A phase transformable ultrastable titanium-carboxylate framework for photoconduction

Porous titanium oxide materials are attractive for energy-related applications. However, many suffer from poor stability and crystallinity. Here we present a robust nanoporous metal–organic framework (MOF), comprising a Ti12O15 oxocluster and a tetracarboxylate ligand, achieved through a scalable synthesis. This material undergoes an unusual irreversible thermally induced phase transformation that generates a highly crystalline porous product with an infinite inorganic moiety of a very high condensation degree. Preliminary photophysical experiments indicate that the product after phase transformation exhibits photoconductive behavior, highlighting the impact of inorganic unit dimensionality on the alteration of physical properties. Introduction of a conductive polymer into its pores leads to a significant increase of the charge separation lifetime under irradiation. Additionally, the inorganic unit of this Ti-MOF can be easily modified via doping with other metal elements. The combined advantages of this compound make it a promising functional scaffold for practical applications.

formates and rearrangement of the framework connection, the major channel running along the caxis contracted a little bit, which corresponds well with the decreased value of the BET surface area, Langmuir surface area and total pore volume (p/p0 = 0.990) of 690(10) m 2 /g, 780(8) m 2 /g and 0.45(5) cm 3 /g respectively.
The theoretical N2-accessible surface area was calculated based on the geometric topology of MIL-177 and a Monte Carlo integration technique where the center of mass of the probe molecule with hard sphere is "rolled" over the framework surface. In this method, a nitrogen-sized (3.681 Å) probe molecule is randomly inserted around each framework atom of the adsorbent and the fraction of the probe molecules without overlapping with the other framework atoms is then used to calculate the accessible surface area. The Lennard-Jones size parameters of the framework atoms were also taken from DREIDING force field except that the size parameter for Zr atom was taken from UFF.
The accessible volume was also calculated using a similar geometric method as mentioned above which consists of using a probe molecule with a diameter of 0 Å to determine the volume of the porous solid that is not occupied by the atoms of the framework. One obtains what is usually called the "free volume". The 1 H→ 13 C multiple-contact cross-polarization (MC-CP) 1 experiment was applied, which allows obtaining quantitative 13 C CP spectra in protonated MOFs 2 . 10 CP blocks of 1000 µs each (total contact time of 10.1 ms) with repolarization periods of 1 s were applied. The 1 H→ 13 C CP conditions used radiofrequency (RF) fields of 60 and 50 kHz on 1 H and 13 C, respectively. 256 to 1024 transients were co-added with 4 s recycle delay. 1 H SPINAL-64 decoupling 3 was applied.
The 1 H NMR spectra were recorded at MAS frequency of 30 kHz, using a 90°-180°-90° Hahnecho sequence. The 90° pulse length was 2.5 µs, and the inter-pulse delay was synchronized with one rotor period. The recycle delay was set to 5 s and 16 transients were recorded for each sample.
The 1 H→ 13 C 2D CP-heteronuclear correlation (CP-HETCOR) NMR spectrum was recorded at MAS 30 kHz, using a contact times of 3 ms. 120 t1 slices with 1024 transients each were co-added (recycle delay of 2 s). The spectra were analyzed using the dmfit software 4 . The 1 H and 13 C chemical shifts were referenced to proton and carbon signals in TMS. The band gap was determined from diffuse reflectance data using the Kubelka−Munk (KM) method, which is given by the following equation: Where R is the reflectance, F(R) is the KM function, and K and S are the absorption and scattering coefficients, respectively.
For allowed direct transition evidenced by analysis of the band structure (Fig. S16), the optical band gap for MIL-177-HT is determined by preparing a Tauc Plot ([F(R)  h] 2 vs h). 5 Extrapolation of this line to the photon energy axis yields a band gap of 3.67eV. The final structure model of MIL-177-HT issued from the joint experimental-modelling approach was further optimized with a view to the electronic structure calculations. The initial optimization was carried out using the PBE-sol functional. The subsequent optimization used the HSE06 hybrid DFT-Hartree Fock functional 6 , which has been shown to be accurate for the calculation of band gaps and HOMO-LUMO gaps 7,8,9 . In this functional, 25% of the exchange part is treated using Hartree-Fock. The electronic band structures and density of states (DOS) were calculated using the VASP planewave code 10 , using the same strategy as Hendon et al 11 (PBEsol functional and PAW scalar relativistic pseudopotentials 12 , and an energy cut-off of 500 eV).
The fermi levels of MIL-177-HT and TiO2 anatase are as follows -3.1 eV and -2.6 eV respectively.
The band structures of MOFs mostly feature many blocks, due to the large unit cells and the huge number of bands. They are therefore difficult to glean information from, and it is more useful to analyze the PDOS. This was seen to be the case when we calculated the band structure of MIL-177 HT. This explains why the analysis provided in the paper mostly focused on the PDOS rather than the band structure.
where  is the angular frequency of microwave used in the present system (9.1 GHz). The derived value of c was c ~ 50 ns, thus to address the peak conductivity soon after an excitation by the short-enough pulses, the deconvolution by another Gaussian function with the standard deviation of 50 ns was carried out. Eventually, as seen in the figure, the maximum value of  was derived as 4  10 -4 cm 2 V -1 s -1 , and this is the case of the value of mobility given in the manuscript.
Details of the set of apparatus were described elsewhere 13 . The χMT product is almost constant between 50 and 300 K, indicating a paramagnetic behavior of the Fe centers. This is further confirmed by the evolution of the magnetization as function of magnetic field at various temperatures (linear at 300 K, no saturation at LT). The value of the χMT product of ca. 3.6 cm 3 .K.mol -1 at 300 K is characteristic of HS Fe(III) (S=5/2). The oxidation state of Fe is also confirmed by XAS measurement at the K edge of iron. This result strongly excludes the possibility of encapsulating Fe2O3 in the pore of MIL-177-HT structure. Note that Fe2O3 should be formed during the thermal treatment for structure transformation if there is free Fe(III) species inside the porosity of MOF. Therefore, it is a solid evidence that the Fe(III) ions were doped into the inorganic unit of the MOF framework rather than staying in the pore.  The combination of nitrogen sorption and EDX results clearly show that acidic species residues trapped in the pore of MIL-177-LT after the treatment in various acids even after washing with huge amount of water.
Supplementary Diffraction data were collected for less than 2 h in continuous scanning mode and the diffractogram was obtained from the precise superposition and addition of the 21 channels data.
Extractions from the peak positions, pattern indexing, whole powder pattern decomposition, direct space strategy used to complete the structural models as well as difference Fourier calculations and Rietveld refinements were carried out with the TOPAS program 22 . In both cases, the LSIindexing method converged unambiguously to a hexagonal unit cell without systematic extinctions.
The structural determination of the MIL-177-LT was initialized with the EXPO package 23 , using EXTRA for extracting integrated intensities and SIR97 for direct-methods structure solutions. This allowed in the P622 space group to localize two independent Ti cations with some oxygen atoms of their environment as well as the central carbon atom of the mdip moiety. The direct space strategy was then used to complete the structural model and half of a linker has been considered as rigid body, added to the partial structural model, and allowed to rotate around its central carbon.
Guest water molecules and carbon atoms of formates where then located by difference Fourier maps. Taking into account the atomic coordinates of the framework solely, Platon 24 was used to check for a higher symmetry and it was deduced that the structure was centric (P6/mmm space group). At its final stage (Table S1 and Figure S1) At its final stage, the Rietveld refinement MIL-177-HT (Table S1) The resulting models were used as a starting point for further Rietveld refinement. This leads to satisfied structure models of MIL-177 as evidenced by the Rietveld plots ( Figure S1 and S2).

