Alkyl chain-dependent cyano-stilbene derivative's molecular stacking, emission enhancement and fluorescent response to the mechanical force and thermal stimulus

Article abstract of DOI:10.1016/j.dyepig.2018.12.036

Two cyano-stilbene derivatives with butyl (C4MPA) and octyl groups (C8MPA) were synthesized to investigate the effects of alkyl chains on molecular stacking, emission enhancement, and mechanofluorochromism. The two compounds displayed weak emissions in mo

Full text of DOI:10.1016/j.dyepig.2018.12.036

Accepted Manuscript
Alkyl chain-dependent cyano-stilbene derivative's molecular stacking, emission
enhancement and fluorescent response to the mechanical force and thermal stimulus
Pengchong Xue, Chunyu Zhang, Kunpeng Wang, Meng Liang, Tong Zhang
PII:
S0143-7208(18)32533-6
https://doi.org/10.1016/j.dyepig.2018.12.036
DYPI 7241
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 19 November 2018
Revised Date: 17 December 2018
Accepted Date: 17 December 2018
Please cite this article as: Xue P, Zhang C, Wang K, Liang M, Zhang T, Alkyl chain-dependent cyano-
stilbene derivative's molecular stacking, emission enhancement and fluorescent response to the
mechanical force and thermal stimulus, Dyes and Pigments (2019), doi: https://doi.org/10.1016/
j.dyepig.2018.12.036.
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Graphic abstract
Both cyano-stilbene derivatives exhibited crystallization-induced emission enhancement and the
length alkyl chains determined in luminescent efficiency and mechanochromism.

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Alkyl chain-dependent cyano-stilbene derivative’s molecular
stacking, emission enhancement and fluorescent response to the
mechanical force and thermal stimulus
Pengchong Xue,* Chunyu Zhang, Kunpeng Wang, Meng Liang, Tong Zhang
Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory
of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of Education, College of
Chemistry, Tianjin Normal University
No. 393 Bin Shui West Road, Tianjin, 300387, P. R. China.
E-mail: xuepengchong@126.com.
Abstract: Two cyano-stilbene derivatives with butyl (C4MPA) and octyl groups (C8MPA) were
synthesized to investigate the effects of alkyl chains on molecular stacking, emission enhancement,
and mechanofluorochromism. The two compounds displayed weak emissions in monomolecular
state and emitted an enhanced fluorescence in crystal state. However, the fluorescence quantum
yield of C4MPA (19%) was lower than that of C8MPA (76%). Face-to-face dimer stacking was
found only in the C4MPA crystal, in which molecules adopted a nonplanar configuration. The
C8MPA crystal presented two kinds of antiparallel 1D arrangement. One involved the antiparallel
stacking of nonplanar aromatic moieties. The other consisted of coplanar molecules without π-π
stacking, which were responsible for the higher luminescence yield. Moreover, the emission of the
C4MPA solid was quenched under mechanical force, whereas the C8MPA solid still emitted a
strong fluorescence after grinding. C8MPA lost fluorescence when heated at above 60 °C because
of its low melting point, and the supercooling viscous liquid without fluorescence was observed
even the sample was cooled to room temperature for 10 h, meaning a very slowly crystallization
process. Such non-emissive supercooling viscous liquid might rapidly transform into strongly
emissive solid under the mechanical force shearing stimulus. Thus, C8MPA films could be used as
sensors for mechanical force and thermal stimuli.

