Coordination Chemistry Reviews 460 (2022) 214459 Contents lists available at ScienceDirect Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr 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 a r t i c l e i n f o a b s t r a c t Article history: Triphenylene (TP) based materials have experienced a great expansion in the latest years. TP molecules Received 27 October 2021 have interesting optoelectronic properties, arising from the aromatic core, which have been exploited in Accepted 4 February 2022 functional two-dimensional (2D) Metal-Organic Frameworks (MOFs) and Covalent Organic Frameworks Available online 25 February 2022 (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 Keywords: have great impact on the properties and performance of these materials in functional devices. Triphenylene Specifically, we report a detailed description on the different TP-based 2D structures detailing the influ- MOF COF ence of the chemical and crystalline structure on the electronic properties, specially the in-plane and out- Two-dimensional of-plane contribution to the electrical conductivity. Finally, we give also attention and present several Electronic examples of functional devices made out with these electronic materials with great impact in the liter- Functional device ature as well as in future technological applications. Ó 2022 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 Abbreviations: 1D, One-dimensional; 2D, Two-dimensional; 3D, Three-dimensional; AFM, Atomic Force Microscopy; BDBA:1, 4-benzenediboronic acid; BDT, benzodithiophene; BPDA, 4,40 -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, 2,3,6,7,10,11- hexahydroxy 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, 4,40 ,400 -(1,3,5-triazine- 2,4,6-triyl)trianiline; TBPA, 1,3,5-Tris[(4- dihydroxyboryl)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: dani.ruiz@icn2.cat (D. Ruiz-Molina). https://doi.org/10.1016/j.ccr.2022.214459 0010-8545/Ó 2022 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/). N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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 easy processability into thin films [4], enabling their integration as components in devices for optoelectronic applications such as Triphenylene (TP) is a polycyclic aromatic hydrocarbon consist- organic photovoltaics or organic light-emitting diode (OLEDs) ing of four orthofused benzene rings with one central ring sur- technologies [6]. rounded by the other three in an alternate fashion (see Fig. 1a), According with previous considerations, TP-based conductive resulting in a large electron cloud with 18 p delocalized electrons 1D structures are promising building blocks for advanced optoelec- [1]. There exists a wealth of derivatives of triphenylene as function tronics. Yet, applications in this domain have been hampered of the groups R1 and R2 (see Fig. 1a). For instance, TPs exhibit a because the lack of reproducibility and required robustness of supramolecular packing mainly through p-p stacking, further opti- these architectures. The reason for this is that charge carrier mobil- mized by Van der Waals interactions arising from hexasubstituted ity is strongly dependent on the crystallinity of the mesophase long alkyl chains that do not perturbate neither the planarity nor [6,9], which varies considerably in DLCs because of the interactions the electronic relocation of the TP core. Precisely, the significant between TP moieties. Alternatively, DLCs can also be obtained with electronic delocalization of the resulting supramolecular structures linear and flexible covalent polymers bearing TP moieties that has made TP derivatives as suitable components for different stack into columnar phases, in which the bended side chains are devices, including sensors for neutral aromatic or cationic guests positioned in the outer surface of the column [10]. Though, control [2], molecular rotors [1], or in highly stable organic-oxygen cell over the crystallinity remains strongly dependent on the nature batteries [3]. However, in this review, we will focus mainly on and bendable character of the building blocks used in these inves- those where R1 and R2 are equal between them, and incorporate tigations [11]. –OH (henceforth HHTP), –NH2 (henceforth HATP) or -SH (hence- Recent years have seen a wealth of research devoted to the forth THT) groups. development of TP-based 2D metal–organic frameworks (MOFs) Worth to mention and preceding the description of TP-based and covalent organic frameworks (COFs) with broad electronic MOFs and COFs, TP materials have been significantly applied in bands for enhanced electronic conduction. COFs and MOFs are the area of discotic liquid crystals (DLCs). DLCs are mesophase polymeric crystalline porous network materials that consist of supramolecular materials, that in a way similar to other liquid organic building blocks that polymerize through covalent or coor- crystals, are formed by the columnar stacking of disk-like moieties dination bonds, respectively [12–14]. While covalent bonds ensure (see Fig. 1b) [4,5]. As a consequence of this assembly, DLCs exhibit a larger thermal and pH stability [14,15], the incorporation of broad energetic band gaps, where conduction occurs mainly due to metallic ions can add novel optical, magnetic and/or electrical hopping of charge carriers rather than a band-like effect, relying on properties [16]. Contrary to DLCs, TP-based organic frameworks the interplanar distance between TP molecules [6]. Therefore, DLCs (with a defined crystallinity and long-range order) do not rely on often require electrochemical or photochemical doping or charge weak interactions but on strong covalent and coordination bonds. injection from metallic surfaces moieties for enhanced electronic Hence, they have a stable and robust structure not as much sensi- conductivity [7]. In this case, large electronic delocalization leads tive to external factors such as temperature (Fig. 1c). Moreover, the to strong charge carrier percolation along the columnar axis, which configuration and dimensionality of the polymer can be predicted makes these materials one-dimensional organic semiconductors by a rational selection of the metal nodes and the organic linkers, [5]. For example, the incorporation of nitrogen-doped planar moi- giving rise to a wide variety of 2D structures [17]. These planar eties as an electron acceptor generates an intercalated donor–ac- polymeric layers can further form a 3D supramolecular structure ceptor DLC system with good electrical capacities, and suitable as by interlayer interactions such as p-p stacking, hydrogen bonding, a cathode in Li batteries [8]. DLCs are also characterized by their Van der Waals forces, or combinations of these. 2 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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 in- plane 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. The incorporation of TP moieties in MOFs and COFs is schemat- on the chemical structure and the condensation bonds forming ically represented in Fig. 2. The 2D layers present a honeycomb the 2D layers. Boronate ester condensation leads to COFs with poor graphitic architecture that results in interesting band structures conjugation and hence low charge delocalization along the 2D lay- and properties. Regarding the electronic properties, two main com- ers [21]. On the other hand, phenazine condensed COFs present a ponents arise: (a) conduction of charge carriers along the 2D layers larger conjugation leading to a larger in-plane charge transfer. (in-plane) and, (b) interlayer charge transfer (out-of-plane conduc- Thus, very few examples of conductive COFs have been reported, tion) [18]. In-plane conduction is usually found for TP-based 2D which commonly require of either chemical (including donor MOFs where coordination bonds typically lead to large electron atoms in their chemical structure) or external doping to aim for delocalization and conjugation pathways along the 2D layers, electronic applications [14]. Out-of-plane conductivities are deter- which results in high in-plane conductivities. Strong delocaliza- mined by the supramolecular stacking of the 2D layers. In a similar tions can be a direct consequence of the strong coordination of fashion to DLCs, layer stacking of the TP moieties can lead to one- d-p bonds (normally arising by the rational selection of suitable dimensional charge transfer channels, where charge transfer metallic centers and TP substitution) [19]. Contrarily, covalent occurs in through-space pathways [18]. However, different stack- bonds formed in TP-based COF structures need to maintain the ing geometries (being mostly eclipsed, nearly-eclipsed or slipped- conjugation in order to lead to such conductivities [20]. TP-based parallel) can be found, being always governed by the chemical 2D COFs can present a wide variety of p-conjugation depending structure of the TP-based 2D MOF/COF under investigation. Hence, 3 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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 MX4 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. the crystalline structure has a significant impact on the in-plane heated up in either a sealed or an opened glassware, depending and out-of-plane contributions of the optoelectronic properties of on the requirement of oxygen in the reaction (see Fig. 3). TP-based MOFs/COFs. In the following, the impact of the chemical TP-based solvothermal reactions of MOFs are characterized by a structures of TP-based MOFs/COFs in the final in-plane and out-of- fast uncontrollable crystallization, leading to several nucleation plane charge transfer properties will be discussed. 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 lar- 2. Synthetic methodologies gest TP-based 2D MOFs reported with this approach yielded crys- tals in the order of few micrometers in length being Cu3(HHTP)2 Features such as the crystallinity and crystallite size of MOFs the main example (Fig. 3c) [24]. In the case of COFs, long reaction and COFs can dramatically impact on their conductive properties times are required to ensure the crystallization of the covalently [18,22]. To date, control over these parameters has been investi- bonded porous polymeric structure, which are obtained mostly gated as function of the methodology used for their synthesis. in the form of insoluble powders (Fig. 3b), as first described by Accordingly, the methods and synthetic approaches used for the Yaghi and co-workers for COF-5 [23]. These reaction mixtures are controlled nucleation and growth of these materials has become characterized by their heterogeneity, which causes the occlusion a very relevant topic in this research area. Most representative of reagents within the precipitated material and low synthetic examples obtained with different methodologies are discussed in yields [27,28]. Thus, studies on homogeneous polymerization have the following sections (see the chemical structures of all the exam- been sought aiming for large crystallization rates and reduced ples in the following Sections 3 and 4). reaction times [28]. For instance, the formation rate dependence of COF-5 with temperature, concentration and presence of addi- 2.1. Bulk synthesis tives allowed for the generation of micrometer-long polycrys- talline aggregates. 2.1.1. Solvothermal reactions The most common method for the synthesis of TP-based 2D 2.1.2. Sonochemical reactions MOFs and COFs is the solvothermal synthesis. In this approach, The reaction between monomers assisted by ultrasound in solu- reactants are dispersed or solubilized in a common solvent and tion is controlled by tuning parameters such as solvent, sonication 4 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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 Cu3(HHTP)2 obtained with solvothermal methods. Scale bar is 1 lm. Image reproduced with permission from [24]. 