable that the bulky size and the rigidity of the TPE moieties
prevent TPE-N-PBI molecules from close packing in the
aggregates. As discussed for absorption spectra, close packing
modes of type I, II and III aggregations can be excluded in
DCM/hexane mixtures. But the X-aggregate, or type IV is a
possible being, because the weak intermolecular electronic
coupling can only lead to small spectral variations.
In THF/water mixtures, however, the emission of M1 shows
different trend from that in DCM/hexane system (comparison Fig.
4 and Fig. 5). Due to the high polarity of aqueous environment,
the emission spectra display a monotonous red-shift with addition
of water (Fig. 5B and 5C). Meanwhile, the quantum efficiency
displayed in Fig. 8C, the Φ
F
0
H
em
9
0
9
5
3.4. Morphology of the aggregates
So far, based on the absorption and emission spectra, we have
inferred that TPE-N-PBI and DTPE-N-PBI dyads may take type-
IV aggregation mode or X-aggregate in the low polar and
exhibits continuously dropping from 63.08% to 0.17% at f = 0
H
6
| Journal Name, [year], [vol], 00–00
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Page 7 of 10
Journal of Materials Chemistry C
hydrophobic DCM/hexane mixtures, and TPE-N-PBI takes type-I
polarized and tend to form H-aggregate that has a large θ and a
or H-aggregate in the high polar and hydrophilic THF/water 50 smaller ϕ (Fig. 9). The repulsion barrier is partially overcome by
View Article Online
DOI: 10.1039/C4TC02550D
mixtures. It is a common phenomenon that the H-aggregate of
PBIs tends to organize into ordered microstructures, because of
the strong π-π interaction between the perylene cores. For TPE-
PBI dyads, the bulky size and propeller shape of the TPE units
may hinder the parallel packing of adjacent PBI cores and there
should be an offset angle (ϕ) between the long molecular axis.
Thus, an important issue is: what are effects of TPE moieties
substituted at the imide positions on the morphology of TPE-PBI
dyads?
the favorite static electrical attraction. While in low polar
DCM/hexane mixture, TPE-N-PBI molecules are less polarized
and take a X-shape aggregation to avoid the steric repulsion
between the bulky TPE units.
For DTPE-N-PBI, the morphology of the aggregates is
characterized by brick-like microstructures (Fig. 8D), which is
distinct from those observed for TPE-N-PBI and M1. It seems
that, in the aggregate of DTPE-N-PBI, the molecular packing has
a three-dimensional characteristic. To get further information
5
0
5
0
5
0
55
1
1
2
2
3
To get direct information, we inspected the morphology of the 60 about the aggregate, high resolution transmittance electron
aggregates with scanning electron microscope (SEM) technique.
The typical images of the aggregates of the dyads TPE-N-PBI
formed in the DCM/hexane and THF/water mixtures are
displayed in Figures 6D and 7D, respectively. The non-solvent
microscope (HRTEM) measurement was carried out and the
lattice image is shown as Fig. 9. The structure-less image
indicates an amorphous character of the DTPE-N-PBI aggregate.
It seems that the DTPE-N-PBI molecules randomly pack together.
fraction of 90% for f and f was preferred owing to the evident 65 But this assumption is denied by the absorption spectra of DTPE-
H W
aggregate formation of the dyad in this condition. In the mixture
of DCM/hexane, bundles of microfibres are found in the field of
microscope. The length of the fibers extends to several tens of
N-PBI in DCM/hexane mixtures, which suggest very weak
intermolecular π-π interaction, because we have not recorded
broadened and weakened absorption features corresponding to
the existence of short-range order in random packing state.
