Enhanced mechanofluorochromic properties of 1,4-dihydropyridine-based fluorescent molecules caused by the introduction of halogen atoms

Article abstract of DOI:10.1039/c9ce00348g

Four cyano(ethoxycarbonyl)methylene-1,4-dihydropyridine (CMD) derivatives were designed and synthesized to study the influence of halogen atoms on their mechanofluorochromic (MFC) properties. These CMD derivatives are demonstrated to have twisted molecular conformations and display aggregation-induced emission properties. In the crystalline state, the compounds adopt loose J-aggregates and exhibit yellow-green or green solid-state fluorescence. Upon grinding, the original samples of CMD-H, CMD-F, CMD-Cl, and CMD-Br show 21, 60, 61, and 52 nm red shifts in their fluorescence spectra, respectively, revealing MFC characteristics. The MFC properties are demonstrated to be due to the crystalline-to-amorphous transition and the red shifts in the fluorescence spectra are attributed to the increased molecular conjugations by the planarization of molecular conformations. X-ray crystallographic analyses reveal that halogen-substituted CMD derivatives have more twisted conformations and looser stacking arrangements than CMD-H, which are responsible for their higher contrast MFC properties. This work indicates that the introduction of halogen atoms to a specific fluorescent molecule might be used as a simple and feasible route for developing high contrast MFC-active materials.

Full text of DOI:10.1039/c9ce00348g

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Tiddo J. Mooibroek, Antonio Frontera et al.
Towards design strategies for anion–π interactions in crystal engineering
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DOI: 10.1039/C9CE00348G
CrystEngComm
ARTICLE
Enhanced
mechanofluorochromic
properties
of
1,4-
dihydropyridine-based fluorescence molecules caused by the
introduction of halogen atoms
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Yating Chen, Yibin Zhou, Zhiqiang Wang, Mengzhu Wang, Wenxia Gao, Yunbing Zhou,
Miaochang Liu, Xiaobo Huang* and Huayue Wu*
www.rsc.org/
Four cyano(ethoxycarbonyl)methylene-1,4-dihydropyridine (CMD) derivatives were designed and synthesized to study the
influence of the halogen atoms on their mechanofluorochromic (MFC) properties. These CMD derivatives are
demonstrated to have twisted molecular conformations and display aggregation-induced emission properties. In the
crystalline state, the compounds adopt loose J–aggregations and exhibit yellow-green or green solid-state fluorescence.
Upon grinding, the original samples of CMD-H, CMD-F, CMD-Cl, and CMD-Br show 21, 60, 61, and 52-nm red shift in the
fluorescence spectra, respectively, revealing MFC characteristic. The MFC properties are demonstrated to be due to the
crystalline-to-amorphous transition and the red shifts in the fluorescence spectra are attributed to the increased
molecular conjugations by the planarization of molecular conformations. The X-ray crystallographic analyses reveal that
halogen-substituted CMD derivatives have more twisted conformations and looser stacking arrangements than CMD-H,
which is responsible for their higher contrast MFC properties. The work indicates that the introduction of halogen atoms
to a specific fluorescent molecule might be used as a simple and feasible route for developing high contrast MFC-active
materials.
Considering that the solid-state emission of an organic compound is
4
Introduction
closely related to their stacking arrangement, the molecular
conformations and intermolecular interactions of traditional
Mechanofluorochromic (MFC) materials refer to a kind of smart
stimuli-responsive fluorescent compounds that exhibit solid-state
fluorescence color and/or intensity changes upon mechanical
stimuli and have drawn wide attention owing to their potential
applications in mechanical sensors, security inks, and optical
fluorescent molecules can be adjusted by appropriate structural
modifications to obtain variable stacking arrangements, which is of
great significance for the generation of new MFC materials.
Because halogen atoms have the strong electron-withdrawing
ability, introducing halogen atoms into a specific fluorescent
molecule helps to regulate the intermolecular interactions and
stacking arrangements, which might be beneficial for the
generation or enhancement of MFC phenomena. In 2014, Lu’s
group investigated the effect of a bromine atom on the MFC
1
recording. Since Sagara and Araki et al. reported the first pure
2
organic compound with MFC property in 2007, some organic MFC-
active compounds have emerged in the past decade. Generally,
3
organic compounds that emit solid-state fluorescence and have
variable aggregated states under external pressure are considered
as good candidates for MFC-active materials. Most of MFC activities
are demonstrated to originate from the crystalline-to-amorphous or
crystalline-to-crystalline transition. Although the development of
organic MFC-active molecules with novel structures is fascinating,
this depends largely on accidental discovery and screening of a
large number of compounds because it is difficult to predict
whether a particular molecule can exhibit an obvious solid-state
fluorescence response behavior under mechanical stimuli.
