A General Method for Constructing Optically Active Supramolecular Assemblies from Intrinsically Achiral Water-Insoluble Free-Base Porphyrins

Helicenes at Air/Water Interface: Spreading Film and Metal Ion Induced a Helical Ring Nanostructure

Two enantiomeric hydrohelicenes containing a hydroxyl group and a π-conjugated nonplanar structure are assembled at the air/water interface. These molecules are found to form spreading films with well-defined surface pressure-area isotherms. Upon transferring the spreading film onto the mica surface, porous nanostructures are observed. The spreading film can be transferred onto solid substrates by the Langmuir-Schaefer (LS) method and the transferred LS films display optical activity as revealed by the circular dichroism (CD) spectra. The P- and M-hydrohelicene enantiomers showed mirrored CD spectra, suggesting that the chirality of the LS films was controlled by molecular chirality. When these molecules are spread on the aqueous solution containing metal ions such as Ag⁺, Cu²⁺, and Zn²⁺, a clear twisted ring nanostructure, which is similar to the Möbius strip, is observed. It is suggested that the interaction between the hydroxyl groups of helicenes and metal ions induced such a ring nanostructure.

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Spreading Films of Anthracene-Containing Gelator Molecules at the Air/Water Interface: Nanorod and Circularly Polarized Luminescence

Two enantiomeric gelator molecules containing anthracene moiety were assembled at the air/water interface and several new insights into the films of the gelator molecules were revealed. When these molecules were spread at the air/water interface, they formed the nanorod structured monolayers and could be subsequently transferred to the solid substrate. The formed Langmuir-Blodgett (LB) films showed both optical activity and circularly polarized luminescence (CPL) due to the chirality transfer upon assembling. The dissymmetric factors of the CPL in the LB films were enhanced nearly 5 times than those in gel systems. Through the formation of the organized nanofilms, the arrangement of the molecules become compact and the film showed enantioselectivity to chiral species, whereas the molecular solution could not.

Platinized spherical supramolecular nanoassemblies of a porphyrin: facile synthesis and excellent catalytic recyclability

Porphyrin-based supramolecular nanoassemblies of a spherical morphology have been attracting broad interest owing to their wide application possibilities in numerous fields of paramount significance. Most of the existing assembly protocols, however, either suffer from the requirement of elaborately-designed yet tediously-synthesized ad hoc porphyrins, the use of surfactant templates, or accurate consideration of the experimental parameters etc. The initiation of a facile surfactant-free fabrication protocol performable under ambient conditions using commercial porphyrins as building blocks is strongly desired. We herein report that a commercial metal-free porphyrins, 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TPPNH2), could be facilely organized to form well-defined discrete spherical nanoassemblies at room temperature by means of a simple reprecipitation method. We further find that the as-manufactured TPPNH2 nanospheres could work as photocatalysts towards the reduction of potassium tetrachloroplatinate(ii), leading to their self-platinization and the production of platinum/porphyrin nanosphere nanocomposites, wherein ultrathin Pt nanoparticles of a size of ca. 3 nm are immobilized on the porphyrin nanospheres. Significantly, by taking the advantage of their easy sedimentation from aqueous suspensions, we show that the as-produced composites could serve as qualified heterogeneous nanocatalysts in terms of their excellent catalytic stability and recyclability towards the reduction of 4-nitrophenol, where the catalytic reactivity exhibits only trivial changes even after the reactions have been repeated 8 times continuously. Taking into account the general concerns of porphyrins- and Pt-based nanostructures, this might provide a facile method for the construction of spherical porphyrin nanostructures with self-platinization capability. Meanwhile, considering the high cost and scarcity of Pt, our nanocomposites with excellent stability and recyclability likely have a bright future of potential uses.

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.

