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We report a new scheme for chiral recognition using coordination polymers of Ag+ with a chiral thiol ligand that contains a binding group. N-Benzoyl-l-cysteine ethyl ester equipped with a boronic acid group at the para position of the phenyl ring forms coordination polymers with Ag+ in alkaline aqueous solutions that exhibit excellent selectivity toward a d-glucose enantiomer over l-glucose, while the coordination polymers from the d-cysteine-based thiol ligand are specific for l-glucose. It is assumed that a conformation change occurs upon interaction of a saccharide molecule with the polymeric chain receptor, for which the next binding is promoted, leading to the highly effective chiral recognition, despite the flexible nature of the polymeric receptor.

A C-terminal amidothiourea motif is shown to promote a β-turn-like folded conformation in a series of β-amino acid-based short peptides in both the solid state and solution phase by an intramolecular 11-membered ring hydrogen bond.

Conventionally, z-direction modulation of two-dimensional covalent organic frameworks (2D-COFs) is difficult to achieve because they rely on spontaneous π-π interactions to form 3D architectures. Herein, we report a facile construction of a novel intercalated covalent organic framework (Intercalated-COF) by synchronizing operations of supramolecular donor-acceptor (D-A) interactions (A unit: 2,5,8,11-tetra(p-formylphenyl)-perylene diimide (PDI) 1; D unit: perylene 3, as intercalator) in the vertical directions, with polymerizations (by only reacting 1 with p-phenylenediamine 2) in the lateral directions. In this Intercalated-COF, the PDI-based covalent 2D layers are uniformly separated by perylene guest layers. This supramolecular strategy opens the possibility for z-direction modulation of 2D-COFs through "intercalating" various guest molecules and thus may contribute to the exploration of advanced applications of these porous and crystalline frameworks.

Many synthetic and supramolecular chiral polymeric systems are known to exhibit the “majority rules effect” (MRE), a positive nonlinear response in which a small enantiomeric excess (ee) of the chiral building blocks leads to unproportionally large chiroptical signals near zero ee. In contrast, the opposite “racemate rules effect” (RRE), a negative nonlinear response in which the chiroptical signals are “at near zero ee, while giving large nonlinear chiroptical responses to ee at high values, has only been occasionally observed. The origin of this unusual ee dependence remains elusive largely because few systems have been established that exhibit this effect. Herein, we present a design approach that enables the development of chiral supramolecular polymers with a pronounced negative nonlinear response akin to RRE. This is achieved by in situ generating a bidentate inducer for supramolecular polymerization that exists in both meso- and homochiral forms upon reacting with chiral guests. The presence of the meso-inducer creates an aggregate structure that has a little response in the circular dichroism (CD) spectra as a function of ee at a particular wavelength, but a homochiral inducer gives large changes in response to ee at this wavelength. This allowed for an RRE-like response to be observed when the CD intensity of the supramolecular polymers was plotted against the ee of the chiral guests that generate the meso- and homochiral inducers without the necessity of the racemic guest preferentially being incorporated into the polymer.

Chiral recognition remains a major challenge in the area of molecular receptor design. With this research, we set out to explore the use of proline-based receptors for chiral recognition. Importantly, the proline structure allows for the introduction of at least two different binding groups due to the availability of both an amine and carboxylic acid group. Here we report a proof-of-concept exploration into the chiral recognition of d/l-glucose as a model chiral species, which prefers to bind to at least two boronic acid groups. We evaluated several proline-based receptors incorporating two phenylboronic acid groups, respectively, at the N- and C-termini of the amino acid residue, via amide bonds. We confirmed that the receptors exhibited chiral recognition using CD, 1H NMR, and 19F NMR spectroscopy. Given the derivation diversity available, our strategy to use proline-based receptors for chiral recognition holds significant promise for extension to other chiral systems.

We report a dynamic covalent approach to transmembrane transport of amino acids by the formation of a three-component assembly. A mixture of a squaramide and a lipophilic and electrophilic aldehyde is shown to synergistically transport highly polar glycine (Gly) across vesicle membranes. The transport was investigated by a 13C NMR assay, an osmotic response assay and a newly-developed fluorescence assay suitable for measuring Gly influx, and other fluorescence assays for leakage and pH change. The transport is proposed to occur via a hydrogen-bonded anionic glycine hemiaminal/imine, accompanied by transport of OH- in the opposite direction. Several control experiments support the role of hemiaminal/imine in the observed facilitated Gly transport. Proton NMR studies of a biphasic system show the presence of both the hemiaminal and imine formed between Gly and an aldehyde. Interestingly, the synergistic effect has also been observed for sarcosine (Sar) that can form hemiaminals but not imines. The results demonstrate the potential of hemiaminal formation for the facilitated transport of substrates containing primary and secondary amino groups.

