Fluorescence-enhanced organogelators with mesomorphic and piezofluorochromic properties based on tetraphenylethylene and gallic acid derivatives

Article abstract of DOI:10.1016/j.dyepig.2013.08.026

Two novel aggregation-induced emission compounds derived from tetraphenylethylene and gallic acid were synthesized and characterized. Both of the emission compounds possessed mesomorphic properties and exhibited mesomorphic phases over a wide temperature

Full text of DOI:10.1016/j.dyepig.2013.08.026

Dyes and Pigments 101 (2014) 74e84
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Fluorescence-enhanced organogelators with mesomorphic
and piezofluorochromic properties based on tetraphenylethylene
and gallic acid derivatives
,
c
,
,
,
a
a
a,
Miao Luo a 1, Xie Zhou b 1, Zhenguo Chi a , Siwei Liu , Yi Zhang , Jiarui Xu
*
*
a PFCM Lab, DSAPM Lab and KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of
Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
b School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, China
c College of Basic Education, Zhanjiang Normal University, Zhanjiang, 524037, Guangdong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel aggregation-induced emission compounds derived from tetraphenylethylene and gallic acid
were synthesized and characterized. Both of the emission compounds possessed mesomorphic prop-
erties and exhibited mesomorphic phases over a wide temperature range and the thermal-induced
mesomorphic transition from metastable to stable phases accompanied by a change of the lumines-
cent color. The mesomorphic transition may be ascribed to liquid crystalline phase transition. The
emission compounds possess different gelation behavior in organic solvents. The emission intensities of
the emission compounds can be reversibly changed with the gelesolution transition by alternate cooling
and heating. Moreover, the color and emission of these organogelators can be repeatedly switched by
various external stimuli including pressing, heating and solvent-fuming. These new compounds may
offer potential as external stimuli-responsive materials.
Received 7 May 2013
Received in revised form
28 August 2013
Accepted 28 August 2013
Available online 18 September 2013
Keywords:
Aggregation-enhanced emission
Tetraphenylethylene derivatives
Liquid crystal
Ó 2013 Elsevier Ltd. All rights reserved.
Piezofluorochromism
Organogelator
Gelation-induced fluorescence
enhancement
1. Introduction
Recently, aggregation-induced emission (AIE) organogelators
have attracted considerable research attention because of their
In recent years, low-molecular mass organogelators (LMOGs)
have drawn immense interest because of their potential applica-
tions in cosmetics, catalysis, drug delivery systems, sensors,
switches and other related fields [1,2]. LMOGs can self-assemble
into fibers, rods, ribbons or other aggregates in suitable solvents
via a combination of noncovalent interactions, such as hydrogen
strong gel emission [8]. For example, an AIE organogelator, 1-
cyano-trans-1,2-bis-(30,5-bis-trifluoromethylbiphenyl)
ethylene
(CN-TFMBE) (Scheme 1), was practically nonfluorescent in the so-
lution state, but highly fluorescent in the gel state, with a more than
100-fold enhancement in the fluorescence intensity [9,10]. In 2008,
Chen et al. [11] reported a thermochromic gel generated from a
cholesterol-containing salen-based organogelator, which exhibited
obvious AIE properties in the gel state. The fluorescence quantum
yield of the gel was approximately 600 times greater than that in
solution.
Mechano- or piezo-fluorochromic materials have also drawn a
great deal of attention because of their fundamental research and
practical applications in sensors, optical recording and strain- or
pressure-sensing systems [12e29]. These materials generate a
fluorescent color in response to external force stimuli, such as
grinding, smearing and pressing, and then revert to the original
color by heating, recrystallization or exposure to solvent vapor. To
date, only a few liquid crystalline materials displaying piezo-
fluorochromic property have been reported [30e32]. LMOGs that
bonding,
pep stacking, electrostatic forces, donoreacceptor in-
teractions, metal coordination, solvophobic forces and van der
Waals forces, to form entangled networks and gel solvents [3,4].
Most LMOGs, called “smart” or “intelligent” gels, show reversible
changes in morphology and/or physical properties in response to
various external stimuli, such as temperature, electrical pulse and
light [5,6]. But most of chromophores exhibit aggregation-induced
quenching in the gel states or in the solid states [7].
* Corresponding authors.
E-mail addresses: chizhg@mail.sysu.edu.cn (Z. Chi), xjr@mail.sysu.edu.cn (J. Xu).
These authors contributed equally to the preparation of this work.
1
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.08.026

M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
75
Scheme 1. Synthetic routes for P4 and P5; the chemical structure of CN-TFMBE.
exhibit piezofluorochromic properties are exceedingly rare [33]. In
2012, Ren et al. reported several phosphole-lipids which formed
highly fluorescent ionic organogels and their donor and acceptor
doped systems exhibited piezofluorochromism. However, organic
LMOGs with multimorphic and piezochromic luminescent prop-
erties have not been reported thus far to the best of our knowledge.
In this article we report our recent results of two organogelators
(P4 and P5, Scheme 1) with AIE, multimorphic and piezo-
fluorochromic properties, synthesized from tetraphenylethylene
and gallic acid derivatives.
acid (3) [34] and 1-bromo-4-(1,2,2-triphenylvinyl)benzene (TPE-
Br) [35,36] were prepared according to the literature methods.
