Defect Engineering in Solution-Processed Polycrystalline SnSe Leads to High Thermoelectric Performance

Review Conductive properties of triphenylene MOFs and COFs Noemí Contreras-Pereda a , Salvador Pané b , Josep Puigmartí-Luis c,d , Daniel Ruiz-Molina a, ⇑ a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain b Multi-Scale Robotics Lab, ETH Zurich, Tannenstrasse 3, Zurich, CH-8092 Switzerland c Departament de Ciència dels Materials i Química Física, Institut de Química Teòrica i Computacional, 08028 Barcelona, Spain d ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain article info Article history: Received 27 October 2021 Accepted 4 February 2022 Available online 25 February 2022 Keywords: Triphenylene MOF COF Two-dimensional Electronic Functional device abstract Triphenylene (TP) based materials have experienced a great expansion in the latest years. TP molecules have interesting optoelectronic properties, arising from the aromatic core, which have been exploited in functional two-dimensional (2D) Metal-Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs) aside other organic polymers. In this review we summarize synthetic approaches of TP-based 2D MOFs and COFs emphasizing on the resulting morphology, crystalline domains and orientation, proven to have great impact on the properties and performance of these materials in functional devices. Specifically, we report a detailed description on the different TP-based 2D structures detailing the influence of the chemical and crystalline structure on the electronic properties, specially the in-plane and outof-plane contribution to the electrical conductivity. Finally, we give also attention and present several examples of functional devices made out with these electronic materials with great impact in the literature as well as in future technological applications. Ó2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Contents 1. Molecular characteristics and supramolecular properties . . . . . .................................................................. 2 2. Synthetic methodologies . . . . . . . . . . . . ..................................................................................... 4 2.1. Bulk synthesis . . . . . . . . . ........................................................................................... 4 2.1.1. Solvothermal reactions. . . . . . ................................................................................ 4 2.1.2. Sonochemical reactions . . . . . ................................................................................ 4 2.2. Template-assisted syntheses . . . . . . . . . . . . . ........................................................................... 5 2.2.1. Layer-by-Layer (LbL). . . . . . . . ................................................................................ 6 2.2.2. Vapor-assisted conversion . . . ................................................................................ 7 2.3. Interfacial syntheses of thin films . . . . . . . . . ........................................................................... 7 2.4. Microfludic devices . . . . . ........................................................................................... 8 2.4.1. Pressed substrates . . . . . . . . . ................................................................................ 8 2.4.2. Continuous flow . . . . . . . . . . . ................................................................................ 8 2.4.3. Simulated microgravity environment . . . . . . . . . . . . . ............................................................. 8 2.5. On-surface UHV evaporation . . . . . . . . . . . . . ........................................................................... 8 3. Triphenylene-based 2D MOFs . . . . . . . . ..................................................................................... 9 https://doi.org/10.1016/j.ccr.2022.214459 0010-8545/Ó2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Abbreviations: 1D, One-dimensional; 2D, Two-dimensional; 3D, Three-dimensional; AFM, Atomic Force Microscopy; BDBA:1, 4-benzenediboronic acid; BDT, benzodithiophene; BPDA, 4,4 0 -biphenyldiboronic acid; BTBA, 1,3,5-benzenetriboronic acid; COF, Covalent Organic Framework; DBP, diphenylbutadiyne; DLC, Discotic Liquid Crystal; FPBA, 4-formylphenylboronic acid; GDY, Graphdyine; HATP, 2,3,6,7,10,11-hexaamino triphenylene; HER, Hydrogen Evolution Reaction; HHTP, 2,3,6,7,10,11hexahydroxy triphenylene; HR-TEM, High resolution Transmission Electron Microscopy; LbL, Layer-by-layer; LSG, Laser Scribed Graphene; MOF, Metal-Organic Framework; ORR, Oxygen Reduction Reaction; PEG, Polyethylene glycol; SEM, Scanning Electron Microscopy; TATTA, 4,4 0 ,4 00 -(1,3,5-triazine2,4,6-triyl)trianiline; TBPA, 1,3,5-Tris[(4dihydroxyboryl)phenyl]benzene; TFTPN, tetrafluoroterephthalonitrile; THQ tetrahydroxy-1, 4-quinone; THT, 2,3,6,7,10,11-hexathiol triphenylene; TP, Triphenylene; TPHS, 2,3,6,7,10,11-hexaselenol triphenylene; UHV, Ultra-High Vacuum; vdP, Van der Pauw; XRD, X-ray diffraction. ⇑ Corresponding author. E-mail address: [email protected] (D. Ruiz-Molina). Coordination Chemistry Reviews 460 (2022) 214459 Contents lists available at ScienceDirect Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr 3.1. Chemical families . . . . . . ........................................................................................... 9 3.1.1. HHTP-based 2D MOFs . . . . . . ................................................................................ 9 3.1.2. HATP-based 2D MOFs. . . . . . . ............................................................................... 10 3.1.3. THT-based 2D MOFs. . . . . . . . ............................................................................... 12 3.1.4. Other structures . . . . . . . . . . . ............................................................................... 13 3.2. Applications of triphenylene-based 2D MOFs .......................................................................... 15 3.2.1. Chemiresistive sensor. . . . . . . ............................................................................... 15 3.2.2. Batteries/Supercapacitors. . . . ............................................................................... 15 3.2.3. Electrocatalyst. . . . . . . . . . . . . ............................................................................... 15 3.2.4. Spintronics . . . . . . . . . . . . . . . ............................................................................... 16 3.2.5. Optoelectronics. . . . . . . . . . . . ............................................................................... 17 3.2.6. Tactile sensor . . . . . . . . . . . . . ............................................................................... 18 3.2.7. FET logic . ............................................................................................... 18 3.2.8. Thermoelectric . . . . . . . . . . . . ............................................................................... 18 4. Triphenylene-based 2D COFs. . . . . . . . . .................................................................................... 18 4.1. Chemical families . . . . . . .......................................................................................... 18 4.1.1. HHTP-based 2D COFs (ester-boronate and polyarylether bonds) . . . . . . . . . . ......................................... 18 4.1.2. HATP-based 2D COFs (phenazine bonds) . . . . . . . . . . ............................................................ 20 4.1.3. Other structures . . . . . . . . . . . ............................................................................... 24 4.2. Applications of TP-based 2D COFs . . . . . . . . . .......................................................................... 24 4.2.1. Photovoltaics and photocurrent. . . . . . . . . . . . . . . . . . ............................................................ 24 4.2.2. Li-ion battery . . . . . . . . . . . . . ............................................................................... 24 4.2.3. Electrocatalysis . . . . . . . . . . . . ............................................................................... 24 5. Future perspectives . . . . . . . . . . . . . . . . .................................................................................... 25 Declaration of Competing Interest . . . . .................................................................................... 26 Acknowledgements . . . . . . . . . . . . . . . . .................................................................................... 26 References . . . . ....................................................................................................... 