Chiral AIEgens – Chiral recognition, CPL materials and other chiral applications

Direct Visualization of Chiral Amplification of Chiral Aggregation Induced Emission Molecules in Nematic Liquid Crystals

Chiral amplification in liquid crystals (LCs) is a well-known strategy. However, current knowledge about the underlying mechanism was still lacking; in particular, how it was realized at the nano scale still remained to be revealed. Here, we provide systematical exploration of chiral amplification of chiral aggregation induced emission (AIE) molecules in LCs from direct visualization of their co-assemblies at the nano scale to theoretical calculation of the molecular packing modes on a single molecular level. Using AFM imaging,we directly visualized the co-assembly formed by chiral AIE molecules/LCs at the nano scale: the chiral AIE molecules self-assembled into helical fibers to serve as the helical template for LCs to bind, while the LCs helically bound to the helical fibers to form the co-assembly, giving the morphology of pearled necklaces or thick rods. Theoretical calculation suggested that chiral AIE molecules were packed into left-handed helical fibers with a large volume of empty space between neighboring molecules, which provided the binding cites for LCs. Structural analysis showed that the π-π stacking between aromatic groups from LCs and TPE groups and the σ-π hyperconjugation between LC aromatic groups and cholesterol aliphatic groups play an important role in stabilizing the binding of LCs in the confined space on the surface of the helical assemblies.

Design and Development of Axially Chiral Bis(naphthofuran) Luminogens as Fluorescent Probes for Cell Imaging

Designing chiral AIEgens without aggregation-induced emission (AIE)-active molecules externally tagged to the chiral scaffold remains a long-standing challenge for the scientific community. The inherent aggregation-caused quenching phenomenon associated with the axially chiral (R)-[1,1'-binaphthalene]-2,2'-diol ((R)-BINOL) scaffold, together with its marginal Stokes shift, limits its application as a chiral AIE-active material. Here, in our effort to design chiral luminogens, we have developed a design strategy in which 2-substituted furans, when appropriately fused with the BINOL scaffold, will generate solid-state emissive materials with high thermal and photostability as well as colour-tunable properties. The excellent biocompatibility, together with the high fluorescence quantum yield and large Stokes shift, of one of the luminogens stimulated us to investigate its cell-imaging potential. The luminogen was observed to be well internalised and uniformly dispersed within the cytoplasm of MDA-MB-231 cancer cells, showing high fluorescence intensity.

Poly(amino acid)s-based star AIEgens for cell uptake with pH-response and chiral difference

Chiral aggregation-induced emission luminogens (AIEgens) are the new-generation chiral sensors that regulate chiral signals from the molecular level to the macroscopic assembly. Expanding applications of chiral AIEgens and in-depth understanding of their chiral recognition in biological systems are meaningful. Herein, two star chiral AIEgens, consisting of tetraphenylethene (TPE) as core and poly(N-acryloyl-L(D) valine) (PLV or PDV) as arms, were precisely synthesized via atom transfer radical polymerization (ATRP) technique and named TPE-PLV and TPE-PDV. They possessed typical AIE characteristics and exhibited an increase in concentration-dependent fluorescence intensity. The two AIEgens were pH-responsive and had strong AIE-related emission in acidic solution. Importantly, AIEgens can enter the living cells by ATP dependent endocytosis, then light them up. The interactions between the AIEgens and living human hepatocarcinoma (HepG2) cells revealed that the internalization process of TPE-PLV and TPE-PDV was both chiral-dependent and pH-responsive. This novel strategy for synthesizing poly(amino acid)s functionalized AIEgens could inspire the development of promising fluorescent materials with chirality.

Circularly polarized luminescence from structurally coloured polymer films

Structurally coloured polymer films with circularly polarized luminescence (CPL) properties were prepared by the photopolymerization of cholesteric liquid crystal mixtures doped with aggregation-induced emission (AIE)-active tetraphenylethylene. The films show good CPL performance with the luminescence dissymmetry factor up to 0.58 and enhanced fluorescence efficiency.

Ancillary ligand modulated stereoselective self-assembly of triple-stranded Eu(iii) helicate featuring circularly polarized luminescence

Creating optically pure metal assemblies is a hot research topic in the realms of chiral supramolecules. Here, three new triple-stranded europium(iii) helicates Eu₂L₃(L′)₂ [L = 4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)diphenyl sulphide; L′ = 1,10-phenanthroline (Phen) or R/S-2,2′-bis(diphenylphosphinyl)-1,1′-binaphthyl (R/S-BINAPO)] were synthesized in order to investigate the effects of ancillary ligands on controlling the stereoselective self-assembly of lanthanide helicates. X-ray single crystal structure analysis showed that Eu₂L₃(Phen)₂ crystalized in an achiral space group P1̄ with the equivalent amount of P and M helicates in one single cell. The isolated Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ were verified to be enantiopure by ¹H, ¹⁹F, ³¹P NMR and DOSY NMR analyses. Additionally, the mirror-image CD spectra also demonstrated the successful syntheses of the enantiomers and the presence of an effective chirality transformation from BINAPO to achiral L. Furthermore, the perfect mirror-image circularly polarized luminescence (CPL) spectra of Eu₂L₃(S-BINAPO)₂ and Eu₂L₃(R-BINAPO)₂ indicated the existence of the excited state chirality of the Eu³⁺ center associated with |g_lum| values reaching 0.112. In addition, the photophysical properties of three helicates were also discussed.

