Introduction

Crystalline porous materials made of molecular constituents networked through intermolecular hydrogen bonds (H-bonds) are termed hydrogen-bonded organic frameworks (HOFs) [1] by analogy to metal organic frameworks (MOFs) and covalent organic frameworks (COFs). Reversible bond formation frequently results in framework materials with crystallinities higher than those of other framework materials. HOFs are a subset of porous molecular crystals (PMCs) [2,3,4,5,6,7,8,9] and are particularly excellent materials from the perspective of preorganization of the frameworks. Namely, their structures are more readily designed than those of other PMCs using supramolecular synthons, which were defined by Desiraju as “structural units within supermolecules which can be formed and/or assembled by known or conceivable synthetic operations involving intermolecular interactions.” [10] The frameworks assembled by H-bonding also have other names, such as supramolecular organic frameworks (SOFs) [11], noncovalent organic frameworks (nCOFs) [12], and porous organic salts (POSs) [13], depending on what structural features are of interest.

In 1969, Duchampe and Marsh reported the first crystal structure of a honeycomb network composed of 1,3,5-benzenetricarboxylic acid (trimesic acid) [14]. The molecules were networked through self-complementary H-bonded dimerization of the carboxy groups. Similarly, a H-bonded diamondoid network of adamantane-1,3,5,7-tetracarboxylic acid was reported by Ermer in 1988 [15]. These were the pioneering studies of networked supramolecular architectures using directional H-bonds, although they have no accessible pores inside the crystals due to interpenetration of the networked structures. Many H-bonded, networked architectures encapsulating solvents or other guest molecules have been reported since the 1960s as inclusion crystals made from the supramolecular synthons of 2-pyridone [16], alcohols [17], carboxylic acids [18, 19], diaminotriazine (DAT) derivatives [20], and others [21,22,23,24,25,26,27]. It should also be mentioned that the conventional inclusion crystals are composed of low symmetry molecules such as steroidal bile acid derivatives [28,29,30] and dumbbell-shaped diols [31], which provide inclusion spaces in the crystal.

In 1997, Wuest et al. reported the structure of an inclusion crystal formed from a tetraphenylmethane derivative with DAT groups [20] and implied that the crystal retained the structure after removing solvent molecules from the voids. In 2011, Chen et al. reinvestigated the porosity of the crystals and named the material HOF-1 [1]. HOF-1 was the first example of an HOF to demonstrate selective adsorption of C2H2 over C2H4. Since then, various HOFs have been constructed via H-bonding of various functional groups bonded to highly symmetric host molecules. The general features of HOFs are as follows:

  1. (1)

    Easy to make: HOFs can be prepared via facile solvent processes, such as recrystallization and even mechanical reactions.

  2. (2)

    High crystallinity: The obtained crystals have large domains with high crystallinity and are often formed as large single crystals.

  3. (3)

    Reusability: The HOFs can be reuses and regenerates via solvent processes such as redissolution.

  4. (4)

    Structural flexibility: Weak and reversible H-bonds provide the HOFs with structural diversity and flexibility.

Among them, structural flexibility and diversity are inextricably linked and lead to fragility and low designability. The removal of included molecules frequently causes a structural transition and consequent collapse of the structure, resulting in loss of the porosity and crystallinity. The H-bonded groups may form H-bonds with solvent molecules, resulting in the formation of unexpected, solvated, nonporous structures. This unpredictability limits the designability of HOFs. However, these issues have been overcome with design strategies, including additional use of intermolecular interactions other than H-bonding, such as π-π stacking of rigid π-conjugated skeletons or charge-assisted H-bonds formed between acidic and basic components [32, 33].

To date, several excellent review articles have been published on the design strategies, properties, and applications of HOFs [34,35,36,37,38,39,40,41,42,43]. This review, therefore, is focused on the crystallographic structural details of HOFs composed of carboxylic acid derivatives.

Carboxy groups are easily prepared by hydrolyzing esters or cyano groups. Due to these facile syntheses and moderate directivity (linearity) of the resulting H-bonded motif, the group has been used as a molecular glue for constructing supramolecular architectures. Although it forms some supramolecular synthons [10], the self-complementary dimer shown in Fig. 1a is used for HOF construction in most cases because of its high probability and designability [44, 45]. For example, it is reasonable to expect that planar tritopic, tetratopic, and hexatopic carboxylic acid derivatives can form honeycomb (hcb), square lattice (sql), and hexagonal (hxl) topological networks, as shown in Fig. 1d–f, respectively, where the three-letter symbols in italic bold font denote the network topology of the frameworks and have been used for classification of the frameworks [46]. Note that a H-bonded truncated dimer (Fig. 1b) or ladder-like chain (Fig. 1c) can be formed depending on the molecular structures and crystallization conditions, resulting in other types of network structures. Moreover, the molecular conformation also affects the network topology: for example, twisted tetratopic carboxylic acids tend to form HOFs with diamonded (dia) topological three-dimensional (3D) networks.

