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.     

Dynamic Chirality Control of tropos DPCB‐digold Skeleton by Chiral Binaphthyldicarboxylate

The planar 3,4‐diphosphinidenecyclobutene (DPCB) can be remarkably twisted into a C₂‐type helical structure by dual coordination of a AuCl moiety. A prompt chirality control of the twisted DPCB skeleton ligated by the digold units affords the enantiopure structure by exchanging the chloride ligands for chiral [1,1′‐binaphthalene]‐2,2′‐dicarboxylate. The chirality of the diaurated 2,2′‐bis(diphenylphosphanyl)‐1,1′‐biphenyl (BIPHEP) system can be controlled prior to that of DPCB. Mixing of a DPCB‐bis(chlorogold) complex with the chiral silver salt dynamically leads to a single diastereomer, which was characterized by the ³¹P NMR spectrum and the CD couplet patterns in the visible (DPCB) area. The absolute configuration of the singly induced helical structure was assigned by the theoretical CD spectra determined by TD‐DFT calculations. Intramolecular alkoxycyclization of hexa‐4,5‐dien‐1‐ol catalyzed by the asymmetric DPCB‐digold structure were also attempted.     

New Helical Folds in α‐Peptides with Alternating Chirality

In α‐peptides, the 8/10 helix is theoretically predicted to be energetically unstable and has not been experimentally observed so far. Based on our earlier studies on ‘helical induction’ and ‘hybrid helices’, we have adopted the ‘end‐capping’ strategy to induce the 8/10 helix in α‐peptides by using short α/β‐peptides. Thus, α‐peptides containing a regular string of α‐amino acids with alternating chirality were end capped by α/β‐peptides with 11/9‐helical motifs at the termini. Extensive NMR spectroscopy studies of these peptides revealed the presence of a hitherto unknown 8/10‐helical pattern; the H‐bonds in the shorter pseudorings were rather weak. The approach of using short helical motifs to induce new mixed helices in α‐peptides could provide avenues for more versatile design strategies.     

7,7,8,8‐Tetraaryl‐o‐quinodimethane Stabilized by Dibenzo Annulation: A Helical π‐Electron System That Exhibits Electrochromic and Unique Chiroptical Properties

When two benzene rings are fused to a tetraaryl‐o‐quinodimethane skeleton, sterically hindered helical molecules 1 acquire a high thermodynamic stability. Because the tetraarylbutadiene subunit contains electron‐donating alkoxy groups, 1 undergo reversible two‐electron oxidation to 2 ²⁺, which can be isolated as deeply colored stable salts. Intramolecular transfer of the point chirality (e.g., sec‐butyl) on the aryl groups to helicity induces a diastereomeric preference in dications 2 b ²⁺ and 2 c ²⁺, which represents an efficient method for enhancing circular‐dichroism signals. Thus, those redox pairs can serve as new electrochiroptical response systems. X‐ray analysis of dication 2 ²⁺ revealed π–π stacking interaction of the diarylmethylium moieties, which is also present in solution. The stacking geometry is the key contributor to the chirosolvatochromic response.     

Enantiopure Double‐Helical Phenylene Ethynylene Cyclophynes with the 2,2′‐Binaphthyl Template

New types of enantiopure compounds were synthesized to gain better insight into the structural features of phenylene ethynylene cyclophynes. Besides the previously obtained meta‐substituted arylene ethynylenes, 1 , ortho‐connected phenylene ethynylene units were incorporated to give cyclophynes with ortho/meta and ortho/ortho connection modes, 2 and 3 . Furthermore, a diphenylethyne component was also accommodated in 4 . Both ab initio calculations and NMR spectra suggest a large amount of strain for 2 but less strain for 3 and 1 a , the latter having the smallest ring size among cyclophynes with the meta/meta connection mode. The CD spectra of 2 and 3 showed a characteristic shoulder at around 340 nm, similar to the case of 1 a . This implies that the aromatic acetylene bonds cross over each other in the double‐helical structure. These results indicate that chirality information is useful for probing the persistency of molecular shape.     

1,2‐Dithienyldicyanoethene‐Based,Visible‐Light‐Driven,Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.     

Helical arrays of pendant fullerenes on optically active poly(phenylacetylene)s

Novel, optically active, stereoregular poly(phenylacetylene)s bearing the bulky fullerene as the pendant were synthesized by copolymerization of an achiral phenylacetylene bearing a [60]fullerene unit with optically active phenylacetylene components in the presence of a rhodium catalyst. The C60-bound phenylacetylene was prepared by treatment of C60 with N-(4-ethynylbenzyl)glycine in a Prato reaction. The obtained copolymers exhibited induced circular dichroism (ICD) in solution both in the main-chain region and in the achiral fullerene chromophoric region, although their ICD intensities were highly dependent on the structures of the optically active phenylacetylenes and the solution temperature. These results indicate that the optically active copolymers form one-handed helical structures and that the pendant achiral fullerene groups are arranged in helical arrays with a predominant screw sense along the polymer backbone. The structures and morphology of the copolymers on solid substrates were also investigated by atomic force microscopy.     

