Altering the binding affinities of tetraruthenocycles for polycyclic aromatic hydrocarbons by post-assembly modification.

We developed a strategy for manipulating the binding strength of polycyclic aromatic hydrocarbons (PAHs) via covalent post-assembly modification (PAM) of tetranuclear ruthenium macrocycles containing s-tetrazine ligands. The macrocycles act as efficient receptors for various PAHs. Inverse electron demand Diels-Alder (IEDDA) reaction of the macrocycles was applied to reduce the binding ability significantly.

Grignard coupling-based synthesis of vinyl-substituted hydridopolycarbosilane: effect of starting material and polymerization behavior.

Despite the multitude of available alternatives, the Grignard coupling-based synthesis of polycarbosilanes remains attractive, offering the benefit of easy structural design. Moreover, this method allows one to obtain a polymer precursor with the stoichiometric Si : C ratio required for SiC ceramic production and is also suited for the synthesis of polymers containing curable functional groups (e.g., allyl and vinyl). Herein, vinyl-substituted hydridopolycarbosilane was synthesized from three different starting materials (Cl3SiCH2Cl, (MeO)3SiCH2Cl, and (EtO)3SiCH2Cl) using Grignard coupling, and the effects of starting material on polymer growth behavior were investigated. The alkoxysilane starting materials had the advantage of being safe to handle, but had a limitation in polymer growth. The largest molecular weight was observed for Cl3SiCH2Cl, although side reactions occurred. This behavior was ascribed to the retention of unreacted -SiCH2Cl groups contributing to polymer growth via coupling with neighboring polymer chains.

Strapped calix[4]pyrrole as a lithium salts selective receptor through separated ion-pair binding.

A triazole-bearing strapped calix[4]pyrrole (1) was synthesized as a lithium salt selective ion pair receptor. 1H NMR spectral studies and X-ray crystallography showed that the capture of LiCl by 1 occurs via separated ion pair binding. Furthermore, lithium salts extracted by 1 could be solidified in the form of Li2CO3 through CO2 or K2CO3 treatment.

Triazole-bearing calixpyrroles: strong halide binding affinities through multiple N-H and C-H hydrogen bonds.

Triazole-bearing calixpyrroles (TCPs) were synthesized as artificial anion binding receptors. The additional C-HX hydrogen bonding interaction induced strong binding affinity towards halide ions. Using this strong binding affinity, Cl- was successfully extracted from the aqueous to organic phase.

Side chain-specific 11/9-helix propensity of α/ß-peptides with alternating residue types.

The 11/9-helix is among the most stable and non-traditional helical structures for α/ß-peptides with alternating residue types. The effect of side chain groups of α-residues and ß3-residues on the 11/9-helix propensity was examined under various solvent conditions. An α-amino acid residue with one of the four representative side chain groups was incorporated into the central position of an α/ß-pentapeptide backbone. A ß-branched valine residue did not show any destabilizing effect. α,α-Dimethylsubstituted Aib residue was tolerated under nonpolar conditions, but did not promote 11/9-helical folding. The oligomer with a glycine residue did not show 11/9-helical folding under polar solvent conditions. The single unmatched stereochemistry of d-alanine was deleterious to 11/9-helical folding. Replacement of a cyclic ß-residue with an acyclic ß3-residue in the 11/9-helical structure had a slight destabilizing effect, which could be compensated by a longer peptide sequence with more cyclic ß-residues. These results provide a guidance for incorporating functional groups into an 11/9-helical α/ß-peptide backbone to design functional oligomers.

Helical α/ß-depsipeptides with alternating residue types: conformational change from the 11-helix to the 14/15-helix.

Short α/ß-peptides that consist of alternating l-α-amino acids and trans-2-aminocyclopentanecarboxylic acid are known to adopt both 11- and 14/15-helical conformations in solution. We report short α/ß-depsipeptides containing (S)-lactic acid as the third residue from the N-terminus. The α/ß-depsipeptide pentamers and heptamers adopt 14/15-helical conformations analogous to the α-helix in the crystal state and display 14/15-helical conformations predominantly in solution.

Helical Aromatic Foldamers Functioning as a Fluorescence Turn-on Probe for Anions.

