Hindered Tetraphenylethylene Helicates: Chiral Fluorophores with Deep-Blue Emission, Multiple-Color CPL, and Chiral Recognition Ability

Facile preparation, mechanochromic luminescence and excitation wavelength-dependent emission of tetra(1H-benzo[d]imidazol-2-yl)ethene Zn(II) complexes and their applications

Nowadays, benzimidazole and its derivatives are widely assembled into multifunctional materials with various properties such as mechanochromism, photochromism, thermochromism and electrochromism. Herein, two novel zinc(II) coordination compounds, [Zn2(L2)Br4]·2H2O (1) and [Zn2(L2)Cl4]·2H2O (2) (L2 = tetra(1H-benzo[d]imidazol-2-yl)ethene), have been constructed via one-pot facile synthesis from bis(1H-benzo[d]imidazol-2-yl)methane (L1) and zinc(II) salts. The ligand L2 with a CC double bond was in situ formed by C-C coupling of two sp3-C atoms of L1 in solvothermal synthesis, which provides a new strategy to generate the conjugation system conveniently. Impressively, complex 1 shows distinct fluorescence and phosphorescence dual emission due to the heavy atom effect of bromide ions. Complexes 1 and 2 exhibit high-contrast mechanochromic luminescence properties due to the transformation from a crystalline state to an amorphous state. In addition, they also exhibited excitation wavelength-dependent luminescence in crystalline states and DMF systems. As the excitation wavelength increased, their luminescence color in DMF solution changed significantly from blue to yellow. Theoretical calculations show that the complexes have multiple emission centers due to the locally excited nature and intramolecular charge transfer. Due to their excellent excitation wavelength-dependent luminescence properties, PMMA films based on complexes 1 and 2 were prepared and could be applied in anti-counterfeiting and UV protection.

Chiral Resolution and Chiroptical Properties of Hindered Tetraphenylethylene Helicates

Hindered tetraphenylethylene (hTPE) helicates are resolved into two left-handed (M) and right-handed (P) isomers by linkage and removal of chiral auxiliary (1R,2S,5R)-menthol, furnishing gram-scale hTPE enantiomers via flash silica column chromatography. hTPE helicate enantiomers bearing electron-accepting cyano and electron-donating triphenylamine groups can emit deep-blue CPL signals with a fluorescence quantum yield surpassing 50%. Full-color and white-light emission were achieved by blending them with dyes in a poly(methyl methacrylate) (PMMA) film.

The Designs and Applications of Tetraphenylethylene Macrocycles and Cages

Macrocycles and cages are very attractive for the development of functional materials due to their unique inner cavities. Building blocks with interesting functions and synthetic conveniences are especially attractive. Tetraphenylethylene (TPE) is such an entity with C2 symmetry and tetrakis-functional groups easily modifiable. As a typical aggregation-induced emission (AIE) active compound, TPE perfectly unites the functions of fluorescence and structural building blocks together. The unique marriage of the two roles into one component makes TPE an ideal platform for the development of functional molecular systems including macrocycles and cages. The TPE macrocycles and cages are not merely a simple combination of those two but also generate added values unseen in either component alone. The fluorescence properties of TPE in macrocycles/cages are greatly improved or modulated, which makes them more suitable for various applications compared to their linear counterparts. In this review, the chemistry and design principles of TPE macrocycles/cages are surveyed first. The unique properties of those compounds are also discussed to provide general guidance for their functionalization. A brief discussion of their applications focusing on the utilization of their unique fluorescence is also presented. In the last, outlooks and future perspectives of TPE macrocycles/cages are provided for further developments.

Advances in Chiral Macrocycles: Molecular Design and Applications

Chiral macrocycles have recently emerged as promising materials for enantioselective recognition, asymmetric catalysis, and circularly polarized luminescence (CPL) due to their terminal-free structure, preorganized chiral cavities, and unique host-guest and self-assembly properties. This review summarizes recent advances in the design and synthesis of chiral macrocycles with central, axial, helical, and planar chirality, each imparting distinct structural and chiroptical characteristics. We highlight key strategies for constructing these macrocycles and their applications in optoelectronic and catalytic systems. Emphasis is placed on the balance between rigidity and flexibility in macrocycle design, essential for effective molecular recognition, adaptable catalysis, and CPL. We conclude with perspectives on future opportunities, anticipating ongoing developments in chiral macrocycle research.

