Novel organogel harnessing Excited-State Intramolecular Proton Transfer process with aggregation induced emission and photochromism

Article abstract of DOI:10.1016/j.dyepig.2016.04.036

Two novel aggregation-induced emission compounds harnessing Excited-State Intramolecular Proton Transfer process based on salicylideneaniline derived from tetraphenylethylene and cholesterol moieties were synthesized and characterized. One of the compounds could gelate in cyclohexane exhibiting gelation-induced enhanced emission and the emission intensities can be reversibly changed with the gel-solution transition by alternate cooling and heating. Moreover, this compound showed photochromic behavior both in gel and solid states under UV light irradiation due to the loose packing of the molecules and permitting the molecule to rotate, which may be a potential candidate for external stimuli-responsive materials through tuning the self-assembly process of the functional gelator.

Full text of DOI:10.1016/j.dyepig.2016.04.036

Dyes and Pigments 132 (2016) 48e57
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel organogel harnessing Excited-State Intramolecular Proton
Transfer process with aggregation induced emission and
photochromism
,
Miao Luo a, Sheng Wang a **, Meiling Wang a, Shaorong Huang a, Chengpeng Li a,
,
*
Lin Chen b, Xiang Ma b
a College of Basic Education, School of Chemistry and Chemical Engineering, Lingnan Normal University, Zhanjiang, 524037, Guangdong, PR China
b Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Meilong Road 130, Shanghai,
200237, PR 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 harnessing Excited-State Intramolecular Proton
Transfer process based on salicylideneaniline derived from tetraphenylethylene and cholesterol moieties
were synthesized and characterized. One of the compounds could gelate in cyclohexane exhibiting
gelation-induced enhanced emission and the emission intensities can be reversibly changed with the
gel-solution transition by alternate cooling and heating. Moreover, this compound showed photochromic
behavior both in gel and solid states under UV light irradiation due to the loose packing of the molecules
and permitting the molecule to rotate, which may be a potential candidate for external stimuli-
responsive materials through tuning the self-assembly process of the functional gelator.
© 2016 Elsevier Ltd. All rights reserved.
Received 31 March 2016
Received in revised form
21 April 2016
Accepted 22 April 2016
Available online 25 April 2016
Keywords:
Aggregation-induced emission
Tetraphenylethylene derivatives
Organogelator
Salicylideneaniline
Photochromism
1. Introduction
temperature, light, sound, and shearing stress, thus potentially
applicable in sensors, switches, chemical valves and drug delivery
Low-molecular mass organogelators (LMOGs) have received
immense interest in supramolecular chemistry and materials sci-
ence due to their potential applications in sensors, cosmetics,
catalysis, energy harvesting, drug delivery systems, switches and
other related fields [1e9]. LMOGs consisting of low molecular
weight molecules can self-assemble into various nanostructures,
such as fibers, rods, ribbons and other aggregates, through multiple
noncovalent interactions [10e13], to form entangled three-
dimensional networks preventing the solvent molecules from
flowing [14e16]. Most LMOGs possessing stimulus-responsive
properties show reversible changes in morphology and/or phys-
ical properties in response to various external stimuli such as pH,
systems [7,14,17e20].
Nowadays, a lot of attentions have been paid on organic fluo-
rescent molecules harnessing the ESIPT process, which show
unique photophysical properties and have potential applications in
chemical sensors, proton transfer lasers, fluorescence imaging, and
organic light-emitting diodes (OLEDs) [21]. However, most of
chromophores harnessing the ESIPT process suffer from aggrega-
tion caused quenching and thus limited their practical applications.
Fortunately, a novel phenomenon of aggregation-induced emission
(AIE) was first found by Tang's group in 2001 [22e24]. Then more
and more significant progress has been made via the AIE mecha-
nism and AIE-active molecules. Several ESIPT molecules possessing
AIE characteristics have been recently reported [2,25e32]. How-
ever, the number of organogelator harnessing ESIPT process is still
very limited so far, and LMOGs based on salicylideneaniline derived
from tetraphenylethylene (TPE) and cholesterol moieties have not
been reported to the best of our knowledge.
* Corresponding author. Key Laboratory for Advanced Materials and Institute of
Fine Chemicals, East China University of Science and Technology, Shanghai, 200237,
PR China.