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]. times or power. These parameters are changed to modulate the control on the coverage area and patterning [26]. However, in most crystallinity, dimensions and porosity of the obtained powders of the cases, crystalline powders are not always suitable for device (Fig. 4). Sonochemical syntheses enables faster crystallization, fabrication because they typically form rough interfaces, which which consequently lowers the reaction times to few hours or even lead to charge entrapment and poor ohmic contacts [34]. The minutes [27]. Moreover, TP-based organic frameworks with larger incorporation of TP-based MOFs on surfaces can be eased by add- surface areas than those reported by solvothermal methods can be ing the substrates in the reaction pot (Fig. 5a). A wide variety of achieved using this approach. While sonochemical reactions are suitable substrates for the growth of TP-based MOFs have been widely employed for the synthesis of COFs, they are rarely applied described (Fig. 5c) including flat substrates for thin film formation in MOF synthesis as small sized crystals are already obtained with [35] and coating of textiles [36], carbon paper [37], shrinkable flex- solvothermal synthesis. ible substrates [38], core–shell materials [39–41] or metallic meshes [42]. 2.2. Template-assisted syntheses Solvothermal or sonochemical reactions were also used to induce the epitaxial growth of COFs on substrates capitalizing on In recent years, different techniques have been described interactions between the monomers and the immersed substrate regarding post-synthetic on-substrate deposition, such as drop- (Fig. 5b). As an example, Dichtel et al. grew nanometer-thick casting of colloidal suspensions of TP-based 2D MOFs (either as- COF-5 and Pyr-COF thin films on single layer graphene substrates, synthetized [29] or after sonication [30]), or TP-based 2D COFs, which were dipped into the reaction mixture [43]. The highly onto surfaces [31]. Apart from the typical drop-casting approach, dense TP-based COF films generated showcased smooth surfaces other top-down techniques include spin-coating [32], ball-milling and long-range order with preferential orientation in the c-axis, blending and abrasion of powders onto substrates [33], or spray- independently of the substrate used to support the graphene single coating of colloidal suspension onto flexible substrates with a high layer. Growth of TP-based 2D COFs on a large variety of substrates 5 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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 of a 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 Ni3(HHTP)2 MOF grown on flexible cotton textile through template-assisted method. Images reproduced with permission from [36]. has also been reported, with carbonaceous architectures such as minimized and only heterogeneous nucleation and growth of films carbon nanotubes and free standing graphene [44,45]. This tech- with low roughness occurs on the substrate. nique has also been used to grow COF-5 on the inner surface of polydopamine-coated capillars [46]. The capillary was filled with 2.2.1. Layer-by-Layer (LbL) the reaction mixture, sealed and subjected to high temperature This approach has been mainly explored for TP-based 2D MOFs. similarly as in a solvothermal approach. Higher quality and scal- Typically, substrates are functionalized with active sites to entrap able thin films can be obtained also by combining a template- the metal ion on the surface (Fig. 6a). Next, the metal-loaded sub- assisted method with a colloidal template strategy [47]. During strate is exposed to the TP-substituted moiety saturating all the the template-assisted formation of several TP-based 2D COFs available metallic centers through coordination bonds and leaving (e.g.: COF-5, COF-10, Pyr-COF, and DBP-COF), unwanted COF coordination sites available. Then, the substrate is exposed again nanoparticles are concomitantly form in the bulk of the solvent to a metallic solution. This procedure is repeated in a cyclic fashion upon addition of co-solvents and remain as a stable colloidal sus- inducing a homogeneous and layered growth of thin films of large pension. Consequently, irregular deposition of COF particles is 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 Cu3(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]. 6 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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 Ni3(THT)2 MOF single-layer thin film grown through liquid–air interface. Image reproduced with permission from [56]. the thickness of the films being generated, and it is directly related in different immiscible solvents, thus confining the reaction at to the number of applied cycles. Note that Cu3(HHTP)2 thin films the interface upon the slow diffusion of the reactants (Fig. 7a obtained with LbL methods grow epitaxially having highly oriented and c) [54,55]. By means of this approach, thin films with sub- crystallites (Fig. 6b). Remarkably, the deposition can be performed millimeter lateral sizes can be obtained. Yet, the fabrication of with different methods such as substrate immersion in solutions films with nanometer thicknesses still remains a challenge due [49,50] or spraying the solutions onto the substrate [51]. The main to the lack of homogeneity along the film [54]. One of the most drawback of LbL approaches is their high time consumption. representative approaches used at liquid–gas interface is cer- tainly the Langmuir-Blodget technique, resulting in the growth 2.2.2. Vapor-assisted conversion of sub-millimeter films with nanoscale thicknesses. Main This methodology has been recently reported for TP-based 2D achievements are the growth of a 2D monolayer of Ni3(THT)2 MOFs, more specifically for Ni9(HHTP)4, Co9(HHTP)4 and MOF [56] or Cu3(HHTP)2 MOF thin films with high preferential Cu3(HHTP)2 structures [52], and relies on the reaction between orientation [54] (Fig. 7d). Note that by using this controlled starting monomers on a wet surface at the solvent gas pressure growth and a stamping approach in a repeated fashion, one conditions (Fig. 6c). This leads to a saturated reaction environment can achieve a controlled increase of the film thickness [54]. nearby the substrate’s surface. This method allowed for growing Another approach is the use of reagents in gas phase such as highly oriented centimeter-long thin films on different substrates ammonia vapors [57] or atmospheric oxygen (as reported for both insulating and conducting. However, optimization from the growth of Ni3(HITP)2) [58]. The thin film formed at the inter- substrate to substrate was required. For some specimens, some face grows upon diffusion of the gases into the liquid phase. acidic modulators were needed in the synthesis in order to obtain Hence, control on the thickness is achieved upon the control homogeneous films on the substrates including large coverage. In of the reaction time. Importantly, this approach can generate the case of COFs, the vapor-assisted conversion approach was films with lateral dimensions of hundreds of microns, which introduced by Bein and co-workers in 2015 as a scalable method lead to specimens with lower conductivities than those previ- for the production of thin films of COF-5 and BDT-COF. Specially, ously reported for compressed pellets. Moreover, the obtained the approach allows for the formation of COF thin films at room thin films were polycrystalline and randomly orientated. Interfa- temperature with a high control on their thicknesses ranging from cial syntheses have been largely exploited for TP-based MOFs hundred nanometers to the micrometer scale (Fig. 6d) [53]. whereas regarding TP-based COF structures, synthesis of TP- based graphdyine using Glaser-Hay cross-coupling reactions is 2.3. Interfacial syntheses of thin films the only example of interfacial synthesis reported to date. In this case, the starting monomer is in one phase and the copper cat- In the case of TP-based 2D MOF syntheses using liquid–liquid alyzer in the other leading to thin films with sub-nanometer interfaces, the metallic salt and the organic ligand are dissolved thicknesses (Fig. 7b) [59]. 7 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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 Ni3(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 Ni3(HITP)2 inside the microfluidic device. (e) Micrograph of the obtained Ni3(HITP)2 thin film using a simulated microgravity environment. Images reproduced with permission from [63]. 2.4. Microfludic devices Pyr-COF and DBP-COF thin films grown under continuous flow conditions [62]. In a recent contribution reported by Dichtel and 2.4.1. Pressed substrates co-workers, monomers solutions are pumped and passed over a Recent approaches based on mimicking capillary forces have substrate under heating conditions. The continuous flow prevents enabled control on the crystallization of TP-based 2D MOFs the irregular deposition of COF colloids formed in solution on the [60,61]. In one example reported by Dincă and co-workers, synthe- substrate. The technique is characterized by a constant rate sis of Ni9(HHTP)4 crystals occurs between two silicon substrates. growth, allowing for a high control on the thickness of the thin The flat surfaces are placed face-to-face and pressed one against film. Studies on the crystallinity of the generated thin films con- the other with two attracting magnets. In this system, the organic firmed that growth relies not only on the flow of the monomeric ligand and the metallic ion are previously deposited on either sub- species over the substrate but also on the pre-polymerized oligo- strates (HHTP needs to be deposited through ultra-high vacuum mers obtained from flowed solutions. (UHV) thermal evaporation for a planar deposition on the surface) [60]. Later, the two pressed substrates are immersed in the reac- 2.4.3. Simulated microgravity environment tion solvent and heated as shown in Fig. 8a. The confinement Our groups have recently reported a novel microfluidic device occurring between the two substrates enhances the MOF growth preventing the presence of convection flows simulating thus the perpendicularly to the c-axis (favoring the metal–organic bond effect of microgravity on Earth [63]. The engineered device is made coordination over the stacking of layers). This has enabled the for- of two sandwiched substrates that are spaced by a micrometer- mation of hexagonal plate-like crystals with lateral sizes of 1– thick silicone film (Fig. 8c). Centimeter-large films of Ni3(HITP)2 10 lm (in sharp contrast to the needle-like crystals typically MOFs with nanoscale thicknesses on both insulating and conduct- obtained in in situ reactions) (Fig. 8b). Alternatively, in another ing substrates were obtained with this method with a large control example, the pressed substrates were be previously functionalized on their preferential orientation (obtaining both c-axis and ab-axis with hydroxyl groups using a piranha solution (or UV-ozone treat- oriented films) which allowed to assess the anisotropy in the elec- ment) and were subjected to a layer-by-layer growth [61]. tronic conduction (Fig. 8d and e). Centimeter-scale Ni9(HHTP)4, Co9(HHTP)4 and Cu3(HHTP)2 thin films with very controlled thicknesses were obtained on top of commercial silicon wafers with a preferential orientation. 