A photoactivated chiral molecular clamp rotated by selective anion binding

Developing chiral molecular platforms that respond to external fields provides opportunities for designing smart chiroptical materials. Herein, we introduce a molecular clamp whose chiral properties can be turned on by photoactivation. Selective anion binding achieves rational tuning of the conformations and chiroptical properties of the clamp, including circular dichroism and circularly polarized luminescence. Cyanostilbene segments were conjugated to chiral amines with a rotatable axis. Negligible chiroptical signals were significantly enhanced through a light illumination-induced isomerization. Binding with halide ions (F-, Cl- and Br-) enables chiroptical inversion and subsequent amplification of the resulting opposite handedness state by photo treatment. In contrast, the larger I- and NO3 - ions failed to achieve chiroptical inversion. Also the handedness inversion was hampered in conformationally locked amines. Density-functional theory-based computational studies and experimental results reveal a structural transformation that proceeds from a butterfly-like open geometry to a closed V-shaped state initiated by four hydrogen bonds and the rotatable axis. This work illustrates design protocols for use in smart chiroptical molecular platforms mediated by photo treatment and anion binding.

Rapid room-temperature phosphorescence chiral recognition of natural amino acids

Chiral recognition of amino acids is very important in both chemical and life sciences. Although chiral recognition with luminescence has many advantages such as being inexpensive, it is usually slow and lacks generality as the recognition module relies on structural complementarity. Here, we show that one single molecular-solid sensor, L-phenylalanine derived benzamide, can manifest the structural difference between the natural, left-handed amino acid and its right-handed counterpart via the difference of room-temperature phosphorescence (RTP) irrespective of the specific chemical structure. To realize rapid and reliable sensing, the doped samples are obtained as nanocrystals from evaporation of the tetrahydrofuran solutions, which allows for efficient triplet-triplet energy transfer to the chiral analytes generated in situ from chiral amino acids. The results show that L-analytes induce strong RTP, whereas the unnatural D-analytes produce barely any afterglow. The method expands the scope of luminescence chiral sensing by lessening the requirement for specific molecular structures.

"Point-Line-Plane-Helix" Binuclear Platinum(II) Complexes: Metal-Induced Chirality, Chirality Self-Sorting, and Chiroptical Properties

Chirality is a fundamental property of nature, and thus, building novel chiral molecules plays a crucial role in multidisciplinary fields. Herein, we have developed a straightforward approach to effectively incorporate all four types of point, axial, planar, and helical chiralities into a single molecule for the first time. The resultant "point-line-plane-helix" binuclear Pt(II) complexes exhibit multiple chiralities, including not only point and axial chiralities from the bridging ligands but also planar and helical chiralities from metal coordination. The intramolecular π-π and Pt-Pt interactions will restrict intramolecular rotations, thereby stabilizing the metal-induced planar and helical chiralities. Furthermore, enantiopure (R,R,R,Rp,M) or (S,S,S,Sp,P) molecules could be obtained by chirality self-sorting without the use of chiral high-performance liquid chromatography. Their single-crystal, circular dichroism, and circularly polarized luminescence properties are comprehensively investigated, providing unequivocal insights into the design of multiple-chirality materials for related applications.

Preferential Enrichment of Enantiomer from Amino Acid Schiff Bases by Coordination Interaction and Crystallization

In this paper, preferential enrichment (PE) is described for three pairs of novel amino acid Schiff base Cu(II)/Cu(I) complexes. Single crystal X-ray diffraction indicated that 1-S/R are one-dimensional coordination polymers (CPs) with helical structures, and 2-S/R and 3-S/R are one-dimensional CPs with auxiliary ligands. By tuning the pH, the solvent and second ligands, the 1-S/R, 3-S/R underwent polymorphic transitions, resulting in enantioselective liberation of excess enantiomers into solution, until the deposited crystals were slightly enriched with the opposite enantiomer, thereby successfully exhibiting PE. However, under the effects of Cu(II), the solvent and low pH, 2-S/R did not exhibit PE and resulted in enrichment of racemic compounds, which was attributed to amino acid Schiff base chiral complex mechanisms of PE. The three pairs of Cu complex structures were characterized by UV-vis, MS and X-ray photoelectron spectroscopy (XPS). All chiral properties were studied by circular dichroism (CD) in the solid and liquid.

Solid state emissive azo-Schiff base ligands and their Zn(ii) complexes: acidochromism and photoswitching behaviour

Two novel azo Schiff base ligands L1 and L2 and Zn(ii) complexes C1 and C2 exhibiting reversible acidochromic behavior and photoswitching properties have been described.

