Molecular Recognition of Chiral Zinc Porphyrin with Amino Acid Esters

One kind of novel chiral porphyrin and its zinc complex were synthesized and characterized. The molecular recognition of chiral zinc porphyrin towards amino acid esters in CHCl₃ was investigated by UV‐vis spectral titration method. The associative constants of the molecular recognition reactions were all K_D>K_L and followed the order of K(PheOMe)>K(LeuOMe)>K(ValOMe)>K(AlaOMe) in host (Zn(L‐BocTyr)TAPP). Circular dichroism spectra were used to explain chiral molecular recognition. The minimal energy conformation of host‐guest molecular system was sought by molecular dynamics method. The molecular recognition process of this host‐guest system was calculated by quantum chemistry and the results were explained by the experiments     

Planar Chiral Organoboranes with Thermoresponsive Emission and Circularly Polarized Luminescence: Integration of Pillar[5]arenes with Boron Chemistry

Enantiopure molecules based on macrocyclic architecture are unique for applications in enantioselective host‐guest recognition, chiral sensing and asymmetric catalysis. Taking advantage of the chiral transfer from the intrinsically planar chirality of pillar[5]arenes, we herein present an efficient and straightforward approach to achieve early examples of highly luminescent chiral systems ( P5NN and P5BN ). The optical resolution of their enantiomers has been carried out via preparative chiral HPLC, which was ascribed to the molecular functionalization of pillar[5]arenes with π‐conjugated, sterically bulky triarylamine (Ar₃N) as an electron donor and triarylborane (Ar₃B) as an acceptor. This crucial design enabled investigations of the chiroptical properties, including circular dichroism (CD) and circularly polarized luminescence (CPL) in the solid state. The intramolecular charge transfer (ICT) nature in P5BN afforded an interesting thermochromic shift of the emission over a wide temperature range.     

Emerging Cubic Chirality in γCD‐MOF for Fabricating Circularly Polarized Luminescent Crystalline Materials and the Size Effect

The chiral feature of γCD‐MOF, and especially the emergent cubic void, was not unveiled so far. Now, through the host–guest interaction between γCD‐MOF and achiral luminophores with different charges and sizes, the unique cubic chirality of the emerging void in γCD‐MOF as well as a size effect on CPL induction are revealed for the first time. Numerous achiral luminophores could be integrated into γCD‐MOF and emitted significantly boosted circularly polarized luminescence. While the small sized luminophores preferred to be loaded into the intrinsic void of γCD, large ones were selectively encapsulated into the cubic void. Interestingly, when the size of the guest luminophores was close to the cube size, it showed strong negative CPL. Otherwise, either positive or negative CPL was induced.     

Host–Guest Chemistry between Perylene Diimide (PDI) Derivatives and 18‐Crown‐6: Enhancement in Luminescence Quantum Yield and Electrical Conductivity

Perylene diimide (PDI) derivatives exhibit a high propensity for aggregation, which causes the aggregation‐induced quenching of emission from the system. Host–guest chemistry is one of the best‐known methods for preventing aggregation through the encapsulation of guest molecules. Herein we report the use of 18‐crown‐6 (18‐C‐6) as a host system to disaggregate suitably substituted PDI derivatives in methanol. 18‐C‐6 formed complexes with amino‐substituted PDIs in methanol, which led to disaggregation and enhanced emission from the systems. Furthermore, the embedding of the PDI ? 18‐C‐6 complexes in poly(vinyl alcohol) (PVA) films generated remarkably high emission quantum yields (60–70 %) from the PDI derivatives. More importantly, the host–guest systems were tested for their ability to conduct electricity in PVA films. The electrical conductivities of the self‐assembled systems in PVA were measured by electrochemical impedance spectroscopy (EIS) and the highest conductivity observed was 2.42×10^?5 S cm^?1.     

新型手性酪氨酸修饰的锌卟啉对氨基酸酯的分子识别研究

合成并表征了一种L型Boc酪氨酸修饰的自由卟啉(L-BocTyrTAPP)及其锌卟啉配合物Zn(L-BocTyr)TAPP.通过紫外-可见光谱滴定法,研究了手性锌卟啉配合物与四对对映异构的手性氨基酸酯客体在CHCl₃中的分子识别行为.实验结果表明,在分子识别的过程中,缔合常数顺序均为D型略大于L型,且按K(PheOMe)>K(AIaOMe)>K(ValOMe)>K(LeuOMe)的顺序依次减小.同时,利用园二色光谱进一步阐述手性分子识别过程.此外,采用分子力学方法搜索了主客体体系的最低能量构象,从理论上对实验本质进行较深入的探讨.     