2.5. On-surface UHV evaporation The UHV evaporation of TP-based monomers on metallic sur- 2.4.2. Continuous flow faces has provided new insights information on the synthesis of Less explored methods to obtain TP-based 2D COFs have been supported TP-based 2D MOF/COF layers [64,65]. Note that TP occasionally reported [14]. This is the case of COF-5, COF-10, molecules are planarly adsorbed on the surface in organized and 8 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 oriented domains. The geometry of the packaging is determined by The resulting honeycomb graphitic structures exhibit an hexagonal the supramolecular interaction between TP molecules, typically porous structure that displays a large electron delocalization, repulsive and leading to two closed-packed orientations rotated which leads to high in-plane conductivities along the 2D coordi- 60° respect to each other [64]. Interestingly, the introduction of nated layer [19]. The radius of the donor atom in the TP moiety functional groups (such as hydroxyl groups) has great impact on and the electronic configuration of the metallic ion have great the available packing geometries. Depending on the evaporation impact on the p-d orbital hybridization of the material, as it deter- conditions, strong interactions between introduced functional mines the bond distance and the strength of the MX4 moieties [19]. groups can come into play and direct the growth of typical honey- Accordingly, the electronic conductivity can be tailored by using comb structures of TP-based 2D MOFs and COFs [65]. The success different combination of TP substitutions and metallic centers of these studies prompted the synthesis of TP-based 2D MOF [22]. These features are key factors to ensure the in-plane charge monolayers by the sequential evaporation of the TP-based ligand conduction. For instance, TP-based MOFs incorporating MO4 moi- and a metallic center (see Fig. 9a). Remarkably, the direct evapora- eties typically showcase lower charge transport than the homolo- tion of HATP molecules onto surfaces containing possible coordina- gous TP-based MOFs comprising MN4 moieties. Moreover, the tion centers (e.g., copper substrates) did not lead to an extended Kagomé lattice arrangement of the metallic centers may originate coordination but to discrete coordination clusters. Thus, after the band structures with the presence of Dirac points, where ultrafast thermal evaporation of the TP molecule, co-deposition of a Ni charge carrier mobilities and non-trivial physical properties as metallic center with electron beam evaporation and annealing topological states can arise [19,71]. Additionally, TP can be oxi- are required to obtain unit cells of TP-based 2D MOF Ni3(HITP)2 dized during the MOF formation reaching up to a total charge of monolayers (Fig. 9c). This research culminated in a more robust 3, some of the intermediate species being stable radicals [71]. protocol, where the thermal evaporation of HATP is sequentially Concerning the out-of-plane contribution to conductivity, different followed by the evaporation of the metallic cluster enabling the stacking of the layers and crystalline arrangements can modify the growth of single-layer structures of Ni3(HITP)2 [66] and Fe3(HITP)2 MOF bulk physical properties and the electron conduction arising [67]. from differences in the overlap of the TP moieties within the struc- Supported monolayered domains of TP-based 2D COFs could ture. Further, extended 1D pore channels arise in the case of an also be obtained by thermal evaporation of the starting organic eclipsed or nearly-eclipsed stacking of the layers that can be used monomers in UHV and deposition on metallic surfaces [69]. Cat- to modify the properties through interaction with analytes or dop- alyzed by the substrate and upon annealing treatment, covalent ing encapsulation [70]. Next, a summary of different TP-based 2D bonds between the deposited monomers occur, generating the MOFs reported and their corresponding conductivity properties expected porous organized network (Fig. 9b). This approach leads are grouped as function of the used donor heteroatom. to the formation of atom-thick molecular films, which are very useful for fundamental studies, though still far from practical 3.1.1. HHTP-based 2D MOFs applications. In 2012, Yaghi and co-workers reported the first TP-based 2D MOF using HHTP and Ni2+, Co2+ and Cu2+ as metal ions [72]. X- 3. Triphenylene-based 2D MOFs ray diffraction (XRD) of the Co-based MOF revealed extended lay- ered honeycomb networks combining cobalt ions and HHTP mole- In the following, we will review different examples of TP-based cules (Fig. 10a and b). The metallic center showcased octahedral MOFs described to date with special attention to the chemical fam- geometries with two terminal water ligands in the axial positions ilies and conductive properties. and the HHTP molecules in equatorial positions. These 2D coordi- nation layers pile up by the p-p stacking of the triphenylene units 3.1. Chemical families in an alternated ABAB stacking pattern, leading to a 3D supramolecular structure. Additionally, the coordination layers TP ligands coordinate with either square planar or octahedral are intercalated with layers of molecules with formula [(H2O)4- metal ions with unavailable axial positions, such as Ni2+, Co2+ or Co]3HHTP. Hence, the final formula of the Co-based MOF resulted Cu2+, to keep the planar coordination along the 2D layers [70]. in Co9(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 Fe3(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. 9 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Fig. 10. (a) and (b) Top and side view of the 2D layers in the Co9(HHTP)4 structure, respectively. In (b), the discrete molecular units (i.e. [(H2O)4Co]3HHTP) can be easily appreciated between the layers showing the distortion that they generate to the p-p stacking of the TP moieties in an eclipsed orientation. (c) SEM images of rod-like Ni9(HHTP)4 crystals. Image reproduced with permission from [72]. (d) Temperature dependence of the electrical conductivity of a Ni9(HHTP)4 single crystal and of a corresponding pressed pellet. Defects arising from polycrystalline pressed pellets led to lower conductivities and different temperature dependance when compared to single crystal devices. Insets: optical microscope images of a single-crystal and a pressed-pellet devices. Image reproduced with permission from [60]. (e) Schematic illustration of the structure of Cu3(HHTP)2 crystals. The structure is characterized by a continuous shifting of the layers. (f) I-V curves of plate-like (full line) and rod-like (dashed line) Cu3(HHTP)2 crystals taken at room temperature, respectively. Each shape allowed to assess the in-plane and out-of-plane contributions to electrical conductivity. Images reproduced with permission from [24]. the other hand, the hexagonal rod-like Ni-based MOF resulted and hence, the charge transport properties. Table 1 summarizes isostructural to the Co-based MOF according to powder XRD and the conductivities of HHTP-based TP-based 2D MOFs showing the high-resolution transmission electron microscopy (HR-TEM) impact of the measured morphologies and crystalline orientations. images (Fig. 10c). Conductivity measurements of M9(HTTP)4 The influence of the stacking interaction between the TP moi- (M = Co and Ni) pressed pellets showed low conductivities in the eties in the conductivity properties were assessed by investigating order of 103 Scm1 as the discrete molecular coordination units a series of lanthanide Lnx+1(HHTP) MOFs [73]. In the described are interlocked between the layers, decreasing the charge inter- structures, Ln centers (Ln = La3+, Nd3+, Ho3+, Yb3+) coordinate to layer delocalization [72]. Recently, in their seminal contribution, HHTP molecules forming a 3D coordination network, where HHTP Dincă and co-workers grew single crystals of Ni9(HHTP)4, to allow molecules are located along 2D planes and Ln centers in between for measurements in-plane conductivity (0.4 Scm1) and out-of- layers. Stacking between HHTP moieties remain close to graphitic plane conductivities (104 Scm1) (Fig. 10d) [60]. The remarkable p-p interactions having interlayer distances of 3 Å. Particularly, difference of up to three orders of magnitude between in-plane the use of larger lanthanide ions led to larger interlayer distances, and out-of-plane conductivities highlights the relevant charge which could be directly correlated to a decrease in the conductiv- transport mediated by the conjugated TP-based 2D MOF layers. ity. Finally, essays on crystallization of TP-based 2D MOFs incorpo- Clearly, and based on these results, one can conclude that the inter- rating Fe ions were firstly attempted by Mirica and co-workers layer charge transport in these MOFs hardly contributes to the con- [33]. Despite the lack of crystallinity of the material, the obtained ductivity. This effect probably results from the large distance Fe-based material displayed similar conductivities than other between the layers and their disordered stacking. pressed pellets of crystalline HHTP-based MOFs, of the order of Interestingly, the powder XRD pattern of the Cu-based MOF 103 Scm1. It was only recently that Medina et al. reported the indicated the presence of a different crystalline phase, as con- successful crystallization of HHTP-based MOFs using Fe ions with firmed afterwards by Dincă and co-workers using powder XRD similar conductivities [74]. Interestingly, the obtained structures and HR-TEM. In this case, the layers are stacked in an almost were three-dimensional instead of layered, characterized by eclipsed fashion (Fig. 10e), with a slight continuous shift along supramolecular tetrahedra of four triphenylene units coordinated the ab-direction, without any occlusion of molecular coordination to a trinuclear iron cluster. Other three-dimensional structures units, and an expected formula of Cu3(HHTP)2 [24]. Yaghi et al. using HHTP as organic ligand have been reported by Yaghi and showed that Cu3(HHTP)2 single crystals could led to conductivity co-workers that resulted in Fe, Ti and V-based porous anionic values as high as 0.1 S cm1 [72]. Later, Dincă and co-workers con- frameworks with large proton conductivity [75]. The Fe-based firmed this value by measuring the conductivity of Cu3(HHTP)2 MOF characterized by dimetallic centers, showcased a proton con- plate-like crystals with their 2D layers oriented along the crystal ductivity of 5.0102 Scm1 under 98% of relative humidity at plane. In this case, the Cu3(HHTP)2 plate-like crystals were syn- 25 °C. This was attributed to the presence of both sulfate and thetized using a sonication-assisted synthesis (Fig. 10f) [24]. In- dimethylammonium (DMA) ions, which combination has great plane and out-of-plane conductivities of the 2D layers were impact on the proton charge transfer. Consequently, the Ti-based 0.5 Scm1 and 1.5 Scm1, respectively, confirming equal contribu- MOF with provided lower conductivities of 8.2104 Scm1 under tion of intralayer and interlayer charge transport (arising from the the same conditions since this structure contains only DMA ions. large planar conjugation and eclipsed stacking of the TP mole- cules). As expected, polycrystalline Cu3(HHTP)2 samples provided 3.1.2. HATP-based 2D MOFs conductivities in the range of 101–103 Scm1. This variation One of the first imino-substituted TP-based MOFs reported was depends on the synthetic method, which affects the morphology, Ni3(HITP)2 by Dincă and co-workers in 2014 [35]. The 2D coordina- 10 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Table 1 Conductivities of HHTP-based MOFs. MOF Measuring technique Conditions Conductivity (Scm1) Orientation Ref Ni9(HHTP)4 Single crystal (4-probe) Vacuum 0.4 ab-axis [60] Single crystal (2-probe) Vacuum 1  104 c-axis [60] Thin film (vdP) – 1.1  103 ab-axis [52] Pellet (4-probe) Vacuum 3.6  103 Random [60] Pellet (4-probe) – 1  101 Random [33] Pellet (4-probe) – 1  102 Random [38] Pellet (4-probe) – 0.26 Random [76] Pellet (2-probe) – 6.0  103 Random [77] Pellet (vdP) – 6.8  103 Random [52] Cu3(HHTP)2 Single crystal (4-probe) Vacuum 0.2 c-axis [72] Single crystal (4-probe) Vacuum 1.5 ab-axis [24] Single crystal (2-probe) Vacuum 0.5 c-axis [24] Film (4-probe) Vacuum 0.29 ab-axis [49] Film (4-probe) Vacuum 104 ab-axis [54] Film (4-probe) Argon 2.3  102 ab-axis [26] Film (2-probe) – 2  102 ab-axis [51] Pellet (4-probe) Argon 4.5  102 Random [26] Pellet (4-probe) – 2.0  102 Random [33] Pellet (4-probe) – 7.6  103 Random [38] Pellet (4-probe) 8.0  103 Random [78] Pellet (2-probe) – 1.0  102 Random [79] Pellet (2-probe) – 3.0  103 Random [37] Pellet (vdP) – 2.7  102 Random [52] Co9(HHTP)4 Thin film (vdP) – 3.3  103 ab-axis [52] Pellet (vdP) – 3.2  102 Random [52] Pellet (4-probe) – 2.7  106 Random [33] Pellet (2-probe) – 2.0  103 Random [77] Fe3(HHTP)2 Pellet (4-probe) Amorphous 3.0  103 Random [33] Pellet (vdP) Ambient 5.6  103 Random [74] Zn3(HHTP)2 Pellet (4-probe) Amorphous 5.7  106 Random [78] tion layers display a honeycomb network with a slipped-parallel cally limits the percolation of charge carriers between crystallites structure with a minimal offset direction (see Fig. 11a and b). [35,70]. A linear increase of the conductivity with the temperature Accordingly, an almost eclipsed stacking of TP moieties is obtained, indicated a semiconducting behavior of the material, sustained by converging to large charge delocalization along the material. the oxidation of the amines into iminosemiquinones and the The resulting conductivities were as high as 40 Scm1 in formation of a hole rich p-type semiconductor. Controversial polycrystalline thin films, even when grain boundaries dramati- results were obtained in a more recent study by Day et al. on Fig. 11. (a) and (b) Top and side view respectively of the 2D layers in the Ni3(HITP)2 structure. (c) SEM image of rod-like Ni3(HITP)2 single crystal device. (d) Temperature dependence of electrical conductivity of a Ni3(HITP)2 single crystal proving the metallic behavior. Images reproduced with permission from [24]. 