PH triggered smart organogel from DCDHF-Hydrazone molecular switch

Article abstract of DOI:10.1016/j.dyepig.2016.03.044

The design, synthesis and photophysical properties of a Low Molecular Weight pH-responsive Gelator (LMWG) based on alkoxy group functionalized DCDHF-Hydrazones (DCDHF-H) are described. A straightforward synthesis of DCDHF-Hydrazone (DCDHF-H) chromophores was achieved via simple azo-coupling starting from 2-(dicyanomethylene)-2,5-dihydro-4,5,5-trimethylfuran-3-carbonitrile and alkoxy bearing aryl diazonium chloride derivatives. The DCDHF-H efficiently gelate selected organic solvents and reversibly respond to pH stimuli with a gel-sol conversion and an associated color change from yellow to purple. Self-assembly of these molecules, as indicated by X-ray crystallography, occurs via cooperative π-π stacking and van der Waals interactions producing gelation in some pure and mixed organic solvents. Furthermore, the presence of acidic or alkaline gases leads to a dramatic change in the rheological properties with potential applications in areas such as gas detection devices and drug release systems. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies of the xerogels obtained from n-propanol provide visual images revealing the formation of fibrous nanostructures.

Full text of DOI:10.1016/j.dyepig.2016.03.044

Dyes and Pigments 130 (2016) 327e336
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
pH triggered smart organogel from DCDHF-Hydrazone molecular
switch
, c,
Tawfik A. Khattab a *, Brylee David B. Tiu b, Sonya Adas a, Scott D. Bunge a,
Rigoberto C. Advincula b
a Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio 44242, United States
b Department of Macromolecular Science and Engineering, Case Western Reserve University Cleveland, Ohio 44106, United States
c Dyeing, Printing and Auxiliaries Department, Textile Research Division, National Research Centre, Dokki, Cairo 12311, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
The design, synthesis and photophysical properties of a Low Molecular Weight pH-responsive Gelator
Received 30 December 2015
Received in revised form
22 March 2016
Accepted 23 March 2016
Available online 28 March 2016
(LMWG) based on alkoxy group functionalized DCDHF-Hydrazones (DCDHF-H) are described.
A
straightforward synthesis of DCDHF-Hydrazone (DCDHF-H) chromophores was achieved via simple azo-
coupling starting from 2-(dicyanomethylene)-2,5-dihydro-4,5,5-trimethylfuran-3-carbonitrile and
alkoxy bearing aryl diazonium chloride derivatives. The DCDHF-H efficiently gelate selected organic
solvents and reversibly respond to pH stimuli with a gel-sol conversion and an associated color change
from yellow to purple. Self-assembly of these molecules, as indicated by X-ray crystallography, occurs via
Keywords:
cooperative pep stacking and van der Waals interactions producing gelation in some pure and mixed
DCDHF-Hydrazone
Molecular switch
pH-responsive
Smart organogel
Nanofibers
organic solvents. Furthermore, the presence of acidic or alkaline gases leads to a dramatic change in the
rheological properties with potential applications in areas such as gas detection devices and drug release
systems. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies of the
xerogels obtained from n-propanol provide visual images revealing the formation of fibrous
nanostructures.
Published by Elsevier Ltd.
1. Introduction
Recently, stimuli-responsive low molecular weight gelators have
been attracted attention due to their potential applications such as
The distinctive ability of small organic gelators to immobilize
organic solvents via the cooperative effects of weak non-covalent
interactions such as hydrogen bonding, pep stacking and van der
soft materials for drug delivery in the pharmaceutical industry, in
the environmental area for capture and removal of pollutants and
in reconstructive medicine for tissue engineering [12e17]. Low
molecular weight pH-responsive gelators belong to a distinctive
class in which variation of pH modulates a solegel phase transition.
This behavior can be exploited to develop novel soft materials for
encapsulation and slow release of biologically important molecules
such as vitamins. Hence, it should be noted that development of
stimuli-responsive smart gels is very appealing [11e20].
Many structure prototypes of low molecular weight organic
gelators have been reported in the recent literature [6e10]. In
contrast, only scant attention has been applied to the structures
and properties of pH-responsive organogels. To the best of our
knowledge, there is no report of low molecular weight organic
gelators based on pushepull DCDHF chromophores. DCDHF de-
rivatives have played a significant role in a range of recent advances
of optoelectronic applications [21e25]. They have been intensively
developed as photo- and electroluminescent materials in many
Waals interactions, has received considerable interest. This interest
extends more recently to their tunable physical properties and
response to external stimuli [1e5]. This self-assembly in nanoscale
supramolecular structures is likely to have significant implications
in the creation of functional nanomaterials [6,7]. The types of su-
pramolecular structures generated by these self-assembled fibrillar
networks include nanofibers, nanorods, and nanoribbons. In order
to create these assemblies, it is important to control the intermo-
lecular interactions in such a way that the molecules can self-
organize into fibrous higher order supramolecular structures but
without conversion into a densified crystalline state [8e11].
* Corresponding author. Department of Chemistry and Biochemistry, Kent State
University, Kent, Ohio 44242, United States.
E-mail address: tkhattab@kent.edu (T.A. Khattab).
http://dx.doi.org/10.1016/j.dyepig.2016.03.044
0143-7208/Published by Elsevier Ltd.

328
T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
fields including electro-optics, photorefractives, single molecule
fluorophores, dye lasers, fluorescent sensors and logic memory and
organic light-emitting devices [26e30]. Therefore, the construction
of an organogel based on DCDHF-H will be significant not only in
the concept of designing new organogelators but also in extending
gel-phase materials with appealing photonic and electronic func-
tionality. We have found that long chain alkoxy substituted DCDHF-
H derivatives can self-organize into supramolecular assemblies.