Fig. 1
figure 1

Three types of typical H-bonding with carboxy groups: (a) complementary dimer, (b) truncated dimer, and (c) ladder-like chain bonding of carboxylic acids. (d) Honeycomb (hcb), (e) square (sql), and (f) hexagonal (hxl) topological networks composed of respective planar tritopic, tetratopic, and hexatopic carboxylic acids

Full size image

Tricarboxylic acids

Examples of tritopic carboxylic acid tectons include trimesic acid (1) [14, 18, 19, 47, 48], 1,3,5-tris(4-carboxyphenyl)benzene (2) [49, 50] and its derivatives 3–9 [51, 52] and 13 [53], tris(4-carboxyphenyl)amine (10) [54,55,56,57], 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine (11) [58], and 2,5,8-tris(4-carboxyphenyl)heptazine (12) [59], as shown in Fig. 2. Among these, C3-symmetric molecules with planar π-conjugated cores can form H-bonded hcb networks, which are assembled with π-interactions to construct interpenetrated HOFs in many cases and noninterpenetrated layered HOFs in a few cases.

Fig. 2
figure 2

Chemical structures of the tricarboxylic acid tectons that form HOFs. He names of the resultant HOFs are also presented at the bottom

Full size image

It is well known that 1 gives nonporous C2/c crystals composed of interpenetrated hcb undulated networks connected through dimerized carboxy groups. The crystal is the first structure reported for 1 [14]. Since then, a number of crystal structures have been reported for 1, such as the noninterpenetrated layered structure with the P31 (or P32) space group reported by Herbstein et al. [18]. Also, Day and Cooper et al. discovered eight new “hidden” polymorphs and/or pseudopolymorphs of 1 using crystal structure prediction (CSP) methodology combined with robotic crystallization screening [47], and revealed that one of them, pentane-solvated TMA_2-33 with the P3121 space group, was converted to a new solvent-free HOF, δ-TMA, with the C2/m space group. δ-TMA had a noninterpenetrated layered structure with AB stacking (Fig. 3a). Banerjee et al. also found that the P3121 crystals of 1, TMA Form II, obtained from a THF solution had a noninterpenetrated layered structure with an ABC stacking pattern (Fig. 3b), and they demonstrated that this form converted to the interpenetrated form (TMA Form I) at room temperature via intermediate structures, which was confirmed by PXRD and FESEM analyses [48].

Fig. 3
figure 3

Crystal structures of HOFs composed of tritopic carboxylic acids. a δ-TMA formed from 2, b TMA_2-33 or TMA Form II formed from 2, c TCPB-OMe formed from 7, d TCPB-Me formed from 6, e HOF-12 formed from 3, f TCPB-MeOMe formed from 8, g TCPB-NO2 formed from 4, h TCPB formed from 2, and (i) TCPB-NH2 formed from 5. (Top) Hexagonal network, (Middle) top view of assemblies, and (bottom) side view of assemblies. Disordered moieties were omitted for clarity

Full size image

The HOFs composed of tectons 2 and 3–9 have homotypic hcb network sheet motifs formed through H-bonded dimers. However, the assembling mechanisms of the sheets differed depending on the substituted functional groups bonded to the central benzene ring. The resultant structures were categorized into four distinct types: simple stacking, single-layer offset interpenetration, double-layer offset interpenetration, and rotated-layer interpenetration. Only 7 formed a simply stacked noninterpenetrated assembly for the HOF TCPB-OMe (Fig. 3c) [52], while HOFs composed of tectons 3, 6, and 7 (HOF-12, TCPB-Me, and TCPB-MeOMe, respectively) had structures with single-layer offset interpenetration, which is the most common assembly manner for the tritopic carboxylic acid tectons (Fig. 3d–f). The parallel layers slip along each other, leading to a gap into which one layer is interpenetrated. HOF TCPB-NO2, on the other hand, showed a structure with double-layer offset interpenetration (Fig. 3g) [52]. HOFs TCPB and TCPB-NH2 exhibited rotated-layer interpenetration structures (Fig. 3h, i) [49, 52]. This assembly involved a two-axis interpenetration of specific layered networks, which were classified as two distinct layers (the A and B layers of simple stacking hexagonal sheets). The B layer was a rotated layer comprising the simple stacking hexagonal sheet. The B layer was rotated by 60° relative to the A layer. The number of layers varied depending on the constituent molecules: three and four layers for TCBP and three and three layers for TCBP-NH2.

