Hydrogen‐Bonding‐Induced Chirality Organization and Stabilization of Redox Species of Polyaniline‐Unit Molecules by Introduction of Amino Acid Pendant Groups

The chirality organization of polyaniline‐unit molecules was achieved by the introduction of amino acid pendant groups through intramolecular hydrogen bonding, which plays an important role in the stabilization of the chirality‐organized redox species. Another interesting feature of the synthesized polyaniline‐unit molecules is the luminescent switching properties based on the redox states of the phenylenediamine moiety.     

Chiral Sensing of Various Amino Acids Using Induced Circularly Polarized Luminescence from Europium(III) Complexes of Phenanthroline Dicarboxylic Acid Derivatives

Circularly polarized luminescence (CPL) was observed from [Eu(dppda)₂]^? (dppda=4,7‐diphenyl‐1,10‐phenanthroline‐2,9‐dicarboxylic acid) and [Eu(pzpda)₂]^? (pzpda=pyrazino[2,3‐f][1,10]phenanthroline‐7,10‐dicarboxylic acid) in aqueous solutions containing various amino acids. The selectivity of these complexes towards amino acids enabled them to be used as chiral sensors and their behavior was compared with that of [Eu(pda)₂]^? (pda=1,10‐phenanthroline‐2,9‐dicarboxylic acid). As these Eu^III complexes have achiral D_2d structures under ordinary conditions, there were no CPL signals in the emission assigned to f–f transitions. However, when the solutions contained particular amino acids they exhibited detectable CPL signals with g_lum values of about 0.1 (g_lum=CPL/2 TL; TL=total luminescence). On examining 13 amino acids with these three Eu^III complexes, it was found that whether an amino acid induced a detectable CPL depended on the Eu^III complex ligands. For example, when ornithine was used as a chiral agent, only [Eu(dppda)₂]^? exhibited intense CPL in aqueous solutions of 10^?2 mol dm^?3. Steep amino acid concentration dependence suggested that CPL in [Eu(dppda)₂]^? and [Eu(pzpda)₂]^? was induced by the association of four or more amino acid molecules, whereas CPL in [Eu(pda)₂]^? was induced by association of two arginine molecules.     

Determination of Amino Acid Enantiopurity and Absolute Configuration: Synergism between Configurationally Labile Metal‐Based Receptors and Dynamic Covalent Interactions

Reliable determination of the enantiomeric excess of free amino acids can be obtained by measuring the induced circular dichroism of a multicomponent assembly formed by a modified tris(2‐pyridylmethyl)amine ligand, a zinc salt, and the amino acid of interest. The systems furnish reliable information for all natural amino acids.     

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

We have developed a working strategy for accurate enantiomeric excess (ee) determination based on induced helical aggregation of achiral perylenebisimide (PBI) dyes. PBI dyes functionalized with boronic acid moieties were shown to be effective chirality sensors for α‐hydroxy carboxylates. Seven α‐hydroxy carboxylates tested showed strong induced Cotton effects in the perylene absorption region around λ=500 nm, which were utilized for enantiomeric excess determination and chemo‐discrimination of the analytes, with an average absolute error of 2 % in ee determination and 100 % correctness in analyte classification. Responses in the absorption spectra, which arise from the guest‐enhanced aggregation, allow the determination of the sample concentration, thus enabling analysis of samples of unknown concentration and ee. The simplicity of the strategy, the ease of sample preparation, and the accuracy demonstrated, can potentially facilitate screening procedures in asymmetric synthesis.     

Multimetallic Architectures from the Self‐assembly of Amino Acids and Tris(2‐pyridylmethyl)amine Zinc(II) Complexes: Circular Dichroism Enhancement by Chromophores Organization

Stereodynamic optical probes are becoming very popular for their capability to act as molecular sensors for the determination of the enantiomeric excess (ee) of chiral compounds. Herein, we describe a new molecular architecture formed by the self‐assembly of three zinc metal ions, two modified tris(2‐pyridylmethyl)amine ligands, and two amino acids. This system is the structural and functional serendipitous evolution of our previous probe for the determination of amino acids ee. In the new system, one of the metals templates in close proximity two chromophores enhancing their exciton coupling.     