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Introduction
Stimuli-responsive organic materials have attracted considerable interest and
have been extensively utilized in advanced applications in various sensors,[1]
electronic skin,[2] drug delivery,[3] self-healing materials,[4][5] molecular
motor,[6] and so on. In particular, organic luminescence materials that respond
to mechanical force have been rapidly developed because of their wide
applications
in
sensors,
memory
chips,
and
security
inks.[7][8][9][10][11][12][13] These smart materials exhibit mechanochromism
(MC), which refers to their ability to change their emission color and intensity
after an appropriate external stress was applied.[14][15][16] The molecular
structures of these smart materials consist of pure organic molecules and metal
complexes. Almost pure MC organic molecules emit fluorescence and exhibit
mechanofluorochromic (MFC) behavior.[17][18] By contrast, metal complexes
with Pt(II), Au(I), Cu(I), and Ir(III) generally exist in an excited triplet state and
possess large shifts of emission wavelength because of the alteration of the
metal–metal interaction triggered by the mechanical force stimuli.[19][20][21]
To date, many kinds of pure organic MC molecules, including
tertraphenylethene,
[22][23]
9,10-divinylanthracene,
β-diketone
[24][25]
boron
triphenylamine,[26][27]
phenothiazine,[28][29][30]
complexes,[31][32] diiodo boron diiminates,[33] and cyano-ethylene,[33][34]
have been found to exhibit obvious MFC behavior. The results suggested that
non-planar geometry and electron pull–push structure can typically change the
luminescence color under a mechanical force.
MFC materials with an aggregation-induced emission (AIE) were rapidly
developed because they emitted an enhanced fluorescence in solid state and
were non-emissive in solution or melted state.[35][36][37] If an AIE molecule
possesses a low melting point it is possible for molecule to have a slow
crystallization rate and then would like to exhibit mechanical force-induced
luminescence enhancement (MILE) properties.[38][39][40][41] In other words,

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a non-emissive supercooling solid or liquid will transform into a strongly
emissive solid under a mechanical force stimulus due to mechanical
force-induced crystallization. However, such functional organic molecules still
are rare. In this case, two cyano-stilbene derivatives with butyl (C4MPA) and
octyl groups (C8MPA) were synthesized in the prospect of obtaining MILE
molecules. Their photophysical, MFC, and MILE properties have been
investigated. It was found that both two compounds were AIE molecule.
C4MPA exhibited emission quenching after grinding, whereas C8MPA did not
change its fluorescence intensity and color. More interestingly, C8MPA
displayed a low melting point and a slow crystallization rate, both of which
promoted the MILE behavior of C8MPA. The supercooling C8MPA viscous
liquid was non-emissive, and strongly fluorescent crystals formed under the
mechanical force stimulus. Thus, the luminescence pattern on a supercooling
film could be drawn by a round pen. Moreover, non-emissive patterns were also
written by a hot pen. These patterns would automatically disappear or self-erase
and could be written repeatedly.
Results and discussion
Photophysical properties in solutions
First, their photophysical properties in solution were investigated. The two
compounds were colorless and easily dissolved in common solvents, such as
hexane, CH2Cl2, THF, acetone, DMF, and DMSO, to form colorless solutions.
They have similar absorption and emission spectra in different solvents. Fig. 1
shows the absorption and fluorescence spectra of C4MPA. The maximal
absorption peak was located at 348 nm in cyclohexane, which shifted to 351 nm
in CH2Cl2, and 355 nm in polar DMF. Moreover, the emission band possessed
red shifts in polar solvents. For instance, C4MPA had an emission peak at 400
nm in cyclohexane and 408 nm in CH2Cl2, which slightly red-shifted to 414
nm. Such small shifts of the emission spectra in polar solvents suggest that the

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polarities of C4MPA in excited and ground state have small difference.[42][43]
Moreover, it was found that two compounds emitted very weak fluorescence in
different solvents. For example, the fluorescence quantum yields of C4MPA
and C8MPA in cyclohexane are 0.1% and 0.09%, respectively.
Fig. 1 (a) Absorption (b) emission spectra of C4MPA in different solvents.
Fig. 2 (a) Optimal geometry of C4MPA with twisted angles and (b) corresponding
frontier orbital (HOMO and LUMO) distributions.

