Yoko Sakata

Shape-Memory Nanopores Induced in Coordination Frameworks by Crystal Downsizing

Size Affects Shape Porous molecular framework materials can adopt a different phase when guest molecules absorb and uniformly distort the framework. Usually the framework returns to its original shape when the guests desorb. Sakata et al. (p. 193) noted that because surface stress drives this process, it might be avoided in smaller crystals. Indeed, a flexible porous coordination polymer, [Cu2(dicarboxylate)2(amine)]n, could retain the structure induced by guest molecules such as methanol if crystallites were made sufficiently small (submicrometer scale) and did so to a greater degree as the crystallite dimensions decreased. A porous material retains its framework shape after guest molecules desorb if its crystallites are sufficiently small. Flexible porous coordination polymers change their structure in response to molecular incorporation but recover their original configuration after the guest has been removed. We demonstrated that the crystal downsizing of twofold interpenetrated frameworks of [Cu2(dicarboxylate)2(amine)]n regulates the structural flexibility and induces a shape-memory effect in the coordination frameworks. In addition to the two structures that contribute to the sorption process (that is, a nonporous closed phase and a guest-included open phase), we isolated an unusual, metastable open dried phase when downsizing the crystals to the mesoscale, and the closed phase was recovered by thermal treatment. Crystal downsizing suppressed the structural mobility and stabilized the open dried phase. The successful isolation of two interconvertible empty phases, the closed phase and the open dried phase, provided switchable sorption properties with or without gate-opening behavior.

Ti(IV)-Centered Dynamic Interconversion between Pd(II), Ti(IV)-Containing Ring and Cage Molecules

Heteronuclear, supramolecular ring and cage complexes have been constructed from a pyridyl catechol ligand, TiO(acac)2, and PdCl2(CH3CN)2. These two complexes are quantitatively interconvertible, in which Ti4+-centered coordination changes take place between a well-known Ti(catecholato)3 and a newly established TiH(catecholato)2(acetylacetonato) structures. The Ti4+-centered structural changes arise from the changes in the component fraction and basicity condition.

Crystal morphology-directed framework orientation in porous coordination polymer films and freestanding membranes via Langmuir–Blodgettry

Porous coordination polymers (PCPs), with their ordered nanoporous systems and large surface areas, are very attractive for numerous applications that involve controlled molecular transport properties. To fully exploit their potential, a straightforward processing method to deposit the PCP crystals on various substrates and to create freestanding membranes with a controlled pore orientation is highly desirable. Here, we report a strategy to self-assemble PCP crystals into two-dimensional monolayers using Langmuir–Blodgettry. This approach allows the deposition on various substrates over several square centimeters, uniformly and with controllable density of the crystals. In addition we show that by controlling the morphology of the crystalline building blocks we can program their orientation on the substrates. Using a copper grid as the substrate, these assemblies can also be fabricated as freestanding sheets. This approach represents a very simple and scalable processing method to translate the orientation of the channel network from the individual crystal to the macroscopic scale, and can help to incorporate this interesting class of materials within advanced hierarchical systems.

Host–Guest Complexation of Perethylated Pillar[5]arene with Alkanes in the Crystal State

Activated perethylated pillar[5]arene crystals show an unexpected alkane-shape- and -length-selective gate-opening behavior. Activated crystals were obtained upon removing solvents from perethylated pillar[5]arene crystals by heating. The activated crystals could quantitatively take up n-alkanes with carbon chains containing more than five carbon atoms as a consequence of their gate-opening pressure. As the chain length of the n-alkanes increased, the gate pressure decreased. A transformation into a herringbone structure was induced when n-hexane was used as a guest. By contrast, cyclic and branched alkanes were not taken up and could not induce a crystal transformation because they were too large to fit in the cavities of the pillar[5]arene. Alkane-shape-selective molecular recognition of pillar[5]arenes in the solution state was translated into the vapor/crystal state.

Targeted functionalisation of a hierarchically-structured porous coordination polymer crystal enhances its entire function

Spatiospecific functionalisation of a shell crystal was performed in a core-shell crystal of a porous coordination polymer (PCP) via post-synthetic modification (PSM). The shell crystal allowed the core crystal to selectively accumulate N,N-dimethylaniline (DMA) and afford the intense exciplex fluorescence.

