Induced Helical Chirality of Perylenebisimide Aggregates Allows for Enantiopurity Determination and Differentiation of α-Hydroxy Carboxylates by Using Circular Dichroism

Circular Dichroism Sensing: Strategies and Applications

The analysis of the absolute configuration, enantiomeric composition, and concentration of chiral compounds are frequently encountered tasks across the chemical and health sciences. Chiroptical sensing methods can streamline this work and allow high-throughput screening with remarkable reduction of operational time and cost. During the last few years, significant methodological advances with innovative chirality sensing systems, the use of computer-generated calibration curves, machine learning assistance, and chemometric data processing, to name a few, have emerged and are now matched with commercially available multi-well plate CD readers. These developments have reframed the chirality sensing space and provide new opportunities that are of interest to a large group of chemists. This review will discuss chirality sensing strategies and applications with representative small-molecule CD sensors. Emphasis will be given to important milestones and recent advances that accelerate chiral compound analysis by outperforming traditional methods, conquer new directions, and pioneering efforts that lie at the forefront of chiroptical high-throughput screening developments. The goal is to provide the reader with a thorough understanding of the current state and a perspective of future directions of this rapidly emerging field.

Negative Nonlinear CD-ee Dependence in Polycrystalline BINOL Thin Films

Generally, the relationship between the observed circular dichroism and the enantiomeric excess in chiral systems (CD-ee dependence) is linear. While positive nonlinear behavior has often been reported in the past, examples of negative nonlinear (NN) behavior in CD-ee dependence are rare and not well understood. Here, we present a strong NN CD-ee dependence within polycrystalline thin films of BINOL by using second-harmonic-generation circular dichroism (SHG-CD) and commercial CD spectroscopy studies. Theoretical calculations, microscopy, and FTIR studies are employed to further clarify the underlying cause of this observation. This behavior is attributed to the changing supramolecular chirality of the system. Systems exhibiting NN CD-ee dependence hold promise for highly accurate enantiomeric excess characterization, which is essential for the refinement of enantio-separating and -purifying processes in pharmaceuticals, asymmetric catalysis, and chiral sensing. Our findings suggest that a whole class of single-species systems, i.e., racemate crystals, might possess NN CD-ee dependence and thus provide us a vast playground to better understand and exploit this phenomenon.

Cage-based sensors for circular dichroism analysis

Quantitative chiral sensing relying on circular dichroism (CD) is very important for determining the enantiomeric excess or concentration of small molecules without strong chromophores, because they form chiral complexes with sensors, yielding strong CD signals. Three-dimensional cages are promising platforms for chiral CD due to their stereochemical flexibility and their variety of cavity and external binding sites that can be used as chiral CD sensors. In this minireview, we discuss recent advances, future challenges, and opportunities in the quantitative sensing of small molecules in host-guest and peripheral complexes with cage sensors by chiral CD. We aim to provide inspiration for the rational design of cage sensors for quantitative chiral sensing of small molecules based on CD.

Synthetic approaches to metal-coordination-directed macrocyclic complexes

Metal-coordination-directed macrocyclic complexes, in which macrocyclic architectures are formed by metal-ligand coordination interactions, have emerged as attractive supramolecular scaffolds for the creation of materials for applications in biosensing and therapeutics. Despite recent progress, uncontrolled multicyclic cages and linear oligomers/polymers is the most likely outcome from metal-ligands assembly, representing a challenge to current synthetic methods. Herein we outlined the state-of-art synthetic approaches to the metal-coordination-directed macrocyclic complexes by using foldable ligands or through assembly of amphiphilic ligands. This mini-review offers a guideline for the efficient preparation of metal-coordination-directed macrocyclic complexes with predictable and controllable structures, which may find applications in many biology-related areas.

