Michito Yoshizawa

Cavity‐Directed, Highly Stereoselective [2+2] Photodimerization of Olefins within Self‐Assembled Coordination Cages

Selective encapsulation and isolation of molecules are one of the most attractive features of cagelike molecules.[1] Intermolecular chemical reactions of two or more substrates encapsulated in a molecular cage can be remarkably accelerated and suitably controlled as a result of the dramatically increased concentration and the strictly regulated orientation of the substrates in the cavity.[2] Such systems provide artificial mimics of the sophisticated active site of enzymes.[3] Recently we reported that structurally well-defined coordination cages (1 and 2), which self-assemble from six metal ions and four tridentate ligands, selectively encapsulate large organic molecules at the fixed position of the nanosized cavity.[4, 5] Thus, they are expected to facilitate intermolecular [2 2] photochemical reactions and control the stereoand regiochemistry in stringent geometrical environment. The photodimerization has been studied extensively in some media such as micelles, zeolites, organic hosts (for example, cyclodextrins and cucurbiturils),[6] and crystals.[7] However, a high degree of stereoand regiochemical control is still desired. Here we report remarkably accelerated, highly stereoregulated [2 2] photodimerization of acenaphthylenes (3)[8] and naphthoquinones (4)[9] within the coordination cages (1 and 2) in an aqueous medium that give rise to only syn and head-to-tail isomers. Quantitative formation of a syn dimer of acenaphthylene (3a) within cage 1 was clearly observed in the following experiment: An excess amount of 3a was suspended in a solution of 1 in D2O (2.0 m ) for 10 min at 80 C. Analysis of the D2O solution after filtration of free 3a by 1H NMR spectroscopy showed formation of the encapsulation complex 1 ¥ (3a)2 had occurred (Figure 1a). The signals of 3a were highly upfield-shifted as a result of the efficient encapsulation in the cavity. After the clear solution was irradiated (400 W) for 0.5 h at room temperature, the signals derived from 3a completely disappeared and one set of new signals appeared at 5.84, 5.61, 3.39, and 2.87 (Figure 1b). The signals of 1

An M2L4 Molecular Capsule with an Anthracene Shell: Encapsulation of Large Guests up to 1 nm

A new M(2)L(4) molecular capsule with an aromatic shell was prepared using two Pd(II) ions and four bisanthracene ligands. The self-assembled capsule possesses a cavity with a diameter of ~1 nm that can encapsulate medium-sized spherical and planar molecules as well as a very large molecule (C(60)) in quantitative yields. The encapsulated guests are fully segregated and shielded from the external environment by the large anthracene panels.

Minimal nucleotide duplex formation in water through enclathration in self-assembled hosts

Short nucleotide fragments such as mono- and dinucleotides are generally unable to form stable hydrogen-bonded base pairs or duplexes in water. Within the hydrophobic pockets of enzymes, however, even short fragments form stable duplexes to transmit genetic information. Here, we demonstrate the efficient formation of hydrogen-bonded base pairs from mononucleotides in water through enclathration in the hydrophobic cavities of self-assembled cages. Crystallographic studies and 1H- and 15N-NMR spectroscopy clearly reveals pair-selective recognition of mononucleotides and the selective formation of an anti-Hoogsteen-type base pair in the cages cavity. Within an analogous expanded cage, dinucleotides are also found to form a stable duplex in water. These results emphasize how hydrogen-bonded base pairing is amplified in a local hydrophobic area isolated from aqueous solution.

Molecular paneling via coordination

This article deals with a coordination approach to three-dimensional assemblies via ‘molecular paneling’. Families of planar exo-multidentate organic ligands (molecular panels) are found to assemble into large three-dimensional assemblies through metal-coordination. In particular, cis-protected square planar metals, (en)Pd2+ or (en)Pt2+ (en = ethylenediamine), are shown to be very useful to panel the molecules. Metal-assembled cages, bowls, tubes, capsules, and polyhedra are efficiently constructed by this approach.

