Self-organized photochromic dithienylcyclopentene organogels

Article abstract of DOI:10.1039/c1jm13342j

A series of photochromic organogelators based on dithienylcyclopentene amides with a phenylene unit as a bridge between the amide and long alkyl chain were synthesized, their gelation behaviors were characterized by rheology, FT-IR, ¹H NMR, SEM, optical microscopy, UV-Vis and fluorescence spectroscopy. These organogelator molecules were found to be able to induce gelation in apolar solvents such as benzene, toluene and p-xylene to form entangled networks driven by intermolecular hydrogen bonding together with π-π interactions. Their excellent reversible photochromism with thermal stability in both solution and gel states was observed, and the thermally reversible property in sol-gel transition was exhibited.

Full text of DOI:10.1039/c1jm13342j

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PAPER
Self-organized photochromic dithienylcyclopentene organogels
a
Krishnamurthy Rameshbabu, Lu Zou, Chanjoong Kim, Augustine Urbas and Quan Li
a
a
b
a
*
Received 15th July 2011, Accepted 3rd August 2011
DOI: 10.1039/c1jm13342j
A series of photochromic organogelators based on dithienylcyclopentene amides with a phenylene unit
as a bridge between the amide and long alkyl chain were synthesized, their gelation behaviors were
1
characterized by rheology, FT-IR, H NMR, SEM, optical microscopy, UV-Vis and fluorescence
spectroscopy. These organogelator molecules were found to be able to induce gelation in apolar
solvents such as benzene, toluene and p-xylene to form entangled networks driven by intermolecular
hydrogen bonding together with p–p interactions. Their excellent reversible photochromism with
thermal stability in both solution and gel states was observed, and the thermally reversible property in
sol–gel transition was exhibited.
reversible switching has been the basis for creating new func-
tional materials with applications in photonics, information
storage, fluorescent photoswitches, sensors etc.9 However, only
minor attention has been paid to the developments of dithieny-
lethene based LMWG.3,4,10
Introduction
Organogelators, i.e. functional materials that assemble to
macroscopic three-dimensional (3D) networks and immobilize
organic solvent molecules in the networks, recently have emerged
as a class of fascinating self-organized materials. Self-organiza-
tion of organogelator molecules acting as low molecular weight
gelators (LMWGs) into an entangled supramolecular 3D fibrous
network in organic solvents is often driven by weak intermolec-
ular interactions such as electrostatic, dipole–dipole, hydrogen
bonding and p–p interactions.1–6 Any variation in the molecular
structure of organogelators would affect the corresponding
intermolecular interactions, and hence the morphology and
properties of the resulting gels. In this context, smart molecular
materials, consisting of small molecules responsive to external
stimuli, may provide a promising route towards active control
over the properties and multi-scale structures of organogels.
Compared with stimuli such as electric field, heat, chemical
reaction or electrochemical reaction, light is particularly attrac-
tive due to its advantages of remote, spatial, and temporal
control. The extension to reversible photochromic organogels
has drawn increasing attention due to their potential application
as sensors, molecular electronics and catalysis.7 Among all the
photochromic organogelators that have been reported to date,
dithienylcyclopentenes hold a particular promise because of their
unique fatigue resistance, thermal irreversibility and electrical
conductivity.8,9 Colorless open-ring dithienylcyclopentene can be
transformed photochemically to the colored closed-ring form
upon UV irradiation whereas its reverse process occurs photo-
chemically with visible light. Since the physical and chemical
properties of the two forms are different, the photochemically
Here we report the design and synthesis of four new photo-
chromic organogelators based on dithienylcyclopentene amides
with a phenylene unit as a bridge between the amide and long
alkyl chain (Scheme 1). These organogelator molecules were
found to be able to induce gelation in apolar solvents to form
photochromic entangled networks driven by p–p interactions in
addition to intermolecular hydrogen bonding between the amide
groups. The interest behind the introduction of the phenylene
unit between the amide and the long alkyl chain is to enhance
aggregation behavior by forming the p–p stacking arrangement.
aLiquid Crystal Institute, Kent State University, Kent, OH, 44242, USA.
