Katsuaki Konishi

Protonation-Induced Chromism of Pyridylethynyl-Appended [core+exo]-Type Au8 Clusters. Resonance-Coupled Electronic Perturbation through π-Conjugated Group

A series of [core+exo]-type Au8 clusters bearing two alkynyl ligands on the exo gold atoms ([Au8(dppp)4(C≡CR)2](2+), 2-6) were synthesized by the reaction of [Au8(dppp)4](2+) (1) with alkynyl anions. Although the C≡C moieties directly attached to the Au8 units did not affect the optical properties arising from intracluster transitions, the pyridylethynyl-bearing clusters (4-6) exhibited reversible visible absorption and photoluminescence responses to protonation/deprotonation events of the terminal pyridyl moieties. The chromism behaviors and proton-binding constants of these clusters were highly dependent on the relative position of the pyridine nitrogen atom, such that the 2-pyridyl (4) and 4-pyridyl (6) isomers showed more pronounced responses than the 3-pyridyl isomer (5). These results suggest that the resonance-coupled movement of the positive charge upon protonation is involved in the optical responses, where the formation of extended charged resonance structures causes significant perturbation effects on the electronic properties of the Au8 unit and also contributes to the high binding affinities.

Guest-selective binding of Z-amino acids by a strapped metalloporphyrin receptor with a hydrogen-bonding capability

In competitive complexation of Z-amino acids (N-benzyloxycarbonyl-amino acids) in CDCl3, a synthetic receptor such as zinc N-methylmesoporphyrin II having a xylylenediamido strap (1a) preferred Z-glycine over other Z-amino acids, where an excellent selectivity (> 90 %) was achieved when Z-β-alanine, Z-sarcosine, Z-leucine, or Z-proline was the competitor. In contrast, in competitive binding of Z-amino acid anions in a CHCl3 / water biphasic system, 1a preferred substrates with hydrophobic side chains.

FREE BASES OF CHIRAL N-SUBSTITUTED PORPHYRINS AS CATALYSTS FOR ASYMMETRIC REACTION

Chiral N-substituted porphyrin free bases having a conformationally locked asymmetric nitrogen atom catalyzed Michael addition of thiophenols to cycloalkenones, where the reaction proceeded enantioselectively when the catalyst having a xylylene strap anchored via two secondary amido linkages (2) was employed.

Rotational oscillation of two interlocked porphyrins in cerium bis(5,15-diarylporphyrinate) double-deckers

Dynamic NMR coupled with racemization studies on chiral cerium bis(5,15-diarylporphyrinate) double-deckers demonstrate that the two interlocked porphyrin ligands oscillate rotationally around the metal center.

Aggregation-Induced Fluorescence-to-Phosphorescence Switching of Molecular Gold Clusters

Aggregation-induced optical responses are ubiquitous among a wide range of organic and inorganic compounds. Here, we demonstrate an unprecedented effect of aggregation on the photoluminescence (PL) profiles of [core + exo]-type [Au8]4+ clusters, which displayed a change in the dominant PL emission mode from fluorescence to phosphorescence-type upon aggregation. In solvents in which cluster molecules are highly soluble and exist as monomers, they displayed single PL bands at ∼600 nm at ambient temperatures. However, in solvents in which cluster molecules are less soluble and cluster aggregation is induced, a new PL band at ∼700 nm also emerged. Lifetime measurements revealed that the PL emissions at ∼600 and ∼700 nm had fluorescence and phosphorescence characters, respectively. Studies of the excitation spectra suggested that organized cluster assemblies were responsible for the lower-energy emission at ∼700 nm and had exceptionally high emission activity. Accordingly, intense phosphorescence-type emissions were observed in the solid state in which the quantum efficiencies were higher by two orders of magnitude than those of the corresponding monomeric forms in solution. This work provides an example of the critical effects of cluster aggregation events on their optical properties and shows the potential of such effects in the design of cluster-based materials with unique functions and properties.

Selective synthesis with metalloporphyrin catalysts

Abstract By using the manganese complexes of the antipodes of the conceptually new chiral strapped porphyrin as catalysts coupled with imidazole, asymmetric epoxidation of prochiral olefins by iodosobenzene was achieved. When the external ligand was absent, the epoxides with the opposite configuration were formed in lower % e.e. In the presence of the external ligand, the enantioselectivity of the reaction depended on the structure of the strap in the catalyst, while no such dependence was observed in the absence of imidazole. The iron complex of chiral p -xylylene-strapped porphyrin effectively catalysed the asymmetric oxidation of sulfides by iodosobenzene in the presence of imidazole.

Apocytochrome b562 as a Novel Chiral Host Molecule: The First Enantioselective Reconstitution

A unique helix-bundled architecture with an exceptionally high conformational stability is exhibited by apocytochrome b562. Reconstitution of this new type of chiral host molecule with the racemate of a chiral zinc mesoporphyrin II having a 1,4-xylylenediamide strap resulted in perfect enantioselection of (R)-1.

Nucleobase dendrimer as a multidentate ligand for arare-earth metal ion

Novel dendritic macromolecules consisting of uracil units (LnU-OBn) with the numbers of the nucleobase layers (n) of 2–4 were synthesized; L3U-OBn and L4U-OBn both formed 1∶1 complexes with La3+, where the L4U-OBn–La3+ complex, in particular, was highly robust towards methanolysis.

The first chiral phosphorous porphyrins with molecular asymmetry: conformation studies on porphyrin macrocycles in solution

Abstract Enantiomers of chiral phosphorous porphyrins having two different axial groups at the trans positions [(EtioP)PR(O), 1 (R=Et) and 2 (R=Ph); EtioP=etioporphyrinato I] were resolved by means of chiral HPLC. The enantiomers displayed different circular dichroism (CD) spectra in basic and acidic media due to a H + -driven conformational change of the porphyrin ring.

Supramolecular acid/base catalysis via multiple hydrogen bonding interaction.

2-Ureidoisocytosine bearing a phosphonium ion functionality self-assembles in an anti-parallel manner, to exhibit a cooperative acid/base catalysis for the ring-opening reaction of an epoxide.