Heating and mechanical force-induced "turn on" fluorescence of cyanostilbene derivative with H-type stacking

Article abstract of DOI:10.1039/c3ce41221k

A cyanostilbene-based derivative with aggregation induced-emission was obtained, which exhibited multi-luminescent intensity under external stimuli. The as-prepared crystals of the desired dye showed a rather faint luminescence. Upon heating, its fluoresc

Full text of DOI:10.1039/c3ce41221k

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Heating and mechanical force-induced “turn on”
fluorescence of cyanostilbene derivative with
H-type stacking†
Cite this: CrystEngComm, 2013, 15,
8998
Yujian Zhang,ab Jingwei Sun,b Xiaojing Lv,b Mi Ouyang,b Feng Cao,a Guoxiang Pan,a
Luping Pan,a Guangbo Fan,a Weiwei Yu,a Chao He,a Sishi Zheng,a Feng Zhang,a
a
b
*
Wei Wang and Cheng Zhang
A cyanostilbene-based derivative with aggregation induced-emission was obtained, which exhibited multi-luminescent
intensity under external stimuli. The as-prepared crystals of the desired dye showed a rather faint luminescence.
Upon heating, its fluorescence could be turned on with the quantum yields of 13.3%. The quantum yields of the
melted and solidified powders were further increased to 28.6%. Powder X-ray diffractometry, fluorescence spec-
troscopy, fluorescence lifetime experiments and single crystal XRD analyses were applied to investigate the changes
of molecular packing modes. Such a transformation of fluorescence “dark”–“bright”, easily distinguished by naked
eye, was related to the phase transition. The planar cyanostilbene with H-type packing and multiple mutable inter-
actions may be the essential factor for the multi-luminescent intensity properties.
Received 24th June 2013,
Accepted 10th September 2013
DOI: 10.1039/c3ce41221k
www.rsc.org/crystengcomm
Introduction
recording. Recently, Park's group proposed that an obvious
switching in emission intensity can effectively settle the intrac-
In the aggregated state, the molecular packing arrangement is
of great importance to determine the optoelectronic properties,
especially fluorescence, of organic materials.1 For example, in
the cofacial configuration (H-aggregate), strong intermolecular
interactions result in luminescence quenching relative to iso-
lated monomers.2 In contrast, the J-aggregate and X-aggregate
(cross stacking) generally contribute to enhancing the fluores-
cence efficiency.3 Thus, the fine-tuning of molecular (dipole)
stacking modes through rational molecular design is a promis-
ing approach to control the optoelectronic properties of small
organic molecule materials. Stimuli-responsive fluorescent
materials are receiving unprecedented attention; especially
organic luminophores whose fluorescence can be changed by
external stimuli including vapour, heating and mechanical
force.1a,4 Such interesting behaviour is commonly attributed to
the altering molecular stacking modes of materials at different
phases. Most of the stimuli-responsive materials indicate
switchable emission colours, unfortunately, which is rather dif-
ficult to meet the needs of high-contrast luminescence
table problem.5 However, only limited examples of high-
contrast fluorescent materials have been presented at the
current stage.6 Therefore, the search for innovative materials
and new design strategies is still greatly interesting.
Herein, we reported a new type of stimuli-responsive lumi-
nescent pigment luminophore α-CN-TPA (see Chart 1),
containing a cyano group as an acceptor (A) and a twisty
triphenylamine (TPA) as a donor (D), which shows the aggrega-
tion induced emission (AIE). In the crystalline state, the
luminophore α-CN-TPA exhibits a faint luminescence with
quantum yields (Φf) of less than 0.1% due to the intermolecular
face-to-face stacking (H-aggregate). Upon heating or grinding,
the ordered structure is destroyed, its luminescence turns
“bright” with Φf of over 24%, which can be clearly distin-
guished by naked eye. In addition, the disruption of H-type
aggregation maybe a design strategy for developing stimuli-
responsive luminescent materials with obvious contrast.
