Metal-Assisted and Solvent-Mediated Synthesis of Two-Dimensional Triazine Structures on Gram Scale

Article abstract of DOI:10.1021/jacs.0c02399

Covalent triazine frameworks are an emerging material class that have shown promising performance for a range of applications. In this work, we report on a metal-assisted and solvent-mediated reaction between calcium carbide and cyanuric chloride, as cheap and commercially available precursors, to synthesize two-dimensional triazine structures (2DTSs). The reaction between the solvent, dimethylformamide, and cyanuric chloride was promoted by calcium carbide and resulted in dimethylamino-s-triazine intermediates, which in turn undergo nucleophilic substitutions. This reaction was directed into two dimensions by calcium ions derived from calcium carbide and induced the formation of 2DTSs. The role of calcium ions to direct the two-dimensionality of the final structure was simulated using DFT and further proven by synthesizing molecular intermediates. The water content of the reaction medium was found to be a crucial factor that affected the structure of the products dramatically. While 2DTSs were obtained under anhydrous conditions, a mixture of graphitic material/2DTSs or only graphitic material (GM) was obtained in aqueous solutions. Due to the straightforward and gram-scale synthesis of 2DTSs, as well as their photothermal and photodynamic properties, they are promising materials for a wide range of future applications, including bacteria and virus incapacitation.

Full text of DOI:10.1021/jacs.0c02399

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Article
Metal-Assisted and Solvent-Mediated Synthesis of
Two-Dimensional Triazine Structures on Gram Scale
Abbas Faghani, Mohammad Fardin Gholami, Matthias Trunk, Johannes Müller, Pradip Pachfule, Sarah
Vogl, Ievgen Donskyi, Philip Nickl, Jingjing Shao, Michael R.S. Huang, Wolfgang E. S. Unger, Raul
Arenal, Christoph T. Koch, Beate Paulus, Jürgen P. Rabe, Arne Thomas, Rainer Haag, and Mohsen Adeli
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c02399 • Publication Date (Web): 27 Jun 2020
Downloaded from pubs.acs.org on June 27, 2020
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Metal-Assisted and Solvent-Mediated Synthesis of Two-Dimensional
Triazine Structures on Gram Scale
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Abbas Faghani, Mohammad Fardin Gholami, Matthias Trunk, Johannes Müller, Pradip
Pachfule, Sarah Vogl, Ievgen Donskyi, Mingjun Li, Philip Nickl, Jingjing Shao, Michael
Huang, Wolfgang E. S. Unger, Raul Arenal, Christoph T. Koch, Beate Paulus, Jürgen P.
Rabe, Arne Thomas*, Rainer Haag and Mohsen Adeli*
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ABSTRACT: Covalent triazine frameworks are an emerging material class, which have
shown promising performance for a range of applications. In this work, we report on a metal-
assisted and solvent-mediated reaction between calcium carbide and cyanuric chloride, as
cheap and commercially available precursors, to synthesize two-dimensional triazine
structures (2DTSs). The reaction between dimethylformamide and cyanuric chloride was
promoted by calcium carbide and resulted in dimethylamino-s-triazine intermediates which in
turn undergo nucleophilic substitutions. This reaction was directed into two dimensions by
calcium ions derived from calcium carbide and induced the formation of 2DTSs. The role of
calcium ions to direct the two-dimensionality of the final structure was simulated using DFT
and further proven by synthesizing molecular intermediates. The water content of the reaction
medium was found to be a crucial factor that affected the structure of the products
dramatically. While 2DTSs were obtained at anhydrous conditions, a mixture of graphitic
material/2DTSs or only graphitic material (GM) was obtained in aqueous solutions. Taking
advantage of the straightforward and gram-scale synthesis of 2DTSs, as well as their
photothermal and photodynamic properties, they are promising materials for a wide range of
future applications including bacteria and virus incapacitation.