micrometers. In THF/water mixture with f
W
90%, micro-fibers
can be also observed in the SEM image, but the average size is 70 Furthermore, the diffraction patterns recorded from several
smaller than that observed in DCM/hexane mixture. The size
difference is associated with the fact that the highly hydrophobic
dyad molecules have lower solubility in the polar hydrophilic
environment; thus the aggregation takes place so quickly that
selected sites of the aggregate display only dispersive halos
without any recognizable array (Fig. S11). Accordingly, we
propose a model as depicted in Fig. 9, the vertically adjacent
DTPE-N-PBI molecules form X-aggregate due to the bulky
aggregates cannot grow to larger size. The fibre-like morphology 75 volume of the two TPE units with a large off-set angle (ϕ). It has
is usually observed for PBI derivative, including the PBI
been reported that the C--H--π interaction between the phenyl
[
7]
derivatives with TPE moieties at the bay positions, which can
be correlated to the preferentially one-dimensional molecular
packing.
groups plays key role in the formation of crystal and other kinds
[
20]
of ordered structures for many TPE derivatives. But the C--H--
π interaction in not strong enough thereby cannot extend to long
range. Consequently, DTPE-N-PBI molecules cannot form large-
sized 3-dimensional ordered structures, and the superficial brick-
like structure (Fig. 8D) comes from random packing of the X-
aggregate (Fig. 9).
8
0
3.5. Mechanism of the fluorescence quenching
8
9
9
5
0
5
It is noted that the emission behaviours of these two TPE-PBI
dyads are quite different from that observed for PBI-derivatives
with TPE substitutions at bay area (1,6- or 1,7- positions), which
showed typical AIE behavior in DCM/hexane mixtures. The bay-
substitutions interrupted the symmetric and planar conformation
of the PBI core, and resulted in the absence of vibronic shoulder
in the emission spectrum. In addition, TPE-substituents at the bay
area are partially conjugated to the PBI core, thus resulted in
pronouncedly red-shifted fluorescence emission. At the same
time, their emission behaviors are different from the PBI-
derivatives with alkyl-/alkyloxyl phenyl substitutions at imide
positions, which show ACQ behavior or emission enhancement
depending on the chemical structure of the substituents. For TPE-
N-PBI and DTPE-N-PBI, the fluorescent emission is largely
quenched in solution and aggregates. Therefore, the modification
Figure 9. A schematic illustration of the aggregation of TPE-N-PBI and
DTPE-N-PBI molecules in different situations. The pictures are the SEM
images of TPE-N-PBI aggregates formed in THF/water (upper) and
DCM/hexane (middle) mixtures and the high resolution TEM image of
DTPE-N-PBI aggregate formed in DCM/hexane (bottom) mixtures.
3
5
The morphological observation implies that the π-π interaction
between the perylene cores is still a strong driving force to
organize the TPE-N-PBI molecules into ordered microstructures
like that observed for its reference M1 (Fig. 4D and 5D), despite
the bulky size and propeller shape of the TPE unit at the one N-
term. Correlating the morphological characters in image 6D and
4
0
1
00 of PBI with TPE units at the imide positions has nontrivial
influence on the optical properties of the resultants.
7
D with the absorption spectra in Fig. 6A and 7A, an obvious
We measured the fluorescence spectra of the solid films cast
from the solution of the dyads of TPE-N-PBI and DTPE-N-PBI.
The results demonstrate that both of them are weakly emissive in
05 solid film. Combing the fluorescent data of TPE-N-PBI and
DTPE-N-PBI together, it is found that these two TPE-PBI dyads
4
5
distinction is that the aggregation modes of TPE-N-PBI and
DTPE-N-PBI molecules in the mixtures with high fraction of
non-solvent are different. According to the solvation principle, in
high polar THF/water mixtures, TPE-N-PBI molecules are highly
1
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Journal Name, [year], [vol], 00–00 | 7

Journal of Materials Chemistry C
Page 8 of 10
are non- or faintly emissive in all cases including dilute solution,
electron and hole localize in the LUMO and HOMO of the
aggregates and solid films. Where has the fluorescence gone?
In the case of dilute solution, we have presented explanations
to the fluorescence quenching in Section 3.2. We tentatively
associated the observation with the intramolecular rotations of the
phenyl groups of the TPE-units, which deplete the energy of the
50 central PBI moiety, due to the symmetric requirement of
View Article Online
molecular orbital. The transition of the hot electron from the
DOI: 10.1039/C4TC02550D
LUMO of PBI to the HOMO of TPE is a forbidden process.
