Role of Achiral Nucleobases in Multicomponent Chiral Self‐Assembly: Purine‐Triggered Helix and Chirality Transfer

Chiral self‐assembly is a basic process in biological systems, where many chiral biomolecules such as amino acids and sugars play important roles. Achiral nucleobases usually covalently bond to saccharides and play a significant role in the formation of the double helix structure. However, it remains unclear how the achiral nucleobases can function in chiral self‐assembly without the sugar modification. Herein, we have clarified that purine nucleobases could trigger N‐(9‐fluorenylmethox‐ycarbonyl) (Fmoc)‐protected glutamic acid to self‐assemble into helical nanostructures. Moreover, the helical nanostructure could serve as a matrix and transfer the chirality to an achiral fluorescence probe, thioflavin T (ThT). Upon chirality transfer, the ThT showed not only supramolecular chirality but also circular polarized fluorescence (CPL). Without the nucleobase, the self‐assembly processes cannot happen, thus providing an example where achiral molecules played an essential role in the expression and transfer of the chirality.     

Plasmonic Chirality Imprinting on Nucleobase‐Displaying Supramolecular Nanohelices by Metal–Nucleobase Recognition

Supramolecular self‐assembly is an important process that enables the conception of complex structures mimicking biological motifs. Herein, we constructed helical fibrils through chiral self‐assembly of nucleobase–peptide conjugates (NPCs), where achiral nucleobases are helically displayed on the surface of fibrils, comparable to polymerized nucleic acids. Selective binding between DNA and the NPC fibrils was observed with fluorescence polarization. Taking advantage of metal–nucleobase recognition, we highlight the possibility of deposition/assembly of plasmonic nanoparticles onto the fibrillar constructs. In this approach, the supramolecular chirality of NPCs can be adaptively imparted to metallic nanoparticles, covering them to generate structures with plasmonic chirality that exhibit significantly improved colloidal stability. The self‐assembly of rationally designed NPCs into nanohelices is a promising way to engineer complex, optically diverse nucleobase‐derived nanomaterials.     

A Carbon Dioxide Bubble‐Induced Vortex Triggers Co‐Assembly of Nanotubes with Controlled Chirality

It is challenging to prepare co‐organized nanotube systems with controlled nanoscale chirality in an aqueous liquid flow field. Such systems are responsive to a bubbled external gas. A liquid vortex induced by bubbling carbon dioxide (CO₂) gas was used to stimulate the formation of nanotubes with controlled chirality; two kinds of achiral cationic building blocks were co‐assembled in aqueous solution. CO₂‐triggered nanotube formation occurs by formation of metastable intermediate structures (short helical ribbons and short tubules) and by transition from short tubules to long tubules in response to chirality matching self‐assembly. Interestingly, the chirality sign of these assemblies can be selected for by the circulation direction of the CO₂ bubble‐induced vortex during the co‐assembly process.     

Cooperative Chirality and Sequential Energy Transfer in a Supramolecular Light‐Harvesting Nanotube

By constructing a supramolecular light‐harvesting chiral nanotube in the aqueous phase, we demonstrate a cooperative energy and chirality transfer. It was found that a cyanostilbene‐appended glutamate compound (CG) self‐assembled into helical nanotubes exhibiting both supramolecular chirality and circularly polarized luminescence (CPL). When two achiral acceptors, ThT and AO, with different energy bands were co‐assembled with the nanotube, the CG nanotube could transfer its chirality to both of the acceptors. The excitation energy could be transferred to ThT but only be sequentially transferred to AO. During this process, the CPL ascribed to the acceptor could be sequentially amplified. This work provides a new insight into the understanding the cooperative chirality and energy transfer in a chiral supramolecular system, which is similar to the natural light‐harvesting antennas.     

Tuning Achiral to Chiral Supramolecular Helical Superstructures

The design and synthesis of achiral organic functional molecules which can assemble into a chiral with selective handedness in the absence of chiral substances is an important in understanding the role chirality plays within these systems. In this review, we described general approaches towards supramolecular chiral molecules the synthesis and self‐assembly of achiral molecule to active chiral molecules to investigate controlled supramolecular chiral nanostructures with their photoluminescent properties for rapid, sensitive and selective detection of analytes of choice. Various small molecules have been discussed for achiral to chiral along with induction of chirality and controlled chiral helical structures in detail. We discussed few examples where stimuli used to control the chirality such as temperature, pH etc. Finally, we will also explore on the photo responsive helicity properties of the aggregation induced emission active molecule such as tetraphenylethene conjugates.     

