Effect of stacking mode on the mechanofluorochromic properties of 3-aryl-2-cyano acrylamide derivatives

Article abstract of DOI:10.1039/c4nj01492h

Three structurally simple 3-aryl-2-cyano acrylamide derivatives, 2-cyano-3-(2-methoxyphenyl)-2-propenamide (1), 2-cyano-3-(3-methoxyphenyl)-2-propenamide (2) and 2-cyano-3-(4-methoxyphenyl)-2-propenamide (3) were synthesized. They exhibited different opti

Full text of DOI:10.1039/c4nj01492h

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Effect of stacking mode on the
mechanofluorochromic properties of
Cite this:DOI: 10.1039/c4nj01492h
3-aryl-2-cyano acrylamide derivatives†
Qingbao Song,a Yongsheng Wang,a Chenchen Hu,b Yujian Zhang,*b Jingwei Sun,a
Kunyan Wangb and Cheng Zhang*a
Three structurally simple 3-aryl-2-cyano acrylamide derivatives, 2-cyano-3-(2-methoxyphenyl)-2-
propenamide (1), 2-cyano-3-(3-methoxyphenyl)-2-propenamide (2) and 2-cyano-3-(4-methoxyphenyl)-
2-propenamide (3) were synthesized. They exhibited different optical properties due to their distinct
face-to-face stacking mode. The as-prepared crystals of 1 exhibited green luminescence and the
emission peak did not change after grinding treatment. However, the emission peaks of 2 (Ff = 12%) and
3 (Ff = 16%) exhibited an obvious red-shift upon grinding, and their corresponding quantum yields
decreased to 8% and 10%, respectively. Differential scanning calorimetry and powder X-ray diffractometry
data indicated that the optical properties of 2 and 3 could be attributed to the transformation from the
crystalline phase to the amorphous phase. X-ray crystal structures, infrared spectroscopy and data from
fluorescence lifetime experiments further validated the relationship between fluorescence switching,
stacking mode and molecular interactions.
Received (in Montpellier, France)
4th September 2014,
Accepted 29th October 2014
DOI: 10.1039/c4nj01492h
www.rsc.org/njc
between molecular packing characteristics and the resulting
MCF properties at the molecular level.
Introduction
Mechanochromic fluorescent (MCF) materials revealing rever-
Recently, Yang’s group reported that 9,10-bis(alkoxystyryl)-
sible fluorescent changes under external force stimuli1 could be anthracene isomers8 indicated MCF behaviour which was both
applied to sensors, memory chips and security inks.2 Recently, chain length-dependent9 and position-dependent. Further,
rapid progress has been achieved in these material systems and short alkyl-containing oOC3 and mOC3 exhibited more remark-
in understanding the formation mechanism of MCF behaviour. able MCF activity than pOC3, which was related to the mole-
For instance, luminophores with a highly twisted conformation cular conformation and stacking modes. In addition, our group
were reported recently,3–5 whose molecular structure adopted a prepared new isomers containing arylamines, in which changing
more planar conformation after grinding that induced a longer the position of the cyano group had a great effect on the MCF
wavelength. In addition, the excitonic interaction was substan- properties and packing modes.10 Clearly, employing structural
tially altered in accord with various stacking modes, where isomers to investigate the formation mechanism of MCF proper-
excimer formation resulted in a red-shift of fluorescence.6 To ties is of significant interest.
date, the overwhelming majority of MCF materials exhibited a
We have synthesized three isomers, 2-cyano-3-(2-methoxyphenyl)-
phase transformation from the crystalline state to the (partial) 2-propenamide (o-MPCPA), 2-cyano-3-(3-methoxyphenyl)-2-propen-
amorphous state.7 However, there is some doubt about whether amide (m-MPCPA) and 2-cyano-3-(4-methoxyphenyl)-2-propenamide
organic powders with a transition will reveal MCF behaviour. (p-MPCPA) (Fig. 1), by a simple Knoevenagel reaction. Among
Thus, it is still essential to further understand the relationship them, the m-MPCPA and p-MPCPA samples with a blue fluores-
cence exhibited remarkable MCF properties. Their fluorescence
changed to green after grinding treatment. For the o-MPCPA,
a State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology,
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou,
P. R. China. E-mail: czhang@zjut.edu.cn
the phase transition also underwent an obvious change upon
grinding, but MCF behaviour was not observed. X-ray crystal
structures and data from fluorescence lifetime experiments
further validated the phase transition of MPCPA (o-, m-, p-).