Meanwhile, the irradiative combination of the electron in the
LUMO with the hole left in HOMO is process with very small
5
0
5
0
5
excited state. Now, based on the fluorescent behaviors of the two 55 probability because the PICT (the hole transfer from PBI’s to
dyads in aggregation states, the mechanism of intramolecular
rotations can be discarded. Because in aggregation states, the
intramolecular rotations have been restricted, thus they cannot
play the decisive role of energy-exhausting channel again. But in
TPE’s HOMO) is a predominant process. Owing to the PICT, the
dyads of TPE-N-PBI and DTPE-N-PBI are non-emissive in both
solution and in solid/aggregate states.
1
1
2
2
The photo-induced intermolecular charge transfer process in
fact, the fluorescence of TPE-N-PBI and DTPE-N-PBI are still 60 PBI-based dyads was previously reported by Wasielewski and
[
2c, 2d, 22]
heavily quenched in the cases of aggregates and solid films. The
quantum efficiency of both TPE-N-PBI and DTPE-N-PBI solid
film is only 0.5%.
Formation of H-aggregate is another explanation to the
fluorescence quenching phenomenon. This mechanism is 65 quenched the fluorescence of PBI. When the porphyrin subunit
plausible in the case of TPE-N-PBI in THF/water mixtures but
fails in the explanation of the emission behaviours of TPE-N-PBI
and DTPE-N-PBI in DCM/hexane mixtures. Absorption and
emission spectral features, as well as SEM images provide
other groups.
For example, in a series of porphyrin-PBI
dyads, when the PBI subunit in a dyad was excited under UV-
light, it was proved that hole transport from the photo-excited
PBI to the neutral porphyrin subunit is a dominant process that
was excited, direct and efficient electron transfer from porphyrin
(donor) to PBI (acceptor) occurred and the fluorescence of the
porphyrin subunit was greatly quenched. In our present case, the
porphyrin is replaced by TPE, which is not an electron donor as
sufficient proofs of the formation of X-aggregates. Therefore, 70 strong as porphyrin, but the photo-induced charge transfer
another mechanism, or the photo-induced charge transfer is a
more convincing mechanism for the fluorescence quenching of
TPE-N-PBI and DTPE-N-PBI. The electronic process is depicted
in Fig. 10.
process is still accessible according to their electronic structures.
Fluorescence quenching in solution was also observed for PBI
derivative substituted with trialkoxyphenyl groups at the imide
[
16b, 18c]
positions.
Würthner and co-workers ascribed the observed
7
5
phenomenon to photo-induced electron transfer processes. Herein,
the TPE is not only an electron donor, but also a special
fluorogen that possesses AIE property. As a result, the dyads of
TPE-N-PBI and DTPE-N-PBI bring something new forward to
the PBI-based organic dyes.
8
0
Conclusions
We have demonstrated the effects of TPE-substitutions at the
imide positions of PBI core on the absorption, emission and
aggregation behaviors of the resultant TPE-PBI dyads. As a part
of the sequence works, the properties of the dyads of TPE-N-PBI
85 and DTPE-N-PBI are compared with their counterparts bearing
TPE-substitutions at the bay positions of the PBI core. Due to the
bulky size and rigidity of phenyl groups, TPE-N-PBI and DTPE-
N-PBI molecules form X-aggregate in DCNM/hexane mixtures.
Bearing a thinner alkyl chain, TPE-N-PBI molecules can form
classical H-aggregate in THF/water mixtures when fH is higher
than 70%. In general, the PBI cores in X-aggregate emit strong
fluorescence that is comparable to its solution, because of the
weak dipole-dipole interaction between the adjacent PBI units.
But it has been found that the both of TPE-N-PBI and DTPE-N-
PBI are weakly emissive in dilute solution, microscopic
aggregates and macroscopic solid films. This property is distinct
from the AIE-active counterparts that have TPE-substitutions at
the bay positions of PBI core. It is also distinct from the
counterparts that have alkyl-/alkyloxy-phenyl substitutions at the
00 imide positions, which normally show ACQ behaviour and
sometimes show efficient emission both in solution and in
aggregates. The “abnormal” fluorescent quenching behaviors are
ascribed to the photo-induced electron transfer between TPE (D)
and PBI (A) subunits, and this mechanism has been supported by
05 the overall experimental data and the theoretically calculated
Figure 10. Mechanism of the overall fluorescence quenching behaviour
of the TPE-PBI dyads with substitutions at imide positions. Processes A
and B are corresponding to excitation on TPE and PBI units, respectively.
