AIE Fluorescent Gelators with Thermo-, Mechano-, and Vapochromic Properties

Article abstract of DOI:10.1002/asia.201801689

A series of aggregation-induced emission (AIE) fluorescent gelators (TPE-C_n-Chol) were synthesized by attaching tetraphenylethylene (TPE) to cholesterol through an alkyl chain. The properties of the gel, nano-/microaggregate, and condensed phas

Full text of DOI:10.1002/asia.201801689

CHEMISTRY
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Accepted Article
Title: AIE Fluorescent Gelators with Thermo-, Mechano- and Vapo-
Chromic Properties
Authors: Hui CHEN, Lu Zhou, Xiang Shi, Jun HU, Jia Guo, Pierre-
Antoine Albouy, and min-hui Li
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AIE Fluorescent Gelators with Thermo-, Mechano- and Vapo-
Chromic Properties
Hui Chen,a, b, † Lu Zhou,a, † Xiang Shi,c Jun Hu,a Jia Guo,a Pierre-Antoine Albouy,c and Min-Hui Li*a, b
Abstract: A series of AIE fluorescent gelators TPE-Cn-Chol were
synthesized by attaching the tetraphenylethylene (TPE) to
cholesterol through alkyl chain. The properties of their gels,
nano/micro-aggregates and condensed phases were studied
carefully. TPE-Cn-Chol molecules form AIE fluorescent gels in
acetone and in DMF. Their fluorescence can be reversibly switched
between “on” and “off” state by the gel-sol phase transitions upon
thermal treatment. The AIE properties of aggregated
nano/microstructures in acetone/water mixtures with different water
fractions were studied by fluorescence spectrometry and scanning
electron microscopy (SEM). In different acetone/water mixtures TPE-
Cn-Chol molecules form different nano/micro-aggregates, such as
rod-like crystallites and spherical nanoparticles that show different
fluorescence colours. Finally, the condensed phase behaviours of
TPE-Cn-Chol were studied by polarizing microscopy (POM),
differential scanning calorimetry (DSC), fluorescence spectrometry,
fluorescence optical microscopy and wide-angle X ray scattering
(WAXS). The clover-like TPE unit introduced into the rod-like
cholesterol mesogen inhibits not only the formation of liquid crystal
phase but also the recrystallization upon cooling from isotropic liquid
phase. Very interestingly TPE-Cn-Chol molecules in the condensed
state change their fluorescence colours upon external stimuli like
melting, grinding and solvent fuming. The phase transition is the
origin of these thermo-, mechano- and vapo-chromic properties.
These findings offer a simple and interesting platform for the creation
of multi-stimuli-responsive fluorescent sensors.
and condensed states, which is prerequisite to develop efficient
optical materials. [3b, 4, 7]
The self-assembling of AIEgen-containing molecules into highly
emissive organogels has attracted considerable attention of the
researchers for several years, because of their interesting
8]
properties and potential applications.[7a,
Among various
gelators, cholesterol is a common one that is used to combine
with AIEgens or other functional moieties in the fabrication of
functional gelators.[2, 9] For example, Wang and Ma et al., have
prepared AIE gelators by connecting the tetraphenylethylene
(TPE) and cholesterol with a central salicylideneaniline which
may allow to make use of the Excited-State Intramolecular
Proton Transfer (ESIPT) process with large Stokes shift in gel
state.[9d] On the other hand, cholesterol is also a famous LC
building block (mesogen), cholesteryl benzoate being the first
molecule in which liquid crystal properties were discovered.[10]
The cholesterol moiety has also been used to form complicated
[11]
LC phases. For example, Hardouin et al.
have reported
smectic A, smectic C*, TGB and cholesteric phases in a
compound made of a cholesterol and a Schiff’s base (another
rod-like mesogen) linked by
a 6-oxyhexanoyloxy spacer.
Recently, our group has developed a series of cholesterol-based
AIE gelators which combined gelation, AIE and photoresponsive
LC properties altogether. [9c] These molecules contain a central
azobenzene unit (photo-responsive mesogen) to which the
cholesterol and the TPE are connected through flexible spacers
at the two opposite sides, respectively. In these molecules with
two mesogens (cholesterol and azobenzene), LC phase persists
in spite of the presence of the non rod-like but clover-like TPE.
Now we wonder whether the LC phase will still exist if the TPE is
attached directly to one mesogen (the cholesterol), and we want
to study their gelation and AIE properties.