5
activities of phenothiazine-based benzoxazole derivatives. It was
found that bromine-substituted derivative exhibited higher contrast
and more stable MFC activity than the derivative without the
bromine atom, which was attributed that bromine atom could
result in a looser π-π stacking and a larger sliding angle. In 2015,
Cheng’s group reported that bromine-substituted anthracene
derivative exhibited reversible MFC property, while the anthracene
derivative without the bromine atom was non-MFC, which was
believed that the presence of the bromine atom affected the
intermolecular interactions through the formation of the non-
covalent interactions between the carbon and hydrogen atoms with
1
b
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou,
3
25035, P. R. China. E-mail: xiaobhuang@wzu.edu.cn; huayuewu@wzu.edu.cn
†
Electronic Supplementary Information (ESI) available: Photophysical properties,
6
the bromine atom. In 2016, Wei’s group found that only fluorine-
substituted phenothiazinyl phenylacrylonitrile derivative displayed
1
13
XRD curves, crystal structures,
DOI: 10.1039/x0xx00000x
H
NMR, and
C
NMR. See
This journal is © The Royal Society of Chemistry 20xx
CrystEngComm, 2015, 00, 1-3 | 1

CrystEngComm
Page 2 of 10
ARTICLE
CrystEngComm
an outstanding MFC phenomenon because it had better crystallinity Finnigan DCMDX-30000 LCQ DCMD mass spectrometer. Fourier
View Article Online
7
DOI: 10.1039/C9CE00348G
than chlorine- and bromine-substituted derivatives. In 2016, Du’s transform infrared (FT-IR) spectra were determined using a Nexus
group demonstrated that the introduction of the halogen atoms to 870 FT-IR spectrometer. Ultraviolet-visible (UV-vis) absorption and
the tetraphenylethylene-based difluoroboron β-diketonate fluorescence spectra were conducted using
a
Cary 500
derivatives could improve the contrast of the MFC phenomena and spectrometer and HITACHI F-4500 spectrophotometer,
a
affect the reversibility by adjusting the crystallizability of the solid respectively. Solid-state fluorescence quantum yields (Ф
samples. However, unlike those mentioned above, Fraser’s group resolved fluorescence decay parameters were obtained using a
reported that the introduction of halogen atoms and different Fluoromax-4 spectrophotometer. X-Ray diffraction (XRD) patterns
F
) and time-
8
halogen atoms had little effect on the MFC properties of were recorded using
difluoroboron β-diketonate derivatives in 2015. These results crystallographic analyses were conducted using a Bruker SMART II
indicate that halogen atoms have complex influences on the MFC CCD area detector. Ethyl 2-cyanoacetate, 2,6-dimethyl-4-pyrone (1),
a
Rigaku Dmax 2000. The X-ray
9
activity of fluorescent molecules with specific structures, and the acetic anhydride (Ac
mechanism of their actions is still unclear. The role of halogen chlorobenzaldehyde, 4-bromobenzaldehyde, and ethylamine were
atoms deserves further study. purchased from commercial suppliers.
2
O), benzaldehyde, 4-fluorobenzaldehyde, 4-
1
,4-Dihydropyridine is one of traditional fluorescent molecular Synthesis
of
ethyl
2-cyano-2-(2,6-dimethyl-4H-pyran-4-
structure units and has been proved to be used to construct ylidene)acetate (2)
3
a,10
molecules that emit solid-state fluorescence in recent years.
The
Compound 1 (1.24 g, 10.0 mmol) and ethyl cyanoacetate (30.0
mmol) were dissolved in 8 mL of acetic anhydride. The mixture was
heated for 3 h at 110 °C. After the reaction was over, the mixture
was allowed to stand overnight, a large amount of solids separated
out. After vacuum filtration, the product was washed with ethanol
1
,4-dihydropyridine derivatives often exhibited aggregation-
induced emission (AIE) properties because of their twisted
molecular conformations induced by the introduction of various
substituents into the nitrogen atom. Furthermore, some of them
were reported to display fluorescence stimulus-responsive activities
1
1
1
1b-g
and then dried to afford pure compound 2 as a light brown solid
in
the
solid
state.
Herein,
four
1
(
1.04 g, 47.2% yield). m. p. 187.2-187.8 °C. H NMR (500 MHz,
cyano(ethoxycarbonyl)methylene-1,4-dihydropyridine
(CMD)
6
DMSO-d ): δ 7.81 (s, 1H), 6.60 (s, 1H), 4.14 (q, J = 7.0 Hz, 2H), 2.35
derivatives (CMD-H, CMD-F, CMD-Cl, and CMD-Br) were designed
and synthesized to study the influence of the halogen atoms on
their MFC properties (Scheme 1). These CMD derivatives were
demonstrated to have twisted molecular conformations by X-ray
crystallographic analyses and exhibited obvious AIE activities in the
aggregated state. In the crystalline state, the molecules of these
compounds adopted loose J–aggregation and emitted yellow-green
or green fluorescence. Importantly, although all CMD derivatives
exhibited MFC activities upon grinding, halogen-substituted
derivatives showed higher contrast than CMD-H, which might be
attributed that the former had more twisted conformations and
looser stacking arrangements caused by the introduction of halogen
atoms.
1
3
(
s, 3H), 2.33 (s, 3H), 1.22 (t, J = 7.5 Hz, 3H). C NMR (125 MHz,
DMSO-d
6
): δ 164.3, 163.9, 163.3, 154.2, 118.1, 107.4, 105.7, 77.4,
+
5
9.8, 19.7, 19.4, 14.2. MS (ESI, m/z): 220.09 [M+H] .
Synthesis of compounds 3a–3d
General procedure: Compound
benzaldehyde/4-fluorobenzaldehyde/
2
(219.2 mg, 1.0 mmol),
4-chlorobenzaldehyde/4-
bromobenzaldehyde (4.0 mmol) and sodium hydroxide (3.0 mmol)
were dissolved in 40 mL of ethanol. The mixture was stirred at room
temperature for 4 h under N atmosphere. After the reaction was
2
completed, a large amount of solids separated out. After vacuum
filtration, the product was washed with water and ethanol in turn,
and then dried to give pure compounds 3a-3d. The characterization
data of 3a-3d are as follows:
Ethyl
2-cyano-2-(2,6-di((E)-styryl)-4H-pyran-4-ylidene)acetate
(
°
7
7
3a). Orange-yellow solid (229.8 mg), 58.1% yield, m. p. 150.6-150.7
1
C. H NMR (500 MHz, DMSO-d
.82 (m, 4H), 7.73 (dd, J = 16.0 Hz, J = 5.0 Hz, 2H), 7.47-7.40 (m,
6
): δ 8.05 (d, J = 1.5 Hz, 1H), 7.86-
3
4
H), 7.31 (d, J = 16.5 Hz, 1H), 4.20 (q, J = 7.0 Hz, 2H), 1.26 (t, J = 7.0
Scheme 1. Chemical structures of the CMD derivatives
13
Hz, 3H). C NMR (125 MHz, DMSO-d
6
): δ 164.1, 158.3, 157.7, 152.9,
1
1
36.8, 136.4, 135.2, 135.1, 129.78, 129.75, 128.81, 128.80, 128.1,
28.0, 120.0, 119.7, 118.1, 109.1, 107.6, 79.9, 60.0, 14.2. MS (ESI,
Experimental
Measurements and materials
+
m/z): 396.36 [M+H] . Anal. calcd. for C26
N, 3.54. Found: C, 78.68; H, 5.33; N, 3.56.