新型手性双核Salen Zn(II)配合物的分子识别研究

采用新型Salen中间体合成了新型SalenZn(II)配合物.用紫外-可见光谱滴定法研究了主体双核SalenZn(II)与咪唑、二胺类等含氮小分子的分子识别行为,测定了它们的缔合常数.对咪唑类客体的缔合常数顺序为K(Im)>K(2?MeIm)>K(EMeIm);对二胺类客体缔合常数顺序为K(DAP)>K(DAE).主体与咪唑类和二胺类客体的配位数分别是2和1.主体与这些客体的识别过程为放热、熵减的焓驱动反应.利用圆二色光谱研究了识别过程的Cotton效应.用分子力学方法研究了主客体体系的最低能量构型,通过量化计算对实验事实做了进一步解释.     

Pillararene Host–Guest Complexation Induced Chirality Amplification: A New Way to Detect Cryptochiral Compounds

The contradiction between the rising demands of optical chirality sensing and the failure in chiral detection of cryptochiral compounds encourages researchers to find new methods for chirality amplification. Inspired by planar chirality and the host–guest recognition of pillararenes, we establish a new concept for amplifying CD signals of cryptochiral molecules by pillararene host–guest complexation induced chirality amplification. The planar chirality of pillararenes is induced and stabilized in the presence of the chiral guest, which makes the cryptochiral molecule detectable by CD spectroscopy. Several chiral guests are selected in these experiments and the mechanism of chiral amplification is studied with a non‐rotatable pillararene derivative and density functional theory calculations. We believe this work affords deeper understanding of chirality and provides a new perspective for chiral sensing.     

A fluorescent, shape-persistent dendritic host with photoswitchable guest encapsulation and intramolecular energy transfer

A fluorescent and photoresponsive host based on rigid polyphenylene dendrimers (PPDs) has been synthesized. The key building block for the divergent dendrimer buildup is a complex tetracyclone 12 containing azobenzenyl, pyridyl, and ethynyl entities. The rigidity of polyphenylenes is of crucial importance for a site-specific placement of different functions: eight azobenzene (AB) moieties into the rigid scaffold, a fluorescent perylenetetracarboxdiimide (PDI) into the core, and eight pyridin functions into the interior cavities. AB moieties of host-1 undergo reversible cis-trans photoisomerization and are photostable, as confirmed by various techniques: UV-vis, (1)H NMR, size exclusion chromatography, and fluorescence correlation (FCS). In this system, AB moieties act as photoswitchable hinges and enable control over (i) molecular size, (ii) intramolecular energy transfer between AB and PDI, and (iii) encapsulation and release of guest molecules. The presence of PDI allows not only following the effect of cis-trans photoisomerization on molecular size with highly sensitive FCS but also monitoring the efficiency of the intramolecular energy transfer process (from AB to PDI) by time-resolved optical spectroscopy. Pyridyl functions were incorporated to facilitate guest uptake via hydrogen bonds between the host and guests. Also, we have demonstrated that the photoswitchability of the host can be utilized to actively encapsulate guest molecules into its interior cavities. This novel, light-driven encapsulation mechanism could enable the design of new drug delivery systems.     

Dual Upconverted and Downconverted Circularly Polarized Luminescence in Donor–Acceptor Assemblies

Through mimicking both the chiral and energy transfer in an artificial self‐assembled system, not only was chiral transfer realized but also a dual upconverted and downconverted energy transfer system was created that emit circularly polarized luminescence. The individual chiral π‐gelator can self‐assemble into a nanofiber exhibiting supramolecular chirality and circularly polarized luminescence (CPL). In the presence of an achiral sensitizer Pd^II octaethylporphyrin derivative, both chirality transfer from chiral gelator to achiral sensitizer and triplet‐triplet energy transfer from excited sensitizer to chiral gelator could be realized. Upconverted CPL could be observed through a triplet–triplet annihilation photon upconversion (TTA‐UC), while downconverted CPL could be obtained from chirality‐transfer‐induced emission of the achiral sensitizer. The interplay between chiral energy acceptor and achiral sensitizer promoted the communication of chiral and excited energy information.     