Concepts and principles of self-n-doping in perylene diimide chromophores for applications in biochemistry, energy harvesting, energy storage, and catalysis

Self-doping is an essential method of increasing carrier concentrations in organic electronics that eliminates the need to tailor host-dopant miscibility, a necessary step when employing molecular dopants. Self-n-doping can be accomplished using amines or ammonium counterions as an electron source, which are being incorporated into an ever-increasingly diverse range of organic materials spanning many applications. Self-n-doped materials have demonstrated exemplary and, in many cases, benchmark performances in a variety of applications. However, an in-depth review of the method is lacking. Perylene diimide (PDI) chromophores are an important mainstay in the semiconductor literature with well-known structure-function characteristics and are also one of the most widely utilized scaffolds for self-n-doping. In this review, we describe the unique properties of self-n-doped PDIs, delineate structure-function relationships, and discuss self-n-doped PDI performance in a range of applications. In particular, the impact of amine/ammonium incorporation into the PDI scaffold on doping efficiency is reviewed with regard to attachment mode, tether distance, counterion selection, and steric encumbrance. Self-n-doped PDIs are a unique set of PDI structural derivatives whose properties are amenable to a broad range of applications such as biochemistry, solar energy conversion, thermoelectric modules, batteries, and photocatalysis. Finally, we discuss challenges and the future outlook of self-n-doping principles.

Chiral Recognition by Flexible Coordination Polymers of Ag+ with a Cysteine-Based Chiral Thiol Ligand That Bears a Binding Site

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 chiral spirobifluorene-based bis(salen) zinc(ii) receptor towards highly enantioselective binding of chiral carboxylates

We have designed a new chiral receptor based on two salen zinc(ii) complex units connected with a spirobifluorene framework. The chiral receptor is proven to enantioselectively bind chiral carboxylate guests and the differences between the binding constants of enantiomeric guests were up to more than one order of magnitude.

Solvophobic interaction promoted supramolecular helical assembly of building blocks of weak intermolecular halogen bonding

Bromination of β-turn structured bis(N-amidothiourea) facilitates the formation of supramolecular helical polymers in water through halogen bonding and hydrophobic interaction, exhibiting an unusual negative nonlinear CD-ee dependence.

Helical nanofibers of N-(perfluorooctanoyl)cysteine ethyl ester in coordination polymers of Ag. Chem Commun (Camb) 2020;56:15133-15136. [PMID: 33165457 DOI: 10.1039/d0cc05989g]

We propose using the formation of coordination polymers of Ag+ to probe differences between the perfluorinated alkyl chain and the alkyl chain by deriving a thiol ligand, N-(perfluoroalkanoyl)cysteine. Rapid formation in EtOH of P-/M-helical nanofibrils of high thermostability was found for N-(perfluorooctanoyl)-l-/d-cysteine ethyl esters at the μM level upon mixing with Ag+, but not for the octanoyl counterpart. This difference was also observed in terms of circular dichroism-enantiomeric excess dependence.

Molecular recognition and sensing of dicarboxylates and dicarboxylic acids

The recognition and detection of dicarboxylic acids and dicarboxylates is of significance for a wide variety of applications, including medical diagnosis, monitoring of health and of environmental contaminants, and in industry. Hence small molecule receptors and sensors for dicarboxylic acids and dicarboxylates have great potential for applications in these fields. This review outlines the challenges faced in the recognition and detection of these species, strategies that have been used to obtain effective and observable interactions with dicarboxylic acids and dicarboxylates, and progress made in this field in the period from 2014 to 2020.

Supramolecular Chiral Aggregates Exhibiting Nonlinear CD-ee Dependence

Although a linear relationship between the optical activity (normally the CD signal) and the enantiomeric excess (ee) of chiral auxiliaries has been the most commonly observed dependence in dynamic supramolecular helical aggregates, positive nonlinear CD-ee dependence, known as the "majority-rules effect" (MRE), indicative of chiral amplification, has also been well documented and to some extent understood. In sharp contrast, the negative nonlinear CD-ee dependence has been much less reported and is not well understood. Here, the state of the art of both the positive and negative nonlinear CD-ee dependence in noncovalently bound supramolecular helical aggregates is summarized, with the hope that the vast examples of supramolecular aggregates showing positive nonlinear dependence, in terms of the methods of investigations, variations in the structure of the building block (single species or multiple species), and theoretical modeling using the mismatch penalty energy and helix reversal penalty energy, would help to guide the design of building blocks to form aggregates showing negative nonlinear dependence, and thus to understand the mechanisms. The potential applications of those functional supramolecular aggregates are also discussed.

Boronic acid sensors with double recognition sites: a review

Boronic acids reversibly and covalently bind to Lewis bases and polyols, which facilitated the development of a large number of chemical sensors to recognize carbohydrates, catecholamines, ions, hydrogen peroxide, and so on. However, as the binding mechanism of boronic acids and analytes is not very clear, it is still a challenge to discover sensors with high affinity and selectivity. In this review, boronic acid sensors with two recognition sites, including diboronic acid sensors, and monoboronic acid sensors having another group or binding moiety, are summarized. Owing to double recognition sites working synergistically, the binding affinity and selectivity of sensors can be improved significantly. This review may help researchers to sort out the binding rules and develop ideal boronic acid-based sensors.