27 1. Molecular characteristics and supramolecular properties Triphenylene (TP) is a polycyclic aromatic hydrocarbon consisting of four orthofused benzene rings with one central ring surrounded by the other three in an alternate fashion (see Fig. 1a), resulting in a large electron cloud with 18 p delocalized electrons [1]. There exists a wealth of derivatives of triphenylene as function of the groups R 1 and R 2 (see Fig. 1a). For instance, TPs exhibit a supramolecular packing mainly through p - p stacking, further optimized by Van der Waals interactions arising from hexasubstituted long alkyl chains that do not perturbate neither the planarity nor the electronic relocation of the TP core. Precisely, the significant electronic delocalization of the resulting supramolecular structures has made TP derivatives as suitable components for different devices, including sensors for neutral aromatic or cationic guests [2], molecular rotors [1], or in highly stable organic-oxygen cell batteries [3]. However, in this review, we will focus mainly on those where R 1 and R 2 are equal between them, and incorporate –OH (henceforth HHTP), –NH 2 (henceforth HATP) or -SH (henceforth THT) groups. Worth to mention and preceding the description of TP-based MOFs and COFs, TP materials have been significantly applied in the area of discotic liquid crystals (DLCs). DLCs are mesophase supramolecular materials, that in a way similar to other liquid crystals, are formed by the columnar stacking of disk-like moieties (see Fig. 1b) [4,5]. As a consequence of this assembly, DLCs exhibit broad energetic band gaps, where conduction occurs mainly due to hopping of charge carriers rather than a band-like effect, relying on the interplanar distance between TP molecules [6]. Therefore, DLCs often require electrochemical or photochemical doping or charge injection from metallic surfaces moieties for enhanced electronic conductivity [7]. In this case, large electronic delocalization leads to strong charge carrier percolation along the columnar axis, which makes these materials one-dimensional organic semiconductors [5]. For example, the incorporation of nitrogen-doped planar moieties as an electron acceptor generates an intercalated donor–acceptor DLC system with good electrical capacities, and suitable as a cathode in Li batteries [8]. DLCs are also characterized by their easy processability into thin films [4], enabling their integration as components in devices for optoelectronic applications such as organic photovoltaics or organic light-emitting diode (OLEDs) technologies [6]. According with previous considerations, TP-based conductive 1D structures are promising building blocks for advanced optoelectronics. Yet, applications in this domain have been hampered because the lack of reproducibility and required robustness of these architectures. The reason for this is that charge carrier mobility is strongly dependent on the crystallinity of the mesophase [6,9], which varies considerably in DLCs because of the interactions between TP moieties. Alternatively, DLCs can also be obtained with linear and flexible covalent polymers bearing TP moieties that stack into columnar phases, in which the bended side chains are positioned in the outer surface of the column [10]. Though, control over the crystallinity remains strongly dependent on the nature and bendable character of the building blocks used in these investigations [11]. Recent years have seen a wealth of research devoted to the development of TP-based 2D metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) with broad electronic bands for enhanced electronic conduction. COFs and MOFs are polymeric crystalline porous network materials that consist of organic building blocks that polymerize through covalent or coordination bonds, respectively [12–14]. While covalent bonds ensure a larger thermal and pH stability [14,15], the incorporation of metallic ions can add novel optical, magnetic and/or electrical properties [16]. Contrary to DLCs, TP-based organic frameworks (with a defined crystallinity and long-range order) do not rely on weak interactions but on strong covalent and coordination bonds. Hence, they have a stable and robust structure not as much sensitive to external factors such as temperature (Fig. 1c). Moreover, the configuration and dimensionality of the polymer can be predicted by a rational selection of the metal nodes and the organic linkers, giving rise to a wide variety of 2D structures [17]. These planar polymeric layers can further form a 3D supramolecular structure by interlayer interactions such as p - p stacking, hydrogen bonding, Van der Waals forces, or combinations of these. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 2 The incorporation of TP moieties in MOFs and COFs is schematically represented in Fig. 2. The 2D layers present a honeycomb graphitic architecture that results in interesting band structures and properties. Regarding the electronic properties, two main components arise: (a) conduction of charge carriers along the 2D layers (in-plane) and, (b) interlayer charge transfer (out-of-plane conduction) [18].In-plane conduction is usually found for TP-based 2D MOFs where coordination bonds typically lead to large electron delocalization and conjugation pathways along the 2D layers, which results in high in-plane conductivities. Strong delocalizations can be a direct consequence of the strong coordination of dp bonds (normally arising by the rational selection of suitable metallic centers and TP substitution) [19]. Contrarily, covalent bonds formed in TP-based COF structures need to maintain the conjugation in order to lead to such conductivities [20]. TP-based 2D COFs can present a wide variety of p -conjugation depending on the chemical structure and the condensation bonds forming the 2D layers. Boronate ester condensation leads to COFs with poor conjugation and hence low charge delocalization along the 2D layers [21]. On the other hand, phenazine condensed COFs present a larger conjugation leading to a larger in-plane charge transfer. Thus, very few examples of conductive COFs have been reported, which commonly require of either chemical (including donor atoms in their chemical structure) or external doping to aim for electronic applications [14].Out-of-plane conductivities are determined by the supramolecular stacking of the 2D layers. In a similar fashion to DLCs, layer stacking of the TP moieties can lead to onedimensional charge transfer channels, where charge transfer occurs in through-space pathways [18]. However, different stacking geometries (being mostly eclipsed, nearly-eclipsed or slippedparallel) can be found, being always governed by the chemical structure of the TP-based 2D MOF/COF under investigation. Hence, Fig. 1. (a) Chemical structure of a TP molecule. When having identical hexasubstitutions different ligand arise being HHTP for 2,3,6,7,10,11 hexahydroxytriphenylene, HATP for 2,3,6,7,10,11 hexaaminotriphenylene and THT for 2,3,6,7,10,11 triphenylenehexathiol. (b) Schematic of stacking of TP moieties in DLCs forming 1D conducting channels. The supramolecular mesophase structure relies on weak interactions that can be broken upon external stimuli as temperature. (c) Schematic of the stacking of TP moieties in TP-based 2D MOFs and COFs. The crystalline packing ensures a robust structure formed by several hexagonally distributed 1D conducting channels. Furthermore, large inplane delocalization can be obtained along the 2D layers depending on the coordination or covalent bonds used. Moreover, the structure defines 1D pores suitable for host– guest interactions or ionic conduction. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 3 the crystalline structure has a significant impact on the in-plane and out-of-plane contributions of the optoelectronic properties of TP-based MOFs/COFs. In the following, the impact of the chemical structures of TP-based MOFs/COFs in the final in-plane and out-ofplane charge transfer properties will be discussed. 2. Synthetic methodologies Features such as the crystallinity and crystallite size of MOFs and COFs can dramatically impact on their conductive properties [18,22]. To date, control over these parameters has been investigated as function of the methodology used for their synthesis. Accordingly, the methods and synthetic approaches used for the controlled nucleation and growth of these materials has become a very relevant topic in this research area. Most representative examples obtained with different methodologies are discussed in the following sections (see the chemical structures of all the examples in the following Sections 3 and 4). 2.1. Bulk synthesis 2.1.1. Solvothermal reactions The most common method for the synthesis of TP-based 2D MOFs and COFs is the solvothermal synthesis. In this approach, reactants are dispersed or solubilized in a common solvent and heated up in either a sealed or an opened glassware, depending on the requirement of oxygen in the reaction (see Fig. 3). TP-based solvothermal reactions of MOFs are characterized by a fast uncontrollable crystallization, leading to several nucleation events in the reaction media. Studies to enhance the crystallinity and particle size were attempted by changing the order of addition or modifying the amount of additives [25,26]. In any case, the largest TP-based 2D MOFs reported with this approach yielded crystals in the order of few micrometers in length being Cu 3 (HHTP) 2 the main example (Fig. 3c) [24]. In the case of COFs, long reaction times are required to ensure the crystallization of the covalently bonded porous polymeric structure, which are obtained mostly in the form of insoluble powders (Fig. 3b), as first described by Yaghi and co-workers for COF-5 [23]. These reaction mixtures are characterized by their heterogeneity, which causes the occlusion of reagents within the precipitated material and low synthetic yields [27,28]. Thus, studies on homogeneous polymerization have been sought aiming for large crystallization rates and reduced reaction times [28]. For instance, the formation rate dependence of COF-5 with temperature, concentration and presence of additives allowed for the generation of micrometer-long polycrystalline aggregates. 