Herein, we report two novel AIE compounds (T1 and T2, Scheme
1) by introducing tetraphenylethylene and cholesterol moieties
** Corresponding author. College of Chemistry Science & Technology, Lingnan
Normal University, Zhanjiang, 524048, PR China.
E-mail addresses: dyesws@126.com (S. Wang), maxiang@ecust.edu.cn (X. Ma).
http://dx.doi.org/10.1016/j.dyepig.2016.04.036
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

M. Luo et al. / Dyes and Pigments 132 (2016) 48e57
49
Scheme 1. Synthetic routes for T1 and T2.
into salicylideneaniline system. TPE, a propeller-like luminogen, as
a prototypical AIEgen has been under intensive and extensive in-
vestigations [33e36]. Cholesterol is favorable for facilitating gela-
tion of solvents and thus been widely used for designing new
LMOGs [1,11]. Moreover, a Schiff base bearing an o-hydroxyl group
on the benzene ring is responsible for ESIPT process [25]. The self-
assemble, ESIPT process and photochromism properties are also
elucidated.
phenylmethanone, 2-bromo-1,1,2-triphenylethylene, diphenyl-
methane, cholesterol, succinic anhydride, 2,4-
dihydroxybenzaldehyde, n-butyl lithium, aliquat 336, tetrakis (tri-
phenylphosphine) palladium (0) were purchased from Aladdin
company and used as received. 4-dimethylamiopryidine (DMAP),
4-toluene
sulfonic
acid,
1-(3-dimethylaminopropyl)-3-
ethylcarbodiimide hydrochloride (EDC-HCl) were purchased from
Shanghai Darui company (China) used as received. Ultra-pure wa-
ter was used in the experiments. Tetrahydrofuran (THF) was
distilled from sodium/benzophenone. All other reagents and sol-
vents were purchased as analytical grade from Zhangjiang Kangbai
Company (China) and used without further purification. 1-bromo-
2. Experimental
2.1. Materials and instruments
4-(1,2,2-triphenylvinyl)benzene
(TPE-Br)
[37,38],
4-(1,2,2-
triphenylvinyl)benzene-amine (P4NH2) [7] and 40-(1,2,2-
4-Aminophenylboronic
acid,
4-bromophenyl

50
M. Luo et al. / Dyes and Pigments 132 (2016) 48e57
triphenylvinyl)biphenyl-4-amine (P5NH2) [7] were prepared ac-
cording to the literature methods.
0.36 mmol) in ethanol (50 mL) was heated to reflux for 12 h and
cooled to room temperature. The precipitate formed was collected
by filtration and washed by alcohol for 3 times to get T2 as light
yellow solid. Yield: 86.5%; m.p 228.0e230.0 ꢀC; IR (KBr):
The IR spectra were measured on a Nicolet-6700 FT-IR spec-
trometer by incorporating the samples in KBr disks. Proton and
carbon nuclear magnetic resonance (1H NMR and 13CNMR) spectra
were measured on a Bruker AVANCE III spectrometer [CDCl3, tet-
ramethylsilane (TMS) as the internal standard]. The electronic
spray ionization (ESI) high-resolution mass spectra were tested on
a HP 5958 mass spectrometer. The SEM images were obtained using
a Hitachi S-4800 spectrometer. The CD spectra were recorded on
JACSO J-815 CD spectropolarimeter. The UV/Vis spectra were
y
¼ 3442 cmꢁ1 (hydroxy), 1764 and 1733 cmꢁ1 (ester carbonyl),
1621 cmꢁ1 (C]N); 1H NMR (400 MHz, CDCl3)
d 8.68 (s, 1H), 7.64 (d,
J ¼ 7.8 Hz, 2H), 7.47e7.33 (m, 5H), 7.21e7.05 (m, 19H), 6.84 (s, 1H),
6.76 (d, J ¼ 7.5 Hz, 1H), 5.41 (d, J ¼ 4.1 Hz, 1H), 4.69 (m, 1H), 2.91 (t,
J ¼ 6.6 Hz, 2H), 2.75 (t, J ¼ 6.6 Hz, 2H), 2.36 (d, J ¼ 7.6 Hz, 2H),
2.08e1.82 (m, 6H), 1.41e1.26 (m, 7H), 1.22e1.08 (m, 7H), 1.07e0.86
(m, 17H), 0.70 (s, 3H); 13C NMR (CDCl3, 75 MHz)
d (ppm):171.4,
determined on
a
Shimadzu-2550 spectrophotometer and
a
170.6, 147.0, 143.8, 143.1, 141.3, 140.4, 139.6, 137.8, 134.9, 133.1, 131.9,
131.4, 127.7, 127.6, 126.5, 126.0, 122.8, 121.6, 113.9, 110.6, 74.6, 56.7,
56.1, 50.0, 42.3, 39.7, 39.6, 38.0, 36.8, 36.3, 35.9, 32.0, 29.5, 27.6,
24.4, 23.8, 22.8, 22.4, 21.0, 19.3, 18.9, 12.2; MALDI-TOF MS (ESþ): m/
z 1012.58 ([M]þ, calcd for C70H77NO5,1012.58).