11 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 micrometer-long rod-like crystals obtained using hydrothermal of the TP-based MOF will have a direct impact not only in possible synthesis. In this contribution, the authors report conductivities applications of these materials but also in the control that can be as high as 150 Scm1 with Ni3(HITP)2 single crystals with non- exerted in their crystalline structure (see vide infra). It was only null conductivities at 0 K, indicating a metallic behavior (Fig. 11c recently that large conductivities in Co3(HITP)2 crystals have been and d) [24]. DFT calculations by Foster et al. already demonstrated reported by Zhi and co-workers upon their post-synthetic treat- in 2018 that such differences may arise from interface defects as ment by liquid exfoliation reaching values of 67.8 Scm1 [84]. grain boundaries or stacking displacements, which do not only Interestingly, novel chemical formula Mn3(HITP)2 is also reported break the conjugation of the graphitic layers or the charge delocal- in this work, proving to be isostructural to Co3(HITP)2 crystals. ization across them but also can open a band gap in the otherwise metallic Ni3(HITP)2 MOF [80]. DFT calculations anticipated a semi- 3.1.3. THT-based 2D MOFs conductor behavior for a Ni3(HITP)2 monolayer, whereas the bulk The first reported THT-based 2D MOF consisted of crystals with material is metallic. A related DFT study anticipated an increase Pt as metallic ion grown through an in situ reaction between a of the band gap upon increasing the intralayer distance [81]. metallic salt and THT in the presence of oxygen [90]. After the Importantly, a strong band dispersion is not only obtained in the MOF formation, the THT ligand was in a 4 oxidation state, which in-plane direction but orthogonally to the 2D layers as well, under- caused the uptake of sodium ions to balance the charge, reaching lining the strong charge delocalization across the layers. Therefore, the final formula of Na1.8Pt3(THT)2. The square planar coordination the relevance of the synthetic protocol is highlighted as it leads to geometry favored the formation of hexagonal layered networks materials with different conductivities as shown in Table 2. with a staggered stacking due to the large ionic radius of Pt ions TP-based MOFs as Cu3(HITP)2 [82] and Co3(HITP)2 [25] struc- (see Fig. 12a and b). Importantly, the THT ligands could be further tures were also synthesized and characterized. The XRD patterns oxidized upon I2 treatment reaching a neutral formula of Pt3(THT)2 of both MOFs indicated their isostructural nature. However, slight without no relevant structural changes and a final conductivity of variations on the interlayer distance with the metallic center were the order of 106 Scm1. This low value arises from the lack of out- determined, being 3.30, 3.16 and 3.29 Å for Ni3(HITP)2, Cu3(HITP)2 of-plane contribution as staggered packing alternates the TP moi- and Co3(HITP)2, respectively [25]. Consequently, different conduc- eties, disabling their interaction. Further, Pt is a very large metallic tivity values of 55.4, 0.75 and 0.024 Scm1 for the pellets were ion that cannot form strong d-p bonds leading to a weak intralayer obtained. Controlled modifications of the conductivity and band charge delocalization. gap were also achieved by tuning the metallic ratio in a series of To improve the conductivity, other smaller metallic ions were alloys MnM’3-n(HITP)2 containing pairs of metallic ions (Ni/Co, Ni/ tested, such as Co and Ni, whose bis(dithiolene) complexes have Cu, Cu/Co). Differences on the MOFs behavior depending on the proved a strong bonding with large carrier mobilities and several incorporating metal atom have been predicted by DFT calculations. accessible redox-active states [18,55]. Hence, in 2015, Marinescu As previously commented, calculations on a Ni3(HITP)2 monolayer and co-workers reported the Co3(THT)2 MOF with honeycomb 2D structure proved a semiconducting band structure with a band gap layers stacked with an eclipsed conformation that resulted in con- of 0.13 eV [83]. The metallic Ni2+ center showcases a dsp2 ductivities of 1.4103 Scm1 for pressed pellets and 3.2102- hybridization with the HITP ligand leading to a perfect square pla- Scm1 for thin films formed through an interfacial reaction nar geometry in the 2D layer. Upon substitution of Ni2+ by Cu2+, a method, without any further treatment [55]. DFT calculations sug- sp3 hybridization occurs due to the additional electron when com- gested that these conductivities were associated with an out-of- paring the electronic configuration between both ions. Conse- plane mobility, with low in-plane conductivities arising from the quently, a square-grid coordination geometry, which induces a mixed-valence character of the metallic centers (two Co3+ per distortion in the 2D layers, is obtained. This change in the crystal- Co2+ center). Note that the Co3(THT)2 MOF transitions from a semi- lographic network generates a metallic behavior of the 2D mono- conducting state, attributed to a scattering of charge carriers at layer. However, direct correlation of the different reported high temperatures, to a metallic state upon decreasing the temper- crystalline structures with their conductivity values remains a ature are strongly dependent on the pellet/film thickness. Dong challenge because of the different degree of crystallinity of the et al. also reported the growth of a 2D monolayer of the complex pressed pellets, especially in the case of Co3(HITP)2 crystals [25]. Ni3(THT)2 using the Langmuir-Blodget technique. The single-layer Accordingly, it is clear that the variability of the chemical formula exhibited a hexagonal pattern with pores of 2 nm and large Table 2 Conductivities of HATP-based MOFs. MOF Measuring technique Conditions Conductivity (Scm1) Orientation Ref Ni3(HITP)2 Single crystal (4-probe) Vacuum 150 c-axis [24] Film (2-probe) Ambient 39.7 ab-axis [63] Film (2-probe) Ambient 7.510-4 c-axis [63] Film (vdP) Vacuum 40 Random [35] Film (-) – 37.2 Random [85] Film (2-probe) – 8.5103 0.23 Random [57] Pellet (vdP) Vacuum 50 Random [86] Pellet (vdP) Vacuum and annealing 58.8 Random [87] Pellet (4-probe) Ambient 55.4 Random [25] Pellet (4-probe) – 39 Random [76] Pellet (4-probe) Ambient 1.896 Random [88] Pellet (4-probe) – 1 Random [89] Pellet (2-probe) Vacuum 2 Random [35] Cu3(HITP)2 Pellet (2-probe) Ambient 0.2 Random [82] Pellet (4-probe) Ambient 0.75 Random [25] Co3(HITP)2 Pellet (4-probe) – 67.8 Random [84] Pellet (4-probe) Ambient 8104 Random [88] Pellet (4-probe) Ambient 0.024 Random [25] Mn3(HITP)2 Pellet (4-probe) – 44.92 Random [84] 12 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Fig. 12. (a) and (b) Single layer and layer packing structures of Pt3(THT)2 crystals. Images reproduced with permission from [90]. (c) Temperature dependance of the electrical resistivity of Co3(THT)2 samples passing from a semiconducting to metallic behavior. The eclipsed honeycomb structure of Co3(THT)2 is shown in the inset. Image reproduced with permission from [91]. (d) Electrical (full dots) and optoelectrical (empty dots) conductivity of Fe3(THT)2 thin films. The thin film device and structure of Fe3(THT)2 are shown in the inset. Images adapted from [92]. stability, with a large lateral area in the order of millimeters, and higher values, reaching room temperature metallic behavior and thicknesses of 0.7 nm. final conductivities of 8.0104 Scm1. Table 3 summarizes the In 2017, Mirica and co-workers reported the hydrothermal syn- reported conductivities of THT-based 2D MOFs synthesized under thesis of Co3(THT)2, Cu3(THT)2 and Ni3(THT)2 MOFs under basic different conditions. conditions [93]. The spectroscopic characterization evidenced the mixed valence character of cobalt and copper metal centers 3.1.4. Other structures (Co2+/Co3+ and Cu1+/Cu2+), whereas only the Ni2+ state was found. The development of new structures were firstly anticipated by Pressed pellets of the resulting polycrystalline material showed DFT calculations for systems combining dual ligands or different very low conductivities of 2.4109, 2.4108 and 3.6104 Scm1 hetero-atom substitutions in ortho position of Pt and Pd TP- for Co3(THT)2, Cu3(THT)2 and Ni3(THT)2, respectively. Thin films based MOFs [95,96]. These variations on previously described of the Fe3(THT)2(NH4)3 MOF with good control on their thickness structures evidenced their great impact on the band structure of were reported by Feng and co-workers in 2018 using a liquid–liq- the material as different band gap openings were obtained. These uid interfacial approach [92]. The Fe ions were found in its Fe3+ indications were further explored experimentally by developing state, despite using Fe2+ ions as reactive materials. Hence, in order novel hetero-atom substitutions or by combining different ligands. to compensate the charge of the fully anionic THT ligand, ammonia The quest for novel structures was realized by the combination ions were incorporated in the structure. Crystallographic analysis approach of two or more ligands within a single structure. For revealed hexagonal layers packed in a near-eclipsed inclined struc- example, Feng and co-workers reported the simultaneous incorpo- ture with conductivities of 3.4102 Scm1 and semiconductor ration of HATP and THT ligands into a honeycomb structure behavior with optical band gaps of 245 meV. Hence, time- obtained as a crystalline powder or thin films (either monolayer resolved photoconductivity measurements were performed, reach- or supported on graphene) [97]. Control on coordination spheres ing mobilities of up to 220 cm2 V1 s1. Marinescu and co-workers is hardly achieved obtaining randomly distributed CoN2S2, CoN4 demonstrated similar semiconductor to metallic transitions in this and CoS4 coordination units along the material (Fig. 13a). The material with conductivities of  0.2 Scm1 for as-synthesized CoN2S2 center contributed to an enhanced catalytic activity caused specimens [94]. Importantly, upon oxidation to atmospheric air, by the combination of donor atoms contributing each to the the semiconductor-to-metal transition temperature shifted to absorption and desorption of analytes. Table 3 Conductivities of THT-based MOFs. MOF Measuring technique Conditions Conductivity (Scm1) Orientation Ref Ni3(THT)2 Pellet (-) – 3.6  104 Random [93] Cu3(THT)2 Pellet (-) – 2.4  108 Random [93] Co3(THT)2 Film (vdP) – 3.2  102 Random [91] Pellet (vdP) – 1.4  103 Random [91] Pellet (-) – 2.4  109 Random [93] Fe3(THT)2 Film (vdP) Vacuum 3.4  102 Random [92] Film (vdP) Vacuum 0.2 Random [94] Pt3(THT)2 Pellet (2-probe) – 3.86  104 Random [90] 13 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Fig. 13. (a) Structure of the honeycomb lattice incorporating both THT and HATP molecules. The different available coordination spheres (CoN4, CoN2S2, CoS4) are shown. Image reproduced with permission from [97]. (b) Top view and side view of the Cu3(HHTP)(THQ) structure. (c) Temperature dependance of the electrical conductivity of Cu3(HHTP)(THQ). Inset: Structure of a single layer of Cu3(HHTP)(THQ). Images reproduced with permission from [98]. (d) Structure of a single layer of Co3(TPHS)2. (e) Temperature dependance of electrical conductivity of Co3(TPHS)2. Inset: Eclipsed packing structure of the layers in Co3(TPHS)2. Images reproduced with permission from [99]. Kitagawa and co-workers reported the combination of alternat- A selenol-based triphenylene (TPHS) ligand was used to synthe- ing HHTP and tetrahydroxy-1,4-quinone (THQ) ligand within the size a semiconducting TP-based 2D MOF, i.e. Co3(TPHS)2, with an semiconducting Cu3(HHTP)(THQ) MOF [98]. Enthylenediamine, as eclipsed packing of 2D layered hexagonal honeycomb (Fig. 13d a chelating agent of the metallic center, was required in the syn- and e) [99]. The conductivity of a pressed pellet was in the order thesis in order to ensure the coordination balance between the of 106 Scm1 because of the poor crystallinity of the material in two units, while avoiding the precipitation of Cu3(HHTP)2 or the stacking direction. DFT calculations on the structure revealed Cu3(THQ)2 MOFs. Hexagonal layers pilled in a slipped parallel fash- a poor electronic coupling of the d and p orbitals of the Co and ion (pores of 1.7 nm) with conductivities of 2.53105 Scm1 ligand respectively, probably caused by the large radius of the Se (Fig. 13b and c) were obtained. donor atom. This led to larger interlayer contribution to the 14 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 electronic coupling. Strikingly, Co3(TPHS)2 showed glassy magnetic discharge cycles. Interestingly, the MOF-based supercapacitor properties with low intralayer antiferromagnetic coupling. was used as battery for flashlight. Similar capacitive values were obtained with Cu3(HHTP)2 electrodes used in zinc batteries, retain- 3.2. Applications of triphenylene-based 2D MOFs ing also 75% of the initial capacity after 500 cycles [79]. Notori- ously, Cu3(HHTP)2 electrodes possessed two redox-active sites In this section, some of the most characteristic applications (the copper ion and the quinoid structure) to increase the specific explored for these materials are listed with a correlation (if appli- capacity of the material in a reversible way, storing the Zn2+ ions in cable) of the chemical structure on the performance of the the 1D pores. Nevertheless, pelletization of the TP-based 2D MOFs material. dramatically decreases the available surface area, hindering the internalization of ions into the structure [37,41]. 3.2.1. Chemiresistive sensor Nanostructuration of a TP-based MOF electrode has been TP-based 2D MOFs have proved to be largely sensitive to ambi- recently reported as an approach to increase the available surface ence conditions [100]. The conductivity of this MOF family can area. In a first example, Cu3(HHTP)2 nanowires were grown on car- reach larger values in vacuum while in ambience, inert conditions bon paper by the template-assisted method leading to large sur- or when exposed to acidic/basic media, the conductivity values face areas of 540 m2g1 without the need for further treatment decrease considerably. Thus, this property has naturally opened (Fig. 15a) [37]. Remarkably, the supported carbon paper electrodes the way to chemiresistive gas sensing applications of these mate- displayed capacities of 22 lFcm2, which were retained up to 80% rials. Many examples of TP-based 2D MOF sensing devices can be of the initial value after 5000 cycles. Furthermore, larger capacitive found in the literature, as for instance sensing humidity [26] or responses of 3200 mFcm2 were obtained in hybrid nanostruc- toxic gases (NH3, NO or H2S) [33,36,38,82]. Additionally, devices tured porous arrays obtained by combining Ni3(HHTP)2 nanorods have been adapted to a wide variety of platforms, including elec- grown on top of double layered NiCo hydroxide nanosheets in a tronic textiles [36], flexible polymers [26,38] or pressed pellets Ni foam [41]. Another example of a hybrid material used for the [33]. Note that all devices showcase ppm range sensitivity and fabrication of a supercapacitor was the growth of Ni9(HHTP)4 on detection limits, and fast responses (both detecting and recovery). 3D porous laser-scribed graphene (LSG) interdigitated electrodes Choosing an appropriate combination of metal and ligand has on polyimide substrates (Fig. 16a–c) [103]. LSG was patterned proved to be critical as different responses to the analytes are applying a CO2 laser and a O2-plasma prior to growth of the MOF obtained for each MOF (Fig. 14a and b). Most frequently the ligands on the surface. The hybrid MOF/LSG material provided large areal used in this application are HATP or HHTP incorporating Ni or Cu as capacitance, energy density and power density (15.2 mFcm2, 4. metal ions. 1 lWhcm2 and 7 mWcm2 respectively). Another strategy to In a very recent study, Mirica and co-workers demonstrated increase charge percolation and available surface area consists of that different host guest interactions between MOF and ammonia growing TP-based 2D MOFs on top of flexible structured substrates gas took place depending on the used ligand and metal [101]. In as polypropylene separators [85] (Fig. 15b–d) or cellulose nanofi- all cases, the MOF electronic structure was altered upon the inter- ber papers (Fig. 16d–g) [76]. The obtained composites exhibited action with ammonia, while preserving the crystallinity of the large conductivities along with large areal capacities, which were material. In the case of amino-substituted TP-based MOFs, NH3 is preserved after several charge–discharge cycles. adsorbed at Lewis acid sites without causing structural changes. However, in the case of hydroxyl-substituted TP-based MOFs, 3.2.3. Electrocatalyst ammonia coordinates to Brönsted acid sites inducing structural The large conductivity and porosity of TP-based 2D MOFs changes in the TP aromatic ring, affecting the MOF’s conductivity together with their large metallic active site density have encour- (Fig. 14c). Moreover, Cu-based MOFs seemed to display larger aged their use as electrocatalysts. Note that they have been redox reactivity by coordination to ammonia than Ni-based ones. reported to effectively perform electrochemical redox reactions Thus, Cu-based MOFs are, in principle, more suitable materials in fuel cell batteries for hydrogen evolution reaction (HER) or oxy- for chemiresistive sensors of basic gases than Ni-based. In contrast, gen reduction reaction (ORR). CoS4 metal–organic centers previ- Ni-based MOFs seem more suitable to detect neutral volatile ously proved to have large catalytic activity for HER. In 2015, organic compounds such as ethanol, acetone or aromatic volatile Marinescu et al. reported the use of Co3(THT)2 thin films as an elec- molecules (Fig. 14d) [102]. Consequently, Cu-based MOFs have trocatalytic cathode for hydrogen generation from water [55]. The been more widely used for highly selective sensing of ammonia film was synthetized by the interfacial method and deposited onto traces in interference gases as ethanol or acetone [51] contrarily a variety of substrates (HOPG, glassy carbon. . .). The electrode to Ni-based MOFs. Note that apolar volatile molecules are hardly proved to be suitable for HER reactions as it exhibited both high detected by TP-based 2D MOFs chemiresistive devices. Driven by stability against acids and large cobalt active site density. Tafel the success in chemiresistive sensor applications, TP-based 2D slopes of 189 mVdec1 at pHs of 4.2 were reported. MOFs are currently studied for the detection of other analytes On the other hand, ORR reactions are apparently less efficiently employing potentiometric [29] (ionic salt concentration) and neu- catalyzed depending on the used structure. In a primary study, rochemical voltammetry sensors [30]. Ni3(HITP)2 film grown on glassy carbon electrodes displayed large ORR activity under O2 atmosphere [104]. Interestingly, the elec- 3.2.2. Batteries/Supercapacitors trodes remained essentially stable with values of 88% of the initial The large conductivity and surface area of TP-based 2D MOFs current in the potentiostatic measurement retained over 8 h. How- led to large capacitance when incorporating them in pressed pel- ever, the reaction was strongly dependent on the pH of the media, lets electrodes. Further, their large 1D pores allow to host large suggesting that the electron transfer mechanism was modified and solvated ionic structures making them suitable for supercapac- along with the pH. Theoretical insights on the catalytic mechanism itors or ion batteries [79,86]. In 2017, Dincă and co-workers for the oxygen reduction reaction on M3(HITP)2 (M = Ni, Cu) proved reported the first TP-based MOF electrode for supercapacitors that generation of H2O2 under acidic conditions is more favorable using pressed pellets of Ni3(HITP)2 with capacities of up to in both structures due to the complicated multipath generation 18 lFcm2, exceeding values found for some other carbon-based of H2O [105]. Consequently, an insightful investigation was fol- materials [86]. Moreover, the electrodes proved large stability, lowed, where the effectivity of different TP-based 2D MOFs for retaining almost 90% of the capacitance after 5000 charge and ORR was studied [77]. In this research, Dincă and co-workers 15 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Fig. 14. (a) Saturation sensor response for electronic textiles containing Ni3(HHTP)2 (red) and Ni3(HITP)2 (blue) exposed to 80 ppm of NO and H2S 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 NH3, NO and H2S at 80 ppm using Cu3(HHTP)2 (orange) and Ni3(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 Cu3(HHTP)2, Cu3(HITP)2 and Ni3(HITP)2 to different analytes. Image reproduced with permission from [102]. reported the higher catalytic activity of hexagonal crystalline sys- as O2 generation [106] (enhanced by doping of the structure with tems such as Ni3(HITP)2, Cu3(HITP)2 and Cu3(HHTP)2. The high iron catalytic centers) and CO2 reduction, mimicking photosynthe- electrical conductivity and redox activity of these systems facili- sis processes [107,108]. Large photocatalytic performance was tated the electron transfer to O2. In contrast, trigonal crystallized obtained under light irradiation when using Ni3(HITP)2 as co- MOFs as Ni3(HHTP)2 and Co3(HHTP)2 have a distortion of the p-p catalyzer along with a photosensitizer [108]. stacking of the triphenylene cores, which strongly affects the elec- tron transfer kinetics. Hence, the crystalline structure is critical to 3.2.4. Spintronics determine the electrocatalytic properties of TP-based 2D MOFs. Cu-based MOFs have been the widest explored materials for Interestingly, large electronic density of M3(HITP)2 structures has these properties due to their electronic configuration with an odd also proved to be suitable for other electrochemical reactions such number of d electrons. In a recent study published by Dincă et al., 16 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Fig. 15. (a) Schematical structure of Cu3(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 Ni3(HITP)2 grown on polypropylene separators showing their different shape and sizes and their flexibility. (d) SEM cross-section image of Ni3(HITP)2 films on polypropylene. Images reproduced with permission from [85]. Fig. 16. (a) Schematic and structural illustrations of the interdigital pattern of LSG/Ni9(HHTP)4 hybrid and (b) solvothermal growth of Ni9(HHTP)4 nanorods. (c) SEM image of the structure of LSG/Ni9(HHTP)4 hybrid showing the large available porosity. Images reproduced with permission from [103]. (d) Schematic illustration of the growth of Ni3(HITP)2 and Ni9(HHTP)4 on cellulose nanofiber papers. (e) Cyclic voltammetry measurements of Ni3(HITP)2 on cellulose paper at different folding angles. (f) Photographs of red light-emitting diodes powered by Ni3(HITP)2 on cellulose paper capacitors under different folding or bending geometries. (g) Photograph of origami folded Ni3(HITP)2 on cellulose paper capacitors in flower shape. Images reproduced with permission from [76]. single crystals of trinuclear molecular models of