3
H21NO : C, 78.97; H, 5.35;
1H and 13C nuclear magnetic resonance (NMR) spectra were
determined using a Bruker DRX 500 NMR spectrometer using
dimethyl sulfoxide (DMSO)-d as a solvent and trimethylsilane as a
6
reference. Elemental analyses were recorded using an Elementar
Vario MICRO analyzer. The mass spectrum was determined on a
Ethyl
2-(2,6-bis((E)-4-fluorostyryl)-4H-pyran-4-ylidene)-2-
cyanoacetate (3b). Orange-yellow solid (384.4. mg), 89.1% yield, m.
p. 235.2-235.5 °C. H NMR (500 MHz, DMSO-d
Hz, 1H), 7.89-7.84 (m, 4H), 7.68 (dd, J = 16.0 Hz, J = 5.5 Hz, 2H),
.35-7.22 (m, 6H), 6.80 (d, J = 2.0 Hz, 1H), 4.17 (q, J = 7.0 Hz, 2H),
1
6
): δ 8.00 (d, J = 2.0
3
4
7
2
| CrystEngComm, 2015, 00, 1-3
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Please do not adjust margins

Page 3 of 10
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1
1
1
1
.25 (t, J = 7.5 Hz, 3H). 13C NMR (125 MHz, DMSO-d
6
): δ 164.1,
C
28
H
26
N
2
O
2
: C, 79.59; H, 6.20; N, 6.63. Found: C, 79.27; H, 6.17; N,
View Article Online
DOI: 10.1039/C9CE00348G
62.90 (d, J = 247.5 Hz), 162.88 (d, J = 247.5 Hz), 158.2, 157.6, 6.60.
52.9, 135.5, 135.1, 131.80 (d, J = 5.0 Hz), 131.77 (d, J = 6.3 Hz),
30.25 (d, J = 13.8 Hz), 130.18 (d, J = 13.8 Hz), 119.68 (d, J = 30.0
Ethyl 2-cyano-2-(1-ethyl-2,6-bis((E)-4-fluorostyryl)pyridin-4(1H)-
ylidene)acetate (CMD-F). yellow-green solid (167.3 mg), 56.5%
yield, m. p. 259.0-259.4 °C. H NMR (500 MHz, DMSO-d
Hz), 119.67 (d, J = 30.0 Hz), 118.1, 115.82 (d, J = 21.3 Hz), 115.80 (d,
J = 21.3 Hz), 109.0, 107.5, 79.9, 60.0, 14.2. MS (ESI, m/z): 432.11
1
6
): δ 8.47 (d,
+
J = 2.0 Hz, 1H), 7.88-7.84 (m, 4H), 7.46-7.40 (m, 2H), 7.33-7.27 (m,
H), 6.96 (d, J = 2.0 Hz, 1H), 4.36 (q, J = 8.5 Hz, 2H), 4.08 (q, J = 7.2
[
M+H] . Anal. calcd. for C26
19 2 3
H F NO : C, 72.38; H, 4.44; N, 3.25.
6
Found: C, 72.69; H, 4.42; N, 3.23.
1
3
Hz, 2H), 1.30 (t, J = 7.2 Hz, 3H), 1.20 (t, J = 7.2 Hz, 3H). C NMR (125
2-(2,6-bis((E)-4-chlorostyryl)-4H-pyran-4-ylidene)-2- MHz, DMSO-d ): δ 166.3, 153.4, 148.2, 147.9, 136.8, 134.3, 133.9,
Ethyl
6
cyanoacetate (3c). Orange-yellow solid (378.8 mg), 81.6% yield, m. 129.54, 129.52, 128.82, 128.79, 121.6, 121.3, 121.2, 112.9, 112.2,
1
+
-
p. 236.4-236.7 °C. H NMR (500 MHz, DMSO-d
6
): δ 8.02 (d, J = 1.5 67.0, 58.5, 44.9, 14.6. MS (ESI, m/z): 459.29 [M+H] . FT-IR (KBr, cm
3
4
1
Hz, 1H), 7.85-7.81 (m, 4H), 7.68 (dd, J = 16.0 Hz, J = 5.5 Hz, 2H),
): 2979, 2186, 1656, 1633, 1600, 1534, 1534, 1510, 1471, 1379,
7
6
3
1
1
.52-7.50 (m, 4H), 7.42 (d, J = 16.0 Hz, 1H), 7.32 (d, J = 16.0 Hz, 1H), 1302, 1272, 1052, 978, 852, 826. Anal. calcd. for C28
24 2 2 2
H F N O : C,
.83 (d, J = 15.0 Hz, 1H), 4.18 (q, J = 7.0 Hz, 2H), 1.25 (t, J = 7.0 Hz, 73.35; H, 5.28; N, 6.11. Found: C, 72.99; H, 5.30; N, 6.08.
1
3
6
H). C NMR (125 MHz, DMSO-d ): δ 164.0, 158.0, 157.4, 152.7,
Ethyl 2-(2,6-bis((E)-4-chlorostyryl)-1-ethylpyridin-4(1H)-ylidene)-
35.3, 134.9, 134.24, 134.21, 134.1, 134.0, 129.7, 129.6, 128.83,
28.80, 120.7, 120.4, 118.0, 109.4, 107.9, 80.3, 60.1, 14.2. MS (ESI,
2
-cyanoacetate (CMD-Cl). yellow-green solid (122.1 mg), 68.1%
1
+
yield, m. p. 246.7-247.7°C. H NMR (500 MHz, DMSO-d ): δ 8.47 (d, J
m/z): 465.80 [M+H] . Anal. calcd. for C26
H19Cl
2 3
NO : C, 67.25; H, 4.12;
6
=
1
2.0 Hz, 1H), 7.82 (d, J = 8.0 Hz, 4H), 7.51-7.46 (m, 6H), 7.29 (t, J =
5.5 Hz, 2H), 6.97 (d, J = 2.0 Hz, 1H), 4.35 (q, J = 7.0 Hz, 2H), 4.08 (q,
N, 3.02. Found: C, 66.97; H, 4.10; N, 3.04.