Therefore, we have designed TPE-Cn-Chol as shown in Scheme
1 by connecting the TPE to cholesterol via a spacer -
O(CH2)nCOO-. We have studied the gelation capacity of TPE-
Cn-Chol in organic solvents, the phase behavior of TPE-Cn-Chol
in condensed state, and the fluorescence properties of
organogels and of condensed phases. The clover-like TPE unit
introduced into molecule with only one mesogen (cholesterol)
inhibits the formation of liquid crystal phase, as well as the
recrystallization upon cooling from isotropic liquid phase. The
fluorescence change upon gel-sol transition was examined.
External stimuli like melting, grinding and solvent fuming were
applied to TPE-Cn-Chol in condensed state, and consequently
fluorescence colours change between blue and cyan. We have
demonstrated that the difference of molecular packing in
crystalline phase and in amorphous phase is the cause of this
AIE fluorescence difference. We believe that our systems offer a
simple platform for the design of thermo-, mechano- and vapo-
chromic fluorescent sensors.
Introduction
The design and the creation of luminophores that exhibit efficient
luminescence in the aggregated nano/microstructures and in the
condensed states are of great importance for the development
of advanced optical materials, such as fluorescent vesicles,[1]
fluorescent gels,[2] luminescent liquid crystals (LCs),[3] sensors,[4]
organic light emitting diodes (OLEDs).[5] Despite these promising
applications, the conventional luminophores in the aggregated
nano/microstructures and in the condensed states often suffer
6]
from the aggregation caused fluorescence quenching.[5a,
Thanks to the discovery of the aggregation-induced emission
(AIE) phenomenon, AIE luminophores (also called AIEgens)
offer an interesting remedy to the fluorescence quenching
problem. These luminophores are highly emissive in aggregates
a
Beijing Advanced Innovation Center for Soft Matter Science and
Engineering, Beijing University of Chemical Technology, Beijing,
100029, China. E-mail: min-hui.li@chimieparistech.psl.eu
Chimie ParisTech, PSL University Paris, CNRS, Institut de
Recherche de Chimie Paris, 75005 Paris, France.
b
c
Université Paris-Sud, CNRS, Laboratoire de Physique des Solides,
UMR8502, 91405 Orsay Cedex, France.
†
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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1
Results and Discussion
All the compounds were carefully characterized by H NMR,13C
NMR and high-resolution mass spectrometry (HRMS) as shown
in Figure S1-S6.
Synthesis
The series of TPE-Cn-Chol (n = 1, 4, 5 and 6) was prepared
according to the synthetic route shown in Scheme 1. Briefly,
TPE-OMe was prepared by the coupling reaction of
diphenylmethane and 4-methoxybenzophenone, followed by
acid-catalyzed dehydration. Then the TPE-OH was synthesized
by the deprotection reaction with BBr3. By Williamson reaction,
the TPE-OH was connected to the Br-Cn-COOMe leading to
TPE-Cn-COOMe which was then transformed to TPE-Cn-COOH
by a hydrolysis reaction. Finally, cholesterol was attached to the
TPE-Cn-COOH through esterification to afford the TPE-Cn-Chol.
Gelation Behavior
The gelation behavior of TPE-Cn-Chol in various organic
solvents was recorded by using “stable to inversion of the test
tube” method. In general, a weighed compound of TPE-Cn-Chol
was mixed with a selected solvent in the capped vial, followed
by the heating in an oil bath until a clear solution formed. Then
the sample vial was cooled down to room temperature to assess
the gelation behavior. The results are summarized in Table 1.
O
BBr3
DCM
Br(CH2)nCOOCH3
1) n-BuLi/ THF
+
K2CO3/ MeCN
O
2) TsOH/ Toluene, reflux
O
OH
n
O
O
O
TPE-Cn-COOMe
TPE-OH
TPE-OMe
Cholesterol
EDCI/DMAP
LiOH
THF/ H2O
O
O
n
OH
O
n
O
O
TPE-Cn-Chol
(n= 1, 4, 5, 6)
TPE-Cn-COOH
Scheme 1. The synthetic route for TPE-Cn-Chol (n= 1, 4, 5, 6).
As shown in Table 1, all TPE-Cn-Chol derivatives could form gel
in acetone, whose critical gelation concentration (CGC)
increased with the length of flexible spacer Cn increasing. All
TPE-Cn-Chol derivatives except TPE-C6-Chol could also form
gel in dimethylformamide (DMF). The results indicate that both
the length of flexible spacer Cn and the choice of solvent
influence the formation of gel.