Cu3(HHTP)2 and Interestingly, changes on the oxidation state do not occur on the Cu3(HITP)2 MOFs were obtained, being analogous coordination sys- metallic center but at the TP core giving rise to closed shell, mono- tems to TP-based MOFs [109]. Magnetic measurements on these radical and diradical species. Consequently, the increase of the oxi- crystals displayed antiferromagnetic coupling of neighboring spins, dation state led to a decrease on the spin coupling strength. which could effectively lead to spin frustration if occurring in the TP-based 2D MOFs. This effect has been deeper studied recently 3.2.5. Optoelectronics by Awaga and co-workers, who demonstrated that thin films of Thanks to their large conjugation, this family of MOFs exhibit a Cu3(HHTP)2 exhibit antiferromagnetic interactions between neigh- strong light absorption with chemically tunable band gaps. The boring atoms, and that TP-based 2D MOF can reach a quantum spin large variability on chemical structures in TP-based 2D MOFs also liquid state at ultralow temperature (38 mK) [78]. This property enables the tuning on the optoelectronic properties as shown by has paved the way to novel spintronic applications for their use Zhang et al. recently [111]. In their work, different M3(HITP)2 struc- as organic spin valves [49]. Highly oriented thin films of tures showcased different non-linear optical absorbance due to the Cu3(HHTP)2 were implemented on vertical spin valves devices increasing number of d electrons. These effects gave relevance in between LMSO and Co ferromagnetic electrodes. The chemical optoelectronic devices such as photodetectors or solar cells. In structure and rational c-axis growth of the film enabled the spin- 2019, Bein and co-workers reported on basic photovoltaic devices polarized transport through the MOF. The devices showcased low by implemented highly oriented Ni3(HHTP)2 thin films grown on magnetoresistances at cryostat temperatures that were retained ITO through vapor-assisted conversion [52]. Interestingly, photoin- upon changing the film thickness and increasing the temperature duced charge generation and separation could be observed in the up to 200 K. MOF film, making this material suitable for solar cell devices. The On the other hand, possible presence of spin in Ni-complexes devices showcased open-circuit voltages of 156 mV under irradia- was assessed very recently by the study of a trinickel molecular tion. Furthermore, Feng and co-workers reported recently a model of Ni3(HHTP)2 subjected to different redox reactions [110]. Fe3(THT)2 photodetector capable of detecting radiation ranging 17 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Fig. 17. (a) Schematic illustration of a Fe3(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]. from UV to the NIR (400–1575 nm) (Fig. 17a) [112]. Owing to the reported with an ultralow thermal conductivity of 0.21 Wm1K1 narrow IR band gap of the material, room-temperature detection and a large electrical conductivity of 58.8 Scm1 [87]. Remarkably, was limited. A good performance was reported at cryogenic tem- Ni3(HITP)2 pellets provided negative Seebeck coefficients, indicat- peratures with specificities of 7108 cmHz1/2W1 (Fig. 17b and c). ing an n-type thermoelectric behavior, and high thermoelectric fig- ure of merit (ZT) of 1.19103 at room temperature. In 3.2.6. Tactile sensor complementary studies, crystallinity of the material was crucial The template-assisted growth of the Cu3(HHTP)2 on Cu(OH)2 as it deeply impacts the electrical conductivity and subsequently, microwires grown on a flexible Cu mesh substrate was used to the thermoelectric efficiency [89]. develop a tactile sensor [40]. The rationally designed flexible sen- sor allowed for the modulation of the contact area as a function 4. Triphenylene-based 2D COFs of the applied pressure, having a direct impact on the measured intensity (Fig. 18). This was used to monitor human motion and 4.1. Chemical families pulse, recognizing music through machine learning algorithms and displaying pressure maps when using an array of multiple sen- In this section, we describe different families of TP-based COFs sors. Remarkably, the sensor showcased a large durability, retain- grouped according to the used ligand. ing nearly all the initial signal after 2700 pressure and release cycles. 4.1.1. HHTP-based 2D COFs (ester-boronate and polyarylether bonds) The most common family of HHTP-based COFs, and arguably of 3.2.7. FET logic all TP-based COFs, is that formed from boronic acids that cova- In 2014, Xu et al. reported porous FET transistors based on thin lently react upon dehydrogenation generating five-membered films of Ni3(HITP)2 [58]. The highly smooth and compact thin films BO2C2 rings. A prototype structure of one of such ester-boronate were obtained by interfacial liquid–air method and recovered by COFs is the COF-5 reported by Yaghi and co-workers by condensing stamping on a silicon wafer for its integration into a FET transistor. the hydroxyl derivative triphenylene HHTP and 1,4- The transistor revealed a p-type behavior with large hole mobili- benzenediboronic acid (BDBA) in a three-day solvothermal synthe- ties of up to 48.6 cm2V1s1. Other electronic features of the sis (Fig. 19) [23,34]. Crystallographic characterization revealed a device as large on/off ratios pointed to future possible applications hexagonal unit cell forming layers stacked in an eclipsed boron as voltage-gated ion channels. nitride arrangement with interlayer distances of 0.34 nm as in gra- phitic structures. This structure leads to a large porosity showcas- 3.2.8. Thermoelectric ing 1D mesopores of 27 Å in diameter. Interestingly, the eclipsed The combination of low thermal conductivity derived from the layer stacking is attributed to the presence of triphenylene cores, strong phonon scattering of the porous of this family of MOFs as they interact by p-p stacking. Afterwards, the synthesis protocol together with their high electrical conductivity has open new ave- has been extended to a wide variety of boronic acid derivatives giv- nues for their use in thermoelectric applications. Nanostructura- ing rise to numerous TP ester-boronate COFs, named after COF-5 as tion and grain boundaries also avoid phonon propagation, COF-6, COF-8 and COF-10 [113], most of them summarized in decreasing thus the thermal conductivity. However, only the ther- Fig. 19. Most of them retain the crystalline space group of COF-5 moelectric properties of Ni3(HITP)2 pressed pellets have been with planar layers and porous diameters ranging from the 18 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Fig. 19. (a) Schematic of the structures of ester-boronate TP-based COFs condensing di- or 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]. angstrom to the few nanometers, and therefore variable adsorption condensations would lead to either other boronic structures or surface areas [114]. the lack of precipitate. However, the obtained TP-based COF exhib- Ester-boronate TP bonding can be combined with other conden- ited a large H2 and CO2 uptake with BET surface areas of sation reactions in the same synthesis, which enables a rational 1619 m2g1. Polyarylether TP-based 2D COF structures have been design of novel COFs. Jiang and co-workers reported numerous recently developed by nucleophilic aromatic substitution between TP-based COF structures combining boron-ester and imine conden- ortho-difluoro benzene derivatives and HHTP [117]. Two layered sation reactions [115] in tri-ligand fused synthesis as the one pre- structures, JUC-505 and JUC-506 (Fig. 20b) were obtained using sented in Fig. 20a. Different functionalities were achieved by tetrafluoroterephtalonitrile and 2,3,6,7-tetrafluoroanthraquinone, modifying the employed boronic acid. Simultaneously, Zeng et al. respectively. Both structures exhibit hexagonal pores of 16.8 and reported the synthetic study of the same structure [116]. Despite 28.4 Å, respectively, and large chemical stability in strong acid, these efforts, crystallization could only be achieved in one-pot basic, reductive or oxidant media. Incorporation of cyano groups reaction in the presence of all three ligands, as sequential in JUC-505 as shown in Fig. 20b allowed for post-synthetic 19 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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-NH2 structures via post- synthetic reactions. Image adapted from [117]. functionalization with carboxyl or amino groups forming JUC-505- depth description of the properties and applications of the struc- COOH and JUC-505-NH2 structures, respectively. The same strat- tures described with different acids can be found in Section 4.2. egy was performed by Yaghi and co-workers to synthesize amide and amidoxime groups in the COF structure [118]. 4.1.2. HATP-based 2D COFs (phenazine bonds) A wide variety of TP-based 2D COFs with ester and ether bonds Inspired by the successful synthesis and application of ester and using HHTP can be synthesized depending on the targeted applica- ether COFs, other TP-based 2D COFs have been recently reported tion. Novel structures or structural modifications have been inves- employing other condensation reactions. Phenazine-fused TP- tigated mainly aimed at gas storage applications (H2, CH4, CO2. . .), based COFs is a main example of novel structures obtained by especially with boronic ester COFs, as the boronic acid determines the imine condensation reaction. Phenazine-fused TP-based 2D the porous size and surface area, and therefore, the gas uptake COFs comprise nitrogen-doped graphitic porous structures with capability [23,114]. Analogously, pore size modifications and in-plane charge transfer features. These tailored structures are suit- chemistry surface functionalization on polyarylether structures able for electronic applications owing to their large electronic con- allows for tailoring the uptake of chemical species into the pores, jugation (see Section 4.2 for more information). Typically, they are enabling water treatment and filtration applications at various formed in solvothermal conditions by condensation of HATP with pHs [117,119]. Note that electronic applications of these structures ortho-planar tetraketones [32,149,150]. As previously indicated are hardly explored since ester and ether bonds break the conjuga- for ester/ether COFs, different ketones can be used as starting tion along the 2D layer. A clear example of this is their recent use in monomers to tailor the porous size as shown in Fig. 21. Fig. 21b low-k dielectrics [47]. Thus, charge transfer occurs mainly in the shows the CS-COF structure obtained by Guo et al. in a reaction out-of-plane direction through the one-dimensional stacking of between HATP and tert-butylpyrene tetraone, which led to an TP moieties, similarly as it occurs for DLCs. Hence, doping is often extended p-conjugated system with pores of 1.6 nm [32]. required for optoelectronic applications, either by incorporation of Fig. 21a depicts a structure reported by Meng et al. as a result of electron donor/acceptor atoms into the ester/ether structure [120], condensing HATP and hexaketonecyclohaxane into a layered 2D use of conjugated ligands (creating electron donor–acceptor sys- COF with 1D pores of diameter 1 nm [149]. Note-worthy, in this tems along with TP) [121] or external optoelectronic doping [44]. work, the TP-based 2D COF proved to have low conductivities of All these aspects are summarized in Table 4, which comprises the order of 108–109 Scm1 under anhydrous conditions at investigated TP-based boronic ester-linked COFs and their proper- 323 K whereas under 97% relative humidity, the conductivity ties and applications to date. Note that optoelectronic features are increased to 1.51105 Scm1, indicating a large proton conductiv- not commonly found in the literature for these materials. An in- ity. The conductivity values were further increased by acidification 20 Table 4 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Described boronic COFs. COF /Boronic acid used Form Synthesis method Pore size BET Surface area (m2g1) Properties and application Ref (Å) Powder Solvothermal 27 1590 H2 uptake [23,122] Powder Solvothermal – 1990 H2, CH4, CO2 uptake [114] Powder Solvothermal – 2000 Synthesis studies [28] Powder Solvothermal – 1517 – [123] Powder Solvothermal 2.274– 1421–36 Tunable porosity [124] 1.248 Powder Solvothermal 27 1670 Mechanistic studies of synthesis [125] Powder Solvothermal – – Growth kinetic study in homogenous reaction [126] Powder and oriented Solvothermal and high pressure – – Li-Ion battery [127] pressed pellets pelletization 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 Sonochemistry with substrate – 2122 Synthesis studies [27] thin film imbedded in solution Powder and free-standing Solvothermal and solvent – 840 (Thin film) Synthesis studies (for colloidal suspension) [132] thin film evaporation for thin film Supported thin film Vapor-assisted conversion 27 – Preferential orientation, control of thickness and [53] different substrates COF coating of CNTs and Sonochemistry with substrate – 57.6/9.83 CO2 uptake [45] graphene imbedded in solution COF coating on Solvothermal – – Electrochromatography of small molecules [46] polydopamine coated glass 21 COF coating on CNTs Solvothermal with substrate – – Li-Ion battery [44] imbedded in solution 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 H2, CH4, CO2 uptake [114] Powder Solvothermal – – Synthesis studies [126] Powder Solvothermal – – Synthesis studies (for colloidal suspension) [132] Supported monolayer Solvothermal with substrate – – – [68] imbedded in solution Coordination Chemistry Reviews 460 (2022) 214459 Powder Solvothermal 6.4 980 Gas uptake [113] Powder Solvothermal – 960 H2, CH4, CO2 uptake [114] (continued on next page) Table 4 (continued) N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. COF /Boronic acid used Form Synthesis method Pore size BET Surface area (m2g1) Properties and application Ref (Å) Powder Solvothermal 18.7 1400 Porosity [113] Powder Solvothermal – 1400 H2, CH4, CO2 uptake [114] Nanosheets Liquid phase exfoliation of powder 17 – Nanostructuration [134] assisted by sonication 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 32 – Photoemission [43] imbedded 22 Powder Solvothermal 25.7 904 Compared to other tiophene based COFs and doping [120] with oxidizing agents Powder Solvothermal 29 with 1844 with ethoxys Synthesis studies and tunable porosity [136] ethoxys Powder Solvothermal – 1125 (S) Tunable conductivity (substitution of sulfur with other [137] 1056 (Se) chalcogenides) 302–352 (Te) Powder and thin film Solvothermal and spin-coating for 30 1810 Photovoltaics: Adsorption of fullerenes as host–guest [138] film for photoluminescent quenching Powder and oriented thin Vapor-assisted conversion 32 1946 (powder) 175– Photovoltaics: Adsorption of fullerenes as host–guest [48] 145 cm2/cm2 (thin films) Coordination Chemistry Reviews 460 (2022) 214459 films for photoluminescent quenching Powder and supported Solvothermal with substrate – 1376 (powder) Both randomly and oriented films. Photoconductivity: [31] thin film imbedded Faster response in oriented. Oriented thin films Vapor-assisted conversion 32 – Conductivity anisotropy measurements and hole [139] photoconductivity Thin film Vapor-assisted conversion 32 990 Control of preferential orientation and thickness on [53] different substrates Powder Solvothermal – 2400 Synthesis studies [126] Table 4 (continued) N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. COF /Boronic acid used Form Synthesis method Pore size BET Surface area (m2g1) Properties and application Ref (Å) Powder Solvothermal – – Synthesis studies (for colloidal suspension) [132] Powder and supported Solvothermal with substrate 47 2640 Gas uptake, control on preferentially oriented films [140] thin film imbedded and blue emission Powder Solvothermal 30 810 Isomerization with UV radiation with retention of [141] porosity and structure Supported monolayer UHV evaporation 36 – – [142] Powder and supported Solvothermal with substrate 28 2021 Photoconductivity [121] thin film imbedded Powder Solvothermal 32 1510 Large interlayer interaction with antiparallel building [143] blocks DTP-ANDI-COF Powder and coating of Solvothermal with presence CNTs 50.6 1583 (powder) Li-ion battery [144] CNTs 676 (CNT) DTP-ANDI-COF Powder Solvothermal 53 1504 Photoluminiscence kinectic studies [145] DTP-APyrDI-COF 53 1910 TDB (1) Powder Solvothermal 20.6 (1) 972 Synthesis studies and conductivity by doping with [120] TDB (2) 13.8 (2) 562 oxidizing agents BTDB (3) 32.4 (3) 544 T-COF-OH Powder Solvothermal 41 2000 Post-synthesis chemical modifications [146] 560 (post-functionalization) 23 Porphyrin Powder and supported Solvothermal with substrate 46 8.9 Photovoltaics [147] thin film imbedded Combination of different acids Powder Solvothermal – – Multifunctional COFs [148] Coordination Chemistry Reviews 460 (2022) 214459 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 of the COF by impregnating it with 12 M H3PO4 up to a value of is ensured by the eclipsed packing of the acceptor units. For exam- 1.23103 Scm1 under hydration conditions of 97% at 323 K. ple, Jiang and co-workers reported a boronic ester COF with a ben- Interestingly, this structure could also be obtained by inverting zothiadiazole boronic acid serving as electron acceptor [121]. This the functional groups of the starting materials, in other words, by structure provided large photocurrents with hole and electron condensing HHTP molecules with planar tetraamines [151]. Strik- mobilities of 0.01 and 0.04 cm2 V1 s1, respectively. The same ingly, nitrogen-doped TP-based 2D COFs with larger pores can be strategy was used by Bein and co-workers in a donor–acceptor sys- obtained upon the reaction between HHTP and HATP as reported tem comprising a porphyrin TP-based COF (Fig. 22b and c). A by Jhulki et al. [152] with the structure of C2P-5 COF. Analogous highly oriented thin COF film was incorporated in a photovoltaic structures were reported by Liu and co-workers, by Buchwald- device, which reached open-circuit voltages of 312 meV [147]. Hartwig cross-coupling reaction between hexabromotriphenylene External doping agents such as fullerene derivatives can be used and HATP [153]. Interestingly, both eclipsed and staggered packing as acceptors when hosted in the one-dimensional pores of the elec- were obtained. All the structures hereby described present an tron donor COF (Fig. 22a) [138]. Nonetheless, such heterojunctions eclipsed packing of the layers, thus out-of-plane contribution is can limit the charge separation and mobility, and hence, the pho- expected as well, either by delocalization of the electrons in the tovoltaics performance. TP moieties or by proton conduction along the one-dimensional Large in-plane conjugation of TP-based COFs may also enable porous. the generation of photocurrents. A clear example is the previously introduced phenazine-fused CS-COF structure seen in Fig. 22d and 4.1.3. Other structures e. The extended p-conjugated system exhibited a large hole pho- Research on different polymerized triphenylene structures has tocurrent mobility (4.2 cm2 V1s1) [32]. However, faster photo- enabled the realization of new carbon allotropes such as TP- conductive kinetics were obtained by hosting C60 fullerenes based graphdyine (TP-GDY) analogues, containing both sp and sp2 molecules in the pores of CS-COF as a donor–acceptor system as carbon atoms. This structure is well characterized, including its it occurred for previously described structures. Further, the large electronic conjugation, which makes TP-GDY suitable for CS-COF  C60 system was successfully integrated into functional electronic applications. Nishihara and co-workers reported the photoswitches and photovoltaic cells. Similarly, previously intro- synthesis of TP-GDY by means of Glaser-Hay homo-cross coupling duced C2P-5 thin films showcased an intrinsic large hole mobility catalyzed by copper in a liquid–liquid interfacial system obtaining resulting into conductivities of 1.75 Scm1 [152]. This conductivity thin films of 220 nm thickness [154]. Later, Tan and co-workers is significantly increased upon irradiation with light especially reported modifications of the synthesis, which allowed the manu- after hybridization of the C2P-5 films with graphene. Larger facturing of nanosheets and thin films with thicknesses of few photocurrents were recorded when applying larger powder light monolayers (0.9 nm) and good mechanical strength [59]. sources. Thus, the hybrid device proved to be a suitable photode- tector for UV, visible and NIR light. 4.2. Applications of TP-based 2D COFs 4.2.2. Li-ion battery A particular property of layered COFs is their large p-orbital As a result of the large conjugation and porosity, the phenazine- overlap in the stacking direction conferring them both high exciton fuse COF (Fig. 21a) has a large proton conduction, which can be and charge conduction, which are interesting features for optoelec- enhanced in aqueous and acidic media. This property makes this tronic applications. Furthermore, their large and extended porosity material suitable as an electrode in a battery cell [149]. Perfor- enable chemical encapsulation or electrochemical doping for com- mance of this material as anode in Li-ion batteries was studied plementary functionalities. Finally, their high thermal stability pre- by Shi and co-workers, which reported high reversible capacities vents phase transitions that impair or inactivate their electronic of 701 mAhg1 and long cycling performance (up to 4500 cycles) properties. However, to aim to given properties or applications, [158]. Similarly, piperazine-based structures with larger pores pro- one might tailor the structure of the COF or shape it in a given vided large capacities of 1644.3 mAhg1 [153]. Other conjugated fashion. graphitic structures, such as TP-GDY, can be considered as suitable candidates for working anodes in Li-ion batteries. For example, 4.2.1. Photovoltaics and photocurrent hybridization of TP-GDY with CNTs through p-p stacking was per- Free TP molecules exhibit a characteristic blue emission fluores- formed obtaining free-standing films has been reported [59]. The cence that is usually shifted upon integration in a COF structure as films exhibited good electrochemical properties with a reversible a result of a higher delocalization degree when conjugated co- capacity of 1624 mAhg1, good rate performance and recyclabil- ligands are present [135,140,155]. These structures are character- ity. Furthermore, the hybrid system was incorporated as an anode ized by the overlap of the TP emission band with the co-ligand in a lithium battery. Ester-boronate COFs are rarely used in elec- absorption one, acting as a donor–acceptor system (where the TP tronic applications as the boron ester bond breaks the conjugation is an electron donor and the co-ligand an electron-acceptor). Inter- along the 2D layer. However, one may reach these properties by layer distance is a critical feature in some cases, and enables on/off external doping [44,45,159]. In a recent study, differently charged phosphorescence at cryogenic temperatures upon its reduction 2D COFs (COF-5 among them) were doped with low-molecular [155]. Interestingly, several studies have reported that photolumi- weight polyethylene glycol (PEG) by including it into their 1D nescent kinetics is highly dependent on crystallites size, with channels [159]. Interestingly, PEG significantly enhanced the Li+ shorter exciton life times for suspensions containing smaller parti- ion conduction along the pores of COF thanks to the large amount cles [156,157]. of oxygen atoms in the PEG structure providing multiple electron Control over the charge transfer in donor–acceptor systems donor sites. suggests potential photovoltaics applications. Several ester- boronate COFs have been described including TP and the boronic 4.2.3. Electrocatalysis acid as electron donor and an acceptor moiety, respectively Another interesting use of nitrogen-doped COFs is in electro- [121,145,147]. The eclipsed stacking of the layers in these struc- catalysis. Zhang and co-workers evaluated phenazine-fused COF tures led to