1
3
Ethyl
2-(2,6-bis((E)-4-bromostyryl)-4H-pyran-4-ylidene)-2- J = 7.0 Hz, 2H), 1.30 (t, J = 7.0 Hz, 3H), 1.20 (t, J = 7.0 Hz, 3H). C
cyanoacetate (3d). Orange-yellow solid (453.6 mg), 82.0% yield, m. NMR (125 MHz, DMSO-d
6
): δ 166.3, 153.4, 148.2, 147.9, 136.8,
1
p. 249.7-250.5°C. H NMR (500 MHz, DMSO-d
6
): δ 8.02 (d, J = 1.0 Hz, 134.3, 133.9, 129.54, 129.52, 128.82, 128.79, 121.6, 121.3, 121.2,
+
1
7
3
1
1
H), 7.78-7.74 (m, 4H), 7.69-7.63 (m, 6H), 7.44 (d, J = 16.0 Hz, 1H), 112.9, 112.2, 67.0, 58.5, 44.9, 14.6. MS (ESI, m/z): 492.31 [M+H] .
-1
.34 (d, J = 16.5 Hz, 1H), 4.18 (q, J = 7.0 Hz, 2H), 1.25 (t, J = 7.0 Hz, FT-IR (KBr, cm ): 2977, 2183, 1657, 1635, 1616, 1599, 1534, 1474,
1
3
6 2 2 2
H). C NMR (125 MHz, DMSO-d ): δ 164.0, 158.0, 157.4, 152.7, 1378, 1268, 1052, 968, 815, 808. Anal. calcd. for C28H24Cl N O : C,
35.4, 135.0, 134.43, 134.38, 131.8, 131.7, 129.9, 129.8, 123.1, 68.44; H, 4.92; N, 5.70. Found: C, 68.71; H, 4.94; N, 5.67.
23.0, 120.7, 120.5, 118.0, 109.4, 108.0, 80.4, 60.1, 14.2. MS (ESI,
+
Ethyl 2-(2,6-bis((E)-4-bromostyryl)-1-ethylpyridin-4(1H)-ylidene)-
-cyanoacetate (CMD-Br). yellow-green solid (195.9 mg), 67.5%
yield, m. p. 246.6-247.6°C. H NMR (500 MHz, DMSO-d
m/z): 554.31 [M+H] . Anal. calcd. for C26
3
2 3
H19Br NO : C, 56.45; H,
2
.46; N, 2.53. Found: C, 56.19; H, 3.48; N, 2.52.
1
6
): δ 8.47 (d, J
Synthesis of the CMD derivatives
= 2.0 Hz, 1H), 7.75 (d, J = 8.0 Hz, 4H), 7.64 (d, J = 8.5 Hz, 4H), 7.50
3
4
(dd, J = 16.0 Hz, J = 7.5 Hz, 2H), 7.28 (t, J = 16.0 Hz, 2H), 6.97 (d, J =
General procedure: Compound 3a/3b/3c/3d (0.5 mmol) and
ethylamine (0.8 mL) were dissolved in 6 mL of acetonitrile. The
mixture was heated at 80 ° C for 6 h under N atmosphere. After
2
2
=
.0 Hz, 1H), 4.35 (q, J = 7.0 Hz, 2H), 4.08 (q, J = 7.0 Hz, 2H), 1.29 (t, J
7.0 Hz, 3H), 1.20 (t, J = 7.0 Hz, 3H). C NMR (125 MHz, DMSO-d ):
6
13
δ 166.2, 153.4, 148.1, 147.8, 136.9, 134.6, 131.71, 131.69, 129.8,
cooling to room temperature, the precipitate was filtered and
washed with methanol, and dried to afford pure target compounds.
The characterization data of CMD-H, CMD-F, CMD-Cl, and CMD-Br
are as follows:
1
1
1
8
29.7, 122.7, 121.7, 121.3, 121.1, 112.8, 112.2, 67.0, 58.4, 44.9,
4.6. MS (ESI, m/z): 581.02 [M+H] . FT-IR (KBr, cm ): 2985, 2204,
698, 1641, 1616, 1546, 1511, 1487, 1318.46, 1241, 1043, 946, 851,
+
-1
2
07. Anal. calcd. for C28H24Br N
2
O
2
: C, 57.95; H, 4.17; N, 4.83.
Found: C, 58.22; H, 4.19; N, 4.80.
Ethyl
2-cyano-2-(1-ethyl-2,6-di((E)-styryl)pyridin-4(1H)-
ylidene)acetate (CMD-H). yellow solid (122.1 mg), 57.8% yield, m. p.
