Multi-stimuli responsive supramolecular gels based on a D-π-A structural cyanostilbene derivative with aggregation induced emission properties

Article abstract of DOI:10.1039/c8sm02434k

Developing multi-stimuli responsive fluorescent gel materials in a single system remains challenging. Gelator molecules with classical fluorophores suffer from the aggregation-caused quenching (ACQ) effect, limiting their applications further. Herein, a n

Full text of DOI:10.1039/c8sm02434k

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ISSN 1744-683X
PAPER
Jure Dobnikar et al.
Rational design of molecularly imprinted polymers
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ARTICLE
Multi-stimuli Responsive Supramolecular Gels Based on a D-π-A
Structural Cyanostilbene Derivative with Aggregation Induced
Emission Properties
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Xiaoxu Wang, Zeyang Ding, Yao Ma, Yuping Zhang, Hongxing Shang and Shimei Jiang*
www.rsc.org/
Developing multi-stimuli responsive fluorescent gel materials in a single system remains challenging. Gelator
molecules with classical fluorophores suffer from aggregation-caused quenching (ACQ) effect, limiting their
applications further. Herein, a novel V-shaped cyanostilbene-based molecule (BAPBIA) with aggregation
induced emission (AIE) characteristics and great gelation ability was synthesized and was found to exhibit
multi-stimuli responsive behaviors. Reversible gel-sol phase transitions together with emission quenching are
realized in response to external stimuli including heating, light and fluoride ion. Especially, the introduction
of dimethylaniline group (donor) and cyano group (acceptor) generates a D-π-A structure, further leading to
an intramolecular charge transfer (ICT) effect, which enlarges the emission contrast with the variations of the
distribution of charge. Thus, upon trifluoroacetic acid (TFA) triggered protonation of dimethylaniline group,
not only gel-sol transition but also emission color switching (yellow-to-blue) is achieved due to the loss of the
ICT effect. This work paves an easy way to construct fully reversible multi-stimuli responsive fluorescence
modulation smart materials.
for wider applications. The incorporation of fluorescence
features provides not only valuable optical information and
external responsive signals but also promising applications in
Introduction
Supramolecular gels are highly crucial and attractive as novel
soft materials thanks to their potential applications in drug
delivery, chemosensors and template synthesis of
nanomaterials.1 Through multiple weak non-covalent
interactions such as hydrogen bonding, π-π stacking
interactions, van der Waals forces, etc, these soft materials are
self-assembled by low-molecular weight gelators (LMWGs) to
form specific interlocked 3D networks. The dynamic nature of
non-covalent interactions and unique structures of gels make
it possible, in many circumstances, to rapidly respond to
external environmental stimuli.2 Therefore, distinct gel-sol
phase transitions can be caused by numerous stimuli like heat,
light, pH, ions and so forth, which makes supramolecular gels
are promising to be smart materials.3 Developing stimuli
responsive gel materials especially multi-stimuli responsive
ones is not only of academic interest but also of practical
applications.
optoelectronics devices.4 Nevertheless, gelators containing
traditional planar π-conjugaged fluorophores suffer from
fluorescence quenching in gel (condensed) state on account of
aggregation-caused quenching (ACQ) effect, hindering the
prospect of fluorescent gels. Fortunately, an intriguing
phenomenon named aggregation induced emission (AIE) was
first reported by Tang and co-workers in 2001.5 In contrast
with ACQ effect, non-emissive molecules are induced to emit
by the aggregates formation, which provides more possibilities
for the development of stimuli responsive fluorescent gels.6
-
For instance, Lin and co-workers reported Fe3+/H2PO4
With the development of the field of stimuli responsive gels,
simple gel-sol responsive behaviors cannot fulfill the demand
a State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, 2699 Qianjin Avenue, Changchun 130012, P. R. China.
E-mail: smjiang@jlu.edu.cn
†Electronic Supplementary Information (ESI) available: [NMR spectra, Maldi-TOF
spectra, XRD spectra, UV/Vis and fluorescence spectra]. See
DOI: 10.1039/x0xx00000x
Fig. 1 Detailed synthetic route of BAPBIA (functional groups are marked).
