Fluorescence Regulation and Photoresponsivity in AIEE Supramolecular Gels Based on a Cyanostilbene Modified Benzene-1,3,5-Tricarboxamide Derivative

Article abstract of DOI:10.1002/chem.201804135

Supramolecular interactions play an important role in regulating the optical properties of molecular materials. Different arrangements of identical molecules can afford a more straightforward insight into the contributions of supramolecular interactions.

Full text of DOI:10.1002/chem.201804135

A Journal of
Accepted Article
Title: Fluorescence Regulating and Photoresponsivity in AIEE
Supramolecular Gels Based on a Cyanostilbene Modified
Benzene-1,3,5-tricarboxamide Derivative
Authors: Zeyang Ding, Yao Ma, Hongxing Shang, Houyu Zhang, and
Shimei Jiang
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To be cited as: Chem. Eur. J. 10.1002/chem.201804135
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Fluorescence Regulating and Photoresponsivity in AIEE
Supramolecular Gels Based on a Cyanostilbene Modified
Benzene-1,3,5-tricarboxamide Derivative
Zeyang Ding, Yao Ma, Hongxing Shang, Houyu Zhang and Shimei Jiang*
Abstract: Supramolecular interactions play an important role in
regulating the optical properties of molecular materials. Different
arrangements for identical molecule afford a more straightforward
insight into the contributions of supramolecular interactions. Herein,
stimulus like sound, temperature, light, solvent and so forth,
which makes supramolecular gels have the potential to be smart
materials.[4] Therefore, to gain more knowledge of the nature in
supramolecular gels and to develop functional luminescence
regulating gel materials remain to be seen a crucial challenge.
Cyanostilbene could be supposed to be a good candidate to
help us to face the challenge. Most cyanostilbene-based
backbones could produce typical aggregation induced emission
(AIE) or aggregation induced emission enhancement (AIEE)
behavior.[5] Videlicet, the emission of cyanostilbene derivatives is
quite weak or none in dilute solution but becomes enhanced
accompanied with aggregates or gels formation.[6] Especially,
the conformation and relative arrangement of cyanostilbene
derivatives in aggregates are sensitive to environment owing to
a
novel gelator, BTTPA, composed of
a
benzene-1,3,5-
tricarboxamide (BTA) central unit functionalized by three
cyanostilbenes is designed, which forms two kinds of gels in
DMSO/water mixtures. Depending on the water volume content, the
gels exhibit quite different aggregation-induced emission
enhancement (AIEE) properties with one emitting a green emission
(G-gel), while the other emits a blue emission (B-gel). The main
reason for this observation is that water affects H-bonding and π-π
interactions, further resulting in disparate packing modes of gelators.
In addition, only G-gel displays gel-to-sol transition accompanied
with fluorescence switching according to the trans-cis
photoisomerization of cyanostilbene under UV light irradiation. B-gel
fails to exhibit any gain due to its tight hexagonal packing
arrangement. Such packing modes restricted the space in which
molecules were located and inhibited the configuration
transformation of cyanostilbene. These phenomena underline the
incomparable status of packing modes and molecular configuration
in regulating fluorescence properties and photoresponse behavior in
organic solid-state luminescent materials.
the
distorted
structures
of
molecules.
Furthermore,
cyanostilbene could undergo trans-cis photoisomerization under
UV light irradiation.[7] The transition of configuration can lead to
the photomechanical effect like gel-to-sol or liquid crystal-to-
isotropic liquid transition, making the luminescence in self-
assemblies changed as well.[8] However, there was little known
in photoresponsive gels containing cyanostilbene to data on
account of the narrow space in which cyanostilbene situate.[9]
1. Introduction
Organic solid-state luminescent materials have attracted
considerable attentions because of their underlying applications
in fluorescence sensors, Organic Light Emitting Diodes (OLEDs),
bioimaging and so forth.[1] Fluorescence properties such as
emission color of materials are not only decided by covalent
structures of molecules themselves but also by the effects of
relative supramolecular interactions (H-bonding, π-π interactions,
hydrophobic interactions, etc).[2] That is to say the modulation of
fluorescence properties can be achieved at the unimolecular
level by controlling ordered molecular self-assemblies.[3]
Supramolecular gels are supposed to be formed by gelators
linked with supramolecular interactions and solvents. Various
supramolecular interactions here produce the possibility of
forming different packing structures. Besides, the biggest
advantage of supramolecular gels is their sensitivity to external
Scheme 1. Molecular structure and detailed synthetic route of BTTPA.
