Rapidly Diversifying Hydrogen‐Bonded Organic Frameworks With Permanent Porosity

ABSTRACT

Throughout the long history of porous molecular crystals (PMCs), hydrogen‐bonded organic frameworks (HOFs) have experienced a renaissance and have advanced significantly over the past two decades. HOFs can yield large single crystals with fewer defects due to their reversible bonding and dissociation processes, enabling precise analysis of the structure‐property relationship. They are also attracting attention as candidates for sustainable materials. This article reviews the evolution of HOFs (primarily those with permanent porosity) over the past 15 years, based on the molecular structures of their constituent molecules (tectons). HOFs have been synthesized from a wide variety of tectons, ranging from simple substituted benzene derivatives to those with various π‐conjugated polycyclic aromatic cores, as well as with rigid or flexible macrocyclic cores. Subsequently, we would like to focus on several distinctive aspects and concepts of HOFs. These include permanent porosity of rigid HOFs and flexible HOFs, which is crucial for porous materials; isostructural (isoreticular) HOFs, which can provide a series of continuous HOF libraries; and non‐stoichiometrically mixed multicomponent HOFs, which aim to tune functions and create new ones by using multiple types of constituent molecules.

This article reviews diversifying hydrogen‐bonded organic frameworks (HOFs) with permanent porosity reported over the past 15 years, based on the molecular structures of their constituent tectons. Furthermore, several distinctive aspects and concepts are described, such as permanent porosity, isostructural (isoreticular) HOFs, and multicomponent mixed HOFs, as well as flexible HOFs.

1. Introduction: A Little Bit of History and Recent Trends

The 2025 Nobel Prize in Chemistry was awarded to Susumu Kitagawa, Richard Robson, and Omar M. Yaghi for their pioneering development of Metal‐Organic Frameworks (MOFs) [1]. The construction of regular, infinitely extended frameworks with permanent porosity, composed of metal cations and organic ligands, is making waves beyond basic science and into industry as game‐changing functional materials. The history of porous molecular crystals with internal voids dates back to the 1950s. Crystals of urea with one‐dimensional (1D) channels [2] and crystals of Dianin's compound (4‐p‐hydroxyphenyl‐2,3,4‐triethylchroman) with hourglass‐shaped cavities [3] are representative examples of early structures. Much of the research focused on these crystals from the perspective of inclusion crystals, which incorporate solvent molecules. In 1976, Barrer et al. reported the permanent porosity of Dianin's compound based on gas adsorption experiments on its crystals [4]. However, the complex interplay of multiple interactions, involving van der Waals forces that govern the formation of the structure, has made the systematic construction of porous crystals challenging. In 2011, Chen redefined porous molecular crystals formed via hydrogen bonding as hydrogen‐bonded organic frameworks (HOFs) [5] and demonstrated selective sorption of C₂H₂ over C₂H₄ using Wuest's inclusion compound [6]. It was a renaissance of porous organic crystals. Since then, numerous HOFs have been constructed based on supramolecular synthons formed by functional groups such as diaminotriazine (DAT), carboxy, amino, and pyridyl groups. In recent years, the hydrogen bonds used in the construction of HOFs have expanded to include even weaker interactions, such as those between fluorine and hydrogen atoms, between a nitro group and an aromatic hydrogen atom, and between a cyano group and an aromatic hydrogen atom, enabling the creation of flexible HOFs capable of dynamic changes of pore shapes. So far, guest‐responsive changes in fluorescence color and the separation of hydrocarbon isomers have been demonstrated for such flexible HOFs.

With the rise of HOF chemistry, many related review articles have been published, referring to HOF design [7, 8, 9], dynamic behaviors [10, 11, 12], selective gas adsorption and separation [13, 14, 15], fabrication of separation membranes [16, 17], catalytic function [18, 19], and composites with biomaterials [20, 21]. In this review, we provide an overview of the HOFs reported since our previous review [8], focusing on the rapidly diversifying structures of HOFs with permanent porosity. In the construction of HOFs, charge‐assisted hydrogen bonding is a very powerful tool [22, 23]. However, these will not be discussed in this review due to space limitations. In the latter half of this review, we focus on permanent porosity and isostructurality (isoreticularity) of HOFs, which have been important aspects of porous materials. Moreover, the concept of non‐stoichiometrically mixed multicomponent HOFs will be introduced. Although this has been extensively reported in other types of framework materials, such as MOFs and COFs, there have been few reports on HOFs because they tend to settle into thermodynamically favorable single‐component or stoichiometric cocrystalline HOFs. However, this is now being achieved through various structural innovations.

2. Diversifying Skeletal Structures of Tectons

2.1. Benzene Derivatives and Analogs

Substituted benzene derivatives and their analogs are the simplest entry of tectons for constructing HOFs (Figure 1). Hexatopic tecton 1 with carboxyphenylethynyl arms forms a three‐dimensional (3D) H‐bonded network via H‐bonded dimerization of the carboxy groups, giving HOF‐76 that shows permanent porosity with Brunauer–Emmett–Teller surface area (SA_BET) of 1121 m²g⁻¹ and selective C₂H₄ uptake over C₂H₆ [24]. Two‐dimensional (2D), hcb ‐network‐based, nonpenetrated, layered HOF TCPB‐1 is constructed by tritopic tecton 2a, while 2b with longer arms gives an 11‐fold interpenetrated framework TCBPB‐1 composed of a nonplanar bowl‐like 2D hcb ‐network [25]. Tecton 2c gives a honeycomb ( hcb ) network, which is polycatenated to give HOF‐12 with a degree of catenation (Doc) of (5/5) [26]. The tecton also gave HOF‐22 with a 2‐fold interpenetrated structure, where two carboxylic groups form a H‐bonded dimer, and the other forms a hydrogen bond with EtOH [27]. Tecton 2d was investigated by three groups independently. The tecton yields three types of HOFs JLU‐SOF‐2 [28], JLU‐SOF‐3 [28], and HIAM‐103 [29] with different H‐bonded motifs or interpenetration manners depending on crystallization conditions. Activation of JLU‐SOF‐3 yields ZJU‐HOF‐1, which shows high C₂H₆ uptake capacity (88 cm³ g⁻¹ at 0.5 bar and 298 K) and excellent C₂H₆/C₂H₄ selectivity with the C₂H₄ productivity of 0.98 mmol g⁻¹ [30]. Tecton 2e yields highly porous HOF ZJU‐HOF‐5 with a SA_BET of 3102 m² g⁻¹, due to low interpenetration of the 3D networks [31]. Tecton 2f forms a 3D 4‐connected dia ‐network, which is 2‐fold interpenetrated to give HOF‐TDBB [32]. Tecton 2 g forms a 3D 6‐connected pcu ‐network, which is 8‐fold interpenetrated to yield HOF‐TDCPB [33]. Tecton 2 h forms a highly open 3D noy ‐network with a solvent‐accessible volume of approximately 68% through 20 N─H···N bonds per molecule with five neighboring molecules [34]. Tecton 2i, on the other hand, connects with six neighboring molecules through 12 N─H···N bonds to yield a highly open 3D cds ‐network with an approximately 69% solvent‐accessible volume [34]. Tecton 3a forms a 2D hexagonal sheet, which develops a three‐layered unit sheet through π···π stacking, followed by mutual polycatenation of the unit sheets to give HOF‐BTB‐NH2 with Doc of (6/6), which shows high fluorescence quenching efficiency toward serotonin [35]. For analog 3b, a four‐layered unit sheet is mutually polycatenated to yield 3D networked HOF‐ZJNU‐3 with Doc of (8/8) [36].

FIGURE 1.

Tectons based on benzene and analogous cores.

Nitrogen‐containing analogs, on the other hand, show different assembly behaviors. Triazine‐based tecton 4a forms three types of entanglement isomers of HOFs, which are composed of hcb ‐networks with different undulation periods: HOFs PFC‐11, ‐12, and‐13 exhibit Doc of 24, 18, and 36, respectively [37]. Tecton 4b gives hollowed crystals of HOF M‐TPY with micro‐ and microporosity [38]. The formation mechanism of the crystal is explained by highly anisotropic crystal growth due to the difference in energy among the highly oriented π···π, C─H···N, and side‐on interactions during diffusion‐limited and supersaturated conditions. Pyridine‐based tecton 5 yields 2D‐layered HOF HOF‐FJU‐88 composed of a hexagonal 2D network, which is formed through N─H···N bonds of pyrazole groups [39]. The HOF exhibits a notable selectivity for CO₂ over C₂H₂ with an IAST selectivity index of 1894.