Circularly Polarized Luminescence of Aluminum Complexes for Chiral Sensing of Amino Acid and Amino Alcohol

Determination of the absolute configuration (AC) of chiral molecules is a key issue in many fields related to chirality such as drug development, the asymmetric reaction screening, and the structure determination of natural compounds. Although various methods, such as X-ray crystallography and NMR spectroscopy, are used to determine the AC, a simple and cheap alternative method is always anticipated. So far, electronic circular dichroism (ECD) spectroscopy has been widely used to ascertain the AC and enantiomeric excess (ee) values by applying appropriate organic probes. Here, circularly polarized luminescence (CPL) spectroscopy was applied to determine the AC and ee values of a series of amino acid and amino alcohol. The measurements were conducted by mixing the amino acids or amino alcohols with an achiral 1-hydroxy-2-naphthaldehyde. Upon in situ formation of the Schiff base complexes, the system showed emission enhancement and CPL in the presence of Al³⁺ , whose intensity and sign can be used to assign the chiral sense of the amino acids and amino alcohols. The authenticity of the method was further compared with the established CD spectroscopy, revealing that CPL spectra of formed Al³⁺ complex were effective to determine the AC of chiral species.

Design of Chiral Supramolecular Polymers Exhibiting a Negative Nonlinear Response

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 flat 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.

A diboronic acid fluorescent sensor for selective recognition of d-ribose via fluorescence quenching

Herein we reported a novel boronic acid-based water-soluble sensor. It decreased the fluorescence by 50% when combined with 0.0146 M of d-ribose, while increased or not changed obviously after binding to other carbohydrates.

Supramolecular chirality of coordination polymers of Ag+ with a chiral thiol ligand that bears a β-turn structure

The coordination polymers of Ag⁺ with a β-turn containing chiral thiol ligand exhibit supramolecular chirality showing simultaneously the majority rules effect (MRE) and racemate rules effect (RRE).

Optical Analysis of Reaction Yield and Enantiomeric Excess: A New Paradigm Ready for Prime Time

This Perspective highlights the advances of optical methods for asymmetric reaction discovery. Optical analysis allows for the determination of absolute configuration, enantiomeric excess and reaction yield that is amenable to high-throughput experimentation. Thus, the synthetic organic community is encouraged to incorporate the methods discussed to expedite the development of high-yielding, enantioselective transformations.

A racemate-rules effect supramolecular polymer for ee determination of malic acid in the high ee region

A supramolecular polymeric system that shows an unusual "racemate-rules" chiroptical property, an effect opposite to the well-known "majority-rules", has been utilized for accurate determination of malic acid enantiopurity at high ee values.

Recent development of boronic acid-based fluorescent sensors

As Lewis acids, boronic acids can bind with 1,2- or 1,3-diols in aqueous solution reversibly and covalently to form five or six cyclic esters, thus resulting in significant fluorescence changes. Based on this phenomenon, boronic acid compounds have been well developed as sensors to recognize carbohydrates or other substances. Several reviews in this area have been reported before, however, novel boronic acid-based fluorescent sensors have emerged in large numbers in recent years. This paper reviews new boron-based sensors from the last five years that can detect carbohydrates such as glucose, ribose and sialyl Lewis A/X, and other substances including catecholamines, reactive oxygen species, and ionic compounds. And emerging electrochemically related fluorescent sensors and functionalized boronic acid as new materials including nanoparticles, smart polymer gels, and quantum dots were also involved. By summarizing and discussing these newly developed sensors, we expect new inspiration in the design of boronic acid-based fluorescent sensors.

As Lewis acids, boronic acids can bind with 1,2- or 1,3-diols in aqueous solution reversibly and covalently to form five or six cyclic esters, thus resulting in significant fluorescence changes.