Keywords: Two-dimensional polymer, two-dimensional polymerization, calcium carbide,
triazine framework
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INTRODUCTION
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Reticular chemistry has notably extended the scope of two and three-dimensional porous
structures.1-4 Two-dimensional polymers,5 metal organic frameworks,6 covalent organic
frameworks,7 and polymeric networks8 with complex but known structures are designed and
prepared by this strategy. These materials have shown a high potential for the future
applications ranging from energy storage9 to membrane separation.10
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However, two-dimensional covalent organic frameworks with versatile but defined
structures are at the forefront of this rapidly developing field of chemistry.11-14 Also, triazine-
based structures with unique physicochemical properties have been intensively investigated in
the last decade.15-19 Microporous triazine-based structures20 with diverse applications in gas
adsorption and storage,21-25 catalysis,26-29 and energy storage30-32 have been prepared by a
variety of synthetic methods.33 Trimerization of nitriles, for example, is a straightforward and
one-pot synthetic approach by which triazine-based structures with high surface area have
been prepared.16 However, most of the current synthetic methods require high reaction
temperatures, which lead to partial carbonization and undefined structures.34-35 Triazine-based
structures20 synthesized by such methods do not show an electronic band gap and useful
optoelectronic properties, due to side reactions at high temperatures.35 Therefore, it is crucial
to develop new strategies to synthesize more defined structures with useful optical and
electronic properties.32 Recently, the synthesis of triazine-based structures has been improved
using acid-catalyzed microwave36 and mechano-chemistry,37 which elaborate the current
harsh synthetic conditions of ionothermal trimerization. However, the synthesis of triazine
structures in solution, through which kinetic and thermodynamic parameters of reactions
could be manipulated, has not been considerably developed. The lack of efficient synthetic
methods in solution is a problem that has limited the structural diversity of triazine-based
structures and hampered their practical usage.19 In addition to synthetic challenges, the
preparation of triazine-based structures in mono- and few-layered forms is another problem
for attaining unique characteristics of two-dimensional nanomaterials.28, 38-39 This objection
affects the efficiency of these materials, where thickness and number of layers play a
significant role.
Recently, triazine-based structures have been synthesized by a polycondensation
reaction of aldehydes and amidines at relatively mild conditions.15, 35 Although frameworks
synthesized by this method have shown layered structures, superior to previous works, they
could not be found in mono-layer form.39 Stacking of covalent triazine-based structures in
multi-layered structures diminishes their accessible surface area and changes their
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physicochemical properties. Aldehyde-amidine polycondensation is a straightforward reaction
for gram-scale synthesis of triazine-based structures but is limited by availability of the
synthetic monomers. Furthermore, purification of triazine based structures is a time-
consuming process and requires washing at relatively high temperatures.40 This process is not
compatible with reactive functional groups and less stable frameworks. Very recently,
triazine-based olefin-linked structures41 have been reported, where triazine units can be
stitched on to other organic linkers via covalent bonding.42 Although, these fully conjugated
and crystalline triazine-based structures have shown interesting properties, this research topic
is still in its infancy and need more attention for bringing out a general synthesis protocol for
various structures in bulk-scale. Accordingly, scale-up, cost-efficiency, purification process,
reaction parameters, and environmental aspects of classical synthetic methods require to be
improved to economically obtain covalent mono-layer triazine frameworks with desired
physicochemical properties.
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In this work, we have addressed these challenges by metal-assisted and solvent-
mediated synthesis of two-dimensional triazine structures (2DTSs). Reaction between calcium
carbide and cyanuric chloride in dimethylformamide (DMF) resulted in 2DTSs with several
micrometer lateral sizes. While cyanuric chloride acts as the source of triazine units, calcium
carbide provides both acetylide linkages and calcium ions. The reaction was mediated by
DMF and directed in two dimensions by calcium ions, leading to 2DTSs after trimerization of
triple bonds. Adding water to the reaction medium changed the structure of the product from
2DTSs, in anhydrous conditions, to graphitic materials. Taking advantage of the π-conjugated
system of their triazine backbone, 2DTSs can be developed for future applications such as
photocatalysis,43 electrocatalysis,44 supercapacitors32 and Li-ion battery.45 However, in this
manuscript, we have focused on the synthesis and physicochemical properties of 2DTSs, in
addition to their mechanistic investigations.
RESULTS AND DISCUSSION
The reaction between cyanuric chloride and DMF at different conditions results in versatile
intermediates.46-48 Herein, we demonstrate that this reaction, in the presence of calcium
carbide, yields 2-(N,N-dimethylamino)-4,6-dichloro-s-triazine, which undergoes consecutive
nucleophilic aromatic substitutions by acetylide dianions and produces two-dimensional
triazine structures after trimerization (Figure 1). Calcium ions directed the carbon-carbon
coupling of monomers in two dimensions and were eventually removed by washing (see page
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S7). This reaction was performed under different conditions and the effect of water content on
the structure of the products was monitored (Figure 1).
In order to carry out reactions, calcium carbide and cyanuric chloride were mixed and
stirred in bulk under inert atmosphere and then dispersed in DMF. Depending on the amount
of water present during the reaction, the product varied from crystalline graphitic material to
pure 2DTSs (Figure 1). The structures of the products were analyzed by different
spectroscopy and microscopy methods as well as elemental and thermal analysis. Also, the
reaction mechanisms were investigated by synthesizing different model compounds and
studying the structure of intermediates (vide infra).