Indolocarbazole-pyridine hybrid foldamers are strongly fluorescent in an extended random conformation, but the fluorescence is completely quenched upon folding to a helical conformation due to the compact stacking between aryl planes in the backbone. Anion binding disturbs the helical conformation, thus regenerating the fluorescence of the foldamers. This unique property has been utilized to develop a fluorescence turn-on probe for anions such as sulfate and fluoride.

Anion induced structural transformation in silver-(3,6-dimethoxy-1,2,4,5-tetrazine) coordination polymers under mechanochemical conditions.

Mechanochemical reactions of 3,6-dimethoxy-s-tetrazine (dmotz) with AgCF3SO3 and AgClO4 afforded the 1D linear polymer [Ag(dmotz)(CF3SO3)]n (1) and the 2D grid polymer {[Ag(dmotz)2](ClO4)}n (2), respectively. Mechanochemical processes convert 1 to 2 in the presence of an equivalent of [ClO4](-) and dmotz; even on using excess [CF3SO3](-), conversion from 2 to 1 was not observed.

Stabilization of 11/9-helical α/ß-peptide foldamers in protic solvents.

α/ß-Peptides with alternating α-amino acid and cis-2-aminocyclohexanecarboxylic acid (cis-ACHC) residues adopt 11/9-helical conformations, the folding propensity of which decreases as the solvent polarity increases. We report a new cis-ACHC analogue, cis-2-amino-cis-4-methylcyclohexanecarboxylic acid, which significantly stabilizes the 11/9-helix propensity in protic solvents.

Folding-Generated Molecular Tubes Containing One-Dimensional Water Chains.

A series of indolocarbazole-pyridine (IP) oligomers were prepared that fold into helical conformations, and their folding features in solution and in the solid state were revealed. Helical folding of these IP foldamers is induced by dipolar interactions through the ethynyl bond and π-stacking between two repeating units. Upon helical folding, (1)H NMR signals of aromatic protons were significantly shifted upfield by Δδ = 0.5-2.2 ppm. In addition, hypochromic shifts and fluorescence quenching were observed in the absorption and emission spectra. X-ray crystal structures clearly demonstrated that IP foldamers folded to helical structures with cylindrical internal cavities wherein 3 or 5 water molecules were occupied by hydrogen-bonding interactions in a 1-D array, reminiscent of transmembrane water channels, called aquaporins.

Synthesis of cobalt cluster-based supramolecular triple-stranded helicates.

A cobalt cluster-based triple-stranded helicate, Co8(PDA)6(PTA)3(DMF)3(H2O)3 () (PDA = 2,6-pyridinedicarboxylate, PTA = benzene-1,3-dicarboxylate, DMF = dimethylformamide) was successfully synthesized and fully characterized. Complex can be used as a supramolecular building block in constructing a higher-order helix-of-helix structure, [Co8(PDA)6(PTA)3(DMF)2(H2O)4-0.51(Co(OHn)2)] (n = 1 or 2) ().

Modulation of Axial-Ligand Binding and Releasing Processes onto the Triazole-Bearing Nickel(II) Picket-Fence Porphyrins: Steric Repulsion versus Hydrogen-Bonding Effects.

N-(p-Methoxycarbonylbenzyl) triazole (BTz) substituents have been introduced to Ni(II) porphyrins (NiPs), in which their modulated axial-coordination processes have been investigated. For this study, the two types of ligands, neutral pyridine versus anionic cyanide, were employed to investigate an effect of BTz substituents. The unique microenvironments given by the BTz substituents provided two different effects on the axial-coordination processes of NiPs on the ground and excited states: (1) steric shielding and (2) donation of hydrogen-bonding sites. The steric shielding diminished the binding affinity of pyridine, while the cooperation of hydrogen bonds extraordinarily strengthened the binding affinity of CN(-). Interestingly, it was observed that the binding of CN(-) with the supporting of BTz substituents accompanied nonplanar distortion of NiPs. Such conformational change perturbed the electronic structure of NiPs, which gave rise to the modulation of coordination processes of NiPs in the excited state. As a consequence, photoinudced ligand binding and releasing processes of four- and six-coordinated NiPs were changed into the dominant photoinduced ligand releasing process.

Synthetic K⁺/Cl⁻-selective symporter across a phospholipid membrane.

