Synthesis and Self-Assembly of Novel s-Tetrazine-Based Gelator

Article abstract of DOI:10.1002/hlca.201800009

The design, synthesis and self-assembly of new symmetrical 3,6-bis(4-(3,4,5-tris(dodecyloxy)benzoate)phenyl)-1,2,4,5-tetrazine were described. The novel gelator, sym-tetrazine, was prepared by addition reaction of 4-cyanophenol with hydrazine monohydrate

Full text of DOI:10.1002/hlca.201800009

Article Type: Full Paper
Synthesis and Self-assembly of Novel s-Tetrazine-based Gelator
Tawfik A. Khattab*
Dyeing, Printing and Auxiliaries Department, Textile Research Division, National Research Centre,
Cairo 12622, Egypt
Email: ta.khattab@nrc.sci.eg
The design, synthesis and self-assembly of new symmetrical 3,6-bis(4-(3,4,5-
tris(dodecyloxy)benzoate)phenyl)-1,2,4,5-tetrazine were described. The novel gelator, sym-tetrazine,
was prepared by addition reaction of 4-cyanophenol with hydrazine monohydrate followed by
oxidation reaction to afford the corresponding 3,6-bis(4-hydroxyphenyl)-1,2,4,5-tetrazine which was
then subjected to esterification reaction with 3,4,5-tris(dodecyloxy)benzoic acid. The chemical
structure of the sym-tetrazine gelator was confirmed by elemental analysis, fourier-transform
infrared spectroscopy (FT-IR), and nuclear magnetic resonance (1H and 13C NMR) spectral
measurements. It was confirmed to exhibit relatively strong gelation ability to produce
supramolecular assemblies in several polar alcoholic organic solvents, such as n-butanol, n-octanol
and 1,6-Dihydroxyhexane. The π-π stacking and van der Waals mediated self-assembly of tetrazine-
based organogelator were studied by scanning electron microscopy images of the xerogel to reveal
that the obtained organogel consists of fibrillar aggregates. Investigation of FT-IR and concentration-
1
dependent H NMR spectra confirm that the intermolecular van der Waals interactions and π-π
stacking were the key driving forces for self-assembly during gelation process of s-tetrazine
molecules.
Keywords: s-Tetrazine • Organogel • Self-assembly • Gelation properties
Introduction
s-Tetrazines are a distinguished class of aromatic heterocycles that have been used in different
research fields including anti-tumor activity [1, 2], coordination specially in metallosupramolecular
[7]
assemblies [3], molecular self-assembly [4], insecticides [5], electrochemistry [6], fluorescence
,
conductive materials [8], bioorthogonal labeling [9], and cell detection applications [10]. Tetrazines
represent the most electron deficient aromatic group of materials. They provide unusual high
electron affinity and charge transfer potential as conjugated molecular structures [11]. They display
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bright colors and electron rich materials have the ability to selectively quench their fluorescence
emission [1, 12]. Tetrazines have been valuable as stimuli-responsive smart materials in the molecular
[15]
identification of environmental pollutants [13], as anion binders [14], and in metal complexation
.
Tetrazines readily react in inverse demand Diels-Alder reactions [16]. High-nitrogen energetic 1,2,4,5-
tetrazine-based materials were also recognized for their use in propellants, explosives, and
pyrotechnic ingredients [17-19]
.
Supramolecular gels are soft materials mainly consist of a fluid, but exhibit solid-like viscoelastic
characteristics which can be attributed to the existence of a gelator able to assemble into three-
dimensional arrangements. These systematic networks can be created in the form of chemical
bonding via crosslinking or polymerization, or physical bonding via self-assembly, phase transition or
entanglements. Recently, low molecular-weight organogelators (LMOGs) have attracted
considerable interest due to the large diversity of gelators giving rise to thermally reversible
organogels in different solvents [20-24]. The distinct capability of small organogelator to solidify and
hold organic liquids via weak non-covalent interactions such as π-π stacking, push-pull systems,
hydrogen bonding, and van der Waals interactions, has received significant attention [25-27]. It is
significant to control such intermolecular forces in a way that the gelator molecules can self-
assemble into fibrous higher order assemblies but without switching into dense crystals [25]. LMOGs
are characterized by their potential function in a wide range of applications, such as rheology
improvers in cosmetics and food industries; polymer crystal nucleating agents; production of
inorganic nanoporous materials; multifunctional sensors; soft materials for drug delivery and
medical regeneration in tissue engineering; and removal of pollutants [28, 29]. The structure-property
relationship of LMOGs has proven to be complex. It has been found that small variations in the
molecular structure, such as the number of carbon atoms in an n-alkyl chain or introducing branches
or cyclic moieties can have remarkable influences on the gelation performance, such as the
organogel transition temperature or the range of solvents that are gelled [25-29]
.
