Semiconductive microporous hydrogen-bonded organophosphonic acid frameworks

Herein, we report a semiconductive, proton-conductive, microporous hydrogen-bonded organic framework (HOF) derived from phenylphosphonic acid and 5,10,15,20‐tetrakis[p‐phenylphosphonic acid] porphyrin (GTUB5). The structure of GTUB5 was characterized using single crystal X-ray diffraction. A narrow band gap of 1.56 eV was extracted from a UV-Vis spectrum of pure GTUB5 crystals, in excellent agreement with the 1.65 eV band gap obtained from DFT calculations. The same band gap was also measured for GTUB5 in DMSO. The proton conductivity of GTUB5 was measured to be 3.00 × 10−6 S cm−1 at 75 °C and 75% relative humidity. The surface area was estimated to be 422 m2 g−1 from grand canonical Monte Carlo simulations. XRD showed that GTUB5 is thermally stable under relative humidities of up to 90% at 90 °C. These findings pave the way for a new family of organic, microporous, and semiconducting materials with high surface areas and high thermal stabilities. Research in hydrogen-bonded organic frameworks (HOFs) has gained interest in recent years due to their facile design and synthesis but no semiconducting HOF has been reported to date. Here the authors report a thermally stable and proton-conductive organic semiconductor based on a porphyrin HOF.

The phosphonic acid functional group has two protons and one oxygen from the P = O bond, which allow it to form multiple hydrogen bonds with other phosphonic acid groups and thereby stabilize the resulting HOF. Interestingly, the unique structure and multiple metal-binding modes of the phosphonic acid functional group have led to some of the most thermally 34,[43][44][45][46] and chemically stable 34,[47][48][49] MOFs in the literature. The phosphonic acid functional group contains two deprotonation modes with pK a values of 1.7 and 7.4, respectively 31 . Therefore, in order to synthesize our phosphonate HOF, we adopted a crystallization method at pHs between 1.7 and 7.4 with mixed phosphonic acid linkers of phenylphosphonic acid (PPA) and 5,10,15,20-tetrakis [p-phenylphosphonic acid] porphyrin (H 8 -TPPA) to ensure that at least one of the phosphonic acid moieties is not fully deprotonated. H 8 -TPPA exhibits a planar tetratopic geometry with a 90°angle between the phenylphosphonate tethers 49,50 . Given these starting conditions and materials, it is expected that a mixed linker strategy involving H 8 -TPPA and PPA could produce twodimensional HOFs with hexagonal void channels.

Results
Design and structural characterization. The H 8 -TPPA linker was synthesized according to our previously reported method involving a Pd-catalyzed Arbuzov reaction 50 , in order to avoid the porphyrin core being occupied by Ni(II) after a Ni-catalyzed Arbuzov reaction 49,50 . The synthesized metal-free H 8 TPPA linker eliminated the possibility of metal-ligand interactions that could have triggered the formation of MOFs. GTUB5 was synthesized following a conventional MOF crystallization method in scintillation vials in DMF/EtOH at a pH between 1.7 and 7.44 to ensure the presence of protonated phosphonic acid functional groups 32 . The synthesis of GTUB5 is quite facile, giving dark purple 1-2 mm long needle-shaped crystals in high yield (see "Methods" section for synthesis details). The dark purple color of GTUB5 is an indication of its semiconductive nature. The structure of GTUB5 was characterized using single crystal X-ray diffraction. As seen in Fig. 1a, b, GTUB5 is composed of two-dimensional sheets of hydrogen-bonded H 8 -TPPA and PPA moieties. The structure contains two different hydrogen-bonding patterns, which are observed between different H 8 -TPPA units and between H 8 -TPPA and PPA (see Fig. 1e). In the first pattern, the P = O bond from the H 8 -TPPA unit is exclusively involved in creating the (almost linear) double hydrogen-bonding pattern between each unit. In the second pattern, the hydrogen bond forms between the second protonated hydroxyl group of the H 8 -TPPA and the deprotonated PPA 2− . The four DMF solvent molecules in the HOF structure act as a Lewis base acquiring the PPAs' protons. The Brunauer-Emmett-Teller (BET) surface area of GTUB5 was estimated to be 422 m 2 g −1 from a simulated N 2 adsorption isotherm at 77 K (see Supplementary Fig. 3) obtained using the grand canonical Monte Carlo method (see "Methods" section for simulation details).