Shimadzu-3600 spectrophotometer. Photoluminescence spectra
(PL) were measured on a Cary Eclipse spectrometer with 10 nm and
10 nm slit widths for excitation and emission, respectively.
2.2. Synthesis of 3-cholesteryloxycarbonylpropanoic acid (1)
3. Result and discussion
A solution of cholesterol (5.80 g, 15 mmol), succinic anhydride
(1.50 g, 15 mmol), pyridine (1.00 mL), and dry heptane (150 mL)
were heated to reflux for 21 h and cooled to room temperature. The
resulting precipitate was recrystallized twice from acetone. Yield:
70%.
3.1. Synthesis
The target compounds were synthesized according to the routes
depicted in Scheme 1. The molecular structure of the target com-
pounds consisted of three parts: the tetraphenylethylene, the sal-
icylaldehyde and the cholesterol moieties. The design strategy of
the salicylaldehyde molecule, including a hydrogen bonding site
and a fluorescent tetraphenylethylene core, is expected to make the
molecule display photochromism. One of the major objectives of
this study is to examine the influence of the linker on the properties
of the compounds. Their molecular structures were confirmed by
1H and 13C NMR spectroscopy, mass spectrometry, and Fourier-
transform infrared spectroscopy.
2.3. Synthesis of compound 2-Hydroxy-4-(3-
cholesteryloxycarbonylpropionyloxy) benzaldehyde (2) [3]
Compound
1
(2.50
g,
5.1
mmol)
and
2,4-
dihydroxylbenzaldehyde (0.9 g, 6.5 mmol) were dissolved in dry
CH2Cl2 (50 mL) containing pyridine (1.8 mL). The solution was
cooled to 0e5 ꢀC and a small amount of DMAP and EDC-HCl (2.50 g,
10 mmol) were added. The mixture was stirred for 4 h at 0e5 ꢀC
and left for 24 h at room temperature. A white precipitate was
removed by filtration. After the solvent was evaporated under
reduced pressure, the resultant residue was purified by column
chromatography on silica gel (cyclohexane/ethyl acetate 5:1). Yield:
3.2. Gelation properties
The gelation behavior of compounds T1 and T2 were tested in
different solvents, with 3.0% (w/v) as a standard concentration. The
results are summarized in Table 1. It can be found that a small
difference of molecule structure greatly affected the gelation abil-
ity. For T2, there is one more phenyl unit than T1 between the link
of TPE and o-hydroxyl benzene unit. Surprisely, T1 could not form
stable organogels in any organic solvents, while T2 could only
gelate in cyclohexane with critical gelator concentrations (CGC)
about 20 mg/mL by using the ‘stable to inversion of a test tube’
method [30,39] and the gel-to-sol phase transition temperature
(Tgel) about 68 ꢀC by using ‘the ball dropping method’ [18].