Two HOFs composed of 10 were reported, that is, IISERP-HOF1 [54] and HOF-11 [55], in which the carboxy groups formed H-bonded dimers. Both had almost the same porous structures with 1D channels and 11-fold interpenetrated (10,3)-b topological networks, although the crystallographic lattices of IISERP-HOF1 and HOF-11 were slightly different depending on the crystallization conditions (in acetic acid at 150 °C and in THF at room temperature, respectively). Meanwhile, the pseudopolymorphic HOF HOF-16 [56, 57], contained carboxy groups that did not form intermolecular H-bonds, which allowed efficient separation of C3H6/C3H8 via interactions between the gas molecules and the free carboxy groups [57]. This pseudopolymorph provided an example of functionalization that takes advantage of the weakness of H-bonds and simultaneously showed the difficulty of constructing isostructural HOFs.

1,3,5-Triazine derivative 11 also formed a H-bonded hcb network, which, however, was not a planar hcb sheet but was undulated and polycatenated with other parallel sheets, resulting in intricate crisscrossed 3D structures (PFC-11, -12, -13) [58]. An undulated hcb network was also observed in the first crystal structure of 1 [14]. Heptazine derivative 12, on the other hand, formed no hcb network but 1D strands, where two of the three carboxy groups in 12 formed a truncated H-bonded dimer, although the molecule was the same C3-symmetric tritopic carboxylic acid seen in other tectons [59]. The heteromeric HOF HcOF-101 was obtained by cocrystallization of 1 and the o-alkoxycarboxylic acid derivative 13 [53]. The resultant HOF was subjected to a thiol-ene crosslinking reaction in the presence of ethanedithiol, which formed covalently linked stable porous materials.

Tetracarboxylic acids

4-Connected tectons containing four carboxy groups lying in the same plane are shown in Fig. 4. They have the planar or quasiplanar skeletons of benzene 14 [60, 61], N-containing polycyclic structures 15–17 [60,61,62,63], tetrathiafulvalene 18 [60, 64, 65] and 19 [66], tetraphenylethene 20 [67] and 21 [68], pyrene 22–27 [69,70,71,72] and 32 [73], 1,2,3,6,7,8-hexahydropyrene 33 [73], porphyrin 28 [74,75,76], metalloporphyrin 28Co and 28Cu [77], and terphenyl analogs 29–31 [78,79,80].

Fig. 4
figure 4

Chemical structures of tetracarboxylic acids forming 2D layered HOFs. The names of the resultant HOFs are also presented in bold

Full size image

π-Conjugated planar tectons tend to form rhombic networks with sql topologies. These homotypic networks are assembled by stacking through π-π and/or CH-π interactions to form layered frameworks. Unlike the hcb lattices formed with C3-symmetric tectons, interpenetrated structures are not common for the planar tetracarboxylic acids, presumably because of the lower symmetry of a sql network compared with that of an hcb network and/or the stabilization effect of layered stacking structures. The compounds listed in Fig. 4 basically construct 2D layered porous structures. The 2D sheets accumulate with the following nonuniform assembly manners, that is, AA stacking, AB stacking, and others, such as correlation offset stacking, resulting in the formation of nonisostructural HOFs. Tectons 14 and 20 gave the HOFs X-Ph [60] and CPE [67] with AB stacking of homotypic 2D networks (Fig. 5a, e), while tectons 15, 16, 17, 29, 30, 31 gave isostructural HOFs (CP-PP [62], X-PyQ [60], BrPQ [63], ABTPA-1 [78], BITA-1 [79], and BTTA-1 [80], respectively) with similar AA-stacking structures (Fig. 5b–d, f–h). The HOFs CP-PP, X-PyQ, and BrPQ underwent drastic structural changes involving rearrangement of H-bonds upon removal of the accommodated solvent molecules, while the HOFs ABTPA-1, BITA-1, and BTTA-1 basically retained their porous structures, except for subtle structural changes such as stacking orders and molecular conformations, and they showed Brunauer‒Emmett‒Teller (BET) surface areas of 1183 m2g−1, 720 m2g−1, and 1145 m2g−1, respectively. It should be noted that tecton 21 gave the HOF CBPE, in which 21 formed isomorphic H-bonded sql 2D sheets, and the sheets were interpenetrated three-directionally to give a wvm-like weave porous structure exhibiting mechanochromic photofluorescence. It was proposed that formation of the weave structure in CBPE arose from the disproportionate conformation of the outer four phenylene rings in the peripheral biphenyl arms [68]. Tectons 32 and 33 yielded isostructural HOFs (CP-Py-1 and CP-Hp-1, respectively) with almost identical cell parameters (Fig. 5i, j) [73]. Interestingly, the similarity of these HOFs allowed 32 and 33 to form nonstoichiometric cocrystalline frameworks (CP-HpPy-1) when the compounds were cocrystallized with various composition ratios. CP-Py-1 was also revealed to exhibit static and dynamic flexibility depending on the desorption and adsorption of various guest molecules [81].