This journal is © The Royal Society of Chemistry 20xx
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triggered reversible “On-Off” fluorescence switching based on were dimethylsulfoxide (DMSO-d6) and chlorofVoierwmArt(icCleDOCnlli3n)e.
DOI: 10.1039/C8SM02434K
a
supramolecular gel formed by bis-naphthalimide Mass spectra were carried out by a Bruker Autoflex speed
functionalized pillar[5]arene.7 Another example was described TOF/TOF mass spectrometer. Elemental analysis was
by Voskuhl and co-workers who designed an AIE gel using bis- performed on a Vario micro cube elemental analyzer. Powder
zwitterionic gelator. Upon pH variations, not only gel-sol X-ray diffraction (PXRD) patterns were captured on a Rigaku
transitions but also a loss of emission was realized.8 Whereas, SmartLab 3 X-ray diffractometer. FT-IR spectra were recorded
in a single gel system, to achieve reversible multi-stimuli with KBr pellets on a Bruker Optics VERTEX 80 V Fourier
responsive behaviors together with diversified fluorescence transform infrared spectrometer. Scanning electron
variations is still limited.9
To fulfill this appeal, multiple responsive sites are needed in instrument with voltage of 3 kV. UV–vis spectra were obtained
a single gelator. In this work, we designed a novel molecule by Shimadzu 3600 UV-Vis-near-IR spectrometer.
microscope (SEM) images were collected on a JEOL JEM-6700F
a
(BAPBIA) decorated on amide bonds, dimethylaniline group Fluorescence spectra were performed on a Shimadzu RF-
and cyanostilbene backbone (Fig. 1). BAPBIA shows distinct 5301PC fluorescence spectrophotometer. The light source was
AIE features and great gelation ability in various organic a Spotlight power 100 to 2000 Watt Multiple spotlights/filter
solvents. In particular, amide bonds and dimethylaniline combinations Timer controlled shutter (UVACUVBE 2000).
groups provide the deprotonation site and protonation site, The Lippert-Mataga equation is listed as follow:
respectively, and cyanostilbene units can undergo photo
1
n2
1
-
2n2 + 1
Ɛ -
induced isomerisation. This makes BAPBIA gels exhibit
underlying applications in developing multiple functionalized
materials. Thus, the gels exhibit reversible gel-sol phase
transitions together with emission switching in response to
external stimuli including heat, light, fluoride ion and
trifluoroacetic acid (TFA) (Fig. 2). Moreover, BAPBIA possesses
a D-π-A structure (dimethylaniline group is served as an
electron donating group and cyano group is an electron
accepting group), endowing the system with intramolecular
charge transfer (ICT) feature, which makes BAPBIA gels exhibit
a red shifted emission to enlarge the emission contrast with
the variation of the distribution of charge. In this way, a
remarkable emission color variation (yellow-to-blue) is
achieved when the gel is exposed to TFA stimulus due to the
destruction of the D-π-A structure.
f =
∆
-
2
+ 1
Ɛ
υ = υ
υ
em
∆
-
abs
Where Δυ is Stokes shift in wavenumbers, υabs and υem are
regarded as the maximum absorption wavenumbers and
emission wavenumbers (cm−1), respectively, Δf is the
orientation polarizability of different solvents, Ɛ is dielectric
constant and n is the refractive index.
Synthesis
3-(4-(dimethylamino)phenyl)-2-(4-nitrophenyl)acrylonitrile (1):
1 was synthesized using a reported method.10 Brick red solid
was obtained (3.66 g). 1H NMR (500 MHz, DMSO-d6), δ/ppm =
8.31 (d, J = 8.7 Hz, 2H), 8.11 (s, 1H), 7.96 (t, J = 8.9 Hz, 4H), 6.87
(d, J = 8.8 Hz, 2H), 3.07 (s, 6H).