Tuning the length of the alkoxy chain is instrumental for the
fabrication of functional gels. Herein, we report the first example of
pH-responsive low molecular weight organic gelator built from a
solvatochromic and pH-responsive DCDHF-H molecular switch.
in n-propanol by heating at 85 ꢀC and then cooling to room tem-
perature to form the gel. The gels formed in 10e25 min depending
on the gelator concentration. The gel melting temperature was
determined by the inverted test tube method [8]. The vials con-
taining the gels were kept upside down in a paraffin oil bath. The
temperature was increased at a rate of 2 ꢀC/min and the temper-
ature at which the gel fell under gravity was recorded as the gel
melting temperature.
2.4. Measurements of pH control
A
solution of trifluoroacetic acid in methanol (conc. ca.
2 ꢁ 10ꢂ4 M) was added to either acetonitrile or ethanol solution of
chromophore 5 (conc. ca. 3 ꢁ 10ꢂ6 M) to reduce the pH value. The
hydrazone anions were produced in situ by addition of a base to
solutions of their conjugate acids (DCDHF-H). For example, addition
of a 1 M methanolic solution of tetrabutylammonium hydroxide to
a solution of chromophore 5 in either acetonitrile or ethanol pro-
duced the corresponding hydrazone anion. The mixture was stirred
and the pH values were recorded with a digital pH meter equipped
with a combined glass-calomel electrode.
2. Experimental details
2.1. Apparatus
Melting points were obtained by differential scanning calorim-
etry (TA instruments 2920). Thermal stability was recorded by
thermogravimetric analysis (TA instruments 2950). IR spectra were
recorded with a Bruker Vectra-33 IR-Spectrometer with ATR probe.
Elemental analyses (C, H, N) were performed on PerkineElmer
2400 analyzer (PerkineElmer, Norwalk, CT, USA) at the Microana-
lytical Center, Cairo University. UVeVis absorption spectra were
measured on a HP-8453 (HEWLETT PACKARD) spectrophotometer.
Fluorescence spectra were measured on a VARIAN CARY ECLIPSE
fluorescence spectrophotometer. NMR spectra were recorded using
a BRUKER AVANCE 400 spectrometer at 400 MHz; chemical shifts
are given in ppm relative to internal standard TMS at 295 K. The pH
measurements were recorded with a BECKMAN COULTER digital
pHI340 pH meter, with a combined glass-calomel electrode. In case
of scanning electron microscopy SEM, the gel formed from 5 in n-
propanol was scooped up and placed on carbon tapes pasted on
aluminum stubs and allowed to dry at room temperature in a
desiccator connected to vacuum pump. The dried samples were
then annealed overnight in an oven at 45 ꢀC, followed by applica-
tion of a 10 nm gold-coating before recording images. SEM images
were obtained using Hitachi S-2600N operating at 20 kV. For
Transmission electron microscopy TEM, a piece of gel 5 in n-
propanol (at its minimum gelation concentration) was placed on a
carbon-coated copper grid (200 mesh) and allowed to dry in vac-
uum at room temperature for two days. TEM was done on JEM-
1200EX at an accelerating voltage of 80 kV. All materials and re-
agents were obtained from commercial sources and were used
without further purification.
2.5. Procedure for concentration-dependent 1H NMR measurements
The concentration-dependent 1H NMR was carried out by
gradually increasing the concentration of the CDCl3 solution of
DCDHF-H 5. The initial concentration of 5 is 0.7 mM, the concen-
tration was then adjusted by direct addition of correct quantity of 5
into the CDCl3 solution.
2.6. X-ray crystal structure information
Single crystals of compound 5 suitable for X-ray diffraction
analysis were obtained by slow crystallization in a mixture of
(EtOH/Toluene, 3:1). Crystal data were collected by mounting a
crystal onto a thin glass fiber from a pool of Fluorolube™ and
immediately placing it under a liquid N2 cooled stream, on a Bruker
AXS diffractometer upgraded with an APEX II CCD detector. The
radiation used is graphite monochromatized Mo K
a radiation
(l
¼ 0.7107 Å). The lattice parameters are optimized from a least-
squares calculation on carefully centered reflections. Lattice
determination, data collection, structure refinement, scaling, and
data reduction were carried out using APEX2 Version 2014.11
software package. The data were corrected for absorption using the
SCALE program within the APEX2 software package. The structure
was solved using direct methods. This procedure yielded a number
of the C and N atoms. Subsequent Fourier synthesis yielded the
remaining atom positions. The hydrogen atoms are fixed in posi-
tions of ideal geometry (riding model) and refined within the
XSHELL software package. These idealized hydrogen atoms had
their isotropic temperature factors fixed at 1.2 or 1.5 times the
equivalent isotropic U of the C atoms to which they were bonded. A
few hydrogen atoms could not be adequately predicted via the
riding model within the XSHELL software. These hydrogen atoms
were located via difference-Fourier mapping and subsequently
refined. The final refinement of each compound included aniso-
tropic thermal parameters on all non-hydrogen atoms. Crystallo-
graphic data (excluding structure factors) for the structure in this
paper have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication nos. CCDC 1443921
Copies of the data can be obtained, free of charge, via http://pubs.
acs.org or on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK, (Fax: þ44 (0) 1223 336033 or e-mail: deposit@ccdc.cam.
ac.uk).
2.2. Materials and reagents
Solvents used in this study were obtained from Fluka and
Aldrich. All reactions were monitored by thin layer chromatog-
raphy (TLC) using Merck aluminum plates pre-coated with silica gel
PF254; 20-20, 0.25 mm, and detected by visualization of the plate
under UV lamp (l¼ 254 or 365 nm). Compounds were purified
through recrystallization or using flash column chromatography
which was performed on Scharlau silica gel, packed by the slurry
method. The DCDHF intermediate was prepared in a 68% yield ac-
cording to an early literature method [25]. The starting material 2-
amino-5-nitrophenol is commercially available and was treated
with K2CO3 and 1-iodoalkane in dimethylformamide at reflux to
afford the corresponding 2-alkyloxy-4-nitrobenzenamine in rela-
tively high yield range from 81 to 91% [31,32].
