Chiral assembly from achiral rod-coil molecules triggered by compression at the air-water interface

Aryl-Phenanthro[9,10-d]imidazole: A Versatile Scaffold for the Design of Optical-Based Sensors

Fluorescent and colorimetric sensors are important tools for investigating the chemical compositions of different matrices, including foods, environmental samples, and water. The high sensitivity, low interference, and low detection limits of these sensors have inspired scientists to investigate this class of sensing molecules for ion and molecule detection. Several examples of fluorescent and colorimetric sensors have been described in the literature; this Review focuses particularly on phenanthro[9,10-d]imidazoles. Different strategies have been developed for obtaining phenanthro[9,10-d]imidazoles, which enable modification of their optical properties upon interaction with specific analytes. These sensing responses usually involve changes in the fluorescence intensity and/or color arising from processes like photoinduced electron transfer, intramolecular charge transfer, intramolecular proton transfer in the excited state, and Förster resonance energy transfer. In this Review, we categorized these sensors into two different groups: those bearing formyl groups and their derivatives and those based on other molecular groups. The different optical responses of phenanthro[9,10-d]imidazole-based sensors upon interaction with specific analytes are discussed.

Blending as a Strategy to Attain Chiro-Optically Activity Polymers

Two copolymers, one containing a chiral center and another without any asymmetric site are studied regarding their chiro-optical properties. The pure polymers do not show any signal of chiro-optical activity, only a smooth line is observed in the circular dichroism spectra, even for the chiral material. However, blends containing the achiral one as a major component show striking chiro-optical activity, originating by stable supramolecular structures whose size and shape remain unchanged, regardless of the blend composition. Only the number of such structures (composed by the chiral one), vary with blend composition. The results suggest that working with supramolecular morphology can be an important strategy to attain chiro-optical active polymers.

Unique supramolecular assembly through Langmuir - Blodgett (LB) technique

The Langmuir-Blodgett (LB) technique is a way of making supra-molecular assembly in ultrathin films with a controlled layered structure and crystal parameter, which have many envisioned technological applications for optical and molecular electronic devices as well as signal processing and transformation. Probably LB technique is the best method to manipulate materials at molecular level and provides a scope to realize the molecular electronics in reality. In this review article, we have discussed about the general introduction of LB technique and recent development on LB and related system including (i) LB methodology, (ii) characterizations of LB films, (iii) LB films and molecular electronics, (iv) historical review of LB films, (v) research and applications including fundamental research and application towards devices.

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.

Study of Compression-Induced Supramolecular Nanostructures of an Imidazole Derivative by Langmuir-Blodgett Technique

In this communication, we report the design and synthesis as well as the supramolecular assembly behavior of a 2,4,5-triaryl imidazole derivative (compound 1) at the air-water interface and in thin films using Langmuir-Blodgett (LB) technique. The main idea for such a chemical structure is that the long alkyl chain and N-H of the imidazole core may help to form supramolecular architecture through the hydrophobic-hydrophobic interaction and hydrogen bonding, respectively. Accordingly, the interfacial behavior as well as morphology of 1 in thin films were studied through a series of characterization methods such as surface pressure-area (π-A) isotherm, hysteresis analysis, ultraviolet-visible (UV-vis) absorption and steady-state fluorescence spectroscopies, Fourier transform infrared, X-ray diffraction, Brewster angle microscopy (BAM), and atomic force microscopy (AFM) measurements, and so forth. Pressure-area isotherm is an indication toward the formation of supramolecular nanostructures instead of an ideal monolayer at the air-water interface. This has been confirmed by the hysteresis analysis and BAM measurement at the air-water interface. AFM images of 1 in the LB monolayer exhibits the formation of supramolecular nanowires as well as nanorods. By controlling different film-forming parameters, it becomes possible to manipulate these nanostructures. With the passage of time, the nanowires come close to each other and become straight. Similarly, nanorods come close to each other and form bundles of several rods in the LB films. H-bonding, J-aggregation, as well as compression during film formation might play a key role in the formation of such nanostructures. Electrical switching behavior of compound 1 was also observed because of the presence of an electron donor-acceptor system in 1. This type of organic switching behavior may be promising for next-generation organic electronics.

Unique phenanthrenequinone imidazole-based fluorescent materials with aggregation-induced or two-photon emission

In this work, we developed a single fluorophore core with different substituents that can exhibit either outstanding AIE or two-photon properties.