2.1.2. Sonochemical reactions The reaction between monomers assisted by ultrasound in solution is controlled by tuning parameters such as solvent, sonication Fig. 2. Scheme of the formation of TP-based 2D MOFs and TP-based 2D COFs. TP 2D MOFs are typically formed by coordination of a metallic ion with the electron donor heteroatoms from TP functionalization. TP 2D COFs are formed by covalently linking TP functional substituents with other organic ligands. The three-fold geometry of TP leads to honeycomb hexagonal structures. Exemplary structures of TP 2D MOFs and COFs layers are shown. In the case of TP 2D MOFs, the planar coordination centers MX 4 defines the honeycomb graphitic structure. On the other hand, in the case of TP 2D COFs, the structure depends on the geometry and the covalent bonding with other organic ligands. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 4 times or power. These parameters are changed to modulate the crystallinity, dimensions and porosity of the obtained powders (Fig. 4). Sonochemical syntheses enables faster crystallization, which consequently lowers the reaction times to few hours or even minutes [27]. Moreover, TP-based organic frameworks with larger surface areas than those reported by solvothermal methods can be achieved using this approach. While sonochemical reactions are widely employed for the synthesis of COFs, they are rarely applied in MOF synthesis as small sized crystals are already obtained with solvothermal synthesis. 2.2. Template-assisted syntheses In recent years, different techniques have been described regarding post-synthetic on-substrate deposition, such as dropcasting of colloidal suspensions of TP-based 2D MOFs (either assynthetized [29] or after sonication [30]), or TP-based 2D COFs, onto surfaces [31]. Apart from the typical drop-casting approach, other top-down techniques include spin-coating [32], ball-milling blending and abrasion of powders onto substrates [33], or spraycoating of colloidal suspension onto flexible substrates with a high control on the coverage area and patterning [26]. However, in most of the cases, crystalline powders are not always suitable for device fabrication because they typically form rough interfaces, which lead to charge entrapment and poor ohmic contacts [34]. The incorporation of TP-based MOFs on surfaces can be eased by adding the substrates in the reaction pot (Fig. 5a). A wide variety of suitable substrates for the growth of TP-based MOFs have been described (Fig. 5c) including flat substrates for thin film formation [35] and coating of textiles [36], carbon paper [37], shrinkable flexible substrates [38], core–shell materials [39–41] or metallic meshes [42]. Solvothermal or sonochemical reactions were also used to induce the epitaxial growth of COFs on substrates capitalizing on interactions between the monomers and the immersed substrate (Fig. 5b). As an example, Dichtel et al. grew nanometer-thick COF-5 and Pyr-COF thin films on single layer graphene substrates, which were dipped into the reaction mixture [43]. The highly dense TP-based COF films generated showcased smooth surfaces and long-range order with preferential orientation in the c-axis, independently of the substrate used to support the graphene single layer. Growth of TP-based 2D COFs on a large variety of substrates Fig. 4. Scheme of the sonochemical reaction of TP-based 2D COFs. (b) and (c) SEM images of crystals of the same COF-5 obtained with sonochemical reactions obtained under different conditions. As observed, small and narrow size distribution of particles is obtained. Images reproduced with permission from [27]. Fig. 3. (a) Scheme illustrating solvothermal reaction syntheses of TP-based 2D MOFs and COFs. (b) Scanning Electron Microscopy (SEM) image of crystals of COF-5 obtained with solvothermal methods. Image reproduced with permission from [23]. (c) SEM image of crystals of Cu 3 (HHTP) 2 obtained with solvothermal methods. Scale bar is 1 l m. Image reproduced with permission from [24]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 5 has also been reported, with carbonaceous architectures such as carbon nanotubes and free standing graphene [44,45]. This technique has also been used to grow COF-5 on the inner surface of polydopamine-coated capillars [46]. The capillary was filled with the reaction mixture, sealed and subjected to high temperature similarly as in a solvothermal approach. Higher quality and scalable thin films can be obtained also by combining a templateassisted method with a colloidal template strategy [47]. During the template-assisted formation of several TP-based 2D COFs (e.g.: COF-5, COF-10, Pyr-COF, and DBP-COF), unwanted COF nanoparticles are concomitantly form in the bulk of the solvent upon addition of co-solvents and remain as a stable colloidal suspension. Consequently, irregular deposition of COF particles is minimized and only heterogeneous nucleation and growth of films with low roughness occurs on the substrate. 2.2.1. Layer-by-Layer (LbL) This approach has been mainly explored for TP-based 2D MOFs. Typically, substrates are functionalized with active sites to entrap the metal ion on the surface (Fig. 6a). Next, the metal-loaded substrate is exposed to the TP-substituted moiety saturating all the available metallic centers through coordination bonds and leaving coordination sites available. Then, the substrate is exposed again to a metallic solution. This procedure is repeated in a cyclic fashion inducing a homogeneous and layered growth of thin films of large lateral sizes. This approach is characterized by the control over Fig. 6. (a) and (b) Schematic of a layer-by-layer growth of TP-based 2D MOFs using spray-drying of the TP and metallic moieties and SEM image of the obtained Cu 3 (HHTP) 2 MOF with this technique, respectively. Images adapted with permission from [51]. (c) Schematic illustration of the vapor assisted conversion method. Image reproduced with permission from [52]. (d) SEM image of a BDT-COF grown with vapor assisted conversion. Image reproduced with permission from [53]. Fig. 5. (a) Schematic of template-assisted synthesis based on the immersion of a substrate in a solvothermal reaction mixture. (b) Cross-section SEM image ofa benzothiophene-based COF (BDT-COF) thin film grown on top of a rigid metallic substrate through template-assisted method. Image reproduced with permission from [48]. (c) Picture and SEM image of Ni 3 (HHTP) 2 MOF grown on flexible cotton textile through template-assisted method. Images reproduced with permission from [36]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 6 the thickness of the films being generated, and it is directly related to the number of applied cycles. Note that Cu 3 (HHTP) 2 thin films obtained with LbL methods grow epitaxially having highly oriented crystallites (Fig. 6b). Remarkably, the deposition can be performed with different methods such as substrate immersion in solutions [49,50] or spraying the solutions onto the substrate [51]. The main drawback of LbL approaches is their high time consumption. 2.2.2. Vapor-assisted conversion This methodology has been recently reported for TP-based 2D MOFs, more specifically for Ni 9 (HHTP) 4 ,Co 9 (HHTP) 4 and Cu 3 (HHTP) 2 structures [52], and relies on the reaction between starting monomers on a wet surface at the solvent gas pressure conditions (Fig. 6c). This leads to a saturated reaction environment nearby the substrate’s surface. This method allowed for growing highly oriented centimeter-long thin films on different substrates both insulating and conducting. However, optimization from substrate to substrate was required. For some specimens, some acidic modulators were needed in the synthesis in order to obtain homogeneous films on the substrates including large coverage. In the case of COFs, the vapor-assisted conversion approach was introduced by Bein and co-workers in 2015 as a scalable method for the production of thin films of COF-5 and BDT-COF. Specially, the approach allows for the formation of COF thin films at room temperature with a high control on their thicknesses ranging from hundred nanometers to the micrometer scale (Fig. 6d) [53]. 