However, the intermolecular interactions remained unchanged
for luminophore o-MPCPA, which might be the main factor
causing its abnormal behaviour.
b Department of Materials Chemistry, Huzhou University, Xueshi Road 1#, Huzhou,
P. R. China. E-mail: sciencezyj@foxmail.com
† Electronic supplementary information (ESI) available. CCDC 999359–999361.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4nj01492h
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molecules in their unit cells. As depicted in Fig. S3C and S4C
(ESI†), multiple C–H/N and C–H/O hydrogen bonds as well as
H-type packing were also observed, which induced a molecular
column. Interestingly, both of the luminogens m-MPCPA and
p-MPCPA adopted head-to-head or parallel arrangements. The
aromatic ring was stacked directly above the benzene ring of the
neighbouring molecule, with interplanar distances of 3.5931 Å
and 3.4903 Å for m-MPCPA and p-MPCPA respectively (Fig. 2b
and c). The interplanar distance was less than the range of the
effective intermolecular interaction, but p–p overlap was slightly
reduced due to the slip-stacking along the long axis of the
m-/p-MPCPA molecule (Fig. 2b and c).11 Thus, there was a weak
p–p interaction between aromatic rings, which caused fluores-
cence quenching.
Fig. 1 Molecular structures of 3-aryl-2-cyano acrylamide derivatives
o-MPCPA, m-MPCPA and p-MPCPA.
Results and discussion
Single crystals of MPCPA (o-, m- and p-) were obtained by slow
evaporation of ethanol–n-hexane mixtures. Fig. 2 exhibits the
intermolecular interactions and packing modes of the neigh-
bouring molecules. It was found that the aromatic ring and
cyano group together with the acrylamide for the three MPCPA
isomers were located on the same plane. For o-MPCPA, the
crystal structure contained eight molecules in the unit cell and
was monoclinic with space group I2/a. As depicted in Fig. S2C
(ESI†), there were two types of hydrogen bonds between adjacent
molecules with distances of 2.379 Å and 2.113 Å. The lumino-
gens adopted an antiparallel H-type packing mode in order to fit
into the crystalline lattice. Viewed from the top, the acrylamide
was placed directly above the aromatic ring of the neighbouring
molecule with an interplanar distance of 3.5329 Å (Fig. 2a).
Importantly, the aromatic rings did not overlap between the
adjacent molecules. Such an antiparallel H-type packing mode
did not result in p–p interactions, which contributed to an
enhancement of the quantum yields of the crystals. By compar-
ison, the luminogens m-MPCPA and p-MPCPA contained four
In the crystalline state, the luminophore o-MPCPA exhibited
a relatively high luminescence with a quantum yield (Ff) of
23%. In sharp contrast, the quantum yields of m-MPCPA and
p-MPCPA were decreased to 12% and 16%, respectively. As
depicted in Fig. S4 and Table S1 (ESI†), the radiative rate
constant (kF = Ff/tf) of o-MPCPA was 3.7 Â 107 sÀ1, which was
similar to that of m-/p-MPCPA (approximately 4.0 Â 107
s
À1).
However, the crystalline powder of o-MPCPA yielded a low
non-radiative rate constant [knr = (1 À Ff)/t] of 1.2 Â 108 sÀ1
,
which increased to 3.1 Â 108 sÀ1 for m-/p-MPCPA. The results
indicated the non-radiative deactivation pathways were
obviously unblocked in m-/p-MPCPA, which led to a lower Ff
as a result of the weak p–p interactions. This explanation was in
accordance with the head-to-head arrangements (Fig. 2b and c).
Fig. 3 shows the fluorescence images of o-MPCPA, m-MPCPA
and p-MPCPA samples after a cycle of grinding, heating or
solvent fuming. Of the crystalline powders, o-MPCPA, with
green luminescence, did not exhibit MCF behaviour as there
was not an obvious spectral shift upon grinding (Fig. 4A).
