Atsuomi Shundo

NMR spectroscopic detection of chirality and enantiopurity in referenced systems without formation of diastereomers.

Enantiomeric excess of chiral compounds is a key parameter that determines their activity or therapeutic action. The current paradigm for rapid measurement of enantiomeric excess using NMR is based on the formation of diastereomeric complexes between the chiral analyte and a chiral resolving agent, leading to (at least) two species with no symmetry relationship. Here we report an effective method of enantiomeric excess determination using a symmetrical achiral molecule as the resolving agent, which is based on the complexation with analyte (in the fast exchange regime) without the formation of diastereomers. The use of N,N′-disubstituted oxoporphyrinogen as the resolving agent makes this novel method extremely versatile, and appropriate for various chiral analytes including carboxylic acids, esters, alcohols and protected amino acids using the same achiral molecule. The model of sensing mechanism exhibits a fundamental linear response between enantiomeric excess and the observed magnitude of induced chemical shift non-equivalence in the 1H NMR spectra.

Nuclear Magnetic Resonance Signaling of Molecular Chiral Information Using an Achiral Reagent

Until now NMR spectroscopic detection of guest chirality using an achiral host has not been possible in the absence of a chiral medium or auxiliary since chiral discrimination is principally based on chiral discrimination by host and/or diastereomeric host-guest complex formation. In this paper, we demonstrate that an achiral oxoporphyrinogen works as a host capable of signaling chiral information of alpha-hydroxycarboxylic acids in (1)H NMR spectroscopy. In particular, enantiomeric excess (ee) can be determined by observing the splitting of (1)H NMR resonances of the achiral host. This differs from the case of chiral hosts (shift reagents) where % ee is generally determined from the ratio of peak areas due to diastereomeric host-guest complexes. UV/vis, CD, FT-IR, and NMR spectroscopic investigations suggest that the unusual phenomenon reported here is based on formation of a complex with 1:2 stoichiometry in concert with a protonation-driven tautomerization of the host.

Langmuir Nanoarchitectonics: One-Touch Fabrication of Regularly Sized Nanodisks at the Air–Water Interface

In this article, we propose a novel methodology for the formation of monodisperse regularly sized disks of several nanometer thickness and with diameters of less than 100 nm using Langmuir monolayers as fabrication media. An amphiphilic triimide, tri-n-dodecylmellitic triimide (1), was spread as a monolayer at the air-water interface with a water-soluble macrocyclic oligoamine, 1,4,7,10-tetraazacyclododecane (cyclen), in the subphase. The imide moieties of 1 act as hydrogen bond acceptors and can interact weakly with the secondary amine moieties of cyclen as hydrogen bond donors. The monolayer behavior of 1 was investigated through π-A isotherm measurements and Brewster angle microscopy (BAM). The presence of cyclen in the subphase significantly shifted isotherms and induced the formation of starfish-like microstructures. Transferred monolayers on solid supports were analyzed by reflection absorption FT-IR (FT-IR-RAS) spectroscopy and atomic force microscopy (AFM). The Langmuir monolayer transferred onto freshly cleaved mica by a surface touching (i.e., Langmuir-Schaefer) method contained disk-shaped objects with a defined height of ca. 3 nm and tunable diameter in the tens of nanometers range. Several structural parameters such as the disk height, molecular aggregation numbers in disk units, and 2D disk density per unit surface area are further discussed on the basis of AFM observations together with aggregate structure estimation and thermodynamic calculations. It should be emphasized that these well-defined structures are produced through simple routine procedures such as solution spreading, mechanical compression, and touching a substrate at the surface. The controlled formation of defined nanostructures through easy macroscopic processes should lead to unique approaches for economical, energy-efficient nanofabrication.

Reversible photoredox switching of porphyrin-bridged bis-2,6-di- tert -butylphenols

