Modular Molecular Self‐Assembly for Diversified Chiroptical Systems

Bottom‐up multicomponent molecular self‐assembly is an efficient approach to fabricate and manipulate chiral nanostructures and their chiroptical activities such as the Cotton effect and circular polarized luminescence (CPL). However, the integrated coassembly suffers from spontaneous and inherent systematic pathway complexity with low yield and poor fidelity. Consequently, a rational design of chiral self‐assembled systems with more than two components remains a significant challenge. Herein, a modularized, ternary molecular self‐assembly strategy that generates chiroptically active materials at diverse hierarchical levels is reported. N‐terminated aromatic amino acids appended with binding sites for charge transfer and multiple hydrogen bonds undergo the evolution of supramolecular chirality with unique handedness and luminescent color, generating abundant CPL emission with high luminescence dissymmetry factor values in precisely controlled modalities. Ternary coassembly facilitates high‐water‐content hydrogel formation constituted by super‐helical nanostructures, demonstrating a helix to toroid topological transition. This discovery would shed light on developing complicated multicomponent systems in mimicking biological coassembly events.     

Reversible Quadruple Switching with Optical,Chiroptical, Helicity,and Macropattern in Self‐Assembled Spiropyran Gels

Enantiomeric glutamate gelators containing a spiropyran moiety are designed and found to self‐assemble into a nanohelix through gelation. Upon alternating UV and visible light irradiation, the spiropyran experiences a reversible change between a blue zwitterionic merocyanine state and a colorless closed ring state spiropyran in supramolecular gels. This photochromic switch causes a series of subsequent changes in the optical, chiroptical, morphological properties from supramolecular to macroscopic levels. While the solution of the gelator molecules does not show any circular dichroism (CD) signal in the region of 250–700 nm due to the fact that the chromophore is far from the chiral center, the gel shows chiroptical signals such as CD and circularly polarized luminescence (CPL) because of the chirality transfer by the self‐assembly. These signals are reversible upon alternating UV/vis irradiation. Therefore, a quadruple optical and chiroptical switch is developed successfully. During such process, the self‐assembled nanostructures from the enantiomeric supramolecular gels also undergo a reversible change between helices and fibers under the alternating UV and visible light trigger. Furthermore, a rewritable material fabricated from their xerogels on a glass is developed. Such rewritable material can be efficiently printed over 30 cycles without significant loss in contrast and resolution using UV and visible light.     

Chiral AuCuAu Heterogeneous Nanorods with Tailored Optical Activity

Here, a facile wet‐chemistry route for the selective growth of crystalline copper (Cu) along the sides of gold nanorods (Au NRs) in the presence of a hexadecylamine (HDA) is reported. The resulting heterostructures feature part etching of copper by galvanic replacement reaction and form crystalline AuCu alloy metal on one side of the Au NRs. By virtue of the dipeptide (cysteine‐phenylalanine, Cys‐Phe) ligand used during synthesis, the AuCuAu heteronanorods (HNRs) exhibit strong circular dichroism (CD) in the wavelength range of 400–1000 nm. The plasmonic chirality can be tailored by increasing the length of the Au NRs, the scale of Cu nanocrystals on the Au NRs, and the amount of gold chloride for postgrowth, resulting in an anisotropy factor (g factor) as high as 0.57 × 10^?2. The strong CD signals are attributed to the local electromagnetic field. Under circular polarized light (CPL) illumination, the chiral plasmonic AuCuAu nanostructure exhibits high efficiency for light polarization dependent reactive oxygen species (¹O₂) that is 22.31 times that of Au NRs. The results of this study demonstrate that the chiral enantiomer provides a chirality dependent avenue for highly efficient phototherapy.     

Chiral copolymers of (R)‐N‐(1‐phenylethyl) methacrylamide and 2‐hydroxyethyl methacrylate: copolymerization characteristics and chiroptical properties

The aim of this investigation was the copolymerization of a chiral monomer, (R)‐N‐(1‐phenylethyl) methacrylamide, with an achiral monomer, 2‐hydroxyethyl methacrylate (HEMA). The copolymerization characteristics as well as the chiroptical properties (optical rotation and circular dichroism) and their variation with copolymer composition and temperature are discussed. The copolymers are statistical and enriched in HEMA. The monomer reactivity ratio of the chiral monomer (r₁) is 0.133 whereas that of HEMA (r₂) is 1.042 based on the Kelen–Tudos method. The sequence of consecutive chiral monomer units predominates for a feed composition between 0.5 and 0.9 (mole fraction). On the other hand, the sequence of HEMA is uniform and it predominates for a feed composition of around 0.5 (mole fraction). The chiroptical properties of the copolymers do not vary linearly with the content of chiral units in the copolymers. The optical rotation and circular dichroism attain optimum values above 30–40 mol% of chiral monomer units in the copolymers. However, the circular dichroism of the copolymers varies linearly with the temperature. The chiral monomer being a more bulky structure is less reactive than HEMA. The nonlinear variation of chiroptical properties of the copolymers with the content of chiral units may be due to the secondary interaction in the copolymers associated with the hydrogen bonding involving the amide linkage (CONH) present in the pendant chromophore of the chiral monomer as well as the hydroxyl pendant group of HEMA and also the aromatic π–π interaction. Copyright © 2009 Society of Chemical Industry     