Spin Crossover by Encapsulation

Encapsulation by synthetic hosts can transform the spin states of square planar Ni(II)(acen) and Co(II)(tap) complexes. Upon encapsulation, the red Ni(II) diamagnetic state was converted into a green paramagnetic state, whereas the Co(II) low spin (S = 1/2) state was changed into a coupled (S = 1/2 and S = 3/2) state. The host cages are noninnocent and host-guest interactions within the confined cavity influence the resultant properties of the enclathrated metal complexes.

Selective Host–Guest Interactions of a Transformable Coordination Capsule/Tube with Fullerenes

An M2L4 coordination capsule or an M2L2 coordination tube was selectively formed by the combination of Hg(II) hinges and bent bispyridine ligands. The two structures reversibly interconvert at room temperature in response to modulation of the metal-to-ligand ratio and exhibit different host-guest interaction behavior. The capsule alone encapsulates large spherical molecules, fullerenes C60 and C70, and the bound guests are released upon capsule-to-tube transformation by the simple addition of metal ions.

Facile Catch and Release of Fullerenes Using a Photoresponsive Molecular Tube

A novel M2L2 molecular tube capable of binding fullerene C60 was synthesized from bispyridine ligands with embedded anthracene panels and Ag(I) hinges. Unlike previous molecular cages and capsules, this open-ended tubular host can accommodate a single molecule of various C60 derivatives with large substituents. The fullerene guest can then be released by using the ideal, noninvasive external stimulus, light.

Engineering Double to Quintuple Stacks of a Polarized Aromatic in Confined Cavities

Discrete, well-defined stacks of the polarized aromatic pyrene-4,5-dione (1) were assembled in the cavities of organic-pillared coordination cages (2). The number (n) of stacked guests depends on the pillar length, and up to quintuple stacks (n = 5) were observed when long (16.5 A) organic pillar ligands were incorporated. As previously reported, pyrene-4,5-dione (1) assembles into infinite columnar stacks in the solid state, but the present work demonstrates that the polarized 1 has a strong propensity to stack in layers even in the absence of crystal packing effects. For n = 2 and 3 structures, crystallographic studies revealed that 1 stacks by pi-pi interactions in the cavity in such a way that a net dipole moment is canceled. These results emphasize the important role of dipole-dipole interactions as well as pi-stacking interactions in the formation of columnar stacks of 1.

Nonvolatile liquid anthracenes for facile full-colour luminescence tuning at single blue-light excitation.

Nonvolatile room-temperature luminescent molecular liquids are a new generation of organic soft materials. They possess high stability, versatile optical properties, solvent-free fluid behaviour and can effectively accommodate dopant dye molecules. Here we introduce an approach to optimize anthracene-based liquid materials, focussing on enhanced stability, fluorescence quantum yield, colour tunability and processability, with a view to flexible electronic applications. Enveloping the anthracene core in low-viscosity branched aliphatic chains results in stable, nonvolatile, emissive liquid materials. Up to 96% efficient energy-transfer-assisted tunable emission is achieved by doping a minute amount of acceptor dye in the solvent-free state. Furthermore, we use a thermoresponsive dopant to impart thermally controllable luminescence colours. The introduced strategy leading to diverse luminescence colours at a single blue-light excitation can be an innovative replacement for currently used luminescent materials, providing useful continuous emissive layers in developing foldable devices.

Preparation of Highly Fluorescent Host–Guest Complexes with Tunable Color upon Encapsulation

Unlike previous coordinative host-guest systems, highly emissive host-guest complexes (up to Φ(F) = 0.5) were successfully prepared upon encapsulation of various fluorescent dyes (e.g., BODIPY and coumarin derivatives) by a Pt(II)-linked coordination capsule in water. Picosecond time-resolved spectroscopy elucidates the photophysical behaviors of the obtained complexes. Notably, the emission color of the fluorescent guest within the capsule can be readily modulated upon pairwise encapsulation with planar aromatic molecules.