Proton and carbon nuclear magnetic resonance (1H NMR and 13
C
NMR) spectra were measured on a Mercury-Plus 300 spectrometer
[CDCl3, tetramethylsilane (TMS) as the internal standard. For
P5NH2, using DMSO-d6 as solvent]. Mass spectra (MS) were
measured on a Thermo DSQ EI-mass spectrometer. Elemental an-
alyses were performed with an Elementar Vario EL elemental
analyzer. Photoluminescence spectra (PL) were measured on a
Shimadzu RF-5301pc spectrometer with 1.5 nm and 3.0 nm slit
widths for excitation and emission, respectively. Ultravioletevisible
absorption spectra (UVevis) were recorded on a Hitachi UVevis
spectrophotometer (U-3900). Wide-angle X-ray diffraction
(WAXD) and small-angle X-ray diffraction (SAXD) measurements
were performed by using a Bruker X-ray diffractometer (D8
2. Experimental
2.1. Materials and measurements
4-Aminophenylboronic acid, 4-bromophenyl phenylmethanone,
2-bromo-1,1,2-triphenylethylene, 4-toluenesulfonic acid, diphe-
nylmethane, 1-bromododecane, methyl 3,4,5-trihydroxybenzoate,
ADVANCE, Germany) with an X-ray source of Cu
Ka
(l
¼ 0.15406 nm) at 40 kV and 40 mA, at scan rates of 4ꢀ and 0.6ꢀ
per 1 min for WAXD and SAXD, respectively. Time-resolved emis-
sion decay behavior was measured on an Edinburgh Instruments
Ltd. spectrometer (FLSP920). Differential scanning calorimetry
(DSC) curves were obtained with a NETZSCH thermal analyzer (DSC
204 F1) at a heating rate of 10 ꢀC/min under N2 atmosphere.
Transition temperatures and textures were determined using a
Leitz Orthoplan Pol polarizing optical microscope (POM) equipped
with a Likam heating stage system. The THF/water mixtures with
different water fractions were prepared by slowly adding distilled
water into the THF solution of samples under ultrasound at room
temperature. Pressed samples were prepared by pressing in an IR
1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
(EDC-HCl), 4-dimethylamiopryidine (DMAP), n-butyllithium,
tetrabutyl ammonium bromide (TBAB) and tetrakis(-
triphenylphosphine) palladium (0) were purchased from Alfa-Aesar
company and used as received. Ultra-pure water (18 M .cm) was
hydrochloride
U
used in the experiments. Tetrahydrofuran (THF) was distilled from
sodium/benzophenone. All other reagents and solvents were pur-
chased as analytical grade from Guangzhou Dongzheng Company
(China) and used without further purification. Methyl 3,4,5-
tris(dodecyloxy)benzoate (2) [7], 3,4,5-tris(dodecyloxy) benzoic

76
M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
120
100
80
60
40
20
0
2 M K2CO3 aqueous solution (9 mL) and TBAB (0.01 g) were
P5
added. The mixture was stirred for 30 min under an argon at-
mosphere at room temperature. Then the Pd(PPh3)4 catalyst was
added and the reaction mixture was stirred at 80 ꢀC for 24 h.
After cooling to room temperature, the product was concentrated
and purified by silica gel column chromatography with CH2Cl2/n-
hexane mixed solvent (1:2 by volume) to afford a pale yellow
powder P4NH2 (3.16 g, yield 91.2%). 1H NMR (300 MHz, CDCl3)
(a)
0%
10%
50%
60%
70%
80%
90%
d
(ppm): 6.41e6.46 (d, J ¼ 7.8 Hz, 2 H); 6.76e6.83 (d, J ¼ 7.8 Hz,
2 H); 6.94e7.18 (m, 17 H). MS (EI) calcd for C26H21N 347, found
347. Anal. calcd for C26H21N: C 89.88, H 6.09, N 4.03; Found: C
89.84, H 6.05, N 4.07.
2.3. Synthesis of 3,4,5-tris(dodecyloxy)-N-(4-(1,2,2-triphenylvinyl)
phenyl)benzamide (P4)
400
450
500
550
600
650
700
Wavelength / nm
110
100
90
80
70
60
50
40
30
20
10
0
To P4NH2 (0.18 g, 0.52 mmol) and 3 (0.42 g, 6 mmol) in THF
(50 mL), DMAP (0.01 g) and EDC-HCl (0.01 g) were added. The
mixture was stirred for 3 days under an argon atmosphere at room
temperature. Excess ethanol was added and the resulting precip-
itate was collected. Then crystallized from THF and ethanol
mixture for 3 times to obtain P4 as a white solid (0.48 g, yield
92.8%). Melting point: w78 ꢀC (DSC). 1H NMR (300 MHz, CDCl3)
(b)
P4
P5
d
(ppm): 0.89 (t, J ¼ 6.3 Hz, 9 H), 1.20e1.48 (m, 52 H), 1.68e1.93
(m, 8 H), 3.96e4.07 (m, 6 H), 6.96e7.17 (m, 19 H), 7.36 (d,
J ¼ 8.4 Hz, 2 H), 7.59 (s, 1 H). 13C NMR (CDCl3, 75 MHz)
d (ppm):
165.6, 153.3, 143.8, 141.5, 141.0, 140.4, 140.1, 136.4, 131.5, 130.1,
127.8, 126.7, 126.6, 119.3, 105.8, 73.8, 69.7, 32.3, 30.1,29.7, 26.5, 23.1,
14.5. FT-IR (KBr)
y
(cmꢁ1): 3306, 2921, 2849, 1640, 1580, 1527,
1497, 1467, 1335, 1233, 1113, 748, 699. MS (EI), m/z: 1004 ([M]þ,
calcd for C69H97NO4, 1004). Anal. calcd for C69H97NO4: C 82.50, H
9.73, N 1.39; Found: C 82.44, H 9.81, N 1.32.