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High resolution transmission electron microscopy (HRTEM) and scanning electron
microscopy (SEM) images revealed the formation of layered and bulk structures with lateral
sizes of several micrometers for the products of reactions at different conditions (Figures
2a,b,d,e). Further investigations revealed that water affected the structure of the reaction
products dramatically. The product of the reaction between calcium carbide and water was
crystalline with a measured in-plane lattice constant of the hexagonal crystal of approximately
0.248 nm (min: 0.241 nm, max: 0.254 nm, see Figure S1). This corresponds well to the in-
plane lattice constant of graphite (a = 0.246 nm). These results were consistently obtained by
HRTEM imaging and electron diffraction in the SEM. Furthermore, EDX elemental analysis
of the crystalline particles concluded approximately 99% carbon, which indicates, together
with the hexagonal unit cell, that the material is graphite. In anhydrous conditions (see page
S7), however, a product with an amorphous structure was obtained (Figure 2c, Figure S4),
which we determined to be 2DTSs. Electron energy loss spectroscopy (EELS) spectra showed
discrete peaks at ~285 eV and ~400 eV, which were assigned to carbon and nitrogen in the
backbone of this material (Figure S3).49-52
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a
b
Cl
O
2-
N
N
:C C:
Ca2+
Trimerization
N
H
+
+
2DTSs
Cl
N
Cl
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2-
Cl
N
Cl
:C C:
Ca2+
N
N
+
Cl
2 g; 0.24 USD
2 g; 0.28 USD
GM/2DTSs
mixture
(1.3 g)
2DTSs
Graphitic
Material (GM)
N
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Cl
X
N
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N
N
X= NMe2
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Figure 1. (a) Reaction between triazine and calcium carbide in anhydrous DMF resulted in

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2DTSs. (b) The product of the reaction between calcium carbide and triazine was strongly
influenced by the water content of the solvent. While graphitic material (GM) was the main
product in aqueous solutions (water, 25 °C, 30 minutes), 2DTSs were obtained in anhydrous
DMF (120 °C, 72 hours). Also, a mixture of GM/2DTSs was created in non-anhydrous DMF
(120 °C, 72 hours). The cost-effectiveness of the reaction is shown by the estimated prices
and quantities of the precursors. Prices of reagents were monitored from commercial suppliers
(see page S3).
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The nitrogen content of the reaction products at different conditions, which is an
indicator for the triazine rings, was further investigated by elemental analysis. While the
nitrogen content of the reaction product in water (GM) was close to zero, it increased to ~21%
for the product of reaction under anhydrous conditions (Table S1). Under anhydrous
conditions the carbon/nitrogen ratio of the product was 2.6, yielding approximately a C3N1
molecular formula for 2DTSs (Table S1). This carbon/nitrogen ratio indicated the formation
of graphitic domains53-54 in 2DTSs by acetylide cross-coupling and trimerization (Figure 3d).
The topography and size of the product of reaction under anhydrous condition was
further investigated by scanning force microscopy in quantitative nanomechanical mapping
mode (SFM-QNM) on a freshly cleaved mica surface (Figures 2g,h and Figures S5a,b). It
revealed layered structures with lateral sizes of 2 to 3 micrometers and thicknesses of 100 to
350 nm, with layers and terraces including smaller layers with self-overlapping regions (folds)
and an inhomogeneous distribution of mobile and small interconnected particles above the
larger flakes, with a RMS roughness of 1 nm. Since the heights of the layers were between 5
to 10 nm, we believe that we did not observe a single-layer exfoliation of the layered
structures. However, exfoliated multilayered structures exhibited self-avoiding behavior as
evident in back folding,55-56 which may be simulated on macroscale using a sheet of paper for
Figure 2h (Figure S5c). Self-avoidance of a structure is a direct consequence of its covalent
bonding within a 2D network. This is indeed not the case for self-assembled 2D structures
such as lipid bilayers/membranes, as those structures can self-penetrate and restructure back
to a single layer rather than stabilize in a folded conformation. An observation of folding in
case of 2D covalent structures is possible only when there is an overall gain in the energy of
the system. This can be achieved by van der Waals interlayer forces (similar to those present
within layers of a crystal) triggered by drying process during surface deposition.56 The
product of the reaction between calcium carbide and cyanuric chloride in DMF under ambient
condition (Figure 1, midline), which is called 2DTSs/GM, was also investigated by different
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characterization methods including TEM, EDX, elemental and thermal analysis. It was found
that this product is always a mixture of 2DTSs and graphitic material (Table S1, Figure 3b).