Results and discussion
Synthesis of cyanostilbene derivatives
a Department of Materials chemistry Huzhou University, Xueshi Road 1#, Huzhou,
PR China
The molecular structure of TPA-based cyanostilbene deriva-
tive α-CN-TPA was illustrated in Chart 1. The key interme-
diate (Z)-2-(4-bromophenyl)-3-(4-methoxyphenyl) acrylonitrile
(CSB-Br) was prepared by Knoevenagel condensation reaction
of 4-methoxybenzaldehyde with 2-(4-bromophenyl)acetonitrile.
Then, the palladium-catalyzed Suzuki coupling reaction of
b State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology,
College of Chemical Engineering and Materials Science, Zhejiang University of
Technology, HangZhou, PR China. E-mail: czhang@zjut.edu.cn
† Electronic supplementary information (ESI) available. CCDC 893985. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3ce41221k
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Chart 1 Molecular structure of cyanostilbene derivative α-CN-TPA.
CSB-Br with (4-(diphenylamino)phenyl)boronic acid was car-
ried out at 90 °C in the mixture of toluene/THF, to obtain
the resulting product with the yields of 72.5%. Finally, the
desired luminophore α-CN-TPA was fully characterized by
13C NMR, 1H NMR, MS spectral and elemental analyses.
1H NMR (500 MHz, CDCl3) δ 7.92 (d, J = 8.5 Hz, 2H),
7.72 (d, J = 8.5 Hz, 2H), 7.65 (d, J = 8.5 Hz, 2H),7.52 (d, J =
7.0 Hz, 2H), 7.51 (s, 1H), 7.32–7.28 (m, 4H), 7.16 (d, J = 9.0 Hz,
6H), 7.07 (t, J = 7 Hz, 2H), 7.01 (d, J = 8.0 Hz, 2H), 3.89 (s, 3H)
13C NMR; δ 161.4, 147.7,147.6, 141.2, 141.0, 133.6, 133.2,
129.3, 127.6, 127.0, 126.1,124.6, 123.6, 123.2, 114.4, 108.4,
55.5, MS(EI) m/z 478.2 [M+].
Fig. 1 plots of maximum emission intensity (I) and wavelength (λem) of α-CN-TPA
versus water fraction in THF-water mixtures.
Crystallographic data for α-CN-TPA: C34H26N2O, M =
478.57 g mol−1, monoclinic, a = 6.897(3) Å, b = 45.92(2) Å,
c = 8.080(4) Å; β = 94.83(1)°, V = 2549.95(252) Å3, T = 293(2) K,
R(int) = 0.0442, space group P2(1)/c, Dcalc = 1.2465 Mg m−3,
Z = 4, the final R indices were R1 = 0.0908, wR2 = 0.2373 [I >
2σ(I)], CCDC 893985.
Aggregation induced emission
As depicted in Fig. S1 and Fig. S2,† the absorption and emis-
sion spectra of luminophore α-CN-TPA were obtained in
THF–water mixtures to understand the AIE effect. In pure
THF, the two absorption bands at 307 nm and 364 nm were
obviously observed, which mainly originated from the
absorption of twisty TPA7 and cyanostilbene, respectively. In
the mixture with 90% water, the absorption spectrum became
broad and upward-shifted due to the Mie scattering of
nanoparticles.8 In contrast, the luminescence behaviour was
dramatically changed with altering of the water fraction (fW)
in THF–water mixtures. With the increase in the fW from 0 to
60%, the emission spectra were continually red-shifted as
well as weakened in emission intensity as shown in Fig. 1
and Fig. S2A.† However, the fluorescence intensity was
abruptly enhanced in the mixtures with fW > 70%, reaching
an intensity maximum with a water content of 90%. Mean-
while, the emission colour was gradually blue-shifted (Fig. 1,
Fig. S2B†). These results revealed the emission peaks depended
on the solvent polarity (fW < 60%). The molecular dipole
moment was as large as 3.498 D. Thus, dye α-CN-TPA was a
D–π–A dipole molecule with an evident intramolecular charge-
transfer (ICT) effect.9 Fig. S3† indicated the LUMO orbitals
were mainly located on the cyanostilbene group. However,
electron clouds of HOMO orbitals were mostly spread to the
twisty TPA. Such electron distribution further proved its
Fig. 2 Photographs of the α-CN-TPA powder under 365 nm UV light: (A) pristine
powders, (B) the original powders annealed at 130 °C for 1 min, (C) after grinding
pristine powder and (D) after heating powders to melt and solidification at RT
(Melted-S).
