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Review

Triptycene Based 3D Covalent Organic Frameworks (COFs)—An Emerging Class of 3D Structures

by
Monika Borkowska
1 and
Radosław Mrówczyński
1,2,*
1
Faculty of Chemistry, Adam Mickiewicz University in Poznań, Uniwersytetu Poznańskiego 8, 61-614 Poznań, Poland
2
Centre for Advanced Technologies, Adam Mickiewicz University in Poznań, Uniwersytetu Poznańskiego 10, 61-614 Poznań, Poland
*
Author to whom correspondence should be addressed.
Symmetry 2023, 15(9), 1803; https://0-doi-org.brum.beds.ac.uk/10.3390/sym15091803
Submission received: 14 July 2023 / Revised: 12 September 2023 / Accepted: 18 September 2023 / Published: 21 September 2023
(This article belongs to the Special Issue Symmetry in Organic Chemistry: Synthesis and Properties of Symmetrical Organic Compounds)
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Abstract

:
Covalent Organic Frameworks (COFs) are a newly emerged class of porous materials consisting of organic building blocks linked by strong covalent bonds. The physical and chemical properties of COFs, i.e., modularity, porosity, well-developed specific surface area, crystallinity, and chemical-thermal stability, make them a good application material, especially in the aspects of adsorption and gas separation. The organic compositions of their building blocks also render them with biocompatible properties; therefore, they also have potential in biomedical applications. Depending on the symmetry of the building blocks, COF materials form two-dimensional (2D COF) or three-dimensional (3D COF) crystal structures. 3D COF structures have a higher specific surface area, they are much lighter due to their low density, and they have a larger volume than 2D COF crystals, but, unlike the latter, 3D COF crystals are less frequently obtained and studied. Selecting and obtaining suitable building blocks to form a stable 3D COF crystal structure is challenging and therefore of interest to the chemical community. Triptycene, due to its 3D structure, is a versatile building block for the synthesis of 3D COFs. Polymeric materials containing triptycene fragments show good thermal stability parameters and have a very well-developed surface area. They often tend to be characterized by more than one type of porosity and exhibit impressive gas adsorption properties. The introduction of a triptycene backbone into the structure of 3D COFs is a relatively new procedure, the results of which only began to be published in 2020. Triptycene-based 3D COFs show interesting physicochemical properties, i.e., high physical stability and high specific surface area. In addition, they have variable porosities with different pore diameters, capable of adsorbing both gases and large biological molecules. These promising parameters, guaranteed by the addition of a triptycene backbone to the 3D structure of COFs, may create new opportunities for the application of such materials in many industrial and biomedical areas. This review aims to draw attention to the symmetry of the building blocks used for COF synthesis. In particular, we discussed triptycene as a building block for the synthesis of 3D COFs and we present the latest results in this area.