2-(4-aminophenyl)-3-(4-(dimethylamino)phenyl)acrylonitrile
(2): SnCl2•2H2O (11.3 g, 50 mmol) was added to a solution
containing compound 1 (2.94 g, 10 mmol) in ethanol (50 ml),
which was refluxed for 5 h. The resulting solution was then
neutralized by sodium bicarbonate saturated aqueous solution
and extracted by ethyl acetate. The obtained organic phase
was washed with saturated salt water, dried over with
anhydrous Na2SO4. The drying product was concentrated by
Experimental
Materials and methods
All the reagents were used without purified unless stated
otherwise. The silicon wafers used here were first treated in
concentrated sulphuric acid–hydrogen peroxide solution (v/v,
7/3), then cleaned with pure water. 1H NMR and 13C NMR
spectra were recorded on a Bruker Avance III 500 MHz NMR
spectrometer. The internal standard was TMS and the solvents
1
rotary evaporation to give a yellow solid (2.08 g, 92.2%). H
NMR (500 MHz, DMSO-d6), δ/ppm = 7.77 (d, J = 8.6 Hz, 2H),
7.48 (s, 1H), 7.34 (d, J = 8.3 Hz, 2H), 6.78 (d, J = 8.5 Hz, 2H),
6.59 (dd, J = 27.8, 8.5 Hz, 2H), 5.43 (s, 2H), 2.99 (s, 6H).
5-(tert-butyl)-N,N-bis(4-(1-cyano-2-(4-(dimethylamino)phenyl)-
vinyl)phenyl)isophthalamide (BAPBIA): Compound 2 (3.16 g,
12 mmol) and triethylamine (1.66 ml, 12 mmol) were dissolved
in dry dichloromethane (DCM) (120 ml). Then the mixtures
were added to a DCM solution containing oxalyl chloride
treated 5-(tert-butyl)isophthalic acid (0.89 g, 4 mmol), which
was refluxed for 5 h. The resulting solutions were evaporated
in vacuo and dissolved in ethanol to get suspensions. The
precipitates were filtered and washed by mixed solvents
(ethanol/DCM = 8: 2) for several times to obtain yellow solid
Fig. 2 Reversible multi-stimuli responsive behaviors of BAPBIA gel upon heat,
light, TBAF and TFA (the two capped vials in each group: the left is under daylight
and the right is under UV light).
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Results and discussion
Synthesis and material characterizations
BAPBIA was synthesized by
a
series of knoevenagel
condensation reaction, reduction reaction and amidation
reaction. Synthetic details and corresponding characterization
data were given in the experimental section and supporting
information (Fig. 1). 1H, 13C NMR spectroscopic measurements,
elemental analysis, and mass spectra were employed to
confirm its structure (Fig. S1-S3).
Fig. 3 Emission spectra of BAPBIA in solvents with different polarities (1×10-5 M,
λex = 400 nm).
ICT effect
Due to the fact that donor (dimethylaniline group) and
accepter (cyano group) units were decorated on the molecular
backbone, ICT effect of BAPBIA was activated possibly. The
photophysical properties in various kinds of organic solvents
were studied systematically. As shown in Fig. S4a, two
characteristic absorption bands located at about 300 nm and
405 nm in these solutions with different polarities were
observed in the UV-Vis absorption. The shorter wavelength
absorption band was assigned to the π-π* transition and the
longer wavelength one was regarded as an ICT band, implying
the existence of CT state.11 Meanwhile, the environmentally
sensitive CT state could lead to outstanding solvatochromism
in solutions with different polarities.12 As expected, in the
fluorescence spectra, BAPBIA exhibited a significant red shift
from 463 nm to 495 nm upon increasing the solvent polarity
from THF to DMSO (Fig. 3). To further evaluate the effect of
solvent polarity on the emission characteristics, Lippert–
Mataga plots which told us the relationship between Stokes
shift (υA-υF) and polarity parameter (Δf) was used (Fig. S4b).11
The obtained plus slope of the fitting line indicated positive
solvatochromic effect and confirmed the ICT effect of BAPBIA
due to the D-π-A structure.
