A single crystalline porphyrinic titanium metal–organic framework† †Electronic supplementary information (ESI) available. CCDC [1036868]. For ESI and crystallographic data in CIF or other electronic format. See DOI: 10.1039/c5sc00916b Click here for additional data file. Click here for additional da

We have successfully synthesized a single crystalline porphyrinic titanium MOF, namely PCN-22. PCN-22 represents an important step towards mimicking dye sensitized TiO2 in MOFs.


Introduction
Metal-organic frameworks (MOFs) are a promising class of highly ordered porous materials with potential applications in gas storage, catalysis, and photoelectric devices. 1-8 For heterogeneous catalytic processes, the accessible external surface of the solid-state catalyst usually plays a decisive role on the catalytic efficiency. 9,10 MOFs provide a platform to synthesize new heterogeneous catalysis with highly accessible external and internal surface and evenly distributed active sites. 11 In addition, MOFs offer a bottom-up approach to tune their structures and functionalities by judicious design of inorganic building units and/or organic linkers. [12][13][14][15][16] In previous studies, the functionalization of MOFs has mainly relied on the modication of organic linkers, whereas the functionalization of inorganic nodes has not been well explored thus far, except for utilization of open metal sites as Lewis acid for catalysis. [17][18][19][20][21][22] The main reason for this is that catalytic processes involving labile coordinated metal centers can potentially compromise the structural integrity of the MOFs. Therefore, constructing a stable MOF with reactive metal nodes poses a great challenge. The development of titanium MOF MIL-125 represents an important breakthrough in the utilization of metal nodes as functional moieties. 23 The titanium-oxo clusters in MIL-125 not only form strong bonds with benzenedicarboxylate (BDC) linkers to afford a highly stable framework, but also endow the material with photocatalytic activity. Nevertheless, the BDC linker in MIL-125 did not contribute to the catalytic process. Efforts were made to functionalize the BDC linker with amino groups acting as photosensitizers in MIL-125; however, the incorporation of efficient photosensitizers for visible-light-induced catalysis remains a challenge. [24][25][26] Porphyrin-sensitized TiO 2 has been widely investigated as a photocatalyst due to its high catalytic efficiency and chemical stability. [27][28][29][30][31] This inspired us to mimic dye-sensitized TiO 2 in a MOF matrix. The titanium-oxo clusters can act as photoactive sites with the porphyrinic ligands acting as photosensitizers to extend the optical response of MOF into the visible region through a dye sensitized mechanism. 32 In this way, the organic and inorganic components of MOF can work cooperatively, making it an integrated dye-sensitized photoactive system. By incorporating the porphyrin-sensitized TiO 2 system into a highly ordered porous material, the resulting porphyrinic titanium MOF has the following advantages: rst, the titanium-oxo clusters can be periodically arranged and separated by organic linkers on the molecular level, leading to a much higher accessible surface compared to that of bulk TiO 2 . Second, the visible light photoresponsiveness of the porphyrinic titanium MOF would be much more sensitive than that of MIL-125 and its derivatives because of the high efficiency of the porphyrin antenna. Meanwhile, the Ti 4+ ion has a high Z/r value, which forms a strong electrostatic interaction with the carboxylate ligand, resulting in an ultrastable framework. The exceptional stability and visible light photocatalytic activity should make porphyrinic titanium MOF a suitable platform for visible-light driven photocatalysis.
Despite all these merits, only two titanium MOFs (MIL-125 and NTU-9) have been reported previously. 23,33 The underdevelopment of titanium MOFs is due to the difficulty in achieving high crystallinity. To obtain single crystals, a reversible bond association/dissociation process is required to allow sufficient structure reorganization and defect reparation. The strong Ti-O bond, however, makes the bond dissociation extremely difficult during MOF growth. Our previous work has demonstrated that the reversibility of the MOF crystallization process can be promoted by increasing the temperature, using metal clusters as precursors and carboxylic acids as competing reagents. 34 With the adoption of a preformed titanium-oxo carboxylate cluster as the metal source, TCPP (TCPP ¼ tetrakis(4-carboxyphenyl)porphyrin) as the organic linker and benzoic acid as a modulating reagent, a single crystalline titanium MOF has been successfully obtained, designated as PCN-22 (PCN ¼ porous coordination network). To the best of our knowledge, this is the rst single crystalline titanium MOF based on carboxylate linkers. 35 This work is a breakthrough in the synthesis of highly crystalline titanium MOF, which would be conducive to the development of titanium MOF chemistry. Moreover, the single crystalline product provides an opportunity to thoroughly characterize the structure and understand the relationship between structure and properties, which will be instructive for future design and synthesis of functional titanium MOFs. Moreover, PCN-22 demonstrates an example of mimicking the porphyrin-sensitized TiO 2 in a porous MOF matrix. It shows photocatalytic activities toward alcohol oxidation with extensive optical response in the visible-light region, illustrating the synergetic effect of combining inorganic and organic components in a tailored framework.