Attenuated total reflection infrared spectroscopy (ATR-FTIR) of 2DTSs revealed a broad
absorbance band at ~1394-1577 cm–1 that corresponds to carbon-carbon and carbon-nitrogen
double bonds. The absence of the triple-bond absorbance and the appearance of C=C
vibrations in the IR spectrum of 2DTSs are attributed to the trimerization of triple bonds and
the formation of benzene rings (Figure S6). Also, an absorbance band at 3300 cm-1 indicated
presence of hydroxyl functional groups in the structure of 2DTSs, which have been created by
the reaction between unreacted chlorine atoms of cyanuric chloride and water in the
purification process.
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Figure 2. The products were investigated by different microscopy methods including
HRTEM, SEM and SFM. (a) and (d) HRTEM images of 2DTSs and GM, respectively. (b)
and (e) SEM images of 2DTSs and graphitic material on top of holey carbon TEM grids,
respectively. (Insets are carbon and nitrogen contents of the indicated area obtained by
energy-dispersive X-ray spectroscopy (EDX).) (c) and (f) Diffraction patterns of 2DTSs and
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GM at the indicated areas of (b) and (e) that show amorphous and crystalline characteristics
for these materials, respectively. The in-plane unit cell parameter of the hexagonal lattice of
GM was on average 0.248 nm. (g) SFM-QNM height image of the product of reaction in
anhydrous condition with plateaus and terraces (more images in Figure S5), (h) A close-up
image of a sheet obtained in anhydrous condition with back folding. Planar layer topography
is evident from the height image. (i) SFM-QNM height images of a large plateau of graphitic
material.
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13C solid-state cross-polarization-magic-angle-spinning (CP-MAS) nuclear magnetic
resonance (NMR) spectra of 2DTS showed a broad signal at 148-162 ppm, which is assigned
to the carbon atoms of triazine rings57-58 and a signal at 27 ppm for –NMe2 groups (Figure 3a).
In this spectrum, a broad signal centered at 126 ppm is assigned to benzene rings, which is a
further proof for the trimerization of triple bonds (Figure 3a).
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Figure 3. Characterization of 2DTSs by spectroscopy methods and thermal analysis. (a) C
CP-MAS-NMR spectrum of 2DTSs showed distinguished signals for the triazine and benzene
rings as well as –NMe2 functional groups. (b) TGA thermograms of 2DTSs, cyanuric
chloride GM, and GM/2DTSs. (c) Survey XPS spectrum of 2DTSs with quantification table
of the main elements. Absence of the calcium peak after washing process confirmed the
successful removal of metal ions. Au4f peak originates from the substrate. (d) Highly
resolved XPS C1s spectrum and (e) highly resolved XPS N1s spectrum of 2DTSs. Fit
parameters for each spectrum and interpretations are shown in table S2. (f) A close up of
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Raman spectra overlay of the G peak and D’ peak regions of the 2DTSs, GM, and highly
oriented pyrolytic graphite (HOPG).
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The thermal behavior of 2DTSs was investigated by thermogravimetric analysis
(TGA) in air. Their thermal stability correlated with the nitrogen content of the products.
Materials with lower nitrogen contents, which are the products of reactions in aqueous
solution or non-anhydrous condition, showed higher thermal stability. This is an indicator for
the higher graphitic content of these materials. Based on literature reports, the reaction
between water and calcium carbide is the source of graphitic materials in our reactions.53
While the production of GM occurs at room temperature, the reaction between calcium
carbide and cyanuric chloride requires temperatures around 120 °C. Therefore, the production
of GM is kinetically favored, and cyanuric chloride does not play a role at low temperatures.
In the TGA thermogram of 2DTSs, weight losses in low and high temperature ranges are
assigned to the detachment of –NMe2 functional groups and decomposition of the backbone of
this nanomaterial, respectively (Figure 3b). The structure of 2DTSs was further investigated
by X-ray photoelectron spectroscopy (XPS). Carbon, nitrogen, as the main components, and
oxygen were detected in the survey spectrum of 2DTSs (Figure 3c). The highly resolved C1s
XPS spectrum (Figure 3d) showed three intense C1s component peaks at 288.4 eV,
representing carbon atoms of triazine rings,59 and at 286.2 and 285.0 eV, corresponding to the
–NMe2 groups and carbons of benzene rings, respectively.54, 58, 60 Two components in the N1s
peak at 400.4 eV and 398.9 eV were assigned to the nitrogen atoms of triazine rings and –
NMe2 groups, respectively (Figure 3e).58, 61 The absence of the calcium peak in the survey
spectrum of 2DTSs is a proof of the successful removal of calcium upon purification (Figure
3c).