-10
0
20
40
60
80
100
VH O
%
Fig. 1. (a) PL spectra of P5 in THF/water mixtures with different water fractions (inset:
the images of P5 were taken at room temperature under 365 nm UV light in THF and
90% water); (b) Change in PL peak intensities of P4 and P5 with different water
2.4. Synthesis of P5NH2
TPE-Br (2.50 g, 6.08 mmol) and 4-aminophenylboronic acid
(1.65 g, 12 mmol) were dissolved in the pure THF (50 mL), then
TBAB (0.01 g) and K2CO3 (2 M, aq. soln., 9 mL) were added. The
mixture was stirred at room temperature for 0.5 h under argon gas
followed by addition of the Pd(PPh3)4 catalyst and then heated to
80 ꢀC for 24 h. The reaction mixture was concentrated and purified
by column chromatography on a silica gel column with CH2Cl2/n-
hexane (1:1 by volume) as eluent to give light yellow crystalline
powder P5NH2 (2.35 g, 92.5% yield). 1H NMR (300 MHz, DMSO-d6)
fractions in THF/water mixtures (concentration 10 mM, excitation wavelength 365 nm).
pellet at 1500 psi for 5 min. Annealing experiments were done on a
hot-stage with automatic temperature control system for 5 min, the
annealing temperature was 70 ꢀC. The experiment of solvent vapor
fuming treatment was performed by filling the pressed sample on a
grooved glass slide, which was then placed in a large beaker satu-
rated with CH2Cl2 vapor for 5 min at room temperature.
2.2. Synthesis of 4-(1, 2, 2-triphenylvinyl)benzenamine (P4NH2)
d
(ppm): 5.14e5.24 (s, 2 H); 6.54e6.62 (d, J ¼ 8.4 Hz, 2 H); 6.88e
7.36 (m, 21 H). MS (EI) calcd for C32H25N 423, found 423. Anal.
calcd for C32H25N: C 90.74, H 5.95, N 3.31; Found: C 90.71, H 5.92,
N 3.35.
To 2-bromo-1,1,2-triphenylethylene (3.35 g, 10 mmol) and 4-
aminophenylboronic acid 1.65 g (12 mmol) in THF (50 mL),
Fig. 2. (A) SEM images of THF/water (10/90, v/v) mixture of P5, (B) SEM image of xerogel formed with P5 in ethanol.

M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
77
Table 1
P5 in hexane
Gel
Gelation properties of P4 and P5 in various solvents.
250 (a)
Solvent
P4 P5 Solvent
P4
P5
200
150
100
50
THF
DMSO
DMF
Ethyl acetate
Tetrachloromethane
Chloroform
S
P
S
S
S
S
S
P
S
S
S
S
Dichloromethane
Methanol
n-Hexane
Cyclohexane
Petroleum ether
Ethanol
S
I
S
I
P
S
P
G (41 ꢀC, 10)
G (40 ꢀC, 22)
G (46 ꢀC, 14)
G (41 ꢀC, 14) G (42 ꢀC, 20)
G: stable gel formed at room temperature; S: soluble; I: insoluble; P: precipitate.
(Tgel; CGC, mg/mL, measured at 25 ꢀC).
Sol
2.5. Synthesis of 3,4,5-tris(dodecyloxy)-N-(4-(1,1,2,2-
tetraphenylvinyl)phenyl) benzamide (P5)
0
400
450
500
550
600
650
700
P5NH2 (0.30 g, 1.01 mmol) and 3 (0.98 g, 14.2 mmol) were dis-
solved in THF (50 mL), and then EDC-HCl (0.01 g) and DMAP
(0.01 g) were added. The solution was stirred at room temperature
for 3 days. After removing the solvent under reduced pressure, the
residue was crystallized from ethanol to give white crystalline
powder P5 (0.62 g, 81.6% yield). Melting point: 127e129 ꢀC (DSC).
