HOFs Built from Hexatopic Carboxylic Acids: Structure, Porosity, Stability, and Photophysics

Hydrogen-bonded organic frameworks (HOFs) have attracted renewed attention as another type of promising candidates for functional porous materials. In most cases of HOF preparation, the applied molecular design principle is based on molecules with rigid π-conjugated skeleton together with more than three H-bonding groups to achieve 2D- or 3D-networked structures. However, the design principle does not always work, but results in formation of unexpected structures, where subtle structural factors of which we are not aware dictate the entire structure of HOFs. In this contribution, we assess recent advances in HOFs, focusing on those composed of hexatopic building block molecules, which can provide robust frameworks with a wide range of topologies and properties. The HOFs described in this work are classified into three types, depending on their H-bonded structural motifs. Here in, we focus on: (1) the chemical aspects that govern their unique fundamental chemistry and structures; and (2) their photophysics at the ensemble and single-crystal levels. The work addresses and discusses how these aspects affect and orient their photonic applicability. We trust that this contribution will provide a deep awareness and will help scientists to build up a systematic series of porous materials with the aim to control both their structural and photodynamical assets.

The milestone of H-bonded networked frameworks was trimesic acid (TMA), reported in 1969 [21]. The three-dimensional (3D) superstructure of hexagonal networks is based on neighboring TMA units linked by H-bonds among carboxyl groups. However, the interest in using H-bonds to assemble ordered networks was revived only in the late of families, depending on their H-bonded structural motifs: (1) flexible HOFs formed by lamination of two-dimensional (2D) hexagonal networks; (2) rigid isostructural HOFs formed by interpenetration of 3D networks and shape-fitted molecular docking; and (3) HOFs with unexpected network structures ( Figure 1).
We join in a unique work the synergic combination of experiences in HOFs' chemistry and laser-based time-resolved (from femto-to-millisecond regime) spectroscopy and fluorescence microscopy techniques. There are two principal key factors that are unravelled in this feature article: (1) the chemical aspects that regulate both the chemistry and structures of the HOFs; and (2) their ensemble and single-crystal spectroscopy and photophysics at different time scales. This contribution also focuses on and considers in which way these aspects regulate the photonic applicability of these materials. Last, but not least, we pose questions and provide our vision for the next HOF generations regarding: (1) how to improve the design strategy, synthesis routes, and crystallization of HOFs; (2) how to increase their stability at ambient temperature; (3) how to control and enhance their response to light for real-world applications; and (4) how to design HOF-based composites involving known fluorescent and sensor dyes for photonic applications. We expect that this contribution will provide a deep awareness to build up a systematic series of porous materials, with the aim to control both their structural and photodynamical properties and boost advanced research in this field.

HOFs Constructed through the H-Bonding of π-Conjugated Hexacarboxylic Acids
The presence of carboxyl groups in the building block encourages the construction of geometrically well-defined frameworks thanks to the formation of highly directional H-bonded dimers of the carboxyl moieties. Carboxylic acid-based HOFs with permanent porosity have been intensively investigated since 2015 [53,92]. This section deals with the working hypothesis proposed few years ago by Hisaki's group, with the aim to systematically build up HOFs with isostructural structure motifs. For this purpose, C3-symmetric π-conjugated planar building blocks (C3PIs) bearing three o-bis (4-carboxyphenyl)benzene groups in the periphery were chosen (Figure 2a). It has been proved that these units can generate isostructural 2D H-bonded hexagonal network (H-HexNet) sheets through Hconnected carboxyl dimers, and that the H-HexNets further stack without interpenetration to provide flexible porous layered HOFs (LA-H-HexNets) [19,[93][94][95][96]. The key Figure 1. Examples of carboxylic acids for HOFs construction: (a) tri-and hexa-substituted benzene derivatives; (b) C 3 -symmetric π-conjugated molecules (C 3 PIs) providing layered HOFs; (c) C 3 PIs providing 3D-networked rigid HOFs; (d) C 3 PIs providing HOFs with unexpected H-bonded networks. The methyl ester derivatives corresponding to the carboxylic acids are denoted by the postfixed -COOMe (e.g., the ester derivative of T12-COOH is described as T12-COOMe).
We join in a unique work the synergic combination of experiences in HOFs' chemistry and laser-based time-resolved (from femto-to-millisecond regime) spectroscopy and fluorescence microscopy techniques. There are two principal key factors that are unravelled in this feature article: (1) the chemical aspects that regulate both the chemistry and structures of the HOFs; and (2) their ensemble and single-crystal spectroscopy and photophysics at different time scales. This contribution also focuses on and considers in which way these aspects regulate the photonic applicability of these materials. Last, but not least, we pose questions and provide our vision for the next HOF generations regarding: (1) how to improve the design strategy, synthesis routes, and crystallization of HOFs; (2) how to increase their stability at ambient temperature; (3) how to control and enhance their response to light for real-world applications; and (4) how to design HOF-based composites involving known fluorescent and sensor dyes for photonic applications. We expect that this contribution will provide a deep awareness to build up a systematic series of porous materials, with the aim to control both their structural and photodynamical properties and boost advanced research in this field.