1
2
1
4
74.7-275.4 °C. H NMR (500 MHz, DMSO-d
6
): δ 8.49 (d, J = 2.0 Hz,
Results and discussion
3
H), 7.78 (d, J = 7.5 Hz, 2H), 7.50-7.39 (m, 8H), 7.29 (dd, J = 15.5 Hz,
J = 10.0 Hz, 2H), 6.98 (d, J = 2.0 Hz, 1H), 4.37 (q, J = 6.5 Hz, 2H), 4.08 Synthesis and characterization
(
q, J = 7.5 Hz, 2H), 1.32 (t, J = 7.0 Hz, 3H), 1.21 (t, J = 7.0 Hz, 3H). 13
NMR (125 MHz, DMSO-d ): δ 166.3, 153.5, 148.4, 148.1, 138.2,
C
6
1
1
38.1, 135.3, 129.5, 128.83, 128.80, 127.8, 121.2, 120.8, 120.5,
12.8, 112.1, 66.8, 58.4, 44.9, 14.62, 14.55. MS (ESI, m/z): 423.40
+
-1
[
M+H] . FT-IR (KBr, cm ): 2988, 2186, 1672, 1637, 1618, 1601,
534, 1472, 1379, 1269, 1052, 967, 755, 691. Anal. calcd. for
1
This journal is © The Royal Society of Chemistry 20xx
Scheme 2. Synthetic route towards the CMD derivatives
CrystEngComm, 2015, 00, 1-3 | 3
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ARTICLE
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The chemical structures and synthetic routes of CMD-H, CMD-F, chlorobenzaldehyde/4-bromobenzaldehyde by aVniew Aratidclde iOtinolinne-
CMD-Cl, and CMD-Br are outlined in Scheme 2. The target elimination mechanism to afford theDOinI:t1e0r.m10e3d9i/aCt9eCsE030a3-438dG,
compounds could be easily synthesized from a 3-step reaction respectively. Finally, the intermediates 3a-3d reacted with
procedure using compound 1 as the starting compound. Firstly, the ethylamine to afford CMD-H, CMD-F, CMD-Cl, and CMD-Br in
Knoevenagel reaction of compound 1 with ethyl 2-cyanoacetate in acetonitrile, respectively. All target compounds were fully
Table 1 Crystallographic data and details of collection and refinement for crystals CMD-H, CMD-F, CMD-Cl, and CMD-Br
CMD-H
CMD-F
CMD-Cl
CMD-Br
CCDC (No.)
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Z
1897635
1897637
1897636
1897638
C
H
28 26
N
2
O
2
C
28 24
H F
2 2
N O
2
C
28
H24Cl
2 2
N O
2
28 2 2 2
C H24Br N O
422.51
170.01
458.49
170.02
491.39
173(2)
580.31
173(2)
Orthorhombic
P bca
Monoclinic
Pī 2(1)/c1
4
Monoclinic
P 2(1)/c
4
Monoclinic
P 2(1)/c
4
8
D
calcd [Mg/m3]
F (000)
1.256
1.209
1.228
1024
1.434
1024
1792
960
θ range [°]
3.059-54.994
0.0926
0.1703
20.0543(6)
8.8607(3)
25.1405(8)
90
3.027-55.258
0.0713
2.386-25.499
0.0692
2.821- 25.499
0.0719
1
R [I>2σ(I)]
wR
2
[I>2σ(I)]
0.2004
0.1488
0.1798
a [Å]
16.7550(8)
19.7470(10)
7.6891(4)
90
17.277(2)
19.953(3)
7.8040(11)
90
17.384(4)
19.973(5)
7.823(2)
90
b [Å ]
c [Å]
α [deg]
β [deg]
γ [deg]
90
98.211(3)
90
98.811(5)
90
98.170(10)
90
90
3
4
467.3(2)
2517.9(2)
2658.5(6)
2688.7(12)
V [Å ]
GOF
1.069
0.1373
46033
4263
1.056
0.0647
26256
4795
1.016
0.0601
12237
4884
1.021
0.0433
10000
4834
R (int)
No. of reflcns collected
No. of unique reflcns
R
1
(all data)
2
(all data)
0.1829
0.2082
0.0940
0.2190
0.1443
0.1926
0.1311
0.2155
wR
acetic anhydride gave the intermediate 2.12 Secondly, intermediate characterized by mass spectrometry, nuclear magnetic resonance,
2
reacted
with
benzaldehyde/4-fluorobenzaldehyde/4- elemental analysis, and X-ray crystallographic analyses. The target
4
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compounds have good solubility in common organic solvents, such
as chloroform, DMSO, dichloromethane, tetrahydrofuran, toluene,
and acetonitrile, but low solubility in methanol, ethanol, and water.
View Article Online
DOI: 10.1039/C9CE00348G
Single-crystal structures
The single crystals of CMD-H and CMD-F were cultured from a slow
evaporation of a DMSO/ethyl acetate (EA) (v/v = 1:1) mixture and a
slow diffusion of a CHCl
Those of CMD-Cl and CMD-Br were cultured from a slow
evaporation of a CHCl /EA (v/v = 1:2) mixture. Fig. 1 shows the
3
/methanol (v/v = 1:1) mixture, respectively.
3
crystal structures and single-crystal fluorescent photographs of the
target compounds. The respective crystallographic data and
refinement parameters are depicted in Table 1. The single crystals
of CMD-H and CMD-F belong to an orthorhombic system and a
monoclinic system in a P bca and a Pī 2(1)/c1 space group,
respectively. The unit cells of CMD-Cl and CMD-Br are monoclinic
with a P 2(1)/c space group.
Fig. 2 Crystal CMD-H: (a) Molecular stacking arrangement; (b)
intermolecular interactions in the same column including C–H···O
hydrogen bond, C–H···π bond, and C–H···H interaction; (c) C–H···H
interactions between different columns.
Fig. 1 Crystal structures and single-crystal fluorescent photographs of
CMD-H (a), CMD-F (b), CMD-Cl (c), and CMD-Br (d).
In CMD-H crystal, taking the central dihydropyridine ring as the
basic plane (ring A), the diheral angles between ring A and the
phenyl ring B and C are 26.49° and 41.04° (Fig. 1), respectively,
indicating that crystal structure adopts a twisted molecular
conformation because of the introduction of the N-ethyl group. The
distances between the central of the dihydropyridine rings in the
upper and lower layers are 4.638 and 4.554 Å (Fig. 2b), indicating
that there is no obvious π–π stacking. As displayed in Fig. 2 and
Table 1, the molecules of CMD-H in the same column are stabilized
by various C–H···π bonds (2.459, 3.486, 3.614, and 3.634 Å), one
kind of C–H···O hydrogen bond (2.436 Å), and one kind of C–H···H
interaction (2.281 Å), while those in different columns are stabilized
by C–H···H interactions (2.365 and 2.391 Å) (Fig. 2c and Table 2).
