Helicity Inversion Research Articles

Realizing Abundant Chirality Inversion of Supramolecular Nanohelices by Multiply Manipulating the Binding Sites in Molecular Blocks

AbstractThe induction of diverse chirality regulation in nature by multiple binding sites of biomolecules is ubiquitous and plays an essential role in determining the biofunction of biosystems. However, mimicking this biological phenomenon and understanding at a molecular level its mechanism with the multiple binding sites by establishing an artificial system still remains a challenge. Herein, abundant chirality inversion is achieved by precisely and multiply manipulating the co‐assembled binding sites of phenylalanine derivatives (D/LPPF) with different naphthalene derivatives (NA, NC, NP, NF). The amide and hydroxy group of naphthalene derivatives prefer to bind with the carboxy group of LPPF, while carboxylic groups and fluoride atoms tend to bind with the amide moiety of LPPF. All these diverse interaction modes can precisely trigger helicity inversion of LPPF nanofibers. In addition, synergistically manipulating the carboxy and amide binding sites from a single LPPF molecule to simultaneously interact with different naphthalene derivatives leads to chirality regulation. Typically, varying the solvent may switch the interaction modes and the switched new interaction modes can be employed to further regulate the chirality of the LPPF nanofibers. This study may provide a novel approach to explore chirality diversity in artificial systems by regulating the intermolecular binding sites.

Realizing Abundant Chirality Inversion of Supramolecular Nanohelices by Multiply Manipulating the Binding Sites in Molecular Blocks.

The induction of diverse chirality regulation in nature by multiple binding sites of biomolecules is ubiquitous and plays an essential role in determining the biofunction of biosystems. However, mimicking this biological phenomenon and understanding at a molecular level its mechanism with the multiple binding sites by establishing an artificial system still remains a challenge. Herein, abundant chirality inversion is achieved by precisely and multiply manipulating the co-assembled binding sites of phenylalanine derivatives (D/LPPF) with different naphthalene derivatives (NA, NC, NP, NF). The amide and hydroxy group of naphthalene derivatives prefer to bind with the carboxy group of LPPF, while carboxylic groups and fluoride atoms tend to bind with the amide moiety of LPPF. All these diverse interaction modes can precisely trigger helicity inversion of LPPF nanofibers. In addition, synergistically manipulating the carboxy and amide binding sites from a single LPPF molecule to simultaneously interact with different naphthalene derivatives leads to chirality regulation. Typically, varying the solvent may switch the interaction modes and the switched new interaction modes can be employed to further regulate the chirality of the LPPF nanofibers. This study may provide a novel approach to explore chirality diversity in artificial systems by regulating the intermolecular binding sites.

Control of chirality inversion kinetics of triple-helical metallocryptands

Abstract Dynamic helical molecules, which can undergo a reversible chirality inversion between the P and M forms, are useful as a platform for switchable chiral functional molecules. The chirality inversion of these molecules has been extensively studied. However, it has mostly been discussed from the viewpoint of the equilibrated P/M ratios before and after the inversion; control of the response speeds or kinetic profiles has rarely been explored. In order to construct helical structures with controllable kinetic profiles, triple-helical metallocryptands, LM3, have been designed and synthesized. These molecules can undergo a relatively slow dynamic P/M chirality inversion (helicity inversion) to produce an equilibrated mixture. The P/M equilibration was accelerated or decelerated based on the following two strategies. One is based on the guest binding in the cryptand cavity. The P/M racemization kinetics of LNi3 was significantly decelerated by recognition of guanidinium ion in the cavity. The other strategy is based on the ligand exchange reactions at the octahedral cobalt(III) centers in LCo3(amine)6. The P/M chirality inversion speeds were controlled by changing the initial and entering amine ligands. In addition, a unique transient chirality inversion behavior was observed when chiral amine ligands were removed from the metallocryptand by the ligand exchange reaction with piperidine.

Kinetically controlled chiral metal‐coordinated supramolecular polymerization accompanying helical inversion or morphological transformation

AbstractHere, we describe the recent progress on metal‐coordinated supramolecular structures with unique characteristics and controlled helicity based on coordination geometry and dynamic intermolecular binding events. The current major research area is the self‐assembly of a metal‐containing molecule into nanomaterials. It generated a large number of supramolecular polymers, which have various structures and functions. Although the dynamic coordination complexation reactions between various metal ions and ligands are spontaneous, we successfully achieved unique morphological transformations with helical inversion of metal‐containing supramolecular polymers via the kinetic control of dynamic coordination complexation reactions. Our findings have a significant impact on the development of novel and advanced supramolecular materials, and dynamic coordination bonds in the molecular design of metal‐containing complexes for modulating self‐assembly pathways, which may be relevant for the fabrication of supramolecular materials with interesting properties. Our contributions may be useful to the understanding of chiral assembly processes in biological systems.

PH and Salt-Assisted Macroscopic Chirality Inversion of Gadolinium Coordination Polymer.

The precise adjustment of handedness of helical architectures is important to regulate their functions. Macroscopic chirality inversion has been achieved in organic supramolecular systems by pH, metal ions, solvents, chiral and non-chiral additives, temperature, and light, but rarely in coordination polymers (CPs). In particular, salt-assisted macroscopic chirality inversion has not been reported. In this work, we carried out a systematic investigation on the role of pH and salt in regulating the morphology of CPs based on Gd(NO3)3 and R-(1-phenylethylamino)methylphosphonic acid (R-pempH2). Without extra NO3-, the chirality inversion from the left-handed superhelix R-M to the right-handed superhelix R-P can be achieved by pH modulation from 3.2 to 3.8. The addition of NaNO3 (2.0 eq) at pH 3.8 results in an inversion of chiral sense from R-P to R-M as a pure phase. To our knowledge, this is the first example of salt-assisted macroscopic helical inversion in artificial systems.