HOFs Constructed through the H-Bonding of π-Conjugated Hexacarboxylic Acids
The presence of carboxyl groups in the building block encourages the construction of geometrically well-defined frameworks thanks to the formation of highly directional H-bonded dimers of the carboxyl moieties. Carboxylic acid-based HOFs with permanent porosity have been intensively investigated since 2015 [53,92]. This section deals with the working hypothesis proposed few years ago by Hisaki's group, with the aim to systematically build up HOFs with isostructural structure motifs. For this purpose, C 3 -symmetric π-conjugated planar building blocks (C 3 PIs) bearing three o-bis(4-carboxyphenyl)benzene groups in the periphery were chosen (Figure 2a). It has been proved that these units can generate isostructural 2D H-bonded hexagonal network (H-HexNet) sheets through Hconnected carboxyl dimers, and that the H-HexNets further stack without interpenetration to provide flexible porous layered HOFs (LA-H-HexNets) [19,[93][94][95][96]. The key structure for the formation of H-HexNets is the so-called phenylene triangle (PhT) motif, formed by

HOFs Based on Dehydrobenzo[12]annulene (DBAs) and Triphenylene Derivatives
The most important advantages of DBAs are planarity and high π-conjugation [98][99][100]. Depending on the molecular structure of the DBA unit and the number and kinds of π-π and H-bonding interactions, the resulting HOF will possess a specific morphology, crystallinity, and pore size [101]. Planar rigid tectons, such as T12-COOH (Figure 1), in which the peripheral carboxyl groups lay along the same molecular plane, form 2D hexagonal networked sheets that subsequently stack without interpenetration to form the corresponding layered T12-apo HOF ( Figure 3) [19]. The HOFs deriving from the assembly of C3-symmetry-building blocks present two pores: (1) a narrower one (pore-I), corresponding to the triangular void space involving the PhT motif, with a constant side length of ~11 Å; and (2) a nonregular hexagonal-shaped wider one (pore-II), with longer (L = 15.8 Å) and shorter (r) sides. While L remains constant, r is affected by the size of the C3PI core. Specifically, T12-apo has an r value of 4.6 Å. Additionally, cyclic C3PI molecules can supply a third void (pore-III) due to the intrinsic pores within the molecules. However, pore-III of T12 is too narrow to accommodate a molecule. Activated HOF T12-apo was thermostable up to 360 °C and possessed permanent porosity with a Brunauer-Emmett-Teller surface area SA(BET) of 557 m 2 g −1 [19]. Both solid materials T12-apo and T12-ester, the latter being a solid bulk composed of T12-ester, exhibited a strong S0-S1 transition with intense absorption signals [102]. The use of single-molecule (crystal) fluorescence microscopy to study T12-ester crystals revealed different photodynamic behaviors depending on whether the investigated crystals had a large (>40 µm) or small (~0.5 µm) size, but while having very similar emission spectra [103]. Large crystals showed a

HOFs Based on Dehydrobenzo[12]annulene (DBAs) and Triphenylene Derivatives
The most important advantages of DBAs are planarity and high π-conjugation [98][99][100]. Depending on the molecular structure of the DBA unit and the number and kinds of π-π and H-bonding interactions, the resulting HOF will possess a specific morphology, crystallinity, and pore size [101]. Planar rigid tectons, such as T12-COOH (Figure 1), in which the peripheral carboxyl groups lay along the same molecular plane, form 2D hexagonal networked sheets that subsequently stack without interpenetration to form the corresponding layered T12-apo HOF ( Figure 3) [19]. The HOFs deriving from the assembly of C 3 -symmetry-building blocks present two pores: (1) a narrower one (pore-I), corresponding to the triangular void space involving the PhT motif, with a constant side length of 11 Å; and (2) a nonregular hexagonal-shaped wider one (pore-II), with longer (L = 15.8 Å) and shorter (r) sides. While L remains constant, r is affected by the size of the C 3 PI core. Specifically, T12-apo has an r value of 4.6 Å. Additionally, cyclic C 3 PI molecules can supply a third void (pore-III) due to the intrinsic pores within the molecules. However, pore-III of T12 is too narrow to accommodate a molecule. Activated HOF T12-apo was thermostable up to 360 • C and possessed permanent porosity with a Brunauer-Emmett-Teller surface area SA(BET) of 557 m 2 g −1 [19]. Both solid materials T12-apo and T12-ester, the latter being a solid bulk composed of T12-ester, exhibited a strong S 0 -S 1 transition with intense absorption signals [102]. The use of single-molecule (crystal) fluorescence microscopy to study T12-ester crystals revealed different photodynamic behaviors depending on whether the investigated crystals had a large (>40 µm) or small (~0.5 µm) size, but while having very similar emission spectra [103]. Large crystals showed a monoexponential emission decay (~28 nanoseconds, ns), while small ones presented an extra component (~5 ns) assigned to the emission of species having suffered an intramolecular charge transfer (ICT) reaction from the methoxycarbonylphenyl groups to the unit core. Furthermore, for small (~0.3-1 µm) T12-apo crystals, the emission spectra clearly depended on their shape and size ( Figure 4). monoexponential emission decay (~28 nanoseconds, ns), while small ones presented an extra component (~5 ns) assigned to the emission of species having suffered an intramolecular charge transfer (ICT) reaction from the methoxycarbonylphenyl groups to the unit core. Furthermore, for small (~0.3-1 µm) T12-apo crystals, the emission spectra clearly depended on their shape and size ( Figure 4).    monoexponential emission decay (~28 nanoseconds, ns), while small ones presented an extra component (~5 ns) assigned to the emission of species having suffered an intramolecular charge transfer (ICT) reaction from the methoxycarbonylphenyl groups to the unit core. Furthermore, for small (~0.3-1 µm) T12-apo crystals, the emission spectra clearly depended on their shape and size ( Figure 4).    (1) those based on π-π stacking, which have a photobehavior similar to that of small T12ester crystals (lifetimes: 4.1-4.5 and 20-21 ns associated with the emission of ICT and not H-bonded species, respectively; see Figure 4a ,b ); and (2) crystals governed by H-bonding interactions between the fundamental units, which emit at lower energies and relax faster (1.4-1.5 and 8.5-8.8 ns) with respect to the crystals described in (1) (see Figure 4c ,d ).