Results and discussion
PCN-22 was prepared by the reaction of Ti 6 O 6 (O i Pr) 6 (abz) 6 (abz ¼ 4-aminobenzoate), 36 TCPP and benzoic acid under solvothermal conditions. The preformed Ti 6 O 6 (O i Pr) 6 (abz) 6 cluster was used as a starting material instead of the titanium salt, which not only slows down the crystallization process but also effectively diminishes the hydrolysis of Ti 4+ ions, avoiding the formation of TiO 2 . Moreover, the air stable titanium-oxo carboxylate clusters were also easier to handle compared to titanium salts. When reactions were conducted with Ti(O i Pr) 4 or TiCl 4 instead of titanium-oxo clusters under identical conditions, no crystalline products were obtained. At the same time, an excess amount of benzoic acid was added as a competing reagent, which could further slow down the forward reaction during the MOF growth process, favoring the formation of crystalline products.
Single-crystal X-ray diffraction analyses revealed that PCN-22 crystalizes in the monoclinic P2/m space group. Three Ti atoms are jointed into a Ti 3 O 3 cluster by a m 3 -O 2À ion and six carboxylates. A pair of Ti 3 O 3 clusters is further bridged by a Ti atom to form an unprecedented Ti 7 O 6 cluster (Fig. 1b). Overall, the 12-connected Ti 7 O 6 cluster is composed of seven Ti 4+ ions, two m 3 -O 2À ions, two terminal O 2À ions, two terminal OH À ions and two bridging DEF molecules. Each Ti 3 O 3 subunit is connected with six TCPP linkers to construct a two dimensional layer. The adjacent 2D layers are further linked by bridging Ti atoms, forming a 3D framework. A tetragonal channel with a diameter of $1.5 nm is observed in PCN-22. Topologically, each Ti 3 O 3 can be regarded as a 6-connected node and a TCPP linker can be seen as a 4-connected node. The overall structure is simplied into a novel (4, 6) connected net with a point symbol of {4 4 $6 2 } 3 {4 9 $6 12 } 2 , which has never been reported to the best of our knowledge. 37,38 The phase purity of PCN-22 was conrmed by comparison of the powder X-ray diffraction pattern and the simulated pattern from the crystal structure (Fig. S5 †). The porosity of PCN-22 was examined by nitrogen sorption experiments at 77 K (Fig. 2). A N 2 uptake of 430 cm 3 g À1 (STP) and a Brunauer-Emmett-Teller (BET) surface area of 1284 m 2 g À1 were observed for PCN-22. Density Functional Theory (DFT) calculations from the N 2 sorption curve indicate that there is one type of pore with a diameter of 1.5 nm (Fig. S6 †) assigned to the solvent accessible tetragonal channel, which is consistent with the crystallographic data when van der Waals radius is considered (Fig. S8 †).
PCN-22 was obtained as dark red crystals (Fig. 1a). The diffuse reectance UV-vis spectrum of PCN-22 shows a broad range of absorption from 200 nm to 640 nm (Fig. S10 †). The calculated band-gap of PCN-22 from the Tauc plot 39 is 1.93 eV, which is smaller than the reported Ti-based MOFs MIL-125 (3.6 eV) and MIL-125-NH 2 (2.6 eV). [24][25][26] The photocurrent prole of PCN-22 indicates that this material is active under visible light (>450 nm) illumination (Fig. 3a). The Mott-Schottky measurement was performed to further reveal the at-band potential of PCN-22. The positive slope of the obtained C 2À to potential plot is consistent with that of typical n-type semiconductors (Fig. 3b). It can be observed that the at-band potential (V  ) of PCN-22 is À0.47 V vs. Ag/AgCl (À0.26 V vs.  In order to test the catalytic activity of PCN-22, we designed a PCN-22/TEMPO system (Scheme 1) for a photocatalyzed alcohol oxidation reaction (TEMPO ¼ 2,2,6,6-tetramethylpiperidinyloxyl). According to the relevant research on a dye/TiO 2 / TEMPO system, 40 we proposed a probable mechanism for the PCN-22/TEMPO system. The TCPP linkers are excited by visible light to inject electrons into Ti 7 O 6 clusters, yielding [TCPP] + . Meanwhile, TEMPO is oxidized to TEMPO + by [TCPP] + , which then selectively oxidizes alcohol into aldehyde by a two-electrontransfer mechanism. The conversion of a benzyl alcohol to the corresponding benzaldehyde reaches 28% under visible-light irradiation in two hours with high selectivity (almost 100%). The turnover number (TON) is over 100, indicating a catalytic process. Moreover, as a heterogeneous catalyst, PCN-22 can be easily recovered by centrifugation without obvious decrease in activity and selectivity aer three successive runs (Table S2 †). For comparison, the catalytic performances of TiO 2 , TCPP, a mechanical mixture of TiO 2 and TCPP, as well as PCN-224 (ref. 41) (a previously reported zirconium porphyrinic MOF) were examined under the same conditions. As shown in Fig. 4, PCN-22 stands out to be the most efficient photocatalyst among these materials. PCN-224 is composed of Zr 6 O 8 clusters and TCPP ligands. It contains a channel of 1.9 nm which is similar to that of PCN-22. However, PCN-224 shows low photocatalytic activity which can be attributed to the large energy gap and fast charge recombination of the Zr 6 O 8 cluster. The low activity observed by the physical mixture of TiO 2 and TCPP can be simply ascribed to the aggregation of the active sites. In contrast, the highly porous structure of PCN-22 makes each catalytic center accessible to the substrates. In addition, the titanium-oxo clusters in PCN-22 are periodically arranged and well separated by photosensitizers, which favors fast electron transfer from the photosensitizers to the titanium-oxo clusters and enhances the catalytic performance. The better performance of PCN-22 demonstrates that the incorporation of porphyrin sensitizer and titanium-oxo clusters into MOF matrix provides a new platform for the synthesis of photocatalysts.