Many structure prototypes of LMOGs have been reported recently [25-29]. Surprisingly, to the best of
our knowledge; there is no report of LMOGs based on sym-tetrazines. We observed that long alkoxy
chain substituted sym-tetrazine can be self-assembled into supramolecular architectures. Herein, we
report the first pattern of thermally reversible tetrazine-based LMOGs via cooperative van der Waals
and π-stacking induced supramolecular assembly. In this LMOG, the self-assembly of appropriate
organic molecules by the process of non-covalent forces results in the formation of fibrillar
supramolecular structures that were further intertwined to afford sample-spanning networks
capable of supporting the solvent. The synthesis of the newly designed tetrazine diphenylester
gallate (TZ-DEG) and its gelatinization behavior in a variety of organic solvents was described.
Results and Discussion
Synthesis and characterization
The 3,4,5-tris(dodecyloxy)benzoic acid 3 was prepared according to literature procedures starting
from ethyl 3,4,5-trihydroxybenzoate 1 [30, 31]. The dihydrotetrazine 5 and 3,6-bis(4-hydroxyphenyl)-
1,2,4,5-tetrazine 6 were prepared according to literature procedures starting from 4-
hydroxybenzonitril 4 [32, 33]. As can be seen in Scheme 1, compound 1 was refluxed with 1-
bromododecane (3.2 equiv.) in presence of anhydrous potassium carbonate and potassium iodide as
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a catalyst to afford ethyl 3,4,5-tris(dodecyloxy)benzoate 2 which was subjected to saponification via
hydrolysis under basic conditions, where sodium hydroxide acts as a nucleophile, while an ethoxide
is the leaving group affording compound 3 ethanol by-product. Based on literature procedure, the
dihydrotetrazine 5 was prepared from 4-hydroxybenzonitril 4 via addition reaction with hydrazine
monohydrate followed by oxidation using sodium nitrite to afford compound 6. The reaction of 3,6-
bis(4-hydroxyphenyl)-1,2,4,5-tetrazine 6 with 3,4,5-tris(dodecyloxy)benzoic acid 3 was performed via
Steglich esterification employing N,Nʻ-dicyclohexylcarbodiimide (DCC) as a coupling reagent and 4-
(dimethylamino)pyridine (DMAP) as a catalyst to produce our novel sym-tetrazine gelator (TZ-DEG
7).
RO
HO
HO
RO
RO
O
O
O
O
O
(i)
(ii)
RO
HO
RO
HO
OH
RO
OH
3
2
1
HN NH
N N
(iii)
(iv)
HO
CN
HO
5
4
N N
N N
OH
6
R'
H2
H1
H1
H1
H1
H2 O
A
R'''
R'
N N
C
N N
R''
B
B
O
O
(v)
A
R''
R'''
O H2
H2
7
Scheme 1. Synthetic route for novel sym-tetrazine organogelator 7 (R = C12H25). Reagents and conditions: (i) C12H25Br, acetone, K2CO3, KI, reflux 36 h; (ii) NaOH,
acetone, 12 h; (iii) NH2NH2.H2O, 90˚C 16 h; (iv) CH3COOH, NaNO2 [O]; (v) 3, DCC, DMAP, dry THF.
1
In H NMR spectra, the peak at 7.37 ppm was assigned for the two equivalent gallate moieties (A).