To investigate the aggregation morphology of the organogel,
xerogel of T2 prepared by slow evaporation of cyclohexane from
the corresponding organogel, was studied by field emission scan-
ning electron microscopy (FE-SEM). As shown in Fig. 1, the xerogel
of T2 was composed of fibrous structure about 100 nm in width and
20%; IR (KBr):
y
¼ 3436 cmꢁ1 (hydroxy), 1765 and 1730 cmꢁ1 (ester
carbonyl), 1659 cmꢁ1 (aldehyde carbonyl); 1H NMR (400 MHz)
d
11.22 (s, 1H), 9.86 (s, 1H), 7.57 (d, J ¼ 8.4 Hz, 1H), 6.83e6.76 (m,
2H), 5.38 (d, J ¼ 3.8 Hz,1H), 4.66 (m,1H), 2.88 (t, J ¼ 6.7 Hz, 2H), 2.72
(t, J ¼ 6.6 Hz, 2H), 2.33 (d, J ¼ 7.8 Hz, 2H), 2.06e1.78 (m, 6H),
1.40e1.23 (m, 7H), 1.12 (m, 7H), 1.04e0.85 (m, 17H), 0.67 (s, 3H).
2.4. Synthesis of T1
A solution of 2 (0.1500 g, 0.24 mmol) and P4NH2 (0.0859 g,
0.24 mmol) in ethanol (50 mL) was heated to reflux for 12 h and
cooled to room temperature. The precipitate formed was collected
by filtration and washed by alcohol for 3 times to get T1 as light
yellow solid. Yield: 78.9%; m.p 178.0e180.0 ꢀC; IR (KBr):
y
¼ 3425 cmꢁ1 (hydroxy), 1763 and 1730 cmꢁ1 (ester carbonyl),
1622 cmꢁ1 (C]N); 1H NMR (400 MHz, CDCl3)
d 8.61 (s, 1H), 7.42 (d,
J ¼ 8.4 Hz, 1H), 7.22e7.01 (m, 21H), 6.90 (d, J ¼ 1.8 Hz, 1H), 6.73 (dd,
J ¼ 8.4, 1.9 Hz, 1H), 5.40 (d, J ¼ 4.2 Hz, 1H), 4.68 (m, 1H), 2.89 (t,
J ¼ 6.6 Hz, 2H), 2.74 (t, J ¼ 6.6 Hz, 2H), 2.38e2.32 (m, 2H), 2.07e1.81
(m, 6H), 1.40e0.86 (m, 31H), 0.70 (s, 3H); 13C NMR (CDCl3, 75 MHz)
Table 1
Gelation properties of T1 and T1 in various solvents.
Solvents
T1
T2
Solvents
T1
T2
d
(ppm): 171.5, 170.5, 164.0, 161.0, 143.6, 143.5, 139.5, 132.5, 131.4,
Cyclohexane
DMF
DMSO
1-Octanol
1,4-Dioxane
Ethylene glycol
Ethyl acetate
Acetone
S
P
S
P
S
I
S
I
S
G
S
S
I
S
I
S
I
I
Toluene
Butyl alcohol
p-Xylene
Petroleum ether
Ethanol
THF
Dichloromethane
Acetonitrile
Methyl alcohol
S
P
S
I
S
I
S
I
131.3, 126.6, 122.8, 120.5, 110.4, 71.6, 56.8, 56.1, 50.1, 42.3, 39.9, 39.6,
37.9, 36.9, 36.7, 36.1, 35.7, 31.8, 29.6, 28.3, 28.2, 27.8, 24.4, 23.7, 22.9,
22.7, 21.2, 21.2, 19.4, 18.7, 11.9; MALDI-TOF MS (ESþ): m/z 936.55
([M]þ, calcd for C64H73NO5, 936.55).
I
I
S
S
I
S
S
I
2.5. Synthesis of T2
Diethyl ether
I
I
A solution of 2 (0.2190 g, 0.36 mmol) and P5NH2 (0.0859 g,
G: stable gel formed at room temperature; S: soluble; I: insoluble; P: precipitate.

M. Luo et al. / Dyes and Pigments 132 (2016) 48e57
51
fluorescence enhancement from gels was belonged to gelation-
induced fluorescence enhanced emission [41].
3.4. AIE properties
The UVeVis absorption and PL emission behaviors of the diluted
mixtures of the compounds were studied in a mixture of water-THF
with different water fractions to determine their AIE properties.