With these considerations in mind, we designed a novel
gelator (BTTPA) containing a benzene-1,3,5-tricarboxamide
(BTA) central unit functionalized by three cyanostilbenes as
shown in Scheme 1. BTA structure has a huge advantage in
fabricating hierarchical supramolecular architectures owing to its
unique structure-three amides attached to a benzene core. The
three amide bonds provide not only strong threefold H-bonding
but also a one-dimensional growth tendency.[10] The AIE active
cyanostilbene π-conjugated backbone was chosen because of
the attractive dual advantages of remarkable fluorescent
Z. Ding, Y. Ma, H. Shang, H. Zhang, Prof.S. Jiang
State Key Laboratory of Supramolecular Structure and Materials
College of Chemistry, Jilin University
Changchun 130012, P. R. China
E-mail: smjiang@jlu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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features and photoisomerization, which are very sensitive to the
aggregation behaviors. We supposed that the synergy between
H-bonding and π-π interactions could give rise to ordered self-
assemblies and the effects could influence the packing modes of
π-conjugated cyanostilbene, further leading to distinct
fluorescence properties of the gels. As expected, BTTPA could
form two kinds of gels in the mixture of dimethyl sulfoxide
(DMSO) and water due to the different self-assembly behaviors.
The gels have distinctly different emission behavior with one
emitting green emission (G-gel), while the other emits blue
emission (B-gel). In addition, only G-gel exhibits a gel-to-sol
transformation accompanied with fluorescence diminishment
according to the trans-cis photoisomerization of cyanostilbene
moieties under UV light irradiation. But B-gel exhibits
outstanding photostability. To make full use of this point, an
interesting photoinduced confidentiality pattern is performed.
there is no liquid flow upon the inversion of the sealed glass vial
when hot solution was cooled to room temperature. BTTPA
can’t form gel in any tested organic solvent and only dissloved in
THF, DMF and DMSO (Table S1). Additionally, mixtures of good
solvents with water were used to probe the gelation ability
(Table 1, S2 and S3). However, the gel formation was only
observed when water was added into DMSO solution containing
BTTPA. Depending on the water content, two kinds of gels were
obtained and could endure ambient conditions for several weeks
(Table 1). At 15% water, BTTPA could form yellow gel with
green emission (G-gel). When the water content increased to
between 30% and 50%, white gel with blue emission was
obtained (B-gel) (Figure 1). Detailed gelation test was given in
the experimental section. In addition, corresponding actual
morphologies of the microstructure of xerogels were shown by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) (Figure 1, S3). Both kinds of xerogels
exhibited entangled network nanostructures consisting of
nanofibers, implying that the gel formation was ascribed to the
immobilization of solvent by the winding structure. Crooked and
gracile fibers with diameters of 30-70 nanometers of the xerogel
prepared from G-gel were observed. Nevertheless, fibers of
xerogels prepared from B-gel were more straight and wide and
the width was up to 200-300 nanometers, implying a more rigid
structure.
2. Results and Discussion
2.1. Synthesis
BTTPA was prepared by treating 1,3,5-benzenetricarboxylic
acid chloride with 2-(4-aminophenyl)-3-phenylacrylonitrile (2).
Compound 2 was obtained using a reduction reaction of 2-(4-
nitrophenyl)-3-phenylacrylonitrile (1). Compound
1
was
synthesized by knoevenagel condensation between
a
benzaldehyde and 4-nitrophenylacetonitrile (Scheme 1).
Experimental details and corresponding characterization data
are given in the experimental section and supporting information.
1
The final product was obtained in about 54% yield. H, 13C NMR
spectroscopic measurements, elemental analysis, and mass
spectra were employed to confirm its structure (Figure S1, S2
and S8).
Table 1. Gelation properties of BTTPA in DMSO/water mixed solvents.
