Synthesis and self-assembly of Salen type Schiff based on o-phenylenediamine organogels in response to Zn2+

Article abstract of DOI:10.1016/j.inoche.2020.108402

Two Salen type Schiff based on o-phenylenediamine were synthesized. The prepared organogelators demonstrated excellent gel properties in some selected solvents, such as n-pentanol, chloroform, and 1,2-dichloroethane. The results for thermal stability showed that under concentrations increasing of the gel molecules and then the gel-to-sol transition temperature value is increased. Through various techniques found that the hydrogen bonding between molecules, the van der Waals force, and the π-π stacking provide multiple driving forces for gel self-assembly. The morphology of the xerogel was investigated by Scanning Electron Microscope (SEM). The metal ions responsiveness experiment is completed by adding the metal ions solution dropwise to the gel surface and confirmed by the UV spectrum.

Full text of DOI:10.1016/j.inoche.2020.108402

Inorganic Chemistry Communications 124 (2021) 108402
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Inorganic Chemistry Communications
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Short communication
Synthesis and self-assembly of Salen type Schiff based on
o-phenylenediamine organogels in response to Zn2+
,
,
Yun-Shang Yanga *, Qi Shanga, Ying-Peng Zhanga *, Wei-Ya Niua, Ji-Jun Xueb
a School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China
b State Key Laboratory of Applied Organic Chemistry & College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two Salen type Schiff based on o-phenylenediamine were synthesized. The prepared organogelators demon-
strated excellent gel properties in some selected solvents, such as n-pentanol, chloroform, and 1,2-dichloro-
ethane. The results for thermal stability showed that under concentrations increasing of the gel molecules and
then the gel-to-sol transition temperature value is increased. Through various techniques found that the
Salen type Schiff
Organogels
Self-assembly
Response
hydrogen bonding between molecules, the van der Waals force, and the π-π stacking provide multiple driving
forces for gel self-assembly. The morphology of the xerogel was investigated by Scanning Electron Microscope
(SEM). The metal ions responsiveness experiment is completed by adding the metal ions solution dropwise to the
gel surface and confirmed by the UV spectrum.
1. Introduction
Salen-type Schiff bases and their complexes are usually used in
catalysis [18-22]. At the same time, its luminescence, photochromism,
In the past few decades, low molecular weight organogels (LOMGs)
are important soft materials that have been received extensive attention
due to their potential applications in drug delivery systems, biomedical
fields, tissue repair, and chemical sensors [1-4]. LOMGs can self-
assemble into the structures such as “belt”, “rod”, and “tube” in some
specific solvent. LOMGs usually self-assemble by weak intermolecular
magnetic properties have important research significance in the devel-
opment and application of new materials [23-32]. Salen-type Schiff
bases are easy to prepare and they have high coordination activity to
metal ions [33,34], so several Schiff base derivatives have been reported
which possess excellent gelation abilities and have good metal ions co-
ordination activity [35]. Some metal-containing compounds can exhibit
paramagnetism and/or mixed oxidation state properties key to the for-
mation of magnetic and conductive materials [36], so some metal-
containing compounds such as Schiff metal complex are synthesized.
however, some simple Schiff compounds may exhibit good gelation
abilities.
forces such as hydrogen bond,
tion. It is important to design a gel molecular including hydrogen bond,
stacking, and van der Waals interaction which is necessary to form a
π-π stacking, and van der Waals interac-
π-π
stable gel [5-9]. The self-assembly research of LOMGs may important for
the production of new materials [10]. Organogels are very sensitive to
external stimuli due to weak non-covalent interactions, it can lead to the
transformation of gel to the solution, so it is visual for ion recognition.
Zinc is an important trace element in the human body, which is
widely existed in cells. It is directly involved in the growth and devel-
opment of cells in the body and also involved in procreation, tissue
repair, gene transcription, metal enzyme catalysis, neurotransmission,
immune function, and other metabolic processes [11-14]. Studies have
reported that Zn2+ is closely related to diseases such as Alzheimer’s
disease (AD), epilepsy, ischemic stroke, and infantile diarrhea [15-17].
Therefore, the identification and detection of Zn2+ are of great
significance.
Therefore, we synthesized two Salen-type Schiff bases gel molecular
and explored the responsiveness of Salen-type Schiff bases gel to Zn2+
.