Enantio‐ and Chemoselective Differentiation of Protected α‐Amino Acids and β‐Homoamino Acids with a Single Copper(II) Host

The association between an achiral copper(II)‐containing host 1 and chiral carboxylates has been expanded beyond previous studies to new chiral carboxylate guests, both α‐amino acids and β‐homoamino acids. The observed exciton‐coupled circular dichroism (ECCD) signals for the enantiomers of each carboxylate were equal and opposite, and these signals differed in size and shape between the individual amino acids. Linear discriminant analysis (LDA) was applied as a statistical analysis technique to differentiate the amino acids, both enantioselectively and chemoselectively, giving the absolute configuration and identity of the amino acid. The identity of each of the α‐amino acids and β‐homoamino acids were determined independently by LDA, and then the two were considered together. Each of these analyses showed good differentiation of the amino acid guests with the use of only one host molecule.     

Induced Circular Dichroism in Phosphine Gold(I) Aryl Acetylide Urea Complexes through Hydrogen‐Bonded Chiral Co‐Assemblies

Phosphine gold(I) aryl acetylide complexes equipped with a central bis(urea) moiety form 1D hydrogen‐bonded polymeric assemblies in solution that do not display any optical activity. Chiral co‐assemblies are formed by simple addition of an enantiopure (metal‐free) complementary monomer. Although exhibiting an intrinsically achiral linear geometry, the gold(I) aryl acetylide fragment is located in the chiral environment displayed by the hydrogen‐bonded co‐assemblies, as demonstrated by induced circular dichroism (ICD).     

Chiroptical Probing of Lanthanide‐Directed Self‐Assembly Formation Using btp Ligands Formed in One‐Pot Diazo‐Transfer/Deprotection Click Reaction from Chiral Amines

A series of enantiomeric 2,6‐bis(1,2,3‐triazol‐4‐yl)pyridines (btp)‐containing ligands was synthesized by a one‐pot two‐step copper‐catalyzed amine/alkyne click reaction. The Eu^III‐ and Tb^III‐directed self‐assembly formation of these ligands was studied in CH₃CN by monitoring their various photophysical properties, including their emerging circular dichroism and circularly polarized luminescence. The global analysis of the former enabled the determination of both the stoichiometry and the stability constants of the various chiral supramolecular species in solution.     

Kinetic Control of the Supramolecular Chirality of Porphyrin J‐Aggregates

The aggregation of achiral sulfonatophenyl‐ and phenyl‐meso‐substituted diprotonated porphyrins to chiral J‐aggregates is a hierarchical noncovalent polymerization process preceded by a critical nucleation stage. This allows significant enantiomeric excesses by the formation of a few primary nuclei and the control of their growth by the effect that flows (imperfect mixing) have on the secondary nucleation of the J‐aggregate particles. In addition, the results strongly suggest that when only one species of aggregate predominates, the CD signals of the three excitonic bands in the visible region (around 420, 490, and 700 nm) show the same sign. Thus, differences on their relative sign would be due to the presence of different species.     

Donor‐Induced Helical Inversion of 1,1′‐Binaphthyl Connecting with Two Molybdenum Complexes

An atropisomeric biaryl molecule with a given absolute configuration could present two opposite helical conformations through the rotation around C? C single bond. To the best of our knowledge, the biaryl system is the simplest helical inversion model apart from stereomutation between two enantiomers. Herein, we first report such true helical inversion phenomena of biaryl compounds. Two [Mo^VIO₂(L)]‐type complexes, in which L is a tridentate dioxoanionic pyridine O,N,O‐ligand, are coalesced on the 2,2′,3,3′‐positions of an (R)‐1,1′‐binaphthyl unit and an intramolecular dioxo bridge is formed by two Mo?O???Mo interactions. Exterior strong donors can coordinate to molybdenum to interrupt this dioxo bridge and inversions from negative to positive chirality are explicitly observed by circular dichroism spectroscopy, consistent with single‐crystal X‐ray diffraction analyses.     

