New π-conjugated cyanostilbene derivatives: Synthesis, characterization and aggregation-induced emission

Article abstract of DOI:10.1016/j.cclet.2016.04.020

Two novel π-conjugated cyanostilbene derivatives, FLU-CNPH and TPE-CNPH, were designed and synthesized by introducing the strong electron donor 1, 4-dihydropyrro[3,2-b]indole and AIE electron donor tetraphenylethylene (TPE) to the (3′,5′-bis(trifluorometh

Full text of DOI:10.1016/j.cclet.2016.04.020

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CCLET 3685 1–5
Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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New
p-conjugated cyanostilbene derivatives: Synthesis,
characterization and aggregation-induced emission
a,b
a,b
a,b,
b
c
c,
5
6
Q1 Fang Wang , Xue Li , Sheng Wang *, Chengpeng Li , Huan Dong , Xiang Ma **,
d
a,
Sung-Hoon Kim , Derong Cao **
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School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510641, China
b
Key Laboratory of Applied Chemistry and School of Chemistry and Chemical Engineering, Development Center for New Materials Engineering & Technology
0
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in Universities of Guangdong, Lingnan Normal University, Zhanjiang 524048, China
c
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China
d
Department of Advanced Organic Materials Science and Engineering, Kyungpook National University, Daegu 702701, Repulic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Two novel
p-conjugated cyanostilbene derivatives, FLU-CNPH and TPE-CNPH, were designed and
Received 15 March 2016
Received in revised form 17 April 2016
Accepted 21 April 2016
Available online xxx
synthesized by introducing the strong electron donor 1, 4-dihydropyrro[3,2-b]indole and AIE electron
0
0
donor tetraphenylethylene (TPE) to the (3 ,5 -bis(trifluoromethyl)-biphenyl-4-yl)-acetonitrile, respec-
tively. Both of them were fully characterized and their AIE behaviors were investigated using
fluorescence spectroscopy and FE-SEM images. Their optimized structures and frontier molecular
orbitals were calculated with the DFT by using Materials Studio 7.0 software to study the relationship
between the structure and properties.
Keywords:
p-Conjugated cyanostilbene derivatives
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
1
,4-Dihydropyrro[3,2-b]indole
Tetraphenylethylene
Aggregation-induced emission
Materials Studio
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1. Introduction
ACQ problem of conventional fluorescent molecules. This discov- 28
ery provides a new direction to design organic fluorescent 29
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Fluorescence as a detection signal can be readily recognized
with good sensitivity and easy to detect. Fluorescent products can
be seen everywhere in our daily life, which have made a great
contribution to the various field of society. But most organic
fluorescent molecules will encounter fluorescence quenching or
weakening when they are in high concentrations or aggregated
state, which is known as ‘‘aggregation-caused quenching’’ (ACQ)
and hinders their wide practical applications in polymer film or
solid state to a certain extent [1–4]. It is worth celebrating that in
2001 Tang group firstly discovered a new type of organic
fluorescent molecules [5] showing the aggregation-induced
emission (AIE) properties that the photoluminescence efficiency
was greatly improved in aggregates, which could well solve the
materials with more practical promising application [6–10].
30
In recent years, many researchers have developed various AIE 31
systems [11–16]. At present, the reported AIE compounds can be 32
mainly classified into the following categories: siloles [17], 33
substituted ethenes [18], CN-substituted phenylene vinylenes 34
[19], pyrans [20], biphenyl compounds [21], and polymers 35
[22]. Accordingly, there are several proposed mechanism to be 36
responsible for the AIE effect, restriction of intramolecular 37
rotations (RIR) [23], twisted intermolecular charge transfer (TICT) 38
[24], J-aggregate formation (JAF) [25], and excited-state intramo- 39
lecular proton transfer (ESIPT) [26].