However, the m-MPCPA and p-MPCPA samples exhibited more
remarkable MCF properties (Fig. 4B). The white crystals of
m-MPCPA emitted blue-purple light (lmax = 452 nm) under UV
light, which changed to green (lmax = 470 nm) after the grinding
treatment. Upon fuming with solvent vapours such as CH2Cl2
and ethyl acetate, the luminescence recovered its original state
(see Fig. S6, ESI†). For p-MPCPA, the sample emitted blue-
purple light (lmax = 450 nm) under UV light, which became
green (lmax = 478 nm) after grinding treatment. Clearly, the
MCF behaviour of MPCPA depended on the change in the
Fig. 2 The packing modes and distances between adjacent molecules in
the single-crystal structures of MPCPA (o-, m- and p-).
Fig. 3 Fluorescence images of o-MPCPA, m-MPCPA and p-MPCPA
under a 365 nm UV light: as-prepared, ground, fumed and heated.
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could be recovered to the original states after solvent or heating
treatment. Thus, the MCF properties of m-/p-MPCPA could be
attributed to the transformation from the crystalline phase to
the amorphous phase, which coincided with previous results.7
Conspicuously, an evident phase transition of the o-MPCPA
sample which did not produce MCF behaviour was also observed.
What were the reasons for this abnormal phenomenon?
IR spectra of o-MPCPA powders were identical before and
after grinding (Fig. 6A). In contrast, two IR peaks at 3163 and
3366 cmÀ1 (nN–H) existed for the luminogen m-MPCPA, which
disappeared upon grinding (Fig. 6B). Further, an IR peak at
3163 cmÀ1 (nN–H) existed for the luminogen p-MPCPA, which
drifted upon grinding (Fig. 6C). Previously, researchers usually
chose a strongly twisted conjugated backbone12,13 as the
research object of MCF fluorophores. The disappearance and
drifting of some IR peaks implied that the compounds probably
Fig. 4 Fluorescence spectra of o-MPCPA, m-MPCPA and p-MPCPA in
crystalline form (K) and after grinding treatment (J).
methoxy position, and followed the order p-MPCPA
m-MPCPA 4 o-MPCPA.
4
To further investigate the MCF behaviour, differential scan- had a more planar conformation and stronger molecular inter-
ning calorimetry (DSC) measurements and powder wide-angle actions. However, since we synthesized MPCPA luminogens
X-ray diffraction patterns (PXRD) of MPCPA (o-, m-, p-) powders with planar conformations, the disappearance and drifting of
in various states were obtained. As depicted in Fig. S6 (ESI†), some IR peaks of m-MPCPA and p-MPCPA could only be caused
the ground powders of the three isomers indicated an obvious by stronger molecular interactions. Thus, the disappearance
cold-crystallization peak before the isotropic melting transi- and drifting of some IR peaks implied that the luminogens
tion, which did not exist in the crystal. These were observed at m-MPCPA and p-MPCPA probably had stronger molecular
approximately 109 1C, 85 1C and 132 1C for the ground samples interactions than those of o-MPCPA in the ground state.
of o-MPCPA, m-MPCPA and p-MPCPA, respectively. In previous
reports, the existence of a cold-crystallization peak indicated
that a ground powder was a metastable state transforming
into a more stable packing structure via annealing, which
was related to the MCF behaviour. However, in our case, the
o-MPCPA samples did not exhibit MCF properties even though
the ground o-MPCPA powders showed an evident cold-
crystallization peak (Fig. S7A, ESI†). Powder X-ray diffraction
patterns showed that the pristine powders of MPCPA clearly
exhibited numerous sharp and intense reflection peaks, which
indicated a well-defined microcrystalline structure. These
peaks of the MPCPA samples almost disappeared and became
weak, broad and diffused peaks upon grinding. The results
showed that the crystal lattice was significantly disrupted and a
new amorphous phase was formed (Fig. 5). The ground samples
Fig. 6 IR spectra of o-MPCPA, m-MPCPA and p-MPCPA in crystalline
form (black curve) and after grinding treatment (blue curve).
Fig. 5 PXRD profiles of o-MPCPA, m-MPCPA and p-MPCPA powders in the different states.