intrinsic ICT property. In the mixtures with high water frac-
tions (fW>70%), the molecules became progressively less solu-
ble, and then aggregated into nanoparticles (Fig. S3†). As a
result, the emission spectrum in the aggregate state was blue-
shifted relative to that in the polar solvents due to the disap-
pearance of solvent polarity effect.9b,10 Moreover, the ED pat-
terns further revealed these nanoparticles were amorphous as
depicted in Fig. S4.†
Stimuli-responsive fluorescence switching behaviour
In addition to AIE property, the luminophore α-CN-TPA
exhibits the unexpected “turn-on” fluorescence behaviour
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Fig. 3 Luminescence spectra of α-CN-TPA in different states.
Fig. 4 XRD profiles of α-CN-TPA at different states.
under the stimulus of heating and mechanical force, as
depicted in Fig. 2a. The obtained powders recrystallized from
ethanol solution exhibited quenched luminescence with Φf
less than 0.1%. Surprisingly, upon grinding, the original pow-
der yielded bright green luminescence with Φf as high as
24.1%. The conversion from “dark” state to “bright” state was
clear enough to be distinguished by naked eye (Fig. 2C). The
emission spectrum was red-shifted to 512 nm as shown in
Fig. 3. Upon annealing at 130 °C for 2 min, the pristine pow-
ders were also turned “bright” with sky-blue luminescence
(Φf = 13.3%). In fact, the phase transition with variable fluo-
rescence was related to the first-order endothermic peak in
the differential scanning calorimetry (DSC) thermogram
(Fig. S5†).11 Interestingly, when the annealed powders were
heated to the melted state and quickly cooled down in air, the
Φf of the solidified powders (Melted-S) was further increased
to 28.6%. To the best of our knowledge, most of the reported
stimuli-responsive materials consisting of only one lumi-
nophore exhibited two different fluorescence colours.12 The
conversion of multi-luminescent intensity has rarely been
realized at the current stage. Regrettably, such changes are
irreversible upon heating or solvent fuming process.
As shown in Fig. 4a, The powder X-ray diffraction (PXRD)
pattern of crystalline α-CN-TPA powders was similar to the sim-
ulated XRD from single crystals, which exhibited sharp and
intense reflection peaks due to some microcrystalline struc-
tures. After grinding, the ordered structure was significantly
disturbed, as shown by broad, diffuse and weak peaks. The
results indicated the ground powder had become an amor-
phous structure. When the pristine powders were thermally
treated in air, the new diffraction peaks appeared, which
reflected the transformation from one crystalline phase to
another crystalline phase. For the sample Melted-S with the
highest Φf, the signals of PXRD were rather weak exhibiting
disordered molecular packing. Clearly, the phase transition
process was crucial to the variable luminescent intensity.
X-Ray crystal structure
The crystal packing motifs were also observed in single crystals,
which were obtained from the mixture of dichloromethane
and n-hexane at room temperature The crystal structure of
α-CN-TPA containing four molecules in the unit cell was
found to be monoclinic with the space group P2(1)/c (a =
6.897(3) Å, b = 45.92(2) Å, c = 8.080(4) Å; β = 94.83(1)°). As
shown in Fig. 5, The dihedral angles between the planes of
acrylonitrile and its neighbouring phenyl rings were 6.8° and
12.8°, respectively, which indicated small distortions. These
results exhibited that the luminophore α-CN-TPA adopted
good coplanarity along cyanostilbene. Two types of C–H⋯N
hydrogen bonds are formed between two molecules with a
distance of approximately 2.6 Å, which made the coplanar
cyanostilbene connect together and form the “molecular
sheets” as depicted in Fig. 6A, 6D. In addition, Fig. 6 showed
the two adjacent molecules were packed into H-type (head-to-
tail) dimers in this column. For a D–π–A dipole molecule,
the intermolecular dipole coupling placed the “A” group of
one molecule above the “D” moiety of its neighbouring mol-
ecule. Additionally, strong C–H⋯O inter-actions with a short
distance of 2.620 Å and angle of 140.3° were also observed
Fig. 5 The dihedral angles of the α-CN-TPA single-crystal.