Structural and Theoretical Investigations, Hirshfeld Surface Analyses, and Cytotoxicity of a Naphthalene-Based Chiral Compound

A novel Schiff base compound derived from the condensation of 2-hydroxy-1-naphthaldehyde with (1S,2S)-(-)-1,2-diphenylethylenediamine in 2:1 M ratio was reported and investigated by elemental analyses, Fourier transform infrared and NMR spectroscopic studies, and single-crystal X-ray crystallography. Hirshfeld surface analyses were also carried out to measure the various intermolecular contacts controlling the supramolecular topology, suggesting the H···O (7.6%) contacts to be the most significant interactions, whereas the H···H (48.9%) and C···H (40.2%) interactions are less-significant. The data obtained from the energy calculations revealed the structure observed experimentally to be the most stable isomer and its energy being lower by 18.0441 kcal/mol than the less stable one. Density functional theory calculations were also carried out to analyze the natural charges, reactivity descriptors, and different intramolecular charge transfer interactions. The in vitro anticancer activity of the compound was evaluated by MTT assays against human colorectal cancer cells, HT-29 and SW620. The results showed that the compound has potential anticancer activity against these cells lines.

Reversible Chromatic Change of Supramolecular Gels for Visual and Selective Chiral Recognition of Histidine

In the present work, a chemical reaction has been performed for supramolecular gels to achieve multiple-stimuli-responsive smart soft materials. Simple chiral binaphthalene-based receptors can condense with histidine (His) to yield a Schiff base, which would help to achieve visual chiral recognition of unprotected l/d-His through gel formation along with specific selectivity toward 20 amino acids. Through intermolecular hydrogen bonds, the resultant Schiff base molecules assemble with excess His molecules to form three-dimensional (3D) networks of metastable cross-linked nanospheres and stable nanofibers in EtOH/water and MeOH/water, respectively. Significantly, this condensation reaction exhibits unique reversible and chromatic phenomena between sol-gel phase transitions in EtOH/water, which provide a way to design chemical reaction-based multivisual-change supramolecular gels for sensing and switching applications.

Ultralow-Molecular-Weight Stimuli-Responsive and Multifunctional Supramolecular Gels Based on Monomers and Trimers of Hydrazides

The simpler, the better. A series of simple, neutral and ultralow-molecular-weight (MW: 140-200) hydrazide-derived supramolecular gelators have been designed and synthesized in two straightforward steps. For non-conjugated cyclohexane-derived hydrazides, their monomers can self-assemble to form gels through intermolecular hydrogen bonds and dipole-dipole interactions. Significantly, conjugated phthalhydrazide can self-aggregate into planar and circular trimers through intermolecular hydrogen bonds and then self-assemble to form gels through intermolecular π-π stacking interactions. It is interesting that these simple gelators exhibit unusual properties, such as self-healing, multi-response fluorescence, and visual and selective recognition of chiral (R)/(S)-1,1'-binaphthalene-2,2'-diamine and S^2- through much different times of gel re-formation and blue-green color change, respectively. These results underline the importance of supramolecular gels and extend the scope of supramolecular gelators.

Enantioselective Fluorescent Recognition of Free Amino Acids: Challenges and Opportunities

Fluorescent probes that can discriminate enantiomers of amino acids in organic media or aqueous solution are discussed. This Minireview focuses on recent progress in the studies of three classes of probes including those made of cyclodextrins, 1,1'-binaphthyl compounds, and nanomaterials, and uses them to illustrate the design strategies, applications, and limitations in this area. These probes are potentially useful for rapid analysis of asymmetric reactions for amino acid synthesis as well as the real-time imaging of amino acids in biological systems. The challenges in these applications are analyzed. Working in this field of enantioselective fluorescent recognition of amino acids offers great opportunities to make new scientific discoveries and to develop important practical applications.

Crystal structure of rac-trans-N,N′-bis(3,5-dibromosalicylidene)-1,2-cyclohexanediamine, C20H18Br4N2O2

C20H18Br4N2O2, monoclinic, P21/c (no. 14), a = 9.1119(6) Å, b = 17.8211(12) Å, c = 13.5228(8) Å, β = 90.809(2)°, V = 2195.7(2) Å3, Z = 4, R gt(F) = 0.0448, wR ref(F 2) = 0.0963, T = 273(2) K.