By dropping the metal ion solution onto the surface of the gel and
observing the change of the gel state, the recognition response of the gel
to the Zn2+ can be judged. And further verified by the UV–visible ab-
sorption spectrum. The synthesized gel molecular was analyzed and
characterized by infrared spectroscopy, ultraviolet spectroscopy, X-ray
diffraction, and scanning electron microscopy, and its self-assembly
driving force was explored.
* Corresponding authors.
E-mail addresses: yangyunshang@tom.com (Y.-S. Yang), yingpengzhang@126.com (Y.-P. Zhang).
https://doi.org/10.1016/j.inoche.2020.108402
Received 11 October 2020; Received in revised form 9 December 2020; Accepted 9 December 2020
Available online 24 December 2020
1387-7003/© 2020 Elsevier B.V. All rights reserved.

Y.-S. Yang et al.
Inorganic Chemistry Communications 124 (2021) 108402
Table 3
Gel G1 and G2 gel-sol transition temperature.
Solvent
Tgel of G1(◦C)
Tgel of G2(◦C)
n-butanol
55
52
39
49
54
/
/
n-pentanol
46
/
Dichloromethane
Chloroform
41
50
48
1,2-Dichloroethane
Ethyl acetate
Scheme 1. Synthesis route of gel factor G1 and G2.
Table 1
Gelation ability of G1 in organic solvents.
Solvent
result
CGC(g/
ml)
Solvent
result
CGC(g/
ml)
Methanol
Ethanol
P
P
/
/
Chloroform
Carbon
G
P
2.8%
/
tetrachloride
1,2-
N-butanol
G
1.5%
G
5.2%
Dichloroethane
Cyclohexanone
Acetone
Tert-butanol
N-pentanol
N-hexane
P
G
I
/
S
S
S
P
P
S
/
/
/
/
/
/
2.2%
/
Tetrahydrofuran
Ethyl acetate
Acetonitrile
Dimethyl sulfoxide
N-heptane
I
/
Cyclohexane
Dichloromethane
I
/
G
2.5%
Fig. 1. Tgel is a function of the concentration of G1.
P = precipitate, I = insoluble, G = stable gel, CGC is the critical gelation
concentration.
gel factor G2 can form a gel in 4 solvents. Gel factor G1 is insoluble in
low-polarity solvents such as n-hexane, n-heptane, and cyclohexane, so
it cannot form a gel. Gel factor G1 has excellent solubility in cyclohex-
anone, acetone, and dimethyl sulfoxide can not form a gel as well. It can
be seen that the gel factor G1 can form a thermally stable gel in n-
butanol, n-pentanol, dichloromethane, chloroform, and 1,2-dichloro-
ethane. Among them, the critical gelation concentration (CGC) in n-
butanol is 1.5%, and the lowest gel concentration in n-pentanol is 2.2%.
It can be judged that the gel factor G1 has better gel performance in long-
chain alcohols. Gel factor G2 can form a thermoreversible gel in N-
pentanol, chloroform, 1,2-dichloroethane, and ethyl acetate. Among
them, the lowest gel concentration (CGC) in 1,2-dichloroethane is the
lowest at 2.6%, so the gel factor G2 has good gel properties in 1,2-dichlo-
roethane. Gel factor G1 and gel factor G2 have different alkyl chain
lengths, so the solvents that can form gels are different. G1 which has a
longer alkyl chain can form thermally stable gels in long-chain alcohols
and halogenated hydrocarbons, while G2 which has a shorter alkyl chain
has better gel properties in halogenated hydrocarbons. The alkyl chain
decides the gel solvent.
Table 2
Gelation ability of G2 in organic solvents.
Solvent
result
CGC(g/
ml)
Solvent
result
CGC(g/
ml)
Methanol
Ethanol
I
/
/
Chloroform
Carbon
G
P
6.4%
/
P
tetrachloride
1,2-
n-butanol
P
/
G
2.6%
Dichloroethane
Cyclohexanone
Acetone
tert-butanol
n-pentanol
P
G
I
/
S
P
S
G
P
S
/
4.6%
/
n-heptane
/
/
/
/
Tetrahydrofuran
Ethyl acetate
Acetonitrile
Dimethyl sulfoxide
/
Cyclohexane
Dichlorome
Dichloromethane
I
4.8%
I
/
/
P
2. Experiment
Experiment reagent and instrumentation are listed in supporting
information.