Wavelength (nm)
(b)
1H NMR (300 MHz, CDCl3)
d
(ppm): 0.89 (t, J ¼ 5.1 Hz, 9 H), 1.16e
1.47 (m, 54 H), 1.70e1.90 (m, 6 H), 3.98e4.09 (m, 6 H), 7.10e7.16 (m,
18 H), 7.31e7.38 (d, J ¼ 8.4 Hz, 2 H), 7.53e7.58 (d, J ¼ 8.7 Hz, 2 H),
7.62e7.57 (d, J ¼ 6.9 Hz, 2 H) 7.72e7.76 (s, 1 H); 13C NMR (CDCl3,
75 MHz) d (ppm): 165.7, 153.4, 143.8, 142.8, 141.8, 140.7, 138.2, 137.4,
136.9, 132.0, 131.5, 130.1, 126.7, 126.1, 120.4, 106.1, 73.9, 69.8, 32.3,
30.4, 29.7, 26.4, 23.0, 14.5. FT-IR (KBr)
y
(cmꢁ1): 3306, 2921, 2849,
1640,1580,1527,1497,1467,1335,1233,1113, 748, 699. MS (EI) calcd
for C75H101NO4 1081, found 1081. Anal. calcd for C75H101NO4: C
83.36, H 9.42, N 1.30; Found: C 83.41, H 9.57, N 1.24.
3. Results and discussion
3.1. Synthesis
Gel (Fluorescence-On)
250 (c)
The target compounds were synthesized according to the routes
depicted in Scheme 1. The molecular structure of the target com-
pounds consisted of two parts: the tetraphenylethylene and the
3,4,5-tris(dodecyloxy)benzoic acid moieties. For P5, the two moi-
eties were linked by a phenylene unit. The design strategy of the
200
150
100
50
P5
250
200
150
100
50
21
22
23
24
25
27
29
31
33
35
37
39
41
43
45
47
49
51
0
Sol (Fluorescence-Off)
0
1
2
3
4
5
6
7
8
9
10
11
Cycle
Fig. 4. (a) Fluorescence spectra of P5 in gel state and solution state in n-hexane with
the same concentration (20 mg/mL); (b) Fluorescence images of P5 for various tem-
peratures; (c) Repeated switching of photoluminescence between the gel and solution
states. Excitation wavelength 365 nm.
0
amide-substituted 3,4,5-tris(dodecyloxy)benzoic acid molecule,
including a hydrogen bonding site and a fluorescent tetraphenyl-
ethylene core, is expected to make the molecule display piezo-
fluorochromism. One of the major objectives of this study is to
examine the influence of the linker on the properties of the com-
400
450
500
550
600
Wavelength / nm
Fig. 3. Temperature-dependent fluorescence spectra of P5 in ethanol (20 mg/mL,
excitation wavelength 365 nm), the insets depict changes in PL peak intensity and
emission images of the compound in solution state and gel state under 365 nm UV
illumination.
pounds. Their molecular structures were confirmed by 1H- and 13
C
NMR spectroscopy, mass spectrometry, Fourier-transform infrared
spectroscopy and elemental analysis.

78
M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
Fig. 5. Calculated spatial electron distributions of LUMO and HOMO (left), and the optimized structures of the compounds (right).
3.2. AIE properties
the mixtures. The Mie effect of the nanoparticles has been
considered to cause such leveling-off of tails in the absorption
spectra [36e38]. Therefore, the increase in PL intensity can be
attributed to an AIE effect caused by the formation of molecular
aggregates, in which the restriction of intramolecular rotations
increased the fluorescent emission. The SEM image of P5 parti-
cles obtained from the mixed solution containing 90% volume
fractions of water are shown in Fig. 2A (A, prepared by dropping
the mixture on the quartz slide and slowly evaporating the sol-
vents), which shows that P5 forms particles of a few hundred
nanometers in size.
The UVevis absorption and PL emission spectra of the diluted
mixtures of these compounds were studied in a mixture of THF/
water with different water fractions to determine whether the
compounds possess AIE properties. The compounds are soluble
in THF and insoluble in water; thus increasing the water fraction
in the mixed solvents changes their existing forms from solution
state in the pure THF to aggregated particles in the mixtures with
higher water content, leading to changes in their UVevis and PL
spectra [36]. The PL spectra of 10 mM P5 in THF/water mixtures
with different water contents are shown in Fig. 1 (the PL spectra
for P4 are provided in Supporting Information, SI, Fig. S1). The
figure shows that the PL intensity was very weak and essentially
did not change as the water fraction of the THF/water mixture
increased from 0% to 50%. When the water fraction was >60%, the
PL intensity slightly increased with the increase in water fraction
3.3. Gelation properties
The gelation behavior of compounds P4 and P5 were tested in
different solvents, with 2.5% (w/v) as a standard concentration. The
results are summarized in Table 1. It can be seen that the gelation
ability of compound P4 was generally weak, since it only gelled in
ethanol. By contrast, P5 can be gelated in several solvents such as
ethanol, n-hexane, cyclohexane and petroleum ether. The critical
gelator concentrations (CGC) of these compounds were tested us-
ing the “stable to inversion of a test tube” method [11]. The values
of gelesol transition temperature (Tgel) were determined by the
“falling drop” method [6,39]. The CGC and Tgel data for different
solvents of the two compounds are also listed in Table 1. The CGC
values were between 10 and 22 mg/mL, whereas the Tgel ranged
from 40 ꢀC to 46 ꢀC.