DMSO/water
90/10
Phase
S
DMSO/water
40/60
Phase
I
I
I
I
85/15
G (9.53), G-gel
GI, B-gel
GI, B-gel
GI, B-gel
30/70
70/30
20/80
Figure 1. SEM images of BTTPA self-assembled structures for G-gel and B-
gel (in DMSO/water mixed solvents with different volume content), G-gel: (a)
85/15, B-gel: (b) 70/30 (c) 60/40 (d) 50/50; insets are the photographs of
corresponding gels under ambient light (left) and under illumination at 365 nm
(right).
60/40
10/90
50/50
Note: S, soluble; G, gel; I, insoluble; GI, gel (cannot dissolve completely when
heated). Number in bracket is the critical gel concentration (CGC, mg mL-1).
To provide a more accurate standard of gelation judgment of
the gels with different water content, rheology experiments were
employed. As shown in Figure 2, both G-gel and B-gel exhibited
a higher elastic storage modulus (G’) and a lower loss modulus
(G’’), indicating the authenticity of gelation judgment again.[4c,11]
More interestingly, with the increase of water content from 15%
to 50%, both G’ and G’’ increased significantly, implying that the
2.2. Gelation Properties
The gelation ability of BTTPA was systematically investigated at
first. A traditional strategy by heating-and-cooling a mixture of
the gelator and a selected solvent was performed. The gel
formation was judged by the “inversion-vial” method, that is,
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enhanced mechanical properties of the gels with more water
content. That means B-gel exhibited higher mechanical strength
than G-gel. Furthermore, the recovery experiments of B-gel
were also performed (Figure S4). When exerted a constant
detail. In the fluorescence spectra, G-gel (15% water) revealed a
green emission at 511 nm (absolute PLQY = 5.3%), presenting a
distinct bathochromic shift (Δλ = 81 nm) and an increase in the
intensity in contrast to the weak emission in solution (λ = 430
strain of 100% during one minute, the gel got destroyed (G’’ >G’). nm), implying the special packing modes of the gel (Figure S5a).
And the gel state would be recovered when the strain was
decreased to 1% (G’ >G’’). The circulation could be repeated
many times, indicating that the gel showed good thixotropic
properties.[12]
As far as UV-Vis absorption spectra were concerned, a distinct
tailing peak in the visible region appeared, suggesting the
aggregation behaviors of BTTPA in the gel state (Figure
S5b).[15,16] Moreover, as is known that temperature could
contribute to the dispergation of the gel owing to the
nonaggregation behavior of molecules in
a
higher
temperature.[17] From the temperature-dependent fluorescence
spectra (Figure 3d), a gradual decline of the intensity of
emission peak was observed with the increase of temperature.
Until the hot solution state was reached, the final fluorescence
was almost gone, while the fluorescence would be restored
accompanied with gel reformulation in a lower temperature,
showing typical thermal-reversible AIEE gel characteristics.
Therefore, a reversible emission transition between “On” and
“Off” could be readily achieved by the variation temperature.
Figure 2. (a) Frequency dependence of G’ and G’’ for G-gel and B-gel (10 mg
ml-1) in DMSO/water mixed solvents with the strain at 1%. (b) Dynamic strain
sweep measurement of G-gel and B-gel in DMSO/water mixed solvents with
angular frequency at 10 rad s-1. Legends are same in (a) and (b).
2.3. AIEE Characteristics
As mentioned above, the gels emit strong fluorescence. Hence,
the fluorescent characteristics of BTTPA were worth
investigating. In pure DMSO, BTTPA had an emission peak at
430 nm with a bluish violet color, while the emission was too
weak to be observed under UV light (Figure 3a, 3c). With the
water content increasing, the intensity decreased gradually
accompanied with a little bathochromic shift, which might caused
by the twisted intramolecular charge transfer (TICT) effect.[13]
The intensity of the emission suddenly enhanced when the
water content reached 40%. With the increase of water content,
the intensity became larger and a sky blue fluorescence (λ = 480
nm) was observed. At 60% water, the intensity reached to
maximum, which was almost 7.5-fold greater than that in DMSO
solution. The enhanced emission could be contributed to the
poor solubility of BTTPA in water compared to that in DMSO,
indicating BTTPA exhibited typical AIEE characteristics. In terms
of UV-Vis absorption, an absorption peak at 347 nm was
observed in pure DMSO, which was attributed to π–π* transition
absorption of cyanostilbene moieties.[14] Along with the increase
of water content, a wilder peak as well as level-off tails in the
visible region were observed and the intensity of the main peak
had a distinct decrease, revealing that BTTPA comes into
aggregates definitely in the poor solvent of water (Figure 3b).[15]
The results further supported the AIEE characteristics of BTTPA.