To explore the thermal stability of as-prepared gel, the gel–sol
phase transition temperatures (Tgel) of the TPE-Cn-Chol gels
formed in DMF and acetone at different concentrations were
recorded. Taking TPE-C5-Chol as example, the Tgel increased
from 28 ℃ to 56 ℃ with the increase of the concentration of
TPE-C5-Chol in acetone, which indicated the stability of the gel
was enhanced with the increase of concentration (Figure 1).
Similar trend of Tgel was found for the gel formed in DMF. Based
on the relationship between Tgel and concentration (Figure 1 and
Figure S7), thermodynamic parameters ΔH, ΔS and ΔG at 298 K
for all samples in DMF and acetone were calculated as shown in
Figure S8 and summarized in Table S1. The ΔG values of all gel
samples are larger than 6.5 kJ.mol-1, which means that at
normal room temperature (25 °C, 298K) all gel-sol transitions
are unfavorable and these gels are stable.
Table 1. Gelation behaviour of TPE-Cn-Chol in organic solvents.a
Entry
Solvent
TPE-C1-
Chol
TPE-C4-
Chol
TPE-C5-
Chol
TPE-C6-
Chol
(CGC)
(CGC)
(CGC)
(CGC)
1
2
3
4
5
6
7
8
n-pentane
hexane
toluene
CHCl3
P
P
P
P
P
P
P
P
S
S
S
S
S
S
S
S
acetone
DMF
G (23)
G (31)
G (34)
G (120)
G (19)
G (45)
G (34)
P
I
DMSO
I
I
I
I
I
I
methanol
I
[a] The values in parentheses are critical gelation concentration
(CGC, mg/ml) at 15°C (winter room temperature during the
experiment), I = insoluble when heated, G = stable gel, P =
precipitation when cooled from hot solution, S = soluble.
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gel by concentration-dependent 1H NMR spectra of TPE-C4-Chol
in acetone-d6. As shown in Figure S11, protons of TPE group
(Hb, Hc) shift up-field with the increase of its concentration,
indicating the presence of π-π stacking in these gels.12
AIE Properties of Nano/Micro-aggregates and of Organogels
AIE Properties of Nano/Micro-aggregates
The UV/Vis absorption spectra of TPE-Cn-Chol were recorded
first. All samples in the solution have a similar UV/Vis absorption
curve with a maximum at 314 nm at long wavelength side, which
corresponds to the π-π* electron transition of the TPE unite
(see Figure 3A and Figure S12). Figure 3A shows the UV/Vis
absorption spectrum of TPE-C5-Chol in dichloromethane
(Dichloromethane was used because it has no absorption by
itself in the measured domain). TPE-Cn-Chol solubilized in good
solvents were not fluorescent because of the non-radiative
energy decay from excited to ground state caused by the
intramolecular rotation around the four single bonds of the TPE
moiety. A poor solvent was then added to the TPE-Cn-Chol
solution to induce the aggregation and to activate the AIE
process. TPE-C5-Chol was taken as an example. The AIE
property of TPE-C5-Chol was evidenced by preparing TPE-C5-
Chol solution or dispersion in acetone/water mixtures with
different water fractions (fw in volume). As shown in Figure 3B
and 3C, the acetone solution of TPE-C5-Chol is non-emissive as
the compound is well dissolved in solution. However, the
emission of TPE-C5-Chol solution turns on at fw = 30%. When fw
reaches 90%, its photoluminescence (PL) intensity is more than
900-fold higher than that at fw = 30% (Figure S13). Interestingly,
the maximum of the emission peak red-shifts from 450 to 470
nm, as the water fraction increases from 30% to ≥ 50%. Both PL
intensity measurements and fluorescence images under UV light
show clearly this emission shift (see Figure 3B and C). The
visible color of the solution changes from blue to cyan after the
water fraction over 50%. In order to understand this color
change, we examined then the morphologies and the sizes of
as-formed aggregates by SEM (Figure 3C-D) and DLS (Figure
S14). At fw = 30% rod-like or thick ribbon-like aggregates with
Figure 1. The gel-sol transition temperature Tgel as
a function of the
concentration for TPE-C5-Chol in DMF and acetone measured at a heating
rate of 2 ℃ min-1 in a water bath.
Figure 2. SEM images of gel formed by TPE-Cn-Chol in acetone. (A) TPE-C1-
length typically superior to 2 μm were observed, while at fw
=
Chol. (B) TPE-C4-Chol. (C) TPE-C5-Chol. (D) TPE-C6-Chol.