bicontinous electron donor–acceptor ordering: upon (Fig. 21a) as an ORR catalyst [160]. DFT calculations revealed that light irradiation hole charge transfer is driven through the one the large nitrogen density, crystallinity and stability of the material dimensional stacking of TP moieties, whereas electron conduction supports high availability of electrocatalytic sites. Consequently, 24 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 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. low overpotentials of 349 mV at 10 mAcm2 and Tafel slopes of or plate-like single crystals have allowed to assess the correlation 64 mVdec1 were obtained. The same nitrogen-doped COF have of the anisotropy of conductivity with the crystalline structure. been used to coat composite core–shell structures of Fe/Fe3C However, insights on this correlation are needed as only anisotropy nanoparticles in order to improve its long-term stability and per- of two structures have been reported in single crystals to date. formance [150]. Interestingly, the composite exhibited a good elec- The measurement of conductivities of single layers of TP-based trocatalytic behavior with a 92% retention of the initial current systems is timely as atomic- or nearly atomic-thick films will after 10,000 chronoamperometric cycles. reveal key quantum confinement effects and interlayer interac- tions. The consequences of shrinking the material to single-layer thicknesses have already been observed in other 2D materials, such 5. Future perspectives as transition metal dichalcogenides. For example, the band gap of MoS2 can be tuned from 1.23 eV in the bulk form to 1.8 eV in The bourgeoning research in TP-based organic frameworks that the monolayer [165]. Single layers can be obtained using top- followed last few years has led to several discoveries in different down methods, mainly by exfoliating a single crystal, an approach fields of applications, where these materials are finding their niche. that has already been successfully used for the delamination of 2D Undoubtedly, TP-based organic frameworks are significantly sky- coordination polymers [166–168]. However, this requires the rocketing in the area of molecular conductors, a field that remains growth of crystals sufficiently large, which is a challenging job. vastly unexplored. TP structures are potential candidates not only While exfoliation of TP-based 2D COFs has been demonstrated in because of their electronic properties but also because of their few recent investigations [134], this is not the case for TP-based inherent porosity. While benzene ligands enable conductivities 2D MOFs. Alternatively, bottom-up techniques represent a strategy within the same range or even larger than TP ligands, in compar- that attracts an increasing interest. Among this family of methods, ison to the latter, benzene ligands have small or inexistent poros- two main methods are particularly appealing such as interfacial ity, which significantly hinders the use of these structures in liquid synthesis [56] and UHV evaporation, the latter being more devices such as chemiresistors. Albeit the potential of TP-based suitable to obtain monolayers as the deposition rate allows for MOFs and COFs, the number of reported structures is limited. an exquisite control over the growth. Reports on the UHV forma- Rational design has enabled large p-conjugation along the 2D lay- tion of TP-based 2D MOFs and COFs have demonstrated the rele- ers and interlayer p-p interactions enabling charge transport path- vance of several key parameters, such as the deposition order or ways, but chemical ligand design is still in its infancy. Novel the stoichiometry between the evaporated monomers structures derived from TP substitutions (such as, selenol) or [68,142,169]. However, despite numerous studies, only specimens ligand combinations have been recently reported but novel with nanodomains have been accomplished, leading to a finite extended ligands containing TP or TP derivative moieties are also polymeric array that may be subjected to in-plane quantum con- arising for both MOF and COF structures [161–164], indicating an finement [68]. Thus, the measured properties may differ from exciting yet challenging greenfield. those obtained in an infinite layer. In future, long-range single lay- Most of the conductivities reported for these materials refer to ers should be obtained and investigated in depth. polycrystalline systems, which are not optimal in terms of conduc- Advantages of single crystals prevail for fundamental studies tivity. Polycrystalline samples are formed by several finite crys- while in terms of applications polycrystalline materials are talline domains exhibiting grain boundaries, which disrupt selected due to the geometrical restrictions to adapt a crystal to carrier paths and charge transfer, ultimately affecting their con- a specific device. Thus, development of methodologies that allow ductive properties and, even result in unexpected undesirable for the controlled deposition of the polycrystalline material on sub- properties such as opening a band gap in otherwise metallic mate- strates to achieve homogenous flat interfaces that avoid charge rials. Single crystal is a continuous and defect-free (aside from entrapment have been pursued. Pressing powders into pellets, monomer vacancies) structure formed by the infinite repetition which can provide smoother surfaces and easily manageable sys- of a unit cell. Hence, the properties (including the conductivity) tems is one of the most used methods. However, application of of a single crystal are closer to those intrinsic properties of the high pressures can induce crystalline phase transitions, crystal material. Hence, single crystals are commonly aimed for the study breaking, or cause the occlusion of pores. An alternative to post- of anisotropic properties, such as in-plane and out-of-plane conduc- synthetic treatment consists of directly growing polycrystalline tivities. Some examples have been reported in the last years espe- material on supported thin films. This approach can lead to thin cially in the case of TP-based 2D MOFs. Measurements of rod-like films with preferential crystalline orientations that minimize grain 25 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 Fig. 22. (a) I–V characteristics of a fullerene-doped thiophene TP-based COF (TT-COF) photovoltaic cell measured under illumination with simulated solar light. Insets: Schematic of the host–guest interaction between the TT-COF and the fullerene derivative (left) and schematic of the photovoltaic device (right). Images adapted from [138]. (b) I-V characteristics under irradiation of a porphyrin TP-based COF acting as a donor–acceptor system. Inset: Schematic of electron and hole charge transfer along the one- dimensional conduction paths defined by the TP and porphyrin moieties. (c) External quantum efficiency of the porphyrin TP-based COF. Images adapted from [147]. (d) Schematic representation of CS-COF and its doping with fullerenes upon host–guest interactions in the open one-dimensional porous channels of the CS-COF (white: carbon; red: nitrogen; purple: fullerene). (e) I-V measurements of the fullerene-doped CS-COF in the dark (blue) and under irradiation (red). Images reproduced with permission from [32]. boundaries and energetic defects while defining the parallel COF-based systems. Accordingly, further studies are required to arrangement of one-dimensional pores, charge conduction chan- improve the stability of TP-based MOFs in water as well as the con- nels and orbitals. Films obtained by this approach display ductivity of TP-based COFs. Note that in-depth investigations of the enhanced performance for applications in photovoltaics [31,48] magnetic properties of TP-based organic frameworks also remain or spin-valves [49]. Unfortunately, most of current synthetic meth- widely unexplored. In this vein, TP-based 2D MOFs are formed ods lead to films with sub-millimeter lateral sizes or random crys- by Kagomé lattice structures, where metallic ions are coordinated tal orientation. Few examples of oriented thin films of TP-based in trinuclear arrangements [49,78,109]. Hence, TP-based 2D MOFs MOFs and COFs have been reported (mostly limited to HHTP), are appealing candidates for the generation of geometrically frus- using layer-by-layer [51] or interfacial approaches [54] (as trated spin systems. To date, only few examples of Cu-based MOFs Langmuir-Blodget or vapor-assisted conversion). Therefore, devel- have been investigated, and successfully implemented in spin opment of novel synthetic methods that can produce highly ori- valves. However, both detailed studies to understand and ented thin film morphologies is urgently required. This would approaches to control the magnetic properties of these materials include the need to incorporate different compatible substrates (with the proper choice of ligands and metals) and their interaction aimed at each specific target application. For example, optically with the conductive properties have been reported. Therefore, we transparent or insulating substrates are typically needed for opto- can anticipate a wealth of research in this area over the years to electronic devices, whereas flexible or wearable electronics are come. more suitable for biological applications. Hence, synthesis proto- cols need to be compatible with a wide variety of substrates. Declaration of Competing Interest Finally, methods that can enable the manufacturing of flexible free-standing thin films that can be easily manipulated even in The authors declare that they have no known competing finan- the absence of a support substrate is the need of the hour, which cial interests or personal relationships that could have appeared has been only achieved in one occasion to our knowledge [92]. to influence the work reported in this paper. In addition to all of the above, there are two other relevant areas that deserve further studies. The first one is to improve the stabil- Acknowledgements ity in water. 2D materials are being increasingly used in the biomedical applications, as improved alternatives to established This work was supported by grant RTI2018-098027-B-C21 from biomaterials. However, the poor stability of TP-based MOFs in the Spanish Government funds and by the European Regional water, which causes the material to swell, and possible redox Development Fund (ERDF). The ICN2 is funded by the CERCA pro- activity of both ligand and metal has significantly prevented their gramme / Generalitat de Catalunya. The ICN2 is supported by the use in this area. TP-based COF structures may have more impact in Severo Ochoa Centres of Excellence programme, funded by the biomedicine as these materials are more stable in water, while Spanish Research Agency (AEI, grant no. SEV-2017-0706). N. C.-P. exhibiting proton conductivity in aqueous conditions. Yet, the con- acknowledges the support of fellowship from ‘‘laCaixa” Foundation ductive properties have not been as advanced as they have been for (ID 100010434). The fellowship code is LCF/BQ/ES17/11600012. J. MOFs. Bioelectronic applications of TP-based COFs remain at an P.-L. acknowledges support from the European Research Council earlier stage of development, being the need of doping and lack Starting Grant microCrysFact (ERC-2015-STG No. 677020), the of intrinsic charge carrier conduction the main limitations in Swiss National Science Foundation (project no. 200021_181988), 26 N. Contreras-Pereda, S. Pané, J. Puigmartí-Luis et al. Coordination Chemistry Reviews 460 (2022) 214459 and grant PID2020-116612RB-C33 funded by MCIN/ AEI structural and electrical transport properties, ACS Cent. Sci. 5 (12) (2019) 1959–1964, https://doi.org/10.1021/acscentsci.9b01006. /10.13039/501100011033. S.P. acknowledges support from the [25] T. Chen, J.-H. Dou, L. Yang, C. Sun, N.J. Libretto, G. Skorupskii, J.T. Miller, M. ERC-2017-CoG HINBOTS Grant No. 771565. Dincă, Continuous electrical conductivity variation in M 3 (hexaiminotriphenylene) 2 (M = Co, Ni, Cu) MOF alloys, J. Am. Chem. 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