1
(2.11 g, yield 74 %). H NMR (500 MHz, DMSO-d6), δ/ppm =
10.56 (s, 2H), 8.43 (s, 1H), 8.17 (d, J = 1.1 Hz, 2H), 7.90 (dd, J =
20.1, 8.9 Hz, 8H), 7.81 (s, 2H), 7.72 (d, J = 8.7 Hz, 4H), 6.83 (d, J
= 9.0 Hz, 4H), 3.04 (s, 12H). 13C NMR (126 MHz,DMSO-d6),
δ/ppm = 165.84, 152.06, 151.93, 142.21, 139.37, 135.32,
131.43, 130.65, 128.14, 125.84, 124.91, 121.52, 121.23,
119.79, 112.11, 102.75, 40.07, 35.38, 31.48. Elemental
analysis: Anal. calcd for C46H44N6O2: C 77.5, H 6.22, N 11.79;
found: C 76.02, H 6.24, N 11.51. MALDI-TOF MS: m/z [M+]
calcd for: 712.35; found: 712.30.
Gelation test
A weighed sample of BAPBIA was mixed with a selected
organic solvent (0.5 ml) in a capped vial. The mixtures were
initially heated over an oil bath to dissolve the molecules,
followed by cooling to room temperature. The gel formation
was confirmed when inverting the vial in which no evident
liquid flow was observed. If only the clear solution was
retained, it was classified as soluble state (S). If the gelator
could not be dissolved, it was noted as insoluble state (I). The
critical gelation concentration (CGC) of BAPBIA was
determined by measuring the minimum amount of the gelator
required for the formation of stable gel at room temperature.
AIE properties
BAPBIA was insoluble in water, the emission characteristics of
BAPBIA in aggregation state were further investigated in the
mixture of acetonitrile and water. As shown in Fig. 4a, the
emission peak at 485 nm was very weak in pure acetonitrile
and almost kept constant until the water fraction reached
The gel-to-sol transition temperature (Tmelt
measured. With the increase of temperature, once gel moving
or liquid flow was observed upon the tilting of the vial, the
) was also
temperature at this time was regarded as Tmelt
.
Table 1 Gelation ability of BAPBIA in various solvents.a
Solvent
Phase
Solvent
Phase
Dichloromethane
1,2-Dichloroethane
Acetonitrile
G (8.2 )b (308)c
Toluene
Methanol
Chloroform
THF
Acetone
DMF
I
I
G (6.0) b (308)c
G (7.0) b (323)c
S
S
S
S
S
Dioxane
G (22.9)b (312)c
Ethyl acetate
Cyclohexane
n-Hexane
I
I
I
DMSO
aG, gel; I, insoluble; S, soluble. bNumbers in parentheses are the critical gel
concentrations (CGC, mg/mL), cNumbers in parentheses are the transition
temperatures of BAPBIA in various solvents (Tmelt, K).
Fig. 4 (a) Emission and (b) UV-Vis absorption spectra of BAPBIA in acetonitrile
with different water fractions (1 × 10-5 M, λex = 400 nm). Inset is the emission
intensity at 530 nm versus different water fractions.
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Fig. 6 FT-IR spectra of (a) BAPBIA xerogel and (b) powder.
Fig. 5 SEM images of the microstructures of xerogels obtained from (a)
acetonitrile, (b) dichloromethane, (c) 1,2-dichloroethane and (d) dioxane
gels.
Further investigations were aimed at a better elucidation of
the significant role of non-covalent interactions responsible for
the gelation process. In the FT-IR spectra (Fig. 6), the cyano
vibrations of the BAPBIA xerogel and powder were both at
2204 cm-1 with no shift and BAPBIA powder exhibited N-H
stretching vibration at 3351 cm-1. But a shifted N-H stretching
vibration at 3272 cm-1 was realized in the xerogel, implying
enhanced hydrogen bonding interactions in the xerogel
state.14 In addition, as shown in Fig. S5, the xerogel exhibited a
strong peak at 20.58° (0.432 nm) in the X-ray diffraction (XRD)
patterns. The d-spacing value was in accordance with the
distance of hydrogen bonds, indicating the existence of
hydrogen bonding interactions in the gel state once more.15
50%. However, a red shifted peak at 536 nm accompanied with
distinct enhanced emission intensity (yellow emission) was
reached upon adding over 60% water. At about 70% water, the
emission became the most intense and the maximal emission
intensity was almost 4.5 fold greater than that in dilute
solution. There is no doubt that the enhanced yellow emission
was ascribed to the aggregation behaviors of BAPBIA induced
by high water fractions. To ascertain the aggregation manners
of BAPBIA, UV-Vis absorption spectra were carried out (Fig.