1. Introduction

Covalent Organic Frameworks (COFs) are a new class of porous materials built from building blocks composed of light elements, i.e., C, H, N, and B. The name of this class of materials (Covalent Organic Frameworks) refers directly to the covalent bonds that link the building blocks of these structures together. Covalent bonds in COF synthesis are based on the Dynamic Covalent Chemistry (DCC) regulations. This means that the formation of the crystalline covalent structure of COFs occurs under thermodynamic control and depends only on the relative stabilities of the final products. The forming bonds are subjected to “error checking” and “self-healing processes”; they are being created and broken in reversible reactions between the building blocks until sufficiently strong and thermodynamically stable bonds and crystal architectures are obtained under given conditions [1,2,3]. Examples of the covalent bonds most commonly used to synthesize COFs are shown in Table 1. The strong covalent bonds in Covalent Organic Frameworks provide them with high chemical stability; most COFs are insoluble in common organic solvents and are resistant to strong acids and bases [1,2,4].
The construction and design of COF materials is based on the principles of reticular chemistry. The selected building blocks for COF synthesis should be geometrically and spatially compatible with each other to obtain a thermodynamically-stable crystal structure with an optimal shape and spatial dimensions [5]. Examples of two-dimensional (2D) and three-dimensional (3D) networks formed by COFs, as well as their symmetry, are shown in Figure 1. However, thanks to computational chemistry, the appropriate building blocks for COF synthesis can be easily and more feasibly identified. Many recently discovered COF structures, especially 3D COFs, have been simulated and successfully synthesized with computational predictions, which include thermodynamical stability and even potential application destinations [6,7]. To design and predict new COF structures and their possible capacity for methane storage, Martin et al. [6] and Mercado et al. [8] used grand-canonical Monte Carlo simulations. Other options for designing COF structures include the materials–genomics-method-based QReaxAA (Quasi-Reactive Assembly Algorithms) for structure generation, which mimics the natural growth processes of COFs [9]. In order to categorize and catalogue already synthesized COFs, both 2D and 3D, as well as those simulated and ready to be synthesized, certain approaches have been made to make a proper database for these structures. Moreover, the currently developed libraries and the aforementioned database, which is still being developed, not only collect constructive information about the architecture of COFs, but also focus on their direct application. For example, Ongari et al. prepared a library consisting of CURATED COFs (CURATED = Clean, Uniform, and Refined with Automatic Tracking from Experimental Database) for characterizing and investigating their CO2 adsorption properties [10]. The CoRe COF database, developed by M. Tong et al., was established based on the structure–property relationships of COFs and noble gases in order to investigate the COFs’ abilities in their separation [11]. One of the most updated and newly published COF databases is ReDD-COFFEE (Ready-to-use and Diverse Database of Covalent Organic Frameworks with Force field based Energy Evaluation) [12]. This database has collected an enormous number of structures (268,687 COFs) with a high diversity in terms of geometric CH4 storage properties.
Reticular synthesis makes it possible to obtain COFs in a modular manner, thus their final crystal construction is strictly dependent on the structure of the selected building blocks. This also means that the properties of their specific surface area and pore size can be directly influenced [1,13]. COFs are characterized by a very well-developed specific surface area and porosity, comparable to other classes of porous materials, such as MOFs and zeolites. Nonetheless, with the increasing incidence of cancer and the often-presented side effects of currently used cancer prophylaxes, together with the growing need for innovative therapy, COFs have also begun to emerge with applications in this area. COF building blocks are based on organic compounds with the rare use of heavy metals, allowing them to provide both biocompatible and non-toxic matrix material as a drug delivery vehicle [4]. In addition, the building blocks used to synthesize COFs are readily modifiable and functionalized, both before and after material synthesis, making the material itself adaptable to a wide range of applications. In addition, a significant part of the building blocks are cyclic aromatic compounds, which, due to their flat structure and the presence of p-electron conjugated systems, enrich the crystal structure with inductive interactions [2]. The unique properties of COFs make them mainly applicable in gas storage and selective separation [14,15], dye adsorption [16,17], and also as catalysts [18,19,20]. Such properties create potential applications of COFs for biomarkers, biosensors and photosensors [21,22,23].