2.3. Gelation procedure
The gelation tests were performed by dissolution of compound 5

T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
329
2.7. Rheological measurements
preparation of the DCDHF-H chromophores was simply performed
via azo-coupling starting from 2-(dicyanomethylene)-2,5-dihydro-
4,5,5-trimethylfuran-3-carbonitrile and diazonium salt derived
from the corresponding 2-alkyloxy-4-nitrobenzenamine [33]. The
molecular structures of the produced chromophores 3e5 were
characterized by 1H- and 13C-NMR spectroscopy, IR spectroscopy,
elemental analysis, and X-ray crystal structure.
Rheological examination was performed using
a stress-
controlled rheometer (TA Instruments, ARG2) equipped with steel
parallel plate geometry (40 mm diameter), when the gap distance
was set at 750 mm. All experiments were performed at 283 K. To
determine the linear viscoelastic region (LVER) of the organogel
sample, strain sweep at a constant frequency (6.28 rad/s) was
achieved in the 0.05e200% range. The frequency sweep was con-
ducted at a range of 0.1e100 rad/s at a constant strain of 0.1%, well
within the linear regime determined by the strain sweep. In order
to study the recovery performance of the gel, a thixotropic exam-
ination was performed. Just after the strain sweep progress, the
recovery of the storage modulus Gʹ of the cracked organogel was
watched at a constant frequency (6.28 rad/s) under a low strain
(0.1%). The storage modulus Gʹ and the loss modulus Gʺ were
recorded as functions of time in the recovery processes. To study
the cycle of deformation and recovery, initially, a constant oscilla-
tory strain (100%), that was enough to crack down the organogel,
was applied to freshly prepared organogel for 10 s when the fre-
quency of the measurement was 6.28 rad/s; then, the recovery of
the storage modulus Gʹ was observed at a constant frequency
(6.28 rad/s) under a low strain (0.1%) within 20 s. Both Gʹ and Gʺ
were recorded as functions of time in each process.
3.2. Photophysical properties of chromophores 3e5 in solution
The three colorants 3e5 are soluble in a range of both non-polar
and polar organic solvents. All of the hydrazone chromophores
show fluorescence emission in both polar and nonpolar solvents.
Since the molecular frameworks of the chromophores 3e5 are very
similar, they likewise have nearly identical absorption and emission
behavior in solution. The UVeVis absorption and fluorescence
spectra of the hydrazone (H) and its corresponding hydrazone
anion (HA) formed in some selected solvents are shown in Table 1.
The colors of these DCDHF-H 3e5 in various pure solvents range
from yellow to purple. In general, the main absorption peak un-
dergoes a bathochromic shift (positive solvatochromism) with
increasing solvent polarity (ca. 27 nm shift value for the hydrazone
form and ca. 16 nm shift value for the hydrazone anion form on
going from dichloromethane to dimethylsulfoxide); with diverse
intensities. This indicates a strongly allowed pep* transition with
3. Results and discussion
charge transfer character. The DCDHF-H form showed interesting
solvatofluorochromic property in various solvents as indicated in
Table 1. The emission maximum of the neutral DCDHF-H dyes is
shifted bathochromically as much as 25 nm while, in contrast, the
DCDHF-H anion form did not show any fluorescence emission.
When deprotonated, the entire hydrazone group can function as a
bridge between donor and acceptor fragments in a pushepull
system or function itself as a donor fragment when in conjugation
with strong electron withdrawing group [34]. The 4-
nitrophenylhydrazones possess an acidic NeH group, able to pro-
duce anions with an improved electron donating capability. Hence,
distinct solvatochromic, solvatofluorochromic, and pH responsive
3.1. Synthesis and characterization
The simple synthetic approach utilized for the DCDHF-H chro-
mophores are shown in Scheme 1 in which R represents an alkyl
chain. The starting material 2-amino-5-nitrophenol is commer-
cially available and was treated with K2CO3 and 1-iodoalkane in
DMF at 130 ꢀC to afford 2-alkyloxy-4-nitrobenzenamine in high
yield ranging from 81% to 91% [31,32]. The intermediate 2-(dicya-
nomethylene)-2,5-dihydro-4,5,5-trimethylfuran-3-carbonitrile
was prepared according to the literature method [25]. The
1
OR
OH
R-I
O2N
NH2
O2N
O2N
NH2
DMF / K2CO3 / 130 ºC
2a-c
OR
OR
_
+
NaNO2/HCl
NH2
Cl
O2N
N
N
0-5 ºC
2a-c
CN
OR
CN
N
CN
CN
CN
CN
CH3CN / CH3COONa
N
0-5 ºC
O2N
O2N
O2N
_
+
N
O
1
O
Cl
N
OR
CN
NH
OR
CN
CN
N
O
R = -CH2CH3 3
R = -(CH2)6CH3 4
R = -(CH2)12CH3 5
Scheme 1. Synthesis of chromophores 3e5.

330
T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
Table 1
Optical properties in some selected solvents of the neutral hydrazone 5 (H form) and its corresponding deprotonated hydrazone anion (HA form) created in basic medium.
Solvent
(H form) lmax, abs. (nm); ε (molar absorbativity)
(H form) lmax, em. (HA form) lmax, abs. (nm); ε (molar absorbativity)
(nm)
(HA form) lmax, em.
(nm)
L molꢂ1 cmꢂ1
L molꢂ1 cmꢂ1
Ethanol
n-Propanol
Acetonitrile
Dimethylsulfoxide 498 (40,574)
Dichloromethane 471 (67,255)
1,4-Dioxane
487 (59,793)
489 (55,326)
477 (55,509)
534
536
537
557
538
532
560 (54,436)
561 (61,589)
571 (66,124)
574 (73,678)
558 (64,678)
570 (48,378)
e
e
e
e
e
e
478 (67,255)
(ꢂ) No fluorescence activity noted.
activities were observed in polar protic and aprotic solvents.