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Time-resolved fluorescence experiments further verified this phase transition, which did not induce MCF behaviour. The
hypothesis and the weighted mean lifetimes (t) are illustrated interplanar distance was generally reduced upon grinding.
in Fig. S5 and Table S1 (ESI†). The t values of o-MPCPA before However, the decrease in distance did not lead to an increase
and after grinding were similar. By contrast, the t values of the in p–p interactions, which was the reason for the abnormal
m-MPCPA and p-MPCPA samples in different states revealed behaviour of o-MPCPA. Thus, MCF behaviour is not only
significant differences: the respective values were 2.99 and dependent on molecular conformation and stacking modes,
2.62 ns for the original samples, and 4.61 and 5.22 ns for the but is also strongly related to molecular interaction.
ground samples. The increase in lifetime revealed that the
excitonic coupling was obviously enhanced. The as-prepared
crystals of o-MPCPA exhibited a quantum yield (Ff) of 23%,
which was scarcely changed upon grinding (Ff = 24%). However,
Experimental
for the luminogens m-MPCPA (Ff = 12%) and p-MPCPA (Ff = 16%), 2-Cyano-3-(2-methoxyphenyl)-2-propenamide (o-MPCPA), 2-cyano-
the quantum yields were decreased to 8% and 10% upon grinding, 3-(3-methoxyphenyl)-2-propenamide (m-MPCPA) and 2-cyano-3-(4-
respectively. Furthermore, the radiative rate constants (kF) for methoxyphenyl)-2-propenamide (p-MPCPA) were synthesized
m-MPCPA and p-MPCPA were decreased from 4.0 Â 107 sÀ1 and (Fig. 1) by a simple Knoevenagel reaction under mild conditions
6.1 Â 107 sÀ1 to 1.7 Â 107 sÀ1 and 1.9 Â 107 sÀ1, respectively. The in good yield.
results indicated that the p–p interaction was further enhanced,
Procedure for o-MPCPA: A mixture of 2-(methoxy)benzaldehyde
which blocked the radiative deactivation pathways. However, this (4.08 g, 30 mmol), 2-cyanoacetamide (2.61 g, 31 mmol) and
process did not exist for the o-MPCPA sample. What were the L-proline (0.69 g, 6 mmol) in ethanol (20 ml) was heated under
reasons that mechanical grinding did not alter the excited state of reflux for 2 h; the solid product was filtered, washed with
the o-MPCPA sample? As shown in Fig. 2a, the acrylamide was ethanol, dried (91% yield) and upon crystallization from
placed above the aromatic ring of the neighbouring molecule. The ethanol solution gave rise to the pure product. Finally, the
interplanar distance was effectively reduced upon grinding, which desired luminophore 2-cyano-3-(2-methoxyphenyl)-2-propenamide
has been demonstrated in other MCF molecules. However, the (o-MPCPA) was fully characterized by 1H NMR, 13C NMR and
decrease in distance did not lead to the enhancement of p–p HRMS. 1H NMR (500 MHz, CDCl3) d 8.81 (s, 1H), 8.19 (dd,
interactions. In sharp contrast, for m-MPCPA and p-MPCPA with J = 7.8, 1.2 Hz, 1H), 7.56–7.48 (dd, J = 7.8, 1.6 Hz, 1H), 7.07
head-to-head packing, the intermolecular interaction between the (t, J = 7.6 Hz, 1H), 6.97 (d, J = 8.5 Hz, 1H), 6.35 (d, 2H), 3.91 (s, 3H).
adjacent aromatic rings was enhanced as a result of the excitonic 13C NMR (126 MHz, CDCl3) d 162.5, 159.3, 148.8, 134.7, 129.1,
coupling. Such strong p–p interactions between the neighbouring 120.9, 120.9, 117.4, 111.2, 102.7, 55.7. HRMS (ESI) calcd for
molecules blocked the decay of the excited species through C11H10N2O2 ([M + H]+): 203.0821. Found: 203.0807. Crystallo-
radiative pathways, resulting in the altering of the excited state graphic data for o-MPCPA: C11H10N2O2, M = 202.21 g molÀ1
,
as well as the weak Ff of m-MPCPA (8%) and p-MPCPA (10%). monoclinic, a = 13.7381(9) Å, b = 8.8770(4) Å, c = 17.5640(9) Å,
In summary, luminogen o-MPCPA revealed an obvious phase b = 103.368(6)1, V = 2083.9(2) Å3, T = 293(2) K, R(int) = 0.0214, space
transition but this did not cause any variation in p–p interactions group I2/a, Dcalc = 1.289 Mg mÀ3, Z = 8; the final R indices were
between adjacent molecules, owing to the antiparallel H-type R1 = 0.0448, wR2 = 0.1153 [I 4 2s(I)], CCDC 999360.
arrangement. Hence, the luminogen o-MPCPA did not show
MCF properties despite the change in phase state.