Solvent-Dependent Bending Ability of Salen-Derived Organic Crystals

The formation of plastic or brittle organic crystals of salen derivatives that depend on the solvents employed for crystallization is demonstrated. Large yellow crystals (ranging from mm to cm size) of ten different salen derivatives were obtained and investigated. Among them, (bis(2-hydroxyacetophenone)ethylenediimine) 2, which was recrystallized from dichloromethane, tetrahydrofuran or chloroform, exhibited plastic deformation behaviour when mechanical force was applied to the (001) face. In contrast, when 2 was recrystallized from benzene, brittle crystals were obtained. Face indexing confirmed that different crystal faces were obtained by depending on the solvent employed for recrystallization, which leads to either flexible (plastic) or brittle crystals. Photoluminescence with a band maximum at 510 nm and thermochromism related to tautomerism between OH and NH forms were also investigated, and indicate that 2 is a flexible organic single-crystal material with multifunctional properties.

Unprecedented Water Effect as a Key Element in Salicyl-Glycine Schiff Base Synthesis

Salens, as chelating, double Schiff base ligands, are an important group utilized in transition metal catalysis. They have been used to build interesting functional metal-organic frameworks (MOFs). However, salens interacting with amino acids have also found applications in receptors. Here, we intended to form a “green” glycine-derived salen fragment, but the available literature data were contradictory. Therefore, we optimized the synthetic conditions and obtained the desired product as two different crystallographic polymorphs (orthorhombic Pcca and monoclinic P₂₁/c space groups). Their structures differ in conformation at the glycine moiety, and the monoclinic form contains additional, disordered water molecules. Despite the high stability of Schiff bases, these newly obtained compounds hydrolyze in aqueous media, the process being accelerated by metal cations. These studies, accompanied by mechanistic considerations and solid-state moisture and thermal analysis, clarify the structure and behavior of this amino acid Schiff base and shed new light on the role of water in its stability.

1D-helical platinum(ii) complexes bearing metal-induced chirality, aggregation-induced red phosphorescence, and circularly polarized luminescence

Binaphthyls-linked Pt(ii) complexes with metal-induced chirality self-assemble to build 1D M or P helices and show aggregation/racemization-induced and circularly polarized luminescence.

A simple and visual approach for enantioselective recognition through supramolecular gels with specific selectivity

(S)/(R)1 enantioselectively self-assemble to form a gel or solution with one enantiomer of (S)/(R)BINAM with specific selectivity.

An amide probe as a selective Al3+ and Fe3+ sensor inside the HeLa and a549 cell lines: Pictet–Spengler reaction for the rapid detection of tryptophan amino acid

Newly synthesized amide-based probe for the selective and specific detection of (i) Al³⁺ or Fe³⁺ ions as a cation sensor and (ii) tryptophan as an amino acid sensor.

Smart, chiral, and nonconjugated cyclohexane-based bis-salicylaldehyde hydrazides: multi-stimuli-responsive, turn-on, ratiometric, and thermochromic fluorescence, single-crystal structures via DFT calculations

Multidentate and environmentally sensitive dyes show turn-on, ratiometric, and thermochromic fluorescence.

Nonconjugated Fluorescent Molecular Cages of Trinuclear Fluoroborate Complexes with Salicylaldehyde-Based Schiff Base Ligands

Fluorescent organic materials are commonly π-conjugated planar molecules. In the present work, however, we report a novel class of nonconjugated fluorescent molecular cages of trinuclear fluoroborate complexes (nine samples) with salicylaldehyde-based Schiff base ligands. Owing to the stress from lone pair electrons of N atom in the triethylamine bridge, these B(III) complexes exhibit unusual enantiomers with a tripodlike side-single-opening structure. They emit blue, green, and red emission with large Stokes shifts (up to 159 nm) and high fluorescence quantum yields in both solution (up to 0.24) and solid state (up to 0.25), which might contribute to their strong intramolecular hydrogen bonds and weak intermolecular and intramolecular π-π interactions. Combining their advantages of nonconjugation and biocompatibility, these flexible complexes have potential applications in living cell imaging and anion hosts. We have examined the inherent relationships between their chemical structures and emission properties and afforded a new stage for the design of nonconjugated fluorescent fluoroborate complexes.

Dual responsive salen-type Schiff bases for the effective detection of l-arginine via a static quenching mechanism

The effect of substituents of salen Schiff bases on the detection of l-arginine via a static quenching mechanism is discussed.

A highly selective and sensitive chemosensor forl-tryptophan by employing a Schiff based Cu(ii) complex

Construction of a new Cu(ii) complex (1) based modified glassy carbon electrode (1-GCE) for highly selective and sensitive detection ofl-tryptophan (l-Trp).