The stepwise addition of the tetrabutylammonium hydroxide
base to a solution of 5 and subsequent recording of the corre-
sponding absorption maxima lmax in acetonitrile led to a series of
spectra with one isosbestic point at 516 nm (Fig. 1a). As a result of
hydrazone anion formation, the intensity of the absorption at
shorter wavelength (ca. 477 nm) decreased while simultaneous
longer wavelength (ca. 571 nm) absorption appeared and grew in
intensity. These results suggest that a strong quinoid-type inter-
action exists between the 4-nitrophenyl group and the lone-pair
electrons on the secondary amine nitrogen atom in the hydra-
zone fragment, while such an extra resonance effect is depressed in
the hydrazone anion presumably because the negative charge in-
teracts instead more strongly to the strong DCDHF acceptor frag-
ment (Scheme 2). Interestingly, UVeVis absorption spectra of 5 in
protic solvents (such as ethanol) display a series of spectra with two
isosbestic points at 391 nm and 521 nm (Fig. 1b). One explanation
for this additional isosbestic point appearing in protic solvents in-
volves hydrogen bonding between the hydrazone anion and
ethanol as the band at 273 nm increases with increasing the cor-
responding hydrazone anion band at 560 nm. The hydrogen
bonding between the produced hydrazone anion and ethanol leads
to a new species in equilibrium with other two species; the
hydrazone dye and its corresponding hydrazone anion form
3.3. pH-responsive molecular switching
The reversible sensing potential of the nitro-substituted DCDHF-
H chromophores to undergo molecular switching imparted by pH
stimulus was established by sequential addition of base and acid to
a 3.0 ꢁ 10ꢂ6 M solution of DCDHF-H in either acetonitrile or
ethanol. The reversible UVeVis electronic absorption spectra and
color switching changes of DCDHF-H 5 are shown in Fig.1. The color
evolves gradually from yellow, orange, orange-red, wine-red, violet,
and purple at pH 5.38, 5.87, 6.21, 6.56, 6.78, and 7.04 respectively.
Upon the addition of tetrabutylammonium hydroxide base to a
solution of compund 5 in acetonitrile, the lmax at 477 nm decreased
and a new peak at 571 nm appeared. The addition of trifluoroacetic
acid produced reappearance of the absorption peak at 477 nm.
Likewise, fluorescence emission intensity was varied, reversibly, at
537 nm in acetonitrile solution by the alternate addition of tetra-
butylammonium hydroxide and trifluoroacetic acid as shown in
Fig. 2 (excitation wavelength at 477 nm). Upon the addition of
tetrabutylammonium hydroxide the fluorescence emission peak at
537 nm progressively decreases, while, in contrast, the intensity of
the same peak increases with the addition of trifluoroacetic acid.
OH
H
1.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
pH 5.41
pH 5.66
pH 5.38
pH 5.58
pH 5.87
pH 6.09
pH 6.21
pH 6.56
pH 6.78
pH 7.04
pH 7.13
0.9
pH 5.77
pH 6.28
pH 6.63
pH 6.94
pH 7.02
pH 7.23
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
350
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 1. The pH dependent absorption spectra of DCDHF-H 5 in acetonitrile (a) or ethanol (b) solutions (conc. 3.0 ꢁ 10ꢂ6 mol Lꢂ1) at different pH values at ambient temperature.

T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
331
Table 2
Gelation properties of DCDHF-H 3e5 in different solvents.
Solvent
3 and 4
5
P
Ethanol
n-Propanol
iso-Propanol
tert-Butanol
P
P
P
P
P
P
P
P
P
I
G (2.24 mM)
P
P
G (2.71 mM)
G (3.68 mM)
G (2.07 mM)
G (1.94 mM)
n-Octanol
1,2-Dihydroxyethane
1,3-Dihydroxypropane
1,6-Dihydroxyhexane
Benzyl alcohol
Benzene
P
I
Toluene
Hexane
P
I
P
I
Cyclohexane
I
I
Cyclohexane/chloroform (v/v ¼ 2/3)
P
S
S
S
S
S
S
S
P
G (5.18 mM)
S
S
S
S
S
S
S
Chloroform
Acetone
Ethylacetate
Acetonitrile
THF
Dimethylformamide
Dimethylsulfoxide
Dimethylsulfoxide/cyclohexane (v/v ¼ 1/2)
G (7.42 mM)
G: gel; S: soluble; I: insoluble; P: precipitation. The critical gelation concentration
(mM) is shown in parenthesis.
Fig. 2. Fluorescence emission of compound
5 in acetonitrile solution (conc.
3.0 ꢁ 10ꢂ6 mol Lꢂ1) at different pH values (ambient temperature, excitation wave-
length at 477 nm).
However, 5 does act as a gelator in pure alcoholic solvents. A series
of mixed non-alcoholic solvents were also examined. It was found
that an organogel of 5 could be generated in some mixed solvents
such as chloroform/cyclohexane (v/v ¼ 2/3) and dimethylsulfoxide/
cyclohexane (v/v ¼ 1/2). The results of the gelation studies are
summarized in Table 2. The critical gelation concentration values
were solvent dependent and in the range of 1.9 and 7.4 mM.
Moreover, we found that the n-propanol gel and 1,3-propanediol
gel formed from the DCDHF-H gelator 5 was stable for several
weeks at room temperature, while under similar conditions the
chloroform/cyclohexane gel could be preserved for only several
days. The above results demonstrate the important role of the side
chain in the gelation ability. The alkoxy side chain of 5 increases its
solubility in a variety of solvents. Thus, the strong solvation of the
alkoxy chain probably reduces any intermolecular affinity required
(Scheme 2).
