Control of optically active structure of thioether-phthalocyanine aggregates by chiral Pd(II)-BINAP complexes in toluene and at the toluene/water interface

Chirality Discrimination at Binary Organic|Water Interfaces Monitored by Interfacial Tension Measurements with Preliminary Comparison with Molecular Dynamics Simulations

Chirality discrimination at a binary toluene (organic)/water(aqueous) interface between R- or S-Tol-BINAP (2,2'-Bis(di-p-tolylphosphino)-1,1'-binaphthyl) molecules and the water-soluble serine chiral specie is examined for the first time, using a combination of interfacial tension measurements and molecular dynamic simulations. Experimental interfacial measurements exhibit a clear chirality-controlled difference when a homochiral versus a heterochiral enantiomeric pairs are introduced at the interfaces. The related molecular dynamics simulations support the experimental results and provide further molecular insight of intermolecular interactions at the interfaces. The results indicate that interfacial tension measurements can capture the preferential interactions which exist between different pairs of enantiomers at the binary interfaces, opening up a new way for probing chirality discrimination at liquid-liquid interfaces.

Helical Self-Assembly of Optically Active Glycoconjugated Phthalocyanine J-Aggregates

Four galactoconjugated zinc(II) phthalocyanines (Pcs) have been prepared and fully characterized. The carbohydrate-containing phthalonitrile precursors of the Pcs were synthesized through a copper-catalysed azide-alkyne cycloaddition (CuAAC). The Pcs show a remarkable aggregation behaviour in solution, depending on the nature of the solvent, the temperature and the substitution position on the phthalocyanine. Solvent-dependent CD-spectroscopy experiments show that these Pcs aggregate as chiral helices in solution. Crystal structure data of a phthalocyanine bearing two carbohydrate units substantiate the properties shown by CD spectroscopy. Furthermore, the 1,2,3-triazole moieties of the Pcs play a decisive role in the formation of supramolecular aggregates. The glycoconjugated zinc(II) phthalocyanines described here show molar extinction coefficients ϵ_max >10⁵  M^-1  cm^-1 and absorption maxima λ_max >680 nm, which make them attractive photosensitizers for Photodynamic Therapy (PDT).

Fabrication of Supramolecular Chirality from Achiral Molecules at the Liquid/Liquid Interface Studied by Second Harmonic Generation

We present the investigation into the supramolecular chirality of 5-octadecyloxy-2-(2-pyridylazo)phenol (PARC18) at water/1,2-dichloroethane interface by second harmonic generation (SHG). We observe that PARC18 molecules form supramolecular chirality through self-assembly at the liquid/liquid interface although they are achiral molecules. The bulk concentration of PARC18 in the organic phase has profound effects on the supramolecular chirality. By increasing bulk concentration, the enantiomeric excess at the interface first grows and then decreases until it eventually vanishes. Further analysis reveals that the enantiomeric excess is determined by the twist angle of PARC18 molecules at the interface rather than their orientational angle. At lower and higher bulk concentrations, the average twist angle of PARC18 molecules approaches zero, and the assemblies are achiral; whereas at medium bulk concentrations, the average twist angle is nonzero, so that the assemblies show supramolecular chirality. We also estimate the coverage of PARC18 molecules at the interface versus the bulk concentration and fit it to Langmuir adsorption model. The result indicates that PARC18 assemblies show strongest supramolecular chirality in a half-full monolayer. These findings highlight the opportunities for precise control of supramolecular chirality at liquid/liquid interfaces by manipulating the bulk concentration.

Chiral Recognition of L-Amino Acids on Liposomes Prepared with L-Phospholipid

In this study, we demonstrated that liposomes composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) can recognize several l-amino acids, but not their d-enantiomers, by analyzing their adsorptive behavior and using circular dichroism spectroscopy. Changes in liposomal membrane properties, determined based on fluorescent probe analysis and differential scanning calorimetry, were induced by l-amino acid binding. UV resonance Raman spectroscopy analysis suggested that the chiral recognition was mediated by electrostatic, hydrophobic, and hydrogen bond interactions, where the recognition site could therefore be constructed on the DPPC membrane. Our findings clearly indicate the potential function of liposomes in asymmetric recognition.

