Mechanofluorochromic properties of fluorescent molecules based on a dicyanomethylene-4: H -pyran and indole isomer containing different alkyl chains via an alkene module

Article abstract of DOI:10.1039/c7ra06951k

A series of 3-, 4-, and 5-position indole-substituted dicyanomethylene-4H-pyran (I3PM, I4PM, and I5PM) derivatives with different alkyl chains were synthesized and their mechanofluorochromic (MFC) properties were investigated. Compared with the alkyl chains, the isomerization of the indole unit plays a more important role in regulating the MFC properties of these compounds. The increase of the alkyl chain length and the isomerization of the alkyl chain are favorable to the effective mechanofluorochromism of the I3PM derivatives, however, the former exhibits the opposite effect on that of the I4PM derivatives. The opposite alkyl length-dependent MFC behavior is closely related to the degree of distortion of the molecular conformation, namely the more twisted the molecular conformation, the more pronounced the MFC phenomenon. In particular, the introduction of the alkyl chains is completely detrimental to the MFC properties of the I5PM derivatives. X-Ray diffraction experiments indicate that the MFC properties of some of these compounds should be mainly attributed to the transformation from a crystalline state to an amorphous state because the weak intermolecular interactions and loose molecular packing modes in their solid-state samples can be easily damaged under external force stimuli as revealed by the single crystal X-ray diffraction analysis.

Full text of DOI:10.1039/c7ra06951k

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Mechanofluorochromic properties of fluorescent
molecules based on a dicyanomethylene-4H-pyran
and indole isomer containing different alkyl chains
via an alkene module†
Cite this: RSC Adv., 2017, 7, 42180
a
b
Lebin Qian,a Yibin Zhou,a Miaochang Liu,a Xiaobo Huang,
Ge Wu,
*
a
a
a
*
Wenxia Gao, Jinchang Ding and Huayue Wu
A series of 3-, 4-, and 5-position indole-substituted dicyanomethylene-4H-pyran (I3PM, I4PM, and I5PM)
derivatives with different alkyl chains were synthesized and their mechanofluorochromic (MFC) properties
were investigated. Compared with the alkyl chains, the isomerization of the indole unit plays a more
important role in regulating the MFC properties of these compounds. The increase of the alkyl chain
length and the isomerization of the alkyl chain are favorable to the effective mechanofluorochromism of
the I3PM derivatives, however, the former exhibits the opposite effect on that of the I4PM derivatives.
The opposite alkyl length-dependent MFC behavior is closely related to the degree of distortion of the
molecular conformation, namely the more twisted the molecular conformation, the more pronounced
the MFC phenomenon. In particular, the introduction of the alkyl chains is completely detrimental to the
MFC properties of the I5PM derivatives. X-Ray diffraction experiments indicate that the MFC properties
of some of these compounds should be mainly attributed to the transformation from a crystalline state
to an amorphous state because the weak intermolecular interactions and loose molecular packing
modes in their solid-state samples can be easily damaged under external force stimuli as revealed by the
single crystal X-ray diffraction analysis.
Received 22nd June 2017
Accepted 24th August 2017
DOI: 10.1039/c7ra06951k
rsc.li/rsc-advances
looks more alluring and more easily accepted by researchers.
Generally speaking, the introduction of alkyl chains to the
Introduction
conjugated skeleton of organic uorescence molecules is to
mainly tune the solubility of the materials, but some recent
studies show that alkyl chains can also inuence the intermo-
lecular interactions and molecular packing modes of the
materials in the solid state, and thereby play a crucial role in
tuning and altering their MFC properties.6 Although the change
of the length of alkyl chain can greatly affect MFC behaviors of
some uorescent molecules, this effect is very complicated. For
instance, longer alkyl chains are favorable to the more
remarkable MFC phenomena of 9,10-bis[(9,9-dialkyluorene-2-
yl)vinyl-2]anthracenes,7 9,10-bis(N-alkylphenothiazin-3-yl-vinyl-
Mechanouorochromic (MFC) organic molecules are a class of
“smart” materials whose solid-state uorescence colors can
display obvious changes in response to external force stimuli,
and have received considerable interest owing to their potential
applications in the elds of uorescent sensors,1 data storage,2
security inks,3 optoelectronic devices,4 etc. In general, to make
an organic uorescence molecule have MFC properties, rst the
molecule must be capable of emitting uorescence in the solid
state, and its solid morphology must be able to undergo
a transition from a crystalline to an amorphous state or from
a crystalline state to another crystalline state under pressure
stimuli.5 Although the development new uorescent molecular
structures and the structural modication of traditional uo-
rescent molecular frameworks to meet the above two conditions
are both crucial for obtaining new MFC materials, the latter
2)anthracenes,8
and
9,10-bis(p-alkoxystyryl)anthracenes,9
whereas obviously disadvantageous to those of 9,10-bis[(N-alkyl-
carbazol-3-yl)-vinyl-2]anthracenes10 and 9,10-bis(N-alkylindole-
3-yl-vinyl-2)anthracenes,11 and 2,5-dialkylcarbazole-substituted
terephthalate derivatives.12 Moreover, the branched alkyl
chains are reported to be advantageous to obtain high-contrast
MFC compounds for 9,10-bis(butoxystyryl)anthracenes.13 Addi-
tionally, some other ways have also proven to be very convenient
in regulating the MFC properties of the uorescent molecules,
such as the introduction of appropriate aromatic substituents14
and heavy atoms15 as well as changing the position of the
aCollege of Chemistry and Materials Engineering, Wenzhou University, Wenzhou,
325035, P. R. China. E-mail: xiaobhuang@wzu.edu.cn; huayuewu@wzu.edu.cn
bSchool of Pharmacy, Wenzhou Medical University, Wenzhou 325035, P. R. China
† Electronic supplementary information (ESI) available: UV-vis spectra,
uorescence spectra, photophysical data, XRD curves, and other additional
information. CCDC 1554482–1554487. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c7ra06951k
42180 | RSC Adv., 2017, 7, 42180–42191
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substituent.16 But so far the inuence of the structure of the Fluorescence measurements were carried out with a HITACHI F-
molecule on its MFC property is still not very clear and the 7000 uorometer. X-Ray diffraction (XRD) measurements were
regular pattern is hard to sum up, which is worthy of further carried out with a Bruker D8 Advance X-ray diffractometer.
study for the development of MFC-active materials based on the Single-crystal X-ray diffraction measurements were carried out
traditional uorescent molecular frameworks.