3.4. Gelation properties
The gelation properties of 3e5 were tested in various solvents
by means of the ‘‘stable to inversion’’ technique [8]. It was found
that both 3 and 4 did not form a gel following typical heating-
cooling processes in any of the tested solvents which included
cyclohexane, hexane, chloroform, tetrahydrofuran, ethanol, tert-
butanol as well as some mixed solvents. Likewise, 5 did not form a
gel in tetrahydrofuran, acetonitrile, chloroform, ethylacetate or
dimethylsulfoxide in which the solubility was too high. Likewise, 5
could not gel in hexane or cyclohexane due to poor solubility.
H
CN
CN
N
CN
N
N
OH
H
CN
CN
O
N
N
O
O
N
O
CN
O
O
More stable quinoid form
Hydrazone form
CN
CN
N
N
O
N
O
CN
O
Hydrazone anion
form
Ethanol (Protic Solvent)
H3C
CH2
Hydrogen Bonding
O
H
N
CN
O
CN
CN
N
O2N
Scheme 2. The pH of the environment has a dramatic influence on the electronic properties of the dyes and their respective absorption wavelengths. The hydrogen bonding
between the hydrazone anion and ethanol leads to a new species in equilibrium with the other two species; namely, the hydrazone dye and its corresponding hydrazone anion
form.

332
T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
Fig. 3. Illustration of acid stability and base instability of the organogel of 5 in n-propanol.
for self-assembly. As compared with the gelation properties of 3
and 4, the long alkoxy chains are necessary features on the aromatic
core for creating a good gelator. Indeed, decreasing the length of the
alkoxy chain eliminates the gelation ability from 5 as indicated by
compounds 3 and 4.
Interestingly, it is only the neutral hydrazone form that has any
propensity for organogelation. Compound 5 was found to be a non-
gelator in its hydrazone anion form in all solvents examined,
whereas when it is used as its hydrazone form it formed strong gels
in a subset of the solvents as discussed above. To illustrate the acid/
base switching of this gel, a simple experiment was devised to
show the reversible switching from gel / sol / gel of 5 in n-
propanol. Upon exposure to ammonia vapor, the initial gel of 5
(Fig. 3) formed a solution (this solution did not form a gel on sub-
sequent heating or cooling) with a concomitant color change from
orange to purple. When this purple solution was exposed to, the
less toxic and volatile hydroiodic acid, hydrogen iodide vapor (long
exposure), the gel was reformed (heating is required to re-produce
the hydrazone form and the gel formed upon cooling to room
temperature); with the color turning back to orange. Moreover,
DCDHF-H 5 cannot gelate any of the organic solvents examined at/
below pH 5.86. From the photophysical properties explained above,
it is important to note that chromophore 5 turns orange in n-
propanol at/below pH 5.77 and purple at/above pH 6.94.
From these results, we conclude that the gel is highly stable
around the pH range 5.77e6.94. When the pH is lowered below
5.77 or higher above 6.94, the gel starts breaking slowly and
completely breaks at pH 7.12 and at that pH the DCDHF-H 5 is
NH
N
=C-H
OR
H
N
H
C
CN
CN
CN
O2N
Ha
O
H
R= C12H25
Fig. 4. Partial 1H NMR spectra of DCDHF-H 5 in CDCl3 at different concentrations (a)
Fig. 5. SEM (a) and TEM (b) images for the dried xerogel of 5 obtained from n-propanol
1.74 mM; (b) 8.54 mM; and (c) 16.43 mM.
(2.24 mM).

T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
333
1.2
Gel
Solution
1.0
0.8
0.6
0.4
0.2
0.0
250 300 350 400 450 500 550 600 650 700 750
Wavelength (nm)
Fig. 7. Normalized UV/Vis absorption spectra of 5 in solution (red, 3.0 ꢁ 10ꢂ6 M) and in
gel (black) in n-propanol. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
3.5. Morphology and structure of assemblies
It was observed that the long alkoxy chain of the DCDHF-H
gelator 5 improves its solubility in organic solvents but also sup-
ports association between the fibers, via van der Waals forces, and
eventual organogel formation. Therefore, of all the DCDHF-H
gelators 3, 4, and 5, only the DCDHF-H 5 forms organogels with
thixotropic property. It indicated that the long chain alkoxy group
played an important role in the thixotropy of organogels. In the
concentration-dependent 1H NMR as shown in Fig. 4, the NeH
resonance signals showed apparent downfield shifts as the con-
centration of 5 increased. These results revealed that the NeH
groups formed hydrogen bonds in the gelation process. Similar
behavior was monitored for the aliphatic ¼ CeH resonance signals
displayed apparent downfield shifts as the concentration of 5
increased. Furthermore, with the regular increase of concentration,
the 1H NMR signal of aryl rings (Ha) showed slight upfield shift,
suggesting that the pep stacking interactions among the aryl rings
were involved in the gelation process.
70
65
60
55
50
45
Fig. 6. (a) Crystal structure (ORTEP view) of 5 as derives from the X-ray analysis
(thermal ellipsoid plots are drawn at the 30% probability level); (b) Face-to-face
intermolecular
pep stacking interactions of the aromatic core; (c) Tail-to-tail van
der Waals interactions between the alkoxy chains.
entirely precipitated out into two phases, the solid phase and the
solution phase. At the higher pH > 6.94; the acidic hydrazone NH
group is ionized inducing the gel to sol switch. However, at the
lower pH < 6.94 (but higher than pH 5.77) the hydrazone group are
protonated and this reduces the solubility and the gel-formation
tendency of the DCDHF-H gelator 5 increases to a great extent.
On the other hand, the same explanation can be applied when the
pH is lowered below 5.77; as this leads to higher solubility of
DCDHF-H 5 resulting in gel instability.