The shorter emission lifetimes of crystals described in (2) above showed the importance of H-bonds in their structures that might lead to PT reactions in the building blocks. In contrast, both the spectral and dynamical properties of larger (>30 µm) T12-apo crystals did not significantly change. A biexponential behavior of the emission decays was found with lifetimes of 4-6.5 and~17-22 ns, attributed respectively to ICT and non-H-bonded species. Due to its Commission Internationale de l'Éclairage (CIE) coordinates (0.42, 0.55) and a high (25%) fluorescence quantum yield (Φ F ), T12-apo might be used for the creation of a metal-free white light-emitting diode (WLED) after covering a blue LED with this HOF.
Further expansion of the DBA structure gave birth to a new π-conjugated system, named the phenylene-ethynylene macrocycle, Ex-COOH or its methyl ester derivative (Ex-COOMe) (see Figure 1) [19,104]. In these frameworks, pore-II has an r value of 11.4 Å, which means that the pore has an almost regular hexagonal shape. Furthermore, pore-II has a diameter of 7.4 Å when the phenylene ring of the fundamental unit of Ex is perpendicular to the molecular plane. Ensemble behavior studies revealed a multiexponential emission decay of both solid samples of Ex-ester and Ex-apo, suggesting a heterogeneity of the systems. Emissions from locally excited (LE) and charge-transfer (CT) states were reported and discussed, with dynamics faster than those found in the parent T12-apo HOF (see above). In particular, the LE species displayed the shortest lifetimes (100-200 picoseconds (ps) for Ex-apo and Ex-ester, respectively), while those formed after fast ICT reactions (<15 ps) were slightly longer (420-870 ps and 1.43-3 ns for Ex-apo and Ex-ester, respectively) (see Scheme 1). Furthermore, an additional component of 4.20 ns in the emission decay was found for Ex-apo and was assigned to anions originating from strong H-bonding interactions between the Ex-COOH units. Figure 4a-d shows two kinds of spectra belonging to the emission of distinct crystals: (1) those based on π-π stacking, which have a photobehavior similar to that of small T12ester crystals (lifetimes: 4.1-4.5 and 20-21 ns associated with the emission of ICT and not H-bonded species, respectively; see Figure 4a′,b′); and (2) crystals governed by H-bonding interactions between the fundamental units, which emit at lower energies and relax faster (1.4-1.5 and 8.5-8.8 ns) with respect to the crystals described in (1) (see Figure 4c′,d′). The shorter emission lifetimes of crystals described in (2) above showed the importance of Hbonds in their structures that might lead to PT reactions in the building blocks. In contrast, both the spectral and dynamical properties of larger (>30 µm) T12-apo crystals did not significantly change. A biexponential behavior of the emission decays was found with lifetimes of 4-6.5 and ~17-22 ns, attributed respectively to ICT and non-H-bonded species. Due to its Commission Internationale de l'Éclairage (CIE) coordinates (0.42, 0.55) and a high (25%) fluorescence quantum yield (ΦF), T12-apo might be used for the creation of a metal-free white light-emitting diode (WLED) after covering a blue LED with this HOF.
Further expansion of the DBA structure gave birth to a new π-conjugated system, named the phenylene-ethynylene macrocycle, Ex-COOH or its methyl ester derivative (Ex-COOMe) (see Figure 1) [19,104]. In these frameworks, pore-II has an r value of 11.4 Å, which means that the pore has an almost regular hexagonal shape. Furthermore, pore-II has a diameter of 7.4 Å when the phenylene ring of the fundamental unit of Ex is perpendicular to the molecular plane. Ensemble behavior studies revealed a multiexponential emission decay of both solid samples of Ex-ester and Ex-apo, suggesting a heterogeneity of the systems. Emissions from locally excited (LE) and charge-transfer (CT) states were reported and discussed, with dynamics faster than those found in the parent T12-apo HOF (see above). In particular, the LE species displayed the shortest lifetimes (100-200 picoseconds (ps) for Ex-apo and Ex-ester, respectively), while those formed after fast ICT reactions (<15 ps) were slightly longer (420-870 ps and 1.43-3 ns for Ex-apo and Ex-ester, respectively) (see Scheme 1). Furthermore, an additional component of 4.20 ns in the emission decay was found for Ex-apo and was assigned to anions originating from strong Hbonding interactions between the Ex-COOH units. Confocal fluorescence microscopy experiments on single crystals of Ex-apo disclosed no site-dependent photobehavior. Emission anisotropy experiments proved a good orientation of the emitters in the crystals.
