Masaki Kawano

The isolation and characterization of small sulfur allotropes have long remained unachievable because of their extreme lability. This study reports the first direct observation of disulfur (S₂) with X-ray crystallography. Sulfur gas was kinetically trapped and frozen into the pores of two Cu-based porous coordination networks containing interactive iodide sites. Stabilization of S₂was achieved either through physisorption or chemisorption on iodide anions. One of the networks displayed shape selectivity for linear molecules only, therefore S₂was trapped and remained stable within the material at room temperature and higher. In the second network, however, the S₂molecules reacted further to produce bent-S₃species as the temperature was increased. Following the thermal evolution of the S₂species in this network using X-ray diffraction and Raman spectroscopy unveiled the generation of a new reaction intermediate never observed before, thecyclo-trisulfur dication (cyclo-S₃²⁺). It is envisaged that kinetic guest trapping in interactive crystalline porous networks will be a promising method to investigate transient chemical species.

P ₄ generated from red phosphorus via the gas phase was trapped into an interactive pore of a porous coordination network, showing weak interactions which stabilize the reactive P ₄ in a pore.

Ammonium and sodium salts (ion pairs) of a multi-interactive tri(4-pyridyl)hexaazaphenalenyl anion (TPHAP⁻) showed completely different ion-conductive properties depending on the crystal structure. TPHAP columnar crystals showed a high conductivity of 10⁻³ S cm⁻¹ while retaining their structures even under humid conditions, whereas TPHAP dimer crystals exhibited a conductivity of ∼10⁻⁵ S cm⁻¹ with crystallinity deterioration. The main unit structures induced by multi-interactivity realized different water accessibility, which explains the differences in their ion conductivity and stability against humidity.

Cd-based coordination networks having channels were formed selectively by using a redox-active aromatic ligand 2,5,8-tri(4-pyridyl)1,3-diazaphenalene (TPDAP, H⁺1^–). An electron-conductive network having a π–π stacking columnar structure of TPDAP formed in the presence of a trace amount of TPDAP radical (1^•). In contrast, a nonconductive network having a dimer unit of H⁺1^– formed in the absence of 1^•. These results suggest the presence of a unique oxidation mechanism of TPDAP induced by formation of H⁺1^–-1^• dimer, which was initiated by a trace amount of 1^•. The dimerization increased HOMO level of H⁺1^– moiety within the dimer to generate further radicals that could not form when H⁺1^– was well isolated in CH₃OH.

Iodinated pores of a Zn-based coordination network were modified by Br ₂ oxidation to produce brominated pores in a polycrystalline-to-polycrystalline manner while maintaining the same network topology.

Crystalline powders self-assembled from interactive discrete molecules reversibly transformed from a porous structure to a 2D one with a nanometer-thick H₂O layer by hydration/dehydration. Multi-point weak intermolecular interactions contributed to maintenance of each phase. This structure transformation induced a humidity-dependent ion conductivity change from insulator to 3.4 × 10⁻³ S cm⁻¹.

Self-assembly of the tripyridyl ligand 2,4,6-tris(4-pyridyl)triazine(TPT) and ZnI₂via the gas phase produced three kinds of networks: [(ZnI₂)(TPT)]_n; [(ZnI₂)₃(TPT)₂]_n; and [(ZnI₂)(μ-I)(ZnI)(TPT)]_n. [(ZnI₂)₃(TPT)₂]_n is the first example of the single crystal formation of a porous network via the gas phase and showed the dependence of iodine encapsulation on the crystal size.

A [2+2] cycloaddition reaction has been observed in a number of solids. The cyclobutane ring in a photodimerized material can be cleaved into olefins by UV light and heat. The high thermal stability of the metal–organic salt K₂SDC (H₂SDC=4,4’‐stilbenedicarboxylic acid) has been successfully utilized to investigate the reversible cleavage of a cyclobutane ring. The two polymorphs of K₂SDC undergo reversible cyclobutane formation by UV light and cleavage by heat in cycles. Of these, one polymorph retains its single‐crystal nature during the reversible processes. Polymorphs are known to show different physical properties and chemical reactivities. This work reveals that the retention of single‐crystal nature is strongly associated with the packing of molecules, which is controlled by kinetics and thermodynamics. The photoemissive nature of the products makes this as a promising material for photoswitches and optical data storage devices.

A [2+2] cycloaddition reaction has been observed in a number of solids. The cyclobutane ring in a photodimerized material can be cleaved into olefins by UV light and heat. The high thermal stability of the metal–organic salt K₂SDC (H₂SDC=4,4’‐stilbenedicarboxylic acid) has been successfully utilized to investigate the reversible cleavage of a cyclobutane ring. The two polymorphs of K₂SDC undergo reversible cyclobutane formation by UV light and cleavage by heat in cycles. Of these, one polymorph retains its single‐crystal nature during the reversible processes. Polymorphs are known to show different physical properties and chemical reactivities. This work reveals that the retention of single‐crystal nature is strongly associated with the packing of molecules, which is controlled by kinetics and thermodynamics. The photoemissive nature of the products makes this as a promising material for photoswitches and optical data storage devices.