Based on a T-Hg(2+)-T binding mode, a sensitive ratiometric fluorescent chemosensor for aqueous Hg(2+) was developed with a heptamethine cyanine chromophore containing a thymine moiety.

A detailed investigation of the absorption and CD signals of Ag(I)-cysteine (Cys) aqueous solutions at buffered or varying pH has allowed us to suggest that coordination polymers are formed upon mixing Ag(I) and Cys bearing a Ag(I)-Cys repeat unit. The formation of the coordination polymers are shown to be facilitated by both the Ag(I)···Ag(I) interaction and the interaction between the side chains in the polymeric backbone. The former allows for an immediate spectral sensing of Cys with enantiomeric discrimination capacity with both high sensitivity and selectivity, and the contribution of the side-chain/side-chain interaction serves to guide extended sensing applications by means of modulating this interaction. With our preliminary data on the corresponding Cu(I)-Cys and Au(I)-Cys systems that exhibited similar spectral signals, we conclude that the M(I)-SR coordination polymers (M = Cu, Ag, or Au) could in general function as spectral sensing ensembles for extended applications. This sensing ensemble involves the formation of coordination polymers with practically no spectral background, thus affording high sensing sensitivity and selectivity.

We report single molecule conductance measurement of 4,4‘-bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE), and 1,2-di(pyridin-4-yl)ethane (BPY-EA), in which the two pyridine rings sandwich the conjugated ethene group and the nonconjugated ethane group, respectively. The conductance measurements of the molecules bridged in two Au electrodes have been carried out in aqueous solutions using the scanning tunneling microscopy-based break junction (STM-BJ) technique. All three molecules show two sets of conductance values with the high conductance (HC) values about 8−10 times higher than the low conductance (LC) values. The HC values for BPY, BPY-EE, and BPY-EA are (4.7 ± 1.1) × 10-3 G0, (2.3 ± 0.9) × 10-3 G0, and (4.5 ± 2.2) × 10-4 G0, respectively, in an order of BPY > BPY-EE > BPY-EA. The decrease of conductance of BPY-EE can be attributed mainly to the length increase of the molecule, while the decrease of the conductance of BPY-EA reveals the degree of electron decoupling upon insertion of one CH2CH2...

A series of salicylanilides (1a-h) bearing varied substituents at the 3'- or 4'-position of the anilino moiety (substituent = p-OCH3, p-CH3, m-CH3, H, p-Cl, m-Cl, p-CO2CH3, and p-CN) were synthesized. In acetonitrile all of the substituted salicylanilides 1a-h predominantly adopt the "closed-ring" conformation facilitated by a strong intramolecular OH...O=C hydrogen bond. In the presence of H2PO4-, the conformation of 1a-h was found to be modulated by the substituent. With our proposed proton-transfer fluorescence probing method, we were able to show that the conformation of 1a-f bearing a not highly electron-withdrawing substituent was switched to the "open-ring" form by H2PO4-, whereas 1h bearing a highly electron-withdrawing substituent, p-CN, remained in the "closed-ring" conformation. The significance of these findings for understanding, from a molecular structural point of view, the mechanism of salicylanilide-based inhibitors for inhibiting the protein tyrosine kinase epidermal growth factor receptor was discussed.

[graph: see text] 8-Hydroxyquinoline benzoates were developed as a new set of 8-HQ derivatives for highly sensitive fluorescent chemosensors for transition metal ions. A prominent fluorescence enhancement was found in the presence of transition metal ions such as Hg2+ and Cu2+, and this was suggested to result from the suppression of radiationless transitions from the npi state in the chemosensors.