Figure 3. (a) Fluorescence (λex = 345 nm) and (b) UV-Vis absorption spectra
of BTTPA (10-5 M) in DMSO/water mixtures with different water volume
fractions (from 0% to 90%). (c) Maximum fluorescence intensity of BTTPA (10-
M) as a function of water content. (d) Temperature-dependent fluorescence
spectrum of G-gel.
5
Modulating the water content, another kind of gel with
disparate blue emission (B-gel) was formed and lost the thermal-
reversible characteristics. That means B-gel was insoluble when
being heated. As to the fluorescence spectra (Figure 4a), B-gel
(30%-50% water) exhibited a blue emission at 471 nm (absolute
PLQY = 6.0%), expressing a big difference in comparison to G-
gel (15% water) (λ = 511 nm) described previously (Δλ = 40 nm).
In terms of UV-Vis absorption spectra (Figure 4b), the
absorption peak (λ = 345 nm) of G-gel was realized. But B-gel
displayed a wider absorption peak and a distinct shoulder peak
appeared at 404 nm. Particularly, with the increase of water
content, the intensity of the shoulder peak increased as well,
implying stronger π-π interactions of cyanostilbene moieties.[4f] It
2.4. Optical Properties in Gel State
In accord with the spectroscopic studies in solution,
fluorescence properties in the gel state were also investigated in
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is noticed that both green emission and blue emission of G-gel
and B-gel could be sustained after solvent evaporation (in a
xerogel state), respectively (Figure S6).
prepared from G-gel, the amide bonds could be reflected in two
N-H stretching vibrations at 3412 cm-1, 3322 cm-1 and one sharp
C=O stretching vibration at 1678 cm-1, indicating one-
dimensional aggregates through different types of H-bonds.[10f,21]
But in the spectrum of sample prepared from B-gel, both kinds of
vibration peaks produced great shifts. Two N-H stretching
vibration peaks shifted to 3312 cm-1, 3259 cm-1 and the C=O
stretching vibration peak shifted to 1644 cm-1 respectively,
revealing stronger H-bonding interactions and tighter one-
dimensional aggregates formation.[22] While cyano group
stretching vibrations of both samples was located at 2216 cm-1,
confirming that the effects of water had nothing to do with cyano
group. Thereby, the H-bonding interactions were enhanced in B-
gel in comparison to G-gel due to different water content, which
might lead to quite different packing structures.
Figure 4. (a) Normalized fluorescence (λex
= 345 nm) and (b) UV-Vis
absorption spectra of G-gel and B-gel in the DMSO/water mixtures.
2.5. Packing Modes in Gel State
To assess how water affected the fluorescence of the gel, X-ray
diffraction (XRD) and FT-IR spectroscopy were employed. It is
widely accepted that the fluorescence of aggregates depends on
the molecular structures and relevant packing modes. If the
molecular structures are unaltered, it is packing modes that
affect the fluorescence of aggregates.[18] As expected,
a
significant discrimination of xerogels prepared from G-gel (15%
water) and B-gel (choosing the gel with 30% water for the next
discussion) was observed in the whole XRD patterns (Figure 5a).