4b). In comparison to the absorption peak (λ = 393 nm) in
acetonitrile, a bathochromic shifted (λ = 408 nm) peak as well
as decreased intensity at higher water fraction was observed,
indicating definite J-type stacking of cyanostilbene moieties.13
Multiple stimuli responsive properties
It is well-known that supramolecular gels are sensitive to
external stimuli because of the nature of non-covalent
interactions.2 Therefore, the reversible gel-sol transitions
accompanied with fluorescence switching were studied
systematically in response to heat, light, fluoride ion, TFA. It is
noticed that all the stimuli-responsive experiments were
carried out in acetonitrile gel.
Gelation properties
The gelation abilities of BAPBIA were illuminated in various
organic solvents by the standard heating-and-cooling method
and the results were summarized in Table 1. BAPBIA was able
to form gels in relative high polar solvents like
dichloromethane, 1,2-dichloroethane, acetonitrile and
dioxane. The corresponding critical gelation concentration
(CGC) was as low as 6 mg/ml in 1,2-dichloroethane, implying
an excellent gelation property. The gel-sol transition
temperatures (Tmelt) were also measured and summarized. The
resulting gels could endure ambient conditions for several
weeks. Additionally, we investigated the self-assembled
morphologies of the gels using scanning electron microscopy
(SEM). It is noticed that the morphologies were related to the
corresponding gelation solvents. Thus, fibers or ribbons like
microstructures formed in different solvents were realized
(Fig. 5). For example, in acetonitrile, 3D entangled network
structures comprised of nanofibers with high aspect ratios,
widths of 200–500 nm and lengths of dozens of micrometers,
were obtained, revealing nice gelation properties (Fig. 5a).
Whereas, in dioxane, we observed shorter and wider ribbons
structure (Fig. 5d). The widths were about 1-5 μm.
Heat responsive properties
Fig. 7 Temperature dependent emission spectra of BAPBIA gel (λex = 450 nm).
Inset: pictures of BAPBIA gel before and after heating under daylight (the left)
and UV light (the right), respectively.
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photoisomerization happened.20 As seen in Fig. 8, BAPBIA
View Article Online
1
adopted a Z,Z-form in the initial state byDtOheI: 1r0e.1s0u3lt9s/Co8fSMH02N4M34RK
spectroscopy. Upon UV irradiation, proton peaks of amide
bonds and aromatic rings started displacement, indicating the
occurrence of photoisomerization behaviors. Proton peaks
near amide bonds (Ha) were used as the indicators to
distinguish the isomers. With the increase of irradiation time,
the amount of Z,E and E,E-isomers increased gradually with a
concomitant decrease of the Z,Z-isomer until
a
photostationary state was reached (Ha’ = 8.40 ppm, Ha’’ = 8.41
ppm). By the integral analysis of the 1H-NMR spectra of
BAPBIA at different irradiation time, kinetic profiles were
plotted. After UV irradiation for 20 minutes, the
photostationary state products consisted of the isomeric
mixture could be achieved and the ratio of Z,Z, Z,E and E,E
isomers was 6: 43: 51 (Fig. S6). We then tested the variation of
the gel upon UV irradiation at 365 nm. It was found that the
gel with yellow emission collapsed completely and turned to a
viscous nonluminous solution after 5 minutes. As to the
fluorescence spectra, a distinct decreased intensity could be
also observed (Fig. S7). Based on the above results, it could be
concluded that Z,Z-isomer possessed better gelation ability
and stronger emission than the other two isomers. As a result
of UV irradiation, plentiful Z,E or E,E-isomers emerged and
disturbed the ordered superstructures, leading to the collapse
of the gel. Meanwhile, as Z,E or E,E-isomers revealed no
fluorescence features, corresponding emission was quenched
after UV irradiation.19 Additionally, the photoisomerization
behaviors could be reversed by heating, which was
demonstrated by the results of 1H NMR spectroscopy (Fig. S8).
Thus, the light caused solution converted back to the original
gel after heating for 18 h. It is noticed that the fluorescence
was also recovered.