The SEM image of P5 xerogels obtained from the organogels is
shown in Fig. 2B. The xerogels generated the formation of entan-
gled networks consisting of porous aggregates on mica, which
facilitate the immobilization of large volumes of solvents within
and between the networks, resulting in gelation (Fig. 2B, xerogels
prepared by slow evaporation of ethanol from the corresponding
organogels).
up to 80%.
A dramatic enhancement in luminescence was
observed when the water fraction reached 90%, which was
approximately 44 times (P4) and 101 times (P5) higher than that
in the pure THF. This result indicated that both compounds
exhibited strong AIE activity. The AIE activity may be attributed
to its twisted structure, in which multiple phenyl peripheries
linked to an ethylene core via Cphenyl-Cethenyl single bonds which
enable their free rotation. The molecular size and effect of steric
hindrance influence their rotation; a larger molecule should have
lower freedom of rotation [37]. The results of their molecular
structure analysis showed that when one phenyl ring was
induced for P5, the molecular size and steric hindrance were
expected to be increased.
The UVevis absorption spectra of P4 and P5 in the THF/water
mixtures (10
mM) are shown in Fig. S2. The spectra displayed
absorption tails extending well into the long wavelength region,
indicating that the molecules aggregated into nanoparticles in

M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
79
the decrease in temperature. In addition, when P5 forms a gel, its
fluorescence intensity is very strong.
P4
a) 1st heating
b) 2nd heating
c) cooling
The fluorescence of both gel and solution in n-hexane with the
same concentration was also examined to compare the fluores-
cence intensity of P5 in the gel state and in the solution (Fig. 4a).
The solution of P5 exhibited rather weak fluorescence; however, its
fluorescence intensity increased by almost 120 times after gel for-
mation. The remarkable fluorescence enhancement from the gels
was possibly due to the formation of self-assembly aggregates. The
organogels were completely thermo-reversible in organic solvents.
From the thermoreversible gelation process of P5 gel in n-hexane
(Fig. 4b), the solution became clear after gentle heating to the
higher temperature (to >60 ꢀC). Upon cooling to room temperature,
the formation of immobile gels was observed. The fluorescence
intensity of the gel can be reversibly modulated with the geleso-
lution transition through alternate cooling and heating (Fig. 4c).
These switchable fluorescent organogels may have potential ap-
plications in information storage and optical devices [40e42].
Quantum mechanical computations were conducted using the
Gaussian 03 software to study the lowest energy spatial confor-
mation of the compounds [43]. The highest occupied molecular
orbitals (HOMOs) and the lowest unoccupied molecular orbitals
(LUMOs) of these compounds were obtained (Fig. 5) according to
the density functional method at the B3LYP/6-31G level after
structural optimization. P4 and P5 showed similar electron cloud
distributions in either their HOMO or LUMO orbital, which illus-
trates why P4 and P5 exhibited very close UV and PL spectra. As
shown by their optimized structures, the three long alkyl chains
expanded freely in space, resulting in relatively larger free volume
in the aggregates of the compounds, which facilitates the immo-
bilization of large volumes of solvents that result in gelation. The
tetraphenylethylene moiety adopted a twisted spatial conforma-
a
b
c
-50
0
50
100
150
200
250
Temperature / oC
P5
a) 1st heating
b) 2nd heating
c) cooling
a
b
c
tion, which made the formation of close
pep packing difficult for
the molecule. Due to the twisted conformation and weaker in-
teractions, the interfaces between the molecules are relatively
loose and there exist some defects, which may explain the
following piezofluorochromic behavior of these compounds.
-50
0
50
100
150
200
250
Temperature / oC
3.5. Mesomorphic behavior
Fig. 6. DSC curves of compounds P4 and P5.
The mesomorphic behavior of P4 and P5 were evaluated using
differential scanning calorimetry (DSC), hot-stage polarized light
microscopy (POM), small-angle X-ray diffraction (SAXD) and wide-
angle X-ray diffraction (WAXD) equipped with a temperature
controller. Fig. 6 shows the first heating, cooling and second heating
DSC curves of P4 and P5. Their phase-transition temperatures and
associated enthalpy values are given in Table 2. For the first heating
curve of P4, the lower peak temperature at 85 ꢀC was the transition
between crystalline solid phase and liquid crystalline (LC) phase,
which was confirmed by POM observation. By being heated above
the temperature, the sample exhibited fluidity and color birefrin-
gence. The other transition at 122 ꢀC belonged to liquid crystalline
phase to isotropic phase transition, whereas the transition at 116 ꢀC
was possibly one liquid crystalline phase to the other crystalline
phase transition. For P5, the transition between the crystalline solid
phase and LC phase was found at 99 ꢀC and the other transition
from the LC phase to the isotropic phase at 128 ꢀC. During the
cooling process, P4 had three exothermic peaks, namely, peaks at
20, 25 and 74 ꢀC. The second heating curve of P4 showed compli-
cated phase transitions, with peak temperatures at 27, 80, 116 and
122 ꢀC. Thus, the transition at 122 ꢀC was considered as Ti, indi-
cating that the transition temperatures below 122 ꢀC may exhibit
the different LC phase and polycrystalline phase transitions. The
second heating curve of P5 showed a significant cold crystallization
peak at 85 ꢀC and Ti at 130 ꢀC.