3.2. Gel stability studies
2.1. Synthesis of gel factor G1 and G2
The thermal stabilities of the G1 and G2 gels were analyzed by the
inverted test tube method [37]. Gel phase inversion temperature was
showed in Table 3.
The synthetic route for gel factor G1 and G2 was shown in Scheme 1,
and the detailed synthetic methods are described in supporting
information.
The maximum phase transition temperature of the gel factor G1 is
55 ◦C in n-butanol. The maximum phase transition temperature of the
gel factor G2 is 50 ◦C in 1,2-Dichloroethane. According to reports in the
literature [38], The length of the alkyl chain affects the gel-sol phase
transition temperature of the gel factor. The longer alkyl chain makes
the greater stability of the gel because of the greater van der Waals force,
and the higher gel-sol phase transition temperature. Since the alkyl
chain of stearic acid is longer than palmitic acid, so the corresponding
maximum phase transition temperature is higher.
3. Result and discussion
3.1. Gelation behaviors
The gelation ability of G1 and G2 was examined in various organic
solvents. As summarized in Table 1 and Table 2. Gel factor G1 has good
gel properties which can form a thermoreversible gel in 5 solvents, and
The gel-sol phase transition temperatures (Tgel) of G1 in N-butanol,
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Y.-S. Yang et al.
Inorganic Chemistry Communications 124 (2021) 108402
3.3. FT-IR studies
To evaluate the driving forces for gelation, we measured the FT-IR
spectra. Fig. 2a and Fig. 2b show the FT-IR spectra of gel factor G1
and G2 xerogel and powder respectively. G1 and G2 contain hydroxyl
groups, so an obvious hydroxyl peak appears at around 3400 cmꢀ 1. The
peak at 1157 cmꢀ 1 is the hydroxyl C O bond stretching vibration. Peaks
–
at 2919 cmꢀ 1 and 2850 cmꢀ 1 are the asymmetric and symmetric
stretching vibrations of the methylene group. The peak at 1392 cmꢀ 1 is
the bending vibration of the methyl group, and at 740 cmꢀ 1 is the weak
absorption peak of the in-plane rocking vibration of (CH2)n. The peak at
1751 cmꢀ 1 is the absorption peak of the stretching vibration of the ester
–
group C O double bond. Through a comparison of G1 powder and
–
xerogel FT-IR spectra, it is found that the wavenumber of G1 is shifted
from 3409 cmꢀ 1 to 3405 cmꢀ 1, and the vibration peak of C N moves
–
–
from 1616 cmꢀ 1 to 1604 cmꢀ 1. Both of these two obvious changes
illustrate the formation of hydrogen bonds during the gel formation
process [39]. Observing the G2 powder and xerogel FT-IR spectra, peaks
at 2923 cmꢀ 1, 2850 cmꢀ 1 are the methylene stretching vibration, peak
at 1392 cmꢀ 1 is the methyl bending vibration, and at 740 cmꢀ 1 is the
(CH2)n weak absorption peak of the in-plane rocking vibration. The peak
at 1751 cmꢀ 1 is the absorption peak of the stretching vibration of the
–
ester group C O double bond. The hydroxyl absorption peak of G2
–
powder is at 3414 cmꢀ 1, while the hydroxyl absorption peak of xerogel
moved to 3409 cmꢀ 1. The C N absorption peak of G2 moved from
–
–
1616 cmꢀ 1 towards 1600 cmꢀ 1. It shows that the gel factor G2 also has
hydrogen bonding in the process of gel formation.
3.4. X-ray diffraction studies
The aggregation mode of the gel was further studied by testing XRD
of the gel factor G1 powder and xerogel. Fig. 3a is the X-ray diffraction
pattern of gel factor G1 solid powder and xerogel in 1,2-dichloroethane.
It can be seen from the XRD diagram that the G1 powder peak shape is
dispersed, and the xerogel has sharp diffraction peaks, which proves that
there is a tendency of crystal alignment during the gel formation process.