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To elucidate the emission spectra in different solvents, quantum chemical
calculation was performed by density functional theory calculations at the
B3LYP/6-31G(d) level. As shown in Fig. 2b, the HOMO state density was
uniformly distributed at the whole aromatic unit, and the LUMO state density at
two oxygen atoms was reduced. Moreover, time-dependent DFT calculation
derived an electron transition with large oscillator strength (0.7884) at ca. 361
nm, which was close to the experimental value (348 nm) and mainly ascribed to
the transition from HOMO to LUMO (97%). The transition from HOMO to
LUMO did not induce a significant polarity change in ground and excited state.
So, energy reduction of excited state after solvent relaxation in polar solvent is
not significant, which induces the slight red shifts of the emission wavelength
in larger polar solvents.
Luminescence behavior in solid state
First, the fluorescence spectra of the crystals of two compounds were obtained.
As shown in Fig. 3, C4MPA had a red-shifted emission band with a maximum
at 455 nm relative to that (400 nm) in cyclohexane solution. Therefore, a π-π
interaction existed in the crystal phase,[44] which was confirmed by the
absorption spectrum of C4MPA. As shown in Fig. 4, a red-shifted absorption
band with a maximum peak at 365 nm was observed in C4MPA solid,
indicating π-π interaction and a head-to-tail packing between chromophores.[45]
The fluorescence quantum yield of C4MPA was determined to be 19% (Table
1), which indicated that C4MPA exhibited an enhanced emission in crystal state.
Furthermore, the fluorescence in THF/water (v/v= 1/9) was still very weak,
suggesting that the emission enhancement in the crystal should be ascribed to
ordered stacking in the crystal phase.[46][47]
To gain further insight into fluorescence enhancement, the crystals was
subjected to single-crystal analysis by simple solvent-diffusion method.[41]
Colorless block crystals of C4MPA were obtained and the single-crystal
structure was successfully analyzed. As shown in Fig. 5a, two kinds of C4MPA
with different conformations were found. The two kinds of C4MPA molecules

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presented different twist angles between the phenyl rings and the cyano or
ethylene moieties. The twist angles were between 7.24° and 22.61°, indicating
nonplanar conformations.[48] No long-distance close packing was detected,
and only dimers consisting of different molecules emerged. Many dimers
formed a herringbone packing (Fig. 5b) without extra π-π interaction. Moreover,
the distances between two molecules in the dimers were short. For example, the
shortest distance between two carbon atoms of the ethylene and phenyl
moieties was 3.48 Å. A short distance was observed between the phenyl and
oxygen atoms (3.43 Å), which indicated a strong π-π interaction. The large slip
angle of ca. 67° between the two C4MPA molecules denoted a J-aggregate,[49]
which will induce a bathochromic shift of the absorption spectra. In addition,
the free rotation of single bonds and isomerization will be suppressed, and an
enhanced emission can thus be expected.[50]
Fig. 3 Normalized fluorescence spectra of C4MPA and C8MPA in pristine and
ground states.

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Table 1. Photophysical data of C4MPA and C8MPA in crystal phases.
λabs (nm) λem (nm) ФF
(%) (ns) (109s-1) (109 s-1) (°C)
19 0.67 0.28 0.12 82.5
76 96.46 0.0079 0.0025 55
τ
kr
knr
m.p
C4MPA
365
455
476
C8MPA 345, 361
The luminescence behavior of C8MPA in crystal state was investigated as well.
A rod-like crystal was obtained by solvent-diffusion method. Under 365 nm
light, the rod-like crystal emitted a very strong cyan fluorescence, and the
fluorescence quantum yield reached 76%. The maximal emission peak was
located at 476 nm, suggesting that a π-π interaction probably existed between
the chromophores. The single-crystal structure of C8MPA was successfully
obtained, and two kinds of molecular conformations existed in the C8MPA
crystal.
Fig. 4 Normalized absorption spectra of C4MPA in cyclohexane solution and crystal,
and C8MPA in crystal.

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Fig. 5 (a) Dimer of C4MPA with twist angles and the distances between adjacent two
molecules, and (b) molecular stacking in crystal phase.
Fig. 6 (a) Dimer of C8MPA with twist angles and the distances between adjacent two
molecules, and (b) molecular stacking in crystal phase.
The aromatic moiety of one molecule, named C8MPA-N, adopted nonplanar
conformation (Fig. 6a). The twist angle between the terminal phenyl and cyano
moieties reaches 26.57°, and the long alkyl chain adopted a folded
conformation. The twist angles of the other molecule, named C8MPA-C, were
as small as 0.70° and 3.34°, and the alkyl chain was an all-trans configuration.

