A large dipole moment to promote gelation for 4-nitrophenylacrylonitrile derivatives with gelation-induced emission enhancement properties

Article abstract of DOI:10.1039/c4ob00768a

A series of 4-nitrophenylacrylonitrile and phenylacrylonitrile derivatives consisting of a carbazole moiety was synthesized. Some of these derivatives with longer alkyl chains and a nitro group could gelatinize some organic solvents, such as ethanol, n-butanol, ethyl acetate, and DMSO. By contrast, phenylacrylonitrile derivatives did not form gels in measured solvents. This result proved that the electron-withdrawing nitro moiety was important for gel formation because it conferred the molecules with large dipole moments, which enhanced the intermolecular interaction. Analyses by UV-vis absorption, X-ray diffraction, and scanning electron microscopy showed that the gelator molecules could self-assemble into one-dimensional nanofibers with layer packing, which further twisted into thicker fibers and formed three-dimensional networks in the gel phase. The single crystal structure of C4CNPA implied that the gelators might adopt an anti-parallel molecular stacking because of their larger ground-state dipole moment. Interestingly, the organogels had enhanced fluorescence relative to solutions at the same concentrations. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00768a

Organic &
Biomolecular Chemistry
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A large dipole moment to promote gelation for
4-nitrophenylacrylonitrile derivatives with
gelation-induced emission enhancement properties†
Cite this: Org. Biomol. Chem., 2014,
12, 7110
Pengchong Xue,*a Boqi Yao,a Yuan Zhang,b Peng Chen,c Kechang Li,d Baijun Liud
and Ran Lu*a
A series of 4-nitrophenylacrylonitrile and phenylacrylonitrile derivatives consisting of a carbazole moiety
was synthesized. Some of these derivatives with longer alkyl chains and a nitro group could gelatinize
some organic solvents, such as ethanol, n-butanol, ethyl acetate, and DMSO. By contrast, phenylacrylo-
nitrile derivatives did not form gels in measured solvents. This result proved that the electron-withdraw-
ing nitro moiety was important for gel formation because it conferred the molecules with large
dipole moments, which enhanced the intermolecular interaction. Analyses by UV-vis absorption, X-ray
diffraction, and scanning electron microscopy showed that the gelator molecules could self-assemble
into one-dimensional nanofibers with layer packing, which further twisted into thicker fibers and formed
three-dimensional networks in the gel phase. The single crystal structure of C4CNPA implied that the
gelators might adopt an anti-parallel molecular stacking because of their larger ground-state dipole moment.
Interestingly, the organogels had enhanced fluorescence relative to solutions at the same concentrations.
Received 15th April 2014,
Accepted 18th July 2014
DOI: 10.1039/c4ob00768a
www.rsc.org/obc
amount of low-molecular mass compounds, have been widely
developed in recent years. Their bulk shape and properties can
Introduction
Researchers and engineers have significantly focused on be either switched or tuned by an external chemical or physical
stimulus-active functional materials because these materials stimulus, such as metal ions, anions, small organic com-
can change their physical or chemical properties upon pounds, protons, light irradiation, oxidation or reduction reac-
exposure to particular stimuli, such as heat, electricity, light, tion, sound and temperature by introducing functional units.2
magnetic, solvent, and pH.1 These unique characteristics Thus, such supramolecular gels are considered as smart
enable stimulus-active materials to be used in many fields, and versatile functional materials for applications in solar
such as smart textiles and apparels, intelligent medical instru- cells,3 ion and molecular recognition,4 light switches,5 logic
ments and auxiliaries, artificial muscles, biomimetic devices, gates,6 biomimetic systems,7 and electronics.8 Among various
heat shrinkable materials, self-deployable sun sails in space- supramolecular gels, fluorescent organogels have attracted
crafts, miniature manipulators, actuators, and molecular intense interest and are extensively studied because of their
motors and sensors. In particular, smart functional organo- promising applications in optoelectronics and fluorescent
gels, formed by a large amount of organic solvents and a small sensors.
In this study, we designed and synthesized a series of 4-nitro-
phenylacrylonitrile and phenylacrylonitrile derivatives to obtain
aState Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun, P. R. China.
temperature-controlled fluorescent switches. Some derivatives
with longer alkyl chains and a nitro group could form gels in
some organic solvents. However, phenylacrylonitrile derivatives
without the nitro group were not gelators. This result suggests
that the electron withdrawing nitro moiety is important for gel
formation because the nitro group increases the molecular
dipole moment and then enhances the intermolecular inter-
action, which was confirmed by the single crystal structure of
C4CNPA. Interestingly, the organogels formed by CnCNPA could
be used as fluorescent switches controlled by temperature
because gels have stronger fluorescence relative to solutions.