2.3. Interfacial syntheses of thin films In the case of TP-based 2D MOF syntheses using liquid–liquid interfaces, the metallic salt and the organic ligand are dissolved in different immiscible solvents, thus confining the reaction at the interface upon the slow diffusion of the reactants (Fig. 7a and c) [54,55]. By means of this approach, thin films with submillimeter lateral sizes can be obtained. Yet, the fabrication of films with nanometer thicknesses still remains a challenge due to the lack of homogeneity along the film [54]. One of the most representative approaches used at liquid–gas interface is certainly the Langmuir-Blodget technique, resulting in the growth of sub-millimeter films with nanoscale thicknesses. Main achievements are the growth of a 2D monolayer of Ni 3 (THT) 2 MOF [56] or Cu 3 (HHTP) 2 MOF thin films with high preferential orientation [54] (Fig. 7d). Note that by using this controlled growth and a stamping approach in a repeated fashion, one can achieve a controlled increase of the film thickness [54]. Another approach is the use of reagents in gas phase such as ammonia vapors [57] or atmospheric oxygen (as reported for the growth of Ni 3 (HITP) 2 )[58]. The thin film formed at the interface grows upon diffusion of the gases into the liquid phase. Hence, control on the thickness is achieved upon the control of the reaction time. Importantly, this approach can generate films with lateral dimensions of hundreds of microns, which lead to specimens with lower conductivities than those previously reported for compressed pellets. Moreover, the obtained thin films were polycrystalline and randomly orientated. Interfacial syntheses have been largely exploited for TP-based MOFs whereas regarding TP-based COF structures, synthesis of TPbased graphdyine using Glaser-Hay cross-coupling reactions is the only example of interfacial synthesis reported to date. In this case, the starting monomer is in one phase and the copper catalyzer in the other leading to thin films with sub-nanometer thicknesses (Fig. 7b) [59]. Fig. 7. (a) Illustration showing a liquid–liquid interfacial synthesis. The starting monomers are in different immiscible liquid phases and the reaction between them occurs at the interface. The reaction and growth are mediated by the diffusion of the monomers towards the interface. (b) Atomic Force Microscopy (AFM) image of a TP-based graphdyine COF obtained in a liquid–liquid interface. Image reproduced with permission from [59]. (c) Schematic illustration of a liquid–gas interfacial synthesis. One monomer meets the reaction mixture at the liquid–air interface, it can be added in a gas form. (d) Optical microscopy (OM) image of a Ni 3 (THT) 2 MOF single-layer thin film grown through liquid–air interface. Image reproduced with permission from [56]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 7 2.4. Microfludic devices 2.4.1. Pressed substrates Recent approaches based on mimicking capillary forces have enabled control on the crystallization of TP-based 2D MOFs [60,61]. In one example reported by Dinca ˘and co-workers, synthesis of Ni 9 (HHTP) 4 crystals occurs between two silicon substrates. The flat surfaces are placed face-to-face and pressed one against the other with two attracting magnets. In this system, the organic ligand and the metallic ion are previously deposited on either substrates (HHTP needs to be deposited through ultra-high vacuum (UHV) thermal evaporation for a planar deposition on the surface) [60]. Later, the two pressed substrates are immersed in the reaction solvent and heated as shown in Fig. 8a. The confinement occurring between the two substrates enhances the MOF growth perpendicularly to the c-axis (favoring the metal–organic bond coordination over the stacking of layers). This has enabled the formation of hexagonal plate-like crystals with lateral sizes of 1– 10 l m (in sharp contrast to the needle-like crystals typically obtained in in situ reactions) (Fig. 8b). Alternatively, in another example, the pressed substrates were be previously functionalized with hydroxyl groups using a piranha solution (or UV-ozone treatment) and were subjected to a layer-by-layer growth [61]. Centimeter-scale Ni 9 (HHTP) 4 ,Co 9 (HHTP) 4 and Cu 3 (HHTP) 2 thin films with very controlled thicknesses were obtained on top of commercial silicon wafers with a preferential orientation. 2.4.2. Continuous flow Less explored methods to obtain TP-based 2D COFs have been occasionally reported [14]. This is the case of COF-5, COF-10, Pyr-COF and DBP-COF thin films grown under continuous flow conditions [62]. In a recent contribution reported by Dichtel and co-workers, monomers solutions are pumped and passed over a substrate under heating conditions. The continuous flow prevents the irregular deposition of COF colloids formed in solution on the substrate. The technique is characterized by a constant rate growth, allowing for a high control on the thickness of the thin film. Studies on the crystallinity of the generated thin films confirmed that growth relies not only on the flow of the monomeric species over the substrate but also on the pre-polymerized oligomers obtained from flowed solutions. 2.4.3. Simulated microgravity environment Our groups have recently reported a novel microfluidic device preventing the presence of convection flows simulating thus the effect of microgravity on Earth [63]. The engineered device is made of two sandwiched substrates that are spaced by a micrometerthick silicone film (Fig. 8c). Centimeter-large films of Ni 3 (HITP) 2 MOFs with nanoscale thicknesses on both insulating and conducting substrates were obtained with this method with a large control on their preferential orientation (obtaining both c-axis and ab-axis oriented films) which allowed to assess the anisotropy in the electronic conduction (Fig. 8d and e). 2.5. On-surface UHV evaporation The UHV evaporation of TP-based monomers on metallic surfaces has provided new insights information on the synthesis of supported TP-based 2D MOF/COF layers [64,65]. Note that TP molecules are planarly adsorbed on the surface in organized and Fig. 8. (a) Schematic illustration of the synthesis of TP-based 2D MOF between pressed substrates. The two substrates, each containing a starting monomer, are pressed together and exposed to the solvent. The reaction takes place between the substrates when the solvent is incorporated via capillary forces. (b) OM images of the Ni 3 (HHTP) 2 MOF crystals obtained with pressed substrates system. Images reproduced with permission from [60]. (c) Illustration of the microfluidic devices designed to simulate microgravity conditions. Two substrates are clamped together with a spacer in between. The space left between the two substrates by the spacer define the microfluidic environment used for the synthesis. (d) Optical images of the different reaction stages achieved during the synthesis of Ni 3 (HITP) 2 inside the microfluidic device. (e) Micrograph of the obtained Ni 3 (HITP) 2 thin film using a simulated microgravity environment. Images reproduced with permission from [63]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 8 oriented domains. The geometry of the packaging is determined by the supramolecular interaction between TP molecules, typically repulsive and leading to two closed-packed orientations rotated 60°respect to each other [64]. Interestingly, the introduction of functional groups (such as hydroxyl groups) has great impact on the available packing geometries. Depending on the evaporation conditions, strong interactions between introduced functional groups can come into play and direct the growth of typical honeycomb structures of TP-based 2D MOFs and COFs [65]. The success of these studies prompted the synthesis of TP-based 2D MOF monolayers by the sequential evaporation of the TP-based ligand and a metallic center (see Fig. 9a). Remarkably, the direct evaporation of HATP molecules onto surfaces containing possible coordination centers (e.g., copper substrates) did not lead to an extended coordination but to discrete coordination clusters. Thus, after the thermal evaporation of the TP molecule, co-deposition of a Ni metallic center with electron beam evaporation and annealing are required to obtain unit cells of TP-based 2D MOF Ni 3 (HITP) 2 monolayers (Fig. 9c). This research culminated in a more robust protocol, where the thermal evaporation of HATP is sequentially followed by the evaporation of the metallic cluster enabling the growth of single-layer structures of Ni 3 (HITP) 2 [66] and Fe 3 (HITP) 2 [67]. Supported monolayered domains of TP-based 2D COFs could also be obtained by thermal evaporation of the starting organic monomers in UHV and deposition on metallic surfaces [69]. Catalyzed by the substrate and upon annealing treatment, covalent bonds between the deposited monomers occur, generating the expected porous organized network (Fig. 9b). This approach leads to the formation of atom-thick molecular films, which are very useful for fundamental studies, though still far from practical applications. 3. Triphenylene-based 2D MOFs In the following, we will review different examples of TP-based MOFs described to date with special attention to the chemical families and conductive properties. 3.1. Chemical families TP ligands coordinate with either square planar or octahedral metal ions with unavailable axial positions, such as Ni 2+ ,Co 2+ or Cu 2+ , to keep the planar coordination along the 2D layers [70]. The resulting honeycomb graphitic structures exhibit an hexagonal porous structure that displays a large electron delocalization, which leads to high in-plane conductivities along the 2D coordinated layer [19]. The radius of the donor atom in the TP moiety and the electronic configuration of the metallic ion have great impact on the p -d orbital hybridization of the material, as it determines the bond distance and the strength of the MX 4 moieties [19]. Accordingly, the electronic conductivity can be tailored by using different combination of TP substitutions and metallic centers [22]. These features are key factors to ensure the in-plane charge conduction. For instance, TP-based MOFs incorporating MO 4 moieties typically showcase lower charge transport than the homologous TP-based MOFs comprising MN 4 moieties. Moreover, the Kagomé lattice arrangement of the metallic centers may originate band structures with the presence of Dirac points, where ultrafast charge carrier mobilities and non-trivial physical properties as topological states can arise [19,71]. Additionally, TP can be oxidized during the MOF formation reaching up to a total charge of 3, some of the intermediate species being stable radicals [71]. Concerning the out-of-plane contribution to conductivity, different stacking of the layers and crystalline arrangements can modify the MOF bulk physical properties and the electron conduction arising from differences in the overlap of the TP moieties within the structure. Further, extended 1D pore channels arise in the case of an eclipsed or nearly-eclipsed stacking of the layers that can be used to modify the properties through interaction with analytes or doping encapsulation [70]. Next, a summary of different TP-based 2D MOFs reported and their corresponding conductivity properties are grouped as function of the used donor heteroatom. 