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Moreover, the two kinds of molecules formed individual 1D anti-parallel
stacking (Fig. 6b). A long distance of 3.72 Å was observed between the
C8MPA-N molecules. By contrast, no effective overlap was found between
coplanar C8MPA-C molecules, and the distance between two chromophores of
the two adjacent C8MPA-C molecules reached up to 7.68 Å, suggesting the
absence of π-π stacking between the C8MPA-C molecules.[51] The absorption
spectrum of the C8MPA crystal exhibited a wide band with two maxima at 345
and 361 nm (Fig. 4). The former originated from C8MPA-C in monomolecular
state, and the latter was attributed to C8MPA-N in aggregate state. A longer
emission wavelength of C8MPA relative to C4MPA implies that C8MPA-C
should be responsible for stronger emission because of absence of π-π stacking
and a larger conjugation degree.
Fig. 7 Time-resolved fluorescence spectra of (a) C4MPA and (b) C8MPA crystals.

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To further understand the luminescence difference of the two compounds in
crystal state, their time-resolved fluorescence spectra were analyzed. As shown
in Fig. 7a and Table 1, the emission decay of C4MPA was single-exponential,
and the lifetime was 0.67 ns. Considering a ФF of 19%, the radiative rate
constant (Kr) was determined to be 0.28
constant (Knr) was 0.12
109 s-1. In comparison, the decay of the C8MPA
×
109 s-1, and the non-radiative rate
×
fluorescence was extremely slow (Fig. 7b), and the corresponding lifetime
reached 96.46 ns. The long lifetime indicated the slow deactivation of excited
molecules. Kr was 7.9
× ×
106 s-1, and Knr was 2.5 106 ns-1. Comparison of the Kr
and Knr of the two compounds implied that the high ФF of C8MPA resulted
from the slow non-radiative transition.[52] Combined with the single-crystal
structure, coplanar C8MPA-C should be responsible for the higher ФF of
C8MPA crystals.
Fig. 8 Images of C4MPA in pristine solid state, after shearing by round pen, and then
thermal annealing by heating at 70 °C for 5 min.
Luminescence response to mechanical force and thermal stimuli
As suggested by other research teams, cyano ethylene derivatives could exhibit
mechanical force-responsive properties. Therefore, their MFC behavior was
investigated. As the two compounds had different stacking models in crystal
phase, different responsive features to mechanical force stimuli were expected.
As shown in Fig. 8, the C4MPA solid film emitted blue fluorescence, and the
shearing by a round pen led to a quenched fluorescence stamp. The ФF of
ground solid decreased to 10%. The maximal emission peak of the sheared
solid was located at 450 nm, indicating a blue-shift of only 5 nm (Fig. 3a). The
thermal annealing by heating could induce the disappearance of the shearing
stamp, thereby indicating a fluorescence recovery. Moreover, the recovered

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solid film could be stamped again, and such a process could be reversibly
repeated many times. The nuclear magnetic resonance spectrum of the shearing
solid after shearing-annealing cycles was consistent with the original one,
suggesting that the mechanical pressure did not damage the chemical
structure.[53] The XRD pattern was measured to investigate the phase
transition after the mechanical force stimulus. As shown in Fig. S1a, the
pristine solid possessed many sharp and strong diffraction peaks. Similar to
those of the pristine solid, the diffraction peaks of the ground solid remained
visible, and the peak intensity of only the ground solid weakened. This result
suggested that shearing solid only partially destroyed the ordered arrangement
of crystals, which may explain why the fluorescence change of C4MPA under
mechanical force stimulus was not obvious.[54][55]
The fluorescence response of C8MPA to mechanical force was also
investigated. An invisible change in fluorescence color or intensity after
grinding was observed by the naked eyes. The fluorescence quantum yield of
the ground solid was 75%, and the maximal emission peak was located at 475
nm. The diffraction peaks of the ground solid were similar to those of the
pristine solid. Therefore, grinding did not induce a mechanochromism of
C8MPA. Interestingly, we found that C8MPA formed supercooling viscous
liquid when it was heated at 70 °C and cooled to room temperature, and
supercooling liquid was non-emissive (Fig. S2). Such non-emissive viscous
liquid kept its liquid state even after 10 h without scarping. It was found that
C8MPA started to crystallize when a thin glass rod was used to rub sample.
After several minutes whole droplet turned into white solid and emitted very
strong fluorescence. Moreover, the C8MPA solid film with an intense cyan
fluorescence on a weighing paper lost its fluorescence when heated to 60 °C
and a strong cyan fluorescence began to appear after 5 min. The complete
recovery of the whole fluorescence required 1 h, suggesting a slow
crystallization process too. Such a slow crystallization process was because that
the long alkyl chain and the two kinds of molecular conformations in crystal

