Responsive aggregation-induced emissive supramolecular gels based on bis-cyanostilbene derivatives

Article abstract of DOI:10.1039/c6tc03260e

In aromatic solvents, V-shaped bis-cyanostilbene derivative 1 forms stable and emissive gels which are capable of responding to light and, selectively, to TFA via a gel-to-sol transformation.

Full text of DOI:10.1039/c6tc03260e

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Responsive aggregation-induced emissive
supramolecular gels based on bis-cyanostilbene
Cite this:DOI: 10.1039/c6tc03260e
derivatives†
Yao Ma,a Massimo Cametti,b Zoran Dzolic*c and Shimei Jiang*a
Received 30th July 2016,
Accepted 29th October 2016
ˇ
´
DOI: 10.1039/c6tc03260e
www.rsc.org/MaterialsC
In aromatic solvents, V-shaped bis-cyanostilbene derivative 1 forms and more so those also capable of both gelation and stimuli
stable and emissive gels which are capable of responding to light responsiveness. Among the fluorescent organogels reported to date,
and, selectively, to TFA via a gel-to-sol transformation.
only very few examples exhibit aggregation-induced emission (AIE)
properties. In such cases, fluorophores that exhibit almost no
Low-molecular-weight organic gelators (LMWOGs) have received fluorescence as discrete molecules in dilute solutions become highly
considerable attention in supramolecular chemistry and materials emissive in an aggregated state. As a consequence of the reversible
science due to their unique properties and a wide range of nature of the self-assembly process at the basis of supramolecular
potential applications.1,2 Apart from basic studies on gel formation gelation, AIE gel materials may exhibit significant modulation
and structural morphology, extensive efforts have been devoted to of their emission upon external inputs. Although several AIE
the investigation of functional and stimuli responsive gels.3 The organogels have been reported in the past years, examples of a
preparation of smart soft materials able to react to external stimuli rational application of established design principles for obtaining
and translate them into a well-defined, controllable, and reversible stimuli-responsive supramolecular organogels are rare.
macroscopic response represents one of the most fascinating
Recently, we have developed a class of effective LMOGs based
and challenging goals of current research on gel materials. This on V-shaped cyanostilbene amide derivatives, which readily formed
explains the numerous works present in the literature, where the gels in various organic solvents and also exhibited interesting
bulk shape and properties of supramolecular gels based on anion-binding properties and AIE behaviour.7 Herein, we present
LMWOGs are found to be responsive to a range of external a detailed study on the V-shaped compound 1 (Fig. 1A) based on a
stimuli of chemical and physical nature.4 As to the latter kind, 1,3,5 substituted benzene core decorated with two cyanostilbene
the use of light is of particular interest for it allows both spatial units via an amidic bond and with a hydrophobic C8 tail.
and temporal control of a specific reaction without requiring Compound 1 is capable of forming stable and emissive gels
physical contact.5 Recently, processes like aggregation induced in aromatic solvents which, interestingly, can be transformed
emission (AIE) or aggregation induced emission enhancement into solution in response to either light (365 nm) or its exposure
(AIEE) have been utilized in developing ‘‘stimuli-responsive to trifluoroacetic acid (TFA). The latter transformation is quite
fluorescent gels’’, which exhibit gel-to-sol phase transitions selective in terms of the acidic species involved. Moreover, at
along with remarkable fluorescence modulation.6 The design variance with other acid-responsive gel systems reported,8 no
of stimuli-responsive fluorescent gelators is very challenging and acid–base reaction seems to be the triggering event of the
thus it has only rarely been reported. Indeed, molecules that observed behaviour. In addition, the effect of the chain length
maintain their fluorescence in the aggregated state are quite rare has been taken into account as shorter and longer chain
derivatives, 2 and 3, were also synthesized and tested.