Another C 3 PI unit possessing a good planarity and π-conjugation similar to the previously mentioned DBA-derivatives is the hexakis-(carboxyphenyl)triphenylene (Tp) unit (Figure 1), which is responsible for the formation of a dual-pored H-HexNet. Flexible layered 2D HOFs (LA-H-HexNets) with permanent porosity are then formed by the assembly of several H-HexNet sheets ( Figure 5). Due to a distorted conformation assumed by the PhT molecular units, these structures show a good solubility and provide shape-persistent pores [45,105]. By introducing methyl (Me) or an F atom at the o-positions of the carboxyphenyl groups, two new compounds, named hexakis(4-carboxy-3,5-dimethylphenyl) triphenylene (TpMe) and hexakis(4-carboxy-3,5-difluorophenyl)triphenylene (TpF), were obtained ( Figure 1) [106]. Substitution had no effect on the binding energy of the H-bonded dimerization, which was~15 kcal mol −1 for all the obtained materials. On the other hand, Me and F substituents caused a twisting of the carboxyl and phenylene groups, thus increasing the flexibility of the peripheral conformation. Activation, defined as the desolvation with retaining pores, of LA-H-HexNets of TpMe and TpF leads to the formation of crystalline TpMe-apo and TpF-apo, respectively. Upon desolvation, both emission and excitation intensity maxima of TpMe-2Ds (TpMe-LA-H-HexNets before the activation) were shifted toward lower energies, indicating that TpMe-apo experiences stronger intermolecular interactions than TpMe-2Ds. A similar behavior was observed for TpF-1 (the TpF-LA-H-HexNets before the activation). Confocal fluorescence microscopy experiments on single crystals of Ex-apo disclosed no site-dependent photobehavior. Emission anisotropy experiments proved a good orientation of the emitters in the crystals.
Another C3PI unit possessing a good planarity and π-conjugation similar to the previously mentioned DBA-derivatives is the hexakis-(carboxyphenyl)triphenylene (Tp) unit (Figure 1), which is responsible for the formation of a dual-pored H-HexNet. Flexible layered 2D HOFs (LA-H-HexNets) with permanent porosity are then formed by the assembly of several H-HexNet sheets ( Figure 5). Due to a distorted conformation assumed by the PhT molecular units, these structures show a good solubility and provide shape-persistent pores [45,105]. By introducing methyl (Me) or an F atom at the o-positions of the carboxyphenyl groups, two new compounds, named hexakis(4-carboxy-3,5-dimethylphenyl)triphenylene (TpMe) and hexakis(4-carboxy-3,5-difluorophenyl)triphenylene (TpF), were obtained ( Figure 1) [106]. Substitution had no effect on the binding energy of the Hbonded dimerization, which was ~15 kcal mol −1 for all the obtained materials. On the other hand, Me and F substituents caused a twisting of the carboxyl and phenylene groups, thus increasing the flexibility of the peripheral conformation. Activation, defined as the desolvation with retaining pores, of LA-H-HexNets of TpMe and TpF leads to the formation of crystalline TpMe-apo and TpF-apo, respectively. Upon desolvation, both emission and excitation intensity maxima of TpMe-2Ds (TpMe-LA-H-HexNets before the activation) were shifted toward lower energies, indicating that TpMe-apo experiences stronger intermolecular interactions than TpMe-2Ds. A similar behavior was observed for TpF-1 (the TpF-LA-H-HexNets before the activation).

HOFs Based on Hexaazatriphenylene (HAT) and -Naphthylene (HATN) Derivatives
The use of heterocyclic π-conjugated compounds such as hexaazatriphenylene (HAT) derivatives leads to the formation of significantly robust HAT-based HOF systems thanks to the rigid, planar, and π-conjugated skeleton of their building blocks. The Hbonded dimers of carboxyl groups have the leading roles in the formation of the supramolecular architecture of these HOFs.
The first case of a HAT-based HOF presenting permanent porosity was reported in 2017. The fundamental unit was a HAT bearing six peripheral carboxyphenyl groups (CPHAT, Figure 1), which formed a three-dimensional (3D) framework with permanent porosity (CPHAT-1-(TCB = 1,2,4-trichlorobenzene)) (see Figure 6) [97]. The activation of CPHAT-1-(TCB) led to the corresponding CPHAT-1a, which preserved both permanent porosity and single-crystallinity. The pore's size in this HOF was 6.4 Å, with an SA(BET) of 649 m 2 g −1 . The larger π-conjugation of CPHAT generated by the presence of the phenyls and carboxylic groups was responsible for the redshift of its absorption spectrum with respect to that of pristine HAT. Considering that the CPHAT moieties are rotated by 60°, this arrangement can hinder the hopping of charge-carrier species [107]. This suggestion was then confirmed by flash-photolysis time-resolved microwave (FP-TRMC) measurements of CPHAT-1-(TCB) and CPHAT-1a crystals, which revealed no charge transport

HOFs Based on Hexaazatriphenylene (HAT) and -Naphthylene (HATN) Derivatives
The use of heterocyclic π-conjugated compounds such as hexaazatriphenylene (HAT) derivatives leads to the formation of significantly robust HAT-based HOF systems thanks to the rigid, planar, and π-conjugated skeleton of their building blocks. The H-bonded dimers of carboxyl groups have the leading roles in the formation of the supramolecular architecture of these HOFs.