Band gap measurements. The band gap was estimated from a solid-state diffuse reflectance UV-Vis spectrum of the GTUB5 crystals (see Supplementary Fig. 8). As seen in Fig. 1c, the Tauc plot derived from the spectrum yields a narrow band gap of 1.56 eV. The second jump at 2.88 eV corresponds to the Soret band of the porphyrin core at 430 nm. A similar band gap was also obtained from a UV-Vis spectrum of a dissolved sample of GTUB5 in DMSO (see Supplementary Fig. 7), suggesting that the hydrogen-bonded supramolecular structure is not disrupted in a polar aprotic solvent. From a cyclic voltammetry measurement on GTUB5 in DMSO (see Supplementary Fig. 9), the first oxidation and reduction potentials were measured to be 0.42 and −1.23 V, respectively, yielding a HOMO-LUMO gap of 1.65 eV, thereby supporting this hypothesis.
Density functional theory (DFT) calculations. To gain insight into the semiconductive nature of GTUB5, we performed DFT calculations. The details of the calculations, employing hybrid Gaussian plane-wave (GPW) and Slater-type orbital (STO) basis sets, can be found in the "Methods" section. Figure 2 shows a periodic representation of the optimized geometry, which is in close agreement with the experimental crystal structure (see Supplementary Table 3 and Supplementary Figs. 4-7). A single point calculation on the optimized structure yields a band gap of 1.65 eV, in very good agreement with the experimental result of 1.56 eV. As seen in Fig. 2, the HOMO and LUMO are predominantly localized on some of the porphyrins within the supercell (in which, a single unit cell is delineated by the black rectangle), but not all of them; with the LUMO localized on the same porphyrins as the HOMO. The fact that the HOMO and LUMO are aligned along the axis perpendicular to the plane of the page within the supercell suggests that GTUB5 is only conductive along this direction.
Focusing in on the portions of the structure that have significant HOMO and LUMO density, we see that the HOMO and LUMO are indeed localized on the same porphyrin (see Fig. 3). Moreover, they are mostly confined to a subset of the carbons and nitrogens. The HOMO is composed of π orbitals mostly on sp 2 hybridized carbons and nitrogens, while the LUMO is composed of π* orbitals on some of the sp 2 carbons and nitrogens. As shown in Table 1, ∼75% of the HOMO and LUMO orbital contributions are from the carbon and nitrogen 2p orbitals of the porphyrin. Table 1 also shows that a HOMO-LUMO transition would lead to an increase in the carbon 2p x orbital population, a slight decrease in the carbon 2p y population, and a slight increase in the carbon 2p z population; while the nitrogen  . These results suggest that the semiconductive nature of GTUB5 is predominantly determined by π-π* transitions involving orbitals localized on some of the porphyrin carbons and nitrogens. Inspection of the projected density of states (pDOS) confirms that the HOMO-LUMO gap is predominantly due to orbitals localized on carbons and nitrogens (see Fig. 4).
Thermogravimetric analysis. The thermal decomposition of GTUB5 was investigated via a thermogravimetric analysis (TGA) of hand-picked crystals up to 900°C. The TGA curve (see Supplementary Fig. 10) shows an initial 2% loss between 50 and 100°C, corresponding to the evaporation of the remaining MeOH on the crystal surface after the synthesis. The following 12% step until 250°C corresponds to the loss of dimethylammonium cations (calculated to be 12.9% based on the molecular formula). The remaining organic components of GTUB5 decompose in two steps at ca. 400 and 900°C. The second large weight loss at ca. 900°C suggests the formation of phosphides above 400°C 52 .
Proton conductivity. Given the presence of -PO 3 H 2 groups in its hydrogen-bonded framework, the proton conductivity of GTUB5 was measured. Electrochemical impedance spectroscopy measurements were carried out at 75% and 90% relative humidity (% rh) and temperatures in the range of 25-75°C (see Supplementary Information and ref. 52 for setup details). At 75%rh, we see that the proton conductivity of GTUB5 increases from 8.29 × 10 −7 to 3.00 × 10 −6 S cm −1 as the temperature is increased from 25 to 75°C, while at 90%rh the conductivities are higher but the increase is more moderate (see Table 2 for full data set). The activation energies, i.e., sum of the migration and defect formation energies, were extracted from the slopes of the Arrhenius plots (see Supplementary Fig. 15) to be E A = 0.26 eV and E A = 0.14 eV at 75 and 90°C, respectively. These low activation energies suggest that a Grotthuss mechanism with high proton mobility (and therefore low migration energy) and H-bridges to water molecules is the predominant mechanism for proton conduction through the framework. As seen in Fig. 1f, the XRD pattern of the sample recorded before and after the proton conductivity experiments slightly changes, indicating that the structure was slightly affected by the humidified atmosphere and the applied temperatures up to 75°C during the measurements. Due to the large numbers of phosphonate groups and hydrogen bonds in GTUB5, one might expect higher proton conductivities. However, in addition to the numbers of phosphonate groups and hydrogen bonds, one must also (and more importantly) consider how rigidly the phosphonate groups are connected to the framework, i.e., how much they change their positions (through rotations and vibrations), and, in turn, how strongly they interact with the water molecules. In a previous study, we found comparable proton conductivities (viz., 1.35 × 10 −6 S cm −1 at 75% rh and 80°C and 5.62 × 10 −6 S cm −1 at 90% rh and 80°C) for porous porphyrin-based metal tetraphosphonates of the CAU-29 type whose phosphonates sit similarly close to the network 53 . If, however, proton-conducting groups are linked to the framework via flexible vibrating and rotating alkyl chains, like in SO 3 H-propylsilane modified Si-MCM-41, proton conductivities higher than 10 −3 S cm −1 at 98% rh and 80°C could be achieved 54 . As quantum chemical calculations in that study confirmed, the proton conductivity is not only strongly dependent on the density of the proton-conducting groups per nm 2 , but also on the length of the alkyl chain connecting the proton-conducting group with the rigid framework. In the cases of GTUB5 and CAU-29, no such flexible alkyl chains are present.