C _2v‐Symmetric tritopic tecton 6, where the point group with the highest symmetry is presented without considering molecular conformation and the same applies below, gives a 6³‐connected 2D network, which is slip‐stacked to give layered HOF 123CPB‐1 [40]. C _s‐Symmetric tetratopic tectons 7a and 7b yield ladder‐shaped 1D network structures, which are slip‐stacked and aligned in a coplanar manner to give HOFs, CsBen‐1 and CsPy‐1, respectively [41]. Similar to 7, C _2v‐symmetric tetratopic tectons 8a and 8b yield ladder‐shaped HOFs CPB‐1 and CPB(OMe)‐1, respectively, in which the ladder motifs stack in different manners, forming discrete voids in the former and 1D inclusion channels in the latter [42]. Tetratopic tecton 9 forms a 3D 6‐connected pcu ‐network through two types of 12 intermolecular C─H···N≡C bonds. The network is 2‐fold interpenetrated to yield HOF‐40 [43]. The framework has a gate‐opening nature and exhibits superior separation performance for Xe/Kr mixtures. Tecton 10 with four carboxyphenyl groups directing alternately up and down gives ZJU‐HOF‐8 with 4‐fold interpenetrated dia networks [44]. The HOF has flexible, robust pores and shows significant uptake of C₃H₈ and n‐C₄H₁₀ compared with smaller hydrocarbons C₂H₆ and CH₄ under ambient conditions. Similar to 2i, tectons 11 gave complex frameworks with various topologies, such as cds and vco , due to flexible conformations and versatile H‐bonding patterns of the DAT groups. Among them, 11b formed a framework with a guest‐accessible volume of 74% [34]. Tecton 12a is connected by two kinds of hydrogen bonds, conventional carboxy dimers and alcohol‐bridged carboxy dimers, to yield HOF‐30 that possesses 10‐fold interpenetrated 3D dia ‐networks. The activated HOF afforded the sole usual carboxy dimers and showed the selective adsorption of propyne over propylene [45]. Analogs 12b and 12c with cyano groups form similar wavy, layered HOFs, HOF‐FJU‐99 and HOF‐FJU‐100, respectively, which are constructed through multiple intermolecular interactions such as C─H···π, C─H···N≡C, and π···π interactions [46]. HOF‐FJU‐99 is nonporous, whereas HOF‐FJU‐100 exhibits microporosity, because the methyl groups of 12c serve as the grafting units to form the pore structures between the layers. HOF‐FJU‐100 showed unparalleled separation efficiency and ultrahigh selectivity for C₂H₂/CO₂ mixtures.

Applying a highly symmetric benzene as a platform enables the formation of well‐defined H‐bonded networks. Many tectons were synthesized based on combinations of the benzene cores with different substitution positions and substituent numbers, H‐bonding functional groups, and linker structures, resulting in the development of versatile HOFs. Particularly, carboxylic acid‐based tectons provide systematic construction of HOFs in terms of molecular geometries and network topologies due to their well‐predictable supramolecular synthons, while the incorporation of pyridine and cyano groups with weak H‐bonding capability enables the construction of stimuli‐responsive flexible HOFs.

2.2. Oligophenylene and Related Skeletons

Tectons with biphenyl, terpenyl, and related skeletons are the second simplest entry of tectons (Figure 2). Tecton 13 forms a 3D dia ‐network via H‐bonded dimerization of the carboxy groups. The network is 4‐fold interpenetrated to give HOF BPTCA‐1 with 1D hexagonal‐shaped pores [47]. The activated HOF BPTCA‐2 shows high absorption selectivity toward naphthalene over anthracene. Tecton 14 yields 7‐fold interpenetrated, robust HOF HOF‐15 with SA_BET of 1964 m²g⁻¹ [48]. Tecton 15 gives a 6‐fold interpenetrated, robust HOF ZJU‐HOF‐10 that exhibits high separation ability for C₂H₆ from the mixture of C₂H₆ and C₂H₄ [49]. Tecton 16 yields 4‐fold interpenetrated robust HOF HOF‐20, showing highly efficient and selective turn‐on fluorescent sensing of aniline in water [50].

FIGURE 2.

Biphenyl‐ and terphenyl‐based and related tectons.

Tecton 17a yields 2D‐layered HOF TPTCA‐1, in which the 2D network constructed via H‐bonded dimerization of the carboxy groups is slip‐stacked. Activated HOF TPTCA‐2 exhibits selective absorption of anthracene over naphthalene [47]. Tecton 17b gives a 2D‐layered HOF composed of 2D‐lamellar networks via H‐bonding between DAT groups [51]. Tectons 18a and 18b yield 2D‐layered HOFs, BTIA‐1 [52] and BTTA‐1 [53], respectively, via the dimerization of the carboxy groups and dipole‐dipole interaction of benzo[c][1,2,5]thiadiazole (BT) cores. These HOFs exhibit photoconductivity due to the 1D‐stacked structure of BT moieties [53]. Tecton 18c forms a 2D‐layered HOF isostructural with that constructed from 17b [51]. Tecton 19 yields 2D‐layered HOF ABTPA‐1 through the dimerization of the carboxy groups and π···π stacking between the anthracene cores. The stacking manner changes from parallel (ABTPA‐1) to herringbone (ABTPA‐2) upon guest removal [54]. Tecton 20 yields a 2D‐layered HOF NTTA‐1, in which the naphthalene moieties are accumulated in parallel‐stacking manners [55]. Tectons 21a, 21b, 21c, 21d yield 2D‐layered HOFs, ZJU‐HOF‐59, ZJU‐HOF‐60, ZJU‐HOF‐61 and ZJU‐HOF‐62 respectively. Their staking structures are different depending on the core moieties [56, 57]. Tecton 22 forms a 2D‐layered HOF NDITA‐1 that possesses π‐stacked columnar domains and pore channels surrounded by NDI planes, resulting in significant changes in absorption and emission spectra as well as electron conductivity upon exchange of aromatic guest solvents [58].

When core structures of tectons are changed from benzene to biphenyl, the resulting HOFs tend to have a 3D‐networked and interpenetrated structure with thermal stability and permanent porosity, due to the twisted conformation of the tectons. On the other hand, the tectons with terphenyl moieties generally yield 2D‐networked structures, which is because the central aryl ring is twisted and the terminal aryl rings adopt a coplanar conformation. Furthermore, skeletal elongation can yield layered HOFs with a larger void. However, no HOFs composed of tectones with longer cores than quaterphenylene have been reported, presumably due to the low solubility of the tectones and/or the structural instability of the resulting HOFs.

2.3. PAH and Related Cores

Tectons with various rigid polycyclic aromatic hydrocarbon (PAH) skeletons provide a wide range of HOFs from structural and functional points of view (Figure 3). Pyrene is one of the most frequently used PAH cores for tectons. 1,3,6,8‐Substituted pyrene‐based HOFs tend to yield robust 2D‐layered HOFs stabilized by strong π···π interactions between pyrene cores [59]. Tectons 23a–d give layered HOFs composed of sql ‐topological 2D networks constructed via the dimerization of carboxy groups [60]. Tecton 24a, possessing BT arms, yields sql 2D‐layered HOF PFC‐111 that promotes the generation rate of single oxygen (quantum yield = 0.98) [61]. Tecton 24b possesses carboxy‐naphthalene forms sql ‐networked, 2D‐layered HOF HOF‐14 with a high SA_BET of 2572 m²g⁻¹ [62]. Tecton 24c possessing m‐carboxyphenyl arms also gives isoreticular HOF FDU‐HOF‐3, showing reversible structural transformation between the crystalline and amorphous phases by exposing to NH₃ gas [63]. Tecton 24d yields 2D‐layered HOF FDU‐HOF‐5 via H‐bonding of DAT groups. The HOF displays selective fluorescence quenching toward diethyl chlorophosphate [64]. Tecton 24e forms an infinite H‐bonded helical chain through N─H···N bonds between pyrazole groups, and these chains interconnected the molecules into 3D HOF HOF‐FJU‐168 that features two distinct channels shaped like a parallelogram and rhombus [65]. The HOF exhibited Xe/Kr separation performance with an excellent balance of adsorption capacity and selectivity. Tecton 25, a configurational isomer of 24a, yields sql ‐networked, 2D‐layered HOF CP‐Py‐1, whose interlayer interaction is not a π···π interaction between pyrene cores, but C─H···π interactions between the hydrogen atoms of peripheral phenylene rings and the pyrene cores [66]. Hexahydrogenated analog 26 gives HOF CP‐Hp‐1 with a nearly identical structure to CP‐Py‐1 [67]. These HOFs exhibited framework shrinkages upon guest removal [66, 68]. Tecton 27a yields a sql ‐networked, 2D‐layered HOF with good crystallinity, while 27b gives a low‐crystalline HOF [69]. These HOFs can produce photocatalytic H₂O₂ under visible light irradiation. NDI‐based tectons yield HOFs with photochromic and electrochromic properties. Tecton 28a forms 1D networks through single hydrogen bonds of carboxy groups, and the network assembles via H‐bonding with [NH₂(CH₃)₂]⁺ ions to give electrochromic HOF ECUT‐HOF‐30 [70]. Tecton 28b yields sql ‐networked, 2D‐layered HOF HOF‐DAT through N─H···O bonds between DAT groups and NDI cores, and complementary N─H···N bonds of DAT groups [71]. The HOF showed excellent electrochemical performance for Na⁺ ion storage. Tecton 29a, possessing a hexaazatrinaphthylene (HATN) moiety, yields layered HOF CPHATN‐1 composed of H‐bonded hxl ‐network sheets [72]. The HOF shows reversible color changes from yellow to reddish‐brown in response to acid vapors, due to the protonation of the HATN cores. Tecton 29b, an isomer of 29a, yields HOF CPBTQ‐1 that also responds to acid vapors [73].