Interfacial organization of achiral porphyrins via unidirectional compression: a general method for chiroptical porphyrin assemblies of selected chirality

Porphyrins are considered to be important scaffolds bridging supramolecular chemistry and chiral chemistry, where chirality selection via physical effects such as directional stirring and spin-coating has aroused particular interest. Nevertheless, these protocols could only work on a limited number of achiral porphyrins. It still remains a formidable challenge to pave a general avenue for the construction of chiral assemblies using achiral porphyrins. By means of a unique Langmuir-Schaefer (LS) technique of a unidirectional compression configuration, we herein have demonstrated that a series of achiral porphyrins could be facilely organized to form chiral interfacial assemblies of controlled supramolecular chirality. It has been disclosed that such a fascinating chirality selection scenario is intimately related to the direction of the compression-generated vortex-like flow, while the compression speed, one of the most significant parameters of the Langmuir technique, contributes less to this issue. With regard to a surface-pressure-dependent chirality selection phenomenon, it is suggested that the directional vortex-like flow generated by lateral compression might play a role in promoting the preferential growth of chiral assemblies showing an enhanced yet controlled CD signal. Our protocol might be, to some extent, a general method for achieving chiral porphyrin assemblies of controlled chirality.

Hierarchical transfer of chiral information from the molecular to the mesoscopic scale by Langmuir–Blodgett deposition of tetrasteroid-porphyrins

Transfer of chiral information from the molecular level to the mesoscale has been obtained by hierarchical self-assembly of tetrasteroid-porphyrins.

Dynamic helical cyclophanes with two quadruply-bridged planes arranged in an "obverse and/or reverse" relation

We describe the design of two types of cyclophanes that generate dynamic helicity through the twisting of two planes in a clockwise or counterclockwise direction to give (M)- or (P)-helicity. We used a rectangular and anisotropic plane of 1,2,4,5-tetrakis(phenylethynyl)benzene (TPEB), since it can be stacked in pairs in two ways, in parallel or orthogonally, to be identified as distinct cyclophane molecules. We adopted a synthetic strategy for obtaining these two cyclophanes as a mixture using a macrocyclic intermediate that possessed two rotatable phenyl rings. We introduced necessary parts into the rotators to give a mixture of rotational isomers leading to a parallel or orthogonal arrangement of TPEBs, and then doubly bridged two planes of TPEB to form quadruply-bridged cyclophanes. We consider that such two planes in each cyclophane are in an "obverse and/or reverse" relation. In each cyclophane, we found unique dynamic helical forms with (M)- or (P)-helicity as well as an inherently non-chiral form. Normally, the screw-sense preference of dynamic helicity would be controlled through the intramolecular or supramolecular transmission of central chirality, when a chiral auxiliary is attached to the cyclophanes or a chiral guest is allowed to form a complex with the cyclophanes. In a case where two different substitution groups were used on bridging units to generate planar chirality in each cyclophane, the screw-sense preference was controlled through the arrangement of these substitution groups, and did not depend on the transmission of central chirality. Two different substitution groups desymmetrize the enantiomeric forms with (M)- or (P)-helicity generated in each dynamic helical cyclophane so that two dynamic helical forms with (M)- or (P)-helicity can be in a diastereomeric relation. Thus, a particular screw sense of dynamic helicity can be preferred, regardless of whether or not the two substitution groups possess some chiral element.

Chiral Nanoarchitectonics: Towards the Design, Self-Assembly, and Function of Nanoscale Chiral Twists and Helices

Helical structures such as double helical DNA and the α-helical proteins found in biological systems are among the most beautiful natural structures. Chiral nanoarchitectonics, which is used here to describe the hierarchical formation and fabrication of chiral nanoarchitectures that can be observed by atomic force microscopy (AFM), scanning tunneling microscopy (STM), scanning electron microscopy (SEM), or transmission electron microscopy (TEM), is one of the most effective ways to mimic those natural chiral nanostructures. This article focuses on the formation, structure, and function of the most common chiral nanoarchitectures: nanoscale chiral twists and helices. The types of molecules that can be designed and how they can form hierarchical chiral nanoarchitectures are explored. In addition, new and unique functions such as amplified chiral sensing, chiral separation, biological effects, and circularly polarized luminescence associated with the chiral nanoarchitectures are discussed.