Supramolecular chiroptical sensing of chiral species based on circularly polarized luminescence

Circularly polarized luminescence (CPL) refers to the differentiation of the left-handed and right-handed emissions of chiral systems in the excited state. Serving as an alternative characterization method to circular dichroism (CD), CPL can detect changes in fluorescence in a chiral system, which could be more efficient in recognizing chiral species. Although CPL can be generated by attaching luminophores to a chiral unit through a covalent bond, the non-covalent bonding of fluorescent chromophores with chiral species or helical nanostructures can also induce CPL and their changes. Thus, CPL can be used as an alternative detection technique for sensing chiral species. In this review, we summarize typical recent advances in chirality sensing based on CPL. The determination of the absolute configuration of chiral compounds and encrypted sensing is also discussed. We hope to provide useful and powerful insights into the construction of chemical sensors based on CPL.

Tetraphenylethene-Based Molecular Cage with Coenzyme FAD: Conformationally Isomeric Complexation toward Photocatalysis-Assisted Photodynamic Therapy

Flavin adenine dinucleotide (FAD), serving as a light-absorbing coenzyme factor, can undergo conformationally isomeric complexation within different enzymes to form various enzyme-coenzyme complexes, which exhibit photocatalytic functions that play a crucial role in physiological processes. Constructing an artificial photofunctional system using FAD or its derivatives can not only develop biocompatible photocatalytic systems with excellent activities but also further enhance our understanding of the role of FAD in biological systems. Here, we demonstrate a supramolecular approach for constructing an artificial enzyme-coenzyme-type host-guest complex with photoinduced catalytic function in water. First, we have designed and synthesized a water-soluble tetraphenylethene (TPE)-based octacationic molecular cage (1) with a large and flexible cavity, which can adaptively encapsulate with two FAD molecules with "U-shaped" conformation (uFAD) to form a 1:2 host-guest complex (1⊃uFAD2) in water. Second, based on the conformationally isomeric complexation of FAD within 1, the 1⊃uFAD2 complex facilitates electron and energy transfers to molecular oxygen upon the white-light illumination, efficiently producing reactive oxygen species (ROS) such as superoxide radical (O2•-) and singlet oxygen (1O2). To our knowledge, the 1⊃uFAD2 complex acts as a photocatalyst to achieve the highest turnover frequency (TOF) of 35.6 min-1 for the photocatalytic oxidation reaction of NADH via a photoinduced superoxide radical catalysis mechanism in an aqueous medium. At last, combining the cytotoxic effects of ROS and the disruption of the intracellular redox balance involving NADH, 1⊃uFAD2 as a supramolecular photosensitizer displays an excellent oxygen-independent photocatalysis-assisted photodynamic therapy in hypoxic tumors.

Chiral Macrocycles for Enantioselective Recognition

The efficient synthesis of chiral macrocycles with highly enantioselective recognition remains a challenge. We have addressed this issue by synthesizing a pair of chiral macrocycles, namely, R/S-BINOL[2], achieving total isolated yields of up to 62% through a two-step reaction sequence. These macrocycles are readily purified by column chromatography over silica gel without the need for chiral separation, thus streamlining the overall synthesis. R/S-BINOL[2] demonstrated enantioselective recognition toward chiral ammonium salts, with enantioselectivity (KS/KR) values reaching up to 13.2, although less favorable separations were seen for other substrates. R/S-BINOL[2] also displays blue circularly polarized luminescence with a |glum| value of up to 2.2 × 10-3. The R/S-BINOL[2] macrocycles of this study are attractive as chiral hosts in that they both display enantioselective guest recognition and benefit from a concise, high-yielding synthesis. As such, they may have a role to play in chiral separations.

Highly enhanced chiroptical effect from self-inclusion helical nanocrystals of tetraphenylethylene bimacrocycles

The helical structure is often the key factor for forming and enhancing chiroptical properties, such as circular dichroism (CD) and circular polarized luminescence (CPL) effects. However, no matter whether helical molecules or helical aggregates, they usually display modest chiroptical signals, which limits their practical applications. Herein, chiral tetraphenylethylene (TPE) bimacrocycles prepared in almost quantitative yield show strong and repeatable CD signals up to more than 7000 mdeg, which is very rare for general organic compounds, besides emitting very strong CPL light with an absolute g lum value up to 6.2 × 10-2. It is found that the superhelices formed by self-inclusion between the cavity and outward cyclohexyl ring of TPE bimacrocycles in crystal state are the key factor for highly enhanced chiroptical effect, and the self-inclusion superhelices in assemblies are confirmed by High Resolution Transmission Electron Microscopy (HR-TEM), Powder X-ray Diffraction (XRD) and Fourier Transform Infrared Spectrometry (FT-IR) data. Furthermore, the chiral TPE bimacrocycle shows great potential in chiral recognition and chiral analysis not only for chiral acids but also for chiral amines, chiral amino acids, and neutral chiral alcohol. Using self-inclusion helical nanocrystals of chiral macrocycles, this work provides a new strategy for chiroptical materials with excellent chiroptical properties.