40
p
-Conjugated cyanostilbene molecules are very typical AIE 41
fluorescent materials owing to the J-type stacking and structural 42
complanation [27–29], which have been attracted intensive 43
research works as a platform for the design and fabrication of a 44
variety of nano- and microstructures to use in organic optoelec- 45
tronics in recent years [30–33]. Park’s group reported a special 46
class of aromatic organogelator (CN-TFMBE), which formed 47
highly fluorescent organogels with AIE properties [31]. With 48
some structural modification, it could self-assemble into highly 49
*
Corresponding author at: School of Chemistry and Chemical Engineering, State
Q2
Key Laboratory of Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510641, China.
** Corresponding authors.
E-mail addresses: dyesws@126.com (S. Wang), maxiang@ecust.edu.cn (X. Ma),
drcao@scut.edu.cn (D. Cao).
http://dx.doi.org/10.1016/j.cclet.2016.04.020
1
001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: F. Wang, et al., New
p-conjugated cyanostilbene derivatives: Synthesis, characterization and
aggregation-induced emission, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.020

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fluorescent single-crystalline organic nanowires [34]. Moreover,
some aromatic organogelator had a gel-to-sol phase thermo-
reversible transition with remarkable fluorescence variation that
it was practically nonfluorescent in the sol phase but highly
fluorescent in the gel phase [35], which realized the temperature-
sensitive fluorescent molecule switch.
purification (PE: EA = 2: 1 as eluent) was carried out to give a red
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1
˚
powder with a yield of 57%. Mp: 194–195 C. H NMR (400 MHz,
chloroform-d): 8.04 (s, 2H), 7.88 (s, 1H), 7.78 (d, 2H, J = 8.4 Hz),
d
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7.67 (d, 2H, J = 8.4 Hz), 7.61 (d, 1H, J = 8.4 Hz), 7.56 (s, 1H), 7.42 (s,
1H), 7.38 (s, 1H), 7.21 (d, 1H, J = 8.4 Hz), 4.09 (s, 3H), 4.04–3.91 (m,
2H), 2.09–1.97 (m, 1H), 1.45–1.21 (m, 9H), 0.91 (dt, 6H, J = 14.1,
1
3
In this article, we designed and synthesized two new
p
-
7.2 Hz). C NMR (101 MHz, chloroform-d): d 144.21, 142.24,
conjugated cyanostilbene derivatives by introducing the strong
electron donor 1, 4-dihydropyrro[3,2-b]indole and typical AIE
137.42, 135.97, 135.87, 132.45, 132.12, 132.08, 128.39, 127.79,
126.89, 125.88, 124.68, 121.97, 121.40, 121.17, 119.32, 118.63,
117.04, 113.06, 112.85, 101.50, 93.64, 48.97, 39.34, 32.32, 31.95,
30.68, 29.73, 29.39, 28.59, 24.09, 23.08, 22.72, 14.16, 14.07,
0
0
electron donor tetraphenylethylene (TPE) to the (3 ,5 -bis(trifluor-
omethyl)-biphenyl-4-yl)-acetonitrile, respectively (FLU-CNPH and
TPE-CNPH, shown in Scheme 1). Both of them were fully
characterized by NMR, FTIR, and HSMS. UV–Vis absorption,
fluorescence spectra and SEM images were used to study their
AIE properties. Meanwhile, their optimized structures and frontier
molecular orbitals were calculated using the Materials Studio
software to investigate the relationship between the structure and
properties.
ꢀ
1
10.74. FT-IR (KBr, cm ): 2963.55, 2926.97, 2857.88, 2210.59,
1565.62, 1496.67, 1453.33, 1415.58, 1379.13, 1277.05, 1178.46,
+
1142.04. HRMS (ESI): m/z 699.1678 [M + Na] .
CompoundTPE-CNPH.Themixtureof10(0.18 g,0.5 mmol)and3
(0.16 g, 0.5 mmol) in tert-butyl alcohol (10 mL) and THF (0.5 mL)
was stirred at 50 C for 1 h. Tetrabutylammonium hydroxide (TBAH)
(0.05 mmol, 1 mol/L in methanol) was slowly dropped into the
mixture and stirred for 1 h. The yellow precipitate was collected by
filtration and washed with methanol. Silica column purification (PE:
6
8
9
2. Experimental
EA = 5: 1 as eluent) was carried out to give a light yellow powder
1
˚
with a yield of 61%. Mp: 180–181 C. H NMR (400 MHz, chloroform-
6
2.1. Materials and instruments
d):
d 8.04 (s, 2H), 7.89 (s, 1H), 7.78 (d, 2H, J = 8.2 Hz), 7.69 (t, 4H,
J = 8.5 Hz), 7.50 (s, 1H), 7.14 (t, 11H, J = 8.5 Hz), 7.10–6.97 (m, 6H).