E-mail: xuepengchong@jlu.edu.cn, luran@mail.jlu.edu.cn
bCollege of Chemistry, Beijing Normal University, Beijing, 100875, P. R. China
cKey Laboratory of Functional Inorganic Material Chemistry (MOE), School of
Chemistry and Materials Science, Heilongjiang University, Harbin, P. R. China
dCollege of Chemistry, Jilin University, Changchun, P. R. China
†Electronic supplementary information (ESI) available: The calculated absorp-
tion spectrum and optimal structure, photos of gels, the absorption spectra of
three gelators in solution and gels, the absorption spectra of C4CNPA in solution
and crystals, XRD patterns of DMSO gels, crystal data of C4CNPA, plots of Tgel
s
vs. concentration, and CCDC 993696. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4ob00768a
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Experimental section
Instruments
Infrared spectra were measured using a Nicolet-360 FT-IR
spectrometer by incorporating the samples in KBr disks. The
UV-vis absorption spectra were determined on a Mapada
UV-1800pc spectrophotometer. C, H, and N elemental analyses
were performed on a Perkin-Elmer 240C elemental analyzer.
Photoluminescence measurements were carried out on a
Shimadzu RF-5301 Luminescence Spectrometer. The fluore-
scence spectra of gels were recorded in a 1 mm cell by right
angle observation. NMR spectra were obtained on a Mercury
plus 500 MHz spectrometer. Mass spectra were obtained using
Agilent 1100 MS series and AXIMA CFR MALDI-TOF (Compact)
mass spectrometers. SEM images were obtained on a Japan
Hitachi model X-650 Scan electron microscope. The samples
for SEM observation were prepared by a method of drop-
casting. The samples were then kept overnight in a vacuum
Scheme 1 Synthesis route to CnCNPA and CnCPA.
oven at room temperature followed by coating of gold. Small (0.84 g, 5.2 mmol) and ethanol (20 mL) were added to a
angle X-ray diffraction (XRD) patterns were obtained at λ = 100 mL round-bottomed flask. After the solid was dissolved
1.5418 Å, by employing a scanning rate of 0.02° s−1 in the 2θ upon heating, 1 mL diethylamine was added and the mixture
range of 1.1 to 10°. The samples were prepared by casting the was refluxed for 10 h. And then, the solution was cooled to
wet gels on glass slides and drying at room temperature. The room temperature. The product was obtained by vacuum
fluorescence quantum yields (Φ) of CnCNPA in toluene were suction filtration and washing with cold ethanol. Yield: 88%.
measured by comparing with a standard (9,10-diphenylanthra- mp: 216.5 °C. Elemental analysis (%): calculated for
cene in benzene with a Φ of 0.85). The excitation wavelength C25H21N3O2: C, 75.93; H, 5.35; N, 10.63; Found: C, 76.11;
was 390 nm. The optimal molecular configurations of C1CNPA H, 5.38; N, 10.59. 1H NMR (CDCl3, 500 MHz, ppm), δ = 8.68
and C1CPA were obtained by density functional theory (DFT) (s, 1H), 8.27 (d, J = 8.6 Hz, 2H), 8.16 (t, J = 8.1 Hz, 2H),
calculation at the B3LYP/6-31G level with the Gaussian 09W 7.87–7.78 (m, 3H), 7.53 (t, J = 7.6 Hz, 1H), 7.47 (d, J = 8.5 Hz,
program package.9
1H), 7.45 (d, J = 8.0 Hz, 1H), 7.31 (t, J = 7.4 Hz, 1H), 4.33 (t, J =
Single crystals were obtained in the toluene solution by 7.2 Hz, 2H), 1.93–1.79 (m, 2H), 1.43–1.33 (m, 2H), 0.97 (t, J =
slow solvent vaporization. Single crystals of C4CNPA were 7.3 Hz, 3H). MS (MALDI-TOF), m/z: cal: 395.16, found: 395.10.
selected for X-ray diffraction analysis on a Rigaku RAXIS-
(Z)-3-(9-Octylcarbazolyl)-2-(4-nitrophenyl)acrylonitrile (C8CNPA).