Chiral ancillary ligands (R/S-BINAPO) modulated the stereoselective self-assembly of lanthanide helicates, which presented strong CPL with |g_lum| values up to 0.112 and high luminescence quantum yield up to 34%.

Adaptive chirality of achiral tetraphenylethene-based tetracationic cyclophanes with dual responses of fluorescence and circular dichroism in water

Two tetraphenylethene-based tetracationic cyclophanes 1 and 2 were synthesized via a one-step S_N2 reaction without using any template. Based on the fluorescence and rotational conformation of the tetraphenylethene units, these water-soluble cyclophanes exhibited adaptive chirality with dual responses of turn-on fluorescence and induced circular dichroism when combined with nucleotides and DNA in water.

Stepwise enhancement of fluorescence induced by anion coordination and non-covalent interactions

A multi-level regulation of fluorescence enhancement upon anion coordination and subsequent binding of a guest (methyl viologen) was presented by a bis-bis(urea)-decorated tetraphenylethene (TPE) ligand with an assembly-enhanced emission characteristic.

Side-chain chiral fluorescent liquid crystal polymers with highly efficient circularly polarized luminescence emission in a glassy-state SmC* film

A series of high-efficiency CPL-active homopolymer materials in solid films with modulated properties were fabricated.

Fluorescent AIE-Active Materials for Two-Photon Bioimaging Applications

Fluorescence imaging has been widely used as a powerful tool for in situ and real-time visualization of important analytes and biological events in live samples with remarkably high selectivity, sensitivity, and spatial resolution. Compared with one-photon fluorescence imaging, two-photon fluorescence imaging exhibits predominant advantages of minimal photodamage to samples, deep tissue penetration, and outstanding resolution. Recently, the aggregation-induced emission (AIE) materials have become a preferred choice in two-photon fluorescence biological imaging because of its unique bright fluorescence in solid and aggregate states and strong resistance to photobleaching. In this review, we will exclusively summarize the applications of AIE-active materials in two-photon fluorescence imaging with some representative examples from four aspects: fluorescence detection, in vitro cell imaging, ex vivo tissue imaging, and in vivo vascular imaging. In addition, the current challenges and future development directions of AIE-active materials for two-photon bioimaging are briefly discussed.

Visible Circularly Polarized Luminescence of Octanuclear Circular Eu(III) Helicate

This work reports on the structural and photophysical characterization of D₄-symmetrical octanuclear circular Ln^III helicates, [(R)- or (S)-ⁱPr-Pybox]₈(Ln^III)₈(THP)₈ (where Ln = Eu and Tb, THP = trianionic tris-β-diketonate, and ⁱPr-Pybox = chiral bis(4-isopropyl-2-oxazolinyl)pyridine). X-ray crystallographic analysis revealed that the octanuclear circular helicate possesses square antiprism architecture and consists of four [(R)- or (S)-ⁱPr-Pybox]₂Ln^III₂(THP)₂ asymmetric units arranged in a closed ring form. Ligand-to-ligand interactions between the THP and the ⁱPr-Pybox ligands have successfully directed formation of enantiopure, homoconfigurational (Δ,Δ,Δ,Δ,Δ,Δ,Δ,Δ)-R and (Λ,Λ,Λ,Λ,Λ,Λ,Λ,Λ)-S isomers. All of the nonacoordinated Ln^III ions are identical and exhibit a distorted capped square antiprism (CSAP) geometry. Upon excitation of the ligand absorption band (λ = 360 nm), the circular helicates display characteristic Eu^III (red, ⁵D₀ → ⁷F_J, J = 0-4) or Tb^III (green, ⁵D₄ → ⁷F_J, J = 6-3) core f-f luminescence. The overall emission quantum yields of the circular Eu^III and Tb^III helicates are 0.145 and 0.0013, respectively, in chloroform. The Eu^III and Tb^III complexes exhibit remarkable circularly polarized luminescence (CPL) activity at their magnetic dipole transition with observed luminescence dissymmetry factors |g_lum| of 1.25 (⁵D₀ → ⁷F₁, λ = 592 nm) and 0.25 (⁵D₄ → ⁷F_5, λ = 541 nm), respectively. Exceptional |g_lum| values of the circular Eu^III helicates highlight the visible intensity difference between left and right circularly polarized emissions of R and S isomers in chloroform and PMMA thin film.

A new soft-matter material with old chemistry: Passerini multicomponent polymerization-induced assembly of AIE-active double-helical polymers with rapid visible-light degradability

Mimicking the superstructures and functions of natural chiral materials is beneficial to understand specific biological activities in living organisms and broaden applications in the fields of chemistry and materials sciences. However, it is still a great challenge to construct water-soluble, double-helical polymers with multiple responsiveness. Herein, we report for the first time a straightforward, general strategy to address this issue by taking advantage of Passerini multicomponent polymerization-induced assembly (PMPIA). The polymerization-induced generation of supramolecular interactions in chiral α-acyloxy amides drives the assembly of polymers and improves their stability in various solvents. This double-helical polymer is sensitive to metal ions, temperature, pH, and solvents, making both the superstructure and the AIE effect reversibly adjustable. Meanwhile, the hydrogen-bonding-assisted cyclization of photolabile α-acyloxy amides accelerates the degradation of helical polymers under visible-light irradiation. It is anticipated that this novel PMPIA strategy opens new horizons to inspire the design of advanced chiral/helical polymers with multiple functions.