The PL spectra of T1 and T2 in THF/water mixtures with different
water contents are shown in Fig. 3. For T1, the PL intensity was very
weak in pure THF and in the mixtures with water fraction <30%,
and then the PL intensity increased with the increase in water
fraction. When the water fraction reached 90%,
a dramatic
enhancement in luminescence was observed. However, there were
some differences of T2 with the increase in water fraction. When
the water fraction reached 60% and 70%, it could easily form floccus
aggregation with the PL intensity stronger than the water fraction
of 90%. This phenomenon was often observed in some compounds
with AIE properties, but the reasons remain unclear. There are two
possible explanations for this phenomenon: (1) after the aggrega-
tion, only the molecules on the surface of the nanoparticles emitted
light and contributed to the fluorescent intensity upon excitation,
leading to a decrease in fluorescent intensity. However, the re-
striction of intramolecular rotations of the aromatic rings around
the carbonecarbon single bonds in the aggregation state could
enhance light emission. The net outcome of these antagonistic
processes depends on which process plays a predominant role in
affecting the fluorescent behavior of the aggregated molecules; (2)
when water is added, the solute molecules can aggregate into two
kinds of nanoparticle suspensions: crystal particles and amorphous
particles. The former leads to an enhancement in the PL intensity,
while the latter leads to a reduction in intensity. Thus, the
measured overall PL intensity data depends on the combined ac-
tions of the two kinds of nanoparticles [42e48]. Careful inspection
of the PL spectra of the dye in the aqueous mixtures reveals a slight
red shift (~3 nm) in the emission peak when the water fraction is
increased from 60% to 90%. This phenomenon often observed in
some compounds with AIEE properties and has been observed and
theorized by Tang et al. This is probably due to the change in the
packing mode of the dye molecules in the aggregates. In the
mixture with the “low” water fraction, solute molecules steadily
assemble in an ordered pattern to form more emissive, bluer
crystalline aggregates. In a mixture with “high” water content, so-
lute molecules quickly agglomerate in a random way to form less
emissive, redder amorphous particles [42,43]. This result indicated
that both compounds exhibited obvious AIE activity. The AIE ac-
tivity may be attributed to its twisted structure (Fig. 4), in which
multiple phenyl peripheries linked to an ethylene core via Cpheny-
leCethenyl single bonds which enable their free rotation. The mo-
lecular size and effect of steric hindrance influence their rotation; a
larger molecule should have lower freedom of rotation [7,49].
The UV/vis absorption spectra of T1 and T2 in the THF/water
mixtures (2.5 mM) were provided in the Supporting Information,
Fig. S2. The spectra displayed absorption tails extending well into
the long wavelength region caused by the Mie effect [38,49e51],
indicating that the molecules aggregated into nanoparticles in the
mixtures. The different SEM images of T2 obtained from the mixed
solutions containing 0%, 50%, 60%, 70%, 80% and 90% volume frac-
tions of water shown in Fig. 5. The result shows that T2 forms
particles of a few hundred nanometers in size in water fractions of
50%, 80%, 90%, while it forms fibers about a few hundred nano-
meters in width in the water fraction of 60% and 70%.
Fig. 1. SEM image of xerogel prepared from cyclohexane solution for T2.
tens of micrometers in length, which further cross-link to form a 3-
D network. These fibers formed right-handed helices (Inset in
Fig. 1), which was in accordance with the CD spectroscopy (Fig. S1).
3.3. Gelation-induced fluorescence-enhanced emission
To investigate the interrelationship between the emission and
aggregation modes along with the solegel transition, temperature-
dependent fluorescence spectra of T2 with a high concentration in
the cyclohexane (20 mg/mL) were measured from 70 to 25 ꢀC. As
shown in Fig. 2, the initial solution of T2 exhibited rather weak
fluorescence. And the fluorescence intensity increased along with
the decrease in temperature. The remarkable fluorescence
enhancement from the gels was possibly due to the formation of
self-assembly aggregates [30,40]. Interestingly, the fluorescence
intensity of T2 can be reversibly modulated accompanying the
gelesol transition through alternating cooling and heating (Inset of
Fig. 2). Furthermore, the obvious enhancement of fluorescence in-
tensity after gelating could easily be distinguished even by naked
eyes from the photographic images (Inset of Fig. 2). The remarkable
Fig. 2. Temperature-dependent fluorescence spectra of T2 in cyclohexane (20 mg/mL,
lex ¼ 324 nm). Insets show the reversible variation of the emission intensity at 528 nm
accompanying the solegel transition (top) and the emission images of the T2 in sol
(left) and gel (right) state under UV irradiation at 365 nm (bottom).