3.1.1. HHTP-based 2D MOFs In 2012, Yaghi and co-workers reported the first TP-based 2D MOF using HHTP and Ni 2+ ,Co 2+ and Cu 2+ as metal ions [72].Xray diffraction (XRD) of the Co-based MOF revealed extended layered honeycomb networks combining cobalt ions and HHTP molecules (Fig. 10a and b). The metallic center showcased octahedral geometries with two terminal water ligands in the axial positions and the HHTP molecules in equatorial positions. These 2D coordination layers pile up by the p - p stacking of the triphenylene units in an alternated ABAB stacking pattern, leading to a 3D supramolecular structure. Additionally, the coordination layers are intercalated with layers of molecules with formula [(H 2 O) 4 - Co] 3 HHTP. Hence, the final formula of the Co-based MOF resulted in Co 9 (HHTP) 4 excluding the coordinated water molecules. On Fig. 9. (a) Schematic of the UHV evaporation synthesis. The monomers are evaporated onto a substrate which will catalyze their reaction to form COF or MOF monolayers. (b) and (c) Scanning Tunneling Microscopy of COF-10 and Fe 3 (HITP) 2 MOF monolayers, respectively. Both materials showcase the characteristic honeycomb porous structure expected for these materials. Images reproduced with permission from [68] and [67], respectively. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 9 reported the higher catalytic activity of hexagonal crystalline systems such as Ni 3 (HITP) 2 ,Cu 3 (HITP) 2 and Cu 3 (HHTP) 2. The high electrical conductivity and redox activity of these systems facilitated the electron transfer to O 2 . In contrast, trigonal crystallized MOFs as Ni 3 (HHTP) 2 and Co 3 (HHTP) 2 have a distortion of the p - p stacking of the triphenylene cores, which strongly affects the electron transfer kinetics. Hence, the crystalline structure is critical to determine the electrocatalytic properties of TP-based 2D MOFs. Interestingly, large electronic density of M 3 (HITP) 2 structures has also proved to be suitable for other electrochemical reactions such as O 2 generation [106] (enhanced by doping of the structure with iron catalytic centers) and CO 2 reduction, mimicking photosynthesis processes [107,108]. Large photocatalytic performance was obtained under light irradiation when using Ni 3 (HITP) 2 as cocatalyzer along with a photosensitizer [108]. 3.2.4. Spintronics Cu-based MOFs have been the widest explored materials for these properties due to their electronic configuration with an odd number of d electrons. In a recent study published by Dinca ˘et al., Fig. 14. (a) Saturation sensor response for electronic textiles containing Ni 3 (HHTP) 2 (red) and Ni 3 (HITP) 2 (blue) exposed to 80 ppm of NO and H 2 S in dry nitrogen (solid bars) and in the presence of 5000 ppm water (with water droplet). Relevance of the used ligand in the MOF is shown, as different responses are obtained between the two systems having the same metallic ion. Image reproduced with permission from [36]. (b) Sensing traces of toxic gases NH 3 , NO and H 2 S at 80 ppm using Cu 3 (HHTP) 2 (orange) and Ni 3 (HHTP) 2 (blue) grown on flexible devices. Grey areas represent the exposure of the devices to the respective gas. Relevance of the used metal in the MOF is highlighted in this figure as different responses are obtained for a same analyte. Image reproduced with permission from [38]. (c) Summary of the general trends in acidity and redox activity of MOFs probed by basic gases and proposed mechanism for the shift of oxidation states in Cu-containing species upon binding of a basic gas. Image reproduced with permission from [101]. (d) Sensor responses of chemiresistors arrays of Cu 3 (HHTP) 2 ,Cu 3 (HITP) 2 and Ni 3 (HITP) 2 to different analytes. Image reproduced with permission from [102]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 16 single crystals of trinuclear molecular models of Cu 3 (HHTP) 2 and Cu 3 (HITP) 2 MOFs were obtained, being analogous coordination systems to TP-based MOFs [109]. Magnetic measurements on these crystals displayed antiferromagnetic coupling of neighboring spins, which could effectively lead to spin frustration if occurring in the TP-based 2D MOFs. This effect has been deeper studied recently by Awaga and co-workers, who demonstrated that thin films of Cu 3 (HHTP) 2 exhibit antiferromagnetic interactions between neighboring atoms, and that TP-based 2D MOF can reach a quantum spin liquid state at ultralow temperature (38 mK) [78]. This property has paved the way to novel spintronic applications for their use as organic spin valves [49]. Highly oriented thin films of Cu 3 (HHTP) 2 were implemented on vertical spin valves devices between LMSO and Co ferromagnetic electrodes. The chemical structure and rational c-axis growth of the film enabled the spinpolarized transport through the MOF. The devices showcased low magnetoresistances at cryostat temperatures that were retained upon changing the film thickness and increasing the temperature up to 200 K. On the other hand, possible presence of spin in Ni-complexes was assessed very recently by the study of a trinickel molecular model of Ni 3 (HHTP) 2 subjected to different redox reactions [110]. Interestingly, changes on the oxidation state do not occur on the metallic center but at the TP core giving rise to closed shell, monoradical and diradical species. Consequently, the increase of the oxidation state led to a decrease on the spin coupling strength. 3.2.5. Optoelectronics Thanks to their large conjugation, this family of MOFs exhibit a strong light absorption with chemically tunable band gaps. The large variability on chemical structures in TP-based 2D MOFs also enables the tuning on the optoelectronic properties as shown by Zhang et al. recently [111]. In their work, different M 3 (HITP) 2 structures showcased different non-linear optical absorbance due to the increasing number of d electrons. These effects gave relevance in optoelectronic devices such as photodetectors or solar cells. In 2019, Bein and co-workers reported on basic photovoltaic devices by implemented highly oriented Ni 3 (HHTP) 2 thin films grown on ITO through vapor-assisted conversion [52]. Interestingly, photoinduced charge generation and separation could be observed in the MOF film, making this material suitable for solar cell devices. The devices showcased open-circuit voltages of 156 mV under irradiation. Furthermore, Feng and co-workers reported recently a Fe 3 (THT) 2 photodetector capable of detecting radiation ranging Fig. 15. (a) Schematical structure of Cu 3 (HHTP) 2 on carbon paper supercapacitor (left) and photograph of a red light-emitting-diode powered by the three supercapacitors connected in series (right). Image reproduced with permission from [37]. (b) and (c) Photographs of the Ni 3 (HITP) 2 grown on polypropylene separators showing their different shape and sizes and their flexibility. (d) SEM cross-section image of Ni 3 (HITP) 2 films on polypropylene. Images reproduced with permission from [85]. Fig. 16. (a) Schematic and structural illustrations of the interdigital pattern of LSG/Ni 9 (HHTP) 4 hybrid and (b) solvothermal growth of Ni 9 (HHTP) 4 nanorods. (c) SEM image of the structure of LSG/Ni 9 (HHTP) 4 hybrid showing the large available porosity. Images reproduced with permission from [103]. (d) Schematic illustration of the growth of Ni 3 (HITP) 2 and Ni 9 (HHTP) 4 on cellulose nanofiber papers. (e) Cyclic voltammetry measurements of Ni 3 (HITP) 2 on cellulose paper at different folding angles. (f) Photographs of red light-emitting diodes powered by Ni 3 (HITP) 2 on cellulose paper capacitors under different folding or bending geometries. (g) Photograph of origami folded Ni 3 (HITP) 2 on cellulose paper capacitors in flower shape. Images reproduced with permission from [76]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 17 from UV to the NIR (400–1575 nm) (Fig. 17a) [112]. Owing to the narrow IR band gap of the material, room-temperature detection was limited. A good performance was reported at cryogenic temperatures with specificities of 710 8 cmHz 1/2 W 1 (Fig. 17b and c). 3.2.6. Tactile sensor The template-assisted growth of the Cu 3 (HHTP) 2 on Cu(OH) 2 microwires grown on a flexible Cu mesh substrate was used to develop a tactile sensor [40]. The rationally designed flexible sensor allowed for the modulation of the contact area as a function of the applied pressure, having a direct impact on the measured intensity (Fig. 18). This was used to monitor human motion and pulse, recognizing music through machine learning algorithms and displaying pressure maps when using an array of multiple sensors. Remarkably, the sensor showcased a large durability, retaining nearly all the initial signal after 2700 pressure and release cycles. 