Fig. 5
figure 5

Crystal structures of (a) X-Ph, (b) CP-PP, (c) X-PyQ, (d) BrPQ, (e) CPE, (f) BTIA-1, (g) BTTA-1, (h) ADTPA-1, (i) CP-Py, and (j) CP-Hp. (Top) Square-lattice networks and assembled structures, (middle) viewed down, and (bottom) viewed from the side. The disordered moieties were omitted for clarity. These structures were obtained from the SCXRD experiment except for BTIA-1, which was analyzed from the PXRD experiment. The HOF CPE in (e) was described only in the Supporting Information for ref. [67]

Full size image

The porphyrin-based tectons 28, 28Cu, and 28Co formed H-bonded sql network sheets, which were stacked in an AB-stacking manner to give the series of porous frameworks PFC-71, -72, and -73, respectively. In the HOFs, the offset stacking manner of the sql networked sheets was modulated by the metal species (Fig. 6a–c), resulting in different BET surface areas (600 m2g−1 for PFC-71, 1646 m2g−1 for PFC-72, and 1714–1856 m2g−1 for PFC-73) [77]. In addition to a 2D layered assembly, a 3D porous structure of the interpenetrated sql network (HOF-6) [74] shown in Fig. 6d and a layered porous structure (PFC-5 [75] and PFC-33 [76]) formed by branched H-bonds of the carboxy groups shown in Fig. 6e, f were also reported. Porphyrin-based HOFs and their composites have been used as catalysts for photooxygenation of amyloid-β [74] and for photoinduced CO2 reduction [77] in addition to selective separation materials [75].

Fig. 6
figure 6

Crystal structures of (a) PFC-71, (b) PFC-72, (c) PFC-73, (d) HOF-6, (e) PFC-5, and (f) PFC-33. In the case of (e) and (f), the carboxy groups formed no self-complementary dimers but branched H-bonded motifs. Disordered moieties were omitted for clarity

Full size image

The formation of versatile layered HOFs was reported for TTF-based tectons 18 and 19 (Fig. 7) [60, 64,65,66]. In the structure of X-TTF, the sql layers were accumulated in the AA stacking manner, while the others were accumulated in the AB stacking manner. TTF-based HOFs underwent drastic structural changes upon solvent removal or solvent exchange, as in the cases of CP-PP, X-PyQ, and BrPQ. The PXRD changes indicated that X-TTF changed its framework structure upon removal of solvent molecules [60]. PFC-77, which was obtained from a water and THF mixed solution, transformed into PFC-78 upon immersion in acetone, and moreover, PFC-77 or PFC-78 transformed into PFC-79 with a denser framework upon immersion in dichloromethane [64]. The structure of PFC-79 contained a tetrameric 2D H-bonded node composed of four carboxy groups, which was also formed in BrPQ [63]. TTF-based tectons tend to form versatile porous frameworks, such as MUV-20a and MUV-20b with the abovementioned tetrameric node and MUV-21 with larger honeycomb-like channels [65], the former two of which presented zwitterionic character and showed semiconduction with conductivities up to 1.35 × 10−6 Scm−1 [65]. TTF derivative 19 with 2-carboxynaphth-6-yl groups formed a homotypic network similar to that of 18 with 4-carboxyphenyl groups [66]. The resultant HOF-110 showed offset stacking similar to that of PFC-78 and became conductive after postsynthetic incorporation of iodine in the porous channels of the framework [66].