2.0
2.5
3.0
3.5
4.0
4.5
Concentration in mM
Fig. 8. Gel melting profile of gelator 5 in n-propanol at fixed pH 6.5.

334
T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
The morphology of the n-propanol xerogel of compound 5 (after
molecules packed into H-aggregate in each unit cell, which was
confirmed by the blue shift of the absorption of DCDHF-H 5 gel
phase compared with that in dilute solution as shown in Fig. 7. In
the crystal, each molecule interacts with the adjacent molecules in
complete evaporation of the solvent) was studied by scanning
electron microscopy and transmission electron microscopy. From
the SEM and TEM images, we find a three-dimensional network
consisting of many entangled fine fibers with diameters ranging
face-to-face intermolecular
pep stacking interactions of the aro-
from 220 nm to 1.2
m
m as indicated by Fig. 5. This structure of the
matic core in cooperation with tail-to-tail van der Waals in-
teractions between the alkoxy chains. The investigation of the
crystal structure of 5 suggests that molecules should have the
tendency to aggregate into two dimensional nanostructures.
The thermal stability of the organogel 5 in n-propanol was
investigated by measuring the concentration dependent gel/sol
transition temperature at constant pH 6.5; as shown in Fig. 8. The
results indicate that cooled organogel melting temperature in-
creases within the temperature range 46e66 ꢀC with increasing the
gelator concentration from 2.24 to 3.30 mM. However, the orga-
nogel melting temperature was found to decrease with further
increase of the gelator concentration (gelator concentration
>3.30 mM). The reason for the decrease of thermal stability at
gelator concentration >3.30 mM is not clearly understood. The
increasing thermal stability at the gelator concentration range from
2.24 to 3.30 mM; can be attributed to the incorporation of a higher
number of gelator molecules in the aggregate. In other words,
increasing the gel / sol transition temperature is associated with
the growth of one-dimensional aggregates (nanofibers), which
makes them more flexible, causing more entanglement of the fi-
bers, and thereby increasing the transition temperature value.
xerogel clearly shows the three-dimensional networks formed by
the intertwined fibers that become swollen with immobilized
organic solvent and results in the formation of the gels. Interest-
ingly, the xerogel film responds reversibly to alkaline vapors with a
concomitant color change from orange to blue. Therefore, these
nanofibers may exhibit higher sensitivity in the detection of alka-
line analytes due to the higher ratio of surface-to-volume, and the
larger interspaces in the networks facilitates the convenient
diffusion of guest gas molecules across the matrix. Therefore, the
three dimensional networks based on xerogel from DCDHF-H 5
exhibit an efficient sensing ability (see video abstract). It is antici-
pated that the xerogel detection film can be employed for detection
of toxic and invisible industrial gases such as ammonia used in
industry especially refrigeration.
Supplementary video related to this article can be found at
http://dx.doi.org/10.1016/j.dyepig.2016.03.044.
To understand the molecular structure and crystal packing, we
obtained a single crystal of compound 5 from a mixture of ethanol/
toluene (3:1) and subjected it to X-ray crystallography. The mo-
lecular structure in the crystal and the crystal packing structures of
5 are illustrated in Fig. 6. The crystal structure reveals two
104
1.0x104
G'
G'
G''
8.0x103
6.0x103
4.0x103
103
102
101
G''
2.0x103
0.0
0
1000
2000
3000
4000
5000
1
10
100
Strain / %
Time / s
a.
c.
104
103
102
101
100
104
103
102
G'
G''
G'
G''
0
20
40
60
80 100 120 140 160 180
1
10
100
Ang. frequency / rad s-1
Time / s
b.
d.
Fig. 9. Storage modulus Gʹ and loss modulus Gʺ values of the organogel of 5 (2.24 mM) in n-propanol on the rheological experiments of (a) strain sweep; (b) Frequency sweep; (c)
time sweep; and (d) three cycles of deformation and recovery processes.

T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
335
3.6. Examination of thixotropic properties via rheological
techniques
changing the pH value. These organogels in different pH media
have melting temperatures higher than the physiological temper-
ature. Thus the DCDHF-H organogel may have potential application
in drug delivery.
Of all the gels from homologous DCDHF-H gelators, only
DCDHF-H 5 organogels exhibited the thixotropic property (Table 2).
We found that the organogel of 5 in n-propanol exhibites thixo-
tropic properties (Fig. 3). The organogel of 5 in n-propanol was
changed to a solution by exposing to ammonia (alkaline) gas, and
then regenerated rapidly with acidic (HCl) vapor. This reversible
gel-sol phase transition could be repeated many times. To the best
of our knowledge, there is no reported example of a thixotropic
DCDHF-Hydrazone organogel. Rheological characterizations of
organogels were additionally achieved in dynamic manner to
examine this pH-responsive behavior in detail. Although the alka-
line ammonia gas broke the well-established assemblies, the acidic
vapors lead to reformation of organogel while it retained the
thixotropic properties. The rheological experiments on the orga-
nogel of 5 in n-propanol were then conducted. The variations in
storage modulus (Gʹ) and loss modulus (Gʺ) under shear strain were
recorded (Fig. 9a). At low strain values, the Gʹ values are one order
of magnitude larger than those of Gʺ, suggesting the dominant
elastic nature of the organogel. Both Gʹ and Gʺ continue roughly
constant below the critical strain value (ca. 1.0%). When the strain
value is above 1.0%, both Gʹ and Gʺ regularly reduced, indicating a
partial breakup of the organogel. The changes of storage modulus
(Gʹ) and loss modulus (Gʺ) as a function of angular frequency were
also determined as shown in Fig. 9b. The frequency sweep results
also show that at low.