3.2.7. FET logic In 2014, Xu et al. reported porous FET transistors based on thin films of Ni 3 (HITP) 2 [58]. The highly smooth and compact thin films were obtained by interfacial liquid–air method and recovered by stamping on a silicon wafer for its integration into a FET transistor. The transistor revealed a p-type behavior with large hole mobilities of up to 48.6 cm 2 V 1 s 1 . Other electronic features of the device as large on/off ratios pointed to future possible applications as voltage-gated ion channels. 3.2.8. Thermoelectric The combination of low thermal conductivity derived from the strong phonon scattering of the porous of this family of MOFs together with their high electrical conductivity has open new avenues for their use in thermoelectric applications. Nanostructuration and grain boundaries also avoid phonon propagation, decreasing thus the thermal conductivity. However, only the thermoelectric properties of Ni 3 (HITP) 2 pressed pellets have been reported with an ultralow thermal conductivity of 0.21 Wm 1 K 1 and a large electrical conductivity of 58.8 Scm 1 [87]. Remarkably, Ni 3 (HITP) 2 pellets provided negative Seebeck coefficients, indicating an n-type thermoelectric behavior, and high thermoelectric figure of merit (ZT) of 1.1910 3 at room temperature. In complementary studies, crystallinity of the material was crucial as it deeply impacts the electrical conductivity and subsequently, the thermoelectric efficiency [89]. 4. Triphenylene-based 2D COFs 4.1. Chemical families In this section, we describe different families of TP-based COFs grouped according to the used ligand. 4.1.1. HHTP-based 2D COFs (ester-boronate and polyarylether bonds) The most common family of HHTP-based COFs, and arguably of all TP-based COFs, is that formed from boronic acids that covalently react upon dehydrogenation generating five-membered BO 2 C 2 rings. A prototype structure of one of such ester-boronate COFs is the COF-5 reported by Yaghi and co-workers by condensing the hydroxyl derivative triphenylene HHTP and 1,4benzenediboronic acid (BDBA) in a three-day solvothermal synthesis (Fig. 19)[23,34]. Crystallographic characterization revealed a hexagonal unit cell forming layers stacked in an eclipsed boron nitride arrangement with interlayer distances of 0.34 nm as in graphitic structures. This structure leads to a large porosity showcasing 1D mesopores of 27 Å in diameter. Interestingly, the eclipsed layer stacking is attributed to the presence of triphenylene cores, as they interact by p - p stacking. Afterwards, the synthesis protocol has been extended to a wide variety of boronic acid derivatives giving rise to numerous TP ester-boronate COFs, named after COF-5 as COF-6, COF-8 and COF-10 [113], most of them summarized in Fig. 19. Most of them retain the crystalline space group of COF-5 with planar layers and porous diameters ranging from the Fig. 17. (a) Schematic illustration of a Fe 3 (THT) 2 thin film photodetector device with indium electrodes. (b) Temperature-dependent photoswitching behavior under pulsed illumination of 785 nm laser. (c) Temperature dependence of photodetection at different power densities. Images reproduced with permission from [112]. Fig. 18. (a) Schematic illustration of the working principle of a MOF-based tactile sensor. Larger applied pressure leads to larger contact area between the MOF-containing mesh and the top electrode film and therefore the lower measured resistance. (b) Heart-beat monitoring using the MOF-based tactile sensor. (c) Current monitoring at different finger bending angles. Images reproduced with permission from [40]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 18 angstrom to the few nanometers, and therefore variable adsorption surface areas [114]. Ester-boronate TP bonding can be combined with other condensation reactions in the same synthesis, which enables a rational design of novel COFs. Jiang and co-workers reported numerous TP-based COF structures combining boron-ester and imine condensation reactions [115] in tri-ligand fused synthesis as the one presented in Fig. 20a. Different functionalities were achieved by modifying the employed boronic acid. Simultaneously, Zeng et al. reported the synthetic study of the same structure [116]. Despite these efforts, crystallization could only be achieved in one-pot reaction in the presence of all three ligands, as sequential condensations would lead to either other boronic structures or the lack of precipitate. However, the obtained TP-based COF exhibited a large H 2 and CO 2 uptake with BET surface areas of 1619 m 2 g 1 . Polyarylether TP-based 2D COF structures have been recently developed by nucleophilic aromatic substitution between ortho-difluoro benzene derivatives and HHTP [117]. Two layered structures, JUC-505 and JUC-506 (Fig. 20b) were obtained using tetrafluoroterephtalonitrile and 2,3,6,7-tetrafluoroanthraquinone, respectively. Both structures exhibit hexagonal pores of 16.8 and 28.4 Å, respectively, and large chemical stability in strong acid, basic, reductive or oxidant media. Incorporation of cyano groups in JUC-505 as shown in Fig. 20b allowed for post-synthetic Fig. 19. (a) Schematic of the structures of ester-boronate TP-based COFs condensing dior tri-boronic acids with HHTP. (b) Condensation reactions between boronic acids and HHTP giving rise different TP-based 2D COFs with different porosities. B, orange; O, red; C, black. All COFs have a boron-nitride (bnn) layer packaging. Image adapted from [114]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 19 functionalization with carboxyl or amino groups forming JUC-505COOH and JUC-505-NH 2 structures, respectively. The same strategy was performed by Yaghi and co-workers to synthesize amide and amidoxime groups in the COF structure [118]. A wide variety of TP-based 2D COFs with ester and ether bonds using HHTP can be synthesized depending on the targeted application. Novel structures or structural modifications have been investigated mainly aimed at gas storage applications (H 2 ,CH 4 ,CO 2 ...), especially with boronic ester COFs, as the boronic acid determines the porous size and surface area, and therefore, the gas uptake capability [23,114]. Analogously, pore size modifications and chemistry surface functionalization on polyarylether structures allows for tailoring the uptake of chemical species into the pores, enabling water treatment and filtration applications at various pHs [117,119]. Note that electronic applications of these structures are hardly explored since ester and ether bonds break the conjugation along the 2D layer. A clear example of this is their recent use in low-k dielectrics [47]. Thus, charge transfer occurs mainly in the out-of-plane direction through the one-dimensional stacking of TP moieties, similarly as it occurs for DLCs. Hence, doping is often required for optoelectronic applications, either by incorporation of electron donor/acceptor atoms into the ester/ether structure [120], use of conjugated ligands (creating electron donor–acceptor systems along with TP) [121] or external optoelectronic doping [44]. All these aspects are summarized in Table 4, which comprises investigated TP-based boronic ester-linked COFs and their properties and applications to date. Note that optoelectronic features are not commonly found in the literature for these materials. An indepth description of the properties and applications of the structures described with different acids can be found in Section 4.2. 4.1.2. HATP-based 2D COFs (phenazine bonds) Inspired by the successful synthesis and application of ester and ether COFs, other TP-based 2D COFs have been recently reported employing other condensation reactions. Phenazine-fused TPbased COFs is a main example of novel structures obtained by the imine condensation reaction. Phenazine-fused TP-based 2D COFs comprise nitrogen-doped graphitic porous structures with in-plane charge transfer features. These tailored structures are suitable for electronic applications owing to their large electronic conjugation (see Section 4.2 for more information). Typically, they are formed in solvothermal conditions by condensation of HATP with ortho-planar tetraketones [32,149,150]. As previously indicated for ester/ether COFs, different ketones can be used as starting monomers to tailor the porous size as shown in Fig. 21.Fig. 21b shows the CS-COF structure obtained by Guo et al. in a reaction between HATP and tert-butylpyrene tetraone, which led to an extended p -conjugated system with pores of 1.6 nm [32]. Fig. 21a depicts a structure reported by Meng et al. as a result of condensing HATP and hexaketonecyclohaxane into a layered 2D COF with 1D pores of diameter 1nm[149]. Note-worthy, in this work, the TP-based 2D COF proved to have low conductivities of the order of 10 8 –10 9 Scm 1 under anhydrous conditions at 323 K whereas under 97% relative humidity, the conductivity increased to 1.5110 5 Scm 1 , indicating a large proton conductivity. The conductivity values were further increased by acidification Fig. 20. (a) Synthesis of a multi-valent hexagonal TP-based COF using boron-ester and imine condensation reactions. Image reproduced from [115]. (b) Synthesis of JUC-505 through the condensation of tetrafluobenzenes and HHTP. The CN groups in the structure enable the generation of JUC-505-COOH and JUC-505-NH 2 structures via postsynthetic reactions. Image adapted from [117]. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 20 Table 4 Described boronic COFs. COF /Boronic acid used Form Synthesis method Pore size (Å) BET Surface area (m 2 g 1 ) Properties and application Ref Powder Solvothermal 27 1590 H 2 uptake [23,122] Powder Solvothermal – 1990 H 2 ,CH 4 ,CO 2 uptake [114] Powder Solvothermal – 2000 Synthesis studies [28] Powder Solvothermal – 1517 – [123] Powder Solvothermal 2.274– 1.248 1421–36 Tunable porosity [124] Powder Solvothermal 27 1670 Mechanistic studies of synthesis [125] Powder Solvothermal – – Growth kinetic study in homogenous reaction [126] Powder and oriented pressed pellets Solvothermal and high pressure pelletization – – Li-Ion battery [127] Powder Microwaved assisted synthesis – 2019–2027 Synthesis studies [128] Powder Microwaved assisted synthesis – 1200 Synthesis studies [129] Powder Sonochemistry – – Starting material for B-doped molten salt synthesis [130] Powder Sonochemistry – – Stability in different pyridine percentages [131] Powder and supported thin film Sonochemistry with substrate imbedded in solution – 2122 Synthesis studies [27] Powder and free-standing thin film Solvothermal and solvent evaporation for thin film – 840 (Thin film) Synthesis studies (for colloidal suspension) [132] Supported thin film Vapor-assisted conversion 27 – Preferential orientation, control of thickness and different substrates [53] COF coating of CNTs and graphene Sonochemistry with substrate imbedded in solution – 57.6/9.83 CO 2 uptake [45] COF coating on polydopamine coated glass Solvothermal – – Electrochromatography of small molecules [46] COF coating on CNTs Solvothermal with substrate imbedded in solution – – Li-Ion battery [44] Supported monolayer UHV evaporation 29.8 – – [69] Powder Solvothermal 34.1 2080 Gas uptake [113] Powder Solvothermal 32 1320 Mechanistic studies of synthesis [125] Powder Solvothermal 34 1200 Ammonia uptake [133] Powder Sonochemistry – – Stability in different pyridine percentages [131] Powder Solvothermal – 2080 H 2 ,CH 4 ,CO 2 uptake [114] Powder Solvothermal – – Synthesis studies [126] Powder Solvothermal – – Synthesis studies (for colloidal suspension) [132] Supported monolayer Solvothermal with substrate imbedded in solution –– – [68] Powder Solvothermal 6.4 980 Gas uptake [113] Powder Solvothermal – 960 H 2 ,CH 4 ,CO 2 uptake [114] (continued on next page) N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 21 Table 4 (continued) COF /Boronic acid used Form Synthesis method Pore size (Å) BET Surface area (m 2 g 1 ) Properties and application Ref Powder Solvothermal 18.7 1400 Porosity [113] Powder Solvothermal – 1400 H 2 ,CH 4 ,CO 2 uptake [114] Nanosheets Liquid phase exfoliation of powder assisted by sonication 17 – Nanostructuration [134] Powder Solvothermal 32 868 Gas uptake, Fluorescence and Conductivity [135] Powder Solvothermal – – Synthesis studies [126] Powder Solvothermal – – Synthesis studies for colloidal suspension [132] Supported thin film Solvothermal with substrate imbedded 32 – Photoemission [43] Powder Solvothermal 25.7 904 Compared to other tiophene based COFs and doping with oxidizing agents [120] Powder Solvothermal 29 with ethoxys 1844 with ethoxys Synthesis studies and tunable porosity [136] Powder Solvothermal – 1125 (S) 1056 (Se) 302–352 (Te) Tunable conductivity (substitution of sulfur with other chalcogenides) [137] Powder and thin film Solvothermal and spin-coating for film 30 1810 Photovoltaics: Adsorption of fullerenes as host–guest for photoluminescent quenching [138] Powder and oriented thin films Vapor-assisted conversion 32 1946 (powder) 175– 145 cm 2 /cm 2 (thin films) Photovoltaics: Adsorption of fullerenes as host–guest for photoluminescent quenching [48] Powder and supported thin film Solvothermal with substrate imbedded – 1376 (powder) Both randomly and oriented films. Photoconductivity: Faster response in oriented. [31] Oriented thin films Vapor-assisted conversion 32 – Conductivity anisotropy measurements and hole photoconductivity [139] Thin film Vapor-assisted conversion 32 990 Control of preferential orientation and thickness on different substrates [53] Powder Solvothermal – 2400 Synthesis studies [126] N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 22 Table 4 (continued) COF /Boronic acid used Form Synthesis method Pore size (Å) BET Surface area (m 2 g 1 ) Properties and application Ref Powder Solvothermal – – Synthesis studies (for colloidal suspension) [132] Powder and supported thin film Solvothermal with substrate imbedded 47 2640 Gas uptake, control on preferentially oriented films and blue emission [140] Powder Solvothermal 30 810 Isomerization with UV radiation with retention of porosity and structure [141] Supported monolayer UHV evaporation 36 – – [142] Powder and supported thin film Solvothermal with substrate imbedded 28 2021 Photoconductivity [121] Powder Solvothermal 32 1510 Large interlayer interaction with antiparallel building blocks [143] DTP-A NDI -COF Powder and coating of CNTs Solvothermal with presence CNTs 50.6 1583 (powder) 676 (CNT) Li-ion battery [144] DTP-A NDI -COF DTP-A PyrDI -COF Powder Solvothermal 53 53 1504 1910 Photoluminiscence kinectic studies [145] TDB (1) TDB (2) BTDB (3) Powder Solvothermal 20.6 (1) 13.8 (2) 32.4 (3) 972 562 544 Synthesis studies and conductivity by doping with oxidizing agents [120] T-COF-OH Powder Solvothermal 41 2000 560 (post-functionalization) Post-synthesis chemical modifications [146] Porphyrin Powder and supported thin film Solvothermal with substrate imbedded 46 8.9 Photovoltaics [147] Combination of different acids Powder Solvothermal – – Multifunctional COFs [148] N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 23 of the COF by impregnating it with 12 M H 3 PO 4 up to a value of 1.2310 3 Scm 1 under hydration conditions of 97% at 323 K. Interestingly, this structure could also be obtained by inverting the functional groups of the starting materials, in other words, by condensing HHTP molecules with planar tetraamines [151]. Strikingly, nitrogen-doped TP-based 2D COFs with larger pores can be obtained upon the reaction between HHTP and HATP as reported by Jhulki et al. [152] with the structure of C2P-5 COF. Analogous structures were reported by Liu and co-workers, by BuchwaldHartwig cross-coupling reaction between hexabromotriphenylene and HATP [153]. Interestingly, both eclipsed and staggered packing were obtained. All the structures hereby described present an eclipsed packing of the layers, thus out-of-plane contribution is expected as well, either by delocalization of the electrons in the TP moieties or by proton conduction along the one-dimensional porous. 4.1.3. Other structures Research on different polymerized triphenylene structures has enabled the realization of new carbon allotropes such as TPbased graphdyine (TP-GDY) analogues, containing both sp and sp 2 carbon atoms. This structure is well characterized, including its large electronic conjugation, which makes TP-GDY suitable for electronic applications. Nishihara and co-workers reported the synthesis of TP-GDY by means of Glaser-Hay homo-cross coupling catalyzed by copper in a liquid–liquid interfacial system obtaining thin films of 220 nm thickness [154]. Later, Tan and co-workers reported modifications of the synthesis, which allowed the manufacturing of nanosheets and thin films with thicknesses of few monolayers (0.9 nm) and good mechanical strength [59]. 4.2. Applications of TP-based 2D COFs A particular property of layered COFs is their large p -orbital overlap in the stacking direction conferring them both high exciton and charge conduction, which are interesting features for optoelectronic applications. Furthermore, their large and extended porosity enable chemical encapsulation or electrochemical doping for complementary functionalities. Finally, their high thermal stability prevents phase transitions that impair or inactivate their electronic properties. However, to aim to given properties or applications, one might tailor the structure of the COF or shape it in a given fashion. 4.2.1. Photovoltaics and photocurrent Free TP molecules exhibit a characteristic blue emission fluorescence that is usually shifted upon integration in a COF structure as a result of a higher delocalization degree when conjugated coligands are present [135,140,155]. These structures are characterized by the overlap of the TP emission band with the co-ligand absorption one, acting as a donor–acceptor system (where the TP is an electron donor and the co-ligand an electron-acceptor). Interlayer distance is a critical feature in some cases, and enables on/off phosphorescence at cryogenic temperatures upon its reduction [155]. Interestingly, several studies have reported that photoluminescent kinetics is highly dependent on crystallites size, with shorter exciton life times for suspensions containing smaller particles [156,157]. Control over the charge transfer in donor–acceptor systems suggests potential photovoltaics applications. Several esterboronate COFs have been described including TP and the boronic acid as electron donor and an acceptor moiety, respectively [121,145,147]. The eclipsed stacking of the layers in these structures led to bicontinous electron donor–acceptor ordering: upon light irradiation hole charge transfer is driven through the one dimensional stacking of TP moieties, whereas electron conduction is ensured by the eclipsed packing of the acceptor units. For example, Jiang and co-workers reported a boronic ester COF with a benzothiadiazole boronic acid serving as electron acceptor [121]. This structure provided large photocurrents with hole and electron mobilities of 0.01 and 0.04 cm 2 V 1 s 1 , respectively. The same strategy was used by Bein and co-workers in a donor–acceptor system comprising