FIGURE 3.

PAH‐based and related tectons.

X‐Shaped tetracarboxylic acid derivatives 30a, 30b, 30c, 31a, and 31b give layered HOFs, CP‐NT‐1 [74], CP‐QX‐1 [74], CP‐PP‐1 [74, 75], X‐PyQ‐1 [76], and Br‐PQ‐1 [77], respectively, composed of H‐bonded, sql ‐topological network sheets. These HOFs undergo drastic structural transformation involving cleavage and rearrangement of H‐bonded networks upon guest removal. Tectons 32a and 32b with similar molecular structures yield HOFs TAAQ‐1 and AQ‐1 with different network structures: the former has the sql ‐network, whereas the latter has a triangle network motif [78]. However, both HOFs undergo the structural transformation with the hydrogen bond reformation. TTF is an attractive redox‐active molecular platform for constructing HOFs with electronic functionality. Tecton 33a yields sql ‐networked, 2D‐layered HOF PFC‐77 that exhibits structural changes involving H‐bonding rearrangement and layer slipping upon guest exchanges [79]. Its redox property changed upon the structural transformation. Tecton 33b gives rigid, sql ‐networked, 2D‐layered HOF HOF‐110, and iodine‐incorporated HOF HOF‐110@I₂ afforded pressed pellets conductivity values of up to 6.0 × 10⁻⁷ S·cm^{– 1} [80]. Tecton 33c forms C≡N···H─C bonds to give flexible HOF HOF‐FJU‐106 that exhibits a temperature‐dependent gate opening phenomenon [81]. This behavior enabled enhanced gas adsorption and separation performance for binary C₃H₆/C₃H₈ mixtures.

Tetra‐ and hexacarboxyphenyl‐substituted tectons possessing a twisted PAH core, such as dibenzo[g,p]chrysene (DBC) and hexaazatriphenylene (HAT), form 3D networks with dia ‐ or pcu ‐topology through the dimerization of carboxy groups, and the networks give 3D frameworks via interpenetration of the networks and shape‐fitted docking of the twisted cores. DBC‐based tectons 34a, 34b, 34c, and 34d yield 6‐fold, 8‐fold, 4‐fold, and 4‐fold interpenetrated HOFs CPDBC‐1, C2N6DBC‐1, C1N4DBC‐1 and C1N5DBC‐1, respectively [82, 83]. CPDBC‐1 retains the original structure after activation, whereas C2N6DBC‐1, C1N4DBC‐1, and C1N5DBC‐1 show activation‐induced structural transformation behaviors depending on the naphthyl substituents upon guest removal [83]. HAT‐based tecton 35a, 35b, 35c, 35d form 4‐fold, 6‐fold, 8‐fold, and 8‐fold, interpenetrated isostructural 3D HOFs CPHAT‐1, CBPHAT‐1, TolHAT‐1, ThiaHAT‐1, respectively, although tecton 35e did not give a HOF [84, 85, 86].

A D _3h‐symmetric tecton 36, possessing a tri(dithiolylidene)cyclohexanetrione unit, forms an anomalistic 3D helical H‐bonded network via the dimerization of carboxy groups, which originated from a slightly larger bite angle between adjacent carboxy phenyl groups in the molecule, to yield 3D HOF CPDC‐1 [87]. Tecton 37 yields solvent‐included porous framework CP‐Phen‐1, in which three of four carboxy groups form H‐bonded dimers to form the ladder‐shaped framework, while the remaining one does not due to geometrical mismatch and alternately interacts with guest molecules through weak H‐bonding [88]. Tecton 38, possessing a twist perylenediimide (PDI) core, forms wave‐like 2D networks through two types of H‐bonding of carboxy groups. The networks are interpenetrated and stacked via π···π interactions between PDI cores to give HOF PFC‐55 [89].

PAH tectons bearing cyano groups tend to form 2D‐networked structures through multiple types of C≡N···H─C bonds between cyano groups and aromatic hydrogen atoms. These networks are assembled to yield flexible HOFs with small pores. Tecton 39 yields HOF‐FJU‐8 possessing 1D channels with a 4.6 × 4.2 Å aperture. The activated HOF exhibited high C₂H₂/CO₂ and Xe/Kr separation performances [90, 91]. Tecton 40 gives 2D‐layered HOF HOF‐TT‐1 possessing 1D channels with a 9.0 × 10.9 Å aperture that shows selective absorption of toluene over methylcyclohexane, accompanied by a change in fluorescence color [92]. Tecton 41 yields robust 2D‐layered HOF HOF‐ZJNU‐1 via donor‐acceptor π···π interactions between the thiophen and benzene rings. The HOF possesses 1D channels with a 3.7 × 4.2 Å aperture and exhibits selective absorption of acetylene and carbon dioxide over methane [93]. Tecton 42 forms a 2D‐layered HOF HOF‐FJU‐119 possessing 1D channels with a 4.9 × 9.7 Å aperture. The activated HOF shows framework expansion to restore its original pore structure upon guest absorption and exhibits selective absorption of benzene over cyclohexane [94]. Tecton 43 forms a H‐bonded 3D network through complementary C≡N···H─C bonds. The network is 3‐fold interpenetrated to yield HOF‐FJU‐1 possessing 1D channels with a 3.4 × 5.3 Å aperture [95, 96]. The activated HOF exhibited tunable flexible‐robust framework properties depending on temperature, resulting in selective adsorption of C₂H₄ over C₂H₆ [96].

Tecton 44 forms HOF Spiro‐4N through C─H···O and C─H···π weak bonds. The HOF displays selective fluorescence quenching for aniline and diphenylamine among various aromatic solvents [97]. Tecton 45 forms HOF‐FJU‐2 through multiple weak interactions. The HOF possesses 1D channels with a 4.8 × 6.1 Å aperture surrounded by a carbazole N─H site, allowing separation of acetone from its mixture with another solvent like methanol [98]. Tecton 46 yields flexible HOF HOF‐NKU‐1 through multiple weak hydrogen bonds, such as C─H···O = C and C─H···C bonds. The HOF exhibits high SO₂/CO₂ separation performance through temperature‐dependent gate‐opening [99].

Tecton 47 yields one of the most porous HOF TH5‐A, composed of a 3D network formed through H‐bonding of quinoxalinedione groups [100]. Tecton 48a forms a hexagonal unit through single O─H···O bonding of carboxy groups, which further interpenetrates with adjacent units through hydrogen bonds between imidazole moieties and carboxy groups to yield 7‐fold catenated HOF RP‐H100 [101]. Alternatively, the same tecton utilizes N─H···O and O─H···N bonding between these two functional groups to give the hexagonal unit; two of these hexagonal units then interpenetrate and connect via O─H···O bonding of carboxy groups to yield another HOF, RP‐H200, possessing hexagonal mesopores with a width of 36 Å [102]. Tecton 48b, an extended analogue of 48a, yields HOF RP‐H101 that is isostructural to RP‐H100, displaying highly balanced volumetric and gravimetric capacities for hydrogen storage [101].