Supplementary Note 3, Elemental analysis
The first and second batches of data were obtained from the test carried out by Service de   The rising edge measured at the Fe K edge for the Fe-doped MIL-177-HT sample is at the same position in energy than the one measured for the crystalline α-Fe2O3 sample, strongly suggesting Fe(III) as valence state for iron in the doped MOF-Ti network. Table S6 and S7 report the results of the fits at the Fe and Ti K edges. Additionally the simulation of the EXAFS spectra (Table S6) recorded for the Fe-doped MIL-177-HT sample indicates a first coordination shell of 6 oxygen atoms and as second nearest neighbours ≈ 2 titanium atoms at 3.08 Å. This second contribution is fully consistent with the substitution of titanium by iron. As a matter of fact, a satisfactorily simulation of the Ti K edge EXAFS spectrum of the Fe-doped MIL-177-HT can be achieved by imposing the presence of an iron contribution with structural parameters (R and σ) fully constrained by the results of the fit at the iron K edge (see Table S7) and coordination number in agreement with the complete substitution of 13 at% of titanium by iron. The reduced quality factor χυ 2 with the constrained Iron contribution is found at 9400 against 14740 without this contribution pointing out that the addition of this constrained contribution is necessary to fully described the local order around Ti in the Fe-doped MIL-177-HT sample.  Electron Paramagnetic Resonance (EPR) spectra were recorded at 10 K on a Bruker Elexsys E500 spectrometer operating at X band (9.3993 GHz) equipped with a SHQ cavity. A modulation of the magnetic field at 100 kHz with amplitude of 1 G was applied to detect the absorption first