4H-Pyran is a traditional and excellent building unit for graphite monochromated Mo Ka radiation. 2,6-Dimethyl-4-
constructing donor–p–acceptor (D–p–A) uorescent pyrone (1), malononitrile, 1H-indole-3-carbaldehyde (3a), 1H-
with a Bruker-Nonius Smart Apex CCD diffractometer with
compounds in the applications of organic electroluminescent indole-4-carbaldehyde (4a), 1H-indole-5-carbaldehyde (5a), and
diodes, molecular logic gates, and uorescent sensors.17 various alkyl bromides were purchased from commercial
However, MFC materials based on 4H-pyran unit have rarely suppliers and used directly.
been reported until now. Very recently, we found that some
indene-1,3-dionemethylene-4H-pyran derivatives containing
different lengths of alkoxy groups showed obvious alkoxy
length-dependent MFC properties and the longer alkoxy chain
led to more remarkable MFC phenomena.18 Moreover, for
Synthesis of N-substituted indolecarboxaldehydes 3b–3d/4b–
4d/5b–5d
naphthylethylene-substituted
dicyanomethylene-4H-pyran General procedure: Alkyl bromide (20.0 mmol) was slowly
derivatives and cyano(ethoxycarbonyl)methylene-4H-pyran added to a solution of 3a/4a/5a (2.47 g, 17.0 mmol) and KOH
derivatives, the position isomerism of the naphthylethylene end (1.43 g, 25.60 mmol) in DMSO (30 mL). The mixture was stirred
groups were found to signicantly affect their solid-state uo- overnight at room temperature, and then added water and
rescence color changes under external stimuli.19 To our knowl- extracted with dichloromethane. The combined organic layers
edge, the combined effects of two different factors, namely were washed with water and brine, dried over anhydrous
different alkyl groups and the isomerization of aromatic Na2SO4, and evaporated to dryness in vacuo. The residue was
substituents on the MFC properties of the uorescent mole- puried by column chromatography through silica gel with
cules has not been reported. Herein, a series of indole- petroleum ether/ethyl acetate (v/v ¼ 4/1) to give the pure N-
substituted dicyanomethylene-4H-pyran (IPM) derivatives with substituted indolecarboxaldehyde. The characterization data
different alkyl chains and varying the linking position of the for the intermediates 3b–3d/4b–4d/5b–5d are listed as follows:
indole from 3-, 4- to 5-position at 4H-pyran ring, namely I3PM,
1-Butyl-1H-indole-3-carbaldehyde (3b). Yellow liquid (3.29 g),
I4PM, and I5PM, were synthesized. The combined effects of 95.9% yield. 1H NMR (CDCl3, 500 MHz): d 9.95 (s, 1H), 8.31 (dd,
isomerization and length of the alkyl chains and position 3J ¼ 6.5 Hz, 4J ¼ 2.5 Hz, 1H), 7.65 (s, 1H), 7.36–7.27 (m, 3H), 4.09
isomerization of the indole unit on the MFC properties of the (t, J ¼ 7.5 Hz, 2H), 1.84–1.79 (m, 2H), 1.35–1.31 (m, 2H), 0.93 (t, J
target compounds were systematically investigated. The results ¼ 7.5 Hz, 3H) ppm. 13C NMR (CDCl3, 125 MHz): d 184.2, 138.2,
show the isomerization of indole unit plays a more important 137.1, 125.2, 123.6, 122.6, 121.8, 117.8, 110.0, 46.7, 31.5, 19.8,
role in regulating the MFC properties of these compounds 13.4 ppm. Anal. calcd for C13H15NO: C, 77.58; H, 7.51; N, 6.96.
compared with the alkyl chains. The isomerization and length Found: C, 77.99; H, 7.56; N, 6.91.
of the alkyl chains exhibit different effects on the MFC prop-
1-Isobutyl-1H-indole-3-carbaldehyde (3c). Yellow liquid (3.20
1
erties of these uorescent molecules when the substitution g), 93.3% yield. H NMR (CDCl3, 500 MHz): d 9.96 (s, 1H), 8.29
position of the indole unit is different. The increase of the alkyl (dd, 3J ¼ 6.5 Hz, 4J ¼ 3.0 Hz, 1H), 7.64 (s, 1H), 7.34–7.26 (m, 3H),
chain length and the isomerization of the alkyl chain are 3.91 (d, J ¼ 7.5 Hz, 2H), 2.24–2.16 (m, 1H), 0.92 (d, J ¼ 6.5 Hz,
favorable to the effective MFC properties of the I3PM deriva- 6H) ppm. 13C NMR (CDCl3, 125 MHz): d 184.4, 138.8, 137.4,
tives, however, the former exhibits opposite effects on those of 125.3, 123.7, 122.7, 121.9, 117.8, 110.2, 54.7, 29.0, 20.0 ppm.
the I4PM derivatives. In particular, the introduction of the alkyl Anal. calcd for C13H15NO: C, 77.58; H, 7.51; N, 6.96. Found: C,
chains is completely detrimental to the MFC properties of the 78.02; H, 7.55; N, 7.02.
I5PM derivatives. The results signify that the choice of the alkyl
1-Dodecyl-1H-indole-3-carbaldehyde (3d). White solid (4.93
chains and the isomerization of indole unit can be used to g), 92.3% yield. 1H NMR (CDCl3, 500 MHz): d 10.00 (s, 1H), 8.31
regulate the MFC properties of the IPM derivatives. X-Ray (dd, 3J ¼ 6.5 Hz, 4J ¼ 2.0 Hz, 1H), 7.70 (s, 1H), 7.38–7.29 (m, 3H),
diffraction (XRD) experiments reveal that the reversible MFC 4.15 (t, J ¼ 7.0 Hz, 2H), 1.90–1.87 (m, 2H), 1.33–1.25 (m, 18H),
properties of the IPM derivatives should be mainly attributed to 0.88 (t, J ¼ 7.0 Hz, 3H) ppm. 13C NMR (CDCl3, 125 MHz): d 184.3,
the transformation between amorphous and crystalline states. 138.1, 137.2, 125.5, 123.8, 122.8, 122.1, 118.0, 110.0, 47.3, 31.9,
29.7, 29.53, 29.46, 29.37, 29.27, 29.1, 26.8, 22.6, 14.0 ppm. Anal.
calcd for C21H31NO: C, 80.46; H, 9.97; N, 4.47. Found: C, 80.92;
H, 9.93; N, 4.44.
Experimental
Measurements and materials
1-Butyl-1H-indole-4-carbaldehyde (4b). Dark yellow liquid
NMR spectral measurements were performed on a Bruker DRX (2.56 g), 74.6% yield. 1H NMR (CDCl3, 500 MHz): d 10.25 (s, 1H),
500 NMR spectrometer. Mass spectrometry spectra were per- 7.62 (d, J ¼ 7.5 Hz, 2H), 7.35–7.29 (m, 3H), 4.17 (t, J ¼ 7.0 Hz,
formed with a Finnigan DECAX-30000 LCQ Deca mass spec- 2H), 1.85–1.79 (m, 2H), 1.35–1.30 (m, 2H), 0.94 (t, J ¼ 7.5 Hz,
trometer or a Finngan MAT95XP mass spectrometer. Elemental 3H) ppm. 13C NMR (CDCl3, 125 MHz): d 192.9, 136.6, 131.2,
analysis was recorded on an Elementar Vario MICRO analyzer. 128.4, 126.9, 126.3, 120.6, 115.6, 101.3, 46.1, 32.4, 20.0,
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13.5 ppm. Anal. calcd for C13H15NO: C, 77.58; H, 7.51; N, 6.96. and then dried to give the pure target compound. The charac-
Found: C, 77.29; H, 7.47; N, 6.92.