As described above, PAHs and hetero‐PAHs can provide HOFs with optoelectronic functionality in addition to thermodynamic stability through π···π stacking between PAH cores. Particularly, large π‐conjugated or twisted π‐conjugated skeletons can form a robust π···π stacked domain, which serves as the basis for the construction of isoreticular HOFs as discussed later. It is also significant from a photoelectronic point of view, and active research is underway to apply this to photocatalytic HOFs. Particularly, introducing heteroatoms into PAHs results in even richer electronic properties, such as redox behavior and charge transport capabilities. The interaction between the nonbonding electron pairs of heteroatoms and cations enables the modulation of electronic states, opening up possibilities for applications such as chemical sensors.

2.4. Cyclic and Caged Cores

Cyclic and caged tectons can further diversify structures and properties of HOFs (Figure 4). Porphyrin is a useful molecular platform for porous frameworks because it can exhibit diverse properties simply by changing the central metal in the porphyrin core without altering its molecular geometry. Tecton 49a forms a sql ‐networked 2D sheet through the dimerization of carboxy groups. The sheet is assembled in an ABAB stacking manner to yield layered HOF PFC‐71 [103]. Crystallization of 49b from a DMF/methanol mixed solution leads to the formation of 3D‐networked HOF UPC‐H4a with 1D hexagonal channels through multiple types of H‐bonding between DAT groups [104]. In contrast, crystallization from a DMF/acetone mixed solution produces 2D‐layered HOF FDU‐HOF‐2 with rhombic 1D channels. The HOF is composed of a sql ‐topological 2D network sheet formed through head‐to‐head interaction of DAT groups and further head‐to‐waist interactions of DAT groups [105]. Tecton 49c is connected via C─H···N bonds between nitrogen atoms of the tetrazole and hydrogen atoms of the porphyrin moiety to give 2D networks, which are stacked through N─H···N and C−H···N interactions to give 2D‐layered HOF JZS‐1 [106]. Tecton 49d forms 2D networks through C─H···N≡C bonds, and these networks are slipped stacked to give 2D‐layered HOF HOF‐29, which shows exclusive recognition of p‐xylene over its isomers of o‐xylene and m‐xylene triggered by the HOF's structural transformation with the sliding of the layers [107]. Metalloporphyrin‐based tectons 50a–d yield 2D‐layered HOFs, as in the case of non‐metalloporphyrinic tecton 49a, although the stacking structure of 2D networks varies depending on the metals [103]. Tecton 50e also yields 2D‐layered HOF PFC‐73‐Mn, in which water molecules interact with the Mn ions and bridge the porphyrin layers [108]. Tectons 51a and 51b, possessing tetrakis‐3,5‐dicarboxyphenyl groups, give 4‐fold and 2‐fold interwoven 3D networked HOFs, YSH‐8_Ni and YSH‐8_Zn , respectively [109]. The HOFs are further stabilized by π···π interaction among porphyrin cores.

FIGURE 4.

Cyclic and caged molecules‐based tectons.

18‐Crown‐6‐ether (18C6)‐based tetracarboxylic acid 52 is connected through water‐bridged H‐bonding of the carboxy groups to give HOF 2CT‐18C6‐III, which possesses unique 1D water molecule alignments and exhibits proton conductivity [110]. 18C6‐Based hexacarboxylic acid 53 yields layered HOF 3CT‐18C6‐I composed of a H‐bonded hexagonal sheet [111]. The HOF features 1D channels with bottlenecks composed of the 18C6 rings, resulting in proton conductivity. D _3h‐Symmetric macrocyclic molecules 54a, 54b, 54c, 54d, and 54e form robust H‐bonded hexagonal network sheets with three kinds of voids. The sheets are stacked without interpenetration to yield 2D‐layered HOFs T12‐1, T18‐1, Ex‐1, BPEx‐1, and TPEx‐1, respectively [112]. Notably, BPEx‐1 possesses the largest hexagonal 1D pores with a width of 24 Å due to the large overlap of the hexagonal frameworks [113], and TPEx‐1 features the largest hexagonal void with a size of 30 × 34 Å [114]. Cage‐shaped tecton 55 is connected through N─H···N bonds between the NH₂ group and triazine ring to give low‐density (0.54 g cm⁻³) mesoporous HOF 3D‐Cage‐HOF‐1 [115]. Tecton 56 yields various HOFs with different crystal structures, CageHOF‐2α, CageHOF‐2β, CageHOF‐2·DMA, CageHOF·2‐THF, and CageHOF‐2·H₂O, depending on the crystallization conditions, which are attributed to “hinge‐like” rotational motions of the molecules [116].

D _3h‐Symmetric fluorenylidene‐aza[1₆]cyclophane derivative 57 forms wavelike 2D hcb ‐networks through the dimerization of carboxy groups due to its flattened cone‐like geometry, and the networks are interpenetrated to yield MEP‐HOF that shows significant fluorescence quenching for nitrobenzene [117]. Trimeric pyromellitic diimide derivatives 58a and 58b yield 3D networked HOFs, PMC‐Tri‐PMDI and HOF‐Tri‐PMDI‐Br, respectively [118]. The former is constructed by H‐bonding between the hydrogen atoms of phenylene rings and the O═C groups of the imide moieties, whereas the latter is formed via unusual H‐bonding between the C(sp₃)‐H of the cyclohexane rings and the O═C groups. C ₃‐Symmetric tectons 59a, 59b, and 59c are assembled through 3‐fold self‐complementary N─H···O═C bonds to give planar, wrinkled, and square‐octagonal wavy 2D networks, respectively, which are stacked to give 2D‐layered HOFs iPrSM_planar , iBuSM_wrinkled , and iPenSM_wavy [119]. The distorted frameworks exhibited record‐high acetone vapor uptake (up to 617 cm³/g STP at 298 K) with high acetone/methanol selectivity (up to 7.2:1).

Since macrocyclic molecules inherently possess internal cavities, it is possible to construct HOFs with multiple pores of different properties. On the other hand, relatively lower yields of cyclization reactions required for the synthesis of cyclic molecules often hinder the development of macrocycle‐based HOFs.

2.5. Tetraphenylethene

Tetraphenylethene (TPE), which exhibits aggregation‐induced emission, can provide HOFs that possess rich luminescent properties (Figure 5). Tecton 60a forms a 2D sql ‐network sheet via the dimerization of carboxy groups, and the sheets are assembled via tri‐axially interpenetration to yield mechanically‐responsive fluorescent HOF CBPE‐1 [120]. Tecton 60b, in contrast to 60a, yields non‐interpenetrated 2D‐layered HOF CTPE‐1, possessing large rhombic voids with a 33.6 × 55.2 Å aperture [121]. HOF 8PCOM composed of 60c changes its crystal structure responding to different solvent vapors, and DCB‐solvated HOF 8PCOM‐DCB exhibits strong emission intensity owing to the alignment of the dipole moments of the four ketone groups [122]. HOF 8PZ, composed of 60d, possesses outstanding adaptive guest accommodation abilities for 3‐alkylthiophenes (denoted SCX, where X (X = 1−10) represents the number of carbon atoms in its alkyl chain) and shows reversible fluorescence changes upon the release of SCX [123]. Tecton 60e yields two types of HOFs with different topologies, cage‐type HOF and channel‐type HOF, depending on the guest solvent used for crystallization. These HOFs can reversibly switch their structures upon soaking in the corresponding guest solvents, accompanied by changes in fluorescence colors [124]. Tecton 60f is connected by C─H···π interactions, to yield HOF‐FJU‐6 that shows reversible framework shrinkage and expansion via desorption and adsorption of guest molecules, leading to the reversible fluorescence change between blue and green [125]. Tecton 60 g yields two solvated HOFs, ethyl acetate‐solvated HOF (2D‐90) and methanol‐solvated HOF (2D‐82), which exhibit reversible structural transformation between them upon exposure to the solvent vapors [126]. HOF 8PN, composed of 60 h, shows versatile reversible transformations accompanied by changes in its luminescence property upon guest exchange [127]. Tectons 60i–n, featuring substituents comprising pyridine and carboxylic acid groups conducive to hydrogen bond formation, yield HOF‐LIFM‐6, HOF‐LIFM‐7, HOF‐LIFM‐8, HOF‐LIFM‐10, HOF‐LIFM‐11, and HOF‐LIFM‐12, respectively [128]. This series of HOFs exhibits exceptional cell imaging capabilities due to the excellent one‐ and two‐photon excited luminescence properties. HOF 4CN, composed of 60n, exhibits versatile reversible structural transformations and fluorescence changes under the stimulation of solvent vapor and temperature [129]. Tecton 61 forms a 1D column by linking up and down through N─H···O bonds between the amide groups, and the four columns are interlocked in pairs to yield MT‐HOF‐1 [130]. The HOF showed clear separation trends for C₂H₆/C₂H₄. Tecton 62 is connected through C─H···N bonding between the nitrogen atoms of the imidazole groups and the hydrogen atoms of the benzene ring/imidazole of adjacent molecules to give HOF‐T that possesses the ability to capture UO₂ ²⁺ selectively [131].