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In order to complement the characterization of the synthesized materials through
investigation of their polymorphism and crystallinity, their Raman spectra were individually
evaluated and mapped (Figures 3f and S7). The G peaks of 2DTSs and GM were appeared at
1562 cm-1 and 1582 cm-1, respectively. In the case of 2DTSs, broadening of Raman peaks at
1320-1360 cm-1 area, centered at 1345 cm-1, as well as peaks at 1550-1620 cm- 1 area with a
pronounced peak at 1562 to 1565 cm-1 was observed. As it is previously reported by Ferrari et
al.,62 the C=N vibrational frequencies are very similar to the C=C within the 1000 to 2000 cm-
1
range. Therefore, the observed peaks in the Raman spectra of 2DTSs were assigned to a
structure with the embedded nitrogen atoms. Furthermore, increasing the nitrogen content of
products affected the G peak position in their Raman spectra, when visible laser light was
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used. For example, for a product with 20-30% nitrogen content, a G peak at 1560-1565 cm-1
was observed which is in agreement with the reported data in literature.62 This downshift in
comparison with the G peak of HOPG was assigned to the difference between GM
composition and HOPG.62 The direct correlation between this shift and the nitrogen content of
the materials is a further proof for the preparation of 2DTSs at anhydrous conditions.
Furthermore, 2DTSs/GM mixtures exhibit Raman bands at ∼1332 cm-1 and ∼1564 cm-1 with
an occasional weak and broad band at ∼1610-1620 cm-1 in addition to a broad band centered
at ∼2700 cm-1 (Figure 3f). The Raman bands at ~1345 cm-1 and ∼1620 cm-1, which
correspond to the D peak, are assigned to the domain boundaries or nucleation sites within the
structure. On the other hand, the D' peak observed occasionally at ∼1620 cm-1 demonstrates
the doping by a hetero atom, suggesting nitrogen atoms of triazine in the structure of 2DTSs
(Figure 3f). It is important to note that no band at ∼2100 cm-1 was observed in the spectrum
of 2DTSs. In agreement with IR and NMR spectra, this is a further proof of the trimerization
of triple bonds.63-64
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After analyzing the chemical structure of the two-dimensional structures, the
mechanism of their synthetic pathway and structures of intermediates were investigated. To
avoid polymerization and to constrain the reaction to the molecular level, two active sites of
cyanuric chloride were blocked by phenol, and 2-chloro-4,6-diphenoxy-1,3,5-s-triazine (1)
was synthesized according to reported methods in literature65 (see Figures S8-11 for NMR
and mass spectra). Then the reaction between 2-chloro-4,6-diphenoxy-1,3,5-s-triazine and
calcium carbide was carried out in anhydrous DMF at 120 °C. Different characterization
methods showed that the chlorine atom of 2-chloro-4,6-diphenoxy-1,3,5-s-triazine was
substituted by DMF leading to 2-(N,N-dimethylamino)-4,6-diphenoxy-s-triazine (2) (Figure
4a). The structure of compound (2) was investigated by single X-ray crystallography and
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spectroscopy methods (Figure 4b-d). H-NMR showed a chemical shift for the phenoxy
protons of compound (1) after reaction with DMF. Also, a signal for –NMe2 protons appeared
in the spectrum of compound (2) (Figure 4c). Comparison of 13C NMR spectra of compounds
(1) and (2) showed a distinct C-Cl chemical shift from 173.8 ppm to 167.4 ppm after reaction
with DMF. These results were in agreement with literature66-67 and confirmed substitution of
the chlorine atom of compound (1) by –NMe2 groups, which were derived from DMF (Figure
4d). The structure of intermediate (2) was further proven by mass spectrometry. The signals at
309.1 and 331.1 m/z are related to the molar mass of this compound accompanied by proton
and sodium ion, respectively (calculated for C17H16N4O2 [M+H] and [M+Na]: 309.1 and
331.1, respectively, Figure S11). Accordingly, the signal of methyl groups in the CP-MAS-
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NMR of 2DTSs can be assigned to –NMe2 groups that are created by reaction between DMF
and cyanuric chloride. Reaction between compound (2) and calcium carbide resulted in
compounds (3 and 4). This reaction was monitored at intervals of two hours by Matrix-
Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) and electrospray
ionization mass spectrometry (ESI) for three days. Signal at 328.11 m/z and 655.25 m/z
(Figures S12 and S13) are assigned to compounds 3 and 4, which were converted to
compounds 5 and 6, respectively upon trimerization. Compound 4 was accompanied by one
or two calcium ions (Figures S13 and S14), which showed that metal-ligand interactions were
possible driving forces to inhibit rotation of triazine rings around linkages and direct
polymerization in two dimensions. However, compound 4, which was accompanied by one
calcium ion, could be more reactive toward trimerization, due to less steric hindrance.