terization data for various IPM derivatives are listed as follows:
1-Isobutyl-1H-indole-4-carbaldehyde (4c). Dark yellow liquid
2-(2,6-Bis((E)-2-(1H-indol-3-yl)vinyl)-4H-pyran-4-ylidene)-
(2.34 g), 68.2% yield. 1H NMR (CDCl3, 500 MHz): d 10.24 (s, 1H), malononitrile (I3PM). Red solids (362.4 mg), 85.0% yield. 1H
7.59 (d, J ¼ 7.5 Hz, 2H), 7.32–7.28 (m, 3H), 3.94 (d, J ¼ 7.5 Hz, NMR (DMSO-d6, 500 MHz): d 11.87 (s, 2H), 8.25 (d, J ¼ 8.0 Hz,
2H), 2.21–2.13 (m, 1H), 0.90 (d, J ¼ 7.0 Hz, 6H) ppm. 13C NMR 2H), 8.04–7.98 (m, 4H), 7.51 (d, J ¼ 8.0 Hz, 2H), 7.28–7.19 (m,
(CDCl3, 125 MHz): d 193.0, 137.0, 131.8, 128.4, 126.9, 126.2, 6H), 6.92 (s, 2H) ppm. 13C NMR (DMSO-d6, 125 MHz): d 160.3,
120.6, 115.8, 101.3, 54.1, 29.7, 20.1 ppm. Anal. calcd for 156.3, 137.6, 132.6, 132.5, 124.6, 122.7, 120.9, 120.8, 116.6,
C
13H15NO: C, 77.58; H, 7.51; N, 6.96. Found: C, 77.18; H, 7.46; N, 113.3, 112.7, 112.4, 104.3, 52.6 ppm. MS (ESI, m/z): 427.10 (M+ +
6.90.
H). Anal. calcd for C28H18N4O: C, 78.86; H, 4.25; N, 13.14.
1-Dodecyl-1H-indole-4-carbaldehyde (4d). Yellow liquid (3.86 Found: C, 79.29; H, 4.22; N, 13.22.
g), 72.2% yield. 1H NMR (CDCl3, 500 MHz): d 10.25 (s, 1H), 7.62
2-(2,6-Bis((E)-2-(1-butyl-1H-indol-3-yl)vinyl)-4H-pyran-4-
(d, J ¼ 7.5 Hz, 2H), 7.35–7.30 (m, 3H), 4.16 (t, J ¼ 7.0 Hz, 2H), ylidene)malononitrile (I3PMBn). Red solids (483.2 mg), 89.7%
1.85–1.82 (m, 2H), 1.30–1.25 (m, 18H), 0.90 (t, J ¼ 7.5 Hz, yield. 1H NMR (CDCl3, 500 MHz): d 7.94 (d, J ¼ 7.5 Hz, 2H), 7.72
3H) ppm. 13C NMR (CDCl3, 125 MHz): d 192.9, 136.7, 131.2, (d, J ¼ 15.5 Hz, 2H), 7.51 (s, 2H), 7.23 (d, J ¼ 8.0 Hz, 2H), 7.36–
128.4, 126.9, 126.5, 120.6, 115.6, 101.4, 46.5, 31.8, 30.3, 29.52, 7.30 (m, 4H), 6.67 (d, J ¼ 16.0 Hz, 2H), 6.48 (s, 2H), 4.19 (t, J ¼
29.45, 29.38, 29.25, 29.11, 26.9, 22.6, 14.0 ppm. Anal. calcd for 7.0 Hz, 4H), 1.91–1.87 (m, 4H), 1.43–1.37 (m, 4H), 0.99 (t, J ¼
C
21H31NO: C, 80.46; H, 9.97; N, 4.47. Found: C, 81.05; H, 9.94; N, 7.0 Hz, 6H) ppm. 13C NMR (CDCl3, 125 MHz): d 159.7, 156.2,
4.49.
137.6, 132.1, 131.2, 125.9, 123.2, 121.6, 120.5, 116.5, 113.5,
1-Butyl-1H-indole-5-carbaldehyde (5b). Yellow liquid (2.23 g), 112.9, 110.5, 104.8, 55.6, 46.7, 32.0, 20.2, 13.6 ppm. MS (ESI, m/
65.0% yield. 1H NMR (CDCl3, 500 MHz): d 10.00 (s, 1H), 8.12 (d, J z): 539.25 (M+ + H). Anal. calcd for C36H34N4O: C, 80.27; H, 6.36;
¼ 1.0 Hz, 1H), 7.77 (dd, 3J ¼ 8.5 Hz, 4J ¼ 1.0 Hz, 1H), 7.39 (d, J ¼ N, 10.40. Found: C, 80.81; H, 6.41; N, 10.49.
8.5 Hz, 1H), 7.17 (d, J ¼ 3.5 Hz, 1H), 6.63 (d, J ¼ 3.0 Hz, 1H), 4.11
2-(2,6-Bis((E)-2-(1-isobutyl-1H-indol-3-yl)vinyl)-4H-pyran-4-
(t, J ¼ 7.0 Hz, 2H), 1.83–1.77 (m, 2H), 1.35–1.28 (m, 2H), 0.93 (t, J ylidene)malononitrile (I3PMBi). Orange-red solids (477.2 mg),
¼ 7.5 Hz, 3H) ppm. 13C NMR (CDCl3, 125 MHz): d 192.2, 139.0, 88.6% yield. 1H NMR (CDCl3, 500 MHz): d 7.96–7.94 (m,
129.6, 129.1, 128.2, 126.3, 121.4, 109.8, 103.0, 41.2, 32.1, 19.9, 2H),7.73 (dd, 3J ¼ 16.0 Hz, 4J ¼ 3.5 Hz, 2H), 7.49 (d, J ¼ 3.5 Hz,
3
4
13.4 ppm. Anal. calcd for C13H15NO: C, 77.58; H, 7.51; N, 6.96. 2H),7.42 (dd, J ¼ 7.5 Hz, J ¼ 3.0 Hz, 2H), 7.36–7.27 (m, 4H),
Found: C, 77.97; H, 7.48; N, 7.01.
6.69 (dd, 3J ¼ 16.0 Hz, 4J ¼ 3.5 Hz, 2H), 6.49 (d, J ¼ 3.5 Hz, 2H),
3
4
1-Isobutyl-1H-indole-5-carbaldehyde (5c). Yellow liquid (2.00 3.39 (dd, J ¼ 7.0 Hz, J ¼ 3.5 Hz, 4H), 2.29–2.24 (m, 2H), 0.99
g), 58.3% yield. 1H NMR (CDCl3, 500 MHz): d 10.02 (s, 1H), 8.14 (dd, J ¼ 6.5 Hz, J ¼ 3.5 Hz, 12H) ppm. 13C NMR (CDCl3, 125
(s, 1H), 7.77 (d, J ¼ 8.5 Hz, 1H), 7.40 (d, J ¼ 8.5 Hz, 1H), 7.16 (d, J MHz): d 159.7, 156.2, 137.8, 132.8, 131.3, 125.8, 123.2, 121.6,
¼ 2.5 Hz, 1H), 6.65 (s, 1H), 3.94 (d, J ¼ 7.5 Hz, 2H), 2.24–2.15 (m, 120.5, 116.5, 113.6, 112.7, 110.7, 104.8, 55.5, 54.5, 29.3,
1H), 0.93 (d, J ¼ 7.0 Hz, 6H) ppm. 13C NMR (CDCl3, 125 MHz): 20.3 ppm. MS (ESI, m/z): 539.25 (M+ + H). Anal. calcd for
3
4
d 192.4, 139.6, 130.3, 129.2, 128.2, 126.4, 121.6, 110.1, 103.1,
C36H34N4O: C, 80.27; H, 6.36; N, 10.40. Found: C, 80.92; H, 6.39;
54.3, 29.7, 20.2 ppm. Anal. calcd for C13H15NO: C, 77.58; H, 7.51; N, 10.50.