A chiroptical switch based on DNA/layered double hydroxide ultrathin films

A highly oriented film was fabricated by layer-by-layer self-assembly of DNA and MgAl-layered double hydroxide nanosheets, and its application in chiroptical switch was demonstrated via intercalation and deintercalation of an achiral molecule into the DNA cavity. DNA molecules are prone to forming an ordered and dispersive state in the interlayer region of rigid layered double hydroxide (LDH) nanosheets as confirmed by scanning electron microscopy and atomic force microscopy. The induced chiroptical ultrathin film (UTF) is achieved via the intercalation of an achiral chromophore [5,10,15,20-tetrakis(4-N-methylpyridyl)porphine tetra(p-toluenesulfonate) (TMPyP)] into the spiral cavity of DNA stabilized in the LDH matrix [denoted as TMPyP-(DNA/LDH)20]. Fluorescence and circular dichroism spectroscopy are utilized to testify the intercalation of TMPyP into (DNA/LDH)20 UTF that involves two steps: the electrostatic binding of TMPyP onto the surface of (DNA/LDH)20 followed by intercalation into base pairs of DNA. In addition, the TMPyP-(DNA/LDH)20 UTF exhibits good reversibility and repeatability in induced optical chirality, based on the intercalation and deintercalation of TMPyP by alternate exposure to HCl and NH3/H2O vapor, which can be potentially used as a chiroptical switch in data storage.

Stimuli-Responsive Films Based on N-Substituted Polyurethane with Different Alkyl Length

N-Alkyl-substituted polyurethanes with different alkyl chain lengths were prepared from the corresponding bromoalkane (1-bromooctane, 1-bromotetradecane, and 1-bromooctadecane) and polyurethane consisting of poly(propylene glycol) (PPG), 4,4′-diphenylmethane diisocyanate, and 1,4-butanediol. The synthesised materials were characterised by 1H NMR and FTIR spectroscopy and the degrees of substitution were discussed by changing the reaction conditions. Differential scanning calorimetry and X-ray diffraction were used to characterise the thermal properties and crystalline state. Microphase-separated nanostructures, with hard segments (nanofibre-like) embedded into amorphous PPG soft segments, are observed by atomic force microscopy and scanning electron microscopy. A reversible behaviour of the films was revealed by contact angle measurement. Stimuli-responsive films were realised by solvent vapour annealing and heat treatment, and exhibited a reversible switching in surface wettability with a remarkable change in the water contact angle of 21°.

Porphyrin assemblies through the air/water interface: effect of hydrogen bond, thermal annealing, and amplification of supramolecular chirality

Molecular assemblies of two achiral porphyrins with different substituents, 5-(4-methoxycarbonylphenyl)-10,15,20-triphenyl-21H,23H-porphine (TPPCOOMe) and 5-(4-carboxyphenyl)-10,15,20-triphenyl-21H,23H-porphine (TPPCOOH), have been fabricated by the Langmuir-Blodgett (LB) technique. It is disclosed that although only slight differences exist in the molecular skeleton of these two compounds, their interfacial assemblies display distinct chiroptical properties. It is found that weak circular dichroism (CD) signals are observed from the TPPCOOH assemblies, while in the case of the TPPCOOMe assemblies, only negligible CD signals could be detected. Interestingly, after the assemblies are subjected to a thermal annealing treatment, TPPCOOH assemblies show a distinct amplification of CD signals, while those of TPPCOOMe do not. An explanation in terms of the effect of substituents on the spreading properties of the compounds and the effect of intermolecular hydrogen bonds on the cooperative stacking of the building blocks is proposed to explain these new findings. The investigation suggests that in the present porphyrin systems, besides a nice spreading property, the cooperative interaction of various noncovalent interactions, including hydrogen bonding, π-π stacking, and hydrophobic interactions, is essentially required for the occurrence of symmetry breaking at the air/water interface.

Tunable helical assemblies of L-alanine methyl ester-containing polyphenylacetylene

The self-assemblying behaviors of L-alanine methyl ester-containing polyphenylacetylene (PPA-Ala, in Chart 1 ) were investigated upon the evaporation of its solvent on mica and on air/water interfaces. The introduction of chiral amino acid attachments to the polyphenylacetylene backbone induced a helical conformation of the backbone, which was stabilized by various noncovalent interactions, especially hydrophobic effect and hydrogen bonds. The helicity of the polymer was further amplified in its higher-order self-assemblies as the formation of helical fibers on the surface of mica upon natural evaporation of its THF solution. By LB technique, the polymer chains were guided to form ordered parallel ridges and highly aligned, with their helical conformation still remaining. The reorganization of the chiral polymer chains on air/water interface was associated with the additional hydrophobic effect of PPA-Ala on an air/water interface. The polymer backbones had to adopt different arrangements to minimize their contact with water, and this adjustment led to the formation of aligned polymer ridges under proper surface pressure.