Quantum mechanical computations were conducted using the
Materials Studio 7.0 software to study the lowest energy spatial
conformation of the compounds [52]. The highest occupied

52
M. Luo et al. / Dyes and Pigments 132 (2016) 48e57
Fig. 3. PL spectra of T1 (a) and T2 (b) in THF/water mixtures with different water fractions (Inset: the images were taken at room temperature under 365 nm UV light in THF and 90%
water), concentration 2.5 mM, lex ¼ 321 nm for T1 and lex ¼ 324 nm for T2; the images of T1 (c) and T2 (d) in THF/water mixtures with different water fractions were taken under
room light (top) and 365 nm UV light (bottom).
Fig. 4. The optimized structures of the compounds.
molecular orbitals (HOMOs) and the lowest unoccupied molecular
orbitals (LUMOs) of these compounds were obtained (Fig. 6) after
structural optimization. T1 and T2 showed different electron cloud
distributions in either their HOMO or LUMO orbital, which illus-
trates why T1 and T2 exhibited very different UV and PL spectra. As
shown in Fig. 6, the electron cloud distributions in HOMO of T1 was
mainly localized at the tetraphenylethylene and the salicylaldehyde
moieties, while T2 was localized at the tetraphenylethylene core.
Meanwhile, the electron cloud distributions in HOMO of T2 were
more disperse than T1, respectively. Moreover, the energy gap

M. Luo et al. / Dyes and Pigments 132 (2016) 48e57
53
Fig. 5. SEM images of water/THF (v/v) mixture of T2.
Fig. 6. Molecular orbital amplitude plots of HOMO and LUMO energy levels of T1 and T2.

54
M. Luo et al. / Dyes and Pigments 132 (2016) 48e57
Table 2
T2 gel in cyclohexane
The absorption and emission peaks of T1 and T2.
0 s
2
1
0
5 s
30 s
1 min
5 min
10 min
30 min
Sample
State
labs (nm)
lE* (nm)
lK* (nm)
Stoke shift (nm)
T1
90%
Solid
70%
90%
Gel
368
340
364
375
356
360
426
e
524
565
525
526
528
527,561
156
225
161
151
172
201
T2
427
427
462
e
Solid
3.5. Photochromic properties
The salicylideneaniline crystals usually exhibit two mutually
exclusive properties: either thermochromism or photochromism
[3,53e59]. The UV absorption spectra of the compounds T1 and T2
were measured at three different states (i.e., solution, gel and solid)
with variable irradiation time under 365 nm light (Fig. 7 and
Fig. S3). After irradiation with 365 nm light, T2 in the solutions of
DMF and THF stayed almost unchanged at 280 nm and 358 nm
(Fig. S3). However, the band at 280 nm and 358 nm of T2 in
cyclohexane gel showed decrease under the prolonged irradiation,
and a new peak of T2 in solid powder appeared at 425 nme575 nm
intensified along with the prolonged irradiation. T1 only showed
very slight change from 425 nm to 525 nm (Fig. S3). This spectrum
suggests that T2 shows significant photochromic properties in both
the gel and the solid phase [3]. It is attributed that the TPE moiety
possessing a twisted spatial conformation, which makes the mo-
lecular packing relatively loose. Moreover, T2 has one more phenyl
ring than T1 among the link of TPE and o-hydroxyl benzene ring to
make the molecular packing much looser than T1, so the cis-trans
keto isomerization of the photo-product is possible due to the
very loose packing. Thus T2 exhibits the property of photochro-
mism [52e58].
300
400
500
600
700
800
Wavelength (nm)
0.5
0.4
0.3
0.2
0.1
0.0
T2--solid
0 min
1 min
2min
5 min
10 min
20 min
30 min
60 min
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength/nm
3.6. ESIPT process
Fig. 7. UVeVis spectra of T2 in cyclohexane gel (a) and in solid state (b) by prolonged
irradiation under 365 nm light from a high-pressure mercury lamp. Inset: the plot of
the absorbance at 500 nm as a function of the irradiation time.
