Supramolecular Assembly of Fused Macrocycle-Cage Molecules for Fast Lithium-Ion Transport

Dimensionality Reduction of Metal-Organic Frameworks to Monolayers for Enhanced Electrocatalysis

Metal-organic frameworks (MOFs) are potential candidates for electrocatalysis due to their well-defined, tunable structures, and ability to incorporate diverse active sites. However, their inherent insulating nature restricts electron transfer from electrode to remote active sites, leading to diminished catalytic performance. In this work, we present a novel strategy to overcome this limitation by reducing 3D MOFs (3D_MOFs) into monolayered MOFs (monoMOFs) with a thickness of ∼1.8 nm, maximizing the exposure of catalytic sites to the electrode and enhancing electrocatalytic performance. We designed and synthesized a monoMOF incorporating cobalt(II)-porphyrin sites in the linker (monoMOF-Co) for CO2 electroreduction. After being grafted onto graphene oxide, the monoMOF-Co exhibited a peak faradaic efficiency for CO production (FECO = 93%), surpassing the performance of a 3D_MOF incorporating the same porphyrin-Co-based linker (3D_MOF-Co, FECO = 51%). Additionally, monoMOF-Co achieved a turnover frequency of 10 600 h-1 at -0.8 V versus the reversible hydrogen electrode (RHE) and maintained stability over 47 h in a near-neutral aqueous solution. In situ spectroscopic studies further confirmed the distinct electric field environment in the Stern layer between monoMOF-Co and 3D_MOF-Co. Furthermore, similar enhancement effects of monoMOFs over 3D_MOFs were observed in the nitrate and oxygen electroreduction reactions, highlighting the broader applicability of monoMOFs in electrocatalysis.

A stimuli-responsive dimeric capsule built from an acridine-based metallacycle for ratiometric fluorescence sensing of TNP

Stimulus-responsive luminescent metal-organic architectures have received a lot of attentions in supramolecular chemistry. Herein, we report the synthesis of an acridine-based metal-organic macrocycle that undergoes reversible interconversion between the monomer and the dimer states in response to variations in the concentration and solvent, resulting in a switch between blue and green fluorescence. X-ray structure analysis reveals that hydrogen bonds between benzimidazole C-H and NO3- anions, along with π-π interactions between acridines, are the primary driving forces behind this behavior of the assembly. The stimuli-responsive supramolecular fluorescence switching originates from the monomer and excimer states. The addition of 2,4,6-trinitrophenol (TNP) leads to a fluorescence "turn-off" at 430 nm for the monomer and a "turn-on" at 520 nm for the dimer, thus facilitating the ratiometric detection of TNP with the detection limit being as low as 13 ppb. Our work provides valuable insights into the construction of stimuli-responsive materials for fluorescence sensing.

One-Pot Synthesis of Tridentate Bicyclocalixarene Cage Molecules

Efficient one-pot synthesis of tridentate bicyclocalixarenes was developed by condensing phloroglucinol with dechlorinated pyrimidine or triazine with a yield up to 58%. These cage-like molecules adopt a symmetric three-blade propeller structure with different terminal function groups and arm spans ranging from 7.5 to 15.8 Å. Due to the 1,3-alternate configuration of intermediate heterocalixarenes, the cage molecules can be prepared only with benzene rings as the upper and lower caps. They will be an ideal scaffold for the preparation of ultrathin two-dimensional materials.

Metal-Free Electrocatalytic Alkaline Water Splitting by Porous Macrocyclic Proton Sponges

Macrocycles are unique as they encapsulate and transfer guest molecules or ions and facilitate catalytic processes. Although metalated macrocycles are pivotal in electrocatalytic processes, using metal-free analogs has been rare. Following the strategy of Kanbara et al., we synthesized an azacalixarene macrocycle-N, N', N''-tris(p-aminophenyl)azacalix[3](2,6)pyridine (CalixNH2). The macrocycle encapsulates a proton in its cavity, maintaining the protonation even in highly alkaline media. Notably, it retains almost 50 % protonated form in 1 M KOH (~pH 14)-acting as a proton sponge. As hydrogen evolution is complex in alkaline media owing to sluggish water dissociation, we implemented the proton sponge (CalixNH2) in an alkaline hydrogen evolution reaction. Conjugated Porous polymers, TpCalix and DhaCalix, have been synthesized from the triamine-CalixNH2. The most efficient catalyst, TpCalix, has shown excellent performance in alkaline HER and OER in 1 M KOH (~pH 14), with low overpotentials of only 112(±2) and 290(±2) mV at 10 mA cm-2, respectively, and durable up to 24 hours. A full-cell reaction using TpCalix in both the cathode and anode exhibited a low full-cell voltage of 1.73 V and was stable for 12 hours. DFT calculations verified the tripyridinic core, which acts as the principal site for proton abstraction and binding.