Radical-Driven Crystal-Amorphous-Crystal Transition of a Metal-Organic Framework.

Self-assembly-based structural transition has been explored for various applications, including molecular machines, sensors, and drug delivery. In this study, we developed new redox-active metal-organic frameworks (MOFs) called DGIST-10 series that comprise π-acidic 1,4,5,8-naphthalenediimide (NDI)-based ligands and Ni2+ ions, aiming to boost ligand-self-assembly-driven structural transition and study the involved mechanism. Notably, during the synthesis of the MOFs, a single-crystal-amorphous-single-crystal structural transition occurred within the MOFs upon radical formation, which was ascribed to the fact that radicals prefer spin-pairing or through-space electron delocalization by π-orbital overlap. The radical-formation-induced structural transitions were further confirmed by the postsynthetic solvothermal treatment of isolated nonradical MOF crystals. Notably, the transient amorphous phase without morphological disintegration was clearly observed, contributing to the seminal structural change of the MOF. We believe that this unprecedented structural transition triggered by the ligand self-assembly magnifies the structural flexibility and diversity of MOFs, which is one of the pivotal aspects of MOFs.

π-Stacks of radical-anionic naphthalenediimides in a metal-organic framework.

Radical-ionic metal-organic frameworks (MOFs) have unique optical, magnetic, and electronic properties. These radical ions, forcibly formed by external stimulus-induced redox processes, are structurally unstable and have short radical lifetimes. Here, we report two naphthalenediimide-based (NDI-based) Ca-MOFs: DGIST-6 and DGIST-7. Neutral DGIST-6, which is generated first during solvothermal synthesis, decomposes and is converted into radical-anionic DGIST-7. Cofacial (NDI)2•- and (NDI)22- dimers are effectively stabilized in DGIST-7 by electron delocalization and spin-pairing as well as dimethylammonium counter cations in their pores. Single-crystal x-ray diffractometry was used to visualize redox-associated structural transformations, such as changes in centroid-to-centroid distance. Moreover, the unusual rapid reduction of oxidized DGIST-7 into the radical anion upon infrared irradiation results in effective and reproducible photothermal conversion. This study successfully illustrated the strategic use of in situ prepared cofacial ligand dimers in MOFs that facilitate the stabilization of radical ions.

Synthesis and Photocatalytic Properties of Titanium-Porphyrinic Aerogels.

We present novel titanium-porphyrinic gels (TPGs) and titanium-porphyrinic aerogels (TPAs), in which porphyrinic ligand tetrakis(4-carboxyphenyl)porphyrin is coordinated to Ti-oxo clusters. These hierarchically porous TPAs, with micro-, meso-, and macropores and reactant-concentration-dependent Brunauer-Emmett-Teller surface areas of 407-738 m2 g-1 , are prepared by CO2 critical point drying of TPGs. Although the Ti4+ → Ti3+ photoreduction of TPAs is less efficient than that of crystalline microporous Ti-porphyrinic framework DGIST-1, prompt diffusion of O2 and spin-trapping agents into the TPA pores causes the rapid generation of reactive oxygen species (ROS), as observed by EPR spectroscopy. When used as an ROS scavenger, large 1,3-diphenylisobenzofuran is degraded by the best-performing TPA 10 times faster than by DGIST-1, suggesting that the accessibility of molecules (reactants) to pores (reactive centers) strongly influences photocatalytic activity.

Stimuli-Responsive Ti-Organic Gels and Aerogels Derived from Ti-Oxo Clusters: Hierarchical Porosity and Photocatalytic Activity.

Herein, we report titanium-organic gels (TOGs) as new Ti-oxo-based materials that exhibit stimuli-responsive sol-gel transformations and hierarchical porosity upon the removal of solvent molecules. Heating a solution of Ti-oxo clusters and pyromellitic acid as a tetratopic ligand produces TOGs that readily become sols by applying physical stimuli such as shaking or vortexing under ambient conditions. Porous titanium-organic aerogels (TOAs) were obtained by the CO2 supercritical point drying (CPD) of the TOGs, and their porous structures were characterized by N2 adsorption and desorption isotherm measurements. These TOAs, based on the Ti-oxo clusters, possess hierarchical micro-, meso-, and macropores. Furthermore, accompanying the prominent photochromic phenomena, reduction of Ti4+ to Ti3+ was observed upon UV irradiation. The TOAs were successfully applied in the adsorption and photocatalytic degradation of several dye molecules. This research introduces a versatile method for preparing stimuli-responsive and porous Ti-oxo-based photocatalysts.