The gelation ability of 1–3 was systematically investigated
(Table S1 in the ESI†). Compounds 1 and 2 were able to form
a State Key Laboratory of Supramolecular Structure and Materials, Jilin University,
2699 Qianjin Avenue, Changchun 130012, P. R. China.
gels in all the tested aromatic solvents. In particular, compound 1
showed the best gelation performance, showing a lower critical
gelation concentration (cgc). In general, cgc of 1 and 2, depending
E-mail: smjiang@jlu.edu.cn; Fax: +86-431-85193421; Tel: +86-431-85168474
b Department of Chemistry, Materials and Chemical Engineering ‘‘Giulio Natta’’,
Politecnico di Milano, Via L. Mancinelli 7, 20131 Milano, Italy
c
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´
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Rudjer Boskovic Institute, Bijenicka cesta 54, 10000 Zagreb, Croatia.
on the solvents, varied in the range of 2.9–8.3 and 3.5–10.0 mg mLÀ1
,
E-mail: zoran.dzolic@irb.hr
respectively. The obtained gels were stable and could endure
ambient conditions for several weeks. In contrast, compound 3,
having a shorter chain length, does not form gels with any of the
† Electronic supplementary information (ESI) available: Detailed synthetic pro-
cedures and characterization data, NMR spectra, UV/Vis and fluorescence spectra.
See DOI: 10.1039/c6tc03260e
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Fig. 2 (A) Fluorescence spectra of 1 in toluene solution (dashed line,
1 Â 10À5 M) and in the gel state (solid line, 5.7 mM), lex = 365 nm. The inset
shows the enlarged fluorescence spectrum of 1 in solution. (B) Temperature-
dependent fluorescence spectra of gel 1.
475 nm, redshifted by ca. 23 nm from the weak emission at
452 nm observed in dilute solution. Significant bathochromic
shifts are commonly observed in aggregating systems, or in the
solid state compared to solution, and point to strong interactions
between gelator molecules in the gel state. Notably, when the
toluene gel is heated until the solution state is reached, the
emission peak intensity at 475 nm is greatly reduced (Fig. 2B),
while emission is restored upon gel formation by cooling to r.t. As to
the mechanism of AIE, it is quite established that the cyanostilbene
moiety is practically non-fluorescent in the molecular solution
Fig. 1 (A) Molecular formulae of the V-shaped bis-cyanostilbene derivatives
1–3. (B) TEM image of toluene gel 1 (scale bar = 500 nm); insets are the
photographs of the toluene gel under ambient light (left) and under illumina-
tion at 365 nm (right). (C) Emission and (D) UV-Vis absorption spectra of 1 in
THF/water mixtures with different water volume fractions. The inset shows
the changes in the emission intensity at 458 nm as a function of water fraction
([1] = 1 Â 10À5 M).
tested solvents except in bromobenzene, where though it behaves state, where non-emissive relaxation can easily occur due to
as the best gelator among the three species. The morphologies of rotational freedom. Upon aggregation, such conformational
these cyanostilbene-based organogels in toluene were studied by motions are restricted, the molecule is planarized and, thus,
scanning electron microscopy (SEM) and transmission electron relaxation goes through an emissive pathway.
microscopy (TEM). Both microscopy techniques reveal gelator
Further studies were then aimed at a better elucidation of the
molecules aggregated into nano-fibrils of several microns in length, driving forces responsible for gelation. Temperature-dependent
with a very high aspect ratio and with diameters ranging from 8 to 1H-NMR measurements were conducted on the gel of 1 in
29 nm, respectively, as shown in Fig. 1B (and Fig. S7, ESI†).
toluene (Fig. S10, ESI†). Upon heating to 328 K no significant
Compounds 1–39 are not soluble in several common polar disaggregation of the networks into dissolved, NMR-observable
and weakly polar solvents (MeOH, MeCN, acetone, CH2Cl2 and aggregates occurred. Upon further heating, sharper signals
CHCl3, EtOAc and DCE), while when dissolved in dioxane and appeared gradually; however, negligible shifts, also for the
THF, they are practically non-emissive. However, their fluores- amidic NH group, were observed. Different is the case when a
cence is distinctly enhanced upon addition of an increasing small aliquot of polar solvent is added to the gel sample. For
amount of water. For example in THF, as shown in Fig. 1C, example, in another experiment, 5 mL DMSO-d6 was added to the
1 presents a weak band in emission at 429 nm, whose intensity previous toluene gel 1 (initial gel volume = 1.0 mL) and
increases dramatically upon increasing the water molar fraction temperature-dependent 1H-NMR spectra were again recorded
( fw). In addition, a significant redshift of the emission occurs (Fig. S11, ESI†). The addition of the small amount of DMSO
(D = 29 nm). A similar behaviour can be observed for compounds causes notable changes. First of all, at 298 K the NH signal
2 and 3 (Fig. S8 and S9, ESI†).