Fig. 7
figure 7

Crystal structures of (a) PFC-77, (b) X-TFF, (c) PFC-78, and (d) HOF-110. Disordered moieties are omitted for clarity

Full size image

Isostructural HOFs were formed by the pyrene-based tectons 22–27 [69,70,71,72] via spacer modification approaches (Fig. 8). Pyrene-based tectons were reported to yield a series of isostructural HOFs with AA stacking of homotypic sql networked sheets. In 2018, Cao et al. reported that 23 gave PFC-1, which had a thermally and chemically stable porous framework with a BET surface area of 2122 m2g−1, and demonstrated that the HOF encapsulated doxorubicin for synergistic chemo-photodynamic therapy [70]. Li and Farha et al. reported that pyrene derivatives 23, 24, 25, and 26 with 4-carboxyphenyl, 2-carboxynaphthyl, 3-methyl-4-carboxyphenyl, and 3-amino-4-carboxyphenyl groups, respectively, gave single-crystalline isostructural HOFs, while 22 and 27 with carboxy and 4-carboxy-3-fluorophenyl groups, respectively, gave isostructural crystalline precipitates whose structures were estimated from PXRD experiments [69, 71]. In the isostructural HOFs, the 2D sql networks were assembled by shape matching π-π stacking. Among them, HOF-102 exhibited the largest void channel with an aperture of 2.5 nm × 3.0 nm and a BET surface area of 2500 m2g−1 and was capable of adsorbing biomolecules such as cytochrome c [69]. Chen et al. also reported that 24 yielded HOF-14 with high stability and porosity [72]. HOFs-101-CH3, -NH2, and -F were reported as chemically engineered frameworks of HOF-101. HOF-101-F showed a 10- to 60-fold enhancement in the generation of reactive oxygen species (ROS) and a 10- to 20-fold greater ROS storage ability compared to traditional TiO2 and C3N4 self-cleaning materials [71]. From a structural perspective, HOF-103-CH3 and -NH2 had slightly longer interlayer distances and offsets due to their steric hindrance (former: 3.42 Å and 1.71 Å, latter: 3.46 and 1.73 Å), and those lengths in PFC-1 and HOF-14 were similar (former: 3.39 Å and 1.65 Å, latter: 3.40 and 1.67 Å). Tecton 23 was also reported to give Kagome-like isomeric HOF PFC-2 containing a large channel with a diameter of 2.97 nm [81]. Interestingly, single crystals of PFC-2 were obtained by recrystallization of 23 from a DMF and ethanol solution at 90 °C in the presence of equimolar 1,4-bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)-phenylbenzene (L2), while only a crystalline powder was formed without L2 [81]. In contrast with 1,3,6,8-substituted pyrene derivatives such as 23, 4,5,9,11-substituted pyrene derivative 32 gave a flexible HOF because the substituent aryl groups prevented the pyrene core from π-π stacking: CP-Py-1 was transformed into CP-Py-3 via CP-Py-2 through shrinkage of the framework and rearrangements of the H-bonds [82].

Fig. 8
figure 8

Crystal structures of (a) PFC-1(HOF-101), (b) HOF-14 (HOF-102), (c) HOF-101-CH3, and (d) HOF-101-NH2. Disordered moieties were omitted for clarity

Full size image

4-Connected tectons containing four non-coplanar carboxy groups are shown in Fig. 9. Tectons with the cores methane 34 [83], silane 35 [83], and adamantane 36 [47] and 37 [83] have tetrahedral (T4) symmetry. Others have twisted and/or nonplanar skeletons, as with biphenyls 38–40 [61, 84,85,86], tetra[2,3]thienylene 41 [87], 5’-phenyl-m-terphenyl 42 [88], p-phenylenediamine 43 [89], 18-crown-6-ether 44 [90], and dibenzo[g,p]chrysene 45–48 [91, 92]. Most of these nonplanar tetratopic tectons form diamonded (dia) topological networks through H-bonding, and assembly of the networks also proceeded in parallel with the networking through weaker noncovalent interactions, such as van der Waals interactions, to give interpenetrated frameworks. When the single H-bonded network has an inside pore larger than the size of the molecular components, additional networks are formed in the pore, leading to the formation of interpenetrated, less porous framework structures. Homotypic networks are easily obtained from complementary H-bonded dimers of carboxy groups, while the entire structures are not always isostructural because interactions between the networks depend on the molecular structure.

Fig. 9
figure 9

Chemical structures of tetracarboxylic acids forming 3D H-bonded networked HOFs. Names of the resultant HOFs are also presented in bold

Full size image

T4-symmetric tectons 34–37 formed dia topological H-bonded networks, which assembled into interpenetrated HOFs through nondirectional weak interactions. Tectons 34 and 35 gave the corresponding porous HOFs TCF-1 and TCF-2, both of which showed 6-fold interpenetrated dia networks. On the other hand, TCF-1 had 1D channels, while TCF-2 had 3D-networked channels, depending on the interpenetration manner of the single network [83]. CSP screening was successfully applied with 36 to experimentally observe the solvated HOFs ADTA-2 and -3 with 2-fold and 3-fold interpenetration, respectively, in addition to the known form with 5-fold interpenetration [47]. Expanded tecton 37, on the other hand, gave the nonporous structure TCF-3, in which the presence of THF molecules disrupted the formation of H-bonded dimers by the carboxy groups [83]. Since there are no directional interactions in the networks of non-π-conjugated molecules, the assembly manner is readily variable depending on the molecular structure, guest molecules, and crystallization conditions, resulting in a wide variety of porous structures. Moreover, even if they exhibited persistent porosity, they often underwent structural transitions after guest removal.

