Multivariate Sulfonic-Based Titanium Metal-Organic Frameworks as Super-protonic Conductors

Excited State Transient Phenomena in Two Different Phases of the Photoactive MOF MIP-177(Ti)

The metal organic framework (MOF) MIP-177(Ti) is under the spotlight for its robust photo-response and stability. This MOF can be synthesized in forms: MIP-177(Ti)-LT (LT: low temperature) and MIP-177(Ti)-HT (HT: high temperature). The MIP-177(Ti)-LT version comprises of Ti12O15 units interconnected by 3,3',5,5'-tetracarboxydiphenylmethane (mdip) ligands and interconnecting formate groups. Upon high temperature treatment, MIP-177(Ti)-LT loses its formate groups, thus rearranging into a continuous 1-D chain of Ti6O9 units leading to the MIP-177(Ti)-HT. Based on this 1-D connected structure, one should expect a higher catalytic activity of MIP-177(Ti)-HT. Nevertheless, the hydrogen evolution reaction photoactivity assessment clearly indicates the opposite. Combining transient IR measurements (TRIR), TAS and DFT/TD-DFPT calculations unveils the reasons for this situation. The TRIR measurements evidence that the photoinduced electrons are located in the inorganic part, while the holes are in the mdip ligand. The longer lifetime of MIP-177(Ti)-LT is mapped onto a slower decay of the Ti-O related peaks. A reversible change in the coordination of the carboxylate groups from a bidentate to a monodentate coordination is observed only in MIP-177(Ti)-LT. Complementary DFT and TD-DFPT simulations demonstrate a higher electron delocalization on the inorganic part for MIP-177(Ti)-LT (hence, enhanced mobility and slower recombination), thus explaining its superior photocatalytic activity.

Exploring the Role of Amino Acid-Derived Multivariate Metal-Organic Frameworks as Catalysts in Hemiketalization Reactions

Understanding the host-guest chemistry in MOFs represents a research field with outstanding potential to develop in a rational manner novel porous materials with improved performances in fields such as heterogeneous catalysis. Herein, we report a family of three isoreticular MOFs derived from amino acids and study the influence of the number and nature of functional groups decorating the channels as a catalyst in hemiketalization reactions. In particular, a multivariate (MTV) MOF 3, prepared by using equal percentages of amino acids L-serine and L-mecysteine, in comparison to single-component ("traditional") MOFs, derived from either L-serine or L-mecysteine (MOFs 1 and 2), exhibits the most efficient catalytic conversions for the hemiketalization of different aldehydes and ketalization of cyclohexanone. On the basis of the experimental data reported, the good catalytic performance of MTV-MOF 3 is attributed to the intrinsic heterogeneity of MTV-MOFs. These results highlight the potential of MTV-MOFs as strong candidates to mimic natural nonacidic enzymes, such as glycosidases, and to unveil novel catalytic mechanisms not so easily accessible with other microporous materials.

Multivariate zeolitic imidazolate frameworks with an inverting trend in flexibility

Through systematic linker substitution in a flexible zeolitic imidazolate framework (ZIF) with step-shaped adsorption-desorption, structural intermediates between the known open and closed phases were isolated. Reflecting this, modulative sorption behaviour with an inverting adsorption pressure trend-in which the step pressure decreases and then increases again with increasing mixed linker concentration-is observed, highlighting how linker substitution modifies the energetic landscape of framework flexibility.

Photoresponsive proton conduction in Zr-based metal-organic frameworks using the photothermal effect

A highly stable and porous MOF [Zr₂(H₄TPPP)(OH/F)₂]·xH₂O (1) containing a metal-free porphyrin-phosphonate ligand is reported. It shows high proton conductivity of 1.2 × 10^-3 S·cm^-1 at 25 °C and 95% RH, a photothermal effect over a wide spectral range from UV-vis to NIR, and photo-enhanced and switchable proton conductivity.

Robust ionic liquid@MOF composite as a versatile superprotonic conductor

A highly performing proton conducting composite was prepared through the impregnation of EMIMCl ionic liquid in the mesoporous MIL-101(Cr)-SO₃H MOF. The resulting EMIMCl@MIL-101(Cr)-SO₃H composite displays high thermal and chemical stability, alongside retention of a high amount of EMIMCl even at temperatures as high as 500 K, as well as under moisture conditions. Remarkably, this composite exhibits outstanding proton conductivity not only at the anhydrous state (σ_{473 K} = 1.5 × 10^-3 S cm^-S) but also under humidity (σ_{(343 K/60%-80%RH)} ≥ 0.10 S cm^-1) conditions. This makes EMIMCl@MIL-101(Cr)-SO₃H a unique candidate to act as a solid state proton conductor for PEMFC applications under versatile conditions.