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.     

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.     

Redox‐Switchable 20π‐, 19π‐, and 18π‐Electron 5,10,15,20‐Tetraaryl‐5,15‐diazaporphyrinoid Nickel(II) Complexes

The first examples of air‐stable 20π‐electron 5,10,15,20‐tetraaryl‐5,15‐diaza‐5,15‐dihydroporphyrins, their 18π‐electron dications, and the 19π‐electron radical cation were prepared through metal‐templated annulation of nickel(II) bis(5‐arylamino‐3‐chloro‐8‐mesityldipyrrin) complexes followed by oxidation. The neutral 20π‐electron derivatives are antiaromatic and the cationic 18π‐electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20‐tetraarylporphyrins.     

Temperature‐Induced Chiroptical Changes in a Helical Poly(phenylacetylene) Bearing N,N‐Diisopropylaminomethyl Groups with Chiral Acids in Water

A stereoregular poly(phenylacetylene) bearing an N,N‐diisopropylaminomethyl group as the pendant (poly‐ 1 ) changed its structure into the prevailing one‐handed helical conformation upon complexation with optically active acids in water. The complexes exhibited induced circular dichroism (ICD) in the UV/Vis region of the polymer backbone. Poly‐ 1 is highly sensitive to the chirality of chiral acids and can detect a small enantiomeric imbalance in these acids, in particular, phenyl lactic acid in water. For example, a 0.005 % enantiomeric excess of phenyl lactic acid can be detected by CD spectroscopy. The observed ICD intensity and pattern of poly‐ 1 were dependent on the temperature and concentration of poly‐ 1 , probably due to aggregations of the polymer at high temperature as revealed by dynamic light scattering and AFM. On the basis of the temperature‐dependent ICD changes, the preferred chiral helical sense of poly‐ 1 was found to be controlled by noncovalent bonding interactions by using structurally different enantiomeric acids.     

Herringbone and Helical Self‐Assembly of π‐Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self‐organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self‐assembly in π‐conjugated molecules based on custom‐designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single‐crystal X‐ray structures were resolved for these OPV synthons and the existence of long‐range multiple‐arm CH/π interactions was revealed in the crystal lattices. Alignment of these π‐conjugated OPVs in the solid state was found to be crucial in producing either right‐handed herringbone packing in the crystal or left‐handed helices in the liquid‐crystalline mesophase. Pitch‐ and roll‐angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self‐assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π‐conjugated materials.     

Self‐Assembly of α‐Helical Polypeptides Driven by Complex Coacervation

Reported is the ability of α‐helical polypeptides to self‐assemble with oppositely‐charged polypeptides to form liquid complexes while maintaining their α‐helical secondary structure. Coupling the α‐helical polypeptide to a neutral, hydrophilic polymer and subsequent complexation enables the formation of nanoscale coacervate‐core micelles. While previous reports on polypeptide complexation demonstrated a critical dependence of the nature of the complex (liquid versus solid) on chirality, the α‐helical structure of the positively charged polypeptide prevents the formation of β‐sheets, which would otherwise drive the assembly into a solid state, thereby, enabling coacervate formation between two chiral components. The higher charge density of the assembly, a result of the folding of the α‐helical polypeptide, provides enhanced resistance to salts known to inhibit polypeptide complexation. The unique combination of properties of these materials can enhance the known potential of fluid polypeptide complexes for delivery of biologically relevant molecules.     

Phenyl‐(2‐pyridyl)‐(3‐pyridyl)‐(4‐pyridyl)methane: Synthesis,Chiroptical Properties,and Theoretical Calculation of Its Absolute Configuration

The title compound, a prototypical chiral molecule based on a tetraarylmethane framework, has been synthesized in five steps from (2‐pyridyl)‐(3‐pyridyl)ketone. X‐ray crystallographic analysis revealed the tetraarylmethane framework of the molecule but did not determine the positions of the nitrogen atoms because the crystal is a racemic compound and the aryl groups are disordered in the crystal. The optical resolution of the title compound was achieved by chiral HPLC with a Chiralcel OD column. The CD spectra of the two fractions in acetonitrile exhibited opposite signs as expected for a pair of enantiomers. Their CD spectra are changed in 2 M HCl due to protonation. The calculated CD curve for the target molecule based on time‐dependent density functional theory (TDDFT) reproduces the experimental result very well, thus suggesting that the first eluted fraction is the R isomer in terms of absolute configuration.     