Discussion
Herein, we presented GTUB5, a two-dimensional, microporous phosphonic acid HOF (with a calculated surface area of 422 m 2 g −1 ). Given its low band gap (as confirmed by solid-state/solution measurements and DFT calculations), GTUB5 paves the way for the creation of new semiconductive microporous organic compounds. The use of hydrogen bonds in constructing a framework comes with the advantages of simpler connectivity options and no toxic metal ions (which could possibly lead to environmentally friendly solutions for capacitors and batteries). The hydrogen-bonded framework also renders GTUB5 proton-conductive (with proton conductivities on the order of 10 −6 S cm −1 ). Owing to its structure, the phosphonic acid group can give rise to structurally diverse frameworks, which could increase the number of potential HOF applications. GTUB5 has the same band gap of 1.65 eV in DMSO, suggesting that GTUB5 retains its hydrogen-bonded network in polar aprotic solvents. Therefore, phosphonate HOFs have the potential to revolutionize the semiconductive materials industry with applications in printed electronics, optoelectronics, photovoltaics, and electrodes in supercapacitors. Currently, we are focusing on different linker geometries  and pH modulations to further optimize the pore sizes and conductive behavior of phosphonate HOFs.

Methods
Synthesis. All the reagents and solvents employed were commercially available and used as received without further purification. The linker H 8 TPPA was synthesized according to our previously reported method 50 (8.77 mg, 0.0088 mmol) and phenylphosphonic acid (PPA) (208 mg, 1.3 mmol) in a 1.6 mL mixture of DMF/EtOH or DMF/MeOH (1.36:0.24, v/v) were added to a 5-mL glass vial. The reaction mixture was ultrasonically dissolved and then heated to 80°C in an oven for 48 h. After cooling down to room temperature, dark purple block crystals of GTUB5 formed, which were then isolated by filtration, washed with DMF and acetone, and finally air-dried. The yield of GTUB5 was 5 mg.
Single crystal structure solution. Suitable single crystals of GTUB5 with appropriate dimensions (0.43 mm × 0.14 mm × 0.12 mm) were carefully chosen from the glass vial using a polarizing microscope, coated with perfluoropolyether oil in order to eliminate the possibility of decomposition, and finally mounted to a thin glass fiber. Intensity data collection was performed with a Bruker APEX II QUAZAR three-circle diffractometer equipped with the IμS Incoatec Microfocus Source with Mo-Kα radiation (λ = 0.710723 Å) at room temperature (296 K). Indexing was performed using APEX2 55 . Data integration and reduction were carried out with SAINT 56 . Absorption correction was performed by multi-scan method implemented in SADABS 57 . The structure was solved using SHELXT 58 and then refined by full-matrix least-squares refinements on F 2 using SHELXL 59 in the Olex2 Software Package 60 . The positions of all H-atoms bonded to the carbon, nitrogen, and oxygen atoms were geometrically optimized with the following HFIX instructions in SHELXL: HFIX 23 for the -CH 2moieties, HFIX 137 for the -CH 3 , HFIX 43 for the CH and NH groups of the aromatic rings and porphyrin cores, and HFIX 147 for the -P-OH groups (H1a) of the phosphonic acid moieties. Another O-bound H atom (H3) was located from the difference Fourier-map. Finally, their displacement parameters were set to isotropic thermal displacements parameters [U iso (H) = 1.2U eq for CH, NH, and CH 2 groups and U iso (H) = 1.5U eq for OH and CH 3  Also, in this crystal structure, the rotationally disordered phosphonate part (-PO 3 ) in PPA was refined as 0.77:0.23. Crystallographic data and refinement details of the data collection for GTUB5 are given in Supplementary Table 4. Crystal structure validations and geometrical calculations were performed using PLATON 61 . The Mercury software package 62 was used for visualization of the cif files.