Boosting Circularly Polarized Luminescence from Alkyl-Locked Axial Chirality Scaffold by Restriction of Molecular Motions

Boosting the circularly polarized luminescence of small organic molecules has been a stubborn challenge because of weak structure rigidity and dynamic molecular motions. To investigate and eliminate these factors, here, we carried out the structure-property relationship studies on a newly-developed axial chiral scaffold of bidibenzo[b,d]furan. The molecular rigidity was finely tuned by gradually reducing the alkyl-chain length. The environmental factors were considered in solution, crystal, and polymer matrix at different temperatures. As a result, a significant amplification of the dissymmetry factor glum from 10-4 to 10-1 was achieved, corresponding to the situation from (R)-4C in solution to (R)-1C in polymer film at room temperature. A synergistic strategy of increasing the intramolecular rigidity and enhancing the intermolecular interaction to restrict the molecular motions was thus proposed to improve circularly polarized luminescence. The though-out demonstrated relationship will be of great importance for the development of high-performance small organic chiroptical systems in the future.

Asymmetric Synthesis of Tetraphenylethylene Helicates and Their Full-Color CPL Emission with High glum and High Fluorescence Quantum Yield

Tetraphenylethylene (TPE) helicates with single helical handedness not only owe high fluorescence quantum yield but also possess good helical chirality, showing an excellent circularly polarized luminescence active material. In this work, a new method for directly obtaining single-handed TPE helicates has been developed. By using chiral p-phenylenediamine derivatives as an intramolecular cyclization reagent of TPE, the single-handed propeller-like conformation and stable helical chirality of the TPE unit were obtained, avoiding complicated and expensive HPLC chiral column separation. The as-prepared chiral TPE helicates displayed strong emission with an almost quantitative fluorescence quantum yield (Φf) and strong circularly polarized luminescence (CPL). In addition, the chirality and CPL signals of the TPE helicates could be significantly magnified by the helical arrangement together with 4'-pentyl-4-biphenylcarbonitrile (5CB) liquid crystal molecules. Moreover, full-color CPL emissions with both a high absolute CPL dissymmetrical factor up to 0.43 and high Φf were afforded.

Multiple Stimuli-Responsive Luminescent Chiral Hybrid Antimony Chlorides for Anti-Counterfeiting and Encryption Applications

Stimuli-responsive circularly polarized luminescence (CPL) materials are ideal for information anti-countering applications, but the best-performing materials have not yet been identified. This work presents enantiomorphic hybrid antimony halides R-(C5 H12 NO)2 SbCl5 (1) and S-(C5 H12 NO)2 SbCl5 (2) showing mirror-imaged CPL activity with a dissymmetry factor of 1.2×10-3 . Interestingly, the DMF-induced structural transformation is realized to obtain non-emissive R-(C5 H12 NO)2 SbCl5  ⋅ DMF (3) and S-(C5 H12 NO)2 SbCl5  ⋅ DMF (4) upon exposure to DMF vapor. The transformation process is reversed upon heating. DFT calculations showed that the DMF-induced-quenched-luminescence is attributed to the intersection of the ground and excited state curves on the configuration coordinates. Unexpectedly, the nanocrystals of the chiral antimony halides 1 and 2 were prepared and indicate the excellent solution process performance. The reversible PL and CPL switching gives the system applications in information technology, anti-counterfeiting, encryption-decryption, and logic gates.

Fluorescein-derived carbon dots with chitin-targeting for ultrafast and superstable fluorescent imaging of fungi

Fluorescence microscopy based on fluorochrome has been rapidly developed as the candidate for morphological identification of pathogenic fungi over recent years, offering superior rapidity and efficacy over traditional culture methods. However, the intrinsic quenching properties of fluorescein limit the clinical application of fluorescence imaging. Herein, we report a nano-strategy by converting a commercial fluorescein dye, fluorescent brightener-33 (FB-33), into carbon dots (FB-CDs) through a one-pot hydrothermal method. FB-CDs exhibit a chitin-targeting capacity allowing the selective recognition and ultrafast imaging of fungi within 30 s. The fluorescence quantum yield of FB-CDs is 51.6% which is 8.6-fold higher than that of commercial dye, FB-33. Moreover, FB-CDs also display superstable fluorescence signals under continuous intense light irradiation for 2 h and long-term storage for more than 2 months. The significantly improved photobleaching resistance meets the prolonged fluorescence observation and quantitative analysis of microbial samples. This work offers a novel nanoconversion strategy of commercial dyes for point-of-care testing of pathogenic organisms.