3.4. Gelation-induced fluorescence-enhanced emission
Gelation-induced fluorescence enhancement was observed for
both P4 and P5. The fluorescence properties of P5 in the gel state
and in solution are shown in Fig. 3 as an example (the PL spectra for
P4 are provided in SI, Fig. S3). It was found that P5 was non-
fluorescent under illumination with UV irradiation when heated
in the solution state, but the fluorescence intensity increased with
Table 2
Phase-transition temperatures T (oC) and associated enthalpy values
for compounds P4 and P5.
O
H (KJ/mol)
T (ꢀC)
Compound
Transition temperatures (ꢀC) [
DH (kJ/mol)]
D
POM
DSC
a
b
TS-LC
Tia
TS-LC
Tib
P4
P5
86
95
130
132
85 (4.24)
99 (2.24)
122 (8.37)
128 (6.23)
44
37
TS-LC: crystalline solid phase to liquid crystalline phase; Ti: liquid crystalline phase to
isotropic phase.
DT: temperature range of transition from the TS-LC phase to the isotropic phase by
POM.
a
Obtained from POM.
The peak value of the first heating DSC curve.
b

80
M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
Fig. 7. Optical micrographs of compounds P4 and P5 between crossed polarizers: liquid crystalline texture for P4 at 100 ꢀC (a) and at 120 ꢀC (b); liquid crystalline texture for P5 at
96 ꢀC (c) and at 128 ꢀC (d).
By being heated above temperature of the solid crystalline
phase to the liquid crystalline phase transition, the sample began
to flow with color birefringence, suggestive of a liquid, indicating
the sample really turned into the liquid crystalline state. The
POM observations showed that both P4 and P5 exhibited stable
LC phases over a relatively wide temperature range (Table 2).
According to the textures obtained during cooling from the
isotropic melt and observed by POM (Fig. 7), the mesophases
formed from P4 and P5 may belong to the frozen smectic LC
phases. It should also be noted that the photomicrographs from
POM look unlike those of typical smectic liquid crystals, perhaps
they may be disordered crystals formed during cooling of the
isotropic melt. The results of DSC and POM studies showed that
the characteristics of the phase transitions observed in the DSC
curves for both P4 and P5 are consistent with those found by
POM observations.
LC phase to the amorphous state as the temperature was further
increased. The diffraction peak completely disappeared when the
temperature reached 100 ꢀC, indicating that P5 dominated the
isotropic state at this temperature.
3.6. Piezofluorochromic properties
The three long alkyl chains in the molecules of P4 and P5
expanded freely in space, leading to relatively large volume
requirement. In addition, the tetraphenylethylene moiety possesses
a twisted spatial conformation. The twisted conformation and the
weak
pep interactions make the molecular packing relatively
loose, resulting in lower lattice energy, which rendered the crystals
to be easily destroyed by the planarization of the molecular
conformation or slip deformation under external pressure. The
release of twist stress and the planarization of molecular confor-
mation may result in increased molecular conjugation, thereby
facilitating a red-shift of the PL spectrum, exhibiting piezo-
fluorochromic phenomena.
SAXD and variable temperature WAXD measurements provided
complementary information on the phase transition and the type of
mesophase. The diffraction patterns are shown in Fig. 8. The SAXD
patterns (Fig. 8a) showed two diffraction peaks at the 2
q
ranging
The images of the two compounds taken at room temperature
under UV light at different conditions are shown in Fig. 9 (left top).
P4 did not exhibit distinct piezofluorochromic properties. However,
with only one more phenyl ring in the linkage than that of P4, the
luminescence color of P5 changed from blue to blue-green upon
pressing in an IR pellet at 1500 psi for 5 min, and then reverted to
blue again upon heating at approximately 70 ꢀC for 0.5 h or
exposing to dichloromethane vapor for 5 min, showing clear pie-
zochromic luminescence. When the original and the pressed
samples of compounds P4 and P5 were heated at the temperatures
of their LC state then naturally cooled to room temperature, they all
converted into strong blue-green luminescence. To date only a few
photofunctional LCs were reported to exhibit an LC phase transition
associated with a dynamic change of photoluminescent color dur-
ing heating [30].
from 1ꢀ to 5ꢀ. The WAXD patterns exhibited higher-order re-
flections at 30 ꢀC, as shown in Fig. 8b. The diffraction peaks
decreased and some peaks disappeared with the increase in tem-
perature. When the temperature reached 120 ꢀC, P4 remained at
the LC phase. Two obvious sharp peaks were observed at 5.3ꢀ and
10.5ꢀ, as well as a diffused broad peak with maximum of about 20ꢀ.
This observation suggests that the mesophase was probably
smectic LC [44].