RAPID diffractometer using graphite-monochromated Mo-Kα By following the synthetic procedure for C4CNPA, C8CNPA was
radiation (λ = 0.71073 Å). The crystals were kept at room temp- synthesized with compound 3b (1.7 g, 5.6 mmol), 4-nitro-
erature during data collection. The structures were solved by phenylacetonitrile (1.0 g, 6.2 mmol), and diethylamine (1 mL).
direct methods and refined on F2 by full-matrix least-square Yield: 85%. mp: 158.5 °C. Elemental analysis (%): calculated
using the SHELXTL-97 program.10 The C, N, O and H atoms for C29H29N3O2: C, 77.13; H, 6.47; N, 9.31; Found: C, 77.16; H;
1
were easily located from the subsequent Fourier-difference 6.43; N, 9.39. H NMR (CDCl3, 500 MHz, ppm), δ = 8.71 (d, J =
maps and refined anisotropically. CCDC 993696 contains the 2.0 Hz, 1H), 8.31 (t, J = 8.6 Hz, 2H), 8.18 (t, J = 8.1 Hz, 2H),
supplementary crystallographic data for this paper.
7.88–7.86 (m, 3H), 7.53 (t, J = 7.6 Hz, 1H), 7.49 (d, J = 8.5 Hz,
1H), 7.46 (d, J = 8.0 Hz, 1H), 7.32 (t, J = 7.4 Hz, 1H), 4.34 (t,
J = 7.0 Hz, 2H), 1.93–1.88 (m, 2H), 1.33–1.23 (m, 10H), 0.87 (t,
Gelation test
The solution containing a weighed compound in an organic J = 7.0 Hz, 3H). MS (MALDI-TOF), m/z: cal: 451.23, found:
solvent was heated in a sealed test tube of 1 cm diameter in an 451.20.
oil bath until the solid was dissolved. After the solution was
(Z)-3-(9-Dodecylcarbazolyl)-2-(4-nitrophenyl)acrylonitrile
allowed to stand at room temperature for 6 h, the state of the (C12CNPA). By following the synthetic procedure for C4CNPA,
mixture was evaluated by the “stable to inversion of a test C12CNPA was synthesized. Yield: 84%. mp: 131.5 °C. Ele-
tube” method.
mental analysis (%): calculated for C33H37N3O2: C, 78.07;
H, 7.35; N, 8.28; Found: C, 78.05; H, 7.30; N, 8.23. 1H NMR
(CDCl3, 500 MHz, ppm), δ = 8.70 (s, 1H), 8.30 (d, J = 8.7 Hz,
Synthetic procedures and characterization
The synthesis route of compounds CnCNPA and CnCPA was 2H), 8.23–8.11 (m, 2H), 7.86 (d, J = 8.9 Hz, 3H), 7.53 (t, J =
shown in Scheme 1. 4-Nitrophenylacetonitrile, 2a–c and 3a–d 7.6 Hz, 1H), 7.48 (d, J = 8.7 Hz, 1H), 7.45 (d, J = 8.2 Hz, 1H),
were synthesized by the procedures reported previously.11
7.32 (t, J = 7.4 Hz, 1H), 4.33 (t, J = 7.2 Hz, 2H), 1.95–1.82
(Z)-3-(9-Butylcarbazolyl)-2-(4-nitrophenyl)acrylonitrile (m, 2H), 1.42–1.20 (m, 18H), 0.87 (t, J = 6.8 Hz, 3H). MS
(C4CNPA). 3a (1.0 g, 4.0 mmol), 4-nitrophenylacetonitrile (MALDI-TOF), m/z: cal: 507.29, found: 507.23.
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(Z)-3-(9-Hexadecylcarbazolyl)-2-(4-nitrophenyl)acrylonitrile (m, 18H), 0.87 (t, J = 7.0 Hz, 3H). MS (MALDI-TOF), m/z: cal:
(C16CNPA). By following the synthetic procedure for C4CNPA, 462.30, found: 462.31.
C16CNPA was synthesized. Yield: 86%. mp: 81.0 °C. Elemental
(Z)-3-(9-Hexadecylcarbazolyl)-2-phenyl-acrylonitrile (C16CPA).