E-mail: qli1@kent.edu; Fax: +1 330 672 2796; Tel: +1 330 672 1537
bMaterials and Manufacturing Directorate, Air Force Research
Laboratory WPAFB, Ohio, 45433, USA
Scheme 1 Light-driven open-ring and closed-ring isomerization of
photochromic dithienylcyclopentene amides 6a–d.
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Table 1 Gelation behavior of photochromic dithienylcyclopentene
amide 6a–d in organic solvent. G: gel; P: precipitation; VS: viscous
solution; CGC: critical gelation concentration in mg mLꢀ1 at 20 ꢁ
C
Solvents
6a (CGC)
6b (CGC)
6c (CGC)
6d (CGC)
Toluene
Benzene
CH2Cl2 : hexane
CCl4
p-Xylene
Dodecane
Ethanol
G (2.5)
G (3.0)
G (3.0)
P
G (2.5)
P
G (2.0)
G (2.0)
G (2.0)
P
G (1.8)
P
G (1.6)
G (1.8)
G (2.0)
G (1.6)
G (1.6)
VS
G (3.0)
G (3.0)
G (3.0)
G (3.0)
G (3.0)
VS
P
P
P
P
Fig. 2 IR spectra of 6c in xerogel (black line) and toluene organogel
state (red line).
Meanwhile, the terminal alkyl chain would improve the solu-
bility and the gelation ability of gelator molecule.
toluene, exhibited weak absorption bands at approximately
3400 cmꢀ1 and 1680 cmꢀ1 (Fig. 2). This indicates the existence of
the intermolecular hydrogen bonding through two amide moie-
ties in the gel state.13 To study the driving force of the gel
1
Results and discussion
formation, we examined temperature dependence of H NMR
spectra. 1H NMR spectra of 25 mg mLꢀ1 6b in toluene-d8 at 24 ꢁC
(gel state), 45 ꢁC (‘‘partial’’ solution state, gel–sol transition
ꢂ40 ꢁC), and 65 ꢁC (solution state) are shown in Fig. 3. On
increasing temperature, the aromatic and amide protons of 6b
developed strong signals with slight shifts. The characteristic
proton in NH group of amide was observed at 7.7 ppm as
a broad singlet peak, while the protons on phenyl group were
located at 7.5 ppm and 6.8 ppm as characteristic doublet peaks.
The singlet at around 7.3 ppm was assigned to the protons on
thiophene moiety. The NMR changes provide clear evidence that
hydrogen bond together with p–p interactions plays a significant
role in the gelation process.14
The target photochromic dithienylcyclopentene amides 6a–
d (Scheme 1) were thermally and chemically stable, and exhibited
the expected reversible photochromic behavior. All the
compounds 6a–d were found to be able to induce gelation in
apolar solvents to form entangled networks at ambient temper-
ature (Table 1, in which a test tube inversion method was
adopted for testing their gelation behavior).11 We evaluated the
temperature dependence of the sol–gel transition using shear
rheometry.12 Fig. 1 shows the rheology of 2 mg mLꢀ1 6b in
toluene. The storage modulus G0 is higher than the loss modulus
G00 at lower temperatures, and the sol–gel transition temperature
is approximately 40 ꢁC. On cooling to room temperature the
solution became gel mainly in apolar solvents and precipitation
was found in polar solvents due to limited solubility. All the
organogels exhibited remarkable stability at room temperature
which were stored even for one year without any decomposition
in gelation properties, although the photochromic gels were
sensitive to light and alteration of the color indicates the trans-
formation from open form to closed form.