Although symmetrical molecular structure of compound 7, the two para-substituted benzene rings
(B) displayed two different doublet peaks at 8.68 and 7.41 ppm. This is due to the different
surrounding electronic environments around protons H1 and H2 as a result of both inductive and
resonance effects applied on both protons H1 and H2. Furthermore, a weak CH---N hydrogen bonding
could be experienced between protons H1 and the corresponding nitrogen atoms of the tetrazine
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ring, and therefore, doublets were only observed and not the expected doublets of doublets [33]. The
IR spectra of 7 display a weak aromatic stretch peak at 3073 cm-1. Another two strong peaks
appeared at 2921 and 2851 cm-1 assigned for CH aliphatic asymmetric stretch and CH aliphatic
symmetric stretch, respectively. The carbonyl ester stretch group appeared at 1734 cm-1.
Gelation properties
The gelation behavior of TZ-DEG were examined in a variety of solvents by means of the ‘‘stable to
inversion’’ technique and the minimum gel concentration (MGC) values were recorded. In the
‘‘stable to inversion’’ technique, the glass vial containing the formed gel is inverted upside down to
indicated the absence of fluid solvent which was immobilized due to the construction of the soft gel
(Fig. 1). Compound 7 selectively formed gels mainly in polar solvents. TZ-DEG showed lower MGC in
protic alcoholic solvents compared to non-protic polar solvents which could be attributed to the
hydrogen bonding between the alcoholic hydrogen of the protic polar solvent and the ester oxygen
of TZ-DEG gelator. The MGCs is known to decrease upon increasing the length of terminal alkyl
chains leading to better gel ability [34]. It was found that 7 did not generate a gel following typical
heating-cooling procedure in tetrahydrofuran, dimethylformamide, ethyl acetate or dimethyl
sulfoxide in which the solubility is too high. Similarly, 7 cannot gel in toluene or cyclohexane owing
to the very low solubility. However, TZ-DEG acts as a gelator in pure alcohols which indicated that 7
is a successful gelator for various alcohols. A series of mixed non-alcohols were also inspected. It
was found that an organogel of 7 can be produced in a number of mixed solvents such as
chloroform/cyclohexane (v/v = 1/3) and acetone/cyclohexane (v/v = 1/1). The gelation studies of the
minimum gel concentration values are displayed in Table 1. The minimum gelation concentration
values demonstrate solvent dependent performance and occur in the range of 2.38 and 9.63 mM
which is comparable to a variety of reported organogelators containing long alkoxy side chains [27, 34-
37]. In addition, the n-butanol gel formed from the TZ-DEG gelator was found to be stable for a
number of weeks at room temperature and atmospheric pressure, while under similar
circumstances, the acetone/cyclohexane gel was stable for only few days.
Figure 1. Reversible sol-gel phase transition of the organogel of TZ-DEG in n-butanol.
The above results elucidate that the long alkoxy side chains of the TZ-DEG gelator not only improve
its solubility in organic liquids but also support assembly between the generated nanofibrous
architectures, via van der Waals forces, and consequently organogel creation. However, the strong
solvation of the alkoxy chain may also reduce the intermolecular affinity required for self-assembly.
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As indicated in the literature, the long alkoxy chain is an essential feature on the aromatic core for
creating a high-quality gelator [34]. These results signify the important function of the side dodecyloxy
chains in the gelation capability. The six dodecyloxy side chains of 7 increase its solubility in various
solvents; therefore, the strong solvation probably brings about the low intermolecular affinity of the
self-association in non-polar and non-protic polar solvents.
Table 1. The minimum gel concentration (MGC) of TZ-DEG in different solvents.
Solvent
MGC in mM
Methanol
Ethanol
sol
ppt
ppt
n-Propanol
n-Butanol
n-Octanol
G (3.06 mM)
G (2.82 mM)
G (4.95 mM)
G (3.21 mM)
G (2.38 mM)
sol
1,2-Dihydroxyethane
1,3-Dihydroxypropane
1,6-Dihydroxyhexane
Acetic acid
Formic acid
sol
Benzyl alcohol
ppt
Toluene
insol
Cyclohexane
insol
Chloroform/Cyclohexane (v/v = 1/3)
Chloroform
G (5.26 mM)
ppt
Acetone
ppt
Ethyl acetate
sol
THF
sol
Dimethylformamide
Dimethyl sulfoxide
Acetone/Cyclohexane (v/v = 1/1)
sol
sol
G (9.63 mM)
G: gel; sol: soluble; insol: insoluble; ppt: precipitation. The
minimum gel concentration in mM is shown in parenthesis.