analysis (%): calculated for C37H45N3O2: C, 78.83; H, 8.05; By following the synthetic procedure for C8CPA, C16CPA was
N, 7.45; Found: C, 78.75; H, 8.00; N, 7.48. 1H NMR (CDCl3, synthesized. Yield: 83%. Elemental analysis (%): calculated for
500 MHz, ppm) δ = 8.74 (d, J = 1.6 Hz, 1H), 8.37–8.31 (m, 2H), C37H45N2: C, 85.66; H, 8.94; N, 5.40; Found: C, 85.63; H, 8.89;
8.22 (dd, J = 8.7, 1.7 Hz, 1H), 8.19 (d, J = 7.8 Hz, 1H), 7.93–7.87 N, 5.43. 1H NMR (CDCl3, 500 MHz, ppm), δ = 8.64 (d, J =
(m, 3H), 7.56 (t, J = 7.2 Hz, 1H), 7.52 (d, J = 8.7 Hz, 1H), 7.49 1.3 Hz, 1H), 8.19–8.12 (m, 2H), 7.74–7.69 (m, 3H), 7.51 (t, J =
(d, J = 8.2 Hz, 1H), 7.35 (t, J = 7.5 Hz, 1H), 4.37 (t, J = 7.3 Hz, 7.7 Hz, 1H), 7.48–7.46 (m, 2H), 7.44 (t, J = 6.0 Hz, 2H), 7.37 (t,
2H), 1.97–1.90 (m, 2H), 1.46–1.23 (m, 26H), 0.90 (t, J = 7.0 Hz, J = 7.3 Hz, 1H), 7.29 (t, J = 7.3 Hz, 1H), 4.32 (t, J = 7.3 Hz, 2H),
3H). MS (MALDI-TOF), m/z: cal: 563.35, found: 563.30.
1.93–1.88 (m, 2H), 1.44–1.21 (m, 26H), 0.88 (t, J = 6.9 Hz, 3H).
(Z)-3-(9-Octylcarbazolyl)-2-phenyl-acrylonitrile (C8CPA). MS (MALDI-TOF), m/z: cal: 518.37, found: 518.30.
After 3b (1.2 g, 3.9 mmol) and phenylacetonitrile (0.49 mL,
4.3 mmol) in ethanol were dissolved upon heating, 0.2 mL
tetrabutylammonium hydroxide (TBAOH, 40% in water) was
added and the mixture was refluxed for 10 h. And then, the
solution was cooled to room temperature. The product was
obtained by vacuum suction filtration and washing with cold
Results and discussion
Synthesis
Scheme 1 shows the synthesis routes to CnCNPA and CnCPA.
Compound 1 was obtained through the nitration reaction of
benzyl cyanide in a mixed acid. 3a–d could be obtained by the
Vilsmeier reaction of the corresponding 2a–d with DMF in the
ethanol. Yield: 85%. mp: Elemental analysis (%): calculated
for C29H30N2: C, 85.67; H, 7.44; N, 6.89; Found: C, 85.69; H,
1
7.41; N, 6.86. H NMR (CDCl3, 500 MHz, ppm), δ = 8.64 (d, J =
presence of POCl3. CnCNPA and CnCPA were synthesized by a
1.7 Hz, 1H), 8.15 (dd, J = 13.3, 4.7 Hz, 2H), 7.72 (dd, J = 5.4,
simple Knoevenagel reaction in the presence of different cata-
3.2 Hz, 3H), 7.54–7.49 (m, 1H), 7.47 (d, J = 7.0 Hz, 2H),
7.46–7.42 (m, 2H), 7.37 (t, J = 7.4 Hz, 1H), 7.29 (dd, J = 11.3,
4.3 Hz, 1H), 4.32 (t, J = 7.3 Hz, 2H), 1.89 (m, 2H), 1.43–1.20
lysts. The stronger acidity of the methylene moiety in com-
pound 1 needs a weak base (diethylamine) as a catalyst, but
TBAOH as a strong base is necessary for benzyl cyanide
because of its weak activity. The products were characterized
(m, 10H), 0.89–0.81 (t, J = 7.0 Hz, 3H). MS (MALDI-TOF), m/z:
cal: 406.24, found: 406.25.
1
by H NMR spectroscopy, MALDI-TOF mass spectrometry, and
(Z)-3-(9-Dodecylcarbazolyl)-2-phenyl-acrylonitrile (C12CPA).
C, H, N elemental analyses.