Reversible and irreversible emergence of chiroptical signals in J-aggregates of achiral 4-sulfonatophenyl substituted porphyrins: intrinsic chirality vs. chiral ordering in the solution

Different origins of the reversible and irreversible emergence of chiroptical signals in the title J-aggregates.

Primary investigation of the optical limiting performance of cyclo[8]pyrrole with a wide optical limiting window

Cyclo[8]pyrrole (CP) was prepared and its UV/vis spectral and optical limiting properties for ns light pulse were investigated withZ-scan technical.

Symmetry Breaking in the Supramolecular Gels of an Achiral Gelator Exclusively Driven by π-π Stacking

Supramolecular symmetry breaking, in which chiral assemblies with imbalanced right- and left-handedness emerge from achiral molecular building blocks, has been achieved in the organogels of a C3-symmetric molecule only via π-π stacking. Specifically, an achiral C3-symmetric benzene-1,3,5-tricarboxylate substituted with methyl cinnamate through ester bond was found to form organogels in various organic solvents. More interestingly, when gels formed in cyclohexane, symmetry breaking occurred; i.e., optically active organogels together with the helical nanofibers with predominant handedness were obtained. Furthermore, the stochastically appeared imbalanced helicity could be driven to desired handedness by utilizing slight chiral solvents such as (R)- or (S)-terpinen-4-ol. Remarkably, the handedness of supramolecular assemblies thus formed could be kept even when the chiral solvents were removed. For the first time, we show that symmetry breaking can occur in supramolecular gel system driven exclusively through π-π stacking.

A general protocol for π-conjugated molecule-based micro/nanospheres: artificial supramolecular antenna in terms of heterogeneous photocatalysis

An oil-in-water-mediated surfactant-assisted assembly is initiated as a general method for π-conjugated molecules-based micro/nanospheres, supramolecular antenna with regard to heterogeneous photocatalysis has been realized using porphyrin spheres.

Porphyrin nanoassemblies via surfactant-assisted assembly and single nanofiber nanoelectronic sensors for high-performance H₂O₂ vapor sensing

Porphyrins are recognized as important π-conjugated molecules correlating supramolecular chemistry, nanoscience, and advanced materials science. So far, as their supramolecular nanoassemblies are addressed, most efforts focus on the photo- or opto-related subjects. Beyond these traditional subjects, it is strongly desired to develop advanced porphyrin nanoassemblies in some other new topics of paramount importance. By means of a surfactant-assisted assembly, we herein show that porphyrins of different central metal ions, 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (H2TPyP), zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (ZnTPyP), and oxo-[5,10,15,20-tetra(4-pyridyl)porphyrinato]titanium(IV) (TiOTPyP), could be organized to form irregular aggregates, short nanorods, and long yet straight nanofibers, respectively. Remarkably, in terms of an organic ribbon mask technique, we show that such long yet straight TiOTPyP nanofibers could be integrated into single nanofiber-based two-end nanoelectronics. Such simple nanodevices could serve as high-performance sensors of a satisfactory stability, reproducibility, and selectivity for an expeditious detection of vapor-phase H2O2. This provides a new alternative for a fast sensing of vapor-phase H2O2, which is currently an important issue in the fields of anti-terrorism, industrial healthcare, etc. In contrast to the traditional investigations focusing on the photo- or opto-related topics, our work endows porphyrin nanostructures with new opportunities as advanced nanomaterials in terms of portable yet high-performance nanoelectronic sensors, which is an issue of general concern in modern advanced nanomaterials.

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.