P5 exhibited multiple diffraction peaks before the temperature
increased to 70 ꢀC, indicating that the compound maintained the LC
state below that temperature, as shown in Fig. 8c. When the tem-
perature was increased to 88 ꢀC, which corresponded to the phase
transition temperature from the solid phase to the LC phase, the
diffraction peak became narrow. The sample transformed from the

M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
81
(a)
P5
P4
0
1
2
3
4
5
6
7
8
9
2θ(deg)
(b)
P4
30oC
50oC
86oC
110oC
120oC
130oC
5
10
15
20
25
30
35
40
45
2θ(deg)
(c)
P5
Fig. 9. WAXD curves, images under UV light (left top) and normalized PL curves (right
top) of P4 under the different conditions: (a) origin, (b) pressed sample (a), (c) heated
sample (b) at 116 ꢀC for 0.5 h, (d) heated sample (a) at 116 ꢀC for 0.5 h. WAXD curves,
images under UV light (left top) and normalized PL curves (right top) of P5 under the
different conditions: (A) Original; (B) Pressed sample (A); (C) Annealed sample (B) at
70 ꢀC for 0.5 h; (D) Fumed sample (B) in CH2Cl2 vapor for 5 min; (E) heated sample (A)
at 100 ꢀC for 0.5 h; (F) heated sample (B) at 100 ꢀC for 0.5 h.
30oC
70oC
recovered (shown in Table S1). When the original and pressed
samples of P4 and P5 were heated at 116 ꢀC (P4 for 0.5 h) and
100 ꢀC (P5 for 0.5 h), respectively, and then naturally cooled at
room temperature, they exhibited a different degree of red-shift.
For P4, the original sample showed a 13-nm red shift, whereas
the pressed one exhibited a 15-nm red shift. For P5, the original
sample showed a 32-nm red shift, whereas the pressed one
exhibited a 22-nm red shift.
WAXD and DSC measurements were conducted to determine
the piezofluorochromism mechanism. The WAXD curves of P4 and
P5 (Fig. 9) exhibited different structures of molecular aggregation
before and after pressing. The diffraction patterns of P4 before and
after pressing displayed sharp and intense reflections, which are
indicative of crystalline order. However, when the original and
pressed samples were heated at 116 ꢀC for 0.5 h and quenched to
room temperature [Fig. 9c for pressed sample and (d) for original
sample] it can been found that some of the diffused peaks
88oC
100oC
5
10
15
20
25
30
35
40
45
2θ(deg)
Fig. 8. (a) SAXD patterns for P4 and P5; (b) variable-temperature WAXD patterns for
P4; (c) variable-temperature WAXD patterns for P5.
The PL spectra of the samples obtained under different treat-
ment conditions are shown in Fig. 9 (right top). The maximum
emission wavelengths of P4 remained almost unchanged after
pressing, with only a 1 nm change. Thus, P4 had no piezo-
fluorochromic property. However, a significant red shift of about
20 nm was observed in P5 after pressing. After the pressed samples
were annealed and fumed, the initial PL wavelengths were partially

82
M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
(a)
P5-pressed 1st heating
P4 1st heating
5000
e
f
a
b
c
d
e
f
g
h
i
j
k
30
60
4000
3000
2000
1000
0
a
g
70
100
105
110
120
130
140
145
150
b
h
c
a
b
d
i
j,k
c
d
400
450
500
550
600
Wavelength / nm
50
100
150
200
550
500
450
Temperature / oC
(b)
P5-pressed
1st heating
cooling
2rd heating
P5 1st heating
A
B
C
D
E
20
40
60
80
100
120
140
160
F
Temperature / oC
Fig. 11. (a) The PL spectra of the pressed P5 sample on different temperature in first
heating; (b) the dependence of PL wavelength of the pressed P5 sample on different
temperature.
50
100
150
200
Temperature / oC
Fig. 10. The first heating DSC curves of P4 under the different conditions.
became smaller. Upon heating the original and pressed samples of
P4 at 116 ꢀC, the peaks at 116 ꢀC and 122 ꢀC changed into one sharp
peak, and the lower temperature peak decreased and almost dis-
appeared. For P5, after pressing (B) and annealing (C), the low
endothermic peak changed from 99 ꢀC to 95 ꢀC. By contrast, the
disappeared, and the diffraction peaks almost disappeared [Fig. 9d],
indicating an amorphous structure.
Compared with P4, the original sample of P5 showed fewer
diffused peaks that indicated low crystalline order. After pressing,
P5 showed a weak, broad and diffused peak that indicated an
amorphous structure. However, the diffraction curves of the
annealed and fumed samples revealed numerous sharp and intense
reflections, indicating the recovery of the crystalline order. More-
over, the diffraction peaks of the original and pressed samples
heated at 100 ꢀC for 0.5 h and quenched to room temperature
almost disappeared, indicating an amorphous structure. The
absence of piezofluorochromic properties of P4 may be due to the
high crystalline order that was difficult to disrupt. These results also
suggest that the structural changes in molecular aggregation from
the crystalline to the amorphous state can induce significant pie-
zofluorochromic properties and photofunctional liquid crystal
properties.
fumed sample (D) exhibited an endothermic peak (at 101 ꢀC, Tm1
)
and re-crystallization (at 105 ꢀC, Tc) before the isotropic tempera-
ture was reached (130 ꢀC, Ti). When the original and pressed
samples of P5 were heated at 100 ꢀC, the lower endothermic peak
disappeared. These results show that the thermally-induced LC
transition from metastable to stable phases was accompanied by a
change of the luminescent color.