Alkane-Shape-Selective Vapochromic Behavior Based on Crystal-State Host–Guest Complexation of Pillar[5]arene Containing One Benzoquinone Unit

Colored crystals of pillar[5]arene containing one benzoquinone unit were found to exhibit alkane-shape-selective vapochromic behavior. Activated pillar[5]arene crystals, prepared by removing solvated methanol from pillar[5]arene crystals, changed color from dark-brown to light-red after exposure to linear alkane vapors; however, no color changes were observed on exposure to branched or cyclic alkanes. Uptake of methanol vapor by the activated crystals induced a different color change, from dark-brown to black. This multi-vapochromism results from the different intermolecular π-stacking interactions between the benzoquinone and 1,4-diethoxybenzene units in the alkane- and methanol-containing crystals. Unlike most known vapochromic materials, these pillar[5]arene-based materials were highly stable; after uptake of n-alkanes or methanol the color of the crystals did not change after storage in air for 3 weeks. This is because the included guests were stabilized in the cavity by multiple CH/π interactions.

Trapping of a Spatial Transient State During the Framework Transformation of a Porous Coordination Polymer

Structural transformability accompanied by molecular accommodation is a distinguished feature of porous coordination polymers (PCPs) among porous materials. Conventional X-ray crystallography allows for the determination of each structural phase emerged during transformation. However, the propagation mechanism of transformation through an entire crystal still remains in question. Here we elucidate the structural nature of the spatial transient state, in which two different but correlated framework structures, an original phase and a deformed phase, simultaneously exist in one crystal. The deformed phase is distinctively generated only at the crystal surface region by introducing large guest molecules, while the remaining part of crystal containing small molecules maintains the original phase. By means of grazing incidence diffraction techniques we determine that the framework is sheared with sharing one edge of the original primitive cubic structure, leading to the formation of crystal domains with four mirror image relationships.

Development of a One-Pot Synthetic Method for Multifunctional Oxazole Derivatives Using Isocyanide Dichloride

A one-pot synthetic method was developed for multifunctional dihydrooxazole and oxazole derivatives. New reaction sequences were developed involving the formation of isocyanide dichloride, an aldol-type reaction with aldehydes, and a nucleophilic addition-elimination reaction, which efficiently afforded the dihydrooxazole and oxazole scaffolds.

Anion-capped metallohost allows extremely slow guest uptake and on-demand acceleration of guest exchange

The switching of molecular recognition selectivity is important for tuning molecular functions based on host–guest binding. While the switching processes in artificial functional molecules are usually driven by changes of the thermodynamic stabilities, non-equilibrium phenomena also play an important role in biological systems. Thus, here we designed a host–guest system utilizing a non-equilibrium kinetically trapped state for on-demand and time-programmable control of molecular functions. We synthesized a bis(saloph) macrocyclic cobalt(III) metallohost 1(OTf)2, which has anion caps at both sides of the cation-binding site. The anion caps effectively retard the guest uptake/release so that we can easily make a non-equilibrium kinetically trapped state. Indeed, we can obtain a long-lived kinetically trapped state {[1·K]3++La3+} prior to the formation of the thermodynamically more stable state {[1·La]5++K+}. The guest exchange to the more stable state from this kinetically trapped state is significantly accelerated by exchange of TfO− anion caps by AcO− in an on-demand manner.

Synthesis of Sterically Fixed Phytochrome Chromophore Derivatives Bearing a 15E-Fixed or 15E-Anti-Fixed CD-Ring Component

To analyze the structure and function of phytochrome chromophores, we have been synthesizing natural and unnatural bilin chromophores of phytochromes. In this manuscript, we report the synthesis of sterically fixed 15 E- fixed 18Et-biliverdin (BV) and 15 E- anti-fixed 18Et-BV derivatives. The key reaction is the introduction of an sp3 carbon alkyl chain bearing a leaving group at the meso-position of the CD-ring component by using the corresponding Grignard reagents in the presence of LiCl.