Shuttle-like supramolecular nanostructures formed by self-assembly of a porphyrin via an oil/water system

In this paper, in terms of the concentration of an aqueous solution of a surfactant, we investigate the self-assembly behavior of a porphyrin, 5, 10, 15, 20-tetra(4-pyridyl)-21H, 23H-porphine [H2TPyP], by using an oil/water system as the medium. We find that when a chloroform solution of H2TPyP is dropwise added into an aqueous solution of cetyltrimethylammonium bromide [CTAB] with a lower concentration, a large amount of irregular nanoarchitectures, together with a small amount of well-defined shuttle-like nanostructures, hollow nanospheres, and nanotubes, could be produced. While a moderate amount of shuttle-like nanostructures accompanied by a few irregular nanoarchitectures, solid nanospheres, and nanorods are produced when a CTAB aqueous solution in moderate concentration is employed, in contrast, a great quantity of shuttle-like nanostructures together with a negligible amount of solid nanospheres, nanofibers, and irregular nanostructures are manufactured when a high-concentration CTAB aqueous solution is involved. An explanation on the basis of the molecular geometry of H2TPyP and in terms of the intermolecular π-π interactions between H2TPyP units, and hydrophobic interactions between CTAB and H2TPyP has been proposed. The investigation gives deep insights into the self-assembly behavior of porphyrins in an oil/water system and provides important clues concerning the design of appropriate porphyrins when related subjects are addressed. Our investigation suggests that an oil/aqueous system might be an efficient medium for producing unique organic-based nanostructures.

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.

Self-assembly of porphyrins on a single crystalline organic substrate

Layers of a free base porphyrin have been deposited from n-heptane solution onto single crystalline potassium acid phthalate substrates and the self-assembled structures have been characterized by atomic force microscopy and X-ray diffraction. Depending on the concentration of the porphyrin solution, different anisotropic structures are found that are epitaxially related to the substrate, with the length axis parallel to the [001] direction of the (010) substrate surface. For undersaturated solutions, islands, cellular structures, and a hole pattern of monolayers of approximately 2.5 nm height are formed, which correspond to porphyrin molecules stacking approximately perpendicular to the substrate surface. From supersaturated solutions, multilayers are formed. At the highest concentrations, nanosized needles develop, which cover the whole substrate, creating a "monocrystalline" film with a length scale of several millimeters.

Hydrodynamic effects on chiral induction

There are few unambiguous reports describing the transfer of chirality from stirring vortices down to the level of electronic transitions. In this tutorial review the methods reported are discussed as well as the structural trends that seem to be necessary conditions in order to detect this phenomenon.

Acidification and assembly of porphyrin at an interface: counterion matching, selectivity, and supramolecular chirality

The interfacial diprotonation and assemblies of a free-base achiral porphyrin, 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-21H,23H-porphine, on various acidic subphases were investigated. It has been shown that the compound could be diprotonated in situ on an acidic subphase and can form assemblies. The interfacially organized supramolecular assemblies were transferred onto a solid substrate, and the assemblies showed supramolecular chirality. Interestingly, the supramolecular chirality of the assemblies of the diprotonated species showed a counterion-dependent behavior. For the assemblies fabricated from the aqueous HCl subphases, a strong Cotton effect (CE) could be observed, although the porphyrin itself is achiral. When an aqueous HBr solution was used as the subphase, the assemblies showed a weak CE, whereas no CE could be detected for the assemblies formulated from the HNO3 or HI subphase. Interestingly, when a mixture of HBr and NaCl, or HNO3 and NaCl, was employed as the subphase, the formed assemblies displayed chiral features similar to those fabricated on the HCl subphase, suggesting that the Cl(-) could be preferentially visualized in terms of supramolecular chirality, although the system itself is composed of achiral species. On the basis of the experimental facts and a theoretical calculation, an explanation with regard to the different sizes of the counterions and the distinct binding affinities of the counteranions to the diprotonated porphyrin species has been proposed. Our findings provide new insights into the assembly of the diprotonated porphyrins as well as the interfacially occurring symmetry breaking.