The morphology of the xerogels of 6a–d, which were prepared
by slow evaporation of toluene from the corresponding orga-
nogels, was investigated by scanning electron microscopy (SEM)
and optical microscopy. The xerogels obtained by drying the
corresponding organogels in toluene were found to have fibrous
architectures formed by self-organization of the organogelators
as shown in Fig. 4. The SEM images provide fine details of the
morphology structure, while the optical microscope images
exhibit coexistence of the intertwisted fibers and ordered fibers in
a larger length scale.
FT-IR experiments were performed to analyze the intermo-
lecular hydrogen bonding motif. For example, FT-IR spectra of
the xerogel 6c showed two strong absorption bands around 3080
and 1610 cmꢀ1 corresponding to N–H and C]O stretching
vibrations, whereas its corresponding organogel, 2 mg mLꢀ1 in
As expected, all the organogelators 6a–d exhibited the
reversible photochromic behavior with thermal stability in both
solution and gel states. For example, a solution of 6d in CHCl3 is
colorless where there is no absorption in the visible region,
Fig. 1 Temperature dependence of the dynamic moduli of 2 mg mLꢀ1 6b
solution in toluene. Storage modulus, G0 ( ), and loss modulus, G00 (B),
were measured at an oscillatory frequency of 1 Hz with an applied strain
of 0.05.
Fig. 3 1H NMR spectra of 25 mg mLꢀ1 6b in toluene-d8 at different
temperatures 24 ꢁC, 45 ꢁC and 65 ꢁC.
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Fig. 7 Fluorescence spectra of 6d in toluene at solution (red line, 50 ꢁC)
and gel state (black line, room temperature).
Fig. 4 SEM images (A for 6c and B for 6d) and optical microscope
images (C for 6c and D for 6d) of xerogels prepared by the evaporation of
toluene from the organogels.
whereas its reverse process to the open-ring form upon irradia-
tion of visible light at 670 nm takes approximately 300 s. The
cycle was repeated many times without fatigue. These organogels
also showed reversible photochromic property in the gel state
with thermally reversible gel–sol transformation (Fig. 6). Inter-
estingly, the gel and its corresponding solution state exhibited an
obvious difference in their fluorescence emission spectra (Fig. 7),
e.g., 6d showed a broad maximum at 400 nm in gel state at room
temperature, while a sharp maximum of its solution state at
50 ꢁC is around 380 nm. This result may root in the different
intermolecular interactions between organogelator molecules in
gel and solution states.
Conclusions
Four photochromic dithienylcyclopentene amides with a phe-
nylene unit as a bridge between the amide and long alkyl chain,
the first of their kind, were synthesized and characterized. They
were found to be able to self-organize into reversible photo-
chromic organogels in apolar solvents. SEM and optical
microscopy studies of their xerogels showed the typical fibrous
networks. Temperature dependence of the sol–gel transition by
Fig. 5 UV-Vis spectra of 6d in chloroform (1.4 ꢃ 10ꢀ5 M) under 310 nm
irradiation with different times.
corresponding to the open-ring 6d (Scheme 1). UV irradiation at
310 nm of this solution resulted in a clean photoisomerization to
the closed-ring 6d, as evidenced by a decrease in the absorbance
at 288 nm and the appearance of new broad absorption bands at
355 nm and 530 nm with an isosbestic point at 330 nm (Fig. 5).
The open-ring 6d is colorless whereas its closed-ring isomer is
colored, which is attributed to the closed-ring form having
a larger p-conjugation than its open-ring form. The photosta-
tionary state of the open-ring 6d to its closed-ring 6d was reached
within approximately 140 s upon UV irradiation at 310 nm,
1
shear rheology was evaluated. FT-IR, H NMR, and fluores-
cence emission spectra indicate that such a fibrous network might
result from the cooperation of the hydrogen bonding between the
amide groups and the p–p interactions between the aromatic
moieties. These materials exhibited excellent reversible photo-
chromism in both solution and gel states. The findings that
hydrogen bonded networks enhanced by p–p interactions
through the insertion of a phenylene bridge between the terminal
alkyl chain and the center dithienylcyclopentene core would
provide insight to design organogels with enhancement of gela-
tion behavior.