The first case of a HAT-based HOF presenting permanent porosity was reported in 2017. The fundamental unit was a HAT bearing six peripheral carboxyphenyl groups (CPHAT, Figure 1), which formed a three-dimensional (3D) framework with permanent porosity (CPHAT-1-(TCB = 1,2,4-trichlorobenzene)) (see Figure 6) [97]. The activation of CPHAT-1-(TCB) led to the corresponding CPHAT-1a, which preserved both permanent porosity and single-crystallinity. The pore's size in this HOF was 6.4 Å, with an SA(BET) of 649 m 2 g −1 . The larger π-conjugation of CPHAT generated by the presence of the phenyls and carboxylic groups was responsible for the redshift of its absorption spectrum with respect to that of pristine HAT. Considering that the CPHAT moieties are rotated by 60 • , this arrangement can hinder the hopping of charge-carrier species [107]. This suggestion was then confirmed by flash-photolysis time-resolved microwave (FP-TRMC) measurements of CPHAT-1-(TCB) and CPHAT-1a crystals, which revealed no charge transport ability [108,109]. CPHAT-1-(TCB) and CPHAT-1a crystals exhibited low emission quan-tum yields (Φ F ) due to efficient nonradiative deactivations triggered by strong stacking molecular interactions. Single-molecule fluorescence microscopy experiments indicated highly anisotropic emission behaviors, reflecting that both CPHAT-1-(TCB) and CPHAT-1a possess ordered crystalline structures, with molecular dipole moments oriented perpendicularly with respect to the long crystal axis. Moreover, CPHAT-1-(TCB) presented a heterogeneous distribution of the molecular interactions due to the presence of TCB, while for CPHAT-1a, the distribution was very homogeneous, with the units forming the crystal having similar interactions. ability [108,109]. CPHAT-1-(TCB) and CPHAT-1a crystals exhibited low emission quantum yields (ΦF) due to efficient nonradiative deactivations triggered by strong stacking molecular interactions. Single-molecule fluorescence microscopy experiments indicated highly anisotropic emission behaviors, reflecting that both CPHAT-1-(TCB) and CPHAT-1a possess ordered crystalline structures, with molecular dipole moments oriented perpendicularly with respect to the long crystal axis. Moreover, CPHAT-1-(TCB) presented a heterogeneous distribution of the molecular interactions due to the presence of TCB, while for CPHAT-1a, the distribution was very homogeneous, with the units forming the crystal having similar interactions. Addition of three benzenes at the HAT unit led to the corresponding carboxyphenylsubstituted hexaazatrinaphthylene derivative (CPHATN) as the fundamental unit ( Figure  1) [79].
This chemical modification had the aim to boost both the electronic and sensing capability of CPHAT-1a. The H-HexNet resulted from the union of the CPHATN building blocks further assemble to result in a layered HOF (Figure 7a), the activated form of which was named CPHATN-1a. The latter had a pore diameter of 7.8 Å, which was larger than the one measured for CPHAT-1a (6.4 Å). Moreover, the three extra benzenes conferred to the core of CPHATN-1a a greater π-conjugation with respect to the case of CPHAT-1a; this resulted in redshifted steady-state absorption and emission spectra of the former compared to the latter. The difference among the excitation and absorption spectra suggested the existence of efficient nonradiative relaxation channels due to the strong coupling of the unpaired core nitrogen electrons with the (n-π*) states. Time-resolved (from fs to ps) experiments proved the existence of photoinduced ICT (ultrafast; ≤100 fs) and intermolecular proton transfer (iPT, fast; 1.1 ps) reactions involving the phenyl groups and the main core in the case of ICT, or the acid groups of the fundamental units of the crystals in the case of iPT (Figure 7b,c). The high anisotropic emission performance (short-width histogram of the emission anisotropy) of a CPHATN-1a single crystal demonstrated a preferential orientation of the molecular dipole moments perpendicular to the long axis with the π-π stacking. The main crystal anisotropy value (−0.44) was accompanied by a second one centered at −0.2, which was assigned to a minor population of the smallest crystals adsorbed on the surface of the larger one and possible defects. The ensemble anisotropy histogram presented a larger width compared to that of the histogram for the single crystal, plainly revealing the existence of differently oriented crystals (Figure 8). Addition of three benzenes at the HAT unit led to the corresponding carboxyphenylsubstituted hexaazatrinaphthylene derivative (CPHATN) as the fundamental unit ( Figure 1) [79].
This chemical modification had the aim to boost both the electronic and sensing capability of CPHAT-1a. The H-HexNet resulted from the union of the CPHATN building blocks further assemble to result in a layered HOF (Figure 7a), the activated form of which was named CPHATN-1a. The latter had a pore diameter of 7.8 Å, which was larger than the one measured for CPHAT-1a (6.4 Å). Moreover, the three extra benzenes conferred to the core of CPHATN-1a a greater π-conjugation with respect to the case of CPHAT-1a; this resulted in redshifted steady-state absorption and emission spectra of the former compared to the latter. The difference among the excitation and absorption spectra suggested the existence of efficient nonradiative relaxation channels due to the strong coupling of the unpaired core nitrogen electrons with the (n-π*) states. Time-resolved (from fs to ps) experiments proved the existence of photoinduced ICT (ultrafast; ≤100 fs) and intermolecular proton transfer (iPT, fast; 1.1 ps) reactions involving the phenyl groups and the main core in the case of ICT, or the acid groups of the fundamental units of the crystals in the case of iPT (Figure 7b,c). The high anisotropic emission performance (short-width histogram of the emission anisotropy) of a CPHATN-1a single crystal demonstrated a preferential orientation of the molecular dipole moments perpendicular to the long axis with the π-π stacking. The main crystal anisotropy value (−0.44) was accompanied by a second one centered at −0.2, which was assigned to a minor population of the smallest crystals adsorbed on the surface of the larger one and possible defects. The ensemble anisotropy histogram presented a larger width compared to that of the histogram for the single crystal, plainly revealing the existence of differently oriented crystals (Figure 8).     