Spectrophotometric and calorimetric titration studies on molecular recognition of camphor and borneol by nucleobase-modified beta-cyclodextrins

A series of modified beta-cyclodextrins with nucleobase substituents, that is, mono(6-ade-6-deoxy)-beta-cyclodextrin (2) and mono(6-ura-6-deoxy)-beta-cyclodextrin (3) as well as mono(6-thy-6-deoxy)-beta-cyclodextrin (4), were selected as molecular receptors to investigate their conformation and inclusion complexation behaviors with some chiral molecules, that is, (+)-camphor, (-)-camphor, (+)-borneol, and (-)-borneol, by spectrophotometric and microcalorimetric titrations in aqueous phosphate buffer solution (pH 7.2) at 298.15 K. Circular dichroism and NMR studies demonstrated that these nucleobase-modified beta-cyclodextrins adopted a co-inclusion mode upon complexation with guest molecules; that is, the originally self-included nucleobase substituents of the host did not move out from the beta-cyclodextrin cavity, but coexisted with guest molecule in the beta-cyclodextrin cavity upon inclusion complexation. Significantly, these nucleobase-modified beta-cyclodextrins efficiently enhanced the molecular binding ability and the chiral recognition ability of native beta-cyclodextrin, displaying enantioselectivity up to 3.7 for (+)-camphor/(-)-camphor pair by 2 and 3.5 for (-)-borneol/(+)-borneol pair by 3. The enhanced molecular/chiral recognition abilities of 2-4 toward (+/-)-camphor were mainly attributed to the increased entropic gains due to the extensive desolvation effects, while the favorable enthalpic gains originating from the good size-fit relationship as well as the hydrogen bond interactions between host and guest result in the enhanced molecular/chiral recognition abilities of 2-4 toward (+/-)-borneol.     

A Red‐Emissive Sextuple Hydrogen‐Bonding Self‐Assembly Molecular Duplex Bearing Perylene Diimide Fluorophores for Warm‐White Organic Light‐Emitting Diode Application

A novel sextuple hydrogen‐bonding (HB) self‐assembly molecular duplex bearing red‐emitting perylene diimide (PDI) fluorophores, namely PDIHB , was synthesized, and its molecular structure was confirmed by ¹H NMR, ¹³C NMR, TOF‐MS and 2D NMR. Compared with the small molecular reference compound PDI , PDIHB shows one time enhanced fluorescence efficiency in solid state (4.1% vs. 2.1%). More importantly, the presence of bulky HB oligoamide strands in PDIHB could trigger effective spatial separation between guest and host fluorophores in thin solid film state, hence inefficient energy transfer occurs between the blue‐emitting host 2TPhNIHB and red guest PDIHB in the 2 wt% guest/host blending film. As a result, a solution‐processed organic light‐emitting diode (OLED) with quite simple device structure of ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/ PDIHB (2 wt%): 2TPhNIHB (50 nm)/LiF (0.8 nm)/Al (100 nm) could emit bias‐independent warm‐white electroluminescence with stable Commission Internationale de L'Eclairage coordinates of (0.42, 0.33), and the maximum brightness and current efficiency of this device are 260 cd·m^?2 and 0.49 cd·A^?1, respectively. All these results indicated that HB self‐assembly supramolecular fluorophores could act as prospective materials for white OLED application.     

Emerging Cubic Chirality in γCD-MOF for Fabricating Circularly Polarized Luminescent Crystalline Materials and the Size Effect

The chiral feature of γCD-MOF, and especially the emergent cubic void, was not unveiled so far. Now, through the host–guest interaction between γCD-MOF and achiral luminophores with different charges and sizes, the unique cubic chirality of the emerging void in γCD-MOF as well as a size effect on CPL induction are revealed for the first time. Numerous achiral luminophores could be integrated into γCD-MOF and emitted significantly boosted circularly polarized luminescence. While the small sized luminophores preferred to be loaded into the intrinsic void of γCD, large ones were selectively encapsulated into the cubic void. Interestingly, when the size of the guest luminophores was close to the cube size, it showed strong negative CPL. Otherwise, either positive or negative CPL was induced.     