For the xerogel prepared from G-gel, weak and tanglesome
diffraction signals were shown in its XRD pattern, implying a low
crystallinity of G-gel. In addition, the first peak at 2.80° with a d-
spacing of 3.15 nm in the small-angle region was considered as
the intermolecular distance in orthogonal direction. In view of the
columnar assemblies which most BTA structural molecules
tended to form and the fibers like microstructure of the gel
(Figure 1a), the intermolecular distance might be supposed to
the intercolumnar distance.[19] And density functional theory
results demonstrated that BTTPA molecule adopted a perfect
equilateral triangle-like structure and the dimension was
estimated to be 2.70 nm (Scheme S1). The d-spacing value of
first peak (3.15 nm) was larger than the molecular dimension,
indicating a kind of relative loose arrangement. While sharper
peaks obtained from B-gel standed for better rigidity and
crystallinity. The xerogel prepared from B-gel exhibited main
peaks at 4.29°, 7.23°, 8.72° and 11.38°, affording a ratio of 1 :
√3 : 2 : √7 (d-spacing values were 2.06 nm, 1.22 nm, 1.04 nm
and 0.78 nm), showing a hexagonal packing arrangement.[20] In
addition, the reflection peak at 22.02°, corresponding to a d-
spacing value of 0.401 nm, was ascribed to π-π stacking
between cyanostilbene moieties.[20c] The d-spacing value of first
peak (2.06 nm) was shorter than the molecular dimension,
indicating tighter hexagonal packing arrangement and overlap of
molecules owing to the enhanced π-π interactions.
Figure 5. (a) X-ray diffraction patterns and (b) FT-IR spectra of xerogels
prepared from G-gel and B-gel.
The structures influenced by water were further investigated
using a computational model. From the 1H NMR spectrum of
BTTPA in DMSO-d6 with D2O (Figure S7), the proton peak of
amide bonds can’t be detected due to the H-D exchange,
suggesting water contacted with the amide bonds and may
influence the packing.[23] Based on the optimized results
(Scheme S2), at high water content, BTTPA dimer was
stabilized by water molecules through N−H···O and O−H···O
hydrogen bonds, an overlapped structure was obtained.
However, at low water content, fewer water molecules can’t form
sufficient hydrogen bonds to sustain such overlapped structure,
leading to a slipped structure.
Considering that most BTA system was supposed to self-
assemble through amide-based H-bonds, FT-IR spectroscopy, a
sensitive tool to detect amide bonds, was employed to identify
the variation of H-bonds (Figure 5b). In the spectrum of sample
Based on the above results and discussion, a schematic
illustration can be proposed for the formation of G-gel and B-gel
(Scheme 2). For G-gel (at low water content), neighboring
molecules stacked aslant and formed a loose packing. In this
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case, the great slippage contributed to a red-shifted emission
(green emission).[18a] Whereas, for B-gel (at high water content),
H-bonding interactions and π-π interactions were enhanced
owing to the high degrees of aggregation of molecules, which
were demonstrated by the FT-IR and UV-Vis absorption results,
resulting in a tight hexagonal packing arrangement of molecules.
Therefore, B-gel exhibited a quite different blue emission in
comparison to the green emission of G-gel because of the
different packing modes of B-gel and G-gel influenced by water.
absorption peak (λ = 347 nm) was blue-shifted to 332 nm while
the intensity of the absorption peak decreased dramatically,
indicating the photoisomerization happened rapidly in dilute
solution. This phenomenon was due to the fact that the
conjugation length of cis-isomers was shorter than that of trans-
isomers.[9a] As to the fluorescence spectra, the original dim
bluish violet emission became weaker when being irradiated.
Scheme 2. Proposed illustration of the self-assembly behaviors of BTTPA in
G-gel and B-gel.
2.6. Photomechanical Effect Accompanied with
Fluorescence Switching
Figure 6. 1H NMR spectral comparison of BTTPA in DMSO-d6 (10-3 M) upon
continuous irradiation with UV light at 365 nm. Ha and Hb represent the proton
peaks of the vinylene for trans-isomer and cis-isomer, respectively.