N, 6.96. Found: C, 77.89; H, 7.55; N, 6.99.
2-(2,6-Bis((E)-2-(1-dodecyl-1H-indol-3-yl)vinyl)-4H-pyran-4-
1-Dodecyl-1H-indole-5-carbaldehyde (5d). White solid (3.94 ylidene)malononitrile (I3PMD). Orange-red solids (662.3 mg),
1
g), 73.8% yield. 1H NMR (CDCl3, 500 MHz): d 10.03 (s, 1H), 8.15 86.8% yield. H NMR (CDCl3, 500 MHz): d 7.94 (d, J ¼ 8.0 Hz,
(s, 1H), 7.78 (dd, 3J ¼ 8.5 Hz, 4J ¼ 1.0 Hz, 1H), 7.41 (d, J ¼ 9.0 Hz, 2H), 7.72 (d, J ¼ 15.5 Hz, 2H), 7.50 (s, 2H), 7.42 (d, J ¼ 8.5 Hz,
1H), 7.19 (d, J ¼ 3.5 Hz, 1H), 6.65 (d, J ¼ 3.5 Hz, 1H), 4.14 (t, J ¼ 2H), 7.36–7.29 (m, 4H), 6.67 (d, J ¼ 16.0 Hz, 2H), 6.47 (s, 2H),
7.0 Hz, 2H), 1.86–1.83 (m, 2H), 1.31–1.24 (m, 18H), 0.88 (t, J ¼ 4.17 (t, J ¼ 7.0 Hz, 4H), 1.92–1.89 (m, 4H), 1.35–1.25 (m, 36H),
7.0 Hz, 3H) ppm. 13C NMR (CDCl3, 125 MHz): d 192.3, 139.3, 0.87 (t, J ¼ 7.0 Hz, 6H) ppm. 13C NMR (CDCl3, 125 MHz): d 159.7,
129.7, 129.3, 128.3, 126.5, 121.6, 109.9, 103.2, 46.7, 31.9, 30.2, 156.2, 137.6, 132.1, 131.2, 125.9, 123.2, 121.6, 120.5, 116.4,
29.56, 29.49, 29.42, 29.28, 29.15, 26.9, 22.6, 14.1 ppm. Anal. 113.5, 112.9, 110.5, 104.7, 55.6, 47.0, 31.9, 30.0, 29.6, 29.5, 29.4,
calcd for C21H31NO: C, 80.46; H, 9.97; N, 4.47. Found: C, 79.75; 29.3, 29.2, 27.0, 22.7, 14.1 ppm. MS (ESI, m/z): 763.45 (M+ + H).
H, 10.01; N, 4.50.
Anal. calcd for C52H66N4O: C, 81.84; H, 8.72; N, 7.34. Found: C,
82.34; H, 8.67; N, 7.40.
2-(2,6-Bis((E)-2-(1H-indol-4-yl)vinyl)-4H-pyran-4-ylidene)-
malononitrile (I4PM). Orange-red solids (300.1 mg), 70.4%
yield. 1H NMR (DMSO-d6, 500 MHz): d 11.50 (s, 2H), 8.06 (d, J ¼
Synthesis of IPM derivatives
General procedure: Compound 2 (172.2 mg, 1.0 mmol) and 16.5 Hz, 2H), 7.58–7.46 (m, 8H), 7.21 (d, J ¼ 7.5 Hz, 2H), 7.10 (s,
compound 3/4/5 (3.0 mmol) were dissolved in the solution of 4H) ppm. 13C NMR (DMSO-d6, 125 MHz): d 159.3, 156.3, 137.4,
acetonitrile (10 mL) and piperidine (0.2 mL). The mixture was 136.5, 127.1, 126.4, 126.0, 121.6, 121.1, 118.6, 115.9, 114.0,
heated at 80 ꢀC under a nitrogen atmosphere for 24 h. Aer 106.9, 100.6, 55.7 ppm. MS (ESI, m/z): 427.10 (M+ + H). Anal.
cooling, the mixture was poured into methanol and suction calcd for C28H18N4O: C, 78.86; H, 4.25; N, 13.14. Found: C,
ltered to give the crude product. The crude product was 79.25; H, 4.23; N, 13.02.
washed with acetone and methanol three times, respectively,
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2-(2,6-Bis((E)-2-(1-isobutyl-1H-indol-5-yl)vinyl)-4H-pyran-4-
ylidene)malononitrile (I4PMBn). Orange-red solids (430.4 mg), ylidene)malononitrile (I5PMBi). Brown solids (223.7 mg), 41.5%
79.9% yield. 1H NMR (CDCl3, 500 MHz): d 7.92 (d, J ¼ 16.0 Hz, yield. 1H NMR (CDCl3, 500 MHz): d 7.85 (s, 2H),7.68 (d, J ¼
2H),7.42 (d, J ¼ 8.5 Hz, 2H), 7.37 (d, J ¼ 7.0 Hz, 2H), 7.28–7.23 15.5 Hz, 2H), 7.48 (d, J ¼ 8.5 Hz, 2H), 7.38 (d, J ¼ 8.5 Hz, 2H),
(m, 4H), 6.86 (d, J ¼ 16.0 Hz, 2H), 6.77 (d, J ¼ 2.5 Hz, 2H), 6.55 (s, 7.13 (s, 2H), 6.70 (d, J ¼ 15.5 Hz, 2H), 6.59 (d, J ¼ 16.0 Hz, 4H),
2H), 4.16 (t, J ¼ 7.0 Hz, 4H), 1.87–1.81 (m, 4H), 1.40–1.32 (m, 3.94 (d, J ¼ 7.5 Hz, 4H), 2.25–2.19 (m, 2H), 0.96 (d, J ¼ 6.5 Hz,
4H), 0.95 (t, J ¼ 7.0 Hz, 6H) ppm. 13C NMR (CDCl3, 125 MHz): 12H) ppm. 13C NMR (CDCl3, 125 MHz): d 159.1, 156.2, 139.8,
d 158.6, 156.0, 136.9, 136.6, 129.6, 127.5, 126.8, 121.6, 119.9, 137.6, 129.8, 129.0, 126.3, 122.6, 120.6, 115.8, 115.4, 110.5,
118.3, 115.6, 111.9, 106.7, 99.3, 58.6, 46.4, 32.4, 20.2, 13.6 ppm. 106.0, 102.1, 57.7, 54.3, 29.7, 20.3 ppm. MS (ESI, m/z): 539.25
MS (EI, m/z): 538.29 (M+, 56%), 495.23 (8%), 394.25 (22%), (M+ + H). Anal. calcd for C36H34N4O: C, 80.27; H, 6.36; N, 10.40.
226.10 (57%), 186.12 (49%). Anal. calcd for C36H34N4O: C, 80.27; Found: C, 79.69; H, 6.33; N, 10.48.
H, 6.36; N, 10.40. Found: C, 80.83; H, 6.33; N, 10.32.