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prolonged ordered stacking time. On the other hand, melted C4MPA could
change into solid within 2 min when it was cooled to room temperature.
Therefore, C8MPA may be used as an excellent thermal responsive
fluorescence material.
As shown in Fig. 9, the C8MPA solid film emitted strong fluorescence, and two
letters, “TJ”, might be easily written by a hot pen (70 °C). The letters remained visible
after 5 min because of the slow crystallization rate and almost disappeared after 15
min. Thus, the information in the film may be automatically erased. Moreover, a new
letter might be rewritten by a hot pen, thereby demonstrating a reversible process.
Moreover, a regular pattern without fluorescence could be imprinted easily by a hot
stamp. When heated using three hot types (70 °C) for 5 s, the fluorescence of the
contact region disappeared, and three Chinese characters emerged in the luminescent
background. These characters remained visible after 5 min and nearly disappeared
after 15 min (Fig. S3). Therefore, such a fluorescence film may be used in temporary
information storage or recycled paper. In addition, shearing a melted C8MPA film in
a filter paper by mechanical force might accelerate the crystallization process. As
shown in Fig. 10, the supercooling film at room temperature was emissive silent. Two
Chinese characters with strong cyan fluorescence rapidly appeared along the track of
the pen. In view of the slow crystallization speed in other regions without mechanical
force shearing, the two characters remained were still visible after 5 min and
completely disappeared after 30 min because the crystallization was completed.
Moreover, this film could be reused when heated at 70 °C for 2 min.

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Fig. 9. Photos of C8MPA film in a filter paper under 365 nm light. (a) Pristine solid
film, (b) after written “TJ” by a hot pen, naturally left for (c) 5 min and (d) 15 min.
Fig. 10 Photos of supercooling C8MPA filter paper film under 365 nm light. (a)
Pristine supercooling film, (b) written by ground pen, naturally left for (c) 5 min and
(d) 60 min.
Conclusions
Two cyano-stibene derivatives with different alkyl chains were prepared, and their
photophysical properties and response to mechanical force were investigated. Both

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compounds exhibited a crystallization-induced emission enhancement. They
displayed a very weak fluorescence in solution state and emitted an enhanced
fluorescence in crystals. C4MPA with a butyl group possessed a 19% ΦF, and a high
ΦF of 76 % for C8MPA with octyl unit was observed. The single-crystal structure and
the time-resolved fluorescence spectra suggested coplanarity, and the 1D sacking in
the absence of π-π stacking in C8MPA crystal suppressed the non-radiative transition,
which improved the photoluminescence performance. Moreover, they exhibited
different responses to mechanical force. The fluorescence of C4MPA was quenched
after shearing or grinding, whereas that of C8MPA remained nearly unchanged under
mechanical force stimuli. More interestingly, the long alkyl chain endows C8MPA
with a low melting point and slow crystallization rate, which make it a promising
candidate for optical recording and temperature or force sensing. This work suggests
that cyano ethylene moiety is a better building block for constructing intense
fluorescence materials, and that the alkyl chain length may endow molecules with
distinct properties.
Experimental Section
4.1 Measurements and characterization
The UV-vis spectra were obtained on a Mapada UV-1800pc spectrophotometer. C, H,
and N elemental analyses were performed on a PerkinElmer 240C elemental analyzer.
Mass spectra were obtained with AXIMA CFR MALDI-TOF (Compact) mass
spectrometer. Fluorescence emission spectra were obtained on a FL-3 fluorescence
spectrophotometer. The absolute fluorescence quantum yields were measured on an
Edinburgh FL-3 steady state spectrometer using an integrating sphere. Luminescent
decay experiments were measured on an Edinburgh FL980 spectrometer. EPLED-360
picosecond flash lamp with 898ps pulse duration was used to measure time-resolved
fluorescent spectra. NMR spectra were recorded on a Mercury plus 500 MHz
instrument. X-ray powder diffraction patterns were obtained on a Bruker D8
ADVANCE X-ray diffractometer equipped with scintillation detector using copper