Control of Conformation and Chirality of Nonplanar π‐Conjugated Diporphyrins Using Substituents and Axial Ligands

Nonplanar conformations of pyrazine‐fused Zn^II diporphyrins could be controlled by the choice of the meso‐aryl substituents and an axial ligand on the central metals. Zn^II diporphyrins bearing sterically demanding meso‐aryl groups with ortho‐substituents led to a twisted chiral D₂ conformation, while an achiral C_2h form was preferred in the case of aryl groups without ortho‐substituents. Helical chirality induction on Zn^II diporphyrins in the twisted conformation was achieved by controlling their handedness of the molecular twist through coordination of optically active 1‐phenethylamine.     

Synthesis and Helical Structures of Poly(ω‐alkynamide)s Having Chiral Side Chains: Effect of Solvent on Their Screw‐Sense Inversion

New ω‐alkynamides, (S)‐HC?CCH₂CONHCH₂CH(CH₃)CH₂CH₃ ( 1 ) and (S)‐HC?CCH₂CH₂CONHCH(CH₃)CH₂CH₂CH₂CH₂CH₃ ( 2 ) were synthesized and polymerized with a rhodium catalyst in CHCl₃ to obtain cis‐stereoregular poly(ω‐alkynamide)s (poly( 1 ) and poly( 2 )). Polarimetric, CD, and IR spectroscopic studies revealed that in solution the polymers adopted predominantly one‐handed helical structures stabilized by intramolecular hydrogen bonds between the pendent amide groups. This behavior was similar to that of the corresponding poly(N‐alkynylamide) counterparts (poly( 3 ) and poly( 4 )) reported previously, whereas the helical senses were opposite to each other. The helical structures of the poly(ω‐alkynamide)s were stable upon heating similar to those of the poly(N‐alkynylamide)s, but the solvent response was completely different. An increase in MeOH content in CHCl₃/MeOH resulted in inversion of the predominant screw‐sense for poly( 1 ) and poly( 2 ). Conversely, poly( 3 ) was transformed into a random coil, and poly( 4 ) maintained the predominant screw‐sense irrespective of MeOH content. The solvent dependence of predominant screw‐sense for poly( 1 ) and poly( 2 ) was reasonably explained by molecular orbital studies using the conductor‐like screening model (COSMO).     

Enhanced Electron‐Transfer Properties of Cofacial Porphyrin Dimers through π–π Interactions

π–π assisted : Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).

     

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.     

Helix Nucleation by the Smallest Known α‐Helix in Water

Cyclic pentapeptides (e.g. Ac‐(cyclo‐1,5)‐[KAXAD]‐NH₂; X=Ala, 1 ; Arg, 2 ) in water adopt one α‐helical turn defined by three hydrogen bonds. NMR structure analysis reveals a slight distortion from α‐helicity at the C‐terminal aspartate caused by torsional restraints imposed by the K(i)–D(i+4) lactam bridge. To investigate this effect on helix nucleation, the more water‐soluble 2 was appended to N‐, C‐, or both termini of a palindromic peptide ARAARAARA (≤5 % helicity), resulting in 67, 92, or 100 % relative α‐helicity, as calculated from CD spectra. From the C‐terminus of peptides, 2 can nucleate at least six α‐helical turns. From the N‐terminus, imperfect alignment of the Asp5 backbone amide in 2 reduces helix nucleation, but is corrected by a second unit of 2 separated by 0–9 residues from the first. These cyclic peptides are extremely versatile helix nucleators that can be placed anywhere in 5–25 residue peptides, which correspond to most helix lengths in protein–protein interactions.     

Control of Circularly Polarized Luminescence by Orientation of Stacked π‐Electron Systems

Planar chiral building blocks based on 4,7,12,15‐tetrasubstituted [2.2]paracyclophanes were obtained via a synthetic route involving an optical resolution step. Planar chiral enantiomers, comprising two fluorophores that were stacked to form a V‐shaped higher‐ordered structure, were synthesized from these building blocks. The V‐shaped molecules emitted intense circularly polarized luminescence (CPL). Their chiroptical properties were compared with those of X‐shaped molecules bearing the same two fluorophores stacked together. The CPL sign of the X‐shaped molecule was opposite to that of the V‐shaped molecule, which is supported by the theoretical results, indicating that the CPL sign can be controlled by the orientation of the stacked fluorophores.     

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.     

Oxa[7]superhelicene: A π‐Extended Helical Chromophore Based on Hexa‐peri‐hexabenzocoronenes

A novel π‐extended “superhelicene” based on hexa‐peri‐hexabenzocoronenes (HBCs) has been synthesized by an efficient four‐step synthetic procedure starting from diphenyl ether. Comprehensive structural analysis of the helicene was performed by NMR spectroscopy and mass spectrometry measurements together with X‐ray analysis. Physicochemical analysis of the superhelicene and suitable HBC references revealed it had outstanding fluorescent features with quantum yields of over 80 %.