90% short rods and small spheres with size smaller than 2 μm
were detected. Eventually in nearly pure water (fw = 99%),
spherical nanoparticles with diameters lower than 1 μm were
obtained. The measurement by DLS showed an average
diameter of 380 nm for these spherical nanoparticles. Therefore,
the fluorescence color is related to the morphology of the
aggregates. We can further attribute these differences in
fluorescence color to the different packing ways of TPE-C5-Chol
in the solvent mixture with different fw. We speculate that in
water fraction of 30%, TPE-C5-Chol packed readily into rod-like
crystallites showing blue emission, while in water fraction of 99%,
this cholesterol-based AIE gelator quickly agglomerated in a
random way to form amorphous nanoparticles with cyan
emission. This hypothesis will be proved by the studies in the
section of Characteristics in Condensed State. Moreover, similar
results were found for TPE-C1-Chol, TPE-C4-Chol and TPE-C6-
Chol as shown in Figure S15-S17.
Morphologies of these gels were studied by SEM. TPE-C1-Chol
and TPE-C5-Chol formed in acetone long ribbon-like structures
with thickness of ~ 200 nm, width ranging from hundreds
nanometer to a few of micrometers and length ranging from
several micrometers to several dozen micrometers (even over a
hundred micrometers, see Figure 2A and C). However, TPE-C4-
Chol formed in acetone sheet-like structures with thickness of
about 100 nm (Figure 2B), and similar sheet-like morphologies
were observed for TPE-C6-Chol (Figure 2D). Compared to TPE-
C1-Chol and TPE-C5-Chol, the difference in morphologies could
be ascribed to the odd-even effect of the alkyl spacer between
cholesterol and TPE units on the molecular packing (see Figure
S10 for their molecular models by Chem3D Pro). In addition, π-
πstacking was found to be the driving force in the formation of
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Figure 3. (A) UV−vis absorption spectra of TPE-C5-Chol in dichloromethane (c = 1 × 10−4 M). (B) Photoluminescence (PL) spectra of TPE-C5-Chol in acetone/H2O
mixtures with different water fractions (c = 1 × 10−4 M; λex = 355 nm). (C) Image of TPE-C5-Chol in acetone/H2O mixtures with different water fractions under 365
nm UV light. SEM images of aggregates at the water fraction of (D) fw =30%, (E) fw= 90% and (F) fw= 99%.
AIE Properties of Organogels
All organogels formed by TPE-Cn-Chol in acetone and in DMF
showed AIE properties. Figure 4A demonstrates the evolution of
fluorescence intensity (decreasing) of TPE-C5-Chol gel in
acetone upon heating from 25 to 55°C. At 55°C the gel
transformed completely to
a solution whose fluorescence
intensity decreased to 1/64 of that at 25°C (Figure S13B).
Obviously, the fluorescence intensity decrease can be ascribed
to the gradual disassembling of the ribbon nanostructures upon
heating and their eventual disappearance at 55°C. Figure 4B
shows the fluorescent gel at 25°C and non-fluorescent solution
at 55°C. The fluorescence can be restored when the organogel
is formed gain by cooling the non-fluorescent solution to room
temperature with ultra-sonication. This finding indicates the
fluorescence of the system is thermos-responsive and can be
reversibly modulated by the gel-sol phase transitions upon
thermal treatment.
Figure 4. (A) Temperature-dependent fluorescence spectra of TPE-C5-Chol
gel in acetone. (B) Photograph of TPE-C5-Chol in the gel state at 25°C (left)
and solution state at 55°C (right) in acetone under UV light of 365 nm (c = 40
mg/mL).
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Characteristics in Condensed States: Mechano- Thermo-
and Vapo-Chromic Properties
TPE-Cn-Chol (see Figure S19). In conclusion, the series of TPE-
Cn-Chol doesn’t show liquid crystal phase; the clover-like TPE
unit introduced into the rod-like cholesterol mesogen inhibits the
formation of LC phase. Moreover the recrystallization upon
cooling from isotropic liquid phase also disappears.