appears at ca. 10.44 ppm, a considerably downfield shifted value
As far as UV-Vis absorption is concerned, the UV-Vis absorption in comparison with the 7.90 ppm signal seen in pure toluene.
band of 1 is red-shifted from ca. 350 nm, in pure THF solution, to Also, under co-solvent conditions, gelator signals become more
ca. 400 nm upon increasing the water molar fraction ( fw) along evident and less broadened, but most importantly, upon increasing
with a gradual decrease in the intensity (Fig. 1D). This clearly T, a dramatic upfield shift in the amide protons (from 10.44 to
indicates that the environment can affect the aggregation of 1–3, 9.49 ppm) is observed. This large temperature dependence of
and these data are consistent with cyanostilbene moieties stacking amide protons (DdNH/DT = 13.6 Â 10À3 ppm KÀ1), not observed
together in J-type arrays.10 As compared to our previous report,7b in the gel sample made in pure toluene, can be rationalized by a
where no obvious AIE was observed for a cyanostilbene derivative progressive disruption of the intermolecular hydrogen bonds (HBs)
with a NO2 group in various solvents, it is apparent that the alkyl between DMSO and 1 molecules. If that is the case, this points to a
chains in 1–3 significantly promote the aggregation.
minor contribution of HBs to the overall stabilization of the gel.
Notably, in line with the spectroscopic studies above, the gel of
Finally, we analyzed toluene gel 1 by the FT-IR technique.
1 in toluene exhibits a significantly enhanced emission (Fig. 2A), Selected regions of the temperature-dependent FT-IR measure-
in contrast to the weak emission in solution. In addition, the gel ments for organogel 1 in toluene are shown in Fig. S12 (ESI†).
of 1 in toluene emitted blue light with an emission maximum at The band at 3325 cmÀ1, in the N–H stretching region, can be
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assigned to the hydrogen-bonded N–H stretching vibration,
while the shoulder at 3440 cmÀ1 can be assigned to the free
(not-bonded) N–H. Instead, the amide I band for CQO appears
at 1678 cmÀ1. Upon increasing the temperature, the hydrogen-
bonded N–H band gradually decreases while the amide I band is
blue-shifted to 1691 cmÀ1 (ca. 13 cmÀ1), confirming the participation
of the amide groups in the formation of hydrogen bonds in the
organogel. Overall, aggregation of 1 can be viewed, no surprise about
that, as the resultant of several different intermolecular inter-
actions, with a major contribution of aromatic stacking, while
HB interactions surely have a lower impact.
Cyanostilbene could be switched from the initial trans (Z-) form
to the cis (E-) form upon UV light irradiation. However, compared
to the more common stilbene and azobenzene molecules, the
Z–E isomerization process of cyanostilbene derivatives is little
known and literature data are especially scarce regarding the
corresponding fluorescence modulation in supramolecular
gels.5c As compound 1 is a very good gelator, we were interested
in verifying whether the gelator configuration would have an
effect on its gelation properties or not. Thus, we first studied in
more detail the light induced Z-to-E isomerization process of 1
Fig. 3 (A) 1H-NMR spectral changes of 1 upon photoirradiation (7 mM in
DMSO-d6 at 298 K) starting with the Z,Z isomer followed by irradiation with
UV light (365 nm) at 298 K. (B) Time evolution of the isomer ratio of 1 as
estimated by integral analysis. (C) Time-dependent fluorescence spectra
of toluene gel 1 upon irradiation with 365 nm light. (D) Photographs of
toluene gel 1 after photoirradiation for (from left to right) 0, 5, 10, 20, 30
and 40 minutes under UV light.