Conclusions
In summary, we report the rst single crystalline titanium MOF, PCN-22, which was synthesized from preformed titanium-oxo carboxylate clusters and porphyrinic ligands. PCN-22 possesses high porosity and photocatalytic activity. It represents an important step towards mimicking dye sensitized TiO 2 in   MOFs, which will extend the potential applications of MOFs to clean energy generation.
Powder X-ray diffraction (PXRD) was carried out with a BRUKER D8-Focus Bragg-Brentano X-ray Powder Diffractometer equipped with a Cu sealed tube (l ¼ 1.54178Å) at 40 kV and 40 mA. Thermogravimetric analyses (TGA) were carried out on a Shimadzu TGA-50 thermal analyzer from room temperature to 600 C at a ramp rate of 5 C min À1 in a owing nitrogen atmosphere. Fourier transform infrared (IR) measurements were performed on a SHIMADZU IR Affinity-1 spectrometer. Nuclear magnetic resonance (NMR) data were collected on a Mercury 300 spectrometer. Samples were activated by supercritical carbon dioxide using MADRIDE prior to gas adsorption. Gas sorption measurements were conducted on a Micrometritics ASAP 2020 system. Energy dispersive X-ray spectroscopy was carried out by JEOL JSM-7500F with Oxford EDS system equipped with X-ray mapping.