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Most of chemicals were purchased from Aladdin and Aldrich.
Solvents were purified by normal procedures and handled under
moisture free atmosphere. The other materials were commercial
products and were used without further purification. NMR spectra
were recorded by using a Buker Avance & 400 MHz spectrometer
with TMS as internal standard. Melting points were determined
using a Beijing Tech X-4 apparatus with a digital thermometer and
are uncorrected. UV–visible absorption spectra were measured on
a Shimadzu UV-2550 spectrophotometer. Fluorescence spectra
were measured on an Agilent Cary Eclipse Fluorescence Spectro-
photometer. Infrared spectra were measured on a Nicolet 6700 FT-
IR spectrometer using KBr pellets. Mass spectra were recorded on a
Shimadzu QP1000 spectrometer. FE-SEM images were acquired on
a Hitachi S-4800 field emission scanning electron microscopy by
dropping the sample liquid on the foil. A Canon EOS 60D digital
camera was used to take photographs. The fluorescent quantum
1
3
CNMR(101 MHz,Chloroform-d):d146.95, 143.28, 143.13,142.55,
1
1
1
1
1
7
42.53, 142.14, 139.94, 138.64, 135.30, 132.49, 132.16, 132.02,
31.37, 131.34, 131.29, 128.94, 127.93, 127.89, 127.85, 127.71,
27.10, 126.95, 126.78, 126.76, 126.73, 124.65, 121.94, 121.38,
ꢀ1
17.92, 109.58. FT-IR (KBr, cm ): 3056.33, 3026.33, 2215.73,
592.94, 1492.36, 1445.46, 1381.63, 1280.30, 1179.92, 1136.61,
+
00.63. HRMS (ESI): m/z 694.1954 [M + Na] .
3. Results and discussion
129
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3.1. The design and synthesis of FLU-CNPH and TPE-CNPH
The synthetic routes leading to compounds FLU-CNPH and TPE-
CNPH are shown in Scheme 1. The key compound 8 was prepared
by a crafty route from 1-methyl-1H-pyrrole, which had experi-
enced an excellent regioselectivity C–H bond activation reaction,
one step direct cyclization reaction, alkylation, and formylation
reaction in turn (Scheme S1 in Supporting information). Compared
to the structure of carbazole, the structure of 1, 4-dihydro-
pyrro[3,2-b]indole contains more nitrogen atoms which endow it
the stronger electron donating ability. Moreover, another structure
TPE as a typical AIE luminogen is also an electron donor. In our
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yields (
F
f
) of FLU-CNPH and TPE-CNPH solutions were measured
= 89%) as the standard [36].
with rhodamine B in ethanol (
F
f
8
8
2.2. Synthesis procedures of FLU-CNPH and TPE-CNPH
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90
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93
94
95
96
97
The intermediate compounds 3, 5–8, 10 were prepared by the
routes outlined in Scheme S1. The detailed procedures and
characterization data are deposited in supporting information.
Compound FLU-CNPH. The mixture of 8 (0.16 g, 0.4 mmol) and
3 (0.13 g, 0.4 mmol) in tert-butyl alcohol (10 mL) and THF (0.4 mL)
was stirred at 50 C for 1 h. Tetrabutylammonium hydroxide
(TBAH) (0.04 mmol, 1 mol/L in methanol) was slowly dropped
into the mixture and stirred for 1 h. The red precipitate was
collected by filtration and washed with methanol. Silica column
work, we tried to respectively connect the two structures to
p-
conjugated cyanostilbene group, which was another typical AIE
luminogen with a strong electron-withdrawing cyano group, and
discussed the AIE properties of the convergence structure.