When 2θ = 13.52◦ corresponding d value is 0.33 nm, it can infer the
Fig. 2. (a) FT-IR spectra of G1 powder and xerogel (b) FT-IR spectra of G2
powder and xerogel.
N-pentanol, Dichloromethane, Chloroform, and 1,2-Dichloroethane are
plotted against the gelation concentration in Fig. 1. It can be found that
as the concentration of the gel factor increases, the gel-sol phase tran-
sition temperature increases. This is because the increase of the con-
centration makes the gel molecule pack more compact, so the
corresponding gel-sol phase transition temperature increases. It also can
be seen that as the concentration increases, the tendency of the gel-sol
phase transition temperature to increase becomes slower. Therefore,
to obtain a more stable gel, we can increase the concentration of the gel
factor to improve its gel-sol phase transition temperature.
occurrence of
π-
π
stacking [40]. However, the G1 powder does not show
stacking
the corresponding peak, which indicates that there is no
π-π
between the gel factor G1. Gel factor G2 xerogel has sharp diffraction
peaks (Fig. 3b), while gel factor G2 powder peaks are relatively diffuse,
so it also can indicate the aggregation arrangement of gel factors during
gel formation, A new diffraction peak was found at 2θ = 13.48◦, and the
corresponding d value is 0.34 nm, indicating the occurrence of
π-π
stacking in G2 gel. The gel factor G1 model was constructed with
gauss09w, and the structure of the gel factor G1 was optimized. The
optimized result is shown in Fig. 4. It can be seen from Fig. 4. that there
Fig. 3. XRD spectra of G1(a) and G2(b) gel molecule and xerogel.
3

Y.-S. Yang et al.
Inorganic Chemistry Communications 124 (2021) 108402
Fig. 4. Energy-minimized structures of G1 and the possible packing modes for the aggregates in organic solvents.
Fig. 5. SEM images, (a) G1 xerogel formed by 1,2-Dichloroethane, Magnitudes are 2000×, (b) G1 xerogel formed by 1,2-Dichloroethane, Magnitudes are 10,000×,
(c) G2 xerogel formed by 1,2-Dichloroethane, Magnitudes are 2000×, (d) G2 xerogel formed by 1,2-Dichloroethane, Magnitudes are 10,000×.
is a
π
-
π
stacking in the G1 molecule, and the distance between the aro-
Fig. 4a, it can be seen that the gel factor G1 is tangled in fiber strips.
Fig. 4b clearly shows that the gel factor G1 is gathered in thin fiber strips
and arranged neatly. It is inferred that the gel factors are arranged
regularly during the self-assembly process, and then begin to entangle,
and finally form a gel with solvent molecules. Although the self-
assembly of the gel is not arranged as neatly as the crystal, there are
certain rules in the assembly process, and the XRD results can also
explain it. The result of gel factor G2 is similar to that of G1, both gel
factors are arranged in fiber strips. By observing the morphology of the
gel, the aggregation and self-assembly form of the gel can be inferred.
matic ring is 3.61 nm. This value is approximately 10 times the value of
d calculated by XRD. This indicated the weak force between the aro-
matic rings of the gel factor has an important influence on the formation
of the gel.
3.5. Morphology of the gels
The morphology of the G1 and G2 xerogel and the powder was
studied by SEM(Fig. 5). The self-assembly of the gel should have a
certain arrangement rule to form a gel. The regularity of the gel factor
arrangement is confirmed by scanning electron microscopy. From
4

Y.-S. Yang et al.
Inorganic Chemistry Communications 124 (2021) 108402
Fig. 6. Normalized UV absorption spectra of compound G1(a) and G2(b) in different solvents(1 × 10ꢀ 5 mol/L).
Fig. 7. The photo of G1 gel added cations.
3.6. UV spectrum studies
Fig. 8. UV spectra of gel factor G1 added with different metal ions (1 × 10ꢀ 2
The ultraviolet spectrum of gel factor G1 and G2 in the solvent which
can form gel is shown in Fig. 6a and Fig. 6b. From Fig. 6a, it can be seen
that there are three absorption bands of gel factor G1 at 234 nm, 320 nm,
and 332 nm, the strongest absorption band is around 234 nm, which can
mol/L).