3
0
When a TPE unit is photo-excited, the photogenerated electron
and hole localize in the LUMO and HOMO of TPE moiety. The
transition from the HOMO of TPE to the LUMO of PBI is a
forbidden process, though it is an energetically allowable process
(cf. Fig. 3). The electron in the LUMO cannot recombine with the
hole left in HOMO to give off fluorescence because the electron
transfer from TPE’s to PBI’s LUMO is a predominated process,
according to the ultrafast photoinduced charge transfer (PICT)
9
0
3
4
4
5
0
5
9
5
[
21]
mechanism.
Besides PICT, TPE is non-emissive in solution,
according to the RIR mechanism of AIE phenomenon.
Consequently, when TPE moiety is excited, the dyads are non-
fluorescent in solutions; while in aggregates and in solid films,
RIR process is achieved, the fluorescence quenching can be
ascribed predominately to the PICT process. AIE takes a weak
effect, which is reflected by the fact that the two dyads show
higher fluorescent efficiency in aggregate than in solution (Fig.
1
6
C and 8C).
When the PBI unit is photo-excited, the photogenerated
1
8
| Journal Name, [year], [vol], 00–00
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Page 9 of 10
Journal of Materials Chemistry C
6
5
Zheng, J.-Y. Wang, D. Zhao and J. Pei, J. Mater. Chem. A 2013, 1,
electronic structure of the dyads. In summary, TPE-substitutions
6
4
609; (d) Y. Lin, Y. F. Li and X. W. Zhan, Chem. Soc. Rev., 2012, 41,
245; (e) X. W. Zhan, A. Facchetti, S. Barlow, T. J.ViMewaArkrtsic,leMO.nAlin.e
at the imide positions of PBI core contributed new PBI
derivatives, and they exhibit distinct optical properties and
aggregation behaviors from a variety of TPE and PBI derivatives.
These characteristics may help us, as the work is going on in our
laboratory, to design novel TPE-PBI dyads and find applications
in optical limiting materials and efficient charge transfer arrays.
Ratner, M. R. Wasielewski and S. R. Mar dD e Or ,I :A 1d0 v. .1 0M 3 a9 t/eCr 4. ,T 2C0 01 21 5, 52 03 D,
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Acknowledgement
7
8
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9
9
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0
5
0
5
This work was partially supported by the National Basic
Research Program of China (973 program; 2013CB834704) and
the National Science Foundation of China (51273175); the
Research Grants Council of Hong Kong (16301614, N_HKUST
1
0
8
9
1
(a) J. B. Birks, Photophysics of Aromatic Molecules, Wiley, London,
6
04/14, N_HKUST 620/11), the Innovation and Technology
1
970; (b) S. W. Thomas III, G. D. Joly and T. M. Swager, Chem.
Commission (ICTPD/17-9) and the University Grants Committee
of Hong Kong (AoE/P-03/08). J. Z. Sun thanks the support of the
Open Project of State Key Laboratory of Supramolecular
Structure and Materials (sklssm201322).
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Engineering, The Hong Kong University of Science & Technology, Clear
Water Bay, Kowloon, Hong Kong, China. Fax: +852-23581594; Tel:
+852-23587375; E-mail: tangbenz@ust.hk
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Journal Name, [year], [vol], 00–00 | 9

Journal of Materials Chemistry C
Page 10 of 10
View Article Online
DOI: 10.1039/C4TC02550D
Graphical Abstract
Perylene-bisimides (PBIs) modified with tetraphenylethene (TPE) at N-terms show unique
X-aggregation and are non-fluorescent in solution, aggregate and solid film due to the
photoinduced charge transfer between TPE and PBI subunits.