Synthetic molecules which selectively transport sodium or potassium chloride across a lipid membrane have been prepared. The salt carriers consist of two heteroditopic binding sites, an anion-binding cavity with three hydrogen bond donors and an azacrown ether for binding an alkali metal cation. The association constants between the carriers and chloride ion have been enhanced by 1 order of the magnitude in the presence of sodium or potassium ion in 10% (v/v) CD3OH/CD3CN, due to the formation of a contact ion-pair between the bound cation and chloride as demonstrated by the single-crystal X-ray structure of a sodium chloride complex. A series of transport experiments have demonstrated that the synthetic molecule functions as a mobile carrier of transporting salts via M(+)/Cl(-) symport. Among alkali metal chlorides, the carrier with an 18-azacrown-6 exhibits a strong selectivity toward potassium chloride, while the carrier with a 15-azacrown-5 displays a moderate selectivity for sodium chloride.

A helically twisted imine macrocycle that allows for determining the absolute configuration of α-amino carboxylates.

Binding of α-amino carboxylates to a helically twisted imine macrocycle based on the indolocarbazole scaffold gives rise to characteristic circular dichroism spectra, and the patterns of the Cotton effects are consistent with the absolute configuration of α-amino carboxylates.

A chiral indolocarbazole foldamer displaying strong circular dichroism responsive to anion binding.

A chiral foldamer that consists of three indolocarbazoles and chiral amide residues folds into a helical conformation with the orientation bias, thus displaying characteristic CD signals. The X-ray crystal structure of its chloride complex was found to be a left-handed (M-) helix which stacks to give one-dimensional columnar arrays.

Multistimuli two-color luminescence switching via different slip-stacking of highly fluorescent molecular sheets.

Color tuning and switching of the solid-state luminescence of organic materials are attractive subjects for both the fundamental research and practical applications such as optical recording. We report herein cyanostilbene-based highly luminescent molecular sheets which exhibit two-color fluorescence switching in response to pressure, temperature, and solvent vapor. The origin for the multistimuli luminescence switching is the two-directional shear-sliding capability of molecular sheets, which are formed via intermolecular multiple C-H···N and C-H···O hydrogen bonds. The resulting two distinctive crystal phases are promoted by different modes of local dipole coupling, which cause a substantial alternation of π-π overlap. These changes can be directly correlated with the subsequent intermolecular excitonic and excimeric coupling in both phases, as demonstrated by an in-depth theory-assisted spectroscopic and structural study. Finally, we have prepared a first device demonstrator for rewritable fluorescent optical recording media which showed multistimuli luminescence tuning with fast response. Our multistimuli responsive system is unique in terms of the slip-stacking of molecular sheets and thus provides a novel concept of rewritable fluorescent optical recording media.

Self-organized spiral columns in laterally grafted rods.

Attachment of a flexible coil on the mid-part of a rigid rod block generates T-shaped rod-coil block molecules that self-assemble into a stepped column. These layers, in turn, self-curve into a spiral column with tunable core structure in the solid state.

Imidazole-based excited-state intramolecular proton-transfer materials: synthesis and amplified spontaneous emission from a large single crystal.

We have synthesized a novel class of imidazole-based excited-state intramolecular proton-transfer (ESIPT) materials, i.e., hydroxy-substituted tetraphenylimidazole (HPI) and its derivative HPI-Ac, which formed large single crystals exhibiting intense blue fluorescence and amplified spontaneous emission (ASE). Transparent, clear, and well-defined fluorescent single crystals of HPI-Ac as large as 20 mm x 25 mm x 5 mm were easily grown from its dilute solution. From the X-ray crystallographic analysis and semiempirical molecular orbital calculation, it was deduced that the four phenyl groups substituted into the imidazole ring of HPI and HPI-Ac allowed the crystals free from concentration quenching of fluorescence by limiting the excessive tight-stacking responsible for intermolecular vibrational coupling and relevant nonradiative relaxation. Fluorescence spectral narrowing and efficient ASE were observed in the HPI-Ac single crystal even at low excitation levels attributed to the intrinsic four-level ESIPT photocycle.

Non-interpenetrating honeycomb-like 2D [6,3] network built by a novel trigonal metalloligand.

The combination of a novel trigonal metalloligand and linear linker affords the large non-interpenetrating honeycomb-like 2D [6,3] network in spite of the large hexagon lattice dimension.