Cyanostilbene unit could undergo trans-cis photoisomerization
upon UV light irradiation at 365 nm. It is worth mentioning that
the occurrence of photoisomerization needs sufficient space in
which molecules located in.[9a] The photoisomerization happened
easily in solution owing to the free state of molecules. Herein, 1H
NMR spectroscopic measurements were carried out to examine
the photoisomerization process of BTTPA in DMSO-d6 (Figure
6). In the initial state, the cyanostilbene unit adopted a trans-
form evidenced by 1H NMR signals and the peaks were
assigned in Figure S1. Proton peaks of amide bonds and
aromatic rings shifted to higher field upon UV light irradiation
from the results of the 1H NMR spectroscopy, indicating the
photoisomerization process happened authentically.[9b,24] Proton
peaks from the double bond containing cyano group were used
as the indicator to tell us the proportion between trans-isomers
and cis-isomers. With the irradiation time increasing, the proton
peak (Ha) for trans (8.05 ppm) decreased but that (Hb) for cis
(7.68 ppm) increased. When the photostationary state was
achieved, about 72% cis-isomers were obtained through the
integral analysis of the proton peaks. It is noted that no new
peaks could be found from the mass spectrum upon UV light
irradiation, implying that there wasn’t any other photoproducts
like the [2 + 2] photodimer but the cis-photoisomers generated
(Figure S8).[24b,25] Optical properties were also changed during
the trans-cis photoisomerization (Figure S9). When a solution of
BTTPA in DMSO was irradiated with UV light at 365 nm, the
To gain more insight into the contributions of supramolecular
interactions in the photoresponse behaviors of self-assembly
materials, the photoresponsivity of the gels were further
investigated. Whether the photoisomerization happened or not in
the condensed state was attributed to the space in which
cyanostilbene moieties situated, which was mentioned above.
Most compounds based on cyanostilbene in the aggregation
state are stable upon UV light irradiation due to the close
packing modes of molecules or strong interactions.[5a,18a,26] In
this work, two kinds of gels exhibited distinct photoresponse
behavior. Upon UV light irradiation, G-gel collapsed and
transformed into viscous sol quickly (Figure 7a). There is no
doubt that the driving force for the gel-to-sol transition was
ascribed to the photoisomerization of the cyanostilbene moieties,
which was additionally proved by 1H NMR spectroscopic
measurements of G-gel in DMSO-d6/H2O (v/v, 85/15) before and
after UV light irradiation (Figure S10). Proton peaks shifted to
higher field like the results in pure DMSO-d6 solution, suggesting
the occurrence of photoisomerization in the gel state.[24] The
conversion rate was about 57% calculated by the proton peaks
variation of cyano group. And all proton peaks became
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sharpened and clear, implying the disappearance of aggregates
after UV light irradiation, as well as the gel-to-sol transition.[27]
However, B-gel failed to exhibit any change upon UV light
irradiation. Thus, we think that no photoisomerization happened
in B-gel. As we know that the occurrence of photoisomerization
was related to the space in which molecules located in. And the
space was influenced by the arrangement of molecules. The
loose packing structures of G-gel provided sufficient space for
the photoisomerization to allow the switching event to happen.
2.93 ns (22.08%), τ2 = 19.92 ns (77.92%) for the gel and τ3 =
0.63 ns (67.08 %), τ4 = 4.22 ns (32.92%) for the sol. The trans-
isomers had great gelation abilities and exhibited green
emission in the gel state. Based on the above results and
discussion, it could be concluded that the trans-isomers
possessed better gelation ability and emission intensity in
contrast to the cis-isomers. As a result of UV light irradiation,
plentiful cis-isomers emerged and disturbed the ordered
architectures, leading to the collapse of the gel and weaker
Nevertheless, B-gel exhibited tight hexagonal packing structures. fluorescence.[9]
Such packing structures restricted the freedom of molecules and
inhibited configuration transformation of cyanostilbene. Thereby,
B-gel showed outstanding photostability, while G-gel showed
superior photosensitivity because of the different packing modes
as well.
Figure 8. Photographs of photoinduced confidentiality pattern made from G-
gel (at the corner) and B-gel (in the center).
The disparate photoresponse behavior of G-gel and B-gel
could be extended to graphic photoinduced confidentiality
pattern (Figure 8). Four circles at the corner were equipped with
G-gel and one circle in the center was equipped with B-gel. Five
of circles with green and blue fluorescence were visible under
UV light. Upon UV light irradiation, green fluorescence parts
decayed but blue fluorescence part still sustained. Only the
circle in the center was presented to us.
Figure 7. G-gel before and after UV irradiation: (a) Photographs; (b)
Fluorescence spectra (λex = 345 nm); (c) Normalized UV-Vis absorption
spectra; (d) Fluorescence kinetic profiles (λex = 365 nm).