On the Helical Motif of the Complexes Created by Association of Helix‐Forming Schizophyllan (SPG) and Helix‐Forming Polythiophene Derivatives

π‐Conjugated polymers can finely tune their electrical and optical properties in response to their conformational changes. We believe that a deeper understanding of their higher‐order structures will stimulate further development of their applications. We had revealed that one helix‐forming natural polysaccharide (SPG) and one polythiophene derivative (PT‐1) formed a stable one‐dimensional complex and in the polythiophene main chain a helical conformation was induced through the dynamic conformational changes. The objective of our present research is to obtain a better mechanistic understanding on the interaction between SPG and polythiophenes. Here we have used particular left‐ and right‐handed helix‐forming polythiophene derivatives (D ‐ and L ‐POWTs, respectively) and studied their influence on the helical motif of the complexes. We observed that SPG interacts with both D ‐ and L ‐POWTs through their dynamic conformational changes and both D ‐ and L ‐POWTs form the right‐handed co‐helical complexes with SPG according to the inherent helical motif of SPG. In addition, it was confirmed that 1) the complexes do not coagulate in aqueous solution, and 2) the exchange in the helical motif can occur only when the polymers experience the denature–renature process. We believe, therefore, that the mechanism of the helical induction of the SPG/POWT complexes is very unique, being different from conventional equilibrium reactions.     

Chirality and Template‐Mediated Induction of Helical Preferences in Achiral β‐Peptides

This study describes chirality‐ or template‐mediated helical induction in achiral β‐peptides for the first time. A strategy of end capping β‐peptides derived from β‐hGly (the smallest achiral β‐amino acid) with a chiral β‐amino acid that possesses a carbohydrate side chain (β‐Caa; C‐linked carbo β‐amino acid) or a small, robust helical template derived from β‐Caas, was adopted to investigate folding propensity. A single chiral (R)‐β‐Caa residue at the C‐ or N‐terminus in these oligomers led to a preponderance of right‐handed 12/10‐helical folds, which was reiterated more strongly in peptides capped at both the C‐ and N‐terminus. Likewise, the presence of a template (a 12/10‐helical trimer) at both the C‐ and N‐terminus resulted in a very robust helix. The propagation of the helical fold and its sustenance was found in a homo‐oligomeric sequence with as many as seven β‐hGly residues. In both cases, the induction of helicity was stronger from the N terminus, whereas an anchor at the C terminus resulted in reduced helical propensity. Although these oligomers have been theoretically predicted to favor a 12/10‐mixed helix in apolar solvents, this study provides the first experimental evidence for their existence. Diastereotopicity was found in both the methylene groups of the β‐hGly moieties due to chirality. Additionally, the β‐hGly units have shown split behavior in the conformational space to accommodate the 12/10‐helix. Thus, end capping to assist chiralty‐ or template‐mediated helical induction and stabilization in achiral β‐peptides is a very attractive strategy.     

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.     

Conformational Studies on Peptides of α‐Aminoxy Acids with Functionalized Side‐Chains

Peptides of homochiral α‐aminoxy acids of nonpolar side chains can form a 1.8₈‐helix. In this paper, we report the conformational studies of α‐aminoxy peptides 1 , 2 , 3 , which have functionalized side chains, in both nonpolar and polar solvents. ¹H NMR, XRD, and FTIR absorption studies confirm the presence of the eight‐membered‐ring intramolecular hydrogen bonds (the N‐O turns) in nonpolar solvents as well as in methanol. CD studies of peptides 1 , 2 , 3 in different solvents indicate that a substantial degree of helical content is retained in methanol and acidic aqueous buffers. The introduction of functionalized side chains in α‐aminoxy peptides provides opportunities for designing biologically active peptides.     

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.     

500‐Fold Amplification of Small Molecule Circularly Polarised Luminescence through Circularly Polarised FRET

Strongly dissymmetric circularly polarised (CP) luminescence from small organic molecules could transform a range of technologies, such as display devices. However, highly dissymmetric emission is usually not possible with small organic molecules, which typically give dissymmetric factors of photoluminescence (g_PL) less than 10^?2. Here we describe an almost 10³‐fold chiroptical amplification of a π‐extended superhelicene when embedded in an achiral conjugated polymer matrix. This combination increases the |g_PL| of the superhelicene from approximately 3×10^?4 in solution to 0.15 in a blend film in the solid‐state. We propose that the amplification arises not simply through a chiral environment effect, but instead due to electrodynamic coupling between the electric and magnetic transition dipoles of the polymer donor and superhelicene acceptor, and subsequent CP Förster resonance energy transfer. We show that this amplification effect holds across several achiral polymer hosts and thus represents a simple and versatile approach to enhance the g‐factors of small organic molecules.     

Chirality of Anisotropic Metal Nanowires with a Distinct Multihelix

Two‐dimensional (2D) anisotropic silver nanowire (AgNW) arrays, fabricated inside chiral mesoporous silica (CMS), exhibited strong and tunable plasmon circular dichroism (CD) signals in the visible and near‐IR regions due to collective dipole coupling between the anisotropic AgNWs. The multihelix with a helical channel orientation and helical arrays of opposite handedness in CMS played a predominant effects on the transversal and longitudinal chirality of the AgNWs, respectively.This behavior differs from both isotropic‐nanoparticle and single‐helix‐induced CD responses. This system will provide new insight into the optical activity of metal inorganic nanoparticles capped with chiral organic molecules and assembled in chiral environments.     

Predicting the Switchable Screw Sense in Fluorene‐Based Polymers

A chirality‐switching free‐energy landscape was reconstructed on a 43‐mer of poly(9,9‐dioctylfluoren‐2,7‐diyl) (PDOF). The simulations were conducted on amorphous silica surface as well as in the vacuum phase for a single chain or for a group of sixteen chains. The achiral‐to‐chiral transition occurs only on amorphous silica (activation free‐energy 35 kcal mol^?1), where the enantiomeric (homochiral) basins are detected. This was supported by the experiments where effective chirality induction to PDOF using circularly polarized light (CPL) was attained only for a film deposited on a quartz glass and not for a solution or a suspension. These results indicate that interactions of PDOF with amorphous silica play a crucial role in chirality switching. Importance of chain assembling was also indicated. Theoretical ECD spectra of the enantiomeric basins containing a 5₁ helix reproduce the experimental spectra.