FIGURE 5.

TPE‐based tectons.

As described above, TPE‐based HOFs alter their dynamic emission colors by structural transformations with the changes in molecular conformation and aggregation states. TPE‐based tectons with weak H‐bonding supramolecular synthons particularly tend to yield flexible HOFs, whose photophysical properties are sensitively modulated by guest absorption/desorption or guest exchange.

2.6. Amine

Triarylamine is a representative platform of nonplanar tritopic tectons (Figure 6). Tecton 63 yields two different HOFs, HOF‐11 and HOF‐16, that are obtained by crystallization from a THF/hexane mixed solution and a methanol solution, respectively. In HOF‐11, all the carboxyl groups form H‐bonded dimers to give 3D networks, followed by forming a multiply‐interpenetrated framework with small 1D channels [132]. In HOF‐16, two carboxyl groups form a H‐bonded dimer, and the other forms hydrogen bonds with two methanol molecules to yield 1D‐helical chains, and these chains are assembled to form 1D channels with exposed carboxylic acid groups [133]. Tecton 64 yields hcb ‐like 2D‐layered HOF HOF‐NBDA(DMA), in which four carboxy groups form classical neutral dimers and the other two form rarely seen COOH···⁻OOC single hydrogen bonds. Dimethylamine (DMA) cation molecules are trapped in the carboxylate of the 2D networks by N─H···O bonds. HOF‐NBDA(DMA) transforms into HOF‐NBDA upon heating, accompanied by the removal of DMA molecules and the reorganization of hydrogen bond dimers [134]. HOF‐NBDA exhibited much higher absorption selectivity for C₂H₆ over C₂H₄ than HOF‐NBDA(DMA), owing to its nonpolar void surface. Tecton 65 forms N─H···O bonds of pyridone groups to give a hexagonal 2D network, and the four monolayer networks interpenetrated to construct 3D HOF PFC‐65 with a hcb ‐topological network [135]. The HOF modified on a glassy carbon electrode showed high sensitivity, stability, and precise identification of dopamine molecules in various environments. Tectons 66a, 66b, and 66c yield a series of isoreticular HOFs DAT‐C₆ ‐HOF‐1,2,3, with (6,3)‐net hcb ‐topology by integrated monomer synthesis‐framework assembly methodology. DAT‐C₆ ‐HOF‐1 displayed highly selective fluorescence recognition of perfluorooctanoic acid over other homologous molecules [136]. Crystallization of 66b from DMSO/methanol and DMF/water mixed solutions gives the polymorph HOFs X‐HOF‐1 [137] and UPC‐HOF‐6 [138], respectively, featuring nearly the same 2D‐layered structure composed of H‐bonded networks with rhombic voids and abnormal hexagon voids. X‐HOF‐1 possessed pressure‐responsive H₂/N₂ separation performance, and UPC‐HOF‐6 exhibited fluorescence change from green to orange‐yellow upon exposure to acetic acid vapors.

FIGURE 6.

Triarylamine‐based tectons.

3. Permanent Porosity

We investigated the Brunauer–Emmett–Teller surface area (SA_BET) of 126 HOFs reported from 2020 to 2025. To facilitate discussion, we classified the HOFs into rigid HOFs that retain their original structures and flexible HOFs that undergo structural transformation after activation. The values of SA_BET are plotted against their publication year (Figure 7). Note that HOFs whose SA_BET has not been reported are excluded from these plots. The data of SA_BET are summarized in Table S1. The HOFs exhibiting exceptionally high SA_BET are mainly those possessing a triptycene skeleton, highlighted as follows. HOF RP‐H101 [101], composed of 48b, shows the highest SA_BET of 3526 m² g⁻¹ among HOFs reported to date. HOF TH5‐A [100], composed of 47, shows SA_BET of 3284 m² g⁻¹, which is the third highest ever reported after T2‐γ [139]. HOF RP‐H200 [102], composed of 48a, exhibits the largest pore size reported to date (3.6 nm) with a SA_BET of 2313 m² g⁻¹. Other HOFs with high SA_BET are ZJU‐HOF‐5 [31], composed of 1,3,5‐tris(3,5‐dicarboxyphenyl)‐2,4,6‐triethylbenzene 2e, with SA_BET of 3102 m² g⁻¹, and HOF‐14 [62], composed of tetrakiscarboxynaphthyl pyrene derivative 24b, with SA_BET of 2573 m² g⁻¹.

FIGURE 7.

BET surface areas of (a) robust HOF and (b) flexible HOF reported in the period from 2020 to 2025.

The SA_BET values are compared among the HOFs constructed by the molecules with common moieties (Table 1). The SA_BET of 1,3,8,11‐substituted pyrene‐based HOFs, which have rigid 2D‐layered structures, tends to increase as their pore size becomes larger. However, this is not always the case; for example, FDU‐HOF‐5 [64] has larger pores than PFC‐1 [59] and HOF‐14 [62], while its SA_BET is smaller, suggesting that framework rigidity after activation also plays a role in SA_BET. The HAT‐based HOFs with rigid structures exhibit large SA_BET as the arm lengths increased (CPHAT‐1< CBPHAT‐1< ThiaHAT‐1), whereas fragile HOF TolHAT‐1 showed small SA_BET [86]. Similarly, in DBC‐based HOFs, structural rigidity significantly influences the SA_BET; robust HOFs such as CPDBC‐1 and C1N5DBC‐1 exhibit larger SA_BET, whereas fragile or flexible HOFs such as C1N4DBC‐1 and C2N6DBC‐1 show smaller SA_BET [82, 83]. In the case of porphyrin‐based HOFs, metalloporphyrin HOFs tend to show larger SA_BET compared to the corresponding metal‐free HOF PFC‐71 [103], due to enhanced framework rigidity, while the type of metal has little effect on SA_BET. The HOFs composed of 5,5′‐substituted bis‐isophthalate derivatives, which have 2D‐layered structures, show larger SA_BET as the core lengths increased (ZJU‐HOF‐59< ZJU‐HOF‐60< ZJU‐HOF‐61< ZJU‐HOF‐62) [56]. Based on the above discussion, not only pore size but also framework rigidity plays an important role in achieving large SA_BET.

TABLE 1.

Comparison of SA_BET values of HOFs composed of tectons with the same skeleton.

Skeleton	HOF	SABET (m2g−1)	Stability	Tecton	Refs.
1,3,6,8‐Substituted pyrene	PFC‐1	2122	robust	23a	[59]
	HOF‐101HCF	1148	robust	23a, b, d a	[60]
	PFC‐111	1177	robust	24a	[61]
	HOF‐14	2573	robust	24b	[62]
	FDU‐HOF‐3	310	robust	24c	[63]
	FDU‐HOF‐5	396	robust	24d	[64]
	HOF‐FJU‐168	1016	robust	24e	[65]
Hexaazatriphenylene (HAT)	CPHAT‐1	649	robust	35a	[84]
	CBPHAT‐1	1288	robust	35b	[85]
	TolHAT‐1	440	fragile	35c	[86]
	ThiaHAT‐1	1394	robust	35d	[86]
Dibenzo[g,p]chrysene (DBC)	CPDBC‐1	1548	robust	34a	[82]
	C2N6DBC‐1	113	fragile	34b	[83]
	C1N4DBC‐1	281	flexible	34c	[83]
	C1N5DBC‐1	1300	robust	34d	[83]
Porphyrin	PFC‐71	600	robust	49a	[103]
	PFC‐72‐Co	1646	robust	50a	[103]
	PFC‐73‐Cu	1714	robust	50b	[103]
	PFC‐73‐Zn	1856	robust	50c	[103]
	PFC‐73‐Ni	1720	robust	50d	[103]
	PFC‐73‐Mn	786	robust	50e	[108]
	JZS‐1	222	flexible	49c	[106]
	YSH‐8Ni	280	robust	51a	[109]
	YSH‐8Zn	1665	robust	51b	[109]
5,5′‐Substituted bis‐isophthalate	ZJU‐HOF‐59	938	robust	21a	[56]
	ZJU‐HOF‐60	1057	robust	21b	[56]
	ZJU‐HOF‐61	1424	robust	21c	[56]
	ZJU‐HOF‐62	1534	robust	21d	[56]

^a HOF‐101HCF is a mixed‐HOF composed of tectons 23a, 23b, and 23d.