锌卟啉对氨基酸甲酯手性分子识别的理论研究

用Tripos力场和分子力学方法研究了手笥锌卟啉的最低能量构象,并用分子动力学模拟了锌卟啉对氨基酸甲酯的识别过程,发现锌卟啉与D-氨基酸酯结合能力强于L-氨基酸,这与热力学实验结果一致.     

Simultaneous enhancement of phosphorescence and chirality by host–guest recognition of molecular tweezers

A novel type of host–guest recognition systems have been developed on the basis of a Au(III) molecular tweezer receptor and chiral Pt(II) guests. The complementary host–guest motifs display high non-covalent binding affinity (K_a: ～10⁴ L/mol) due to the participation of two-fold intermolecular π–π stacking interactions. Both phosphorescence and chirality signals of the Pt(II) guests strengthen in the resulting host–guest complexes, because of the cooperative rigidifying and shielding effects rendered by the tweezer receptor. Their intensities can be reversibly switched toward pH changes, by taking advantage of the electronic repulsion effect between the protonated form of tweezer receptor and the positive-charged guests in acidic environments. Overall, the current study demonstrates the feasibility to enhance and modulate phosphorescence and chirality signals simultaneously via molecular tweezer-based host–guest recognition.     

Molecular recognition in cyclodextrin complexes of amino acid derivatives. 1. Crystallographic studies of beta-cyclodextrin complexes with N-acetyl-L-phenylalanine methyl ester and N-acetyl-L-phenylalanine amide pseudopeptides

Cyclodextrins (CDs) are widely utilized in studies of chiral and molecular recognition. By changing the functionality of the guest molecule, the effect of such changes on recognition by the host CD molecule can be examined. We report crystal structure determinations for two nearly isomorphous complexes of phenylalanine derivatives: beta-CD/N-acetyl-L-phenylalanine methyl ester and beta-CD/N-acetyl-L-phenylalanine amide. The complexes crystallize as hydrated head-to-head host dimers with two included guest molecules in space group P1. The crystal packing is such that it presents a nonconstraining hydrophobic pocket adjacent to a hydrophilic region, where potential hydrogen-bonding interactions with hydroxyl groups of neighboring cyclodextrin molecules and waters of hydration can occur. The two host molecules display very similar conformations; only a few of the primary hydroxyl groups are conformationally disordered. There are a number of changes in the location of water of hydration molecules, some of which are the result of different hydrogen-bonding interactions. For the different guest molecules, similar modes of penetration are observed in the CD torus; however, there is a 0.985-A shift in the position of the guest molecules in the host torus, which takes place without changing the hydrophobic interactions displayed by the phenyl side chains. This observation and the thermal motion of the guest molecules in the ester complex are taken as evidence that complex binding forces are weak. The pseudopeptides experience a significant degree of flexibility in the crystalline environment provided by CD dimers. Conformational differences of the pseudopeptide backbones and the presence of disordered water molecules in the host-guest interface provide examples of different hydrogen-bonding schemes of similar potential energy. The crystal system presents an opportunity to establish a database of molecular interactions for small peptides and peptide analogues with waters of hydration and functional groups in nonconstraining binding environments.     

Dynamic Switching of the Circularly Polarized Luminescence of Disubstituted Polyacetylene by Selective Transmission through a Thermotropic Chiral Nematic Liquid Crystal

The circularly polarized luminescence (CPL) of chiral disubstituted liquid‐crystalline polyacetylene (di‐LCPA) can be dynamically switched and amplified from left‐ to right‐handed CPL and vice versa through the selective transmission of CPL across a thermotropic chiral nematic liquid crystal (N*‐LC) phase. By combining a chiral di‐LCPA CPL‐emitting film with an N*‐LC cell and tuning the selective reflection band of the N*‐LC phase to coincide with the CPL emission band, a CPL‐switchable cell was constructed. The phase change induced by the thermotropic N*‐LC cell by varying the temperature leads to a change in the selective transmission of CPL, which enables the dynamic switching and amplification of CPL. It is anticipated that CPL‐switchable devices might find applications in switchable low‐threshold lasers and optical memory devices.     