3.2. AIE properties of FLU-CNPH and TPE-CNPH
145
Fig. 1 shows the UV–Vis absorption and PL spectra of FLU-CNPH
and TPE-CNPH in solution. The maximum absorption peaks at
77 nm and 461 nm were mainly assigned to the absorption of the
structure of 1, 4-dihydropyrro[3,2-b]indole and TPE, respectively.
Their maximum emission wavelengths were both at 522 nm, but
the Stokes shift of TPE-CNPH (145 nm) is much larger than that of
FLU-CNPH (61 nm), which means TPE-CNPH can be selected as an
ideal fluorophore. Meanwhile, TPE-CNPH is not completely non-
emissive in dilute solutions but not weak fluorescence emission
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3
(
f
F = 1.5%). This was probably because the rotation of the benzene
ring directly connected to cyanostilbene group was restricted to
some extent. As shown in the Fig. 2, by adding water to the
solution, the luminescence intensity of TPE-CNPH was highly
Scheme 1. Synthetic procedures of FLU-CNPH and TPE-CNPH.
Please cite this article in press as: F. Wang, et al., New
p-conjugated cyanostilbene derivatives: Synthesis, characterization and
aggregation-induced emission, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.020

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F. Wang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
3
1
0
0
0
0
0
.0
for 0 vol%, 0.12% for 50 vol%) was observed in the THF-H
2
O
168
400
300
200
100
0
mixtures of FLU-CNPH when the water fraction was no higher than 169
0 vol%. However, the fluorescence intensity was swiftly enhanced 170
with a water fraction of 60 vol% ( = 2.4%). Meanwhile, the 171
.8
.6
.4
.2
.0
5
F
f
fluorescent colors turned to yellow-orange with many strong 172
fluorescent granular precipitates formation in the mixture. But the 173
fluorescence intensity decreased when the water fraction was 174
f
increased to 70 vol% (F = 0.6%) or more, the mixtures all became 175
homogeneous suspensions with no precipitates and the fluores- 176
cent colors finally turned red with a water fraction of 90 vol% 177
300
400
500
600
700
800
(F
= 0.6%).
178
f
Wavelength (nm)
To better understand the AIE properties of TPE-CNPH and FLU- 179
CNPH, FE-SEM images of their aggregates were studied (Fig. 4). 180
Surprisingly, the microstructure like morning glory of FLU-CNPH 181
was obtained by 60 vol% fractions of water addition, a flower or a 182
Fig. 1. UV–Vis absorption (dashed lines) and PL (solid lines) spectra of TPE-CNPH
red) and FLU-CNPH (blue) THF solutions (concentration: 10 mmol/L). The
(
excitation wavelength was kept at 375 nm for TPE-CNPH and 462 nm for FLU-
CNPH.
2
bunch of flowers was dispersed in the THF-H O mixture (Fig. 4a). 183
With a water fraction of 90 vol%, more aggregates were obtained 184
because of its poorer solubility. The regular ‘‘flower’’ shape 185
disappeared and was replaced by FLU-CNPH nanoparticles, which 186
were similar to the shape of small round cakes and stuck together 187
with each other (Fig. 4b). The strong
p–p interactions between 188
molecules would prompt the formation of excimers and this 189
detrimental species led to the observed fluorescence quenching of 190
FLU-CNPH in 90 vol% mixture. FE-SEM pictures in Fig. 4c show that 191
TPE-CNPH nanoscale sticks obtained with a water fraction of 90 vol% 192
were cross or parallel to stick together with each other. Compared to 193
the nanoparticles in Fig. 4b, the nanosticks have more regular 194
geometric structure and the TPE-CNPH molecular packing are 195
relatively more loose leading to the weaker
p–p interactions, so the 196
RIR process is most responsible for the AIE behavior of TPE-CNPH. 197
3.3. Theoretical calculations of geometrical and electronic properties 198
Fig. 2. (a) PL photos under UV (365 nm) light, (b) PL spectra of TPE-CNPH in different
water-THF (v/v) mixtures, (c) The dependence of the fluorescence emission intensity
on the water fraction (inset: luminescent picture of the solid powder of TPE-CNPH
under 365 nm). Concentration: 20 mmol/L, excitation wavelength: 375 nm.