To further confirm the identification response characteristics of G1
gel to metal ions, Ultraviolet spectroscopy tests on each metal ion of gel
factor G1 are completed. First, a 1,2-dichloroethane solution to gel
factor G1 (1 × 10ꢀ 2mol/L) was prepared. 2 ml HEPES buffer solution
be assigned to n → π* transitions [41]. From the polar solvent (n-pen-
tanol) to the non-polar solvent (1,2-dichloroethane), the absorption
band is red-shifted from 234 nm to 244 nm. Gel factor G2 also has three
absorption bands around 232 nm, 320 nm, and 334 nm(Fig. 6b). The
strongest absorption band is around 232 nm, which is also assigned to n
was added to the cuvette, 2 μl G1 1,2-dichloroethane solution was then
added, and finally, 2
μ
l different metal ion solution (1 × 10ꢀ 2mol/L) was
added for the UV spectrum test. The test results are shown in Fig. 8. It
can be seen that after the addition of metal Zn2+ and Co2+, a strong
absorption band appears at 240 nm, and the absorption band at 318 nm
is red-shifted to 350 nm, the absorption band at 332 nm is red-shifted to
364 nm, but the band does not change after the addition of other metal
ions, so it can indicate the gel factor G1 interacts with Zn2+ and Co2+. To
determine the coordination ratio of G1 to Zn2+ and Co2+, this section
draws the Job’s plot curve, as shown in Fig. 9(a), it can be seen that the
coordination ratio of G1 and Zn2+ is 2:1 at the maximum absorbance.
From Fig. 9(b), the coordination ratio of G1 and Co2+ is 1:1.
→
π* transitions. Similarly, from a polar solvent (n-pentanol) to a
non-polar solvent (1,2-dichloroethane), the absorption band is
red-shifted from 232 nm to 246 nm. This indicates that the formation of
the gel is not related to the polarity of the solvent.
3.7. Responsive recognition of metal ions studies
Since Salen-type Schiff bases are tetradentate ligands, they are easy
to coordinate with metal ions to form stable complexes [42]. Therefore,
this article studied the responsive recognition of G1 gels to metal ions to
obtain specific recognition results. The metal ion responsiveness test is
completed according to the literature [43]. The photo of the G1 gel after
adding the nitrate solution is shown in Fig. 7. The nitrate solutions of
different metal ions are prepared in equimolar according to the gel
factor qualities, and then a 0.1 ml solution of different metal ions is
added dropwise to the surface of the prepared G1 gel, and the gel is left
to observe whether the gel collapses. Adding 0.1 ml water to the surface
of the G1 gel to eliminate the influence of water on the gel state. It was
observed that water did not collapse the gel. After about 20 min, it was
found that the gel G1 with Zn2+ dropped was destroyed, but the other
gels were not affected. After 2 h, the G1 gel with the addition of Co2+
was destroyed. Therefore, it is preliminarily judged that G1 gel can
4. Conclusion
Thus, two gel factor G1 and G2 have been designed and synthesized,
and their assembled structures are investigated in a xerogel state. From
the gelation behavior studies, it is found that gel factor G1 and G2 can
form a gel in solvents such as alcohols and chloroalkane. The self-
assembly driving force such as hydrogen bonding between molecules,
strong van der Waals force between long alkyl chains, and π-π stacking
between naphthalene rings. The morphology of the gels shows the gel
factor is gathered in thin fiber strips and arranged neatly. Interestingly,
the addition of Zn2+ and Co2+ will destroy the state of the gel while
other cations do not. UV spectra confirm this result. From the collapse
time of the gel, quick response to Zn2+ can be obtained.
identify Zn2+ and Co2+
.
5

Y.-S. Yang et al.
Inorganic Chemistry Communications 124 (2021) 108402
Fig. 9. Job’s plot for G1 with (a) Zn2+, (b) Co2+
.
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CRediT authorship contribution statement
Yun-Shang Yang: Investigation, Validation. Qi Shang: Writing -
original draft. Ying-Peng Zhang: Resources, Methodology, Supervision.
: . Wei-Ya Niu: Software. Ji-Jun Xue: Software, Writing - review &
editing.
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Declaration of Competing Interest
[17] S. Dasgupta, S. Karim, S. Banerjee, et al., Designing of novel zinc(II) Schiff base
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The authors declare that they have no known competing financial
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