Temperature-dependent fluorescence spectroscopy was con-
ducted to further explore the PL wavelengths and the intensities of
the pressed samples for P5 at variable temperatures (Fig. 11). From
the first heating process, the PL wavelengths and intensities
decreased with the increase in temperature (T > 90 ꢀC). When the
temperature reached 105 ꢀC, the sample remained in the LC state
and exhibited a significant red shift at 504 nm. From 115 ꢀC to
130 ꢀC, the PL wavelengths and intensities again decreased, and
then increased with the decrease of temperature. Below 100 ꢀC, the
pressed samples exhibited a blue shift similar to that in annealing
condition with the increase in temperature. From 100 ꢀC to 105 ꢀC,
the pressed sample transformed into the LC state, resulting in an
instant red shift, and then the PL wavelengths slightly changed. At
The thermal properties of P4 and P5 were then examined by
DSC. In the first heating process (Fig. 10), both original and pressed
samples of P4 have three endothermic peaks at 86.2, 116.8 and
122.6 ꢀC, respectively. Compared with the original sample, the high
temperature peak (at 122.6 ꢀC) of the pressed sample almost un-
changed, however, the two low temperature peaks significantly

M. Luo et al. / Dyes and Pigments 101 (2014) 74e84
83
in Fig. 12 and Table 3. In all cases, the emission can fit the double
exponential decay. The lifetimes may be correlated to the various
ground state aggregates, excimers and charge-transfer dimer
emissions [45]. The weighted mean lifetimes <s> of the original
and pressed samples for P4 were similar, and the time-resolved
fluorescence curves were almost coincidental with each other.
40000
35000
30000
25000
20000
15000
10000
5000
P4
a
b
d
c
Original
Pressed
Origin heat at116 C for 0.5h
Pressed heat at 116 C for 0.5h
However, the <s> values of the original, pressed, annealed and
fumed P5 samples were significantly different. The pressed sam-
ples showed longer lifetimes, and after annealing and fuming, their
lifetimes were found to be nearly identical to that of the original
sample. When heated to LC phase temperature, the original and
pressed samples of P4 and P5 showed longer lifetimes with a sig-
nificant red shift. Thus, pressing and heating may disrupt the or-
dered assemblies, generating amorphous regions with increasing
the formation probability of excimer or intermolecular charge-
transfer state, corresponding to the red shift emission [45].
0
0
20
40
Time / ns
4. Conclusions
40000
35000
30000
25000
20000
15000
10000
5000
Two new gelation-induced enhanced emission compounds, P4
and P5 based on tetraphenylethylene and gallic acid derivatives,
have been designed and synthesized. These compounds exhibit AIE
behavior, as indicated by the significant fluorescence enhancement
in the aggregated states. Moreover, their fluorescence intensities
can be reversibly changed with gelesolution transition by alternate
cooling and heating. The POM and DSC studies revealed that both
P4 and P5 exhibit mesomorphic properties over a relatively wide
temperature range. Furthermore, P5 has significant piezo-
fluorochromic properties due to the crystalline-amorphous phase
transformation. To the best of our knowledge, a fluorescence-
enhanced organogel with both mesomorphic and piezochromic
luminescent properties has not yet been reported in the literature.
As the compounds show thermally-induced LC transition from
metastable to stable phases accompanied by changes in the lumi-
nescent color, they may be potential candidates for external
stimuli-responsive materials.
P5
A
Original
Pressed
Annealed
Fumed
Original heat at 100 C for 0.5h
Pressed heat at 100 C for 0.5h
B
C
D
E
F
0
0
20
40
Time / ns
Fig. 12. Time-resolved emission decay curves of P4 and P5 samples under different
treatment conditions.
Acknowledgments
the temperature above 130 ꢀC, the pressed sample went into the
isotropic phase with a red shift. During the cooling process and the
second heating process, the pressed samples exhibited a blue shift
and a red shift with the decrease and increase in temperature,
respectively.
The time-resolved emission-decay behavior of these com-
pounds under different conditions was also studied. The time-
resolved fluorescence curves and the lifetime data are illustrated
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (51173210,
51073177), the Science and Technology Planning Project of
Guangdong Province, China (Grant numbers: 2007A010500001-2,
2008B090500196), Construction Project for University- Industry
cooperation platform for Flat Panel Display from the Commission of
Economy and Informatization of Guangdong Province, the Funda-
mental Research Funds for the Central Universities and Natural
Science Foundation of Guangdong (S2011020001190).
Table 3
Solid-state fluorescence lifetime data of P4 and P5 samples under different
conditions.
Appendix A. Supplementary data
Type
s1 (ns)a
s
2 (ns)a
A1
A2
<s
> (ns)c
b
b
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2013.08.026.
P4
P5
Original
Pressed
1.01
0.99
1.25
1.21
0.88
1.03
0.98
0.94
0.97
1.11
2.27
2.28
4.81
4.86
1.99
2.55
2.42
2.17
3.47
3.44
77.79
70.71
41.34
34.69
76.76
72.90
72.29
79.93
19.68
13.81
22.21
29.29
58.66
65.31
23.24
27.10
27.71
20.07
80.32
86.19
1.29
1.37
3.34
3.59
1.13
1.44
1.38
1.19
2.98
3.12
Original-116 ꢀCd
Pressed-116 ꢀCd
Original
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