The UV-Vis absorption spectra of TZ-DEG in both solution and gel states were recorded in n-butanol
(Fig. 2). The main absorption peak undergoes a bathochromic shift from 310 nm for the solution
phase to 316 nm for the gel phase (ca. 6 nm shift value). Such red-shift of the absorption peak of TZ-
DEG gel phase to longer wavelength compared with that in dilute solution indicates that the
molecules packing have the J-aggregate form.
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1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
Sol
Gel
250
275
300
325
350
375
400
Wavelength (nm)
Figure 2. Normalized UV-Vis absorption spectra of TZ-DEG 7 in both solution (green curve) and gel (blue) states in n-butanol.
Morphology and structure of assemblies
To gain a macroscopic insight of the aggregation mode of TZ-DEG gelator in organogelation process,
the n-butanol xerogel (after complete solvent evaporation) was investigated by using scanning
electron microscopy. The SEM images display the development of 3D entangled arrangements (Fig.
3) in which the self-assembled tape-like structural morphology was monitored with widths of 180-
350 nm and lengths. SEM inspection of the xerogel gives an insight into higher order nanofibrous
arrangements able to bundle and generate the network. Creation of long nanofibers signifies that
strong directional intermolecular interactions might exist. The xerogel displays strongly developed
supramolecular morphology with high porosity responsible for holding and immobilizing solvent
molecules. This microscopic aspect of the xerogel affords three-dimensional nanofibrous network
produced by the intertwined nanofibers able to immobilize and hold organic solvent and result in
the construction of the gel.
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Figure 3. SEM images for the dried xerogel of TZ-DEG obtained from n-butanol (3.06 mM).
In order to discover the motivating forces for the self-assembly of the supramolecular nanofibrous
1
architectures, FT-IR and concentration-dependent H NMR spectroscopic experiments were carried
out. The development of expanded tape-like nanofibrous aggregates proves that the self-assembly
of TZ-DEG is motivated by strong directional intermolecular interactions. The π-π stacking
interaction forces of the aromatic gallate unit in solution were verified by 1H NMR spectroscopy. No
gelation behavior was monitored upon increasing the minimum gel concentration of TZ-DEG in
chloroform. However, nanofibrous precipitate was formed upon cooling. Therefore, chloroform can
1
be considered as a suitable solvent to study self-assembly behavior using H NMR spectroscopy.
Upon regular increase of concentration (Deuterated chloroform solution of 7) from 0.5 to 25 mmol L-
1, the gallate ring protons slightly shifted upfield from 7.37 to 7.34 ppm (Fig. 4). This is strong
evidence for the creation of stacked molecules in which the protons of the gallate aromatic ring are
shielded by aromatic rings of adjacent molecules due to the π-π stacking interactions between the
aryl rings are engaged in the gel development process.
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Figure 3. Set of partial 1H NMR spectra of TZ-DEG in CDCl3 at different concentrations of (a) 0.5 mmol L-1, (b) 10 mmol L-1, and (c) 25 mmol L-1 at room
temperature.
Furthermore, both of symmetric and antisymmetric methylene stretching vibrational modes of TZ-
DEG was employed to detect alkyl chain conformations. With the alkyl chain length expanding, for
TZ-DEG xerogel, the (CH2)sym and (CH2)anti were shifted from 2851 cm-1 (solid) to 2849 cm-1 (xerogel)
and from 2921 cm-1 (solid) to 2917 cm-1 (xerogel), respectively, indicating a significant population of
the trans conformation (Fig. 8S) [34]. The decreased wavenumber values reflect a reduced fluidity of
the alkyl chains due to the physically powerful association of the alkyl units via van der Waals forces.
Accordingly, the motivating forces of the organogelation process are π-π stacking interaction forces
of the aromatic gallate ring in cooperation with van der Waals forces among the terminal alkyl
chains.