frequency values (ca. below 1.0 rad/s), the Gʹ values are one
order of magnitude larger than the values of Gʺ, suggesting the
performance of a “true gel”. However, both Gʹ and Gʺ demonstrate
apparent frequency dependency in the 1.0e100 rad/s frequency
range, which indicates that the organogel has poor tolerance to
external forces in the 1.0e100 rad/s frequency range. The recovery
property of the organogel of 5 in n-propanol was further studied. As
shown in Fig. 9c, after the gel is continually deformed at 100%
deformation for 30 s, its Gʹ and Gʺ are monitored over 5000 s to
follow the structural recovery of the gel. The gel recovers its elastic
character rapidly and proceeds as a gel-like material once the
external force is removed. Furthermore, the thixotropic process of
the gel is completely reproducible for at least three cycles as shown
in Fig. 9d. Generally, the rheological measurements demonstrate
that the organogel of 5 is a robust thixotropic gel with a fast-
recovery ability, which might be attributed to the rapid reorgani-
Acknowledgments
This research was supported by the National Science Foundation
through the Center for Layered Polymer Systems (CLiPS) under
Grant No. 0423914. This material was also supported by the USDA
grant # 58-3148-2-113.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.03.044.
References
[1] An Byeong-Kwan, Lee Deug-Sang, Lee Jong-Soon, Park Yil-Sung, Song Hyung-
Su, Park Soo Young. Strongly fluorescent organogel system comprising fibrillar
self-assembly of a trifluoromethyl-based cyanostilbene derivative. J Am Chem
Soc 2004;126(33):10232e3.
[2] Tsou Chia-Chen, Sun Shih-Sheng. New fluorescent amide-functionalized
phenylethynylthiophene low molecular weight gelator. Org Lett 2006;8(3):
387e90.
[3] Wang Sisi, Xue Pengchong, Wang Panpan, Yao Boqi, Li Kechang, Liu BaiJun.
Fluorescence-enhanced gels of D-p-A diphenylacrylonitrile derivatives: in-
fluence of nitro group and alkoxy chain. Dyes Pigments 2015;122:302e9.
[4] Li Xue-Qing, Stepanenko Vladimir, Chen Zhijian, Prins Paulette,
Siebbeles Laurens DA, Würthner Frank. Functional organogels from highly
efficient organogelator based on perylene bisimide semiconductor. Chem
Commun 2006;37:3871e3.
[5] Jeong Yeonhwan, Hanabusa Kenji, Masunaga Hiroyasu, Akiba Isamu,
Miyoshi Kentaro, Sakurai Shinichi, et al. Solvent/gelator interactions and su-
pramolecular structure of gel fibers in cyclic bis-urea/primary alcohol orga-
nogels. Langmuir 2005;21(2):586e94.
[6] Zhang Xiaofei, Lu Ran, Jia Junhui, Liu Xingliang, Xue Pengchong, Xu Defang,
et al. Organogel based on b-diketone-boron difluoride without alkyl chain and
H-bonding unit directed by optimally balanced
Commun 2010;46(44):8419e21.
pep interaction. Chem
[7] He Yabing, Bian Zheng, Kang Chuanqing, Cheng Yanqin, Gao Lianxun. Novel C
3-symmetrical triphenylbenzene-based organogelators with different linkers
between phenyl ring and alkyl chain. Tetrahedron 2010;66(19):3553e63.
[8] Wang Xiao-Jun, Xing Ling-Bao, Cao Wei-Ning, Li Xu-Bing, Chen Bin,
Tung Chen-Ho, et al. Organogelators based on TTF supramolecular assemblies:
synthesis, characterization, and conductive property. Langmuir 2010;27(2):
774e81.
[9] Xue Pengchong, Lu Ran, Chen Guojun, Zhang Yuan, Nomoto Hiroyuki,
Takafuji Makoto, et al. Functional organogel based on a salicylideneaniline
derivative with enhanced fluorescence emission and photochromism. Chem A
Eur J 2007;13(29):8231e9.
[10] Yoshida Masaru, Koumura Nagatoshi, Misawa Yoshihiro, Tamaoki Nobuyuki,
Matsumoto Hajime, Kawanami Hajime, et al. Oligomeric electrolyte as a
multifunctional gelator. J Am Chem Soc 2007;129(36):11039e41.
[11] Sangeetha Neralagatta M, Maitra Uday. Supramolecular gels: functions and
uses. Chem Soc Rev 2005;34(10):821e36.
zation of these molecules via cooperative
Waals interactions.
pep stacking and van der
4. Conclusions
[12] Ray Sudipta, Das Apurba K, Banerjee Arindam. pH-responsive,
bolaamphiphile-based smart metallo-hydrogels as potential dye-adsorbing
agents, water purifier, and vitamin B12 carrier. Chem Mater 2007;19(7):
1633e9.
[13] Miao Wangen, Qin Long, Yang Dong, Jin Xue, Liu Minghua. Multiple-stimulus-
responsive supramolecular gels of two components and dual chiroptical
switches. Chem A Eur J 2015;21(3):1064e72.
Three pH-responsive DCDHF dyes containing a hydrazone unit
as part of the -system have been successfully designed, synthe-
p
sized and characterized. We have demonstrated an interesting
protonation and deprotonation induced gelation of a DCDHF-H
gelator. It was possible to demonstrate the acid stability and base
instability of the obtained organogel. It was also found that the
organogels showed high thermal stability and their morphology
[14] Kim Sung-Hoon, Hwang In-Jeong, Gwon Seon-Yeong, Son Young-A. The
thermoresponsive behaviour of a poly (N-isopropylacrylamide) hydrogel with
a D-p-A type pyran-based fluorescent dye. Dyes Pigments 2010;87(1):84e8.
[15] Cornwell Daniel J, Smith David K. Expanding the scope of gelsecombining
polymers with low-molecular-weight gelators to yield modified self-
assembling smart materials with high-tech applications. Mater Horiz
2015;2(3):279e93.