Experimental section
Materials and methods
All chemicals and solvents were used without further purifica-
tion. Melting points were uncorrected. The rheology of 6b
organogel in toluene was obtained using a stress-controlled
rheometer (HAAKE MARS II, Thermo Scientific) on a Peltier
plate with a solvent trap to ensure homogeneous temperature
and to prevent solvent evaporation from the organogel solution.
Storage and loss moduli are measured by applying shear strain of
0.05 with a cone-and-plate geometry (cone angle ¼ 4ꢁ, diameter
Fig. 6 Images of 6d in toluene (4 mg mLꢀ1) at the gel state and the
solution state. Top: open form (left) and closed form (right) in solution.
Bottom: open form (left) and closed form (right) in gel.
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¼ 35 mm) at an oscillatory frequency of 1 Hz, while varying
temperature to determine gelation temperature. The temperature
ꢁ
ꢁ
is varied from 50 to 10 C by 2 C steps with a 2 minute equi-
librium time for each temperature step. FTIR spectra were
recorded with KBr pellets on Nicolet Instruments. For
measuring gels, it is deposited in KBr pellets and dried until
1
complete evaporation under vacuum. H and 13C NMR spectra
were recorded on a Bruker 400 spectrometer or Varian Gemini
200. Chemical shifts are in d units (ppm) with the residual solvent
peak as internal standard. The coupling constant (J) is reported
in hertz (Hz). NMR splitting signal are designated as s, singlet; d,
doublet; t, triplet; and m, multiplet. Column chromatography
was carried out on silica gel (60–200 mesh). Elemental analysis
was performed by Robertson Microlit Laboratories, Inc. Mass
spectrum was taken by Mass Spectrometry & Proteomics Facility
of Ohio State University. The intertwisted fibers were observed
by optical microscopy using an Olympus microscope. UV-Vis
spectra were recorded using a Perkin-Elmer Lambda spectro-
photometer in the range of 200–800 nm. Fluorescence measure-
ments were performed using Varian Cary Eclipse fluorescence
spectrophotometer controlled with slit excitation of 5 nm and
scan ranges from 300–600 nm. The irradiation studies were
carried out using a high-pressure 100 W Xenon lamp (Asahi
spectra co Ltd, Japan) with mirror module of UV and visible
range of 300–400 and 400–700 nm with respective cutoff filters.
SEM images were taken using a JEOL and a Hitachi S–2600 N
model with an operating voltage of 20–25 kV. SEM samples were
prepared by dropping dilute gels onto a clean wiped glass slide.
The slow evaporation of solvent in air for 15 min afforded
xerogels. All the samples for SEM experiments were sputtered
with gold thin films.
Scheme 2 Synthesis of photochromic dithienylcyclopentene amide 6. (a)
Glutaryl dichloride, AlCl3, CS2; (b) TiCl3(THF)3, Zn, THF; (c) n-BuLi,
THF, solid CO2; (d) SOCl2, DMF, CH2Cl2.
Anal. calcd for C47H62N2O2S2: C 75.15, H 8.32, N 3.73, S 8.54;
found: C 75.11, H 8.63, N 3.63, S 8.34%.
Data for 6b
Gelation
1H NMR (CDCl3): d ¼ 0.88 (t, J ¼ 6.3 Hz, 6H), 1.28–1.51 (m,
24H), 1.75 (t, J ¼ 7.4 Hz, 4H), 1.99–2.05 (m, 8H), 2.73 (t, J ¼ 7.2
Hz, 4H), 3.90 (t, J ¼ 6.4 Hz, 4H), 6.83 (d, J ¼ 8.8 Hz, 4H), 7.29 (s
2H), 7.44 (d, J ¼ 9.0 Hz, 4H), 7.72 (s, 2H). 13C NMR (CDCl3):
d ¼ 14.1, 14.8, 22.7, 26.0, 29.3, 29.4, 29.5, 31.9, 38.4, 68.3, 114.8,
122.1, 130.0, 130.6, 134.6, 134.9, 136.5, 140.6, 156.2, 159.8. TOF
MS ES (m/z) (M + Na+) calcd for C47H62N2O4S2Na: 805.4043;
found: 805.4040. Anal. calcd for C47H62N2O4S2: C 72.08, H 7.98,
N 3.58, S 8.19; found: C 72.11, H 8.31, N 3.54, S 8.11%.