A Chiral Recognition System Orchestrated by Self-Assembly: Molecular Chirality, Self-Assembly Morphology, and Fluorescence Response

The newly developed oligophenylenevinylene (OPV)-based fluorescent (FL) chiral chemosensor (OPV-Me) for the representative enantiomeric guest, 1,2-cyclohexanedicarboxylic acid (1,2-CHDA: RR- and SS-form) showed the high chiral discrimination ability, resulting in the different aggregation modes of OPV-Me self-assembly: RR-CHDA directed the fibrous supramolecular aggregate, whereas SS-CHDA directed the finite aggregate. The consequent FL intensity toward RR-CHDA was up to 30 times larger than that toward SS-CHDA. Accordingly, highly enantioselective recognition was achieved. Application to the chirality sensing was also possible: OPV-Me exhibited a linear relationship between the FL intensity and the enantiomeric excess through the morphological development of stereocomplex aggregates. These results clearly show that the chiral recognition ability is manifested by the amplification cascade of the chirality difference through self-assembly.

Ag+ coordination polymers of a chiral thiol ligand bearing an AIE fluorophore

The Ag⁺ coordination polymers of a chiral thiol ligand containing an AIE fluorophore differ very much in their supramolecular structures compared to the aggregates of the thiol ligand itself. The differences in the extent of fluorescence enhancement are detailed for the first time.

Recent advances in boronic acid-based optical chemosensors

This minireview highlights the developments in optical chemosensors from 2014 to 2016 that utilise the boronic acid interaction with polyols or Lewis bases.

Self and directed assembly: people and molecules

Self-assembly and directed-assembly are two very important aspects of supramolecular chemistry. As a young postgraduate student working in Canada with Tom Fyles my introduction to Supramolecular Chemistry was through the self-assembly of phospholipid membranes to form vesicles for which we were developing unimolecular and self-assembling transporter molecules. The next stage of my development as a scientist was in Japan with Seiji Shinkai where in a “Eureka” moment, the boronic acid templating unit (directed-assembly) of Wulff was combined with photoinduced electron transfer systems pioneered by De Silva. The result was a turn-on fluorescence sensor for saccharides; this simple result has continued to fuel my research to the present day. Throughout my career as well as assembling molecules, I have enjoyed bringing together researchers in order to develop collaborative networks. This is where molecules meet people resulting in assemblies worth more than the individual “molecule” or “researcher”. My role in developing networks with Japan was rewarded by the award of a Daiwa-Adrian Prize in 2013 and I was recently rewarded for developing networks with China with an Inaugural CASE Prize in 2015.

Determining Chiral Configuration of Diamines via Contact Angle Measurements on Enantioselective Alanine-Appended Benzene-Tricarboxamide Gelators

Spectroscopic techniques exist that may discern between enantiomers and assess chiral purity. A nonspectroscopic approach that may be directly observed could provide numerous benefits. Using chiral alanine-appended benzene-tricarboxamide gelators, we reveal a methanol gel system that is capable of providing visual discrimination between enantiomers of various diamines. Specifically, gelation is induced by supramolecular nanofiber assembly resulting from interaction between a chiral gelator and a diamine of opposing chirality (i.e., a heterochiral system). Upon further implementing the chiral gelator in electrospun fibers as solid state films, we revealed enantioselective surface wetting properties that allowed for determining chirality through contact angle measurements. While these two approaches of observable gelation and surface wetting offer nonspectroscopic approaches, we also find that the supramolecular nanofiber assembly was able to enhance the induced circular dichroism signal resulting from addition of chiral diamines, allowing precise quantification of their enantiomeric purity.

Supramolecular aggregates as sensory ensembles

Recent research progress in sensing based on induced supramolecular aggregation or disaggregation.

Optical chirality sensing using macrocycles, synthetic and supramolecular oligomers/polymers, and nanoparticle based sensors

Optical sensors that respond to enantiomeric excess of chiral analytes are highly demanded in chirality related research fields and demonstrate their potential in many applications, for example, screening of asymmetric reaction products. Most sensors developed so far are small molecules. This Tutorial Review covers recent advances in chirality sensing systems that are different from the traditional small molecule-based sensors, by using macrocycles, synthetic oligomers/polymers, supramolecular polymers and nanoparticles as the sensors, in which supramolecular interactions operate.

Multicomponent covalent dye assembly for tight binding and sensitive sensing of l-DOPA

Exploitation of dye aggregation enables tight binding and sensitive sensing of bifunctional analyte l-DOPA, simply by mixing two monofunctional dyes.

Hierarchical supramolecules and organization using boronic acid building blocks

Current progress on hierarchical supramolecules using boronic acids has been highlighted in this feature article. The feasibility of the structure-directing ability is fully discussed from the standpoint of the generation of new smart materials.