Fabrication of a "turn-on"-type enantioselective fluorescence sensor via a modified achiral MOF: applications for synchronous detection of phenylalaninol enantiomers

Homochiral metal-organic frameworks (HMOFs) have garnered considerable attention due to their extrachiral properties and broad application for chiral recognition. However, assembling a pair of high-quality chiral MOFs for sensing enantiomers precisely is a formidable challenge because of the complicated chiral environment and uncontrollable coordinated conditions. Herein, one pair of homochiral UiO-66 analogues, S-1 (l-AP@UiO-66-(COOH)₂) and R-1 (d-AP@UiO-66-(COOH)₂), are reported for chiral recognition. They were fabricated via a condensation reaction between the carboxyl groups of UiO-66-(COOH)₂ and amino groups of l/d-amino propanol (l/d-AP). These novel fluorescent probes exhibited highly enantioselective fluorescence enhancement towards l/d-phenylalaninol (l/d-PA). For example, when S-1 and R-1 were treated with l-PA or d-PA, they displayed different fluorescence responses: the enantiomeric fluorescence enhancement ratio (ef) was 2.51 and 0.41 for S-1 and R-1, respectively. Hence, a visible difference in fluorescence enhancement for l-PA and d-PA and excellent enantioselective behavior between S-1 and l-PA (or R-1 and d-PA) was displayed. Measurements of fluorescence lifetime, powder X-ray diffraction, molecular-dynamic simulations and Benesi-Hildebrand plots were employed to determine the observed high enantioselectivity for l/d-PA. In brief, we found that two post-modified HMOFs, S-1 and R-1, were outstanding enantioselective sensors for detecting l-PA and d-PA. They had a prominent difference in ef and remarkable enantioselectivity factor α and ΔΔG based on steric hindrance and stereochemical difference.

Recent advances of BINOL-based sensors for enantioselective fluorescence recognition

Enantioselective fluorescent sensors show large potential for fast, real-time, and highly sensitive measurement of the concentration and enantiomeric composition of chiral molecules. Among all of the sensors, BINOL-based sensors have been actively investigated and extensively used to carry out highly enantioselective, sensitive recognition of chiral α-hydroxycarboxylic acids, amino acids, amino acid derivatives, amino alcohols and amines. In this manuscript, the recent progress of chiral BINOL-based sensors for enantioselective fluorescence recognition of different substrates is reviewed and discussed. The structure of BINOL is tuned by introducing various groups or molecules which systematically changed its fluorescence properties and offered potential for rapid assays of chiral organic molecules. From the development of this area, we gain fresh insight into the challenges and chances of BINOL-based sensors.

1,1′-Bi(2-naphthol-4,5-dicarboximide)s: blue emissive axially chiral scaffolds with aggregation-enhanced emission properties

Novel electron-deficient 1,1′-binaphthol derivatives bearing two dicarboximide groups at the peri-positions show intense blue luminescence and aggregation-enhanced emission characteristics.

Synthesis and studies of axial chiral bisbenzocoumarins: Aggregation-induced emission enhancement properties and aggregation-annihilation circular dichroism effects

Axial chiral bisbenzocoumarins were synthesized for the first time by converting naphthanol units in 1,1'-binaphthol (BINOL) molecule to the benzocoumarin rings. The substitute groups on 3,3'-positions of bisbenzocoumarins showed significant influence on their aggregation-induced emission enhancement (AEE) properties. It was also found that BBzC1 with ester groups on 3,3'-positions exhibit an abnormal aggregation-annihilation circular dichroism (AACD) phenomenon, which could be caused by the decrease of the dihedral angle between adjacent benzocoumarin rings in the aggregation state. The single crystal structure of BBzC1 showed that the large dihedral angle in molecule prohibited the strong π-π stacking interactions, which could be main factors for its AEE properties.

Chiral carbon dots and their effect on the optical properties of photosensitizers

In this work, we have successfully prepared intrinsically chiral carbon dots from chiral precursors. We have also demonstrated that the chirality of these carbon dots can affect the optical properties of photosensitizer molecules like azobenzene.

Remarkably selective and enantiodifferentiating sensing of histidine by a fluorescent homochiral Zn-MOF based on pyrene-tetralactic acid

A highly luminescent and water-stable homochiral Zn-MOF, i.e., Zn-PLA, developed based on pyrene-tetraacetic, selectively senses histidine amongst all other amino acids and also differentiates d and l isomers, as revealed by quenching of the fluorescence of its aqueous suspension.

A highly luminescent and water-stable homochiral Zn-MOF, i.e., Zn-PLA, has been developed based on a pyrene-tetralactic acid, which inherently features concave shapes for guest inclusion, to explore sensing of amino acids by fluorescence quenching; the solid-state fluorescence quantum yield of the MOF was found to be 46%. The fluorescence of an aqueous suspension of Zn-PLA was shown to be quenched specifically by histidine amongst all the other amino acids. Selective sensing of histidine is of prime importance due to its relevance in a variety of biological functions. The lack of quenching of fluorescence of Zn-PLA by all the amino acids other than histidine has been rationalized based on the exchange of the cationic dimethylammonium species in the MOF crystals with histidine that is protonated in water; the latter is envisaged to quench the fluorescence via charge transfer in the excited state. Furthermore, the homochiral crystals of Zn-PLA were found to permit enantiodiscrimination in the quenching by the d- and l-forms such that the ratio of enantioselectivity, i.e., K_d/K_l, is 1.8, as determined by Stern–Volmer quenching plots. The highly selective as well as enantiodifferentiating sensing of amino acids by MOFs is unprecedented for any sensor type.

A highly effective and stable CuZn0.3MgxAlOycatalyst for the manufacture of chirall-phenylalaninol: the role of Mg and its hydrotalcite-like precursor

A highly effective and stable CuZn_0.3Mg_0.1AlO_ycatalyst derived from a Cu-rich hydrotalcite-like precursor was prepared for the catalytic hydrogenation ofl-phenylalanine methyl ester tol-phenylalaninol with ~100% ee selectivity.

Salen-based enantiomeric polymers for enantioselective recognition

In a simple way, the spatial arrangement of the building blocks in a main chain polymer determines its recognition properties.

Chiral, fluorescent microparticles constructed by optically active helical substituted polyacetylene: preparation and enantioselective recognition ability

Novel chiral fluorescent microparticles derived from helical substituted polyacetylene were prepared. The microparticles showed enantioselective recognition ability in both heterogeneous and homogeneous systems.

l-Amino acid derived pyridinium-based chiral compounds and their efficacy in chiral recognition of lactate

A series ofl-amino acid derived pyridinium-based chiral compounds have been designed and synthesized. Among them, somel-valine derivatives show enantioselective recognition ofd-lactate in fluorescence.

A new synthesis strategy for chiral CdS nanotubes based on a homochiral MOF template

We present a convenient and rapid way to prepare helical CdS nanotubes, with a MOF as the template.

Stereospecific generation of homochiral helices in coordination polymers built from enantiopure binaphthyl-based ligands

The novel enantiopure spacer 2,2′-dimethoxy-1,1′-binaphthyl-3,3′-bis(4-pyridyl-amido) has been designed to prepare helical coordination polymers here investigated by means of experimental and theoretical data.

Axially chiral BODIPYs

The synthesis and resolution of a class of chiral organic fluorophores, axially chiral 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (Ax*-BODIPY), is described. Ax*-BODIPYs were prepared through a modular synthesis, resolved by preparative chiral HPLC and absolute stereochemistry assigned by ECD.