2-(2,6-Bis((E)-2-(1-dodecyl-1H-indol-5-yl)vinyl)-4H-pyran-4-
2-(2,6-Bis((E)-2-(1-isobutyl-1H-indol-4-yl)vinyl)-4H-pyran-4- ylidene)malononitrile (I5PMD). Brown solids (162.7 mg), 21.3%
ylidene)malononitrile (I4PMBi). Orange-red solids (360.7 mg), yield. 1H NMR (CDCl3, 500 MHz): d 7.85 (s, 2H), 7.70 (d, J ¼
67.0% yield. 1H NMR (CDCl3, 500 MHz): d 7.99 (d, J ¼ 16.0 Hz, 15.5 Hz, 2H), 7.49 (d, J ¼ 8.5 Hz, 2H), 7.39 (d, J ¼ 8.5 Hz, 2H),
2H), 7.42 (dd, 3J ¼ 14.0 Hz, 4J ¼ 8.0 Hz, 4H), 7.29–7.27 (m, 4H), 7.15 (s, 2H), 6.72 (d, J ¼ 16.0 Hz, 2H), 6.63 (s, 2H), 6.57 (s, 2H),
6.93 (d, J ¼ 16.0 Hz, 2H), 6.82 (d, J ¼ 2.5 Hz, 2H), 6.64 (s, 2H), 4.14 (t, J ¼ 7.0 Hz, 4H), 1.88–1.85 (m, 4H), 1.33–1.26 (m, 36H),
3.99 (d, J ¼ 7.5 Hz, 4H), 2.28–2.20 (m, 2H), 0.97 (d, J ¼ 6.5 Hz, 0.86 (t, J ¼ 6.5 Hz, 6H) ppm. 13C NMR (CDCl3, 125 MHz): d 159.1,
12H) ppm. 13C NMR (CDCl3, 125 MHz): d 158.7, 156.0, 137.0, 156.2, 139.9, 137.3, 129.2, 129.1, 126.3, 122.6, 120.7, 115.8,
136.9, 130.2, 127.4, 126.9, 121.6, 120.0, 118.4, 115.6, 112.1, 115.4, 110.3, 106.0, 102.2, 57.8, 46.7, 31.9, 30.3, 29.6, 29.5, 29.4,
106.7, 99.2, 58.8, 54.4, 29.7, 20.3 ppm. MS (ESI, m/z): 539.25 (M+ 29.3, 29.2, 27.0, 22.7, 14.1 ppm. MS (EI, m/z): 761.24 (M+ ꢁ H,
+ H). Anal. calcd for C36H34N4O: C, 80.27; H, 6.36; N, 10.40. 100), 382.23 (31%), 298.23 (78%), 207.03 (69%), 186.10 (73%),
Found: C, 79.86; H, 6.32; N, 10.35.
158.08 (67%). Anal. calcd for C52H66N4O: C, 81.84; H, 8.72; N,
2-(2,6-Bis((E)-2-(1-dodecyl-1H-indol-4-yl)vinyl)-4H-pyran-4- 7.34. Found: C, 81.23; H, 8.69; N, 7.29.
ylidene)malononitrile (I4PMD). Orange-red solids (600.3 mg),
78.7% yield. 1H NMR (CDCl3, 500 MHz): d 7.94 (d, J ¼ 16.0 Hz,
Results and discussion
Synthesis and features of IPM derivatives
2H), 7.39 (dd, 3J ¼ 19.0 Hz, 4J ¼ 7.0 Hz, 4H), 7.26 (d, J ¼ 3.5 Hz,
2H), 7.24 (d, J ¼ 7.0 Hz, 2H), 6.88 (d, J ¼ 16.0 Hz, 2H), 6.77 (d, J ¼
2.5 Hz, 2H), 6.59 (s, 2H), 4.14 (t, J ¼ 7.0 Hz, 4H), 1.86–1.83 (m,
The chemical structures and synthetic route of IPM derivatives
4H), 1.31–1.23 (m, 36H), 0.85 (t, J ¼ 6.5 Hz, 4H) ppm. 13C NMR
(CDCl3, 125 MHz): d 158.7, 156.0, 137.0, 136.6, 129.6, 127.5,
126.9, 121.6, 120.0, 118.4, 115.6, 111.9, 106.7, 99.3, 58.7, 46.7,
31.9, 30.3, 29.6, 29.5, 29.4, 29.3, 29.2, 27.0, 22.7, 14.1 ppm. MS
(ESI, m/z): 763.45 (M+ + H). Anal. calcd for C52H66N4O: C, 81.84;
H, 8.72; N, 7.34. Found: C, 82.16; H, 8.77; N, 7.38.
are depicted in Scheme 1. The intermediate 2-(2,6-dimethyl-4H-
pyran-4-ylidene)malononitrile (2) was synthesized according to
the previous literature procedure using 2,6-dimethyl-4-pyrone
(1) as the starting material.20 The intermediates 3b–3d/4b–4d/
5b–5d were obtained from the alkylation of 3a/4a/5a with the
corresponding alkyl bromide in the presence of KOH, respec-
tively, adopting a method similar to the previous literature
procedures (Scheme S1, ESI†).21 And then the IPM derivatives
could be facilely prepared from a Knoevenagel reaction of the
2-(2,6-Bis((E)-2-(1H-indol-5-yl)vinyl)-4H-pyran-4-ylidene)
malononitrile (I5PM). Orange-red solids (311.7 mg), 73.1%
yield. 1H NMR (DMSO-d6, 500 MHz): d 11.39 (s, 2H), 8.07 (s, 2H),
7.92 (d, J ¼ 16.0 Hz, 2H), 7.63 (d, J ¼ 16.5 Hz, 2H), 7.48 (d, J ¼
8.5 Hz, 2H), 7.43 (s, 2H), 7.29 (d, J ¼ 16.0 Hz, 2H), 6.81 (s, 2H),
6.54 (s, 2H) ppm. 13C NMR (DMSO-d6, 125 MHz): d 159.6, 156.2,
140.0, 137.1, 128.0, 126.7, 126.3, 122.5, 120.6, 116.0, 115.5,
112.1, 105.7, 102.1, 54.7 ppm. MS (ESI, m/z): 427.10 (M+ + H).
Anal. calcd for C28H18N4O: C, 78.86; H, 4.25; N, 13.14. Found: C,
78.51; H, 4.28; N, 13.06.
2-(2,6-Bis((E)-2-(1-butyl-1H-indol-5-yl)vinyl)-4H-pyran-4-
ylidene)malononitrile (I5PMBn). Orange solids (245.2 mg),
45.5% yield. 1H NMR (CDCl3, 500 MHz): d 7.84 (s, 2H), 7.67 (d, J
¼ 16.0 Hz, 2H), 7.48 (d, J ¼ 8.5 Hz, 2H), 7.38 (d, J ¼ 8.5 Hz, 2H),
7.15 (s, 2H), 6.69 (d, J ¼ 16.0 Hz, 2H), 6.58–6.56 (m, 4H), 4.14 (t, J
¼ 7.5 Hz, 4H), 1.88–1.82 (m, 4H), 1.40–1.34 (m, 4H), 0.97 (t, J ¼
7.5 Hz, 6H) ppm. 13C NMR (CDCl3, 125 MHz): d 159.1, 156.2,
139.8, 137.3, 129.2, 129.0, 126.2, 122.6, 120.6, 115.8, 115.3,
110.2, 106.0, 102.2, 57.6, 46.4, 32.4, 20.1, 13.6 ppm. MS (ESI, m/
z): 539.25 (M+ + H). Anal. calcd for C36H34N4O: C, 80.27; H, 6.36;
N, 10.40. Found: C, 79.75; H, 6.31; N, 10.33.
intermediate
2 with 3a–3d/4a–4d/5a–5d, respectively. The
chemical structures of IPM derivatives were conrmed by NMR
spectroscopy, mass spectrometry, and elemental analysis.