Tectons 38–40 formed H-bonded, three-dimensionally networked frameworks because of the twisted conformations of their sterically hindered biphenyl cores. Their HOFs, HOF-TCBP, ZJU-HOF-10, and HOF-15, showed the same dia topological network, while the shape, assembly manner, and number of interpenetrated network structures depended on the substituent groups. Pristine tecton 38 exhibited π-π stacking in the 5-fold interpenetrated HOF-TCBP (Fig. 10a) [84], while the methyl and methoxy substituents in 39 and 40, respectively, forced the central biphenyl moieties to assemble via CH-π interactions instead of π-π interactions, resulting in the 6-fold and 7-fold interpenetrated HOFs ZJU-HOF-10 [86] and HOF-15 [85], respectively (Fig. 10b, c). Tecton 41 formed the 6-fold interpenetrated HOF TT-TC, in which dia network structures were assembled by π-π stacking (Fig. 10d) [87]. Interestingly, a change in the molecular structure of 41 triggered by the removal of the solvent molecules resulted in jumping behavior of the HOF crystals. The heterobiphenyl tecton 42 with carboxy and 4-carboxyphenyl groups gave HOF-20, which had a 4-fold interpenetrated ThSi2 topological H-bonded network (Fig. 10e) [88]. HOF-20 exhibited efficient turn-up fluorescent sensing of aniline in water with a detection limit of 2.24 μM. Tecton 43 had p-phenylenediamine moieties with a perpendicular orientation of the phenylene core relative to the amine moieties and formed the 10-fold interpenetrated framework HOF-30 with a topological H-bonded network (Fig. 10f) [89]. Tecton 44 included a flexible 18-crown-6-ether moiety, and therefore, the resultant HOF 2CT-18C6-III was nonporous, and the H-bonded network in the HOF was very complicated [90]. Tectons 45–48 had dibenzo[g,p]chrysene cores and formed robust 1D stacked columnar structures due to shape-filled docking of the twisted π-conjugated skeleton, which gave the isostructural porous HOFs CPDBC-1 [91], C1N4DBC-1, C1N5DBC-1, and C2N6DBC-1 [92]. Although the structures of their solvated frameworks were similar, their dynamic behaviors during desorption and adsorption of solvent molecules were different depending on the chemical structures of the spacer moieties.

Fig. 10
figure 10

Crystal structures of (a) HOF-TCBP, (b) ZJU-HOF-10, (c) HOF-15, (d) TT-TF, (e) HOF-20, and (f) HOF-30 with 5-, 6-, 7, 6-, 4-, and 10-fold interpenetrated frameworks. (Top) Assembled porous structures and (Bottom) a single dia-network colored gray and 1D assembled columnar structures, for which the number of colors in the stacked molecules correspond to the number of interpenetrated frameworks. Guest molecules and disordered moieties were omitted for clarity

Full size image

Hexacarboxylic acids

The molecular structures of hexatopic tectons are shown in Fig. 11. They typically have π-conjugated cores with benzene 49 [93], 50 [94], and 67 [95], triphenylene 51 [96], 52 [97], and 53 [97], acetylene bridged macrocycles 54 [98], 55 [99], 56 [98, 100], and 57 [98], bowl-shaped sumanene 58 [101], hexaazatriphenylene (HAT) 59 [102], 60 [103], 61 [104], and 62 [104], hexaazatrinaphthylene (HATN) 63 [105], benzotriquinoxaline 64 [106], quinoxaline-annelated dehydro[12]annulene 65 [107], tri(dithiolylidene)cyclohexanetrione 66 [108], and triptycene 68 [109] and 69 [110], as well as 18-crown-6-ether 70 [111].

Fig. 11
figure 11

Chemical structures of hexatropic carboxylic acids forming HOFs. Names of the resultant HOFs are also presented in bold

Full size image

These tectons have 4,4’-dicarboxy-o-terphenyl and analogous substructures, which form robust H-bonded cyclic trimers, or so-called phenylene triangles (PhT) (Fig. 12a). Formation of a PhT motif consequently results in expansion of the hxl topological hexagonal network. Tectons with planar π-conjugated hydrocarbon cores (49, 51, 52, 53, 54, 55, 56, and 57) and those with a planar N-hetero π-conjugated core (63) formed homotypic hlx sheets, which were stacked in an inverted fashion through π-π, CH-π and CH-O interactions to give noninterpenetrated, layered HOFs with AB stacking, although optimization of the recrystallization conditions, such as the temperature and the combination of solvents, was needed. The resulting offset arrangements of the AB stacked layers, on the other hand, differed from each other depending on the core structures [98] and substituents at the ortho-positions relative to the carboxy groups [97, 99].