Restricted Conformational Flexibility of a Triphenylamine Derivative on the Formation of Host–Guest Complexes with Various Macrocyclic Hosts

Herein, we report the host–guest‐type complex formation between the host molecules cucurbit[7]uril (CB[7]), β‐cyclodextrin (β‐CD), and dibenzo[24]crown‐8 ether (DB24C8) and a newly synthesized triphenylamine (TPA) derivative 1 X₃ as the guest component. The host–guest complex formation was studied in detail by using ¹H NMR, 2D NOESY, UV/Vis fluorescence, and time‐resolved emission spectroscopy. The chloride salt of the TPA derivative was used for recognition studies with CB[7] and β‐CD in an aqueous medium. The restricted internal rotation of the guest molecule on complex formation with either of these two host molecules was reflected in the enhancement of the emission quantum yield and the average excited‐state lifetime for the triphenylamine‐based excited states. Studies with DB24C8 as the host molecule were performed in dichloromethane, a medium that maximizes the noncovalent interaction between the host and guest fragments. The Förster resonance energy transfer (FRET) process involving DB24C8 and 1 (PF₆)₃, as the donor and acceptor fragments, respectively, was established by electrochemical, steady‐state emission, and time‐correlated single‐photon counting studies.     

Molecular arrangement and assembly guided by hydrophobic cavities inside DNA

DNA is a unique yet useful material to organize nanoscale molecular arrays along the helix axis. In this study, we demonstrate a useful approach for creating molecular arrays inside a double helical DNA. Our approach is based on a host-guest system. Introducing abasic sites into DNA afforded a hydrophobic cavity that serves as a host. A planar aromatic molecule (cationic perylenediimide, PDI) was used as the guest molecule. In an aqueous solution, the PDI molecules tend to aggregate with themselves due to the strong hydrophobicity. In the presence of DNA with the cavity, the binding of the PDI was found to site-specifically occur in the hydrophobic cavity. The unique assembly and arrangement for more than two PDI molecules was achieved by controlling the sizes and positions of the cavities. Our approach would provide a simple and convenient way to construct one-dimensional aromatic arrays in DNA.     

A Rapidly Self‐Healing Host–Guest Supramolecular Hydrogel with High Mechanical Strength and Excellent Biocompatibility

It is still a challenge to achieve both excellent mechanical strength and biocompatibility in hydrogels. In this study, we exploited two interactions to form a novel biocompatible, slicing‐resistant, and self‐healing hydrogel. The first was molecular host–guest recognition between a host (isocyanatoethyl acrylate modified β‐cyclodextrin) and a guest (2‐(2‐(2‐(2‐(adamantyl‐1‐oxy)ethoxy)ethoxy)ethoxy)ethanol acrylate) to form “three‐arm” host–guest supramolecules (HGSMs), and the second was covalent bonding between HGSMs (achieved by UV‐initiated polymerization) to form strong cross‐links in the hydrogel. The host–guest interaction enabled the hydrogel to rapidly self‐heal. When it was cut, fresh surfaces were formed with dangling host and guest molecules (due to the breaking of host–guest recognition), which rapidly recognized each other again to heal the hydrogel by recombination of the cut surfaces. The smart hydrogels hold promise for use as biomaterials for soft‐tissue repair.     

Spectroscopy, NMR and DFT studies on molecular recognition of crown ether bridged chiral heterotrinuclear salen Zn(II) complex

A barium-containing crown ether bridged chiral heterotrinuclear salen Zn(II) complex BaZn2L(ClO4)2, where L is a folded dinuclear chiral (R,R)-salen ligand, has been synthesized and characterized by elemental analysis, 1H NMR, UV-vis, IR, circular dichroism (CD) spectra, and mass spectra. As a folded dinuclear chiral host, its recognition with achiral guests (imidazole derivatives), rigid bidentate guest (1,4-diazobicyclo[2,2,2]octane, DABCO) and chiral guests (amino acid methyl esters) was investigated by means of UV-vis spectrophotometric titration, CD spectra. The association constants of D-amino acid methyl esters are found to be higher than those of their L-enantiomer. The sandwich-type binding of BaZn2L(ClO4)2-DABCO supramolecular assembly was specially studied via 1H NMR titration and 1H ROESY. To understand the recognition on molecular level, density functional theory (DFT) calculations on B3LYP/LanL2DZ were performed on the minimal energy conformations of host, guests, and host-guest complexes. The minimal energy conformations were obtained by molecular mechanics (MM) optimization and molecular dynamics (MD) simulation. The results of single point energy, HOMO energy, and charges transfer were analyzed. The results of theoretical calculations are in good agreement with the experimental data.