The thermal stability of the TZ-DEG organogel in n-butanol was explored by determining the
concentration-dependent gel-to-sol transition temperature (Fig. 5). An increase in the organogel
melting temperature was detected within the temperature range 33-47°C upon increasing the
gelator concentration in n-butanol from 3.06 to 10.17 mmol L-1. However, the organogel melting
temperature decreases upon increasing the gelator concentration (gelator concentration > 10.17
mmol L-1). The reason for the reduced thermal stability at gelator concentration > 10.17 mmol L-1 is
not obviously understood. The improved thermal stability at gelator concentration lies between
3.06-10.17 mmol L-1; can be assigned to the higher amount of gelator molecules incorporated in the
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nanofibrous aggregates. In other words, the increased gel→sol transition temperature is correlated
to the expansion of one-dimensional nanofibrous aggregates, which exhibits them more flexibility,
leading to more fiberous entanglements, and that way rising the transition temperature.
48
46
44
42
40
38
36
34
32
3
4
5
6
7
8
9
10
Concentration (mM)
Figure 5. Gel-to-sol transition temperature profile of TZ-DEG gelator in n-butanol as a function of gelator concentration.
To gain insight into the sol-to-gel transition reversibility, we conduct the temperature-dependent
gel-to-sol transition. The gel was subjected to heating around the boiling point (118˚C for n-butanol)
until a clear homogeneous solution appears, and the gel-to-sol transition temperature was reported
at the initial collapse of the gel. The mixture was then allowed to stand at room temperature (at
25˚C) within few minutes to allow the gel to reform. The formation of gels was confirmed by “stable
to inversion of a glass vial” technique which was indicated by the absence of fluid solvent. The
process can be repeated by numerous cycles of heating and cooling (Fig. 9S). No changes were
observed in the gel-to-sol transition temperature, indicative of the fully thermal reversibility of the
sol-to-gel conversion.
Conclusions
Novel sym-tetrazine containing gallate unit was successfully designed, synthesized and
1
characterized. According to SEM clarification, FT-IR and H NMR spectroscopic techniques, we have
demonstrated the systematic investigation of the gelation properties in different solvents for novel
sym-tetrazine-based gelator assembled to form the macromolecular morphology. Upon studying the
minimum gel concentration, assemblage morphology of organogel, and gel formation driving forces,
the results show that both of π-π stacking and van der Waals forces are the key driving forces for the
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development of self-assembled organogel. The temperature-dependent gel-to-sol transition
indicated fully thermal reversibility of the sol-to-gel conversion.
Experimental Section
Materials and methods
All experimental results were collected at room temperature (25˚C) unless stated otherwise.
Solvents used in this study were obtained from Fluka and Aldrich for syntheses and spectroscopic
measurements (spectroscopic grade) and were employed without further purification. Compound 3
was prepared according to literature procedures starting from ethyl 3,4,5-trihydroxybenzoate 1
commercial material [30, 31]. Compounds 5 and 6 were prepared according to literature procedures
starting from the commercial compound 4 [32, 33]. Compounds 1 and 4 were purchased from Sigma-
Aldrich. All reactions were monitored by thin layer chromatography (TLC) using Merck aluminum
plates pre-coated with silica gel PF254; 20x20 cm, 0.25 mm, and detected by visualization of the
plate under UV lamp (254 or 365 nm). Compounds were purified through crystallization or using
flash column chromatography which was performed on Scharlau silica gel, packed by the slurry
method.