Fig. 8 (a) Pictures of the reversible responsive behaviors of BAPBIA acetonitrile
gel (8 mg/ml) upon UV irradiation at 365 nm and heating; (b) 1H NMR spectra
variations of BAPBIA (1 × 10-3 M) in DMSO-d6 solution with the increase of UV
irradiation time until 20 minutes.
Heat induced gel-sol transition is common owing to the labile
nature of supramolecular motifs. The increased temperature
could contribute to disappearance of aggregates.16 Herein, in
view of the emission properties of BAPBIA, the heat induced
emission changing was worth expecting. From the
temperature dependent fluorescence spectra (Fig. 7), the gel
showed a yellow emission (530 nm) at room temperature. A
gradual decline of the intensity of emission peak was observed
as the temperature increased. Until Tmelt was reached, the gel
with yellow emission converted to nonluminous solution
completely, while the emission would be restored
accompanied with gel reformulation by cooling procedure. In
this way, the emission can be reversibly modulated by
alternating cooling and heating, which had underlying
applications in thermally driven fluorescence molecular switch.
Flouride ion responsive properties
Light responsive properties
Light stimuli responsive materials have aroused considerable
attentions since light as a stimulus has not only green
pollution-free nature but also the capacity of remote
controlling.17
Cyanostilbene
could
go
through
photoisomerization from Z-form to E-form upon UV
irradiation. Especially, the photoisomerization could induce
outstanding fluorescence variation in comparison to other
photoswitchable molecules like azobenzene or stilbene.18
However, cyanostilbene based photoresponsive gels are rarely
reported on account of the high degrees of aggregation.14b,19
Considering the nice gelation ability of BAPBIA, we were
interested in whether the transformation of configuration
1
would influence the gel or emission properties. Therefore, H
NMR spectroscopic measurements were used to examine the
detailed photoisomerization process in solution at first upon
UV irradiation at 365 nm. As BAPBIA possessed two
cyanostilbene moieties, three isomers (Z,Z, Z,E, E,E) with
different chemical shifts would be discovered if the
Fig. 9 (a) Pictures of the reversible responsive behaviors of BAPBIA acetonitrile
gel (8 mg/ml) upon TBAF/water stimuli; (b) 1H NMR titration experiments of
BAPBIA in DMSO-d6 solution with the increase of the amount of TBAF.
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completely finally. A new triple peak emerged around 16.14
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DOI: 10.1039/C8SM02434K
−
ppm, which was ascribed to the HF2 dimer, indicating the
deprotonation reaction between fluoride ion and amide
bonds.22,23 More interestingly, when a spot of water was
added to the resulting solution, gel reformation as well as
fluorescence recovery were realized. Better explanations were
state as follows. The hydrogen bonding interactions formed by
amide bonds, the main driving force of the gelation, were
destroyed due to the deprotonation effect induced by fluoride
ion, leading to the collapse of the gel. Super polar protic
solvent-water, a more effective competitor for fluoride ion,
made fluoride ion can’t work with amide bonds and blocked
amide based hydrogen bonds.24 Thereby, reconstituted
protonation and hydrogen bonding interactions contributed to
the recovery of yellow emission gel.
Fig. 10 (a) Time dependent pictures of BAPBIA acetonitrile gel (8 mg/ml) with the
addition of 10 equiv. TFA; (b) 1H NMR titration of BAPBIA (5 × 10−4 M) with the
addition of TFA in CDCl3 solution.