Procedure for m-MPCPA: A mixture of 3-(methoxy)benzaldehyde
(4.08 g, 30 mmol), 2-cyanoacetamide (2.61 g, 31 mmol) and
L-proline (0.69 g, 6 mmol) in ethanol (20 ml) was heated under
reflux for 2 h; the solid product was filtered, washed with
ethanol, dried (92% yield) and upon crystallization from
ethanol solution gave rise to the pure product. Finally, the
Conclusions
In summary, we have synthesized novel luminogenic 3-aryl-2- desired luminophore 2-cyano-3-(3-methoxyphenyl)-2-propenamide
cyano acrylamide derivatives (o-MPCPA, m-MPCPA and p-MPCPA), (m-MPCPA) was fully characterized by 1H NMR, 13C NMR and
with differences in the methoxy position. It was found that the HRMS. 1H NMR (500 MHz, CDCl3) d 8.31 (s, 1H), 7.54–7.48 (m, 2H),
optical properties and stacking mode depended on the change in 7.42 (t, J = 7.8 Hz, 1H), 7.11–7.09 (m, 1H), 6.43 (s, 1H), 6.19 (s, 1H),
the methoxy position. The luminogen o-MPCPA adopted an anti- 3.87 (s, 3H). 13C NMR (126 MHz, CDCl3) d 162.1, 160.0, 154.1,
parallel H-type packing mode. However, both of the luminogens 132.8, 130.3, 123.8, 119.7, 117.1, 114.6, 103.3, 55.4. HRMS
m-MPCPA and p-MPCPA adopted head-to-head or parallel (ESI) calcd for C11H10N2O2 ([M + H]+): 203.0821. Found:
arrangements. Thus, the luminogen o-MPCPA exhibited a rela- 203.0822. Crystallographic data for m-MPCPA: C11H10N2O2,
tively higher quantum yield than the luminogens m-MPCPA M = 202.21 g molÀ1, monoclinic, a = 15.6778(10) Å, b =
and p-MPCPA due to the distinct packing mode and molecular 3.9598(3) Å, c = 17.4076(14) Å, b = 108.928(8)1, V = 1022.26(13) Å3,
interactions. The molecular conformation and stacking modes T = 293(2) K, R(int) = 0.0235, space group P2(1)/c, Dcalc = 1.314 Mg mÀ3
,
of the luminogens MPCPA (o-, m-, p-) were altered under the Z = 4; the final R indices were R1 = 0.0430, wR2 = 0.1138 [I 4 2s(I)],
stimulation of mechanical force. However, the luminogen CCDC 999359.
o-MPCPA did not exhibit MCF behaviour. As far as we know,
Procedure for p-MPCPA: A mixture of 4-(methoxy)benzalde-
this is the first example of a luminogen showing an evident hyde (4.08 g, 30 mmol), 2-cyanoacetamide (2.61 g, 31 mmol)
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and L-proline (0.69 g, 6 mmol) in ethanol (20 ml) was heated
under reflux for 2 h; the solid product was filtered, washed with
ethanol, dried (94% yield) and upon crystallization from ethanol
solution gave rise to the pure product. Finally, the desired lumino-
phore 2-cyano-3-(4-methoxyphenyl)-2-propenamide (p-MPCPA) was
fully characterized by H NMR, 13C NMR and HRMS. H NMR
(500 MHz, DMSO) d 8.11 (s, 1H), 7.97 (d, J = 8.8 Hz, 2H), 7.79
(s, 1H), 7.72–7.59 (m, 1H), 7.14 (t, J = 5.8 Hz, 2H), 3.86 (s, 3H).
13C NMR (126 MHz, DMSO) d 163.1, 162.6, 150.1, 132.4, 124.4,
117.0, 114.8, 102.9, 55.6, 54.7. HRMS (ESI) calcd for C11H10N2O2
([M + H]+): 203.0821. Found: 203.0824. Crystallographic data
for p-MPCPA: C11H10N2O2, M = 202.21 g molÀ1, monoclinic,
a = 3.920(2) Å, b = 10.792(6) Å, c = 23.124(12) Å, b = 93.463 (8)1,
V = 976.5 (9) Å3, T = 293(2) K, R(int) = 0.1143, space group P2(1)/c,
Dcalc = 1.375 Mg mÀ3, Z = 4; the final R indices were R1 = 0.0824,
wR2 = 0.2367 [I 4 2s(I)], CCDC 999361.
1
1
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Natural Science Foundation of China (51403060, 51273179),
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