By following the synthetic procedure for C8CPA, C12CPA was
synthesized. Yield: 86%. Elemental analysis (%): calculated for
C33H37N2: C, 78.07; H, 7.35; N, 8.28; Found: C, 78.03; H, 7.21;
Gelation ability in organic solvents
N, 8.22. 1H NMR (CDCl3, 500 MHz, ppm), δ = 8.65 (d, J = Through the standard heating-and-cooling method,12 the gela-
1.3 Hz, 1H), 8.15 (dd, J = 13.7, 4.8 Hz, 2H), 7.75–7.69 (m, 3H), tion abilities of CnCNPA and CnCPA were investigated. We found
7.51 (t, J = 7.2 Hz, 1H), 7.47 (d, J = 6.3 Hz, 2H), 7.45 (dd, J = that the gelation properties were strongly affected by the nitro
10.6, 5.0 Hz, 3H), 7.37 (t, J = 7.4 Hz, 1H), 7.29 (t, J = 7.3 Hz, group and the alkyl chain (Table 1). For example, CnCNPA with
1H), 4.32 (t, J = 7.2 Hz, 2H), 1.94–1.84 (m, 2H), 1.43–1.20 C8, C12, and C16 alky chains could form gels in DMSO and
Table 1 Gelation properties in different organic solventsa
Solvent
C4CNPA
C8CNPA
C12CNPA
C16CNPA
C8CPA
C12CPA
C16CPA
Chloroform
Cyclohexane
Dichloromethane
n-Hexane
Petroleum ether
THF
Acetone
Ethyl acetate
Toluene
o-Dichlorobenzene
DMF
DMSO
Methanol
Ethanol
S
I
S
I
S
I
S
I
I
S
S
P
S
P
I
S
P
P
S
P
S
P
P
S
P
P
S
S
S
P
P
S
S
S
S
S
S
S
P
P
S
S
P
S
S
S
S
S
S
S
S
S
S
S
S
P
S
S
S
P
S
S
S
S
S
S
S
S
S
S
S
P
P
P
S
S
S
I
S
P
P
S
S
S
P
P
P
P
P
P
G (18)b
G (18)
S
S
S
S
S
S
S
S
S
G (10)
G (1.2)
G (2.0)
P
G (4.1)
G (2.5)
G (1.8)
G (8.8)
G (6.7)
P
G (4.7)
G (5.4)
G (7.1)
P
n-Butanol
t-Amyl alcohol
tert-Butanol
P
P
G (7.9)
G (10)
a G: stable gel formed at room temperature; S: soluble; I: insoluble; P: precipitate. b Minimal gelation concentration (MGC, mM).
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some alcohols. However, the CnCPA solutions in these solvents to 413 nm in CHCl3, and to 421 in DMF. The red-shifted
remained in the solution state, or the precipitate was separated absorption peaks in polar solvents suggest that C8CNPA has a
from the solutions. Although the two types of compounds had larger dipole moment in the excited state than in the ground
high solubility and maintained the solution state in some sol- state,15 which can be confirmed by solvent-dependent fluo-
vents, such as CH2Cl2, CHCl3, THF, benzene, and DMF, the solu- rescence spectra. An emission peak at 449 nm for C8CNPA in
bility of CnCPA and CnCNPA was different in n-hexane, cyclohexane was found (Fig. 1b), which indicates a Stokes shift
cyclohexane, acetone, ethyl acetate, and n-butanol. CnCNPA with of 2480 cm−1. In THF, the emission peak was located at
the nitro unit had lower solubility in these solvents. These results 542 nm, implying green fluorescence and a Stokes shift of
suggest that the nitro moiety is very important for compounds to 5881 cm−1. With the further increase in the solvent polarity,
form gels in organic solvents.13 The gelation ability of CnCNPA the fluorescence exhibited a continued red shift, whereas the
in the given solvents is relative to the length of the alkyl chain.14 emission color changed to yellow, orange, and red in CHCl3,
C4CNPA is not the gelator for selected solvents. Only C8CNPA CH2Cl2 and DMF, respectively.16 The largest Stokes shift was
could gelatinize acetone and ethyl acetate at high concentrations, observed in DMF (7386 cm−1). Such a significant red-shift of
whereas C12CNPA and C16CNPA separated as depositions from the emission band in polar solvents indicates an intramolecu-
their solutions in two solvents. C12CNPA is formed alone in lar charge-transfer (ICT) characteristic for the excited state.17
A
n-butanol and t-amyl alcohol. The minimal gelation concen- linear relationship between the emission maximum energy
tration of C8CNPA in gelation solvents except that in ethanol was and the Lippert solvent polarity instead of cyclohexane was
generally lower than that of C12CNPA and C16CNPA.
found (Fig. 1c), and the emission band in cyclohexane was
narrow and had a shoulder peak. It indicates that the emission
in cyclohexane is the locally excited one and the fluorescence
Photophysical properties in solution
Because only CnCNPA could form gels in measured solvents, in other solvents is attributed to ICT emission.18 Therefore,
their spectral characteristics rather than those of CnCPA in C8CNPA is a typical D–π-A molecule.