Salt‐Induced Adaptation of a Dynamic Combinatorial Library of Pseudopeptidic Macrocycles: Unraveling the Electrostatic Effects in Mixed Aqueous Media

Dynamic combinatorial libraries are powerful systems for studying adaptive behaviors and relationships, as models of more complex molecular networks. With this aim, we set up a chemically diverse dynamic library of pseudopeptidic macrocycles containing amino‐acid side chains with differently charged residues (negative, positive, and neutral). The responsive ability of this complex library upon the increase of the ionic strength has been thoroughly studied. The families of the macrocyclic members concentrating charges of the same sign showed a large increase in its proportion as the ionic strength increases, whereas those with residues of opposite charges showed the reverse behavior. This observation suggested an electrostatic shielding effect of the salt within the library of macrocycles. The top‐down deconvolution of the library allowed us to obtain the fundamental thermodynamic information connecting the library members (exchange equilibrium constants), as well as to parameterize the adaptation to the external stimulus. We also visualized the physicochemical driving forces for the process by structural analysis using NMR spectroscopy and molecular modeling. This knowledge permitted the full understanding of the whole dynamic library and also the de novo design of dynamic chemical systems with tailored co‐adaptive relationships, containing competing or cooperating species. This study highlights the utility of dynamic combinatorial libraries in the emerging field of systems chemistry.     

Multi‐input–Multi‐output Molecular Response System Based on Dynamic Redox Behavior of Hexaphenylethane‐type Electron Donors with the Tetrahydrophenanthrazepine Skeleton: Strong Chiroptical Signals through the Transmission of Point Chirality to Helicity

The title heterocyclic donors undergo reversible C? C bond formation/cleavage upon electron transfer (dynamic redox behavior). The helical sense in both neutral and cationic states is interconvertible by facile ring flipping. The π‐type asymmetric center on the azepine nitrogen atom induces a significant degree of diasteromeric preference, thus endowing strong CD activity based on exciton coupling. Chiroptical properties could be modified not only by redox reactions but also by heat and protonation. The present redox pairs can serve as unprecedented three‐way‐input (e, H⁺, Δ) and two‐way‐output (UV/Vis, CD) response systems.     

One‐Handed Helical Screw Direction of Homopeptide Foldamer Exclusively Induced by Cyclic α‐Amino Acid Side‐Chain Chiral Centers

Chiral cyclic α,α‐disubstituted amino acids, (3S,4S)‐ and (3R,4R)‐1‐amino‐3,4‐(dialkoxy)cyclopentanecarboxylic acids ((S,S)‐ and (R,R)‐Ac₅c^dOR; R: methyl, methoxymethyl), were synthesized from dimethyl L ‐(+)‐ or D ‐(?)‐tartrate, and their homochiral homoligomers were prepared by solution‐phase methods. The preferred secondary structure of the (S,S)‐Ac₅c^dOMe hexapeptide was a left‐handed (M) 3₁₀ helix, whereas those of the (S,S)‐Ac₅c^dOMe octa‐ and decapeptides were left‐handed (M) α helices, both in solution and in the crystal state. The octa‐ and decapeptides can be well dissolved in pure water and are more α helical in water than in 2,2,2‐trifluoroethanol solution. The left‐handed (M) helices of the (S,S)‐Ac₅c^dOMe homochiral homopeptides were exclusively controlled by the side‐chain chiral centers, because the cyclic amino acid (S,S)‐Ac₅c^dOMe does not have an α‐carbon chiral center but has side‐chain γ‐carbon chiral centers.