Compounds 5 and 6 were detected by MALDI-TOF (Figure 4e). These compounds proved
formation of benzene rings in the structure of 2DTSs upon trimerization of triple bonds.
Furthermore, fragments at 336 m/z and 639 m/z in the ESI mass spectrum of this model
reaction indicated building blocks consisting benzene rings with six substituents (Figures S14
and S15). This result showed that calcium carbide can reacted with triazine rings from both
sides because trimerization of the product of such a reaction results in a 2D polymer
consisting of benzene rings with six substituents. It is worth noting that in compound 6
triazine rings were able to rotate to minimize the repulsion of electron pair of adjacent
nitrogen atoms. However, in 2DTSs rotation of such units is restricted leading to an
energetically unfavorable structure. Therefore, the units created by di-substituted calcium
carbide can be mostly at the edges or defect sites, where the electron repulsion of adjacent
nitrogen atoms is reduced. The backbone of 2DTSs consist mostly of graphitic domains or
regions created by trimerization of mono-substituted calcium carbide or a mixture with its di-
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substituted analog (Figure 1b). In addition to MALDI-TOF spectra, H NMR indicated the
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formation of mono-substituted calcium carbide. H NMR of the product of reaction between
2-chloro-4,6-diphenoxy-1,3,5-s-triazine and calcium carbide showed clear signals for
compounds 5, which could be created by trimerization of mono-substituted calcium carbide
(Figure S16). The reaction mechanism was further investigated by synthesizing 2,4-dichloro-
6-phenoxy-1,3,5-s-triazine and polymerization of this monomer in DMF under the same
reaction conditions (Scheme S1). In the MALDI-TOF spectrum of the product of this
reaction, distinguished peaks at 243n m/z intervals, where n is a natural number, indicated a
repeating unit with benzene substitutes (Figure S17). This repeating unit is a further proof of
the nucleophilic substitution of chlorine atoms of triazine followed by trimerization. The role
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of DMF in the reaction between cyanuric chloride and calcium carbide was further proven by
changing the solvent to N,N-dimethylacetamide toluene and mesitylene. Accomplishment of
reaction in these solvents did not result in any detectable product. This experiment proved the
critical role of DMF in the production of two-dimensional triazine structure. As indicated by
compound (3), complexation of intermediates with calcium ions hampered the rotation of
triazine rings around acetylide linkages and directed the consequent reactions in two
dimensions (Figure S18). The directing role of calcium ions was further investigated by DFT
functional in GPAW program 68-70 (see page S23 and S24).
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Figure 4. Investigation of the mechanism of the reaction between cyanuric chloride and
calcium carbide in DMF using model compounds. (a) Reaction between compound (1) and
DMF resulted in intermediate (2). Consequently, nucleophilic reaction between compound (2)
and calcium carbide resulted in compounds 3 and 4 that converted to compounds 5 and 6
upon trimerization. (b) Single crystal XRD structure of compounds (1) and (2). (c) and (d)
1H- and 13C-NMR spectra of compound (1) and compound (2), respectively. (e) Expanded
MALDI-TOF spectrum of mixture of reaction between compound 1 and cyanuric chloride in
DMF. (f) The simulated structure of model compound coordinated with two calcium ions (on
both sides) and the total energy of the system with different dihedral angle with step size of 5°
(see details in Figure S18 and Table S3).
Coordination of one or two calcium ions with nitrogen atoms of the triazine rings in
compound 4 (phenyl groups were replaced by hydrogen) caused 0.05 eV and 1.6 eV barrier
energy, respectively, for rotation around C1-C2 and C3-C4 and bonds (Figure 4f). These energy
barriers are driving forces for growing triazine-benzene structures in two dimensions.
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According to the literature, calcium ions are able to form complexes with nitrogen containing
ligands such as 1,10-phenanthroline, 4,4ˊ-bipyridine and 2,2ˊ-bipyridine.71-72 While
interactions between these ligands and calcium ions are not as strong as oxygen containing
ligands,73-74 they are able to induce enough rigidity for the two-dimensional coupling of
monomers.68-70 The weakness of the interactions on the other hand enables that the metal ions
can be removed completely from the final product. Another proof of the directing role of
calcium ions was achieved by adding sodium carbonate to the reaction medium. When the
reaction between cyanuric chloride and calcium carbide was performed in DMF and in the
presence of sodium carbonate, no sheet-like structure was achieved, due to the production of
sodium carbonate and absence of any driving force to conjugate monomers in a sheet-like
structure (Figure S19).