These multiple weak interactions cause that the molecules are
aligned in a zigzag shape with a head-to-tail J-aggregation in the
same column (Fig. 2a).
Fig. 3 Crystal CMD-F: (a) Molecular stacking arrangement; (b) C–H···π
bond and C–H···O hydrogen bond in the same column; (c) C–H···O (N)
hydrogen bonds between different columns.
The molecules of CMD-F in the same column are connected by C–
H···π bond (2.616 Å), C–H···O hydrogen bonds (2.375, 2.624, and
.718 Å), and π–π stacking interaction with a distance of 3.504 Å
between the dihydropyridines (Fig. 3b). The molecules in different
columns are connected by C–H···N hydrogen bonds (2.538 and
2
2
.673 Å) and C–H···O hydrogen bond (2.709 Å) (Fig. 3c). CMD-F
adopts an arrangement of zigzag shape with a J-type aggregation of
the molecules in the same column (Fig. 3a). Crystal CMD-Cl (Fig.
S1†) has almost exactly the same intermolecular interactions and
stacking arrangement as CMD-F. The difference is that the distances
of the weak interactions in CMD-Cl are slightly longer than those in
CMD-F, revealing that the former has the more loose arrangement
than the latter. Although CMD-Br (Fig. S2†) has the same stacking
arrangement as CMD-F and CMD-Cl, there exists a C–H···π bond
with a distance of 2.611 Å between aromatic C–H and phenyl ring
except C–H···O hydrogen bonds (2.500 and 2.650 Å), C–H···π (C=O)
bond (2.666 Å), and π–π interaction (3.555 Å).
For CMD-F, CMD-Cl, and CMD-Br, the diheral angles between the
dihydropyridine unit and the two phenyl units are 36.96° and
5
6.17°, 35.52° and 50.04°, 35.73° and 49.25° (Fig. 1), respectively.
These results indicate that halogen-substituted CMD derivatives
have more twisted conformations than CMD-H. Furthermore, the
intermolecular interactions and stacking arrangements in the single
crystals of CMD-F, CMD-Cl, and CMD-Br were investigated. The
distances of their intermolecular interactions are listed in Table 2.
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Table 2 The distances of the intermolecular interactions in crystals CMD-H, CMD-F, CMD-Cl, and CMD-Br
View Article Online
DOI: 10.1039/C9CE00348G
Crystal
Weak interaction
C28-H28A···π(Pyridine)
C5-H5···π(Ph)
d/Å
Crystal
Weak interaction
π(Pyridine)···π(Pyridine)
C15-H15···π(C=O)
C8-H8···N2
d/Å
CMD-H
3.634
3.486
3.614
2.459
2.391
2.365
2.281
2.436
3.561
2.668
2.830
2.681
2.814
2.470
2.707
2.637
CMD-F
3.504
2.616
2.673
2.538
2.709
2.718
2.375
2.624
3.555
2.666
2.611
2.500
2.650
C1-H1···π(Pyridine)
C28-H28A···π(C=O)
C18-H18···H26A
C18-H18···H25A
C18-H28A···H12
C18-H28A···O1
C1-H1···N2
C23-H23C···O2
C17-H17···O1
C21-H21···O1
C15-H15···O1
CMD-Cl
π(Pyridine)···π(Pyridine)
C15-H15···π(C=O)
C6-H14···N2
CMD-Br
π(Pyridine)···π(Pyridine)
C15-H15···π(C=O)
C9-H9···π(Ph)
C13-H13···N2
C17-H17··O1
C21-H21···O1
C28-H28C···O2
C17-H17···O1
C15-H15···O1
C28-H28A···O2
As mentioned above, CMD-H, CMD-F, CMD-Cl, and CMD-Br have
twisted molecular conformations, and their molecules are
connected by various weak intermolecular interactions and adopt
loose stacking arrangements. Based on these results, these
compounds are expected to exhibit AIE and MFC properties.
AIE properties
Considering that DMSO and water were good and poor solvents for
the CMD derivatives, respectively, the UV-vis absorption and
fluorescence spectra of these compounds in the DMSO-water
mixtures by changing the volume percentage of water (f ) in the
w
range of 0-99% were investigated to determine whether they
exhibited AIE properties. The DMSO-water mixtures (10.0 μmol/L)
of CMD-H hardly emitted fluorescence when f
w
was in the range of
0
-50% (Fig. 4a and Fig. 4b). This should be attributed that the
molecules with low volumetric ratios of water were dispersed and
thus the free rotations of the phenyl rings greatly weakened the
emissions through the non-radiative relaxation of the excited
Fig. 4 CMD-H: Fluorescence spectra (a), changes in the fluorescence
intensity (b), and UV-vis absorption spectra (c) in DMSO-water mixtures
1
1a,13
states.
When the f
w
value went up to 60-99%, the UV-vis
absorption spectra of mixtures showed level-off tails in the long-
wavelength region because of the Mie scattering effect (Fig. 4c),
which indicated the formations of nanosized aggregates.
(10.0 μmol/L) with f
mixtures (10.0 μmol/L) with the glycerol volume fraction from 0 to 90%.
The insets in (a) show digital photographs of the mixtures with f = 0 and
9% under a 365 nm ultraviolet lamp.
w
= 0-99%. Fluorescence spectra (d) in DMSO-glycerol
1
4
w
9
Correspondingly, the fluorescence intensity began to increase at f
w
=
w
60% and reached the maximum at f = 99% which was 11-fold to
volume fraction enhanced the viscosity of the mixtures, which
prevented the free rotations of the phenyl rings and thus enhanced
the fluorescence intesities. The result indicated that the AIE
activity of CMD-H was ascribed to the restriction of intramolecular
rotation. Similarly, CMD-F, CMD-Cl, and CMD-Br also exhibited
obvious AIE properties. Different from CMD-H, the fluorescence
intensities of CMD-F, CMD-Cl, and CMD-Br exhibited a process of
that of the single-molecule state. The results indicated that the
aggregation of the molecules led to the fluorescence enhancement,
revealing that CMD-H was AIE-active molecule. To gain insights into
the mechanism of AIE activity, the change of fluorescence spectra
of CMD-H DMSO-glycerol mixtures (10.0 μmol/L) upon the varying
glycerol volume fractions was conducted. It could be found that the
increase of the glycerol volume fraction led to a gradual increase in
fluorescence intensities of the mixtures (Fig. 4d). Because the
glycerol was a liquid with high viscosity, the increase of glycerol
15
first enhancing and then weakening with the increasing f
w
value.