solutions were studied. We found that CnCNPA with different
Quantum chemical calculations were performed on
alkyl chains showed the same spectral behavior, so the C1CNPA by density functional theory calculations at the
C8CNPA was selected to show the spectral characteristics in B3LYP/6-31G(d) level to further clarify the ICT transition.
solution. As shown in Fig. 1a, C8CNPA shows an absorption Fig. 1d shows that the HOMO state density is distributed at the
band with a maximum of 404 nm in cyclohexane, which red- 4-nitrobenzene and vinyl units. By contrast, the LUMO density
shifted to 408 nm in toluene, to 411 nm in THF and CH2Cl2, is mainly localized on the carbazole and vinyl moieties. The
Fig. 1 Normalized absorption (a) and emission (b) spectra of C8CNPA in different solvents (1 × 10−6 M). λex = 390 nm. (c) Lippert–Mataga plot:
fluorescence emission maximum energy of C8CNPA as a function of solvent polarity. (d) Frontier orbital plots of the HOMO and LUMO of C1CNPA.
The long alkyl chain was replaced by the methyl group to simplify the calculation.
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simulated absorption spectrum of C1CNPA shows an absorp-
tion peak at ca. 451 nm (Fig. S1†) ascribing to the HOMO→
LUMO transition. Therefore, light excitation clearly induces a
charge distribution change from the donor to the acceptor.
This result further proves that the maximum absorption and
emission are due to the ICT transition.19 We also obtained the
optimal conformation of C1CPA and compared their dipole
moments to understand the role of the nitro unit on gel for-
mation (Fig. S2†). The result suggests that the dipole moment
of C1CNPA (10.6269 Debye) is larger than that of C1CPA
(4.3811 Debye), implying that the larger dipole moment may
be responsible for the gel formation.20
We also found that C8CNPA was a weak emissive fluoro-
phore. The fluorescence quantum yield (Φ) in toluene was as
low as 0.21%. In other solvents, the Φs were also lower than
1%. Φs of C4CNPA, C12CNPA and C16CNPA in toluene were
0.21%, 0.25%, and 0.25%, respectively. Thus, small Φs are
ascribed to the intramolecular rotation of a single bond and
the existence of a cyano moiety, which induces the twisted con-
formation in the isolated state because of the steric interaction
between the bulky cyano unit and the neighboring hydrogen
atoms.21 This condition could be confirmed by the optimal
geometry of C1CNPA (Fig. S2a†), in which the dihedral angles
of the vinyl group with carbazole and 4-nitrobenzene units are
5.8° and 24.4°, respectively.
Fig. 2 SEM images of C8CNPA (a, b), C12CNPA (c, d) and C16CNPA
(e, f) gels in ethanol and DMSO. Insets show the enlarged images.
Self-assembly in the gel state
First, the morphologies of the gelators in the gel phase were
observed and compared. Fig. 2 shows the scanning electron
microscopy (SEM) images of C8CNPA, C12CNPA, and
C16CNPA in ethanol and DMSO. Three compounds had
similar morphologies in DMSO gels and self-assembled into a
one-dimensional (1D) nanoribbon with a large aspect ratio
(Fig. 2b, d and f). However, the self-assemblies in ethanol gels
were different. Larger ribbons with larger widths were observed
for C8CNPA (Fig. 2a). Some ribbons were more than 10 μm in
width. C12CNPA and C16CNPA could form thinner and longer
ribbons in ethanol gel, whereas right-handed and left-handed
twisted ribbons were found in C16CNPA ethanol gel. Because a
longer alkyl chain will increase the disorder of molecular
packing, gelators with long chains can self-assemble into thin
and soft fibers.22 This result suggests that the length of the
alkyl chain strongly affects the gelator morphology in solvents.
When the yellow sols of C8CNPA in ethanol and DMSO
were cooled to room temperature and transferred into turbid
gels (Fig. S3†), the system color changed to orange, suggesting
the existence of π–π interactions between aromatic units.
Therefore, the absorption spectrum was used to monitor this
process. As shown in Fig. 3, when the hot sols in the two sol-
vents were cooled, the absorption peaks started first to
increase and slightly red-shifted because of the increase in the
π-conjugation degree at low temperatures. After 1 min, the
absorption peaks gradually decreased and further shifted to a
longer wavelength region. Weak absorption bands at approxi-
mately 500 nm appeared and gradually heightened, which
Fig. 3 Absorption spectra of C8CNPA in (a) ethanol and (b) DMSO
during gelation, C = 3.2 mM in ethanol and 19 mM in DMSO. The interval
explains the orange color of gels. Temperature-dependent time is 1 min.