Now we study the fluorescence properties of TPE-Cn-Chol in
different condensed phases. Figure 6 and Figure S20 show that
TPE-C5-Chol in crystalline phase, in isotropic phase and in glass
state are all fluorescent but with different colors, deep blue
emission for needle-like crystals and cyan emission for
amorphous glass (prepared by cooling the liquid phase to room
temperature) and isotropic phase. The measurement by
fluorescence spectroscopy proved also this observation (see
Figure 7A). That means all TPE-Cn-Chol samples present
thermo-chromic properties. We then check the mechano-
chromic property of TPE-C5-Chol. As shown in Figure 7A, the
maximum emission of the pristine sample of TPE-C5-Chol is
located at 453 nm, while this maximum of emission peak red-
shifts to 473 nm after grinding the pristine sample, showing
typical mechano-chromic property. Notably, TPE-C1-Chol, TPE-
C4-Chol and TPE-C6-Chol also present mechano-chromic
properties (see Figure S21). As shown in Figure 7C, TPE-C5-
Chol exhibits also vapo-chromic property. The blue-emissive
pristine solid transformed to cyan-emissive state (red-shift) upon
fuming with dichloromethane (DCM). In contrast, when the
pristine sample or the sample in glass state was fumed with
hexane (a non-polar solvent), a slight blue-shift of the maximal
fluorescence peak was observed.
Figure 5. DSC thermograms of TPE-C5-Chol. the blue line shows the 1st
heating from pristine powder sample at 25 ℃ to 200 ℃ (at 2 ℃/min) and 1st
cooling from 200 to 25 ℃ (at -2 ℃/min). The red line refers to the 2nd heating
and 2nd cooling between -25 °C and 150 °C at 2 ℃/min. The green line
indicates the 3rd heating and 3rd cooling between -25°C and 150°C at 10 ℃
/min.
Since the cholesterol moiety is not only a good gelator block, but
also a liquid crystal (LC) unit, [10, 13] it is interesting to study the
phase behavior of TPE-Cn-Chol in condensed state to see if they
exhibit still LC properties with the presence of a non-rod-like but
clover-like TPE unit attached to the cholesterol mesogen. Firstly,
we used DSC to record the phase transitions. As shown in
Figure 5, the first heating scan from pristine powder sample
(afforded by precipitation method using dichloromethane/
methanol as solvent/precipitant) reveals a melting point with a
peak temperature at 116.8°C and an enthalpy of 33.5 KJ/mol.
Upon cooling to 25°C, no recrystallization was recorded. We
then performed 2nd heating/cooling scan between -25°C and
150°C (Detail image see Figure S18). A small peak at 46°C was
detected upon 2nd heating, while only a step-like signal around
43°C corresponding to a second order transition was observed.
It is worth noting that when a higher cooling rate was used to
make the DSC measurement, this second order transition was
more clearly detected (Figure 5 and Figure S18B). We then
attribute the transition at 43°C to a glass transition. Figure 6A
shows the POM image of pristine powder sample of TPE-C5-
Chol with needle-like crystals. After the melting followed by the
cooling to room temperature (RT), the TPE-C5-Chol exhibits an
isotropic glass state, as shown in Figure 6B. In summary, the
phase sequences of TPE-C5-Chol are Cr-116.8°C-Iso on 1st
heating; g-46°C-Iso on 2nd heating; Iso-43°C-g on cooling (Cr =
crystalline phase, Iso = isotropic liquid phase; g = glass). Similar
phase behaviors were observed in other derivatives of the series
Figure 6. (A) POM image of pristine powder sample of TPE-C5-Chol after
synthesis and purification. (B) POM image of TPE-C5-Chol in isotropic phase
at 150°C and in glass state at 25°C (no modification was observed upon
cooling from 150°C to room temperature). (C) Fluorescence optical
microscope image of pristine powder sample of TPE-C5-Chol. (D)
Fluorescence optical microscope image of TPE-C5-Chol glass at 25°C after
cooling from the isotropic liquid phase. (excitation wavelength = 365 nm)
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Figure 7. (A) Fluorescence spectra of TPE-C5-Chol at different condensed states: pristine sample, ground sample and sample in glass state at room temperature
after having heated to isotropic phase (120°C). (B) Wide angle X-ray scattering (WAXS) patterns of TPE-C5-Chol powder sample at different condensed states as
shown in (A),  being the scattering angle. (C) Fluorescence spectra of TPE-C5-Chol with different vapor treatments: pristine sample, after fuming with DCM and
hexane. (D) WASX patterns of TPE-C5-Chol with different vapor treatments as shown in (C). The inserted images correspond to the photographs of samples in
different states under UV light.