Spontaneous chiral resolution directed by symmetry restriction and π-π interaction

In order to understand and rationally construct homochiral self-assembled structures from racemic molecules, two novel crystalline metal-organic frameworks with chiral cavities were developed. The homochirality of the layers in both MOFs was achieved by forming strong coordinate bonds between the C₃-symmetric cyclotriveratrylene and Zn₄O(CO₂)₆ cluster. By changing weak π-π interactions between organic building blocks, the achiral assembly of ZnCTV-1 was successfully transformed into a chiral assembly in ZnCTV-2. This study demonstrated a possible route for designing the synthesis of chiral MOF through weak interactions.

Self-assembled nanostructures of optically active phthalocyanine derivatives. Effect of central metal ion on the morphology, dimension, and handedness

Four optically active {(2,3,9,10,16,17,23,24-octa[(S)-2-methylbutoxy]} phthalocyanine derivatives with different central metal ions, namely (S)-H2Pc(β-OC5H11)8 (1), (S)-ZnPc(β-OC5H11)8 (2), (S)-CuPc(β-OC5H11)8 (3), and (S)-NiPc(β-OC5H11)8 (4) have been synthesized and their self-assembly behaviors systematically investigated by electronic absorption and circular dichroism (CD) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) techniques, and X-ray photoelectron spectroscopy (XPS). Metal free phthalocyanine together with its zinc and copper congeners self-assembles into right-handed helical arrangements in a stack of phthalocyanine chromophores according to the CD spectroscopic result, while the nickel complex does so into the left-handed helical arrangements. These helical arrangements, acting as elemental primary structures, further pack in a hierarchical fashion into either highly ordered right-handed fibrous nanostructures with average 300 μm length, 4.8 μm width, and 4.4 μm helical pitch for 1 and 300 μm length, 2.4 μm width, and 1.8 μm helical pitch for 2 but left-handed fibrous nanostructures with average 4 μm length, 0.48 μm width, and 0.12 μm helical pitch for 3 and 300 μm length, 2 μm width, and 1.2 μm helical pitch for 4, clearly revealing the effect of central metal ion on the distance and relative orientation of neighboring phthalocyanine chromophores and in turn the supramolecular chirality, morphology, dimension, and handedness of the self-assembled nanostructures. The present result not only represents a unique phenomenon in the self-assembly of phthalocyanine compounds but more importantly denotes the transcription and amplification of molecular chirality to supramolecular helicity with different helical bias during the self-assembly processes without changing the chiral handles attached to the phthalocyanine chromophore.

Origin and correction of magnetic field inhomogeneity at the interface in biphasic NMR samples

The use of susceptibility matching to minimize spectral distortion of biphasic samples layered in a standard 5 mm NMR tube is described. The approach uses magic angle spinning (MAS) to first extract chemical shift differences by suppressing bulk magnetization. Then, using biphasic coaxial samples, magnetic susceptibilities are matched by titration with a paramagnetic salt. The matched phases are then layered in a standard NMR tube where they can be shimmed and examined. Linewidths of two distinct spectral lines, selected to characterize homogeneity in each phase, are simultaneously optimized. Two-dimensional distortion-free, slice-resolved spectra of an octanol/water system illustrate the method. These data are obtained using a 2D stepped-gradient pulse sequence devised for this application. Advantages of this sequence over slice-selective methods are that acquisition efficiency is increased and processing requires only conventional software.