Chiral assemblies of achiral rigid-flexible molecules at the air/water interface induced by silver(I) coordination

Chiral films are obtained from achiral rigid-flexible molecules. Due to hydrogen bonding and steric constraints, these molecules can self-assemble into chiral assemblies at the air/water interface upon compression. When the molecules are spread on a subphase containing AgNO(3), they form a stable monolayer through coordination with Ag(I) ions, as confirmed by surface pressure-area isotherms, UV/Vis and FTIR spectroscopy, and AFM. More interestingly, macroscopic chirality was detected in the Ag(I)-coordinated films by circular dichroism measurements. The formation mechanisms of the two kinds of chiral films are briefly discussed.

Chiral biointerface materials

Chiral phenomena are ubiquitous in nature from macroscopic to microscopic, including the high chirality preference of small biomolecules, special steric conformations of biomacromolecules induced by it, as well as chirality-triggered biological and physiological processes. The introduction of chirality into the study of interface interactions between materials and biological systems leads to the generation of chiral biointerface materials, which provides a new platform for understanding the chiral phenomena in biological system, as well as the development of novel biomaterials and devices. This critical review gives a brief introduction to the recent advances in this field. We start from the fabrication of chiral biointerface materials, and further investigate the stereo-selective interaction between biological systems and chiral interface materials to find out key factors governing the performance of such materials in given conditions, then introduce some special functionalities and potential applications of chiral biointerface materials, and finally present our own thinking about the future development of this area (108 references).

Construction of two 3D homochiral frameworks with 1D chiral pores via chiral recognition

The reactions of a pair of enantiomers of macrocyclic nickel(II) complexes with racemic penicillamine generated two 3D hydrogen-bonded homochiral frameworks of {[Ni(f-(SS)-L)](2)(l-pends)(ClO(4))(2)}(n) (Λ-1) and {[Ni(f-(RR)-L)](2)(d-pends) (ClO(4))(2)}(n) (Δ-1). The frameworks possess 1D tubular pores and opposite right/left-handed helical porous surfaces (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; pends(2-) = penicillaminedisulfide anion).

Interfacial assemblies of atypical amphiphilic porphyrins: hydrophobicity/hydrophilicity of substituents, annealing effects, and supramolecular chirality

Interfacial molecular assemblies of eight atypical amphiphilic porphyrins, substituted with hydrophobic or hydrophilic substituents but without long alkyl chains on their molecular skeletons, have been investigated in terms of supramolecular chiralty generation and amplification. It is found that all of the originally organized interfacial molecular assemblies display very weak or undetectable supramolecular chirality. Interestingly, for those porphyrins bearing hydrophilic substituents, it is found that their molecular assemblies display distinct supramolecular chirality when applying a thermal annealing treatment. In contrast, for those bearing hydrophobic substituents, the formed assemblies remain achiral after similar treatment. This investigation suggests that the hydrophobicity/hydrophilicity of the substituents of the atypical amphiphilies could affect the occurrence of the interfacial mirror symmetry breaking substantially. It discloses that, although some of the interfacial assemblies are superficially achiral, a thermal annealing treatment could endow them with evident supramolecular chirality.

Monte Carlo study of the self-assembly of achiral bolaform amphiphiles into helical nanofibers

It is shown by coarse-grained off-lattice Monte Carlo simulations that a geometrically induced frustration of the parallel arrangement of rigid achiral bolaform amphiphiles can cause chirality in self-assembled nanostructures. The amphiphilic molecules are represented as rigid linear chains of 8 equally sized hydrophobic spheres (tail) and a hydrophilic sphere (head) at each end. The hydrophilic and hydrophobic spheres differ in size. A very simple interaction scheme consisting of only hard-core repulsion between all spheres and square-well attraction between hydrophobic spheres is sufficient for self-assembly into helical fibers for molecules with head/tail diameter ratios ranging from 1.3 to 1.8.