Flexible HOFs have rapidly increased in number since around 2022. They tend to exhibit smaller SA_BET than rigid HOFs because they exhibit a reduction in pore size and, sometimes, decreasing of crystallinity upon solvent removal (i.e., activation). On the other hand, by utilizing their small pore sizes and/or flexible pore transformations (e.g., pore expansions and gate‐opening behaviors), many flexible HOFs have achieved selective adsorption of target molecules and efficient separation of mixed gases (Table 2). For example, in ZJU‐HOF‐8a, its flexible pore pockets selectively open to accommodate larger alkanes like n‐C₄H₁₀ and C₃H₈ due to their stronger binding affinities. This adsorbate‐induced framework expansion allows for significantly higher uptake of these molecules, resulting in high adsorption selectivity over C₂H₆ and CH₄, which cannot trigger the gate‐opening behavior [44]. HOF‐NKU‐1 exhibits a shape‐memory effect accompanied by a guest‐adaptive pore expansion that is specifically triggered by strong intermolecular interactions with SO₂ molecules. Because CO₂ interacts too weakly to induce this structural transformation, the material achieves highly selective SO₂ capture through the dense packing of SO₂ dimers within the expanded cavities [99].

TABLE 2.

Flexible HOFs exhibiting pore expansion and gate‐opening upon gas adsorption and their gas selectivity.

HOF	SABET (m2g−1)	Tecton	Gas selectively	Ref
ZJU‐HOF‐8	863	10	C3H8, n‐C4H10 > CH4, C2H6	[44]
ZJU‐HOF‐60	1057	21b	C2H6 > C2H4	[57]
HOF‐FJU‐8	375	39	C2H2 > CO2, Xe > Kr	[90, 91]
HOF‐FJU‐100	328	12c	C2H2 > CO2	[46]
HOF‐FJU‐119	106	42	C6H6 > C6H12	[94]
HOF‐FJU‐106	198	33c	C3H6 > C3H8	[81]
HOF‐FJU‐1	385	43	C3H6 > C3H8, C2H4 > C2H6	[95, 96]
HOF‐40	234	9	Xe > Kr	[43]
HOF‐NKU‐1	200	46	SO2 > CO2	[99]

Next, we overview the permanent porosity of HOFs from the perspective of their H‐bonded supramolecular synthons. Carboxylic acid‐based HOFs tend to exhibit larger SA_BET than HOFs using other synthons, because self‐complementary hydrogen bonds of the carboxy groups are highly directional and relatively stable (ca. 65 kJ mol⁻¹), allowing facile formation of stable HOFs with large voids. In particular, 2D‐layered HOFs and 3D‐HOFs stabilized by strong π···π interactions and shape‐fitted π‐conjugated cores often possess large channels, resulting in a high SA_BET value. Recently, carboxylic acid‐based flexible HOFs have attracted considerable attention, which show the rearrangement [45, 74, 75, 76, 77, 78, 79], shrinkage/expansion [66], and sliding [44, 140] of H‐bonded networks upon guest absorption and desorption. Such flexible HOFs enable selective gas absorptions and guest‐responsive emissions. Most of the DAT‐based HOFs reported in recent years showed small SA_BET, because these HOFs were constructed through head‐to‐waist interactions of DAT groups, resulting in a small pore size. Cyanide‐based HOFs tend to possess flexible pores due to weak C≡N···H─C bonds between a cyano group and a hydrogen atom of peripheral phenylene groups. Their flexible structures enable selective gas absorption and separation of mixed gases via pore expansion and gate opening mechanisms. While amide‐based HOFs have been extensively studied primarily using triptycene‐based mesoporous frameworks [139, 141], recent efforts have led to the construction of HOFs from amide derivatives with diverse core moieties, exhibiting a wide range of porosities. Over the past five years, HOFs using novel hydrogen bonding synthons, such as pyridine [38, 142], aldehyde [143], pyrazole [65, 144, 145], tetrazole [106], ketone [122], and chloro groups [99], have been reported. Most of them have relatively small pores and small SA_BET, and several systems have been demonstrated to show selective gas absorption properties [65, 98, 99, 144, 145].

4. Updated Concepts of HOFs

4.1. Isostructural (Isoreticular) HOFs

Isostructural porous frameworks, whose molecular arrangements and network topology are identical but with different pore sizes and/or chemical and physical properties, provide a systematic library of functional porous materials [146, 147]. Therefore, constructing such frameworks is considered a touchstone for reticular chemistry [148]. In the fields of MOFs and COFs, pore expansion can be readily achieved by elongating the length of ligand molecules or spacer moieties [149, 150]. In contrast, in the case of HOFs, it remains difficult to obtain isostructural frameworks even when using tectons with similar structures, due to the weakness of hydrogen bonds and the diversity in molecular packing modes. For example, tectons 54, D _3h‐symmetric π‐conjugated planar building blocks possessing three o‐bis(4‐carboxyphenyl)benzene moieties, form the same hxl ‐topological sheets; however, their stacking manners are different due to the lack of specific interlayer interactions (Figure 8) [112]. Similarly, tetraphenylethene‐based tetracarboxylic acids 60a and 60b form common 2D sql ‐networks but give different packing structures: the former has a polycatenated structure, whereas the latter forms a slipped‐stacked, layered structure [120, 121].

FIGURE 8.

Selected crystal structures of T12‐1 (54a), T18‐1 (54b), Ex‐1 (54c), BPEx‐1 (54d), and TPEx‐1 (54e). The tectons form the same hxl ‐topological sheet, while the sheets stack in different ways depending on the cores. The butoxy side chains of molecule 54e are omitted.

On the other hand, utilizing shape‐fitted docking of the molecules is a useful strategy to produce isostructural frameworks. HAT‐base hexacarboxylic acids 35a–d give nearly isostructural HOFs due to the shape‐fitted docking of the HAT cores, and the size of the triangle pore can be expanded up to 20.4 × 19.2 Å (Figure 9a) [84, 85, 86]. DBC‐base tetracarboxylic acids 34a–c that possess 1,4‐, 1,5‐, and 2,6‐substituted carboxynaphthyl groups also yield isostructural HOFs with different pore geometries due to the docking of the DBC cores (Figure 9b). They show different structural stabilities against activation [82, 83]. Most porphyrin‐based tetracarboxylic acids yield sql ‐networked, 2D‐layered HOFs, in which stacking manners of the 2D networks can be either identical or different depending on the metal encapsulated within the porphyrin ring [103]. 1,3,6,8‐Substituted pyrene‐based tetracarboxylic acids 23a, 23c, and 24b yield isostructural 2D‐layered HOFs through strong π···π stacking of the pyrene core, which enables the modification of pore size and pore environment by changing the arm moieties of the tectons (Figure 9c) [59, 60, 61, 62, 63]. Tetracarboxylic acids 30a–c with naphthalene, quinoxaline, and pyrazinopyrazine cores, respectively, yield isostructural 2D‐layered HOFs, where sql ‐networked sheets were slip‐stacked in a closely similar manner through C─H···π interactions (Figure 9d), while these HOFs transformed into different structures upon the guest removal [74]. Terphenyl analogs 18b, 19, and 20 with π‐conjugated moieties yield nearly isostructural 2D‐layered HOFs, where the 2D networks were stacked through π···π interactions between the core moieties (Figure 9e) [53, 54, 55, 58].

FIGURE 9.

Recently reported isostructural HOFs. (a) HAT‐based HOFs CPHAT‐1 (35a), CBPHAT‐1 (35b), TolHAT‐1 (35c) and ThiaHAT‐1 (35d). (b) DBC‐based HOFs CPDBC‐1 (34a), C2N6DBC‐1 (34b) and C1N5DBC‐1 (34d). (c) Pyrene‐based HOFs PFC‐1 (23a), HOF‐101N (23c), and HOF‐14 (24b). (d) HOFs with naphthalene and its analogue cores CP‐NT‐1 (30a), CP‐QX‐1 (30b), CP‐PP‐1 (30c). (e) Terphenyl analogs‐based HOFs ABTPA‐1 (19), NTTA‐1 (20), BTTA‐1 (18b).