The gelation test was carried out with various solvents using
a test tube inversion method. The compound and solvent were
added in a septum capped test tube and heated (>85 ꢁC) until the
solid dissolved. In order to obtain gels, the sample was cooled to
ambient temperature and left for a period of time. No gravita-
tional flow while inverting the test tube of the sample indicates
successful gelation.
Synthesis
Data for 6c
The photochromic organogelator molecules 6a–d were prepared
starting from 2-chloro-5-methylthiophene 1 in a facile synthesis
following the general procedure as shown in Scheme 2.15 Their
structures were identified by 1H NMR, 13C NMR, elemental
analysis and HR-MS.
1H NMR (CDCl3): d ¼ 0.88 (t, J ¼ 7.0 Hz, 6H), 1.25–1.57 (m,
40H), 1.99–2.09 (m, 8H), 2.55 (t, J ¼ 7.7Hz, 4H), 2.76 (t, J ¼ 7.9
Hz, 4H), 7.13 (d, J ¼ 8.8 Hz, 4H), 7.31 (s, 2H), 7.47 (d, J ¼ 8.8
Hz, 4H), 7.63 (s, 2H). 13C NMR (CDCl3): d ¼ 14.1, 14.8, 22.7,
29.2, 29.4, 29.5, 29.7, 31.5, 31.9, 35.3, 38.3, 115.2, 120.1, 128.8,
129.0, 129.9, 134.6, 134.8, 135.2, 136.5, 139.2, 140.7, 159.7. TOF
MS ES (m/z) (M + Na+) calcd for C53H74N2O2S2Na: 857.5084,
found 857.5081. Anal. calcd for C53H74N2O2S2: C 76.21, H 8.93,
N 3.35; found C 76.41, H 9.28, N 3.40%.
Data for 6a
1H NMR (CDCl3): d ¼ 0.88 (t, J ¼ 6.3 Hz, 6H), 1.28–1.49 (m,
28H), 1.72 (t, J ¼ 7.4 Hz, 4H), 1.95–2.01 (m, 8H), 2.73 (t, J ¼
7.3 Hz, 4H), 6.80 (d, J ¼ 8.7 Hz, 4H), 7.30 (s, 2H), 7.44 (d, J ¼
8.8 Hz, 4H), 7.60 (s, 2H). 13C NMR (CDCl3): d ¼ 14.1, 14.3, 22.5,
25.9, 29.2, 29.1, 29.3, 31.1, 38.1, 113.2, 114.2, 122.0, 129.1, 130.2,
134.7, 134.8, 135.2, 140.1, 155.2, 158.8. TOF MS ES (m/z) (M +
Na+) calcd for C47H62N2O2S2Na: 773.4145, found: 773.4140.
Data for 6d
1H NMR (CDCl3): d ¼ 0.88 (t, J ¼ 7.0 Hz, 6H), 1.28–1.33 (m,
16H), 1.58–1.63 (m, 4H), 2.02–2.10 (m, 8H), 2.61 (t, J ¼ 7.6 Hz,
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¼ 8.4 Hz, 4H), 7.37 (s, 2H), 7.74 (d, J ¼ 9.2 Hz, 4H), 7.97 (s, 2H),
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Acknowledgements
The work is partially supported by the Air Force Office of Scientific
Research (AFOSR FA9550-09-1-0254), the Department of Energy
(DOE DE-SC0001412), and the National Science Foundation
(NSF IIP 0750379). We thank A. Gericke for the use of his char-
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