1
in solution by H-NMR. In the initial state, the cyanostilbene
unit in 1 adopts a Z configuration, as evidenced by well-assigned
1H-NMR signals (Fig. S13, ESI†). The Z-to-E photoisomerization
occurs when the compound is irradiated using UV light at
365 nm. The appearance of new NH peaks corresponding to
the formation of new isomers can be observed. As 1 possesses
two photoisomerizable double bonds, the starting material in
configuration Z,Z can give rise to two different photoproducts,
E,Z- and E,E-isomers, all easily identifiable in the NH region
(10.5–10.8 ppm). As seen in Fig. 3A, over a prolonged irradiation
time, the amount of Z,E- and E,E-isomer products increased
gradually with a concomitant decrease of the Z,Z-isomer until a
photostationary state is reached. By the integral analysis of the
1H-NMR spectra of 1 in DMSO-d6 at different irradiation times
kinetic profiles could be plotted (Fig. 3B). After ca. 1 h irradiation
time, the photostationary state is composed of the following
isomeric mixture: Z,Z : Z,E : E,E ratio of ca. 7 : 38: 55. We then
tested the response of the gel state to irradiation. Toluene gel 1
(ctol = 4.1 mM) was irradiated with 365 nm light (Fig. 3C and D).
After 40 minutes, as documented by photographs in Fig. 3D, the
gel was transformed into a viscous solution. The obtained
solution state was also analyzed by 1H-NMR, confirming the
presence of significant amounts of Z,E and E,E isomers (Fig. S14,
ESI†). As the temperature was maintained constant during
irradiation, the gel-to-sol transition can be unambiguously con-
sidered to be photoinduced. Hence, it can be concluded that the
Z,Z isomer has a considerably stronger gelating ability than the
other two isomers which are gradually formed by irradiation.
Interestingly, the isomerization reaction can be reversed by
heating (Fig. S14, ESI†). Thus, if the irradiated solution is heated
for a short time, it can be transformed back into a stable gel.
Toluene gel 1 was also found to be selectively responding to
TFA exposure in a way which could be interesting for TFA
sensing purposes (Fig. 4). Fluorescent gelators capable of acid
literature so far, the gelator molecules contain clearly defined
basic sites that can specifically interact with acidic species,
resulting in shifts in emission wavelength or fluorescence
quenching.8 The gel of 1 in toluene could be converted into
fluid solution upon exposing to TFA vapors (Fig. 4C-b). Conse-
quently, the emission at 475 nm, as said to have been induced
by aggregation, was also quenched immediately. Under the
same conditions, other examined volatile acid vapors, such as
HCl, HBr, formic acid and acetic acid, could not lead to gel
decomposition and had no effects on the emission spectra of
toluene gel 1 (Fig. 4A). Therefore, the gel possesses excellent
selectivity for TFA. The same behaviour was also observed upon
addition of a few drops of TFA on the top of the preformed gel
Fig. 4 (A) Fluorescence spectra of toluene gel 1 (5.7 mM) upon its exposure
to different acid vapors at 298 K (lex = 365 nm); (B) response of the gel
system to the addition of TFA drops onto its top layer over a period of a few
minutes; (C) photographs under natural light (left) and under UV light (right)
of (a) toluene gel 1, (b) the system after exposure to TFA vapors and
sensing are rare. Moreover, in all the systems reported in the (c) restoration of the gel state upon addition of TEA followed by sonication.
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(Fig. 4B), while, again, other acids did not perturb the gel multistimuli-responsive fluorescence switching materials applied
phase. This clearly highlights the selective response of the gel here may be widely applicable to the development of other func-
towards TFA over other common acidic species. The gel sample tional materials, and further studies along this research line are
transformed to solution upon exposure to TFA vapors was currently in progress in our laboratories.
analyzed by 19F-NMR in order to determine the actual amount
This work was supported by the National Natural Science
of acid needed to produce such a conspicuous transition Foundation of China (21374036), the National Basic Research
(Fig. S15, ESI†). Interestingly, the amount of TFA calculated Program of China (2012CB933800) and by a Croatian-Chinese
by integral analysis in the presence of an internal standard of bilateral project.
known concentration was found to be quite high, ca. 75 times
the gelator molecule in terms of molar ratio. On the other hand,
Notes and references
the 1H-NMR spectrum recorded on the same sample reveals
small downfield shifts of the aromatic proton signal (Fig. S16,
ESI†). A more accurate titration experiment of compound 1
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TFA–TEA cycle can be repeated more than three times, thus
showing the potential of this system for fluorimetric sensing of
acid vapors.
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