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CuKα (λ= 1.5406 Å) radiation and a scanning rate of 0.02 s−1 (from 5 to 30°).
Geometrical optimizations were performed by density functional theory (DFT)
calculations at the B3LYP/6-31G level with the Gaussian 09W program package.
Single crystals of C4MPA and C8MPA were obtained by solvent-diffusion
(n-hexane/dichloromethane) method and were selected for X-ray diffraction studies in
a Rigaku RAXIS-RAPID diffractometer using graphite-monochromated MoKα
radiation (λ= 0.71073 Å). The crystals were kept at room temperature during data
collection. The structures were solved by direct methods and refined on F2 by
full-matrix least-squares using the SHELXTL-97 program. The C, N, O and H atoms
were easily placed from the subsequent Fourier difference maps and refined
anisotropically. CCDC 1062078 and 1869506 contain the supplementary
crystallographic data for C4MPA and C8MPA.
CnMPA were synthesized by two-step reactions. Synthesis route is shown in
Scheme 1. Compound An were synthesized by previously reported
procedure.[56] CnPMA were obtained by a condensation reaction in the
presence of tetrabutylammonium hydroxide (TBAOH) as catalyst.
Scheme 1. Synthesis route of
CnMPA.
(Z)-3-(4-butoxy-3-methoxyphenyl)-2-phenylacrylonitrile (C4MPA)
A4 (2.0 g, 9.6 mmol), phenylacetonitrile (1.66 mL, 14.4 mmol) and 0.2 mL
tetrabutylammonium hydroxide (TBAOH, 40% in water) were added in 20 mL
ethanol. The mixture was heated and refluxed for 10 h. The mixture was cooled to

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room temperature and the solid separated out. The product (2.3 g) was obtained by
vacuum suction filtration and washing by cooled ethanol. Yield: 79 %. m.p: 82-83 °C.
1H NMR (500 MHz, CDCl3) 7.73 (d, J = 1.6 Hz, 1H), 7.68 (d, J = 7.5 Hz, 2H),
7.50-7.42 (m, 3H), 7.40 (d, J = 7.2 Hz, 1H), 7.37 (dd, J = 8.4, 1.9 Hz, 1H), 6.94 (d, J =
8.4 Hz, 1H), 4.16-4.03 (m, 2H), 3.98 (s, 3H), 1.96-1.78 (m, 2H), 1.63-1.47 (m,
2H),1.02 (t, J = 7.4 Hz, 3H). MS (MALDI-TOF), m/z: cal: 307.2, found: 307.5.
Element analysis (%): calculated for C20H21NO2: C, 78.15; H, 6.89; N, 4.56; Found: C,
78.18; H, 6.82; N, 4.59.
(Z)-3-(3-methoxy-4-(octyloxy)phenyl)-2-phenylacrylonitrile (C8MPA)
By following the synthetic procedure for C4MPA. Yellowish solid (3.0g) was
obtained. Yield: 76 %. m.p: 54-56 °C. 1H NMR (500 MHz, CDCl3), 7.71 (d, J =
2.0 Hz, 1H), 7.65 (d, J = 7.5 Hz, 2H), 7.46-7.40 (m, 3H), 7.37 (d, J = 7.3 Hz,
1H), 7.34 (dd, J = 8.4, 2.0 Hz, 1H), 6.92 (d, J = 8.4 Hz, 1H), 4.08 (t, J = 6.9 Hz,
2H), 3.96 (s, 3H), 1.93-1.81 (m, 2H), 1.50-1.40 (m, 2H), 1.41-1.21 (m, 8H),
0.89 (t, J = 6.9 Hz, 3H). MS (MALDI-TOF), m/z: cal: 419.3, found: 419.5.
Element analysis (%): calculated for C24H29NO2: C, 79.30; H, 8.04; N, 3.85;
Found: C, 79.30; H, 8.04; N, 3.85.
Acknowledgements
This work was supported by the Scientific Research Foundation of Tianjin Normal
University (Grant No. 5RL151), the Foundation of Development Program of Future
Expert in Tianjin Normal University (WLQR201804) and the Science &Technology
Development Fund of Tianjin Education Commission for Higher Education
(2018ZD12).
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1. Two cyano-stilbene derivatives exhibited crystallization-induced emission
enhancement.
2. Crystal stacking models of two compounds are different.
3. Supercooling liquid C8MPA emitted enhanced fluorescence after shearing.