3D COFs—Synthesis and Properties

COF-type materials, depending on the chosen building blocks, form crystal lattices in two ways: two-dimensional (2D), yielding 2D COFs, and three-dimensional (3D), yielding 3D COFs. A two-dimensional spatial network requires the use of planar building blocks, rich in conjugated p-electron systems. In a single 2D layer, the building blocks connect to each other through covalent bonds. The p-p electron interactions, produced by the conjugated p-electron systems of the building blocks, connect successive overlapping 2D layers, forming a 2D COF crystalline spatial network stabilized by two types of interactions. Through interlayer interactions, 1D modular channels are produced between the layers [24,25]. In order to synthesize three-dimensional crystal materials (3D COFs), one of two options for building block combination must be achieved. The first requires at least one non-planar building block (tetrahedral or triangular-pyramid shaped).
The second option is similar to the synthesis of 3D MOF materials; the chosen combination of planar building blocks, e.g., 4-c and 3-c, must generate two types of vertices and one type of edge, resulting in three 3D structures [26,27,28]. However, 3D COF synthesis from planar building blocks is highly dependent on forming linkages of conformational flexibility, because under different conditions, 3D COF material or 2D COFs can be obtained from the same building blocks [26].
The interconnections that stabilize the significantly more developed crystal network of 3D COFs rely solely on covalent interactions [29]. 3D COFs appear to have a better-developed specific surface area than 2D COFs, due to their larger spatial dimension. The higher porosity of 3D COFs makes them more volumetric and lighter than 2D COFs, which should increase their application in areas such as gas storage and separation, or catalysis. However, it is 2D COFs that enjoy more widespread identification and are much more widely used, in contrast to 3D COFs. There are a number of reasons for this, the most important of which is the lack of readily available building blocks with the right structures to form a 3D network and the complex topology of the spatial network itself [24,25]. In addition, obtaining 3D COFs in an interpretable crystalline form is a significant problem. Most of the 3D COFs obtained are in amorphous form and have much lower stability due to the lack of additional p-p interactions that stabilize the crystal structure (as in the case of 2D COFs) [30]. As a result, since 2007, 3D COF materials have been obtained only in one-element topologies, i.e., dia [31], ctn, bor [32], pts [33], ffc [9], rra [34], srs [35], lon [36], stp [28], acs [37], tbo [27], bcu [38], and fjh [39]. Between 2020 and 2021, five new topologies for 3D COF materials were documented. Li et al. [40] obtained a 3D COF (3D-hea-COF) material exhibiting a hea-type topology through a reaction between the precursor [2,3,6,7,14,15-hexakis(4-formylphenyl)] triptycene (HFPTP) and [tetrakis(4-amino biphenyl)methane (TABPM)]. X. Xu et al. performed a reaction between 5,10,15,20-tetra(4-aminophenyl)porphyrin (TAPP) and hexa(4-formylphenyl) benzene (HFPB), resulting in a porphyrin-based 3D COF (TAPP-HFPB-COF) with an she-type topology [41]. A paper by Xie et al. was also published, in which they obtained two highly crystalline 3D COF-type materials (3D-TPB-COF-OMe and 3D-TPB-COF-Ph) from their sterically controlled synthesis, designed using electron diffraction techniques. The latter, 3D-TPB-COF-Ph, exhibited an ljh-type topology not previously documented in the ToposPro database [42]. Wang et al., through a stratigraphic strategy, obtained a 3D COF (SPB-COF-DBA), constructed from flat square units of cobalt (II) phthalo cyanate (PcCo), which also exhibited an nbo-type topology that was not yet documented [43]. The first 3D COF-type material based on anionic titanium (Ti-COF-1) was also obtained [44]. A highly crystalline material with remarkable stability, Ti-COF-1 was obtained from octahedral Ti (IV) complex units and exhibited an soc-type topology. However, despite promising results and simulation calculations, new topologies of three-dimensional covalent organic structures still remain a synthetic challenge to realize, despite obtaining and designing them.

2. Triptycene in Polymeric Materials

Triptycene is an aromatic hydrocarbon, belonging to the iptycene group. It consists of three aromatic rings condensed into a [2,2,2] bicyclooctatriene grouping resembling a paddle wheel (Figure 2). Due to this unique form, the triptycene molecule exhibits considerable rigidity and provides a good base for the synthesis of polymers.
The presence of triptycene in the structure of polymeric materials increases their thermal strength. Triptycene polymers begin to degrade mostly at about 400 °C and retain more than 50% of their weight even at 800 °C. R. Bera et al., obtained nanoporous azo-polymers (NAPs) whose thermal degradation occurs between 528–531 °C. Moreover, at 800 °C, the char yield of these polymers was greater than 67%. The authors justify the thermal stability of NAPs with the presence of triptycene fragments in the structure of azo-polymers. [45] Nanoporous networks based on triptycene and amine bonds obtained by A. Alam et al. (TBOSBLs), showed similar charring efficiency to NAPs at 800 °C (<50%), which the authors also justify with the presence of triptycene units in the polymer structure [46]. Triptycene-based microporous polymers (TMPs) degraded at a lower temperature range, 397–460 °C, but their carbon yield at 800 °C was in the 56–68% range. In the results of measuring the thermal stability of TMPs, the authors explain both the triptycene molecules embedded in the structure of the polymers and their crosslinking [47]. On the other hand, T_COPs (covalent-organic polymers based on hydroxy-functionalized triptycene) had the highest thermal stability compared to the previously mentioned triptycene-based polymeric materials—thermal degradation occurred only at 600 °C, retaining a carbon yield of more than 60% up to that point [48]. In all of the aforementioned examples of triptycene-based polymeric materials, they were found to be mainly in amorphous form, although there are reports of such materials in graphite-like form [49,50]. Polyphenylene networks containing triptycene units (TPPs), obtained by S. Shetty et al., exhibited very high thermal stability, as they began to degrade in the 558–604 °C range, losing only 10% of their weight [49]. Polymeric materials based on triptycene are characterized by a high specific surface area. The presence of triptycene fragments contributes to the formation of meso- and micro-pores in the spatial network of the polymer molecule, as triptycene units are characterized by the phenomenon of so-called “internal molecular free volume” (IMFV) [51,52]. This means that in addition to the spaces formed initially between the joined building blocks of the polymer, the triptycene fragments present in the structure provide additional volume due to their three-dimensional structure. A summary of the specific surface area values along with the maximum pore volume of some polymeric materials containing triptycene is shown in Table 2. The large specific surface areas as well as the presence of more than one type of porosity in them, give tryptic polymer materials very good adsorption properties. Hence, many of these types of materials perform well in gas storage (especially for CO2, H2, and CH4) [45,46,49,53], selective gas separation, as well as in the adsorption of dyes [50]. A. Hassan et al. show that polymeric triptycene materials (T_COPs) can also be used to absorb radioactive iodine from the environment [48].