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Fig. 6 Crystal structures of α-CN-TPA: (A) Side view of the antiparallel dipole arrangement, (B) view of the antiparallel arrangement along the short molecular axes, (C) view
of the antiparallel arrangement along long molecular axes and C–H⋯O hydrogen bonds interactions and (D) top view and illustration of C–H⋯π and C–H⋯N hydrogen
bonds interactions.
(Fig. 6C). Thus, π–π stacking interactions, dipole-dipole
interactions and C–H⋯O interactions together stabilized
the antiparallel face-to-face stacking. The interplanar distance
between the neighbouring molecules was 3.478 Å, which is
within the range for an effective intermolecular interaction.
As a result, these interplanar interactions, existing in the
dimer molecules, were sufficient to cause intermolecular exci-
tonic coupling. This proposal was further verified by a rather
long fluorescence lifetime of the pristine powders relative to
the samples under external stimuli, as depicted in Fig. S6.†
In their crystalline powder, the excited state yielded a single-
exponential decay with the lifetime of 20.8 ns. Upon external
stimuli, the sample decayed bi-exponentially with the lifetime
shortened to less than 4 ns. Clearly, the pristine powders
adopted face-to-face stacking, which caused fluorescence
quenching due to strong intermolecular interactions. In con-
trast, upon amorphization, the excitonic coupling was obvi-
ously weakened by the disappearance of H-type packing
leading to the higher Φf. Interestingly, the fluorescence decay
behaviour of samples under external stimuli is fitted by a
double-exponential. With sample Melted-s, for example, the
decay in one pathway is short (τ = 1.45 ns), and that in
another is relatively long(τ = 11.45 ns) as shown in Fig. S6.†
The results revealed the mixture of two distinguished phases
in the amorphous state. The long fluorescence lifetime of the
melted powders in one pathway implied that the strong inter-
molecular interactions still existed in spite of the disappear-
ance of H-type packing. Nevertheless, such interactions didn't
take a dominant place. As depicted in Fig. 5 and 6, the ter-
minal TPA was highly twisted with dihedral angles of over
68°. We inferred that the luminogen may adopt a more
distorted conformation to fit into the crystalline lattice in
the crystalline state. However, once the crystalline lattice was
disrupted by external stimuli, the molecules in the amor-
phous state might readjust themselves and adopted a more
planar conformation.13 This extended their effective conjuga-
tion length. As result, the crystalline powders with
excimeric coupling or excited state dimeric coupling exhibited
a shorter emission wavelength than that of the amorphous
phase (see Fig. 3).
a
In summary, a novel cyanostilbene derivative α-CN-TPA with
AIE effect was effortlessly obtained, exhibiting thermochromic
and mechanochromic fluorescence with high contrast. Espe-
cially, the non-luminescent crystalline powders could trans-
form to sky-blue crystals (Φf = 13.3%) upon heating, further to
deep-green amorphous solids (Φf = 28.6%) when thermally
melted and then quickly solidified in air. On the basis of
photophysical, structural, and optical studies, we concluded
that the variable fluorescence intensity could be attributed to
the phase transition between the crystalline phase and amor-
phous phase. In the crystalline state, the as-prepared powders
adopted the H-type (head-to-tail) stacking, which caused fluo-
rescence quenching due to excimeric coupling. After external
stimuli, the intermolecular interactions were weakened due to
the disruption of the ordered structure. As a result, the fluores-
cence intensity was obviously enhanced. These results showed
that employing “H-aggregated molecules” with multiple muta-
ble interactions to prepare high-contrast stimuli-responsive
materials was rational and successful.