TFA responsive properties
Acid recognition is always the research focus in industrial and
healthcare. In particular, protons released by acid are utilized
by scientists to develop smart acid responsive materials.25 In
our previous work, gel-sol transformation and the consequent
fluorescence quenching upon TFA stimulus were realized.20a
Nevertheless, no reaction site but the lipophilic diffusion of
TFA was observed in the process. Herein, the introduction of
dimethylaniline group, for one thing, provides a definite
proton-binding site, improving the sensitivity towards TFA. For
the other thing, the ICT effect enlarges the emission contrast
before and after the protonation. At first, the protonation
effect triggered by TFA was confirmed by 1H NMR titration
experiments. As shown in Fig. 10, all the peaks showed various
degrees of shift with the addition of TFA. And in particular, the
signals of dimethylaniline aromatic protons (Hh) and methyl
protons (Hi) revealed obvious downfield shifts from 6.76 and
3.07 to 7.64 and 3.31 ppm, respectively, demonstrating a
significant structural change because of the protonation of
dimethylaniline group.26 The protonation process was further
expressed in the gel state. With the addition of TFA, the yellow
emission gel collapsed gradually and corresponding collapsed
part was replaced by brilliant blue emission solution. After only
30 seconds, complete blue emission solution was obtained, as
shown in Fig. 10a. As far as fluorescence spectra were
concerned, with the increase of the amount of TFA, the
original emission peak at 530 nm decreased and blue shifted
gradually (Fig. 11b). Finally, a new peak (λ = 463 nm) with a
Fluoride ion plays an important role in medical, environmental
and chemical aspects.21 In particular, the strong
electronegativity of fluoride ion provides the possibility of the
deprotonation of N-H or O-H group, in this way, we realized
colorimetric variations toward fluoride ion based on amide
functionalized gel in our previous work.22 However, there were
no fluorescence signals displaying. In this work, we adopted an
AIE supramolecular gel system also modified by amide bonds
to study fluoride ion triggered emission as well as gel
properties. Gel-sol transformation together with fluorescence
decaying was observed with the addition of only 1 equiv.
tetrabutylammonium fluoride (TBAF) (Fig. 9a). The change of
emission was also reflected in the fluorescence spectra. The
intensity exhibited an obviously decrease (Fig. S9). The
variation was proved through the 1H NMR titration
experiments (Fig. 9b). All the shifted proton peaks manifested
that fluoride ion worked with the molecule. Especially, with
the increase of the amount of TBAF, proton peak of amide
bonds (Hc, 10.56 ppm) shifted downfield and vanished
Fig. 11 (a) Pictures of the reversible responsive behaviors of BAPBIA acetonitrile
gel (8 mg/ml) upon TFA/TEA stimuli; (b) emission spectra of the gel with the
increase of the amount of TFA (λex = 400 nm).
Fig. 12 The possible mechanism of the TFA/TEA caused fluorescence color
transitions.
6 | J. Name., 2012, 00, 1-3
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Soft Matter
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significant enhanced intensity was reached. The process could
be also reversible with the assistance of triethylamine (TEA)
owing to the deprotonation effect. The outstanding
fluorescence colour variation must be related to the special
structure of BAPBIA. The original BAPBIA possessed a D-π-A
structure and displayed yellow emission in the gel state.
Because of the TFA caused protonation effect, the
dimethylamino group (electron donating group) transformed
into an electron withdrawing group, contributing to the whole
conjugation structure variations as well as the loss of the ICT
effect (Fig. 12).26 Thus, the protonated structure generated a
blue-shifted emission and gel collapse. And TEA triggered
deprotonation could result in the transformation of the
structure. In this way, remarkable reversible gel-sol transitions
accompanied with emission color variations (yellow-blue) were
achieved.
2
3
4
Conclusions
A novel V-shaped cyanostilbene-based gelator (BAPBIA) with
AIE characteristics has been synthesized. BAPBIA could form
stable gels in various organic solvents. Due to the fact that the
molecule is decorated on amide bonds, dimethylaniline group
and cyanostilbene backbone, which provides deprotonation
site, protonation site and photoisomerisation unit, BAPBIA
gels show multi-stimuli responsive behaviors. Upon external
stimuli including heating, light, fluoride ion and TFA, reversible
gel-sol phase transitions together with emission switching are
realized. In particular, the TFA triggered protonation of
dimethylaniline group can generate an emission color
transition (yellow-to-blue) accompanied with enhanced
intensity, which is attributed to the variation of the ICT effect.
The initial gel exhibits a yellow CT emission, while the
protonation makes the dimethylamino group (electron
donating group) transforms into an electron withdrawing
group, contributing to the loss of the ICT effect to obtain a
blue-shifted emission. The interesting results show that this
kind of soft material has large potential applications in
multifunction aspect for its richness, visuality and reversibility.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51673082 and 21374036).
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Reversible gel-sol transitions accompanied with fluorescence switching of BAPBIA View Article Online
DOI: 10.1039/C8SM02434K
gel upon multiple stimuli including heat, light, TBAF and TFA.
TOC Figure