Among these compounds, the structures of I3PM, I3PMBn,
I3PMBi, I4PMBn, I4PMBi, and I5PM were further conrmed by
single-crystal X-ray diffraction analysis. These target
compounds are soluble in common organic solvents, such as
chloroform, dichloromethane, toluene, tetrahydrofuran (THF),
and ethyl acetate (EA).
Photophysical properties of IPM derivatives in solution and
solid state
The absorption and uorescence spectra of the IPM derivatives
in THF solution were measured as shown in Fig. S1 and S2
(ESI†), respectively, and the data are presented in Table S1
(ESI†). The IPM derivatives in THF solution have similar
absorption and emission spectra, and exhibit three or four
absorption peaks in the range of 333–487 nm, and one emission
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Scheme 1 Chemical structures and synthetic routes of IPM derivatives.
peaks in the range of 529–580 nm. Compared with I3PM, I4PM,
and I5PM, the corresponding N-alkylated IPM derivatives show
a slight redshi in absorption spectra as well as uorescence
spectra because of the introduction of the electron-donating
alkyl group. However, the different alkyl groups or isomerized
indole units do not show obvious inuence on their absorption
and uorescence spectra in THF solution.
MFC properties of IPM derivatives
The MFC behaviors of the original samples of IPM derivatives
were explored (Fig. 1). When the original solid samples of I3PM
derivatives were put in a mortar and ground with a pestle, the
change of the uorescence color was not obvious for I3PM and
I3PMBn. Although grinding led to an obvious red shi in the
emission spectra of I3PM and I3PMBn (Fig. 2 and Table 1) and
Furthermore, the uorescence spectra of the IPM derivatives
in the solid state were investigated. As shown in Fig. S3 (ESI†)
and Table 1, the original solid samples of I3PM, I4PM, and
I5PM emit red or orange uorescence, and appear emission
peaks at 634, 607, and 608 nm, respectively. Compared with
I3PM and I4PM, the corresponding N-alkylated IPM derivatives
in the solid state exhibit an obvious blue shi in emission
spectra, whereas, the emission spectra I5PM derivatives are
related irregularly to the change of the alkyl chain. It is worth
mentioning that the increase of alkyl chain length (from n-butyl
group to n-dodecyl group) leads to the blue shi for the I3PM
derivatives and the red shi for the I4PM derivatives in the
emission spectra, showing an interesting correlation with their
MFC behaviors which will be mentioned below. Contrary to the
situation in the solution, the alkyl groups exhibit an obvious
inuence on the solid-state uorescence properties of IPM
derivatives.
Table 1 Emission wavelengths of the IPM derivatives in solid state
Compound lOriginal (nm) lGround (nm) DlMCa (nm) lFumed (nm)
I3PM
634
622
601
591
607
588
586
590
608
613
655
631
632
604
634
621
626
599
618
614
21
9
n.d.b
n.d.
618
592
613
583
585
n.d.
615
n.d.
n.d.
n.d.
I3PMBn
I3PMBi
I3PMD
I4PM
I4PMBn
I4PMBi
I4PMD
I5PM
31
13
27
33
40
9
10
1
5
I5PMBn
I5PMBi
I5PMD
588, 675 (s)c 593, 675 (s)
603, 657 (s) 603, 663 (s)
6
a
b
lMC ¼ lGround ꢁ lOriginal
.
n.d. ¼ no detection. c s ¼ shoulder peak.
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naked eye. However, a color change was clearly observed from
orange to red for I3PMBi (DlMFC ¼ 31 nm) and I3PMD (DlMFC
¼
13 nm), and the uorescence colors of their ground samples
could be changed into orange by solvent-fuming (exposed to EA
vapor), suggesting these two compounds exhibited reversible
MFC properties. The results indicated that the isomerization of
the alkyl chain and the increase of the alkyl chain length were
favorable to the effective mechanouorochromism of the I3PM
derivatives.
For I4PM derivatives, by grinding the original samples of
I4PM, I4PMBn, and I4PMBi with orange uorescence, the cor-
responding red-emitting ground samples were obtained and
their DlMFC values were 27, 33, and 31 nm, respectively. Different
from I3PMD, I4PMD containing a dodecyl chain did not show
the uorescence color change in response to press stimuli,
indicating that the increase of the alkyl chain length were
disadvantageous to the MFC properties of the I4PM derivatives.
As for I5PM derivatives, interestingly, only the original samples
of I5PM exhibited reversible uorescence color change upon
grinding (DlMFC ¼ 10 nm) and solvent-fuming, the uorescence
spectra and colors of the other homologues had no signicant
change (DlMFC ¼ 5–6 nm). Thereby the introduction of the alkyl
chain was not advantageous for the MFC properties of the I5PM
derivatives. As mentioned above, the alkyl chains exhibited
different effects on the MFC properties of the IPM derivatives
when the substitution position of the indole unit was different.
To understand the different MC behaviors of the IPM
derivatives upon external stimuli, XRD measurements were
Fig. 1 Photographs of the solid samples of IPM derivatives upon
pressing and solvent-fuming (EA vapor) under a 365 nm UV lamp.
the grinding-induced spectral shis (DlMFC ¼ lGround ꢁ lOriginal
)
were 21 and 9 nm, respectively, these two compounds did not
show obvious MFC phenomena because the uorescence color
change from red to more red could not be well recognized by the
Fig. 2 Normalized fluorescence spectra of the solid samples of IPM derivatives upon external stimuli.
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carried out for the samples (Fig. 3, S4, and S5, ESI†). As shown
in Fig. 3, XRD measurements indicated all the I3PM derivatives
showed different structures of molecular aggregation before
and aer grinding. The XRD curves of the original samples of
I3PMBi and I3PMD displayed sharp and intense diffraction
peaks, indicating they were crystalline structures. These
diffraction peaks disappeared or decreased obviously aer
grinding, indicating the destruction of crystalline structures
and the formation of amorphous structures. Thus, we could
conclude that the original samples underwent a morphological
change from a crystalline state to an amorphous state upon
grinding. The diffraction peaks were recovered when the
ground samples were fumed by EA vapor and the recovery led to
the blue-shied uorescence spectra as well as the recovery of
the uorescence colors. It was noted that the morphology of
I3PM and I3PMB was also signicantly changed under the
stimulation of pressure, which was consistent with the red-
shied emission wavelengths observed in the uorescence
spectra (Fig. 2), but the uorescence color change was hardly
distinguished. For the I4PM and I5PM derivatives, the grinding
treatment led to the transformation from crystalline state to
amorphous state of the MFC-active compounds, such as I4PM,
I4PMBn, I4PMBi, and I5PM, whereas, the morphology of the
MFC-inactive compounds including I4PMD, I5PMBn, I5PMBi,
and I5PMD still maintained (Fig. S4 and S5, ESI†). The results
indicated the MFC properties of the IPM derivatives should be
mainly attributed to the transformation between amorphous
and crystalline states.