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Figure 5. (a) Singlet oxygen generation kinetics of water dispersion of 2DTSs and (b)
photothermal properties of 2DTSs under laser irradiation (808 nm, 0.5 W/cm2). Thermally
reduced graphene oxide (TRGO) was used as control, the concentration was 0.5 mg/mL in
water (more details can be found in page S27). (c) and (d) antibacterial activities of samples
(0.5 mg/mL) against E. coli and B. subtilis. E. coli and B. subtilis in PBS were used as
controls. Data are presented as mean ± SD, n = 3. Statistically significant differences are
indicated by *p < 0.05 and **p < 0.01 compared with the control. (e-h) biofilm eradication
test: the bacteria entrapped in biofilm were destroyed by 2DTSs upon laser irradiation. In
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live/dead assay, live and dead bacteria are shown by green and red colors respectively (details
are available in page S27).
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The photochemical properties of 2DTSs were investigated by different methods.
2DTSs showed a maximum UV absorption at 235-276 nm corresponding to n→π* transitions
(Figure S20). Due to their large π-conjugated system, 2DTSs were able to produce singlet
oxygen under irradiation with near infrared (NIR) laser (808 nm). This was confirmed using
1,3-Diphenylisobenzofuran (DPBF) as a specific singlet oxygen scavenger (Figure 5a).
2DTSs demonstrated a higher singlet oxygen production rate as observed by absorbance
change of DPBF in comparison to methylene blue as a photosensitizer reference in similar
solvent. This property could be used for different applications ranging from photodynamic
therapy to photocatalyst applications. Laser irradiation of a water dispersion of 2DTSs
elevated the medium temperature from 20 °C to 40 °C in 2 minutes (Figure 5b). The
production of heat is less efficient than by thermally reduced graphene oxide but still in a
suitable range for some applications including photothermal therapy or antibacterial activity.
Accordingly, the antibacterial activity of 2DTSs under laser irradiation was investigated.
While laser irradiation did not show a significant adverse effect against bacteria, 2DTSs (0.5
mg/mL) followed by 10 minutes laser irradiation resulted in incapacitation of 46% and 67%
E.coli and B. subtilis respectively (Figures 5c,d). Prolongation of the laser irradiation to 30
min resulted in destruction of both gram positive and gram negative bacteria completely (in
the detection limit). The bacteria entrapped in biofilm are more resistant to biocide, due to
protection and sufficient nutrition inside the biofilm. Therefore, antibiofilm property of an
antibacterial agent is of high relevance and determines its efficiency for the treatment of
contaminated surfaces. Incubation of 2DTSs with an 18 μm thick biofilm for 1 h followed by
laser irradiation for 30 min resulted in destruction of bacteria inside the biofilm, proven by red
fluorescence (Figures 5e, g). As Figures 5f and 5h show, 2DTSs with black color penetrated
into the biofilm and incapacitated bacteria (more details in page S27).
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Our results showed that the reaction between calcium carbide and triazine in DMF is
an efficient strategy to develop versatile two-dimensional triazine structures through the
concepts of reticular chemistry. Deep understanding of the mechanism of this reaction can
help us to manipulate the structure of the products by changing reaction parameters including
precursors, metal ion, solvent, and water content.
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CONCLUSION
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The reaction between cyanuric chloride and calcium carbide under anhydrous conditions was
mediated by N, N-dimethylformamide to produce 2D triazine-benzene structures on the gram
scale. A mechanistic study revealed a dual role of the calcium carbide, first as a source of
acetylide ions, which created benzene rings upon trimerization, and second the calcium ions
could act as a two-dimensional directing force upon interaction with triazine rings. Adding
water showed a high impact on the reaction route and in the presence of water, graphitic
material was created. Taking advantage of its interesting physicochemical properties,
straightforward synthesis and cheap precursors, the presented two-dimensional nanomaterial
is a promising candidate for a wide range of future applications including photodynamic and
photothermal therapy.
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ASSOCIATED CONTENT
Supporting Information
Experimental details; SEM, TEM, HRTEM and SFM images; EELS spectra and PXRD
diffractograms; IR, Raman & NMR spectra; Mass spectrometry; XPS interpretation;
computational analysis (DFT calculation).