a porphyrin TP-based COF (Fig. 22b and c). A highly oriented thin COF film was incorporated in a photovoltaic device, which reached open-circuit voltages of 312 meV [147]. External doping agents such as fullerene derivatives can be used as acceptors when hosted in the one-dimensional pores of the electron donor COF (Fig. 22a) [138]. Nonetheless, such heterojunctions can limit the charge separation and mobility, and hence, the photovoltaics performance. Large in-plane conjugation of TP-based COFs may also enable the generation of photocurrents. A clear example is the previously introduced phenazine-fused CS-COF structure seen in Fig. 22d and e. The extended p -conjugated system exhibited a large hole photocurrent mobility (4.2 cm 2 V 1 s 1 )[32]. However, faster photoconductive kinetics were obtained by hosting C 60 fullerenes molecules in the pores of CS-COF as a donor–acceptor system as it occurred for previously described structures. Further, the CS-COF C 60 system was successfully integrated into functional photoswitches and photovoltaic cells. Similarly, previously introduced C2P-5 thin films showcased an intrinsic large hole mobility resulting into conductivities of 1.75 Scm 1 [152]. This conductivity is significantly increased upon irradiation with light especially after hybridization of the C2P-5 films with graphene. Larger photocurrents were recorded when applying larger powder light sources. Thus, the hybrid device proved to be a suitable photodetector for UV, visible and NIR light. 4.2.2. Li-ion battery As a result of the large conjugation and porosity, the phenazinefuse COF (Fig. 21a) has a large proton conduction, which can be enhanced in aqueous and acidic media. This property makes this material suitable as an electrode in a battery cell [149]. Performance of this material as anode in Li-ion batteries was studied by Shi and co-workers, which reported high reversible capacities of 701 mAhg 1 and long cycling performance (up to 4500 cycles) [158]. Similarly, piperazine-based structures with larger pores provided large capacities of 1644.3 mAhg 1 [153]. Other conjugated graphitic structures, such as TP-GDY, can be considered as suitable candidates for working anodes in Li-ion batteries. For example, hybridization of TP-GDY with CNTs through p - p stacking was performed obtaining free-standing films has been reported [59]. The films exhibited good electrochemical properties with a reversible capacity of 1624 mAhg 1 , good rate performance and recyclability. Furthermore, the hybrid system was incorporated as an anode in a lithium battery. Ester-boronate COFs are rarely used in electronic applications as the boron ester bond breaks the conjugation along the 2D layer. However, one may reach these properties by external doping [44,45,159]. In a recent study, differently charged 2D COFs (COF-5 among them) were doped with low-molecular weight polyethylene glycol (PEG) by including it into their 1D channels [159]. Interestingly, PEG significantly enhanced the Li + ion conduction along the pores of COF thanks to the large amount of oxygen atoms in the PEG structure providing multiple electron donor sites. 4.2.3. Electrocatalysis Another interesting use of nitrogen-doped COFs is in electrocatalysis. Zhang and co-workers evaluated phenazine-fused COF (Fig. 21a) as an ORR catalyst [160]. DFT calculations revealed that the large nitrogen density, crystallinity and stability of the material supports high availability of electrocatalytic sites. Consequently, N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 24 low overpotentials of 349 mV at 10 mAcm 2 and Tafel slopes of 64 mVdec 1 were obtained. The same nitrogen-doped COF have been used to coat composite core–shell structures of Fe/Fe 3 C nanoparticles in order to improve its long-term stability and performance [150]. Interestingly, the composite exhibited a good electrocatalytic behavior with a 92% retention of the initial current after 10,000 chronoamperometric cycles. 5. Future perspectives The bourgeoning research in TP-based organic frameworks that followed last few years has led to several discoveries in different fields of applications, where these materials are finding their niche. Undoubtedly, TP-based organic frameworks are significantly skyrocketing in the area of molecular conductors, a field that remains vastly unexplored. TP structures are potential candidates not only because of their electronic properties but also because of their inherent porosity. While benzene ligands enable conductivities within the same range or even larger than TP ligands, in comparison to the latter, benzene ligands have small or inexistent porosity, which significantly hinders the use of these structures in devices such as chemiresistors. Albeit the potential of TP-based MOFs and COFs, the number of reported structures is limited. Rational design has enabled large p -conjugation along the 2D layers and interlayer p - p interactions enabling charge transport pathways, but chemical ligand design is still in its infancy. Novel structures derived from TP substitutions (such as, selenol) or ligand combinations have been recently reported but novel extended ligands containing TP or TP derivative moieties are also arising for both MOF and COF structures [161–164], indicating an exciting yet challenging greenfield. Most of the conductivities reported for these materials refer to polycrystalline systems, which are not optimal in terms of conductivity. Polycrystalline samples are formed by several finite crystalline domains exhibiting grain boundaries, which disrupt carrier paths and charge transfer, ultimately affecting their conductive properties and, even result in unexpected undesirable properties such as opening a band gap in otherwise metallic materials. Single crystal is a continuous and defect-free (aside from monomer vacancies) structure formed by the infinite repetition of a unit cell. Hence, the properties (including the conductivity) of a single crystal are closer to those intrinsic properties of the material. Hence, single crystals are commonly aimed for the study of anisotropic properties, such as in-plane and out-of-plane conductivities. Some examples have been reported in the last years especially in the case of TP-based 2D MOFs. Measurements of rod-like or plate-like single crystals have allowed to assess the correlation of the anisotropy of conductivity with the crystalline structure. However, insights on this correlation are needed as only anisotropy of two structures have been reported in single crystals to date. The measurement of conductivities of single layers of TP-based systems is timely as atomicor nearly atomic-thick films will reveal key quantum confinement effects and interlayer interactions. The consequences of shrinking the material to single-layer thicknesses have already been observed in other 2D materials, such as transition metal dichalcogenides. For example, the band gap of MoS 2 can be tuned from 1.23 eV in the bulk form to 1.8 eV in the monolayer [165]. Single layers can be obtained using topdown methods, mainly by exfoliating a single crystal, an approach that has already been successfully used for the delamination of 2D coordination polymers [166–168]. However, this requires the growth of crystals sufficiently large, which is a challenging job. While exfoliation of TP-based 2D COFs has been demonstrated in few recent investigations [134], this is not the case for TP-based 2D MOFs. Alternatively, bottom-up techniques represent a strategy that attracts an increasing interest. Among this family of methods, two main methods are particularly appealing such as interfacial liquid synthesis [56] and UHV evaporation, the latter being more suitable to obtain monolayers as the deposition rate allows for an exquisite control over the growth. Reports on the UHV formation of TP-based 2D MOFs and COFs have demonstrated the relevance of several key parameters, such as the deposition order or the stoichiometry between the evaporated monomers [68,142,169]. However, despite numerous studies, only specimens with nanodomains have been accomplished, leading to a finite polymeric array that may be subjected to in-plane quantum confinement [68]. Thus, the measured properties may differ from those obtained in an infinite layer. In future, long-range single layers should be obtained and investigated in depth. Advantages of single crystals prevail for fundamental studies while in terms of applications polycrystalline materials are selected due to the geometrical restrictions to adapt a crystal to a specific device. Thus, development of methodologies that allow for the controlled deposition of the polycrystalline material on substrates to achieve homogenous flat interfaces that avoid charge entrapment have been pursued. Pressing powders into pellets, which can provide smoother surfaces and easily manageable systems is one of the most used methods. However, application of high pressures can induce crystalline phase transitions, crystal breaking, or cause the occlusion of pores. An alternative to postsynthetic treatment consists of directly growing polycrystalline material on supported thin films. This approach can lead to thin films with preferential crystalline orientations that minimize grain Fig. 21. (a) and (b) Examples of nitrogen-doped graphitic COFs obtained through condensation of HATP with ortho-planar ketones. Images adapted from [149] and [32] respectively. N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 25