[16] Kim Tae Hyeon, Kim Dai Geun, Lee Minjung, Lee Taek Seung. Synthesis of
reversible fluorescent organogel containing 2-(20-hydroxyphenyl) benzox-
azole: fluorescence enhancement upon gelation and detecting property for
nerve gas simulant. Tetrahedron 2010;66(9):1667e72.
was fibers of diameters ranging from 220 nm to 1.2 mm. This low
molecular weight gelator, that can gelate organic solvents, repre-
sents a new class of gelators that have the ability to respond to acid/
base stimuli and are potentially valuable in emerging fields. Com-
pound 5 can assemble to form the molecular stacks in solution;
however, further stacking to generate the supramolecular
morphology needs the support of van der Waals interactions of the
long alkoxy chain. Although the organogel is quite stable in the
presence of alcohol, the gelation properties can be modulated by
[17] Guan Xidong, Fan Kaiqi, Gao Tongyang, Ma Anping, Zhang Bao, Song Jian.
A novel multi-stimuli responsive gelator based on d-gluconic acetal and its
potential applications. Chem Commun 2016;52(5):962e5.
[18] Lin Qi, Zhu Xin, Fu Yong-Peng, Yang Qing-Ping, Sun Bin, Wei Tai-Bao, et al.

336
T.A. Khattab et al. / Dyes and Pigments 130 (2016) 327e336
Rationally designed supramolecular organogel dual-channel sense Fꢂ under
gel-gel states via ion-controlled AIE. Dyes Pigments 2015;113:748e53.
[19] Arndt Karl-Fr, Schmidt Thomas, Richter Andreas, Kuckling Dirk. High response
smart gels: synthesis and application. In: Macromolecular Symposia. Vol. 207.
No. 1. WILEY-VCH Verlag; 2004.
Enzymatic activation of nitro-aryl fluorogens in live bacterial cells for enzy-
matic turnover-activated localization microscopy. Chem Sci 2013;4(1):220e5.
[27] Park Jeesook, Kim Hyunjin, Choi Yongdoo, Kim Youngmi. A ratiometric fluo-
rescent probe based on a BODIPYeDCDHF conjugate for the detection of
hypochlorous acid in living cells. Analyst 2013;138(12):3368e71.
[20] Grove Tijana Z, Forster Jason, Pimienta Genaro, Dufresne Eric, Regan Lynne.
A modular approach to the design of protein-based smart gels. Biopolymers
2012;97(7):508e17.
[28] Willets Katherine A, Nishimura Stefanie Y, Schuck P James, Twieg Robert J,
Moerner WE. Nonlinear optical chromophores as nanoscale emitters for
single-molecule spectroscopy. Acc Chem Res 2005;38(7):549e56.
[21] Lu Zhikuan, Lord Samuel J, Wang Hui, Moerner WE, Twieg Robert J. Long-
wavelength analogue of PRODAN: synthesis and properties of Anthradan, a
fluorophore with a 2, 6-donor-acceptor anthracene structure. J Org Chem
2006;71.26:9651e7.
[29] Cheng Yen-Ju, Luo Jingdong, Hau Steven, Bale Denise H, Kim Tae-Dong,
Shi Zhengwei, et al. Large electro-optic activity and enhanced thermal sta-
bility from diarylaminophenyl-containing high-b nonlinear optical chromo-
phores. Chem Mater 2007;19(5):1154e63.
[22] Lord Samuel J, Lu Zhikuan, Wang Hui, Willets Katherine A, Schuck PJames,
Lee Hsiao-lu D, et al. Photophysical properties of acene DCDHF fluorophores:
long-wavelength single-molecule emitters designed for cellular imaging.
J Phys Chem A 2007;111(37):8934e41.
[23] Luo Jingdong, Zhou Xing-Hua, Jen Alex K-Y. Rational molecular design and
supramolecular assembly of highly efficient organic electro-optic materials.
J Mater Chem 2009;19(40):7410e24.
[30] Gubler Ulrich, He Meng, Wright Daniel, Roh Yeonsuk, Twieg Robert,
Moerner WE. Monolithic photorefractive organic glasses with large coupling
gain and strong beam fanning. Adv Mater 2002;14(4):313.
[31] Su Bin, Diaz-Cruz Edgar S, Landini Serena, Brueggemeier Robert W. Novel
sulfonanilide analogues suppress aromatase expression and activity in breast
cancer cells independent of COX-2 inhibition.
J Med Chem 2006;49(4):
1413e9.
[24] Lu Zhikuan, Liu Na, Lord Samuel J, Bunge Scott D, Moerner WE, Twieg Robert J.
Bright, red single-molecule emitters: synthesis and properties of environ-
mentally sensitive dicyanomethylenedihydrofuran (DCDHF) fluorophores
with bisaromatic conjugation. Chem Mater 2009;21(5):797e810.
[25] Davis Matthew C, Chafin Andrew P, Hollins Richard A, Baldwin Lawrence C,
Erickson Eric D, Zarras Peter, et al. Synthesis of an isophorone-based nonlinear
optical chromophore. Synth Commun 2004;34(18):3419e29.
[32] Su Bin, Chen Shiuan. Lead optimization of COX-2 inhibitor nimesulide analogs
to overcome aromatase inhibitor resistance in breast cancer cells. Bioorg Med
Chem Lett 2009;19.23:6733e5.
€
[33] Khattab Tawfik A, Abdelmoez Sherif, Klapotke Thomas M. Electrospun
nanofibers from
a tricyanofuran-based molecular switch for colorimetric
recognition of ammonia gas. Chem A Eur J 2016;22(12):4157e63.
ꢀ
ꢀ
[34] Millan Daniela, Domínguez Moises, Rezende Marcos Caroli. Solvatochromic
[26] Lee Marissa K, Williams Jarrod, Twieg Robert J, Rao Jianghong, Moerner WE.
hydrazone anions derived from chalcones. Dyes Pigments 2008;77(2):441e5.