Triptycene in 3D COFs

The addition of triptycene fragments makes it possible to improve the stability and strength parameters of polymeric materials. However, despite the reported use of triptycene for the synthesis of polymeric materials, there are not many reports on the use of triptycene for analogous purposes in the synthesis of COFs. The reason for this phenomenon is most likely due to the fact that the class of porous COF-type materials was established relatively recently (2005) [56] and is only beginning to develop. Moreover, of the COF structures obtained so far, 2D structures predominate, while 3D COF structures are more difficult to obtain. Based on publications on the addition of triptycene fragments to polymeric materials, it can be expected that the introduction of these fragments into crystalline structures increases the crystallinity of COF materials, enriches the specific surface area with additional internal volume (IMFV), and the incorporation of a rigid triptycene backbone guarantees the stability and resistance of the system to thermal and chemical effects. Between 2020 and 2021, five articles were published in which the authors describe the synthesis of 3D COFs based on triptycene. All of the published syntheses are based on the same starting reagent: (six-connected) [2,3,6,7,14,15-hexakis(4-formylphenyl)Triptycene] (HFPTP), whose spatial structure resembles a triangular prism. In 2020, H. Liu et al. published the reaction between HFPTP and synergistic 4-connected 2D D2h monomer 1,3,6,8-tetrakis(4-aminophenyl)pyrene (TAPPy) which resulted in the first 3D COF material based on triptycene with stp topology, JUC-564 (Figure 3) [57]. It is the only example of such a material to date.
The material was characterized by high thermal stability (TGA: ~450 °C), very well-developed specific surface area, and low density (0.108 g/cm3); BET analysis results confirmed a surface area of 3383 m2/g, and N2 adsorption measurements showed the presence of two types of porosity, with pore diameters of 15 Å and 41–43 Å. In the case of the second value, it is the largest obtained so far for 3D COF-type materials (in comparison, the previously largest values for 3D COFs were 15.4 Å for JUC-518 [58] and 28 Å for DBA-3DCOF [29]). A material with such large pores provides an opportunity for the development of 3D COF structures and increases the potential applicability in biomedicine, among other fields, as the authors also found that large biologically active molecules, such as proteins, can be adsorbed in the pores of the materials [57]. In 2021, four more publications were published on newly-obtained triptycene-based 3D COF structures.
H. Liu et al. obtained two new 3D COF materials by reacting HFPTP sequentially with 6- and 1,3,5-tris(4-aminophenyl)triazine (TAPT) JUC-568 with a ceq topology (Figure 3) connected 2,3,6,7,14,15-hexa(3′,5′-diisopropyl-4′-amino) Triptycene (HDIATP) JUC-569 with an acs topology (Figure 4) [28]. JUC-569 exhibited good thermal stability (TGA: ~400 °C). BET analysis and measurement of N2 adsorption showed that the specific surface area was equal to 1254 m2/g and was mainly rich in micropores with a diameter of about 13 Å (1.27 nm). JUC-568 was obtained in an analogous manner to the material published in the same year by Z. Liu et al. [3D-ceq-COF] [59], which showed similar structural parameters and a ceq topology (Figure 5).
The material had a higher thermal stability than JUC-568 (TGA ~550 °C). Its specific surface area was equal to 1148.6 m2/g, and it was also rich in micropores with diameters between 10–16 Å. The obtained 3D COF materials [28,59] were investigated for adsorption of gases such as CO2, CH4, and H2 (see Table 3). The adsorption levels of both 3D-ceq-COF and its analogue JUC-568 for CO2 and CH4 at two different temperatures (273 and 298 K) proved to be extremely high, especially for H2 adsorption (JUC-568—274 cm3/g at 77 K, 3D-ceq-COF—178.49 cm3/g at 77 K and 131.27 cm3/g at 87 K), which is significantly higher than for other materials of this type such as PPN-3 (1.58 wt%) [60], PAF-1 (1.66 wt%) [61], SPT-CMP1 (1.72 wt%) [62], and DL-COF-1 (2.09 wt%) [63]. The adsorption values of CO2, CH4, and H2 also compare favorably to the results obtained for JUC-569: CO2 (98 cm3/g at 273 K and 81 cm3/g at 298 K); CH4 (48 cm3/g at 273 K and 32 cm3/g at 298 K); and H2 (274 cm3/g at 77 K). In the same year, the team of Z. Li et al. published another new structure in addition to 3D-ceq-COF. The 3D-hea-COF material was obtained using a reaction between HFPTP and [tetrakis(4-amino-biphenyl)methane] (TABPM) and had an hea topology not yet reported for 3D COF materials (Figure 5) [40].
Like the previously described structures, 3D-hea-COF exhibited good thermal stability properties (TGA: ~480 °C) as well as specific surface area (BET: 1804.0 m2/g). Its surface area, like that of JUC-568-569 and 3D-ceq-COF, was mainly rich in micropores with a diameter of about 16 Å. The paper mentions that 3D-hea-COF was tested mainly for H2 adsorption. The 3D COF material shows good hydrogen adsorption (193.48 cm3/g at 77 K and 131.03 cm3/g at 87 K), which is comparable to the results obtained with PAF-1 (186 cm3 g−1) [61], Trip-PIM (185 cm3 g−1) [64], and SPT-CMP1 (193 cm3 g−1) [62]. According to the authors, such a high adsorption value is due to the presence of triptycene fragments and aromatic systems in the 3D-hea-COF structure, which, by forming an internal microstructure in the material, cause greater gas adsorption [40]. The use of triptycene, as a building block that provides a stable framework for the crystal structure, was also presented in an article published in 2021 by Y. Wang et al. Two different 3D COF materials were obtained using an imine condensation reaction [6 + 4]: Trip-COF-1 and Trip-COF-2 with stp topologies [65]. Both materials had good crystalline properties, with each forming a structure that resembled a honeycomb (Figure 6 and Figure 7).
The surface areas for Trip-COF-1 and Trip-COF-2 were determined by measuring the adsorption of gaseous N2, obtaining values of 1474 m2/g and 1624 m2/g, respectively. For Trip-COF-1, BET analysis showed the presence of three types of porosity: micro-, meso- and macro-porosity. The pore values occurred in the ranges of about 13 Å, 30 Å, and 40 Å. Trip-COF-2 had two types of porosity, with pore diameters of about 13 Å and 29 Å. The authors explain the differences in the BET analysis as due to the presence of the 2-fold interpenetrating structure in Trip-COF-1, which causes the phenomenon of interpenetration and, consequently, greater variation in porosity. This structural feature is missing in Trip-COF-2, resulting in the structure forming only two types of porosity [65]. In this way, the authors show how, by handling the size and interpenetration of each reactant and reaction conditions, the structural properties of 3D COF materials can be influenced, which can contribute to a better understanding of their complexity. In addition, materials in which it is possible to design and control the formation of pores of a specific size represent a good application potential in the catalysis, separation and adsorption of large molecules. The authors also believe that the unusual honeycomb structure of Trip-COFs can be used for optoelectronic applications, due to the delocalization of p-electrons along the wall of 1D nanowires [65].
Table 3. Triptycene based 3D COF properties.
Table 3. Triptycene based 3D COF properties.
NameTopologyTGA
(°C)		SBET
(M−1 g−1)		Pore Size Distribution (Å)CO2 Uptake (cm3g−1)CH4 Uptake (cm3g−1)H2 Uptake (cm3g−1)Reference
Trip-COF 1stp-1473.0012.6, 29.6, 39.9---[65]
Trip-COF 21624.0012.6, 29.3
JUC-568
JUC-569		ceq/acs~520
~400		1433.00
1254.00		~19.2
~18.7		98.00 (273 K), 81.00 (298 K)
47.00 (273 K), 31.00 (298 K)		48.00 (273 K), 32.00 (298 K)
19.00 (273 K), 11.00 (298 K)		274.00 (77 K)
167.00 (77 K)		[28]
3D-ceq-COFceq~5501148.610, 1691.27 (273 K), 330.33 (298 K)36.28 (273 K), 23.22(298 K)178.49 (77 K), 131.27 (98 K)[59]
3D-hea-COFhea 1804.001680.01 (273 K)21.77 (273 K)193.48 (77 K)[40]
JUC-564stp 3383.0015, 43---[57]