Usually, an o-hydroxyl group on the benzene ring of schiff base
was introduced to form an intramolecular hydrogen bond, which
was responsible for ESIPT process and necessary for their AIE
property at high concentrations or in the aggregate state due to
their large Stokes shift [25e28]. To further verify the mechanism of
ESIPT process, absorption and emission spectra of T2 under
different conditions were measured (Fig. 8, Table 2). Both T1 and T2
70%-Abs
90%-Abs
T2
Gel-Abs
Solid-Abs
70%-Em
90%-Em
Gel-Em
exhibited typical ESIPT behavior with a
p-
p* transition peak at
356e375 nm in the absorption spectrum and dual bands in the
emission spectrum, which comprised a weak E* emission at
426e462 nm and a large Stokes' shifted (>150 nm) K* emission at
525e565 nm, respectively. The detailed photoinduced isomeriza-
tion and photoluminescence processes are shown in Fig. 9. T2 has
one phenyl ring more than T1 between the link of TPE and o-hy-
droxyl benzene ring to make the molecular packing much looser
than T1. T1 might undergo a transition from the excited state to the
ground state accompanied by a longer emission wavelength, and
then return to the E-OH ground state rapidly with a large Stokes
shift (225 nm) but no photochromism. For T2, there is enough space
to permit the molecular to rotate, thus make the Z-NH form
transform into the E-NH form possible exhibiting photochromism.
These results were in agreement with the time-resolved emission-
decay spectra.
Solid-Em
400
500
600
700
Wavelength / nm
Fig. 8. Absorption and emission spectra of T2 under different conditions.
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
in Fig. 10 and Table 3. In all cases, the emission can fit the double
exponential decay. The lifetimes may be correlated to the various
between the HOMO and LUMO of T2 (0.57 ev) is larger than T1
(0.43 ev), which may explain the following photochromism
behavior of these compounds.

M. Luo et al. / Dyes and Pigments 132 (2016) 48e57
55
Fig. 9. Photoinduced isomerization and photoluminescence processes of a typical salicylideneaniline moiety.
Table 3
Solid-state fluorescence lifetime data of T1 and T2 samples under different
conditions.
1.0
0.8
0.6
0.4
0.2
0.0
T1
T2
b
b
Sample
lem
t1 (ns)a
t2 (ns)a
A1
A2
<t
> (ns)c
T1
T2
565
527
561
0.35
1.05
1.05
1.80
3.72
3.72
85.00
92.00
92.00
15.00
8.00
8.00
0.57
1.26
1.26
a
Fluorescence lifetime.
Fractional contribution.
Weighted mean lifetime.
b
c
ground state aggregates, excimers and charge-transfer dimer
emissions [7,60]. The weighted mean lifetime < > of T2 (1.26 ns)
t
was much longer than T1 (0.57 ns), which are in good agreement
with its optical energy gap, suggesting that energy transfer of T1 in
the excited state provided by the proton transfer across the OH … N
hydrogen bond is faster than T2.
400
450
500
550
600
650
700
750
800
Wavelength / nm
4. Conclusion
10000
T1-565 nm
T2-527 nm
T2-561 nm
In summary, two novel AIE compounds, T1 and T2 based on
salicylideneaniline harnessing ESIPT process have been designed
and synthesized. T2 could form gel in cyclohexane, exhibiting
gelation-induced enhanced emission behavior attributed to the
formation of self-assembly aggregates. Their fluorescence in-
tensities could be reversibly changed with gelesol transition by
alternatively cooling and heating. SEM images and CD spectra
revealed that the gelator molecule self-assembled into 1D helical
fiber with diameters of approximately 100 nm and further twisted
into 3D networks. Meanwhile, T2 shows significant photochromic
properties in both the gel and the solid phase under UV irradiation
due to the loose packing of the molecules and permitting the
molecule to rotate, thus indicating that it may be a potential
candidate for external stimuli-responsive materials through tuning
the self-assembly process of the functional gelator.
1000
100
10
1
980
1980
2980
3980
Channel
Fig. 10. (a) Fluorescence spectra (lex: 380 nm) and (b) Fluorescence decay (lem: T1
(565 nm), T2 (527 nm and 561 nm)) of the compounds T1 and T2 in the solid state at
25 ꢀC.
Acknowledgments
This work was financially supported by NSFC (21372194,
21476075 and 21272072) and the Guangdong Yangfan Talent Plan
(2013). M. Luo and C. Li also acknowledge the Research Funds of

56
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