In order to understand more about the relationship between the 199
molecular structure and performance of FLU-CNPH and TPE-CNPH, 200
their optimized structures and electronic distribution of HOMO 201
and LUMO energy levels were obtained by molecular orbital 202
calculations with the DFT by using Materials Studio 7.0 software on 203
GGA/BLYP levels, as shown in Fig. 5. The four benzene rings on 204
double bond of TPE-CNPH were all in different plane but the 205
benzene ring directly connected to the cyanostilbene structure was 206
on the plane of cyanostilbene with a small dihedral angle, which 207
explained the weak fluorescence of TPE-CNPH in THF solution. 208
Similarly, the 1, 4-dihydropyrro[3,2-b]indole and cyanostilbene 209
structure can be considered to be in the same plane but the long 210
branched alkanes was approximatively perpendicular to the plane. 211
On this basis, we could explain the different aggregation formation 212
which caused the unique AIE properties of FLU-CNPH. With the low 213
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f
enhanced by 10 times with water fraction 90 vol% (F = 16%).
Because TPE-CNPH was not so solvable in water, the molecules
aggregated in the mixture, which activated the RIR process and the
PL intensity was enhanced.
Different from the AIE properties of TPE-CNPH, a red-shift of
fluorescence emission was obviously observed in FLU-CNPH
solutions with water volume fraction increasing, accompanied
by the formation of different aggregates (Fig. 3). As shown in
Fig. 3a, only very weak yellowish green fluorescence (
f
F = 0.09%
Fig. 3. (a) PL photos under UV (365 nm) light, (b) PL spectra of FLU-CNPH in different
water-THF (v/v) mixtures, (c) The dependence of the fluorescence emission intensity
on the water fraction (inset: luminescent picture of the solid powder of FLU-CNPH
under 365 nm). Concentration: 20 mmol/L, excitation wavelength: 462 nm.
= 6:4, 2.0 ꢁ 10ꢀ mol/L)
5
Fig. 4. FE-SEM images of FLU-CNPH (a) in H
2
5
O-THF (v:
v
ꢀ
mixture, (b) in H
2
O-THF (v:
v
= 9:1, 2.0 ꢁ 10 mol/L) mixture, (c) FE-SEM images of
ꢀ5
TPE-CNPH in H
2
O-THF (v:v = 9:1, 2.0 ꢁ 10 mol/L) mixture.
Please cite this article in press as: F. Wang, et al., New
p-conjugated cyanostilbene derivatives: Synthesis, characterization and
aggregation-induced emission, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.020

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FLU-CNPH.
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water fraction 60 vol% the molecules of FLU-CNPH would slowly
assemble in an ordered fashion, like J-type aggregates, which was
more emissive. While in the water fraction above 60 vol%, the more
tightly packed structure was obtained in a random way, which
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26981–26986.
branched alkanes could reduce this effect to a certain extent so that
the fluorescence intensity in the water fraction above 60 vol% had
decreased but still stronger than that of the 0 vol% mixture,
accompanied by the red shift of fluorescence spectra.
The HOMO of TPE-CNPH was mainly localized over the TPE
moiety and its LUMO was located on the cyanostilbene group. As
shown in Fig. 5, the HOMO and LUMO energy levels were well
separated in TPE-CNPH molecule, which was in favor of a facile
charge migration. But, for FLU-CNPH, the LUMO and HOMO were
localized over the whole molecule which suggested a poor charge
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In conclusion, we designed and synthesized two novel
-conjugated cyanostilbene derivatives, FLU-CNPH and TPE-
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45
Acknowledgment
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247
248
249
This work was financially supported by NSFC (Nos. 21372194,
21476075 and 21272072) and the Guangdong Yangfan Talent Plan
(2013). Chengpeng Li also acknowledge the Research Funds of
Lingnan Normal University (No. QL1402).
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in
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