Isostructural HOFs using supramolecular synthons other than carboxylic acid dimers have also been reported. Tetrahedral pyrazole derivatives with different cores yield isostructural HOFs, in which the molecules are connected through N─H···N bonding tetramers to give 4‐fold interpenetrated diamond networks [145]. Tetra‐imine oligomers form isostructural HOFs with 1D quasi‐square pores, formed through C─H···π interactions and further stabilized via multiple hydrogen bonds [143].

4.2. Mixed HOFs

Multicomponent mixed frameworks, in which two or more kinds of tectons are combined in various ratios, have attracted attention due to their ability to fine‐tune material properties and generate new functions via compositional variation, and have been widely studied in MOFs and COFs [151, 152]. Their strong coordination or covalent bonds facilitate the construction of mixed systems, leading to numerous reported examples. In contrast, HOFs tend to form thermodynamically stable single‐component frameworks due to their relatively weak, reversible hydrogen bonds, making the construction of mixed HOFs challenging. This challenge, however, has recently been addressed by using tectons that can form isostructural HOFs as components of mixed HOFs (Figure 10) [153]. In particular, carboxylic acid‐based mixed HOFs often exhibit high crystallinity, allowing detailed structural analysis and revealing correlations among composition, structure, and properties, thereby offering promising potential for further development.

FIGURE 10.

A combination of tectons to prepare mixed HOFs.

As a pioneering work, Xue and coworkers reported TC‐HOFs composed of 17b and 18c with DAT moieties [51]. The two molecules homogeneously distribute within the HOF, and the fluorescence color of bulk crystals of the HOF can be tuned by the component ratio due to the energy transfer from donor 17b to acceptor 18c (Figure 11). In this system, however, the precise structural analysis using single crystals was not reported, presumably due to the low crystallinity or size limitation of crystals. Our group demonstrated, for the first time, that pyrene‐ and hexahydropyrene‐based tectons 25 and 26, respectively, give single crystals of mixed HOFs CP‐HpPy‐1 [67]. Single‐crystal X‐ray diffraction analysis and microscopic Raman spectroscopy revealed that the two kinds of tectons are contained in a single crystal with a similar composition ratio to the mixing ratio of the initial solution, while they are distributed heterogeneously and/or gradationally rather than homogeneously throughout the single crystal. (Figure 12). Since CP‐HpPy‐1 was reported, various mixed HOFs and their property modulations by compositional variation of tectons have been demonstrated. They are mainly composed of carboxylic acids. 1,3,6,8‐Substituted tetrakis(carboxyphenyl)pyrene derivatives bearing various functional groups (─H, ─CH₃, ─NH₂, and ─F) on phenylene arms (23a, 23b, 23c, and 23d, respectively) yield multicomponent mixed HOFs MTV‐HOF with rigid structures and permanent porosities [60]. MTV‐HOF exhibits tunable hydrophobicity depending on the component ratio in the frameworks and superior selective sorption properties for mustard gas simulant, 2‐chloroethyl ethyl sulfide (CEES), from moisture (Figure 13). [1,1′:4′,1″‐Terphenyl]‐3,3″,5,5″‐tetracarboxylic acids 67a, 67b, and 67c, possessing functional groups (─H, ─NH₂, and ─Cl, respectively) on the central benzene, yield mixed HOFs PFC‐76‐X with tunable water adsorption performance by component ratios, and PFC‐76‐NH2–67% with an optimized balance of pore size and adsorption sites achieves the highest water uptake within a narrow humidity range [154]. Terphenyl‐based tetracarboxylic acids 18b and 20, possessing naphthalene and benzothiadiazole cores, yield fluorescent mixed HOF B_TN_T‐1 (Figure 14) [55]. Depending on the composition and distribution of the tectons, single crystals of B_TN_T‐1 exhibit a wide range of fluorescent colors, ranging from single colors such as blue, white, green, and yellow to dual emission colors such as blue‐green and green‐yellow.

FIGURE 11.

(a) Schematic representation of the co‐crystallization of 17c and 18c and energy transfer in TC‐HOF with a low content of benzothiadiazole (BTAD) derivative 18c. (b) Fluorescence images, CIE dots, and plot of emission maxima vs the fraction of 18c for TC‐HOF with various component ratios. Reproduced with permission from ref [51]. Copyright 2022, American Chemical Society.

FIGURE 12.

Formation of mixed HOF CP‐HpPy‐1 composed of tectons 25 and 26. (a) Crystal structure of CP‐HpPy‐1, where yellow‐colored parts indicate disordered propylene moieties of hexahydropyrene. (b) Anisotropic displacement ellipsoid plot with 50% probability for CP‐HpPy‐1(0.5, 0.506), in which the carbon atoms at the 1,2,3‐ and 6,7,8‐positions are disordered in two structures corresponding to CP‐Hp and CP‐Py, with occupancies of 0.506 and 0.494, respectively. (c) Composition in single‐crystals of CP‐HpPy‐1(x, y), where x and y denote the molar fraction or occupancy of CP‐Hp in the initial solution and the resultant single crystal, respectively. For a certain x, the plots in different colors (i.e., cyan and dark blue) indicate that single crystals were selected from different batches. (d) microscopic Raman spectra of single‐component HOF crystals of CP‐Hp‐1 and CP‐Py‐1 and mixed HOF CP‐HpPy‐1(x = 0.5), where spectra 1, 2, 3, and 4 were recorded at different positions numbered on the photographs of the single crystal shown at the top. Reproduced with permission from ref [67]. Copyright 2022 Wiley‐VCH GmbH.

FIGURE 13.

Formation of multi‐component HOF MTV‐HOF based on 23a, 23b, 23c, and/or 23d. (a) Four H₄TBAPy−R (R = H, CH₃, NH₂, and F) tectons and MTV‐HOF‐101 are synthesized with each other. (b) Schematic diagram of the structure of MTV‐HOF‐101. (c) Comparison of the water and CEES adsorption of MTV‐HOF‐101 at 0.8 kPa. HOF‐101‐C, HOF‐101‐N and HOF‐101‐F means HOF‐101‐R (R = CH₃, NH₂ and F). Reproduced with permission from ref [60]. Copyright 2023 Wiley‐VCH GmbH.

FIGURE 14.

The crystal structure and solid‐state fluorescence properties of mixed HOF B_TN_T‐1, composed of tectons 18b and 20. (a) Packing diagrams of B_TN_T‐1. (b) Anisotropic displacement ellipsoid plot of B_TN_T‐1 (0.3, 0.395) with 50 % probability, in which the core moieties are disordered in two structures corresponding to BTTA (18b) and NTTA (20), with occupancies of 0.395 and 0.605, respectively. (c) Fluorescence photographs of bulk crystals of NTTA‐1, BTTA‐1, and B_TN_T‐1(x) (x = 0.1, 0.3, 0.5, 0.7) HOFs, where x values show the molar fraction of BTTA in the initial solution. (d) Fluorescence photographs of single crystals whose composition ratios were determined by SCXRD. (e) Determination of composition ratio for several positions in inhomogeneous single crystals showing blue‐green and light green‐dark yellow by local SCXRD analysis. y denotes the crystallographically determined occupancy of 18b. Emission photographs and spectra were obtained of representative single crystals of the synthesized and studied HOFs: NTTA‐1 (crystal‐A), BTTA‐1(crystal‐B), and B_TN_T‐1(0.3): blue (crystal‐C), white (crystal‐D), blue‐green (crystal‐E), and green (crystal‐F). The points on the crystal indicate the position from which the emission spectra were recorded. Reproduced with permission from ref [55]. Copyright 2024 Wiley‐VCH GmbH.

Not only static property modulation but also dynamic property modulation has been achieved in mixed HOFs. CP‐HpPy‐1 undergoes structural transformation from the open framework into the shrunk framework (CP‐HpPy‐2) by spontaneous release of the solvent molecules included in the pores. Interestingly, the transformation behaviors strongly depend on the composition ratio (Figure 15) [68]. The transformation speed changed not proportionally but nonlinearly with the composition ratio. The maximum speed of the transformation is observed when the composition ratio of tectons 25 and 26 is 1:1.

FIGURE 15.