3. Conclusions
A
novel gelator BTTPA containing
a
benzene-1,3,5-
tricarboxamide (BTA) central unit functionalized by three
cyanostilbenes has been synthesized. The special structure
results in favourable gelation ability in the mixture of DMSO and
water. Depending on the water volume content, two kinds of gels
exhibit disparate AIEE properties with one emitting a green
emission (G-gel, containing 15% water), while the other emits a
blue emission (B-gel, containing 30%-50% water). Meanwhile,
G-gel and B-gel show different photoresponse behaviors as well.
Further investigations were designed to characterize the
nature of the sol state obtained by G-gel after UV light irradiation.
Comparing the XRD data and SEM images of G-gel before and
after UV light irradiation, we found that there was no noticeable
diffraction in wide-angle region of the XRD pattern (Figure S11)
and also disordered planar aggregates were observed in the
SEM image (Figure S12), indicating the amorphous feature in
the sol state.[28] Next, optical properties of the sol were
investigated. In the fluorescence spectra, the sol displayed a dim
dark blue emission (λ = 441 nm) with a low intensity in
comparison to the green emission with a high intensity of the
initial gel (Figure 7b). As far as UV-Vis absorption spectra were
concerned, a significant blue shift from the absorption peak of
the gel (λ = 345 nm) to that of the sol (λ = 280 nm) was
observed (Figure 7c). Meanwhile, the time-resolved
fluorescence revealed that the average fluorescence lifetime (τ =
19.24 ns) of the initial gel state reduced to 3.38 ns upon UV light
irradiation (Figure 7d). The fluorescence lifetimes are
determined by two component exponential fitting as follows; τ1 =
G-gel exhibits
a
gel-to-sol transition accompanied with
fluorescence switching upon UV light irradiation. But B-gel fails
to exhibit any gain upon UV light irradiation, revealing great
photostability. The observation is ascribed to the different
packing structures influenced by water. Loose structures and
great freedom of molecules are disclosed for G-gel, providing
sufficient space for the configuration transformation
(photoisomerization) of cyanostilbene. For B-gel, H-bonding
interactions and π-π interactions are enhanced, resulting in tight
hexagonal packing structures and restricting the space in which
cyanostilbene moieties locate and inhibiting the configuration
transformation. Consequently,
a
graphic photoinduced
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D/MAX 2500/PC X-ray diffractometer with CuKa radiation (λ = 0.15418
nm). Scanning electron microscopy (SEM) images were collected on a
JEOL JEM-6700F at 3 kV, with the samples coated with Au. Transmission
electron microscopy (TEM) was performed on a JEOL JEM-2100F
transmission electron microscope. Samples were prepared by wiping a
small amount of gel samples onto a carbon-coated copper grid followed
by naturally evaporating the solvent. Mass spectra were recorded on a
Brucker Autoflex speed TOF/TOF mass spectrometer. The solution of
BTTPA was irradiated using two 365 nm-handheld UV lamps (12 W). The
model was WFH-204B. BTTPA gel was irradiated using Tank 007 UV-
AA01 flashlight (25000 μW cm-2).
confidentiality pattern is performed. Furthermore, this work helps
people to gain more insight into the contributions of
supramolecular interactions in regulating luminescence of the
self-assembly materials and opens a door to a new class of
developing novel smart materials.
4. Experimental Section
Synthesis of 2-(4-nitrophenyl)-3-phenylacrylonitrile (1): Benzaldehyde
(2.04 ml, 20 mmol) and pyrrolidine (3.31 ml, 40 mmol) were added to a
solution containing 4-nitrophenylacetonitrile (3.24 g, 20 mmol) in ethanol
(80 ml). The reaction mixture was stirred at room temperature for 6h and
filtered to remove liquid. The solid was washed with ethanol for 3 times to
give a faint yellow powder (4.66 g, yield 93%). 1H NMR (500 MHz,
DMSO-d6) δ/ppm = 8.40 – 8.35 (m, 2H), 8.34 (s, 1H), 8.08 – 8.05 (m,
2H), 8.02 – 8.00 (m, 2H), 7.62 – 7.56 (m, 3H).
Gelation test: BTTPA was dissolved in DMSO in a glass vial beforehand.
When water was added into the solution, white precipitates appeared.
For G-gel (15% water), white precipitates dissolved completely when
being heated at 90 ℃. Yellow gel was formed when cooled to room
temperature. For B-gel (30%-50% water), the glass vial needed to be
stirred slightly to make sure the precipitates were fully dispersible in
solution. Then, the mixture was heated at 90 ℃ for only 15 seconds.