Crystal structures of IPM derivatives
To gain deep insight into the underlying reasons for the pho-
tophysical behavior of the IPM derivatives in the solid state,
single crystals of some of the target compounds were obtained
and the crystal structure analysis was performed.22 Single crys-
tals of I3PM and I5PM were obtained from a THF/1,4-dioxane
solution (v/v ¼ 1 : 1), and those of I3PMBn, I3PMBi, I4PMBn,
and I4PMBi were cultured from a CHCl3/ethanol solution (v/v ¼
1 : 1), respectively. The crystal structures of these compounds
are shown in Fig. 4 and the selected crystal data are given in
Table 2. The unit cells of I3PM, I3PMBn, I4PMBi, and I5PM are
ꢀ
all triclinic with space group P1, and those of I3PMBi and
I4PMBn are monoclinic with space group P121/c1 and C 2/c,
respectively.
For I3PM, the dihedral angles between the 4H-pyran cycle
and the two indole units are 3.61ꢀ and 9.64ꢀ, respectively,
indicating its molecule shows a certain degree of distortion.
And there are no obvious p–p stacking interactions in the
˚
crystal of I3PM. One kind of aliphatic C–H/N bonds (2.688 A)
restrict the two adjacent I3PM molecules in the same layer
(Fig. 5 and Table 3). Because of the unit cell of I3PM includes
three 1,4-dioxane molecules, these solvent molecules connect
the I3PM molecules of the upper and lower layers by two kinds
˚
of C–H/O bonds (2.645 and 2.687 A), one kind of N–H/O
˚
˚
hydrogen bond (2.899 A), and C–H/p bond (2.796 A). The
intermolecular interactions in crystal I3PM promote the
formation of a stairs-appearance structure. In comparison with
I3PM, I5PM has a more twisted conformation because the
Fig. 3 XRD curves of the solid samples of I3PM derivatives upon external stimuli.
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Fig. 5 Intermolecular interactions (a) and molecular packing (b) in
I3PM.
spectra in THF solution (Table S1†) as well as in solid state
(Table 1). There are weak p–p stacking in crystal I5PM because
the distances between the indole ring and the 4H-pyran ring in
the upper and lower layers are measured to be 3.642 and
Fig. 4 Single-crystal structures of I3PM (a), I3PMBn (b), I3PMBi (c), I4PMBn
(d), I4PMBi (e), and I5PM (f). Hydrogen atoms are omitted for clarity.
˚
3.698 A. Furthermore, the crystal structure of I5PM is stabilized
˚
by one kind of C–H/N bond (2.239 A) between the two adjacent
I5PM molecules in the same layer, and one kind of C–H/O
dihedral angle between the 4H-pyran cycle and one of the indole
units reaches 27.82ꢀ, which can use as an explanation for the
facts that I5PM has blue-shied absorption and emission
˚
˚
bond (2.705 A) and ve kinds of C–H/p bonds (2.727–3.362 A)
between the 1,4-dioxane solvent molecules and I5PM molecules
Table 2 Crystal data of I3PM, I3PMBn, I3PMBi, I4PMBn, I4PMBi, and I5PM
I3PM
I3PMBn
I3PMBi
I4PMBn
I4PMBi
I5PM
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Z
C
38H38N4O6
C36H34N4O
538.67
293(2)
Triclinic
ꢀ
P1
C36H34N4O
538.67
296(2)
Monoclinic
P121/c1
4
C36H34N4O
538.67
296(2)
Monoclinic
C 2/c
C36H34N4O
538.67
273(2)
Triclinic
ꢀ
P1
C38H38N4O6
646.72
298(2)
Triclinic
ꢀ
P1
646.72
298(2)
Triclinic
ꢀ
P1
2
2
8
4
2
Dcalcd [mg/m3]
F (000)
1.231
684
2.20–26.00
0.0724
0.2047
10.4610(14)
12.7008(17)
14.931(3)
101.290(3)
103.112(3)
109.122(2)
1745.0(5)
0.971
1.187
572
1.76–25.00
0.0832
0.1684
8.3257(12)
12.1141(17)
15.672(2)
100.531(3)
90.188(4)
103.883(3)
1506.8(4)
1.054
0.893
1144
1.159
2288
1.019
1144
3.10–64.99
0.0917
0.2407
11.5539(4)
17.9477(6)
17.9799(5)
105.002(2)
96.230(2)
99.152(2)
3511.4(2)
0.954
1.234
684
q range [ꢀ]
2.38–66.78
0.0978
0.2622
19.030(3)
21.991(3)
9.8034(12)
90
102.551(7)
90
4004.5(9)
0.950
2.31–65.00
0.0793
0.2138
39.856(3)
8.1733(4)
19.7806(10)
90
106.633(2)
90
6174.0(6)
0.913
2.32–24.92
0.0811
0.2091
9.885(5)
13.056(6)
15.400(8)
70.424(8)
80.392(9)
68.547(8)
1740.7(15)
1.037
R1 [I > 2s(I)]
wR2 [I > 2s(I)]
˚
a [A]
˚
b [A]
˚
c [A]
a [deg]
b [deg]
g [deg]
3
˚
V [A ]
GOF
R(int)
0.0300
13 631
6740
0.1338
0.0477
8415
5293
0.1608
0.0837
22 108
6829
0.1414
0.3153
0.1099
20 863
5158
0.1746
0.2966
0.0896
40 675
11 555
0.1375
0.0654
14 608
6358
0.1275
0.2355
No. of recns collected
No. of unique recns
R1 (all data)
wR2 (all data)
0.2363
0.2008
0.2779
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Table 3 Summarization of the intermolecular interactions in I3PM, I3PMBn, I3PMBi, I4PMBn, I4PMBi, and I5PM
Crystal
No.
IIa
Crystal
No.
IIa
˚
d/A
˚
d/A
I3PM
1
2
3
4
5
1
2
3
4
5
6
7
8
9
1
2
3
4
5
N1–H1/O5
H35B/p(pyrrole)
H22/O4
H16/O4
H10/N3
p(pyran)/p(pyrrole)
p(pyran)/p(Ph)
H32B/p(C]C)
H16/O2
H30A/p(Ph)
H30A/p(pyrrole)
H1A/N4
H34B/p(Ph)
H34B/p(pyrrole)
H29B/p(pyran)
p(pyrrole)/p(C]C)
H2/H23
2.889
2.796
2.687
2.645
2.688
3.698
3.642
3.118
2.705
2.844
3.062
2.239
3.362
2.727
3.373
3.654
2.340
2.827
3.083
I3PMBi
1
2
3
1
2
3
4
5
6
7
1
2
3
4
5
6
7
H32/N4
2.740
4.530
4.798
2.837
3.836
2.710
2.244
2.351
1.943
2.829
2.943
2.632
2.699
2.560
3.296
3.590
2.634
2.747
2.493
3.374
p(Ph)/p(pyran)
p(pyrrole)/p(pyran)
H34A/p (CN)
p(Ph)/p(pyran)
H25/N3
H29C/H36B
H29C/H36A
H28A/H36A
H20/p(Ph)
H32/p(pyrrole)
H17/N5
H9/N5
H2/N5
p(C]C)/p(C]C)
p(C]C)/p(pyran)
H38/N1
I4PMBn
I5PM
I4PMBi
I3PMBn
H30B/p(Ph)
H29A/p(pyrrole)
8
9
10
H45/N1
H53/N1
p(CN)/p(CN)
a
Intermolecular interaction.