Self‐Assembled Luminescent Quantum Dots To Generate Full‐Color and White Circularly Polarized Light

The design and fabrication of quantum dots (QDs) with circularly polarized luminescence (CPL) has been a great challenge in developing chiroptical materials. We herein propose an alternative to the use of chiral capping reagents on QDs for the fabrication of CPL‐active QDs that is based on the supramolecular self‐assembly of achiral QDs with chiral gelators. Full‐color‐tunable CPL‐active QDs were obtained by simple mixing or gelation of a chiral gelator and achiral 3‐mercaptopropionic acid capped QDs. In addition, the handedness of the CPL can be controlled by the supramolecular chirality of the gels. Moreover, QDs with circularly polarized white light emission were fabricated for the first time by tuning the blending ratio of colorful QDs in the gel. The chirality transfer in the co‐assembly of the achiral QDs with the gelator and the spacer effect of the capping reagents on the QD surface are also discussed. This work provides new insight into the design of functional chiroptical materials.     

Helical Nano‐crystallite (HNC) Phases: Chirality Synchronization of Achiral Bent‐Core Mesogens in a New Type of Dark Conglomerates

Spontaneous generation of macroscopic homochirality in soft matter systems by self‐assembly of exclusively achiral molecules under achiral conditions is a challenging task with relevance for fundamental scientific research and technological applications. Dark conglomerate phases (DC phases), being optically isotropic mesophases composed of conglomerates of macroscopic chiral domains and formed by some non‐chiral bent‐core mesogens, represent such a case. Here we report two new series of non‐symmetric bent‐core molecules capable of forming a new type of mirror symmetry broken DC phases. In the synthesized molecules, a bent 4‐bromoresorcinol core is connected to a phenyl benzoate wing and an azobenzene wing with or without additional peripheral fluorine substitution. The self‐assembly was investigated by DSC, polarizing microscopy, electro‐optical studies and XRD. Chiral and apparently achiral DC phases were observed besides distinct types of lamellar liquid crystalline phases with different degree of polar order, allowing the investigation of the transition from smectic to DC phases. This indicates a process in which increased packing density at first gives rise to restricted rotation and thus to growing polar order, which then leads to chirality synchronization, layer frustration and nano‐scale crystallization. Topological constraints arising from the twisted packing of helical conformers in lamellar crystals is proposed to lead to amorphous solids composed of helical nano‐crystallites with short coherence length (HNC phases). This is considered as a third major type of DC phases, distinct from the previously known liquid crystalline sponge phases and the helical nano‐filament phases (HNF phases). Guidelines for the molecular design of new materials capable of self‐assembly into these three types of DC phases are proposed.     

Enantioselective Synthesis of a Chiral Coordination Polymer with Circularly Polarized Visible Laser

Circular dichroism is known to be the feature of a chiral agent which has inspired scientist to study the interesting phenomena of circularly polarized light (CPL) modulated molecular chirality. Although several organic molecules or assemblies have been found to be CPL‐responsive, the influence of CPL on the assembly of chiral coordination compounds remains unknown. Herein, a chiral coordination polymer, which is constructed from achiral agents, was used to study the CPL‐induced enantioselective synthesis. By irradiation with either left‐handed or right‐handed CPL during the reaction and crystallization, enantiomeric excesses of the crystalline product were obtained. Left‐handed CPL resulted in crystals with a left‐handed helical structure, and right‐handed CPL led to crystals with a right‐handed helical structure. It is exciting that the absolute asymmetric synthesis of a chiral coordination polymer could be enantioselective when using CPL, and provides a strategy for the control of the chirality of chiral coordination polymers.     

Evolution from lyotropic liquid crystal to helical fibrous organogel of an achiral fluorescent twin-tapered bi-1,3,4-oxadiazole derivative

We report an unprecedented hierarchical self‐assembly of an achiral twin‐tapered bi‐1,3,4‐oxadiazole derivative (2,2‐bis(3,4,5‐trioctanoxyphenyl)‐bi‐1,3,4‐oxadiazole, BOXD‐T8). This molecule can form a layer‐structured lyotropic liquid crystal and further forms a helical fibrous organogel in DMF at concentrations above 0.6 wt %. The self‐assembly process of BOXD‐T8 in DMF is accompanied by a change in its fluorescence. The pitches of the helical fibers are non‐uniform, and both left‐ and right‐handed helical fibers are observed in equal quantities. Intermolecular π–π interactions between aromatic segments have been demonstrated to be the driving force for aggregate formation. This helical structure of BOXD‐T8 is dependent on the solvent, concentration, and the layer‐structured intermediate liquid‐crystalline state.     