Remarkably, CPHATN-1a has been shown to be an efficient and reversible sensor for HCl vapors [79]. Under HCl-saturated atmosphere, a clear color change (from yellow to reddish brown) was observed only in the solid-state as the consequence of the HATN core protonation and additional intermolecular interactions between protonated and neutral HATN units (Figure 9(ai,aii)). The pristine color was practically recuperated after heating at 150 • C for 30 min (Figure 9(aiii)). The absorption spectrum of acidified CPHATN-1a revealed a new band at 500-600 nm (Figure 9b), while the emission signal was strongly reduced (Figure 9c). Moreover, by removing the HCl exposure and clearing the crystals in air at ambient temperature for 48 h, 90% of both the absorption and emission intensities were recovered (Figure 9b,c). Remarkably, CPHATN-1a has been shown to be an efficient and reversible sensor f HCl vapors [79]. Under HCl-saturated atmosphere, a clear color change (from yellow reddish brown) was observed only in the solid-state as the consequence of the HATN co protonation and additional intermolecular interactions between protonated and neutr HATN units (Figure 9(ai,aii)). The pristine color was practically recuperated after heatin at 150 °C for 30 min (Figure 9(aiii)). The absorption spectrum of acidified CPHATNrevealed a new band at 500-600 nm (Figure 9b), while the emission signal was strong reduced (Figure 9c). Moreover, by removing the HCl exposure and clearing the crysta in air at ambient temperature for 48 h, 90% of both the absorption and emission intensiti were recovered (Figure 9b,c). The structure of CPHAT was also modified by adding another benzene ring to ea carboxyphenyl group, thus obtaining a HAT bearing six peripheral carboxybiphen groups (CBPHAT, Figure 1) [110]. Remarkably, this HOF presented pore (12.4 Å) an SA(BET) (1288 m 2 g −1 ) values of twice those corresponding to CPHAT-1. The solid-sta UV-visible absorption spectra of CBPHAT-1a ( Figure 6) and its butylated derivativ (CBPHAT-C4H9) were quite similar, consisting of broad bands indicative of the presen of different ground-state species. On the other hand, the fluorescence spectrum CBPHAT-1a showed an additional emission assigned to anions produced by strong H bonding interactions between the networking HOF's units. Figure 10a,b exhibits the p emission decays and time-resolved emission spectra (TRES) of solid CBPHAT-1a HO An ICT of 60 ps time constant was observed in the excited CBPHAT-1a molecules, an which was decaying within 380 ps. Two different populations, relaxing at 1.02 and 2. ns, were produced after the ICT process. The recorded TRES of CBPHAT-1a proved th formation of a new emission band at 566 nm at longer times (>350 ps) (Figure 10b), a signed to anions. As in the case of CPHATN-1a, the CBPHAT-1a crystal was HCl-respo sive (Figure 10c,d). The new absorption band at ~600 nm suggested a robust interaction The structure of CPHAT was also modified by adding another benzene ring to each carboxyphenyl group, thus obtaining a HAT bearing six peripheral carboxybiphenyl groups (CBPHAT, Figure 1) [110]. Remarkably, this HOF presented pore (12.4 Å) and SA(BET) (1288 m 2 g −1 ) values of twice those corresponding to CPHAT-1. The solid-state UV-visible absorption spectra of CBPHAT-1a ( Figure 6) and its butylated derivative (CBPHAT-C 4 H 9 ) were quite similar, consisting of broad bands indicative of the presence of different groundstate species. On the other hand, the fluorescence spectrum of CBPHAT-1a showed an additional emission assigned to anions produced by strong H-bonding interactions between the networking HOF's units. Figure 10a,b exhibits the ps-emission decays and time-resolved emission spectra (TRES) of solid CBPHAT-1a HOF. An ICT of 60 ps time constant was observed in the excited CBPHAT-1a molecules, and which was decaying within 380 ps. Two different populations, relaxing at 1.02 and 2.66 ns, were produced after the ICT process. The recorded TRES of CBPHAT-1a proved the formation of a new emission band at 566 nm at longer times (>350 ps) (Figure 10b), assigned to anions. As in the case of CPHATN-1a, the CBPHAT-1a crystal was HCl-responsive (Figure 10c,d). The new absorption band at 600 nm suggested a robust interaction of the nitrogen atoms of the HAT core with the acid protons. Moreover, the new band intensity increased with the exposure time. The color change could be easily appreciated by the naked eye (upper pictures in Figure 10c,d). Once in contact with HCl vapors, the emission spectrum of CBPHAT-1a shifted toward the red (Figure 10d) due to a protonation of the nitrogen sites of the core of CBPHAT-1a. Remarkably, the CBPHAT-1a/HCl interaction (adsorption, desorption) was reversible even at ambient temperature.
Int. J. Mol. Sci. 2022, 23, x FOR PEER REVIEW 11 of the nitrogen atoms of the HAT core with the acid protons. Moreover, the new band inten sity increased with the exposure time. The color change could be easily appreciated by th naked eye (upper pictures in Figure 10c,d). Once in contact with HCl vapors, the emissio spectrum of CBPHAT-1a shifted toward the red (Figure 10d) due to a protonation of th nitrogen sites of the core of CBPHAT-1a. Remarkably, the CBPHAT-1a/HCl interactio (adsorption, desorption) was reversible even at ambient temperature. Modification of the arms of CBPHAT-1a with 1,2-diphenylethyne and 4,7-diph nylbenzo-2,1,3-thiadiazole resulted in the formation of more expanded HOFs (TolHAT and ThiaHAT-1) [111]. Particularly, ThiaHAT-1 ( Figure 6) showed a great stability up t 305 °C and a high BET surface area of 1394 m 2 g −1 , with a pore diameter of 15.5 Å. Th photophysical properties of Tol-HAT-1 and ThiaHAT-1 were elucidated by steady-sta and time-resolved spectroscopy, as well as single-crystal fluorescence microscopy. Whi TolHAT-1 was not stable under UV light excitation, probably due to the presence of th ethynyl moieties in the fundamental unit, ThiaHAT-1 exhibited an outstanding stabilit under light irradiation. Crystals of ThiaHAT-1 showed two broad absorption bands cen tered at ~435 and 600 nm together with a bright yellow emission upon 365 nm-illumin tion, with a ΦF of 8%.