Grand canonical Monte Carlo (GCMC) surface area calculations. The surface area of GTUB5 was calculated by force-field based atomistic simulations, which were performed with the RASPA molecular simulation package 63 . For these simulations, the GTUB5 unit cell was replicated by 1 × 2 × 4 times in the x, y, and z directions, respectively, and the replicated framework atoms were fixed in their crystallographically determined positions. Lennard-Jones (LJ) and Coulomb potentials were employed to determine the non-bonded interaction energies between atoms: where r ij is the distance between atoms i and j, ε ij and σ ij are the LJ well depth and diameter, respectively, q i is the partial charge of atom i, and ε 0 is the dielectric constant. The LJ parameters between different types of sites were calculated using the Lorentz-Berthelot mixing rules, and the Ewald summation method was employed to compute the electrostatic interactions. The LJ interactions were shifted to be 0 at a cutoff distance of 12.0 Å. For the real part of the Ewald summation, the cutoff was also set to 12.0 Å. A N 2 adsorption isotherm for GTUB5 was computed by performing GCMC simulations at 77 K and up to 0.4 bar. In the GC ensemble, the chemical potential, volume, and temperature of the system are fixed; however, the number of molecules fluctuates. For all GCMC simulations, a 100,000 cycle initialization and a 100,000 cycle production run were performed. Each cycle is N steps, where N is equal to the number of molecules in the system. Random insertions, deletions, translations, rotations, and reinsertions of the N 2 molecules were sampled with equal probability. The TraPPE force field was used to model the N 2 molecules 64 , which was originally fit to reproduce the vapor-liquid coexistence curve of N 2 . In this force field, the N 2 molecule is rigid with the N-N bond length fixed at its experimental value of 1.10 Å. This model reproduces the experimental gas-phase quadrupole moment of the N 2 molecule by placing partial charges on nitrogen atoms and on a point located at the center of mass (COM) of the molecule. Supplementary Table 2 shows the LJ parameters and partial charges for the N 2 molecule. In GCMC simulations, one computes the absolute adsorption (N total ); whereas, in adsorption experiments, the excess adsorption (N excess ) is measured. Therefore, the simulated excess adsorption of N 2 was calculated using the following expression: where ρ gas is the bulk density of the gas at simulation conditions which were calculated using the Peng-Robinson equation of state and V p is the accessible pore volume. The BET surface area of GTUB5 was obtained from the simulated N 2   Fig. 3) and estimated to be 422 m 2 g −1 .
When applying the BET theory, we made sure that our analysis satisfied the two consistency criteria as detailed by Walton and Snurr 65 .
Density functional theory (DFT) calculations. The geometry optimization of GTUB5 was performed using DFT and the conjugate gradient method 66 within the Quickstep-CP2K program 67,68 , starting from the experimental crystal structure and with the lattice vectors set to their experimental values. Since GTUB5 is a bulk material, periodic boundary conditions were applied to a reoriented 1 × 1 × 1 cell (a = 25.452 Å, b = 22.863 Å, c = 7.1798 Å, α = γ = 90.0°, β = 102.325°). The Perdew-Burke-Ernzerhof (PBE) 69 generalized gradient approximation (GGA) functional was used in conjunction with the Grimme D3 dispersion correction 70 and BJ damping 71 . The Gaussian and plane waves method 70,72 was used, with the valence orbitals expanded in terms of molecularly optimized Gaussian basis sets of double-ζ plus polarization (MOLOPT-DZVP) 73 quality and the core electrons represented by norm-conserving Goedecker-Teter-Hutter pseudopotentials 74,75 . Γ-point sampling was used and the plane-wave cutoff in reciprocal space was set to 550 Ry, with a Gaussian mapping of 60 Ry over five multi-grids. The self-consistent field was converged to 10 −6 Ry with the FULL_ALL preconditioner using the orbital transformation method with a HOMO-LUMO gap of 1.67 eV. Single point calculations were performed using CP2K to obtain the HOMO-LUMO iso-surface plots (Figs. 2 and 3), orbital populations (Table 1), and the HOMO-LUMO gap. Another single point calculation was performed using the Slater-Type Orbital (STO) software ADF-BAND 2018.104 76,77 to obtain the projected density of states (pDOS) (Fig. 4), band structure ( Supplementary Fig. 8), and the HOMO-LUMO gap. The periodic ADF-BAND calculations were performed using an all-electron double-ζ plus polarization (DZP) basis set, PBE-D3-BJ, and Γ-point sampling for the 1 × 1 × 1 unit cell, with a good numerical quality. The HOMO-LUMO gaps obtained from CP2K and ADF-BAND were both 1.65 eV (thus, the HOMO-LUMO iso-surfaces and orbital populations obtained from CP2K are expected to be the same as those from ADF-BAND).

Data availability
All data is available in the main text and the Supplementary Information. The source data for Table 1, Fig. 1c