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absorption spectra in two solvents showed similar spectral
change (Fig. S4†). There is a red-shift of 4 nm during gelation
in ethanol and a red-shift of 2 nm in DMSO. In addition, the
dilute solutions of C8CNPA in two solvents showed blue-
shifted absorption bands (9 nm and 3 nm in ethanol and
DMSO, respectively) relative to those of gels (Fig. S5†). These
spectral changes illustrate the J-aggregate formation during
gelation.23 The absorption spectra of C12CNPA and C16CNPA
during the gel process were also recorded. As shown in
Fig. S6,† maximal absorption peaks for C12CNPA and
C16CNPA had blue-shifts of 7 nm and 10 nm relative to those
of hot sols. Moreover, the gels in ethanol possessed blue-
shifted absorption bands relative to those of dilute solutions
(Fig. S7†). This observation suggests that the packing models
of C12CNPA and C16CNPA in gels are different from that of
C8CNPA, although the absorption band at 500 nm also
appeared in their gels. In the C12CNPA and C16CNPA gels, the
H-aggregate formation is responsible for the blue-shifted
absorption.24
Considering that X-ray diffraction (XRD) always shows the
stack model of the gelator in gels, small-angle XRD patterns of
the three gelators in ethanol gel were investigated (Fig. 4).
C8CNPA, C12CNPA, and C16CNPA clearly adopts a lamellar
packing structure with packing periods evaluated to be 1.75,
2.30, and 2.78 nm, respectively.25 The XRD patterns of the
DMSO xerogels were also obtained. C8CNPA and C16CNPA
DMSO xerogels possessed similar lamellar structures and
stacking periods to the corresponding ethanol xerogels
(Fig. S8†). However, two types of layer periods in the DMSO
xerogel of C12CNPA were observed. One layer was similar to
that of ethanol xerogel, whereas the other one had a longer
period of 2.13 nm, indicating two types of molecular packing
models. These results show that three gelators self-assembled
into ribbons with a layer packing structure in the gel phase.
We attempted to obtain single crystals of three gelators to
further understand the detailed stacking model in gels.
However, only thin and longer orange fibers were grown from
their solutions. Fortunately, sufficient large orange single crys-
tals of C4CNPA from toluene solution were obtained, and its
Fig. 5 (a) 1D packing of C4CNPA in crystal and (b) top view of one
dimer.
single-crystal structure and crystal data are shown in Fig. 5 and
Table S1,† respectively. In the crystal, the dihedral angles
between the vinyl and two benzene rings were very small at
2.9° and 9.9°, respectively, indicating that the planarity of the
molecule is good. The molecules were also arranged into 1D
stacking, in which one anti-parallel dimer appeared repeatedly.
Molecules adopt anti-parallel packing rather than parallel ones
in crystals because of the inherent large dipole moment of
C4CNPA. Anti-parallel stacking allows sufficient electrostatic
attraction force between molecules. The distance between two
molecules in the dimer was 3.48 Å, suggesting a strong inter-
molecular interaction between molecules. We found a new
peak at 500 nm and a blue-shifted absorption band at 383 nm
in the absorption spectrum of the crystal (Fig. S9†) relative to
its solution. The crystal absorption spectrum could be
explained by the crystal structure. Two molecules in the dimer
were stacked together face-to-face, producing an H-aggregate
and then inducing a blue-shifted absorption peak. The sliding
angle between the two dimers is larger than that in the dimer
itself, so a red-shifted absorption band appears that indicates
a J-aggregate (Fig. 5a). This spectral change is similar to those
in C12CNPA and C16CNPA gels, so C12CNPA and C16CNPA in
gels have similar intermolecular packing models (Fig. 6).
Considering that C8CNPA had red-shifted absorption
during gelation, a large dipole moment, and the existence of
an absorption peak at 500 nm, C8CNPA reasonably stacked
together in the anti-parallel model. A different dimer structure
from C4CNPA also exists. As shown in Fig. 6, the sliding
angles between two molecules in the dimer and between two
dimers are large enough to induce two red-shifted absorption
Fig. 4 XRD patterns of C8CNPA, C12CNPA, and C16CNPA xerogels
from ethanol.
Fig. 6 Packing models in gels for (a) C12CNPA and C16CNPA, and for
(b) C8CNPA.
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Organic & Biomolecular Chemistry
bands. The difference of van der Waals interactions between
alkyl chains should be in charge of different molecular stack-
ings in gels because aromatic moieties of three compounds are
the same, and only the length of the alkyl chain is different.