Synthesis
Ti 6 O 6 (O i Pr) 6 (abz) 6 . The Ti 6 O 6 (O i Pr) 6 (abz) 6 cluster (Habz ¼ 4-aminobenzoic acid) was synthesized according to a reported procedure. 34 To a 2-propanol solution (6.0 mL) containing 4-aminobenzoic acid (192.1 mg, 1.40 mmol) added was titanium(IV) isopropoxide (103.6 mL). Aer stirring for 30 min at RT, the orange-colored slurry was heated to 100 C for 77 h inside a sealed glass tube. The bright yellow crystalline product was collected, washed with 2-propanol and dried under vacuum for 3 h.

Photoelectrochemical studies
To prepare the working electrode, 10 mg PCN-22 was ground and dispersed in 2 mL acetone by sonication for 30 min. About 10 mL of the obtained slurry was coated on uorine-doped tin oxide (FTO) glass with a xed area of 0.5 cm 2 . The electrode was dried in air at 85 C for 10 min. The photoelectrochemical tests were performed using an electrochemical workstation (CHI 830b). The photocurrent test was carried out using a threeelectrode setup, in which the working electrode (PCN-22/FTO electrode) and the counter electrode (Pt plate electrode) were short-circuited. The 0.5 M Na 2 SO 4 solution was used as the electrolyte. A 30 W uorescent light bulbs was used as the visible light source. The Mott-Schottky curves were measured in dark using a three-electrode cell at frequencies of 10 Hz. The Pt plate was used as counter electrode and Ag/AgCl electrode (3 M KCl) was used as reference electrode. The at band position (V  ) determined from the intersection is approximately À0.47 V vs. Ag/AgCl (i.e. À0.26 V vs. NHE) for PCN-22. Since it is generally believed that the bottom of the conduction band in many n-type semiconductors is more negative by about 0.10 V than the at band potential, the conduction band (LUMO) of PCN-22 can be estimated to be À0.36 V vs. NHE. According to a band gap of 1.93 eV as described above, the valence band (HOMO) can be calculated to be 1.57 V vs. NHE.

Alcohol oxidation reaction
The photocatalytic selective oxidation of benzoic alcohol was performed as follows. A mixture of benzyl alcohol (200 mL, 1.78 mmol), PCN-22 (10 mg) and TEMPO (3 mg, 0.02 mmol) in CH 3 CN was transferred into a 30 mL Pyrex bottle and purged with 30 mL min À1 O 2 ow for 5 minutes. For controlled experiments, 10 mg of TiO 2 , TCPP, PCN-224, as well as a mechanical mixture of TiO 2 (2 mg) and TCPP (8 mg) were used instead of PCN-22 under the same conditions. The suspension was continuously stirred and irradiated by a 300 W Xe lamp with a light lter to cut off light of wavelength <450 nm. Every 5 minutes, the reaction mixture was sampled with a 20 mL pipettor aer 1, 5, 10, 20, 30, 40 and 120 minutes respectively. The samples were analyzed with HPLC (Shimadzu HPLC System with LC-20AD pump, SIL-20A Autoinjector and SPD-20A UV-vis Detector). The concentration of benzyl alcohol and benzaldehyde are calibrated by standard samples. The conversion and selectivity aer two hours for three successive runs is shown in Table S2. † There was no obvious decrease in activity and selectivity aer three successive runs, demonstrating a good recyclability of PCN-22 as a recyclable heterogeneous catalyst.