Interfacial activity of metal β-diketonato complexes: in situ generation of amphiphiles by water coordination

Symmetric transition metal complexes of 2,4-pentanedione (acetyl acetone) are interfacially active: Spinning drop tensiometry reveals lowering of the interfacial tension at the water-organic interface, most pronounced for [Cr(acac)(3)], [Fe(acac)(3)], [Zr(acac)(4)], and [Hf(acac)(4)]. The interfacial activity is explained by the in situ generation of amphiphilic species. Based on tensiometry and (1)H and diffusion-ordered NMR spectroscopy (DOSY NMR), hydrogen bonding of the organically dissolved complexes with water, in some cases in combination with inner-sphere hydrolytic coordination, is identified as the primary origin of this amphiphilicity. The complexes are a rare example of symmetric molecules that turn amphiphilic only upon interfacial interaction with water.

Study of liquid-liquid interfaces by an easily implemented localized NMR sequence

To selectively extract heavy metals from solutions containing fission products, it is essential to optimize the liquid-liquid extraction processes. Such an objective requires improving the fundamental knowledge of the different mechanisms that are involved in these processes. In that respect, we propose a localized NMR sequence named LOCSY to assess the concentration profiles of different species involved in these processes. One of the goals of this sequence is to study the products as close as possible to the liquid-liquid interface with the help of a standard NMR spectrometer of chemistry labs. The one-dimensional spatial localization along the NMR tube is obtained by a discrete stepping of the frequency-selective excitation pulses under a pulsed field gradient. Specific data processing has been developed to obtain the 1D NMR spectra as a function of the vertical position in the NMR tube. The LOCSY sequence has been tested and evaluated on three different systems: (i) a cylindrical phantom inserted in the NMR tube containing 4-methylsalicylic acid solution, (ii) D(2)O/olive oil biphasic system, and (iii) the dissolution of solid saccharose in D(2)O. These examples illustrate potential applications of the LOCSY sequence, particularly the possibility to measure concentration profiles and to study phenomena such as diffusion, provided the dynamic range is compatible with NMR timescale and sensitivity.

Helical nanostructures self-assembled from optically active phthalocyanine derivatives bearing four optically active binaphthyl moieties: effect of metal-ligand coordination on the morphology, dimension, and helical pitch of self-assembled nanostructures

(R)- and (S)-enantiomers of optically active metal free tetrakis[11,12:13,14-di(1',2'-naphtho)-1,4,7,10,15,18-hexaoxacycloeicosa-2,11,13-trieno]-phthalocyanine and their zinc complexes, (R)- and (S)-H(2)Pc (1) and (R)- and (S)-ZnPc (2), were prepared from the tetramerization of corresponding phthalonitriles, (R)- and (S)-2,3-(4',5'-dicyanobenzo)-11,12:13,14-di(1',2'-naphtho)-1,4,7,10,15,18-hexaoxacycloeicosa-2,11,13-triene, in the absence and presence of Zn(OAc)(2).2H(2)O template, respectively, promoted by organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Their self-assembly behavior in the absence and presence of 4,4'-bipyridine has been comparatively investigated by electronic absorption and circular dichroism (CD) spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) technique, and X-ray photoelectron spectroscopy (XPS). The metal free phthalocyanine self-assembles into highly ordered fibrous nanostructures (ca. 3 microm length, 70 nm width, and 125 nm helical pitch) with left-handed and right-handed helicity for (R)-1 and (S)-1, respectively, through the hierarchical manner via one-dimensional helices with chirality determined by the optically active binaphthyl side chains. In contrast, self-assembly of the phthalocyaninato zinc analogue leads to the formation of nanoparticles. However, in the presence of 4,4'-bipyridine, additionally formed metal-ligand Zn-N(4,4'-bipyridine) coordination bonds between the nitrogen atoms of additive 4,4'-bipyridine molecule and the zinc center of (R)- and (S)-2 molecules together with pi-pi interaction and chiral discrimination of chiral side chains induce a right-handed and left-handed helical arrangement in a stack of (R)- and (S)-2 molecules, respectively, which further hierarchically packs into highly ordered fibrous nanostructures of average tens of micrometers in length, 30 nm width, and 106 nm helical pitch with the same helicity to the stack, revealing the effect of metal-ligand coordination bonding interaction on the morphology, dimension, handedness, and helical pitch of self-assembled nanostructures.