White suspension turned into white stable gel. For all of the
measurements, the gel concentration was set to 10 mg ml-1.
Synthesis of 2-(4-aminophenyl)-3-phenylacrylonitrile (2): SnCl2·2H2O
(18.11 g, 80 mmol) was added to a solution containing compound 1 (4.66
g, 18.6 mmol) in ethanol (120 ml), which was refluxed at 75 ℃ for 5h.
The resulting solution was then neutralized with sodium bicarbonate
saturated aqueous solution and extracted by ethyl acetate. The obtained
organic phase was washed with saturated salt water for 3 times, dried
over with anhydrous Na2SO4 and filtered. The resulting solution was
concentrated by rotary evaporation to give a yellow solid (2.91 g, yield
71%). 1H NMR (500 MHz, DMSO-d6) δ/ppm = 7.87 – 7.83 (m, 2H), 7.70
(s, 1H), 7.49 (dd, J = 8.3, 6.8 Hz, 2H), 7.44 (dd, J = 8.4, 6.6 Hz, 3H), 6.68
– 6.62 (m, 2H), 5.62 (s, 2H).
Theoretical and computational method: The molecular structure of
BTTPA was optimized in gas phase using density functional theory
calculations at the B3LYP/3-21G* level with the Gaussian 09 package.[29]
For the simulation of the stabilized stacking dimer of BTTPA, a semi-
empirical method (PM7) was employed, as implemented in the Mopac
2016 program.[30]
Acknowledgements
Synthesis of benzene-1,3,5-tricarboxylic acid tris-{[4-(1-cyano-2-
phenyl-vinyl)-phenyl]-amide} (BTTPA): Triethylamine (1.77 ml, 13.3 mmol)
and a solution of 1,3,5-Benzenetricarboxylic acid chloride (1.07 g, 4.1
mmol) in DCM (10 ml) were slowly added to a solution containing
compound 2 (2.91 g, 13.3 mmol) in DCM (60 ml), which was stirred at
room temperature for 20h. The resulting mixture was filtered to remove
the liquid and the solid was washed with DCM and water for many times
to obtain the final yellow powder (2.71 g, yield 82%). 1H NMR (500 MHz,
DMSO-d6) δ/ppm = 10.85 (s, 3H), 8.80 (s, 3H), 8.05 (s, 3H), 8.02 (d, J =
8.5 Hz, 6H), 7.95 (d, J = 7.6 Hz, 6H), 7.84 (d, J = 8.5 Hz, 6H), 7.58 – 7.50
(m, 9H); 13C NMR (126 MHz, DMSO-d6) δ/ppm = 165.07, 142.08, 140.39,
135.75, 134.35, 130.92, 130.55, 129.62, 129.47, 126.84, 121.08, 118.38,
110.46; MALDI-TOF-MS: calcd for C54H36N6O3: 816; found: 839.177 [M+
+ Na] Elemental analysis: Anal. calcd for C54H36N6O3: C 79.41, H 4.41, N
10.29; found: C 79.09, H 4.51, N 10.08.
This work was supported by the National Natural Science
Foundation of China (51673082 and 21374036).
Keywords: fluorescence switching
•
photoresponsivity
•
aggregation-induced emission enhancement • supramolecular
interactions • cyanostilbene
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Entry for the Table of Contents
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The modulation of fluorescence
properties can be achieved at the
unimolecular level by controlling
ordered molecular self-assemblies.
Zeyang Ding, Yao Ma, Hongxing Shang,
Houyu Zhang and Shimei Jiang*
Page No. – Page No.
This paper reports that
gelator, BTTPA, can form two kinds of
gels with one emitting green
emission (G-gel), while the other emits
a blue emission (B-gel) depending on
the water content in DMSO/water
mixtures. Only G-gel displays highly
photosensitive, while B-gel fails to
exhibit any gain upon UV irradiation.
a novel
Fluorescence Regulating and
Photoresponsivity in AIEE
Supramolecular Gels Based on a
Cyanostilbene Modified Benzene-
1,3,5-tricarboxamide Derivative
a
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