In order to understand the mechanism behind all these stimuli-
sensitive chromic properties, experiments of wide-angle X-ray
scattering (WAXS) were carried out on different powder samples
of TPE-C5-Chol. As revealed by Figures 7B, the WAXS profile
from 2θ = 5 – 33° (for distance measurement from 1.76 – 0.27
nm) of the pristine sample display many sharp and intense
reflection peaks, indicating that the pristine sample was mainly
composed of crystals in agreement with the POM observation
(Figure 6A). By grinding, the size of the powder-like crystals will
be reduced and eventually the long-range crystalline ordering
can be disrupted, which is proved by WAXS showing no sharp
reflection peaks but a broad signal centered at 2θ = 18° with
small peaks nearly drown in the background noise (Figure 7B).
The maximum of the broad signal gives a distance of 0.49 nm
corresponding to the average lateral distance between two
molecules of TPE-C5-Chol. The same WAXS signal was
obtained for TPE-C5-Chol powder sample in glass state.
Therefore, both thermo- and mechano-chromic properties are
caused by the crystallization or amorphization which induces the
change of molecular packing in the condense state. [14]
Figure 7D shows the WAXS of powder samples treated by DCM
and hexane compared with pristine sample. Fuming with
dichloromethane destroyed the crystals of pristine solid of TPE-
C5-Chol, leading to formation of amorphous solid. In contrast,
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fuming with hexane could not destroy the crystals of pristine
powder; even it could improve the crystalline organization.
Figure 7D shows the sample fumed by hexane exhibits sharper
WAXS peaks. Again, we found the amorphous phase (obtained
by fuming with DCM) emitted cyan fluorescence. Moreover, the
improved crystalline phase (obtained by fuming with hexane)
fluorescence from blue to cyan color, while fuming the pristine
powder with non-polar solvent like hexane leads to slight blue-
shifting of the fluorescence. Again, the changes of molecular
packing in condensed phases were the cause for these
fluorescence color changes. Dichloromethane can destroy the
crystalline structures, while hexane can even improve the
crystalline ordering. In conclusion, TPE-Cn-Chol is a family of
simple molecules with gelation and AIE features as well as
multiple stimuli-responsive chromic properties. We believe that
our systems offer a simple platform to create interesting thermo-,
mechano- and vapo-chromic fluorescent sensors.
emitted
a
slightly blue-shifted blue fluorescence. Very
interestingly, fuming the sample in glass state with hexane can
also turn back the fluorescence color from cyan to blue (Figure
S22). Thus, the emission color of TPE-C5-Chol can also be
switched between blue and cyan through repeating heating and
fuming.
As above-discussed, we succeeded in connecting the crystalline
phase to the blue emission and amorphous phase to the cyan
emission in condensed state. This proved also the hypothesis
made in the previous section for the nano/micro-aggregates, i.e.,
rod-like crystallites showing blue emission and amorphous
nanoparticles showing cyan emission. The molecular packing
should play a key role in all these stimuli-responsive chromatic
properties, because this packing dictated the energy gaps
between HOMO and LUMO of excimers as shown by other
researchers. [15] Further investigation with the aid of theoretical
calculations is in progress in order to understand the precise
reason of the different emission properties in amorphous and
crystalline phases.
Experimental Section
Materials. Diphenylmethane (99%, J&K Scientific), n-butyllithium (J&K
Scientific), 4-methoxybenzophenone (97%, Accela), p-toluene sulfonic
acid (PSTA, 98%, Beijing HWRK Chem. Co), boron tribromide (Acros),
all the other chemicals were purchased commercially and used without
purification.
Instruments. 1H NMR and 13C NMR spectra were recorded on a Bruker
AV 400 spectrometer. HRMS (ESI) mass spectra were obtained on
XEVO-G2QTOF with addition of KCl and NaCl salt. Scanning electron
microscopy (SEM) was performed using a JSM-7610F with 10 kV
acceleration voltage. Samples were prepared by dropping the diluted gel
onto a clean foil, followed by drying in air. A thin layer of Au was
deposited onto samples before measurement. UV-vis absorption spectra
were measured on
a
Milton Ray Spectronic 3000 Array
spectrophotometer. Photoluminescence (PL) spectra were recorded on
Spex FluoroMax Spectrofluorometer. Average size distributions of TPE-
C5-Chol in acetone/H2O mixture were measured at 25 °C by dynamic
light scattering (DLS, Malvern zetasizer 3000HS, UK) with a 633 nm
laser. A 90° scattering angle was used for all measurements. Differential
scanning calorimetry (DSC) was performed using a Perkin-Elmer DSC7
calibrated for temperature and enthalpy changes. Samples were scanned
with three cycles. The phase transition temperatures were taken at the
peak temperatures in DSC thermograms with heating and cooling rates.