(Fig. 6 and Table 3). As a result, the molecules of I5PM pack in
In the case of crystals I3PMBn, I3PMBi, and I4PMBn, the two
a reverse lamellar structure. For the crystals of I3PM and I5PM, dihedral angles between the two indole units and the 4H-pyran
the presence of solvent molecules indicates that there are cycle are 7.67 and 14.32ꢀ (I3PMBn), 5.89 and 19.79ꢀ (I3PMBi),
a large number of defects or holes in their offset lamellar crystal and 10.01 and 13.04ꢀ (I4PMBn), respectively. For crystal I4PMBi,
structures.23 This kind of loose crystal structure is easily there were two independent conformational molecules whose
destroyed by planarization of the molecular conformation or corresponding dihedral angles are 5.77 and 23.58ꢀ, and 5.23
slip deformation upon grinding, which increases molecular and 30.22ꢀ, respectively. Based on the above mentioned, we can
conjugation and promotes a red shi of the emission spectra.23a conclude the following results: (i) the introduction of n-butyl
The destruction of crystalline structures means the trans- group and i-butyl group on the nitrogen atom of the indole unit
formation from a crystalline state to an amorphous state, which enhances the degree of distortion of the molecules, which can
is conrmed by the results of XRD experiments (Fig. 3 and S5†). be used to account for the obvious blue shi of the solid-state
emission spectra of these four molecules relative to those of
the corresponding parent molecules I3PM and I4PM (Table 1);
(ii) compared with n-butyl group, i-butyl group is more favorable
for the formation of a twisted conformation, and therefore the
solid-state emissions of I3PMBi and I3PMBi show blue shi
compared to those of I3PMBn and I3PMBn, respectively; (iii)
I4PM derivatives possess a more twisted conformation than the
corresponding I3PM derivatives, which is consistent with the
fact that the former has shorter emission wavelength in the
solid state.
Furthermore, we investigated the intermolecular interac-
tions and packing arrangements of these compounds. For
crystal I3PMBn, the distance between the indole unit and the
olenic double bond in the upper and lower molecules is
˚
measured to be 3.654 A, indicating the existence of weak p–p
stacking (Fig. 7 and Table 3). Three kinds of aliphatic C–H/p
˚
bonds (2.827, 3.083, and 3.373 A), and one kind of van der
Waals force between aromatic hydrogen atoms in the phenyl
˚
units with a distance of 2.340 A stabilizes the crystal structure.
Similar to I3PM, I3PMBn adopts a reverse lamellar packing
pattern. However, there is only one kind of C–H/N bond
Fig. 6 Intermolecular interactions (a) and molecular packing (b) in
I5PM.
˚
(2.740 A) in the crystal of I3PMBi, which results in
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Fig. 7 Intermolecular interactions (a) and molecular packing (b) in
I3PMBn.
Fig. 9 Intermolecular interactions (a) and molecular packing (b) in
I4PMBn.
a herringbone arrangement with a herringbone angle of 86.03ꢀ
(Fig. 8 and Table 3). Herein, the i-butyl group obviously
decreases the intermolecular interactions between the mole-
cules of I3PMBi, which causes that I3PMBi has larger DlMFC
value and shows more signicant solid-state uorescence
change compared with I3PMBn containing n-butyl group.
As shown in Fig. 9, the molecules of I4PMBn in the crystal are
(Table 3). The molecules of I4PMBn pack in a herringbone sharp
including a lamellar arrangement and the herringbone angle is
79.50ꢀ, which is similar to those of I3PMBi. For crystal I4PMBi,
˚
six kinds of C–H/N bonds (2.493–2.747 A) restrict the mole-
cules in the same layer, three kind of p–p stacking interaction
˚
(3.296, 3.590 and 3.374 A) and two kinds of C–H/p bonds
˚
(2.874 and 2.943 A) stabilize the molecules in the upper and
˚
stabilized by one kind of p–p stacking interaction (3.836 A)
lower layers (Fig. 10 and Table 3). These intermolecular inter-
actions cause the molecules of I4PMBi to adopt a face-to-face
lamellar stacking mode. Herein, compared with the herring-
bone packing containing a lamellar arrangement of I4PMBn,
purely lamellar arrangement of I4PMBi makes it easier to break
down by external pressure, which leads to its larger DlMFC value.
For the introduction of different alkyl chains, two factors,
namely the isomerization of the alkyl chain and the alkyl chain
length, are considered. In view of the I3PM and I4PM deriva-
tives, according to the above crystal structure analysis, i-butyl
group leads to a weaker intermolecular interaction and/or
a looser stacking mode than n-butyl group due to a more
twisted molecular conformation, which can be used to account
for the fact that the isomerization of the alkyl chain is benet
for the remarkable MFC properties. But for the length of alkyl
chain, its inuence is obviously controlled by the substitution
position of the indole unit according to the experimental
results. The increase of the alkyl chain length is favorable to the
MFC properties of the I3PM derivatives, whereas is unfavorable
to those of the I4PM derivatives. This unusual phenomenon is
likely to be related to the change of the molecular conformation
before and aer grinding. Because there is no strong p–p
stacking as well as other intermolecular interactions in the
original crystalline samples of the I3PM and I4PM derivatives,
their emissions should mainly depend on the molecular
conformations. As shown in Table 1, the original crystalline
samples of I3PMD and I4PMD show a blue shi and red shi
between the 4H-pyran ring and the phenyl unit, one kind of
˚
C–H/N bond (2.710 A), two kinds of C–H/p bonds (2.829 and
˚
2.837 A), and three kinds of van der Waals force between the
˚
hydrogen atoms in n-butyl groups (1.943, 2.244 and 2.351 A)
Fig. 8 Intermolecular interactions (a) and molecular packing (b) in
I3PMBi.
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specially, the introduction of the alkyl chains is completely
detrimental to those of the I5PM derivatives. Based on the
results of crystal structure analysis, the weak intermolecular
interactions and loose packing modes in MFC-active molecules
result in that the crystalline structures of the molecules are
easily destroyed by pressure stimulation and thus the
morphological transformation of the molecular aggregates are
realized. Although the transformation from a crystalline state to
an amorphous state cannot fully determine whether a molecule
has MFC properties, for example, I3PM and I3PMBn do not
exhibit obvious solid-state uorescence color change upon
grinding, there is no doubt the transformation is crucial for the
formation of MFC properties. This work provides a good design
idea for designing and obtaining new MFC-active materials
based on the traditional uorescent molecular frameworks.
Conflicts of interest
There are no conicts to declare.
Fig. 10 Intermolecular interactions (a) and molecular packing (b) in
I4PMBi.
Acknowledgements
compared with those of I3PMBn and I4PMBn, respectively,
indicating I3PMD and I4PMBn might have a more twisted
conformation and a poorer p-conjugation. Aer grinding,
a better p-conjugation for the ground samples of I3PMD and
I4PMBn is anticipated, and thus I3PMD and I4PMBn have
a larger DlMFC value. Based on the results in the case, a regular
pattern can be concluded as follows: if the emission wave-
lengths of the original crystalline samples of a series of uo-
rescent compounds gradually blue-shied with the increase of
alkyl chain length, the homologue with longer alkyl chain will
exhibit higher contrast MFC behavior aer grinding;7–9,24
otherwise shorter alkyl chain is more benecial to the MFC
property,10–12 which is in good agreement with the previous
reports.
This research work was supported by National Natural Science
Foundation of China (Grants 21572165, 21472140, and
21372177) and Zhejiang Provincial Natural Science Foundation
(Grant LY16B040005).
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