Gelation-induced emission enhancement
Based on the above discussion, CnCNPA is known to exhibit
weak emission and small Φs in solution because of the exist-
ence of the cyano moiety and free intramolecular rotation. We
also found that the hot sols of the three gelators in ethanol
and DMSO emitted weak fluorescence, but stronger emission
for the corresponding gels was observed. Thus, our gels exhibi-
ted gelation-induced emission enhancement. Given that high
temperature always quenches fluorophore emission, we
selected the hot DMSO sol (100 °C) of C8CNPA at a low con-
centration of 40 mM, which requires 30 min to form a gel.
When the sol changed into a gel, the sol had been cooled to
room temperature. The fluorescence of the sol was very weak
before gelation. When the sol started to become turbid and
form a gel, the emission intensity of the system increased
swiftly. This result suggests that the emission enhancement
originated from the molecular aggregate.26
Fig. 7 shows the fluorescence spectral change for C8CNPA
in ethanol and DMSO during gelation. The hot ethanol sol of
C8CNPA emitted weak green fluorescence with a maximum of
539 nm. After 2 min, the emission band at approximately
600 nm appeared and gradually enhanced. After 15 min, the
system changed into a gel and showed an orange red emission
with more than 6-fold intensity compared with the sol. In
DMSO, the system also exhibited similar emission enhance-
ment during gel formation. Weak and red fluorescence for the
hot DMSO sol was observed, but the corresponding gel
emitted blue-shifted orange fluorescence with a maximum at
594 nm and a 4-fold increase in the emission intensity. Temp-
erature-dependent fluorescence spectra showed the same
result (Fig. S10†). At low temperatures the system showed
strong emission, and a fluorescence intensity decrease at
higher temperatures. By comparison of the optimal configur-
ation from theory calculation and the molecular structure in
the crystal state, the enhanced emission of the gel is attributed
to the synergetic effect of the restricted molecular motion and
the formation of more coplanar conformations.27 In addition,
similar emission wavelengths of gels in ethanol and DMSO
should correspond to the same molecular stacking in two
solvents.
Fig. 7 Fluorescence spectra of C8CNPA in (a) ethanol and (b) DMSO
during gelation. C = 3.2 mM in ethanol, 19 mM in DMSO, and λex
=
410 nm. The interval time is 1 min. Insets show photos of C8CNPA in
ethanol and DMSO under 365 nm light.
with the increasing gelator concentration. For example, the
C8CNPA gel (11 mM) changed into a sol at 45 °C, accompanied
by a fluorescence decrease. As the concentration increased to
16 mM and 22 mM, the Tgel also increased to 59 °C and 70 °C,
respectively. The solvent is also important for the fluorescence
color change. In ethanol, the fluorescence color changed from
orange red to green, whereas the DMSO gel with stronger
orange emission transformed into a red solution with a weak
red fluorescence.
As shown above, the gels with enhanced emission were
obtained when the hot sols were cooled. Therefore, when the
Conclusion
gels were heated to form a sol, the obvious fluorescence A series of 4-nitrophenylacrylonitrile and phenylacrylonitrile
change was anticipated. As expected, when the temperature derivatives consisting of a carbazole moiety was synthesized.
was higher than the sol–gel phase transition temperatures Some of these derivatives with longer alkyl chains and a nitro
(Tgels) of gels and gels transferred into sols, the emission group could gelatinize some organic solvents. However, phenyl-
intensity sharply decreased, showing temperature-controlled acrylonitrile derivatives were not gelators. This result suggests
fluorescent switches. Given that Tgels could simply be adjusted that the electron withdrawing nitro moiety is important for gel
by the gelator concentration, the sensing temperatures of formation because it provides molecules with large dipole
these switches could also be decided by the gelator concen- moments, which was proved by theory calculation and the
tration. As shown in Fig. S6,† the Tgel values rise nonlinearly single-crystal structure. The single crystal structure of C4CNPA
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implies that gelators might adopt an anti-parallel molecular
stacking given the larger ground-state dipole moment.
The organogels also exhibited enhanced fluorescence relative
to solutions. This result indicates that the gels can be used
as temperature-sensitized fluorescent switches. The switch
temperature can be easily tuned by adjusting the gelator
concentration.
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21103067, 21374041, and
51102081), the Youth Science Foundation of Jilin Province
(20130522134JH), the Science and Technology Development
Plan of Jilin Province, China (20130521003JH), the Open
Project of the State Key Laboratory of Supramolecular Structure
and Materials (SKLSSM201203), and the Open Project of State
Key Laboratory of Theoretical and Computational Chemistry
(K2013-02).
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