Experimental section
Instruments
The UV–vis spectrum was recorded on a Perkin Elmer
Lambda 35 spectrophotometer. Fluorescent spectra were
obtained using a Varian, Cary Eclipse Fluorescence spectro-
photometer. Powder XRD measurements were conducted on
X'Pert PRO diffractometer (CuKα) in the range 5° < 2θ < 35°
(PANalytical, Netherlands). X-ray crystallographic intensity
data were collected using a Xcalibur, Eos, Gemini Ultra CCD
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diffractometer equipped with a graphite monochromated
Enhance (Mo) X-ray source (λ = 0.71073 Å). The time-resolved
fluorescence lifetime experiments were recorded on time-
correlated single photo-counting technique with an Flsp 920
spectro-fluorometer. Digital photographs were taken by
Canon 550D (Canon, Japan) digital cameras. Differential
scanning calorimetry (DSC) was performed on a TA Instru-
ments DSC 2920 at a heating rate of 10 °C min−1. The
ground-state geometry was fully optimized by density func-
tional theory (DFT) with the hybrid B3LYP functional and
3-21G** as the basis set. SEM and ED patterns were
obtained using a Tecnai G2 F30 S-Twin microscope at an
accelerating voltage of 200 kV.
3 (a) W. Feast, P. W. Lövenich, H. Puschmann and C. Taliani,
Chem. Commun., 2001, 505–506; (b) N. Sanyal and
P. M. Lahti, Cryst. Growth Des., 2006, 6, 1253–1255; (c)
F. Würthner, T. E. Kaiser and C. R. Saha-Möller, Angew.
Chem., Int. Ed., 2011, 50, 3376–3410; (d) Z. Q. Xie, B. Yang,
F. Li, G. Cheng, L. L. Liu, G. D. Yang, H. Xu, L. Ye, M. Hanif,
S. Y. Liu, D. G. Ma and Y. G. Ma, J. Am. Chem. Soc.,
2005, 127, 14152–14153.
4 (a) Y. Sagara and T. Kato, Nat. Chem., 2009, 1, 605–610; (b)
Z. G. Chi, X. Q. Zhang, B. J. Xu, X. Zhou, C. P. Ma, Y. Zhang,
S. Liu and J. R. Xu, Chem. Soc. Rev., 2012, 41, 3878–3896.
5 M. S. Kwon, J. Gierschner, S. J. Yoon and S. Y. Park, Adv.
Mater., 2012, 24, 5487–5492.
6 (a) J. Luo, L. Y. Li, Y. L. Song and J. Pei, Chem.–Eur. J.,
2011, 17, 10515–10519; (b) M. J. Teng, X. R. Jia, S. Yang,
X. F. Chen and Y. Wei, Adv. Mater., 2012, 24, 1255–126.
7 (a) P. I. Shih, C. H. Chien, F. I. Wu and C. F. Shu, Adv. Funct.
Mater., 2007, 17, 3514–3520; (b) Y. J. Zhang, Y. X. Jin, R. Bai,
Z. W. Yu, B. Hu, M. Ouyang, J. W. Sun, C. H. Yua and
C. Zhang, J. Photochem. Photobiol., A, 2012, 227, 59–64.
8 H. Auweter, H. Haberkorn, W. Heckmann, D. Horn,
E. Lüddecke, J. Rieger and H. Weiss, Angew. Chem., Int. Ed.,
1999, 38, 2188–2191.