Dynamic structural transformation of mixed HOF CP‐HpPy‐1(0.5) to CP‐HpPy‐2(0.5) composed of tectons 25 and 26. Representative crystal structures of (a) CP‐HpPy‐1(0.5, 0.506) and (b) CP‐HpPy‐2(0.5, 0.254) revealed by SCXRD analysis, where molecules of included DCB are omitted. Disordered structure of the hydropyrene (yellow and green) and pyrene (red) core in (c) CP‐HpPy‐1(0.5, 0.506) and (d) CP‐HpPy‐2(0.5, 0.254). (e) Morphological changes of a single crystal of CP‐HpPy‐1(0.5) after wiping off bulk solvent from the surface of the crystal. (f) Cracking in a single crystal of the framework upon the structural transformation. (g) Schematic diagram for the transformation speed of CP‐Hp‐1, CP‐Py‐1, and CP‐HpPy‐1(x). The speed of CP‐Hp‐1 is larger than that of CP‐Py‐1, while the speed of CP‐HpPy‐1(x) changes in a nonlinear manner depending on the composition ratio of CP‐Hp and CP‐Py. CP‐HpPy‐1(0.5) exhibits the maximum transformation speed. Reproduced with permission from ref [68]. Copyright 2025 Wiley‐VCH GmbH.

Mixed HOFs with heterogeneous structures, such as block‐type or core–shell‐type architectures, have also been reported, although only in a few cases. Block‐type HOF systems that exhibit multimodal luminescent responses to external stimuli have been demonstrated for application in high‐security anti‐counterfeiting and information coding (Figure 16). Tetraphenylethene derivatives 68a and 68b produce the triblock single crystals of HOF heterostructure by a H‐bond‐assisted epitaxial growth method. The adopted HOFs, HOF‐FJU‐39 and HOF‐FJU‐40, exhibit distinct responsive characters of thermochromism via the framework transformation and acidichromism via protonation, respectively, which endow the HOF heterostructures with multiple emission modes responsive to external stimuli [155]. Tetraphenylethene‐based 69a and 69b with thiophen ring and furan ring, respectively, are integrated by an epitaxial growth method to yield monochromatic HOF heterostructures composed of HOF‐FJU‐107 and HOF‐FJU‐109. The hetero HOF exhibits partial fluorescence response under heating, due to the ring‐opening reaction of furan moieties [156]. Two dumbbell‐shaped tectons 70 and 71 yield bulk crystals of mixed HOFs BHJ HOFs, showing composition‐dependent fluorescence color from blue to red via white. Furthermore, sequential crystallization of the tectons gives rod‐like triblock heterojunction HOFs THJ HOFs that act as an excellent platform for information encryption and anticounterfeit [157].

FIGURE 16.

Multi‐block HOF crystals composed of tectons 68a and 68b. (a) PL spectra of HOF‐FJU‐39 and HOF‐FJU‐39b. (b) PL spectra of HOF‐FJU‐40 and HOF‐FJU‐40H. (c) Schematic illustration of the hydrogen bond‐assisted epitaxial‐growth process of the HOF heterostructure. (d) PL images of HOF heterostructures under thermal and acid stimulations in different orders. (e) Proof‐of‐concept demonstration of the smart‐responsive HOF‐heterostructure‐based photonic barcodes for anticounterfeiting application. Reproduced with permission from ref [155]. Copyright 2023 Wiley‐VCH GmbH.

Tectons 2a and 4b yield three types of core–shell HOFs by rapid sequential crystallization (Figure 17). Core‐shell HOFs DFA‐TPY@P2‐TPY and P2‐TPY@P2‐TPY are constructed by the single component of 4b and exhibit different crystal morphologies. On the other hand, HOF‐3COOH@P2‐TPY is composed of two kinds of tectons, 2a and 4b [158]. These core‐shell HOFs exhibit superhydrophobicity and trapping abilities for the capture of persistent water contaminants such as oils and microplastics.

FIGURE 17.

Morphological modulation of mixed HOFs. (a) Schematic representation of the sequential crystallization of FA‐TPY and DFA‐TPY with tecton 4b. SEM images of (b) FA‐TPY and (c) DFA‐TPY. Scale bars (from left to right): (b) 100 µm, 100 µm, (c) 100 µm, 10 µm. (d) Schematic representation of the sequential crystallization of HOF‐3COOH@P2‐TPY using tectons 2a and 4b. Optical (Left) and SEM (Right) images of (e) HOF‐3COOH and (f) HOF‐3COOH@P2‐TPY. Reproduced from ref [158].

5. Conclusion and Perspectives

Fifteen years have passed since porous molecular crystals formed by the assembly of tectons via hydrogen bonding were redefined as “hydrogen‐bonded organic frameworks” in 2011. During this time, a wide variety of constituent molecules have been synthesized, and numerous HOFs have been reported. Tectons, which feature a benzene ring as a core and incorporate three to six aryl arms bearing H‐bonding groups at various positions, are one of the simplest families of tectons. PAH‐ and hetero‐PAH‐based HOFs are thermodynamically stabilized by the shape‐persistent, rigid skeletons and the extensive π–π stacking between the cores. Furthermore, due to the tuned electronic states of the cores, such HOFs exhibit photoelectronic properties, which can lead to applications such as photocatalysis.

• ▪Focusing on H‐bonding functional groups that form supramolecular synthons, the DAT group was the mainstream choice for HOFs with permanent porosity in the early days. After it was demonstrated that even carboxylic acid dimers could maintain permanent porosity, numerous HOFs composed of carboxylic acid derivatives have been reported. In addition, pyrazole and pyridine have also been continuously utilized. A notable trend in recent years is applying tectons in which cyano groups are introduced as H‐bonding sites. The weak H‐bonding of such groups facilitates structural changes in HOFs, making it ideal for constructing open‐gate or breathing‐type HOFs. These HOFs are attracting significant attention as materials capable of selectively adsorbing and separating specific chemical species from mixtures of isomers and analogs of gases and volatile hydrocarbons.
• ▪Some important concepts on HOFs have also been developed. A series of isoreticular (isostructural) HOFs can be constructed by utilizing a combination of arms with various lengths and skeletons with similar geometric structures but different properties. Isoreticular HOFs share identical geometric characteristics of their voids, allowing the series to undergo stepwise variation in pore size alone, or conversely, maintaining the same pore shape and size while altering the chemical properties of the pore surfaces. Such an isoreticular HOF library can be applied as tailor‐made molecular sieves and catalysts, which is expected to grow significantly. To tune functionality and create new ones, multicomponent HOFs with non‐stoichiometric mixing ratios of tectons are being actively researched. Although there are still challenges in controlling how these components mix, if this can be achieved, it will become a powerful platform for constructing a library of functional porous crystals. As mentioned in the section on cyanide‐based HOFs, flexible HOFs that utilize the reversible formation and breaking of hydrogen bonds have been the subject of very active research in recent years. The correlation between their dynamic behavior and the chemical structure of the tectons has finally begun to emerge. This is a category of HOFs that is poised for further development. Currently, porous frameworks, centered on MOFs, are beginning to transition into industrial applications.

Due to their inherent characteristics, HOFs are relatively easy to synthesize as large single crystals, despite their low mechanical strength, making them an ideal platform for demonstrating various new properties and functions of porous materials. Furthermore, because they can be easily recovered, purified, and recycled, demand for them as sustainable materials is expected to grow significantly in the future.

Author Contributions

Taito Hashimoto: writing – original draft, writing – review and editing, and conceptualization. Ichiro Hisaki: writing – original draft, writing – review and editing, and conceptualization.

Conflicts of Interest

The authors declare no conflicts of interest

Supporting information

Supporting File 1: anie73521‐sup‐0001‐SuppMat.docx.

Table for detailed information on HOFs with permanent porosity [159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172].

Biographies

Taito Hashimoto received his B.Eng. degree in 2022 and M.Eng. degree in 2024 from Osaka University. He is currently a doctoral student at the Graduate School of Engineering Science, the University of Osaka. His research interests include the design and structural investigation of multicomponent porous crystals and their dynamic behaviors.

Ichiro Hisaki received his Ph.D. degree from Osaka University in 2005 under the supervision of Professor Dr. Yoshito Tobe. After being a JSPS postdoctoral researcher in the group of Professor Dr. Atsuhiro Osuka at Kyoto University, he joined the group of Professor Dr. Mikiji Miyata at Osaka University in 2005 as an assistant professor. In 2018, he moved to the group of Professor Dr. Takayoshi Nakamura at Hokkaido University as an associate professor. Since 2020, he has been a professor at the Graduate School of Engineering Science, the University of Osaka. His current interest is the development of porous functional crystalline materials.

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article