The mesomorphic properties of trans-Cn-Chol in the bulk state were
observed by Motic Polarizing Microscope BA310 Pol equipped with a
Mettler FP82 hot stage. The molecular packing of TPE-C5-Chol in
different state were studied by Wide angle X-ray scattering (WAXS) with
a Bruker D8 diffractometer using Cu K(a) radiation (= 1.5418 Å). The
diffractometer reflections were taken at room temperature and the value
of 2θ were swapped between 5˚ and 35˚, recorded on a position sensitive
detector (PSD).
Conclusions
We have synthesized a series of cholesterol-based AIE gelator
TPE-Cn-Chol (n = 1, 4, 5, 6) by connecting the TPE and the
cholesterol through alkyl chains. All TPE-Cn-Chol derivatives
form AIE fluorescent gels in acetone and in DMF (except that
TPE-C6-Chol doesn’t form gel in DMF). The fluorescence of the
gels is thermo-responsive and can be reversibly modulated by
the gel-sol phase transitions upon thermal treatment. The AIE
properties of the nano/micro-aggregates prepared by TPE-Cn-
Chol in acetone/water mixture with different water fraction (fw in
volume) were also characterized in detail. For instance, TPE-C5-
Chol self-assembles into rod-like crystallites showing blue
emission at the water fraction of 30%, but into amorphous
spherical nanoparticles emitting cyan fluorescence at the water
fraction of 99%. Finally, the phase behaviors of TPE-Cn-Chol in
condensed state were studied. The clover-like TPE unit
introduced into the rod-like cholesterol mesogen inhibits not only
the formation of liquid crystal phase but also the recrystallization
upon cooling from isotropic liquid phase. Very interestingly TPE-
Cn-Chol in the condensed state show thermo-, mechano- and
vapo-chromic properties. The crystalline powder samples emit
blue fluorescence. In contrast, both the liquid phase obtained by
heating the sample above its melting point and the amorphous
glass phase obtained by cooling the liquid phase to room
temperature emit cyan fluorescence. Grinding the crystalline
powder samples causes also the red-shift of the fluorescence
from blue to cyan color because of the partial disruption of the
crystalline structures. Fuming the crystalline powder samples
with polar solvent like dichloromethane red-shifts the
Synthesis. TPE-OMe and TPE-OH were synthesized according to our
previous works.[1a]
The synthesis of TPE-Cn-COOMe will be described in detail based on the
representative compound TPE-C5-COOMe. Compound TPE-OH (1.498 g,
4.31 mmol) was dissolved in 100 mL of acetonitrile, K2CO3 (0.89 g, 6.47
mmol) and methyl 6-bromohexanoate (0.990 g, 4.74 mmol) were added,
and the mixture was refluxed for 4h before cooling and filtering. The
filtrate was concentrated and purified on a silica gel column to give the
product (1.74 g, 85.00%). 1H NMR (400MHz, CDCl3), δ (ppm): 6.99-7.12
(m, 15H, ArH), 6.91 (d, J = 8.8 Hz, 2H, ArH), 6.61 (d, J = 8.8 Hz, 2H,
ArH), 3.88 (t, J = 6.4 Hz, 2H, OCH2), 3.67 (s, 3H, OCH3), 2.34 (t, J = 7.5
Hz, 2H, CH2COO), 1.64-1.79 (m, 4H, CH2), 1.43-1.51 (m, 2H, CH2). 13C
NMR (100MHz, CDCl3), δ (ppm): 174.08, 157.55, 144.05, 144.00, 140.55,
139.99, 135.98, 132.50, 131.39, 131.36, 131.32, 127.70, 127.57, 126.32,
126.20, 113.53, 67.40, 51.52, 33.99, 28.97, 25.68, 24.71.