Porphyrin derivatives bearing 2,6-di-tert-butylphenol substituents at their 5,15-positions undergo reversible photoredox switching between porphyrin and porphodimethene states as revealed by UV-vis spectroscopy, fluorescence spectroscopy, and X-ray single-crystal analyses. Photoredox interconversion is accompanied by substantial variations in electronic absorption and fluorescence emission spectra and a change of conformation of the tetrapyrrole macrocycle from planar to roof-shaped. Oxidation proceeds only under photoillumination of a dianionic state prepared through deprotonation using fluoride anions. Conversely, photoreduction occurs in the presence of a sacrificial electron donor. Transient absorption spectroscopy and electron spin resonance spectroscopy were applied to investigate the processes in photochemical reaction, and radical intermediates were characterized. That is, photooxidation initially results in a phenol-substituent-centered radical, while the reduction process occurs via a delocalized radical state involving both the macrocycle and 5,15-substituents. Forward and reverse photochemical processes are governed by different chemical mechanisms, giving the important benefit that conversion reactions are completely isolated, leading to better separation of the end states. Furthermore, energy diagrams based on electrochemical analyses (cyclic voltammetry) were used to account for the processes occurring during the photochemical reactions. Our molecular design indicates a simple and versatile method for producing photoredox macrocyclic compounds, which should lead to a new class of advanced functional materials suitable for application in molecular devices and machines.

Toward Volatile and Nonvolatile Molecular Memories: Fluorescence Switching Based on Fluoride‐Triggered Interconversion of Simple Porphyrin Derivatives

Written in the gel: A fluoride-writable memory system has been demonstrated by using two porphyrin derivatives. Structural modification enabled both volatile and nonvolatile modes. For both porphyrins, fluorescence is quenched by addition of F(-) with only one of the derivatives returning to its initial fluorescent state following removal of F(-) (see scheme). Nonvolatile behaviour is based on anion-induced conversion from porphyrin to oxoporphyrinogen.

Controllable heterogeneity in a supramolecular hydrogel

A dispersion of a peptide amphiphile into water forms hierarchical fibril structures, leading to a supramolecular hydrogel. We here report that there exists dynamic heterogeneity in the gel, which might be induced by the heterogeneous fibril network. The network, and therefore, the heterogeneity, can be easily regulated by changing the temperatures used to dissolve the gelator in water.

Colorimetric visualization of acid–base equilibria in non-polar solvent

Visualization of an acid-base equilibrium in a non-polar solvent (dichloromethane), which may be extended to other solvents, is reported. It is based on an oxoporphyrinogen as a multichromic indicator of prevailing acidity in solution and presents up to six distinct hues depending on degree of protonation, tautomeric state or presence of a basic guest.

Spatial heterogeneity in the sol–gel transition of a supramolecular system

Heating and then cooling down a dispersion of a peptide amphiphile in water forms hierarchical fibril structures leading to a supramolecular hydrogel. When the gel was physically broken apart by shaking, it transformed into a sol state. After aging it at room temperature for a given time, it returned to the gel state (re-gelation). To obtain a better understanding of such re-gelation processes, we have applied particle tracking to the sol obtained by disrupting the gel, as a function of aging time. The sol was more heterogeneous at the micrometer scale than the initial gel in terms of its viscoelastic properties, and the extent of the heterogeneity in the sol decreased as the re-gelation proceeded. The origin of the heterogeneity could be directly associated with a fibril network confirmed from Fourier-transform infrared spectroscopic, small-angle X-ray scattering and fluorescence microscopic measurements. The particle tracking study using different particle sizes suggested that the characteristic length scale of the heterogeneous network was not larger than 3 μm. This knowledge might be useful for understanding and controlling the gelation, thereby leading to the design and functionalization of soft materials.

Spatial heterogeneity in a lyotropic liquid crystal with hexagonal phase

Non-ionic surfactant hexaethylene glycol, C(12)E(6), in water self-assembles into various kinds of mesophases by varying the surfactant concentration. A spatial heterogeneity was discussed on the basis of the diffusion of probe particles dispersed in the C(12)E(6)-water solution. Interestingly, at 50 wt% C(12)E(6) where the hexagonal structure was formed, two kinds of motion of probe particles were observed: some particles normally diffused while others were restricted, indicating the existence of a heterogeneity in the physical properties. Such heterogeneity can be explained in terms of heterogeneous structures composed of hexagonal domains with isotropic-like regions.

Hydrogen-Bond-Assisted “Gold Cold Fusion” for Fabrication of 2D Web Structures

Keeping their cool: Fabrication of a 2D weblike nanonetwork of gold was successfully demonstrated through a two-step procedure including complexation of gold precursors to a weblike supramolecular assembly of surfactant followed by in situ reduction of the precursors to gold. Molecular assemblies stabilized by hydrogen bonding provided a sound template, leading to the highly integrated structure of gold through room-temperature (cold) nanostructure fusion.