Synthesis of 3,6-bis(4-(3,4,5-tris(dodecyloxy)benzoate)phenyl)-1,2,4,5-tetrazine 7
In a 250 ml round bottom flask; a mixture of 3,6-bis(4-hydroxyphenyl)-1,2,4,5-tetrazine 6 (650 mg,
2.24 mmol), 3,4,5-tris(dodecyloxy)benzoic acid 3 (3.37 g, 5.0 mmol, 2.05 equiv.), 4-
dimethylaminopyridine DMAP (30 mg, 0.25 mmol) and dicyclohexylcarbodiimide DCC (1.03 g, 5.0
mmol) was stirred in an anhydrous THF (80 mL) overnight. The mixture was concentrated under
reduced pressure, filtered over multiple times from dichloromethane, and re-crystallized from
1
methanol to afford a purple solid (2.78 g, yield 72%). mp 107-108˚C; H NMR (400 MHz, CDCl3), δ
(ppm): 8.68 (d, 4H Ar-H), 7.41 (d, 4H Ar-H), 7.37 (s, 4H Ar-H), 4.00 (m, 12H aliphatic), 1.74 (m, 12H
aliphatic), 1.43 (m, 12H aliphatic), 1.20 (m, 96H aliphatic), 0.81 (t, 18H aliphatic);
13C NMR (400 MHz, CDCl3), δ (ppm): 164.60 (C=O), 163.38 (C), 154,84 (B), 153.07 (A), 143.40 (A),
129.45 (B), 129.30 (B), 123.41 (A), 122.89 (B), 108.75 (A), 73.64 (R’’), 69.36 (R’ and R’’’), 31.96 (R’’),
31.93 (R’ and R’’’), 30.38 (R’’), 29.77 (R’ and R’’’), 29.75 (R’’), 29.74 (R’ and R’’’), 29.71 (R’’), 29.67 (R’
and R’’’), 29.65 (R’’), 29.58 (R’ and R’’’), 29.41 (R’’), 29.37 (R’ and R’’’), 29.33 (R’’), 26.10 (R’ and R’’’),
26.08 (R’, R’’ and R’’’), 22.71 (R’, R’’ and R’’’), 22.69 (R’, R’’ and R’’’), 14.11 (R’, R’’ and R’’’). IR (neat,
ν/cm-1): 3073 (CH aromatic stretch), 2921 (CH aliphatic asymmetric stretch), 2851 (CH aliphatic
symmetric stretch), 1734 (C=O ester stretch), 1591 (C=N), 1506 (C=C aromatic bend), 1333 (C-O
ester), 1190 (CH2 aliphatic deformation bend), 1124 (CH3 aliphatic deformation bend), 863 (CH
aromatic bend), 720 (CH2 aliphatic bend rocking); Elemental analysis calculated for C100H162N4O10
(1579.23): C 76.00, H 10.33, N 3.55; Found: C 75.89, H 10.36, N 3.61
Gelation procedure
The gelation tests were performed by dissolution of sym-tetrazine in solvent by heating in a sealed
glass vial around the boiling point until a clear solution appeared. The mixture was then allowed to
stand at room temperature for 1 hour. The gels formed in 10-25 minutes depending on the gelator
concentration and the employed solvent. The formation of gels was confirmed by “stable to
inversion of a glass vial” technique which was indicated by the absence of fluid solvents when the
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glass vial was inverted. The sol-to-gel transition reversibility was also investigated as the vial
containing the gel was kept upside down in a paraffin oil bath. The temperature was increased at a
rate of 2°C/minute and the temperature at which the gel fell under gravity was recorded as the gel
melting temperature. This procedure was repeated for several cycles.
Characterization and measurements
Melting points
All melting points were determined on Stuart SMP30 digital apparatus and were reported
uncorrected in °C.
FTIR spectra were recorded using a JASCO FT/IR-4700 spectrophotometer with a resolution of 4 cm-1
and in the wavenumber region of 4000-400 cm-1. The measurements were made at room
temperature (25˚C).
Elemental analyses (C, H, N) were performed with PerkinElmer 2400 analyzer (PerkinElmer, Norwalk,
CT, USA) at the Microanalytical Center, Cairo University.
1H and 13C NMR spectra
NMR spectra were recorded using a BRUKER AVANCE 400 spectrometer at 400 MHz; chemical shifts
were reported in ppm relative to tetramethylsilane internal standard at 295°K.
UV/Vis absorption spectra
The UV/Vis absorption spectra were recorded with an HP-8453 spectrophotometer.
Scanning Electron Microscope (SEM)
In case of scanning electron microscopy, the gel formed from TZ-DEG in n-butanol 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 sample was then annealed overnight in an oven at
45°C, followed by application of a 10 nm gold-coating before recording images. SEM images were
obtained using Hitachi S-2600N operating at 20 kV.
Procedure for concentration-dependent 1H NMR measurements
1
The concentration-dependent H NMR was carried out by gradually increasing the concentration of
TZ-DEG solution in CDCl3. The initial concentration of TZ-DEG 7 was 5 x 10-4 mol L-1, the
concentration was then adjusted by direct addition of correct quantity of 7 into the CDCl3 solution at
concentrations of 10 and 25 mmol L-1.
Supplementary Material
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/MS-number.
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Acknowledgements
Technical support from the National Research Centre, Cairo, Egypt, is gratefully acknowledged.
Authors Contribution Statement
T. A. Khattab designed and performed the experiments, analyzed the data, and wrote the
manuscript.
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