Inversion of the Supramolecular Chirality of Nanofibrous Structures through Co‐Assembly with Achiral Molecules

To understand the behavior of chiral nanostructures, it is of critical importance to study how achiral molecules regulate the chirality of such nanostructures and what the main driving forces for the regulation processes are. In this work, the supramolecular chirality of helical nanofibers consisting of phenylalanine‐based enantiomers is inverted by achiral bis(pyridinyl) derivatives through co‐assembly. This inversion is mainly mediated by intermolecular hydrogen bonding interactions between the achiral additives and the chiral molecules, which may induce stereoselective interactions and different reorientations for the assembled molecules, as confirmed by calculations. This work not only exemplifies a feasible method to invert the helicity of chiral nanostructures by the addition of achiral molecules, but also provides a method to explore their functions in environments where chiral and achiral molecules are in close proximity.     

Crystallization‐Driven Asymmetric Helical Assembly of Conjugated Block Copolymers and the Aggregation Induced White‐light Emission and Circularly Polarized Luminescence

Controlling the self‐assembly morphology of π‐conjugated block copolymer is of great interesting. Herein, amphiphilic poly(3‐hexylthiophene)‐block‐poly(phenyl isocyanide)s (P3HT‐b‐PPI) copolymers composed of π‐conjugated P3HT and optically active helical PPI segments were readily prepared. Taking advantage of the crystallizable nature of P3HT and the chirality of the helical PPI segment, crystallization‐driven asymmetric self‐assembly (CDASA) of the block copolymers lead to the formation of single‐handed helical nanofibers with controlled length, narrow dispersity, and well‐defined helicity. During the self‐assembly process, the chirality of helical PPI was transferred to the supramolecular assemblies, giving the helical assemblies large optical activity. The single‐handed helical assemblies of the block copolymers exhibited interesting white‐light emission and circularly polarized luminescence (CPL). The handedness and dissymmetric factor of the induced CPL can be finely tuned through the variation on the helicity and length of the helical nanofibers.     

1,2‐Dithienyldicyanoethene‐Based,Visible‐Light‐Driven,Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.     

1,2‐Dithienyldicyanoethene‐Based,Visible‐Light‐Driven,Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.     

In Situ Controlled Construction of a Hierarchical Supramolecular Chiral Liquid‐Crystalline Polymer Assembly

Hierarchical supramolecular chiral liquid‐crystalline (LC) polymer assemblies are challenging to construct in situ in a controlled manner. Now, polymerization‐induced chiral self‐assembly (PICSA) is reported. Hierarchical supramolecular chiral azobenzene‐containing block copolymer (Azo‐BCP) assemblies were constructed with π–π stacking interactions occurring in the layered structure of Azo smectic phases. The evolution of chirality from terminal alkyl chain to Azo mesogen building blocks and further induction of supramolecular chirality in LC BCP assemblies during PICSA is achieved. Morphologies such as spheres, worms, helical fibers, lamellae, and vesicles were observed. The morphological transition had a crucial effect on the chiral expression of Azo‐BCP assemblies. The supramolecular chirality of Azo‐BCP assemblies destroyed by 365 nm UV irradiation can be recovered by heating–cooling treatment; this dynamic reversible achiral–chiral switching can be repeated at least five times.     

Unidirectional helical assembly via triple hydrogen bonds between chiral tris(oxazoline) and achiral tris(imidazoline)

Tris(imidazolines) are assembled via triple hydrogen bonding interactions to give rise to a stacked polymeric structure. A mixture of chiral tris(oxazoline) and achiral tris(imidazoline) generates a helical assembly in which the helical direction of the assembly is unidirectionally induced by the chirality of tris(oxazoline).     

Chirality Synchronization of Hydrogen‐Bonded Complexes of Achiral N‐Heterocycles

2,4‐Diamino‐6‐phenyl‐1,3,5‐triazines carrying a single oligo(ethylene oxide) (EO) chain form an optically isotropic mesophase composed of a conglomerate of macroscopic chiral domains with opposite sense of chirality even though the constituent molecules are achiral. This mesophase was proposed to result from the helical packing of hydrogen‐bonded triazine aggregates, providing long‐range chirality synchronization. The results provide first evidence for macroscopic achiral symmetry breaking upon conglomerate formation in an amorphous isotropic phase formed by hydrogen‐bonded associates of simple N‐heterocycles that are related to prebiotic molecules.     