3. Future Perspectives

3D COF materials, in which it is possible to design and control pore formation to a specific size, represent a good application potential in the catalysis, separation, and adsorption of large molecules. The promising results of these structures in gas adsorption measurements demonstrate the possible potential of using triptycene-based 3D COF materials for environmental action in the adsorption of heating gases from the atmosphere [40,59] as well as in the development of work on the acquisition and storage of hydrogen-based renewable fuels [40]. In addition, obtained 3D COFs in the form of unusual structures, such as the honeycomb structure of Trip-COFs, can be used for optoelectronic applications due to the delocalization of p-electrons along the wall of 1D nanochannels [65]. Therefore, the field of 3D triptycene based COFs is an unexplored area and requires further studies in terms of controlled synthesis of such structures. An important issue that has not arisen in the literature is the toxicity of triptycene based COFs, which is of high importance in the case of their application in the medical, biological, and environmental fields. These studies are crucial to understand the biological and environmental fates of these 3D structures and their impacts on living organisms. In particular, the medical applications of triptycene-based structures seem to be underappreciated and almost unexplored, but these materials might be used as platforms for a drug delivery system with a high loading capacity, biocompatibility and straightforward surface functionalization.

4. Conclusions

Covalent Organic Frameworks are a new class of crystalline porous materials showing great application potential due to their physicochemical properties. Due to their high specific surface areas, porosities, crystallinities, chemical-thermal stabilities, and biocompatible morphologies, COFs find applications not only in industrial areas, but also in biomedical areas. 3D COFs are characterized by better physical parameters than 2D-COFs, but are less frequently used, due to the difficulty of synthesizing them and finding suitable building blocks for their construction.
The triptycene skeleton addition to 3D COF structures enhances their chemical and physical properties. The rigid, three-paddle-wheel triptycene form provides 3D COFs with architectural stability, extra volume (IMFV), and lower density, which results in high thermal stability (up to 500 °C) and good adsorption of gases such as CO2, H2, and CH4. In addition, some of them have a specific surface area rich in pores of more than one type, with some macro-scale CFOs able to adsorb not only gases but also large biomolecules the size of proteins (JUC-564). Moreover, the addition of the triptycene building block not only enriches the library of 3D COF building blocks, but also extends new application possibilities for these materials. The impressive adsorption properties of gases, including heating gases such as CO2 and CH4, allow these types of materials to be used in environmental rescue applications. The presence of pores with a diameter of more than 41 Å, capable of adsorbing large molecules, makes 3D COF materials a good alternative in the design of nanocarriers with therapeutic applications. In order to develop triptycene 3D COFs and explore their great application potential, further investigations and analyses are required. Moreover, the development of COF databases and their theoretical tailoring seems to be an area of high interest and importance for the development of 3D COFs and 3D COFs based on triptycene.

Author Contributions

Conceptualization, R.M. and M.B.; writing—original draft preparation, M.B. and R.M.; writing—review and editing, M.B. and R.M.; supervision, R.M.; project administration, R.M.; funding acquisition, R.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Sciences Centre, grant number UMO-2018/31/B/ST8/02460.

Conflicts of Interest

The authors declare no conflict of interest.