Time-resolved ps studies under 371 and 515 nm excitation revealed comparable mu tiexponential photobehavior, with lifetimes of 160-180 ps, 710-720 ps, and 2.3-2.5 n Modification of the arms of CBPHAT-1a with 1,2-diphenylethyne and 4,7-diphenylbenzo-2,1,3-thiadiazole resulted in the formation of more expanded HOFs (TolHAT-1 and ThiaHAT-1) [111]. Particularly, ThiaHAT-1 ( Figure 6) showed a great stability up to 305 • C and a high BET surface area of 1394 m 2 g −1 , with a pore diameter of 15.5 Å. The photophysical properties of Tol-HAT-1 and ThiaHAT-1 were elucidated by steady-state and time-resolved spectroscopy, as well as single-crystal fluorescence microscopy. While TolHAT-1 was not stable under UV light excitation, probably due to the presence of the ethynyl moieties in the fundamental unit, ThiaHAT-1 exhibited an outstanding stability under light irradiation. Crystals of ThiaHAT-1 showed two broad absorption bands centered at~435 and 600 nm together with a bright yellow emission upon 365 nm-illumination, with a Φ F of 8%.
Time-resolved ps studies under 371 and 515 nm excitation revealed comparable multiexponential photobehavior, with lifetimes of 160-180 ps, 710-720 ps, and 2.3-2.5 ns. These components were respectively assigned to the emissions of LE, CT, and anionic species, similar to what was observed for the isostructural CBPHAT-1. Fs-emission experiments on ThiaHAT-1 in a DMF suspension allowed us to determine the time constants for the excited-state PT (4.4 ps) and CT (450 fs) reactions ( Figure 11). ThiaHAT-1 also exhibited an excellent response to the presence of HCl vapors, and which could be easily observed by the naked eye under daylight or UV irradiation. The exposure of this HOF to HCl vapors for short times (5 min to 1 h) led to a strong bright yellow-to-dark-brown color change (Figure 12a). The ThiaHAT-1-HCl interaction could be clearly appreciated in both the absorption and emission spectra of the investigated HOF (Figure 12b-d). The additional emission band at 700 nm recorded in the presence of HCl reflected the protonation of ThiaHAT-1. Interestingly, ThiaHAT-1 preserved the dark brown color and quenched emission after the HCl treating, thus making this HOF a promising candidate for the construction of an HCl vapochromic smart sensor.
These components were respectively assigned to the emissions of LE, CT, and anionic species, similar to what was observed for the isostructural CBPHAT-1. Fs-emission experiments on ThiaHAT-1 in a DMF suspension allowed us to determine the time constants for the excited-state PT (4.4 ps) and CT (450 fs) reactions ( Figure 11). ThiaHAT-1 also exhibited an excellent response to the presence of HCl vapors, and which could be easily observed by the naked eye under daylight or UV irradiation. The exposure of this HOF to HCl vapors for short times (5 min to 1 h) led to a strong bright yellow-to-dark-brown color change (Figure 12a). The ThiaHAT-1-HCl interaction could be clearly appreciated in both the absorption and emission spectra of the investigated HOF (Figure 12b-d). The additional emission band at 700 nm recorded in the presence of HCl reflected the protonation of ThiaHAT-1. Interestingly, ThiaHAT-1 preserved the dark brown color and quenched emission after the HCl treating, thus making this HOF a promising candidate for the construction of an HCl vapochromic smart sensor.

HOFs with Unusual H-Bonding Topology
In the previous sections, we highlighted the importance of highly symmetric plana π-conjugated cores that, armed with directional H-bonding interactions, can reasonabl provide well-defined, 2D-networked porous sheets that then pile into crystalline layere frameworks. However, 2D frameworks made of nonplanar π-conjugated molecules ar likewise interesting constructions. Sheets with bent and/or bumpy surfaces are in fact sup posed to possess exclusive electronic, chemical, or physical properties due to the curve π-systems [112][113][114][115]. Nowadays, it is still a stimulating challenge to develop such new classes of functional materials. Basing on these considerations, we proposed the creatio of H-bonded 2D frameworks by utilizing the C3-symmetric buckybowl, sumanene [116 118], which presents a distorted triphenylene moiety, and thus is an appropriate system to compare to our previous system of Tp HexNet frameworks [94]. The H-bonded 2D a chitecture of the buckybowl hexakis(carboxyphenyl)sumanene derivative CPSM provide two kinds of H-bonded 2D HexNet frameworks: (1) a waved HexNet structure with hx (hxl = hexagonal lattice) network topology composed of alternate alignment of up-an downward bowls (CPSM-1); and (2) a bilayered HexNet structure (CPSM-2) in which a of the six carboxy groups contribute to form H-bonds to give hamburger-shaped su manene dimers. Furthermore, CPSM-2 crystals underwent highly anisotropic shrinkin along the c axis by about 11% after applying a uniform hydrostatic pressure of 970 MPa This shrinking was caused by interlayer slithering of the bilayered HexNet sheets alon the curved sumanene surfaces. This behavior can provide new understanding of 2D frameworks built from nonplanar π-conjugated systems.
To continue with another example of an HOF with unusual topology, we recentl reported the first 3D-networked HOF based on a carboxyphenyl-substitute (b) Absorption and (c) emission spectra of ThiaHAT-1 before and after being exposed to vapors of HCl.