Oligopeptide‐Assisted Self‐Assembly of Oligothiophenes: Co‐Assembly and Chirality Transfer

The biomolecule‐assisted self‐assembly of semiconductive molecules has been developed recently for the formation of potential bio‐based functional materials. Oligopeptide‐assisted self‐assembly of oligothiophene through weak intermolecular interactions was investigated; specifically the self‐assembly and chirality‐transfer behavior of achiral oligothiophenes in the presence of an oligopeptide with a strong tendency to form β‐sheets. Two kinds of oligothiophenes without (QT) or with (QTDA) carboxylic groups were selected to explore the effect of the end functional group on self‐assembly and chirality transfer. In both cases, organogels were formed. However, the assembly behavior of QT was quite different from that of QTDA. It was found that QT formed an organogel with the oligopeptide and co‐assembled into chiral nanostructures. Conversely, although QTDA also formed a gel with the oligopeptide, it has a strong tendency to self‐assemble independently. However, during the formation of the xerogel, the chirality of the oligopeptide can also be transferred to the QTDA assemblies. Different assembly models were proposed to explain the assembly behavior.     

Self‐Assembly of Achiral Shape Amphiphiles into Multi‐Walled Nanotubes via Helicity‐Selective Nucleation and Growth

Soft nanotubes are normally constructed from chiral amphiphiles through helical self‐assembly. Yet, how to self‐assemble achiral molecules into nanotubes is still a challenge. Here, we report the nanotube construction with achiral shape amphiphiles through helical self‐assembly and also unravel the formation mechanisms. The amphiphiles have a dumbbell shape and are composed by covalently linking three achiral moieties together: two unlike clusters and an organic tether. The difference in polarity between the unlike clusters drives the amphiphiles to self‐assemble into single‐ and multi‐walled nanotubes as well as intermediates. Analysis of the key intermediates unravels the self‐assembly mechanism of helicity‐selective nucleation and growth. Meanwhile, direct visualization of the individual clusters in the ribbons displays a two‐dimensional deformed hexagonal lattice. Thus, we speculate that it is the lattice deformation that creates anisotropic tension along different directions of the ribbon which further results in the formation of helical ribbons towards nanotubes by amphiphiles.     

Water‐Dependent Optical Activity Inversion of Chiral DNA–Silica Assemblies

Chirality is widely found in nature and is expressed hierarchically in many organic–inorganic hybrid materials. Optical activity (OA) is the most fundamental attribute of these chiral materials. In this study, we found that the OA of impeller‐like chiral DNA–silica assemblies (CDSAs) was inverted with the addition of water. The state of DNA under dry and wet conditions, and the dual chirality of chiral DNA layers and twisted helical arrays of opposite handedness in CDSAs were considered to exert predominant effects on the OAs. The circular dichroism (CD) responses for the dry CDSAs were mostly attributed to the chiral arrangement of DNA layers, whereas the opposite CD responses for the wet CDSAs primarily originated from twisted helical arrays of DNA molecules. The observed CD signals were a super‐position of the two opposing OA responses. The increase in the longitudinal relation of DNA molecules due to the recovery of a double‐helical structure of DNA in the presence of water was considered to be the reason for the increase in intensity of the CD signals that originated from the twisted helical array, which led to the inversion of OA of the CDSAs. The inversion of the plasmon‐resonance‐based OAs for the chiral‐arranged achiral Ag nanoparticles (NPs) located in the channels of the CDSAs in dry and wet states further confirmed the dual chirality of DNA packing. Such research on DNA assemblies and metal NPs with dual, opposite chirality assists in the understanding of DNA hierarchical chirality in living systems and the creation of macroscopic ordered helical materials and biosensors.     

Transfer of chirality from molecule to phase in self-assembled chiral block copolymers

Here, we report the mechanisms of chiral transfer at various length scales in the self-assembly of enantiomeric chiral block copolymers (BCPs*). We show the evolution of homochirality from molecular chirality into phase chirality in the self-assembly of the BCPs*. The chirality of the molecule in the BCP* is identified from circular dichroism (CD) spectra, while the handedness of the helical conformation in the BCP* is determined from a split-type Cotton effect in vibrational circular dichroism spectra. Microphase separation of the BCP* is exploited to form a helical (H*) phase, and the handedness of helical nanostructure in the BCP* is directly visualized from transmission electron microscopy tomography. As examined by CD and fluorescence experiments, significant induced CD signals and a bathochromic shift of fluorescence emission for the achiral perylene moiety as a chemical junction of the BCPs* can be found while the concentration of the BCPs* in toluene solution is higher than the critical micelle concentration, suggesting a twisting and shifting mechanism initiating from the microphase-separated interface of the BCPs* leading to formation of the H* phase from self-assembly.