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Symmetry 15 01803 g001
Figure 1. Topology diagram representing a general basis for design and construction of (A) 2D COFs and (B) 3D COFs [2].
Figure 1. Topology diagram representing a general basis for design and construction of (A) 2D COFs and (B) 3D COFs [2].
Symmetry 15 01803 g001
Symmetry 15 01803 g002
Figure 2. Triptycene structure with the three-fold symmetry axis.
Figure 2. Triptycene structure with the three-fold symmetry axis.
Symmetry 15 01803 g002
Symmetry 15 01803 g003
Figure 3. The new COF (termed JUC-564) has a high specific surface area (up to 3300 m2g−1), the largest pore size among 3D COFs (43 Å), and record-breaking low density among crystalline materials reported to date (0.108 gcm−3) [56].
Figure 3. The new COF (termed JUC-564) has a high specific surface area (up to 3300 m2g−1), the largest pore size among 3D COFs (43 Å), and record-breaking low density among crystalline materials reported to date (0.108 gcm−3) [56].
Symmetry 15 01803 g003
Symmetry 15 01803 g004
Figure 4. A 3D triptycene-based COF, JUC-569, with an acs topology [27].
Figure 4. A 3D triptycene-based COF, JUC-569, with an acs topology [27].
Symmetry 15 01803 g004
Symmetry 15 01803 g005
Figure 5. A 3D-COF with a ceq topology utilizing a D3h-symmetric triangular prism vertex with a planar triangular linker [58].
Figure 5. A 3D-COF with a ceq topology utilizing a D3h-symmetric triangular prism vertex with a planar triangular linker [58].
Symmetry 15 01803 g005
Symmetry 15 01803 g006
Figure 6. A 3D COF with an hea topology [39].
Figure 6. A 3D COF with an hea topology [39].
Symmetry 15 01803 g006
Symmetry 15 01803 g007
Figure 7. (a) TEM image of Trip-COF 1; (b) TEM image of Trip-COF 1 giving an enlarged view of the selected area in panel (a); (c) Fourier-filtered image of selected areas of Trip-COF 1; inset: fast Fourier transform (FFT) from the selected areas; for Trip-COF 2 see (d–f), respectively; (g) iDPC image of Trip-COF 1; (h,i) enlarged views of a selected area in panel (g) [65].
Figure 7. (a) TEM image of Trip-COF 1; (b) TEM image of Trip-COF 1 giving an enlarged view of the selected area in panel (a); (c) Fourier-filtered image of selected areas of Trip-COF 1; inset: fast Fourier transform (FFT) from the selected areas; for Trip-COF 2 see (d–f), respectively; (g) iDPC image of Trip-COF 1; (h,i) enlarged views of a selected area in panel (g) [65].
Symmetry 15 01803 g007
Table 1. COF linkage types.
Table 1. COF linkage types.
BORONIC ESTERSymmetry 15 01803 i001
BOROXINESymmetry 15 01803 i002
IMINESymmetry 15 01803 i003
HYDRAZONESymmetry 15 01803 i004
IMIDESymmetry 15 01803 i005
AMIDESymmetry 15 01803 i006
AZINESymmetry 15 01803 i007
β-KETOENAMINESymmetry 15 01803 i008
β-KETOENAMINESymmetry 15 01803 i009
BORAZINESymmetry 15 01803 i010
SQUARAINESymmetry 15 01803 i011
PHENAZINESymmetry 15 01803 i012
C=C BONDSymmetry 15 01803 i013
1,3,5-TRIAZINESymmetry 15 01803 i014
Table 2. Properties of triptycene based materials.
Table 2. Properties of triptycene based materials.
PolymerSABET
(m2g−1)		SALANG
(m2g−1)		Vtotal
(cm3 g−1)		Reference
NAP 1109516221.060[45]
NAP 292313530.690
TMP192312110.490[47]
TMP2109414570.700
TMP3137218170.860
TAP14747360.740[53]
TAP277211731.410
TAP372910931.040
TBPAL177510360.401[54]
TBPAL27299450.369
TBPAL36029420.446
TBPAL462010270.529
TBPAL581514110.760
T_COP-1206-0.218[48]
T_COP-22590.320
T_COP-38260.533
TPP1380-0.250[49]
TPP24680.290
TPP32400.250
STP-25417360.320[55]
STP-33785150.340
TBOSBL164910510.527[46]
TBOSBL25708100.384
TBOSBL34938170.467
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Borkowska, M.; Mrówczyński, R. Triptycene Based 3D Covalent Organic Frameworks (COFs)—An Emerging Class of 3D Structures. Symmetry 2023, 15, 1803. https://0-doi-org.brum.beds.ac.uk/10.3390/sym15091803

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Borkowska M, Mrówczyński R. Triptycene Based 3D Covalent Organic Frameworks (COFs)—An Emerging Class of 3D Structures. Symmetry. 2023; 15(9):1803. https://0-doi-org.brum.beds.ac.uk/10.3390/sym15091803

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Borkowska, Monika, and Radosław Mrówczyński. 2023. "Triptycene Based 3D Covalent Organic Frameworks (COFs)—An Emerging Class of 3D Structures" Symmetry 15, no. 9: 1803. https://0-doi-org.brum.beds.ac.uk/10.3390/sym15091803

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