Supramolecular Chiroptical Switches Based on Achiral Molecules

Chirality plays an important role in biological and material sciences. By introducing chiral elements into functional materials, new properties are created and an increase in information density can be achieved. Chiral properties of functional materials do not only rely on molecular structure, but also on supramolecular interaction between the building blocks. In contrast to the generally accepted opinion that chiral systems should include chiral molecules, this Research News introduces the role of achiral molecules in realizing chiral properties in films and gel‐like materials. Even a system that is entirely composed of achiral molecules can exhibit interesting chiroptical properties in supramolecular ultrathin films. This article demonstrates how achiral molecules can be assembled into supramolecular chiral films and organogels. It further shows how the incorporated achiral molecules can be used to switch the chiral properties of these supramolecular films and organogels.     

Supramolecular Nanostructures of Chiral Perylene Diimides with Amplified Chirality for High‐Performance Chiroptical Sensing

Chiral supramolecular nanostructures with optoelectronic functions are expected to play a central role in many scientific and technological fields but their practical use remains in its infancy. Here, this paper reports photoconductive chiral organic semiconductors (OSCs) based on perylene diimides with the highest electron mobility among the chiral OSCs and investigates the structure and optoelectronic properties of their homochiral and heterochiral supramolecular assemblies from bottom‐up self‐assembly. Owing to the well‐ordered supramolecular packing, the homochiral nanomaterials exhibit superior charge transport with significantly higher photoresponsivity and dissymmetry factor compared with those of their thin film and monomeric equivalents, which enables highly selective detection of circularly polarized light, for the first time, in visible spectral range. Interestingly, the heterochiral nanostructures assembled from co‐self‐assembly of racemic mixtures show extraordinary chiral self‐discrimination phenomenon, where opposite enantiomeric molecules are packed alternately into heterochiral architectures, leading to completely different optoelectrical performances. In addition, the crystal structures of homochiral and heterochiral nanostructures have first been studied by ab initio X‐ray powder diffraction analysis. These findings give insights into the structure–chiroptical property relationships of chiral supramolecular self‐assemblies and demonstrate the feasibility of supramolecular chirality for high‐performance chiroptical sensing.     

Gold Core‐DNA‐Silver Shell Nanoparticles with Intense Plasmonic Chiroptical Activities

The strong plasmonic chiroptical activities of gold core‐DNA‐silver shell nanoparticles (NPs) are reported for the first time, using cytosine‐rich single‐stranded DNA as the template for the guidance of silver shell growth. The anisotropy factor of the optically active NPs at 420 nm reaches 1.93 × 10^?2. Their chiroptical properties are likely induced by the DNA–plasmon interaction and markedly amplified by the strong electromagnetic coupling between the gold core and silver shell.     

Optical and chiroptical switches based on photoinduced photon and proton transfer in copolymers containing spiropyran and azopyridine chromophores in their side chains

New methacrylic copolymers, bearing in the side chain spiropyran moieties and/or the optically-active (S)-3-hydroxy pyrrolidinyl group linked through the nitrogen atom to an azopyridine chromophore, along with an opportune molar content of methyl methacrylate co-units, have been prepared by radical polymerization of the corresponding monomers.The resulting macromolecules have been fully characterized with particular attention to the study of their thermal stability, optical activity, chiroptical and photoinduced properties.In the presence of acid, it is possible to modulate the protonation of the azopyridine groups by photo-isomerization (with UV and/or Vis light) of the spiropyran component. The resulting signal communication between these macromolecular switches can be monitored by UV-VIS as well as CD spectroscopy and is completely reversible and reproducible.The results are discussed in terms of copolymer composition and different cooperative behaviour, which lead to a variation of interactions between spiropyran and azoaromatic chromophores and reveal that the best proton transfer is actually obtained in the random terpolymer with lower amounts of co-units of spiropyran and azopyridine chromophores in the side chain, which displays improved sensitivity to proton-transfer process and, in addition, exhibits good stability to repeated cycles of irradiation with UV and Vis light.