Measuring the optical chirality of molecular aggregates at liquid-liquid interfaces

Some new experimental methods for measuring the optical chirality of molecular aggregates formed at liquid-liquid interfaces have been reviewed. Chirality measurements of interfacial aggregates are highly important not only in analytical spectroscopy but also in biochemistry and surface nanochemistry. Among these methods, a centrifugal liquid membrane method was shown to be a highly versatile method for measuring the optical chirality of the liquid-liquid interface when used in combination with a commercially available circular dichroism (CD) spectropolarimeter, provided that the interfacial aggregate exhibited a large molar absorptivity. Therefore, porphyrin and phthalocyanine were used as chromophoric probes of the chirality of itself or guest molecules at the interface. A microscopic CD method was also demonstrated for the measurement of a small region of a film or a sheet sample. In addition, second-harmonic generation and Raman scattering methods were reviewed as promising methods for detecting interfacial optical molecules and measuring bond distortions of chiral molecules, respectively.

Interfacial assembly of an achiral zinc phthalocyanine at the air/water interface: a surface pressure dependent aggregation and supramolecular chirality

The aggregation and supramolecular chirality of the interfacial assemblies of an achiral phthalcyanine derivative, zinc 2,3,9,10,16,17,23,24-octakis(octyloxy)-29 H,31 H-phthaloxyanine (ZnPc), were investigated, and a surface pressure dependent behavior was observed. It was found that ZnPc could be spread as a Langmuir film on water surface and transferred onto solid substrates by the horizontal lifting method. The compound formed mixed J- and H-aggregates in the transferred Langmuir-Blodgett (LB) films. Deconvolution of the broaden Q-band revealed three main components of the spectra, which corresponded to H- and J-aggregates and medium transition aggregates, whose relative contents could be modulated by the surface pressure at which the films were transferred. On the other hand, the transferred LB films composed of these aggregates showed Cotton effects in circular dichroism spectra when the floating film was compressed over a certain surface pressure although the compound itself was achiral. The cooperative arrangement of the macrocylic ring in a helical manner through the interfacial organization was suggested to be responsible for such optical activity in the LB films. A possible explanation based on the cooperative arrangement of the ZnPc building blocks in a helical sense stacking in the aggregates was proposed.

Binding Behavior of Subphthalocyanine-Tagged Testosterone with Human Serum Albumin at then-Hexane/Water Interface

Subphthalocyaninatoboron(III) was applied for the first time as a novel marker-tag of testosterone (Subpc-test) for the binding analysis with human serum albumin (HSA) at a liquid/liquid interface. The binding interaction of Subpc-test with HSA at the n-hexane/water interface was studied by UV-visible absorption and circular dichroism (CD) spectroscopy combined with a centrifugal liquid membrane cell at different pHs of aqueous solutions. Complementary studies by a high-speed stirring experiment and an interfacial tension measurement were also performed to characterize the interfacial adsorptivity of Subpc-test and HSA molecules, respectively. The n-hexane solution of Subpc-test showed no optical chirality, but the contact with the aqueous solution of HSA induced its optical chirality, clearly suggesting the formation of Subpc-test/HSA complexes at the n-hexane/water interface. Furthermore, pH profiles of CD signals showed that the interaction between Subpc-test and HSA was very sensitive to the neutral-to-base transition (N-B transition). Interfacial formation of the Subpc-test/HSA complex was studied in the presence of the site-selective ligand, furosemide (site I) or cefaclor (site II). The experimental results suggested that Subpc-test is bound to site I of the HSA molecule at the n-hexane/water interface. This technique using subphthalocyanine as a tag molecule and the liquid/liquid interface as a two-dimensional nanoreaction field will be useful for the evaluation of the interaction between drugs or hormones and proteins.