A series of neutral N-(substituted-benzamido)-N'-phenylthioureas (substituent = p-OC(2)H(5), p-CH(3), m-CH(3), H, p-Cl, p-Br, m-Cl, and p-NO(2)) were designed as anion receptors, in which the thiourea binding site was attached to the benzamido moiety via an N-N bond. The absorption spectra of these N-benzamidothioureas in acetonitrile peaked at ca. 270 nm were found to show unprecedented red shifts by 7 373 to 14 325 cm(-1) in the presence of anions such as AcO(-), F(-), and H(2)PO(4)(-). Under the same conditions, the classic neutral thiourea receptors, N-(substituted-phenyl)-N'-phenylthioureas, showed absorption spectral shifts in most cases of less than 800 cm(-1) with one exception of 6501 cm(-1). Control experiments, effects of protic solvent, and (1)H NMR titration confirmed the formation of hydrogen-bonding complexes between the new N-benzamidothiourea receptors and anions. The binding constants with AcO(-), for example, are at 10(5)-10(7) mol(-1) L order of magnitude, which are 13 to 590 times those of the corresponding classic N-phenylthioureas in the same solvent. It was found that, whereas the absorption of the N-benzamidothiourea receptors showed essentially no dependence on the substituent, the substantially red-shifted new absorption band of the N-benzamidothiourea-anion binding complex was sensitively subject to the substituent. A linear relationship was found between the absorption energies of the N-benzamidothiourea-acetate binding complexes and the Hammett constants of the substituents with a negative slope of -0.34 eV. This led to the assignment that the substantially red-shifted absorption band was the ground-state intramolecular charge-transfer absorption with the substituent locating in the electron acceptor moiety. It was concluded that anion binding to the thiourea moiety of the N-benzamidothiourea receptors switched on their ground-state charge transfer. An anion-binding induced structural change was suggested to occur around the N-N bond in N-benzamidothioureas, which resulted in a substantially increased electron donating ability of the electron donor in the receptor molecules. As a consequence, the ground-state charge transfer takes place in the N-benzamidothiourea-anion binding complexes, leading to unprecedented red shifts in the absorption spectra and substantially enhanced anion binding affinities than those of the corresponding N-phenylthiourea receptors. N-Benzamido-N'-phenylthioureas represent a new generation of neutral thiourea-based anion receptors that show substantially improved anion binding performance important for anion sensing and recognition.

[reaction: see text] A substantially red-shifted fluorescence emission in 3-hydroxyl-2-naphthanilide in acetonitrile was developed and drastically enhanced upon addition of anions such as F(-), AcO(-), and H(2)PO(4)(-), with the enhancement depending on anion basicity. Excited-state intermolecular proton transfer in the sensor-anion hydrogen-bonding complex was suggested to be the signaling mechanism.

p-Dimethylaminobenzanilides with a para or meta substituent at the amido anilino moiety were designed to generate a series of dual fluorescent molecules of variable electron acceptors. Ab initio calculations indicated that the anilino substitution did not lead to an obvious change in the ground-state structures of the fluorophores, and the 1H NMR signal of the amido −NH proton was found to experience a linear downfield shift with increasing σ of the substitutent, supporting the fact that the amido anilino moiety was indeed varied comparably by the substitution. The intramolecular charge transfer dual fluorescence was indeed observed in solvents over a large polarity range from the nonpolar cyclohexane (CHX) through diethyl ether (DEE) and tetrahydrofuran (THF) to highly polar acetonitrile (ACN). It was found that the CT emission shifted to the blue with increasing electron-withdrawing ability of the amido anilino substituent up to a Hammett constant σ of ca. +0.39 and to the red at higher σ. The solvent p...

A series of benzoyl para- and meta-substituted benzanilides (BAs) and N-methylbenzanilides (MBAs) were synthesized and their absorption and fluorescence spectra in nonpolar solvent cyclohexane were investigated. Quantum mechanical calculations indicated that the ground state BAs existed preferentially in the trans configuration, whereas MBAs existed in the cis configuration, and benzoyl substitution hardly changed the ground-state structure in the same series. It was observed that all of the synthesized compounds displayed dual fluorescence in cyclohexane, i.e., a normal emission at ca. 330 nm and an abnormal long-wavelength fluorescence at around 500 nm. Although the normal emission of both BAs and MBAs did not show obvious variation, the long-wavelength emission shifted strongly to the red with increasing electron-withdrawing ability of the substituent at the benzoyl moiety. The emission energies of the long-wavelength fluorescence of BAs and MBAs in cyclohexane were found to vary linearly with the Hammett substituent coefficient U with almost the same slopes of -0.34 eV for both lines. In diethyl ether, similar linear correlation was also found for BAs with a slope of -0.36 eV that is close to that in cyclohexane. It was concluded that the long-wavelength emission of both BAs and MBAs in cyclohexane originated from a single intramolecular charge transfer state of similar configuration for both BAs and MBAs and of high extent of charge separation. The latter was supported by a linear slope of -0.67 for the correlation of long-wavelength emission energies of BAs against their reduction potentials. It was suggested that the reaction constant F might reflect the extent of charge separation of the CT reaction and the F value for a full charge separation was calculated to be -8.5 and +8.5 for a substituent at the electron donor and acceptor moiety, respectively.