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The synthesis of TPE-Cn-COOH will be described in detail based on the
representative compound TPE-C5-COOH. The compound of TPE-
2COOMe (1.34 g, 2.81 mmol) was dissolved in 80mL of THF/H2O (v/v =
7:1) mixture. LiOH (0.674 g, 28.10 mmol) was then slowly added, and the
mixture was stirred for overnight. THF was then removed under reduced
pressure and added CH2Cl2 100 mL, then adjusting the pH to 1.0 with 1
M HCl before extracting 3 times with DCM. The organic phase was dried
over anhydrous MgSO4 and then concentrated to give compound TPE-
2COOH (1.17 g, 90% yield) with flash chromatography. 1H NMR
(400MHz, CDCl3), δ (ppm): 6.99-7.12 (m, 15H, ArH), 6.91 (d, J = 8.8 Hz,
2H, ArH), 6.61 (d, J = 8.8 Hz, 2H, ArH), 3.88 (t, J = 6.4 Hz, 2H, OCH2),
2.39 (t, J = 7.4 Hz, 2H, CH2COO), 1.66-1.80 (m, 4H, CH2), 1.46-1.52 (m,
2H, CH2). 13C NMR (100MHz, CDCl3), δ (ppm): 179.27, 157.53, 144.04,
144.00, 140.55, 140.01, 136.01, 132.52, 131.39, 131.36, 131.33, 127.70,
127.58, 126.33, 126.21, 113.55, 67.38, 33.84, 28.96, 25.62, 24.44.
The synthesis of TPE-Cn-Chol will be described in detail based on the
representative compound TPE-C5-Chol. A flask containing compound
TPE-COOH (0.54 g, 1.17 mmol), cholesterol (0.48 g, 1.23 mmol), EDCI
(0.27 g, 1.41 mmol), DMAP (0.20 g, 1.41 mmol) and dried CH2Cl2 (100
mL). And the mixture was stirred overnight at room temperature. The
organic layer was washed with saturated brine solution three times. The
combined organic phase was dried over anhydrous Na2SO4. The filtrate
was concentrated and purified on a silica gel column to give the final
compound TPE-C5-Chol (0.75 g, 85 % yield). 1H NMR (400MHz, CDCl3),
δ (ppm): 6.99-7.14 (m, 15H, ArH), 6.91 (d, J = 8.7 Hz, 2H, ArH), 6.61 (d,
J = 8.7 Hz, 2H, ArH), 5.37 (d, J = 4.4Hz, 1H, CH=C), 4.58- 4.66 (m, 1H,
OCH), 3.87 (t, J = 6.4, 2H, CH2O), 2.28-2.32 (m, 4H, CH2COO and
CH2CHO), 0.68- 2.02 (m, H for cholesterol and OCH2CH2CH2CH2). 13C
NMR (100MHz, CDCl3), δ (ppm): 173.15, 157.68, 144.17, 144.13, 140.68,
140.10, 139.79, 136.06, 132.63, 131.51, 131.48, 131.44, 127.82, 127.69,
126.44, 126.33, 126.31, 122.75, 113.66, 77.16, 73.92, 67.53, 56.82,
56.27, 50.16, 42.45, 39.87, 39.65, 38.29, 37.13, 36.73, 36.32, 35.93,
34.70, 32.04, 29.10, 28.37, 28.15, 27.95, 25.77, 24.94, 24.43, 23.97,
22.97, 22.71, 21.17, 19.47, 18.87, 12.01. HRMS-ESI: m/z calcd. for
C59H74O3K: 869.5270 [M + K] +; found: 869.5266.
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Gelation Test: 12 mg of the gelator TPE-Cn-Chol was mixed with a
selected solvent (0.2 mL) in a septum-capped vial and heated in an oil
bath until the solid was dissolved. The sample vial was then cooled to
room temperature. If no flow was observed when inverting the vial, a
stable gel was formed and gelation (G) was noted. If the clear solution
was retained, it was marked as soluble (S). If the compound was unable
to dissolve, it was noted as insoluble (I). Short-term sonication was
needed at the beginning of the cooling process to assist the gelation
process. The CGC value of the gelator was determined by measuring the
minimum amount of the gelator required in 0.2 mL of solvent for the
formation of a stable gel at room temperature.
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Acknowledgements
This work is supported by the National Natural Science
Foundation of China (21604001 and 21528402) and the French
National Research Agency (ANR-16-CE29-0028).
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Keywords: Fluorescent gel • AIE gelator • Mechanochromic
property • Vapochromic property • Thermochromic property
[1]
a) N. Zhang, H. Chen, Y. Fan, L. Zhou, S. Trepout, J. Guo and M. H. Li,
ACS Nano 2018, 12, 4025-4035; b) P. P. Ghoroghchian, P. R. Frail, K.
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Hui Chen, Lu Zhou, Xiang Shi, Jun Hu,
Jia Guo, Pierre-Antoine Albouy, and
Min-Hui Li *
Page No. – Page No.
AIE Fluorescent Gelators with
Thermo-, Mechano- and Vapo-
Chromic Properties
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