HOFs with Unusual H-Bonding Topology
In the previous sections, we highlighted the importance of highly symmetric planar π-conjugated cores that, armed with directional H-bonding interactions, can reasonably provide well-defined, 2D-networked porous sheets that then pile into crystalline layered frameworks. However, 2D frameworks made of nonplanar π-conjugated molecules are likewise interesting constructions. Sheets with bent and/or bumpy surfaces are in fact supposed to possess exclusive electronic, chemical, or physical properties due to the curved π-systems [112][113][114][115]. Nowadays, it is still a stimulating challenge to develop such new classes of functional materials. Basing on these considerations, we proposed the creation of H-bonded 2D frameworks by utilizing the C 3 -symmetric buckybowl, sumanene [116][117][118], which presents a distorted triphenylene moiety, and thus is an appropriate system to compare to our previous system of Tp HexNet frameworks [94]. The H-bonded 2D architecture of the buckybowl hexakis(carboxyphenyl)sumanene derivative CPSM provides two kinds of H-bonded 2D HexNet frameworks: (1) a waved HexNet structure with hxl (hxl = hexagonal lattice) network topology composed of alternate alignment of up-and downward bowls (CPSM-1); and (2) a bilayered HexNet structure (CPSM-2) in which all of the six carboxy groups contribute to form H-bonds to give hamburger-shaped sumanene dimers. Furthermore, CPSM-2 crystals underwent highly anisotropic shrinking along the c axis by about 11% after applying a uniform hydrostatic pressure of 970 MPa. This shrinking was caused by interlayer slithering of the bilayered HexNet sheets along the curved sumanene surfaces. This behavior can provide new understanding of 2D-frameworks built from nonplanar π-conjugated systems.
To continue with another example of an HOF with unusual topology, we recently reported the first 3D-networked HOF based on a carboxyphenyl-substituted tri(dithiolylidene) cyclohexanetrione (DC) derivative, CPDC [119]. The related HOF, CPDC-1, is assembled through an anomalistic helical H-bonded motif, instead of the conventional planar or simple helical motifs. This unusual H-bonding motif probably originates from a slightly larger bite angle (θ) of CPDC. As a consequence, CPDC-1 forms a robust 3D, non-interpenetrated network that preserves its stability up to 372 • C. The absorption spectrum of the ester derivative exhibited three mean peaks at 246, 370, and 470 nm, the latter displaying the maximum intensity. The hexabenzoate showed a 491 nm-centered emission band (exciting at 370 nm) with a relative weak Φ F of 0.76%. Additionally, the highly π-delocalized redox core of CPDC promoted intermolecular π-stacking and S-S interactions (S refers to each sulfur atom in the dithiolylidene ring), supporting charge conduction in the HOF. From the absorption edge at 488 nm, an optical band gap of 2.54 eV was determined. Differential pulse voltammetry (DPV) experiments were carried out, aiming to define the redox behavior of the ester. The results indicated that all the three dithiole rings underwent sequential oxidation and reduction reactions. The HOMO and LUMO energies were found to be −5.82 and −3.57 eV, respectively, providing an electrochemical band gap of 2.25 eV. Theoretical calculations performed on CPDC and a methyl ester of CPDC revealed stabilization of the HOMO by electron-withdrawing carboxyl/carboxylate substituents.

Conclusions and Outlooks
In this feature article, we showed and commented on the latest progress and findings achieved by our groups on the use of C 3 -symmetric π-conjugated molecules (C 3 PIs) possessing three o-bis(4-carboxyphenyl)aryl moieties in the periphery for systematic construction of various HOFs. Planar C 3 PIs gave birth to isostructural 2D H-bonded hexagonal network (H-HexNet) sheets via H-bonded carboxyl dimers. These isostructural layers were created thanks to the so-called phenylene triangle (PhT), which is a triangular H-bonded motif. Additional non-interpenetrating stacking of the H-HexNets led to flexible porous layered HOFs (LA-H-HexNets). On the other hand, nonplanar C 3 PIs made feasible the construction of rigid 3D-networked HOFs. In particular, hexaazatriphenylene (HAT) cores formed a π-stacked column through shape-fitted docking, giving more rigid frameworks. Interpenetration of H-bonded networks also promoted the development of rigid HOFs. Interestingly, bowl-shaped C 3 PIs produced H-bonded networked structures with unusual network topology, in which all the carboxy groups were involved in the formation of H-bonded dimers. Furthermore, the use of a carboxyphenyl-substituted tri(dithiolylidene)cyclohexanetrione (DC) derivative, CPDC, also originated an HOF with an uncommon network topology. These exotically structured HOFs presented exclusive electronic, chemical, and photophysical properties. Hexatopic building blocks (i.e., with six H-bonding functional groups) can give robust frameworks with a large variety of topologies and properties. The use of carboxyl groups and simple high directional H-bonds as the molecular glue can empower the systematic construction of a series of HOFs. We join together in a unique work the experiences in HOFs' chemistry and laser-based time-resolved (in the fs-to-ms regime) spectroscopy and microscopy techniques. The HOFs showed ultrafast and slow photoinduced events, ranging in the fs-to-ms regime, and reflecting, for example, ICT, strong H-bond interactions, iPT. We emphasized two main aspects, chemical and photophysical. The latter unravelled at both the ensemble and single-crystal levels. We also referred to and examined how these aspects influenced the photonic applicability of the studied HOFs. The future of HOFs is going to be reliant on a series of key challenges that must be solved; more specifically: (1) improving the design approach and synthesis routes and crystallization of HOFs; (2) increasing their stability at ambient temperature; (3) regulating and boosting their light response for real-world applications; and (4) making feasible the design of HOF-based composites involving known fluorescent and sensor dyes for photonic applications. Computational methods are also an important tool to additionally support the breakthrough of HOFs with appropriate functions.

Conflicts of Interest:
The authors declare no conflict of interest.