Fig. 12
figure 12

a H-bonded motif, the so-called phenylene triangle (PhT), formed from 4,4’-o-terphenyl moieties. Crystal structures of HOFs possessing layered hexagonal network structures: (b) HCBP, (c) Tp-1, (d) TpF-1, (e) TpMe-1, (f) T12-1, (g) T12F-1, (h) T18-1, and (i) Ex-1. Hexagonal network; in (a), the PhT motif contains at least one twisted H-bonded dimer due to conformational frustration. Guest molecules were omitted for clarity

Full size image

Kobayashi et al. reported the first crystal structure of a layered hlx network with hexasubstituted benzene derivative 49. The crystals of the HOF HCPB were obtained by recrystallization from methanol solution with and without the guest molecule 2,7-dimethoxynaphthalene. In the crystal structure, the peripheral phenylene groups of 49 were almost perpendicular to the central benzene ring due to steric hindrance, which resulted in a longer interlayer distance of 5.6 Å for the HOF [93]. Triphenylene-based tecton 51 formed four polymorphic HOFs, Tp-1, -2, -3, and -4 [96]. Generation of the polymorphs originated from versatile locations and conformational frustration of the PhT moieties: namely, adjacent peripheral phenylene groups of the 4,4’-dicarboxy-o-terphenyl in 51 were inclined in the same direction to avoid steric repulsion between them, and therefore, the PhT motif included at least one conformationally frustrated H-bonded carboxyl dimer, as shown in Fig. 12a [96]. The activated HOF Tp-a, which was characterized by a combination of CSP and experimental PXRD measurements, exhibited permanent porosity with a BET surface area of 718 m2g−1 [98]. The triphenylene-based tecton 52 also gave polymorphic HOFs with layered hlx sheets (TpMe-1, -2, and -3) due to the versatile conformations of the sterically hindered peripheral groups [97]. Tecton 53 gave the HOF TpF-1, in which the peripheral groups were disordered in two positions due to steric hindrance [97]. The π-conjugated cyclic tectons 54, 55, 56, and 57 formed homotypic hlx sheets with scalene hexagonal apertures, and the longer side lengths ranged from 4.6 to 11.4 Å depending on the size of the macrocycle [98]. The resultant HOFs T12-1, T18-1, and Ex-1 had solvent accessible voids of 41%, 58%, and 59%, respectively, calculated by PLATON software with a proven radius of 1.2 Å. After activation, T12-1 retained its crystallinity and permanent porosity with a BET surface area of 557 m2g−1 and showed reversible dynamic structural changes among four different crystalline states during CO2 sorption [112]. The photodynanic behaviors of HOFs T12-1 and Ex-1 were also investigated by Douhal et al. with fluorescence microscopy applied to single HOF crystals [113, 114].

HATN derivative 63 gave the HOF CPHATN-1a with a BET surface area of 379 m2g−1 (Fig. 13a). Upon exposure to acids such as HCl, CPHATN-1a changed color from yellow to reddish brown, and the original color was recovered when the acid was removed [105]. Wang and Jiang et al. applied CPHATN-1a as a cathode material in a lithium-ion battery [115]. Analogs 64 [106] and 65 [107] formed the layered HOFs CPBTQ-1 and TQ12-1 with quasi hlx structures (Fig. 13b, c), in which the carboxy groups formed truncated interlayer H-bonds as well as the complementary dimer (Fig. 13d, e). In particular, 64 formed a complicated, low-symmetry, layered structure with ABCD stacking [106]. CPBTQ-1 and TQ12-1 also showed color changes from yellow to reddish brown upon exposure to HCl due to the proton-responsive pyrazine rings incorporated in the π-conjugated systems.