9 (a) W. Z. Yuan, Y. Y. Gong, S. M. Chen, X. Y. Shen,
J. W. Y. Lam, P. Lu, Y. W. Lu, Z. M. Wang, R. R. Hu, N. Xie,
H. S. Kwok, Y. M. Zhang, J. Z. Sun and B. Z. Tang, Chem.
Mater., 2012, 24, 1518–1528; (b) R. R. Hu, E. Lager,
A. A. Aguilar, J. Z. Liu, J. W. Y. Lam, H. H. Y. Sung,
I. D. Williams, Y. C. Zhong, K. S. Wong, E. P. Cabrera and
B. Z. Tang, J. Phys. Chem. C, 2009, 113, 15845–15853.
10 Y. J. Zhang, J. W. Sun, G. F. Bian, Y. Y. Chen,
M. Ouyang, B. Hua and C. Zhang, Photochem. Photobiol.
Sci., 2012, 11, 1414.
Sample preparation
A single crystal of α-CN-TPA was prepared from the mixture of
n-hexane and dichloromethane at room temperature. Organic
nanoparticles were obtained by
a simple reprecipitation
method. The ICT compounds α-CN-TPA were dissolved in THF
to a concentration of 1 × 10−4 M. 1 mL of this solution was
quickly injected into 9 mL of distilled water and kept stirring,
and the mixture was maintained at room temperature. After
being stirred for 3 min, the sample was left undisturbed for
about 3 days to stabilize the nanostructures. Then, one drop of
suspension was put onto a silicon substrate and copper mesh
for SEM and TEM examination, respectively.
Acknowledgements
The authors gratefully give thanks for the support of National
Basic Research Program of China (2010CB635108,
2011CBA00700), International S&T Cooperation Program,
China (2012DFA-51210) and scientific research project of
Huzhou teachers college.
11 S. J. Yoon and S. Y. Park, J. Mater. Chem., 2011, 21,
8338–8346.
12 (a) Y. Sagara, T. Mutai, I. Yoshikawa and K. Araki, J. Am.
Chem. Soc., 2007, 129, 1520–1521; (b) K. Mizuguchi and
H. Nakano, Dyes Pigm., 2013, 96, 76–80; (c) Y. J. Zhang,
G. L. Zhuang, M. Ouyang, B. Hua, Q. B. Song, J. W. Sun,
C. Zhang, C. Gu, Y. X. Xu and Y. G. Ma, Dyes Pigm., 2013, 98,
486–492; (d) J. Kunzelman, M. Kinami, B. R. Crenshaw,
J. D. Protasiewicz and C. Weder, Adv. Mater, 2008, 20,
119–22.
13 (a) Y. Q. Dong, J. W. Y. Lam, A. J. Qin, J. X. Sun, J. Z. Liu,
Z. Li, J. Z. Sun, H. H. Y. Sung, I. D. Williams, H. S. Kwokc
and B. Z. Tang, Chem. Commun., 2007, 3255–3257; (b)
W. Z. Yuan, Y. Q. Tan, Y. Y. Gong, P. Lu, J. W. Y. Lam,
X. Y. Shen, C. F. Feng, H. H. Y. Sung, Y. W. Lu, J. Z. Sun,
Y. M. Zhang and B. Z. Tang, Adv. Mater., 2013, 25,
2837–2843.
Notes and references
1 (a) S. Varghese and S. Das, J. Phys. Chem. Lett., 2011, 2,
863–873; (b) C. Kitamura, T. Ohara, N. Kawatsuki, A. Yoneda,
T. Kobayashi, H. Naito, T. Komatsuc and T. Kitamura,
CrystEngComm, 2007, 9, 644–647; (c) H. Y. Zhang, Z. L. Zhang,
K. Q. Ye, J. Y. Zhang and Y. Wang, Adv. Mater., 2006, 18,
2369–2372; (d) R. R. Wei, P. S. Song and A. J. Tong, J. Phys.
Chem. C, 2013, 117, 3467–3474; (e) T. Mutai, H. Satou and
K. Araki, Nat. Mater., 2005, 4, 685–687; (f) S. P. Anthony,
ChemPlusChem, 2012, 77, 518–531.
2 (a) F. C. Spano, J. Chem. Phys., 2001, 114, 5376–5390; (b)
J. Cornil, D. Beljonne, J. P. Calbert and J. L. Bredas, Adv.
Mater., 2001, 13, 1053–1067.
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