The strongest fluorescence intensity was observed at f = 70, 60,
w
and 70% for CMD-F (Fig. S3†), CMD-Cl (Fig. S4†), and CMD-Br (Fig.
6
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S5†), respectively. With the further increase of water content, the
fluorescence intensity began to weaken gradually. Similar
phenomena were often observed in some previously reported AIE-
active molecules,1 and the possible reason was that the dispersed
molecules aggregated to form aggregates with different
View Article Online
DOI: 10.1039/C9CE00348G
6
w
morphologies at higher f conditions.
MFC properties
The original samples of CMD-H, CMD-Cl, and CMD-Br exhibited
yellow-green fluorescence with the λem values of 544, 529, and 533
nm, respectively, while that of CMD-F emitted green fluorescence
with the λem value of 515 nm (Fig. 5 and Fig. 6). This indicated that
the enhancement of electron-withdrawing ability of the substituent
group could lead to the blue shift in the solid-state emission. As
shown in Fig. 5, upon grinding in a mortar with a pestle, the original
samples of these compounds were converted into orange-emitting
solids. The 21, 60, 61, and 52-nm red-shift in the emission spectra
were observed for CMD-H, CMD-F, CMD-Cl, and CMD-Br,
respectively (Table 3). Although all these compounds exhibited MFC
properties, obviously, the introduction of the halogen atoms
resulted in higher contrast MFC phenomena. The XRD
measurements of these compounds before and after grinding were
conducted (Fig. 7). The original samples of these compounds
exhibited a series of sharp and intense diffraction peaks, revealing
the characteristics of crystalline state, whereas their ground
samples were amorphous because of the disappearance of
diffraction peaks. This indicated that the grinding treatment
destroyed the crystalline structures of these compounds and led to
a crystalline-to-amorphous transition in the morphologies. For
Fig. 6 Normalized fluorescence spectra of the original samples of CMD-H
(a), CMD-F (b), CMD-Cl (c), and CMD-Br (d) upon grinding and fuming
(CH CN vapor).
3
3
CMD-H, after the ground sample was fumed by CH CN vapor, the
sharp and intense diffraction peaks recovered, which accorded well
with those of the XRD pattern of the original sample (Fig. 7a). This
suggested that the fuming treatment resulted in a morphological
change from the amorphous state to the crystalline state, and thus
the recovery of fluorescence. As a result, the fumed sample emitted
Fig. 7 XRD patterns of the original samples of CMD-H (a), CMD-F (b), CMD-
Cl (c), and CMD-Br (d) upon grinding and fuming (CH CN vapor).
3
the same as the original sample (Fig. 6a). Similarly, the fuming
treatment also recovered the fluorescence colours of the ground
samples of the halogen-substituted CMD derivatives. As a result, a
rewritable optical recording medium could be established based on
CMD-F (Fig. 5e). When a picture containing a cat was painted on the
surface of the original sample using a metal spatula, the orange
picture appeared in the green background under a 365 nm UV
3
lamp. Then, the picture was easily erased by fuming with CH CN
vapor. However, it should be pointed out that for CMD-F, CMD-Cl,
and CMD-Br, although most of the diffraction peaks of their fumed
samples appeared again, the intensities were weaker than those of
the corresponding original samples (Figs. 7b-7d), indicating that
amorphous solids were partially present in the fumed samples.
Therefore, the λem values of their emissions could not be completely
Fig. 5 Photographic images of the original samples of CMD-H (a), CMD-F
6
3
(b), CMD-Cl (c), and CMD-Br (d) upon grinding and fuming (CH CN
vapor) under irradiation at 365 nm. (e) A picture containing a cat was
written and then erased based on the original sample of CMD-F under
irradiation at 365 nm.
restored to the original states.
yellow-green fluorescence with the λem value of 544 nm, which was
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Table 3 Fluorescence parameters of the solid samples of CMD-H, CMD-F, CMD-Cl, and CMD-Br
View Article Online
DOI: 10.1039/C9CE00348G
(ns)a
τ
(ns)a
A
b
A
b
<τ> (ns)c
Ф
F
Compound
Type
λem (nm)
τ
1
2
1
2
CMD-H
Original
544
1.26
2.32
0.30
0.70
2.00
26.9%
Ground
Fumed
565
544
0.78
0.95
2.65
2.89
0.83
0.74
0.17
0.26
1.10
1.45
8.2%
10.8%
CMD-F
Original
Ground
Fumed
515
575
539
0.06
0.62
0.06
2.35
2.44
2.16
0.82
0.94
0.83
0.18
0.06
0.17
2.37
0.73
1.28
21.3%
4.4%
11.3%
CMD-Cl
Original
Ground
Fumed
529
590
543
0.11
0.93
0.84
3.26
3.37
3.73
0.29
0.90
0.53
0.71
0.10
0.47
2.35
1.17
2.20
26.5%
3.0%
12.5%
CMD-Br
Original
Ground
Fumed
533
585
543
0.16
0.53
0.35
2.78
2.79
2.56
0.17
0.90
0.29
0.83
0.10
0.71
2.33
0.76
1.92
25.1%
4.6%
13.8%
a
Fluorescence lifetime. b Proportion. Weighted mean lifetime <τ> was determined from <τ> = (A
c
τ +A τ )/(A +A ).