Fig. 13
figure 13

Crystal structures of (a) CPHATN-1, (b) CPBTQ-1, and (c) TQ12-1. (top) Hexagonal networks and assembled structure (middle) viewed down and (bottom) viewed from the side. d Interlayer H-bonds with the truncated tetramer fashion observed in CPBTQ-1. e Interlayer H-bonds with the water-incorporated truncated tetramer observed in TQ12-1. Guest molecules and disordered moieties were omitted for clarity

Full size image

When a tecton with a bowl-shaped π-conjugated core is used to construct a HOF, the resultant H-bonded network differs drastically from those composed of planar tectons. The sumanene derivative 58 gave two HOFs, CPSM-1 and CPSM-2; the former had a layered structure composed of ungulated hlx sheets, while the latter had exotic bilayered structures with a complicated topological network formed through an H-bonded trefoil knot [101]. Even when using a tecton whose π-conjugated core was apparently flat, the resultant HOF may have a three-dimensionally networked framework. For example, HAT derivative 59 exhibited a nonplanar propeller-shaped twisted conformation in the crystalline state, probably due to the packing force, which resulted in the formation of a 3D pcu network through helical H-bonds instead of a 2D hxl network (Fig. 14a, e, f) [102]. The 3D networked structures were assembled uniformly through π-stacking to form a 4-fold interpenetrated porous structure. Importantly, the twisted cores stacked in a shape-fitted fashion to yield a robust 1D columnar architecture, which played a role in stabilizing the HOF. Indeed, the activated HOF CPHAT-1a retained single crystallinity and exhibited a BET surface area of 649 m2g−1 and heat resistance up to 339 °C. A series of HAT derivatives 60, 61, and 61 also formed the isostructural HOFs CBPHAT-1, TolHAT-1, and ThiaHAT-1, respectively (Fig. 14b–d), which had 6-, 8-, and 8-fold interpenetrated structures with the same pcu topological network, and the pore sizes were determined by the arms with different lengths [103, 104]. The BET surface areas of the HAT systems ranged from 649 m2g−1 to 1394 m2g−1. These HOFs also showed HCl-induced color changes due to the pyrazine rings incorporated in the π-conjugated cores [104, 116]. The hexa-substituted benzene derivative 50 formed HOF-76 with a pcu network without interpenetration due to the slightly twisted molecular conformation (Fig. 14g, h). The HOF exhibited a BET surface area of 1100 m2g−1 and showed preferential binding of C2H6 over C2H4 and thus highly selective separation of C2H6/C2H4 mixtures [94]. The HOF CPDC-1 was constructed from the bis(4-carboxyphenyl)dithiol-based tecton 66, in which the angle between the two 4-carboxyphenyl groups was ca. 73° (Fig. 14i, j) [108]. The angle was smaller than that of the 4,4’-dicarboxy-o-terphenyl group. Therefore, 66 formed an anomalistic, noninterpenetrated, helical network denoted by the {82.10} point symbol instead of a hxl or pcu network. Tecton 67 formed the 3-fold interpenetrated HOF ZJU-HOF-1 with a BET surface area of 1465 m2 g−1, which showed a high C2H6 uptake capacity and excellent C2H6/C2H4 selectivity [95]. Another type of hexatropic tectonism is triptycene-based tectonism. Tecton 68 formed the polymorphic HOFs PETHOF-1 and PETHOF-2, in which a single network exhibited the same hexagonal topology acs, while the network was interpenetrated in different ways to form two kinds of HOFs [109]. The tecton 69 yielded the HOF PETHOF-3 with a topological network unlike those of the former two [110]. The 18-crown-6-ether tecton 70 yielded the HOF 3CT-18C6-I possessing a layered structure with hexagonally networked 2D sheets [111].

Fig. 14
figure 14

Crystal structures of HOFs with helical H-bonded strands; (a) CPHAT-1, (b) CBPHAT-1, (c) TolHAT-1, (d) ThiaHAT-1, (g) HOF-76, and (i) CPDC-1. e Selected side view of the 4-fold interpenetrated framework of CPHAT-1. Schematic representation of helical H-bonding in (f) CPHAT-1, (h) HOF-76, and (j) CPDC-1. Guest molecules were omitted for clarity

Full size image

Conclusion

In this review, we reviewed hydrogen-bonded frameworks (HOFs) composed of tritopic, tetratopic, and hexatopic carboxylic acid derivatives from a structural perspective. Since the carboxy groups formed predictable linear dimers through reversible H-bonding in many cases, the dimers were applied as supramolecular synthons to construct designed networked structures; for example, tritopic planar tectons can give an hcb network, nonplanar and planar-shaped tetratopic tectons can give dia and sql networks, respectively, and planar hexatopic tectons can give an hxl network. The higher dimensional structures formed by assembling the network structure, however, are still difficult to design due to the weaker and less-directional interactions between the networks. Moreover, even slight distortion of the tectons results in the formation of unpredicted networks, such as the pcu network formed by apparently planar hexasubstituted tectons. Therefore, these two issues are the next topics to be resolved for well-controlled construction of HOFs.