AUTHOR INFORMATION
Corresponding Authors
Arne Thomas − Department of Chemistry / Functional Materials, Technische Universität
Berlin, Hardenbergstraße 40, 10623 Berlin, Germany; Email: arne.thomas@tu-berlin.de
Mohsen Adeli − Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22,
Berlin 14195, Germany. Faculty of Science, Department of Chemistry, Lorestan University,
Khorramabad, Iran; Email: m.aadeli@fu-berlin.de
Authors
Abbas Faghani − Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee
22, Berlin 14195, Germany
Mohammad Fardin Gholami − Department of Physics & IRIS Adlershof, Humboldt-
Universität zu Berlin, Newtonstr. 15, 12489 Berlin Germany
Matthias Trunk − Department of Chemistry / Functional Materials, Technische Universität
Berlin, Hardenbergstraße 40, 10623 Berlin, Germany
Johannes Müller − Department of Physics & IRIS Adlershof, Humboldt-Universität zu
Berlin, Newtonstr. 15, 12489 Berlin Germany
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Pradip Pachfule − Department of Chemistry / Functional Materials, Technische Universität
Berlin, Hardenbergstraße 40, 10623 Berlin, Germany
Sarah Vogl − Department of Chemistry / Functional Materials, Technische Universität
Berlin, Hardenbergstraße 40, 10623 Berlin, Germany
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Ievgen Donskyi − Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee
22, Berlin 14195, Germany. BAM – Federal Institute for Material Science and Testing,
Division of Surface Analysis and Interfacial Chemistry, Unter den Eichen 44-46, 12205
Berlin, Germany
Mingjun Li − Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22,
Berlin 14195, Germany. Center for Health Science and Engineering, Tianjin Key Laboratory
of Materials Laminating Fabrication and Interface Control Technology, School of Materials
Science and Engineering, Hebei University of Technology, Tianjin 300130, China
Philip Nickl − Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22,
Berlin 14195, Germany. BAM – Federal Institute for Material Science and Testing, Division
of Surface Analysis and Interfacial Chemistry, Unter den Eichen 44-46, 12205 Berlin,
Germany
Jingjing Shao − Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22,
Berlin 14195, Germany
Michael Huang − Department of Physics & IRIS Adlershof, Humboldt-Universität zu Berlin,
Newtonstr. 15, 12489 Berlin Germany
Wolfgang E. S. Unger − BAM – Federal Institute for Material Science and Testing, Division
of Surface Analysis and Interfacial Chemistry, Unter den Eichen 44-46, 12205 Berlin,
Germany
Raul Arenal − Laboratorio de Microscopias Avanzadas (LMA), Instituto de Nanociencia de
Aragon, Universidad de Zaragoza, 50018 Zaragoza, Spain. Fundacion ARAID, 50018
Zaragoza, Spain. Instituto de Ciencias de Materiales de Aragon, CSIC-Universidad de
Zaragoza, 50009 Zaragoza, Spain
Christoph T. Koch − Department of Physics & IRIS Adlershof, Humboldt-Universität zu
Berlin, Newtonstr. 15, 12489 Berlin Germany
Beate Paulus − Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22,
Berlin 14195, Germany
Jürgen P. Rabe − Department of Physics & IRIS Adlershof, Humboldt-Universität zu Berlin,
Newtonstr. 15, 12489 Berlin Germany
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Rainer Haag − Institut für Chemie und Biochemie, Freie Universität Berlin, Arnimallee 22,
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Berlin 14195, Germany
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ACKNOWLEDGMENTS
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We thank the German Science Foundation (DFG) for financial support within the grants SFB
765 and SFB 658. M.F.G. and J.P.R. also acknowledge the support of the Cluster of
Excellence “Matters of Activity. Image Space Material” funded by the DFG under Germany’s
Excellence Strategy EXC 2025. Furthermore, A.T. acknowledges the DFG for funding within
the project TH 1463/12-1. We would like to thank Dr. Andreas Schäfer and Maiko Schulze
for solid-NMR experiments as well as we appreciate the effort of Vahid Ahmadi Soureshjani
for MALDI-TOF experiments. We would like to acknowledge M. Eng. Jörg M. Stockmann
for operating the XPS instrument at the BAM, Prof. Stephanie Reich and Dr. Antonio Setaro
for fruitful discussions. 2DTs-HRTEM and -EELS studies were conducted at the Laboratorio
de Microscopias Avanzadas, Instituto de Nanociencia de Aragon, Universidad de Zaragoza,
Spain. R.A. gratefully acknowledges the support from the Spanish Ministry of Economy and
Competitiveness (MINECO) through project grant MAT2016-79776-P (AEI/FEDER, UE)
and from the European Union H2020 programs ETN projects “Graphene Flagship” (785219)
and “ESTEEM3” (823717).
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