1 1 2 2 1 2
The fluorescence quantum yields (Ф
fluorescence decay parameters of the CMD derivatives in different
solid states were investigated and shown in Table 3. The Ф values
F
) and the time-resolved
F
of the original samples of CMD-H, CMD-F, CMD-Cl, and CMD-Br
were 26.9, 21.3, 26.5, and 25.1%, whereas those of the
corresponding ground samples were 8.2, 4.4, 3.0, and 4.6%,
respectively, indicating that the grinding process could reduce the
efficiency of fluorescence quantum yields. This should be attributed
that grinding destroyed the intermolecular interactions in the J-
aggregation and enhanced the non-radiative relaxations.
Additionally, the weighted mean lifetime (<τ>) values of the original
samples of CMD-H, CMD-F, CMD-Cl, and CMD-Br were 2.00, 2.37,
2
.35, and 2.81 ns, whereas those of their corresponding ground
samples were 1.10, 0.73, 1.17, and 0.76 ns, revealing that the
grinding treatment caused a significant decrease in <τ> values.
Interestingly, in the case of previously reported MFC-active 1,4-
dihydropyridine derivatives, the <τ> values often increased
1
1b,e
significantly after grinding.
Although the reasons for this
Fig. 8 Normalized absorption spectra of the original samples of CMD-H
(a), CMD-F (b), CMD-Cl (c), and CMD-Br (d) upon grinding and fuming
(CH CN vapor).
3
opposite trend were not yet clear, there was no doubt that external
pressure could lead to remarkable changes in <τ> values for MFC-
active organic compounds.
follows. As shown in Table 2, the distances of the intermolecular
interactions in CMD-H were generally shorter than those in CMD-F,
CMD-Cl, and CMD-Br. Futhermore, the herringbone angle in the
zigzag arrangement of CMD-H was 115° (Fig. 2), while that in
halogen-substituted CMD derivatives was about 133° (Fig. 3, Fig.
S1†, and Fig. S2†), suggesting that the latter had looser
arrangements. The weaker intermolecular interactions and looser
arrangements caused that the original samples of the halogen-
substituted CMD derivatives were more easily destroyed, and thus
the diffraction peaks in their XRD curves completely disappeared
upon grinding, while few broad and weak peaks remained in the
XRD curve of the CMD-H ground sample (Fig. 7). Generally, the red
shift in the emission spectrum upon grinding may originate from
the formation of excimers induced by the enhancement of the π–π
interactions or the increased molecular conjugation caused by the
planarization of molecular conformation. According to the report of
According to the single crystal structures of the CMD derivatives,
we can better understand the intrinsic reasons for the generation of
MFC derivatives of the CMD derivatives. As shown in Fig. S6†, the
diffraction peaks of the CMD derivatives original samples in the XRD
patterns accorded well with the simulated peaks of the
corresponding single crystals, indicating that the original samples
should have the same stacking arrangements as the single
crystals. Upon grinding, the loose J-aggregations with the weak
intermolecular interactions in the original samples were easily
destroyed, which resulted in the morphological changes from a
crystalline state to an amorphous state and thus the MFC
properties of these compounds.
1
7
Why did the introduction of halogen atoms help to enhance the
MFC activities? Possible reasons for this phenomenon were listed as
8
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1
8
Chem. C, 2018, 6, 6327–6353; (g) S. MukVhieewrjAereticlaenOdnlinPe.
DOI: 10.1039/C9CE00348G
Ma, Z. Wang, M. Teng, Z. Xu and X. Jia, ChemPhysChem,
Chi’s group , if the red shift of the emission spectrum was caused
by the formation of excimers, the maximum absorption peaks
should remain unchanged, while the increased molecular
conjugation could lead to outstanding changes. Upon grinding, the
CMD derivatives original samples all exhibited obvious red shifts in
the maximum absorption peaks (Fig. 8), indicating that the red
shifts of emission wavelengths in these cases should be mainly
ascribed to the enhancement of the molecular conjugation. As
described above, CMD-F, CMD-Cl, and CMD-Br had more twisted
conformations than CMD-H, so their molecules had more space for
change in the process of the planarization of molecular
conformation after grinding, thus showing greater red shifts in the
emission spectra. The results indicated that more twisted
conformations and looser stacking arrangements could be used to
explain the reason why the introduction of the halogen atoms led
to the enhanced MFC phenomena.
Thilagar, J. Mater. Chem. C, 2016, 4, 2647–2662; (h) Z.
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1
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In conclusion, we synthesized four AIE-active CMD derivatives and
found the introduction of halogen atoms led to the enhanced MFC
activities. These compounds had twisted molecular conformations
and adopted loose J–aggregations, which caused that their
crystalline structures were easily destroyed by the grinding
treatment. Their MFC properties were demonstrated to originate
from the crystalline-to-amorphous transition and the red shifts of
emission wavelengths were ascribed to the enhancement of the
molecular conjugations through the planarization of molecular
conformations. Compared with CMD-H, the introduction of halogen
atoms led to more twisted conformations and looser stacking
arrangements of CMD-F, CMD-Cl, and CMD-Br, which was
responsible for their higher contrast MFC properties. The results
indicated that the introduction of halogen atoms into a specific
traditional fluorescent molecule may contribute to the formation of
high contrast MFC fluorescent materials.
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We thank the National Natural Science Foundation of China (Nos
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1572165 and 21472140) and the Xinmiao Foundation of Zhejiang
Province (No 2018R429057) for financial support.
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Enhanced
mechanofluorochromic
properties
DOI: 10.1039/C9CE00348G
o
f
1
,4-dihydropyridine-based fluorescence molecules caused by the
introduction of halogen atoms
Yating Chen, Yibin Zhou, Zhiqiang Wang, Mengzhu Wang, Wenxia Gao, Yunbing Zhou,
Miaochang Liu, Xiaobo Huang and Huayue Wu
The
halogen-substituted
1,4-dihydropyridine
derivatives
exhibit
higher
contrast
mechanofluorochromic properties than the derivative without the halogen atom because of more
twisted conformations and looser stacking arrangements.