Tuning of the Supramolecular Helicity of Peptide-Based Gel Nanofibers.

Helical supramolecular architectures play important structural and functional roles in biological systems. The helicity of synthetic molecules can be tuned mainly by the chiral manipulation of the system. However, tuning of helicity by the achiral unit of the molecules is less studied. In this work, the helicity of naphthalimide-capped peptide-based gel nanofibers is tuned by the alteration of methylene units present in the achiral amino acid. The inversion of supramolecular helicity has been extensively studied by CD spectroscopy and morphological analysis. The density functional theory (DFT) study indicates that methylene spacers influence the orientation of π-π stacking interactions of naphthalimide units in the self-assembled structure that regulates the helicity. This work illustrates a new approach to tuning the supramolecular chirality of self-assembled biomaterials.

Ruthenium-Locked Helical Chirality: A Barrier of Inversion and Formation of an Asymmetric Macrocycle.

Upon coordination to metal centers, tetradentate ligands based on the 6,6′-bis(2″-aminopyridyl)-2,2′-bipyridine (bapbpy) structure form helical chiral complexes due to the steric clash between the terminal pyridines of the ligand. For octahedral ruthenium(II) complexes, the two additional axial ligands bound to the metal center, when different, generate diastereotopic aromatic protons that can be distinguished by NMR. Based on these geometrical features, the inversion barrier of helical [RuII(L)(RR′SO)Cl]+ complexes, where L is a sterically hindered bapbpy derivative and RR′SO is a chiral or achiral sulfoxide ligand, was studied by variable-temperature 1H NMR. The coalescence energies for the inversion of the helical chirality of [Ru(bapbpy)(DMSO)(Cl)]Cl and [Ru(bapbpy)(MTSO)(Cl)]Cl (where MTSO is (R)-methyl p-tolylsulfoxide) were found to be 43 and 44 kJ/mol, respectively. By contrast, in [Ru(biqbpy)(DMSO)(Cl)]Cl (biqbpy = 6,6′-bis(aminoquinolyl)-2,2′-bipyridine), increased strain caused by the larger terminal quinoline groups resulted in a coalescence temperature higher than 376 K, which pointed to an absence of helical chirality inversion at room temperature. Further increasing the steric strain by introducing methoxy groups ortho to the nitrogen atoms of the terminal pyridyl groups in bapbpy resulted in the serendipitous discovery of a ring-closing reaction that took place upon trying to make [Ru(OMe-bapbpy)(DMSO)Cl]+ (OMe-bapbpy = 6,6′-bis(6-methoxy-aminopyridyl)-2,2′-bipyridine). This reaction generated, in excellent yields, a chiral complex [Ru(L″)(DMSO)Cl]Cl, where L″ is an asymmetric tetrapyridyl macrocycle. This unexpected transformation appears to be specific to ruthenium(II) as macrocyclization did not occur upon coordination of the same ligand to palladium(II) or rhodium(III).

Controlled helicity inversion, selective enantiomer release, and methanol recognition in azobenzene gel

Constructing artificial supramolecular materials is important because they possesses wide application in many fields such as bio-, chiral and information science. Here, we prepared a glutamide–azobenzene gelator (GAZS) which is both chiral and amphiphilic. The GAZS gel exhibited different cotton effects depending on the solvents, in methanol the gel was endowed with chiroptical switch properties under external stimuli. Selective enantiomer release could be achieved by the shrinkage process of the gel in methanol under UV irradiation. In alcohol solvents only the gel in methanol is opaque and has a positive cotton effect. We believe our work could provide some insights into the solvent recognition and the construction of chiroptical supramolecular materials.

Circularly Polarized Luminescence Active Supramolecular Nanotubes Based on PtII Complexes That Undergo Dynamic Morphological Transformation and Helicity Inversion

AbstractCircularly polarized luminescence (CPL) with tunable chirality is currently a challenging issue in the development of supramolecular nanomaterials. We herein report the formation of helical nanoribbons which grow into helical tubes through dynamic helicity inversion. For this, chiral PtII complexes of terpyridine derivatives, namely S‐trans‐1 and R‐trans‐1, with respective S‐ and R‐alanine subunits and incorporating trans‐double bonds in the alkyl chain were prepared. In DMSO/H2O (5 : 1 v/v), S‐trans‐1 initially forms a fibrous self‐assembled product, which then undergoes dynamic transformation into helical tubes (left‐handed or M‐type) through helical ribbons (right‐handed or P‐type). Interestingly, both helical supramolecular architectures are capable of emitting CPL signals. The metastable helical ribbons show CPL signals (glum=±4.7×10−2) at 570 nm. Meanwhile, the nanotubes, which are the thermodynamic products, show intense CPL signals (glum=±5.6×10−2) at 610 nm accompanied by helicity inversion. This study provides an efficient way to develop highly dissymmetric CPL nanomaterials by regulating the morphology of metallosupramolecular architectures.

Circularly Polarized Luminescence Active Supramolecular Nanotubes Based on PtII Complexes That Undergo Dynamic Morphological Transformation and Helicity Inversion.

Circularly polarized luminescence (CPL) with tunable chirality is currently a challenging issue in the development of supramolecular nanomaterials. We herein report the formation of helical nanoribbons which grow into helical tubes through dynamic helicity inversion. For this, chiral PtII complexes of terpyridine derivatives, namely S-trans-1 and R-trans-1, with respective S- and R-alanine subunits and incorporating trans-double bonds in the alkyl chain were prepared. In DMSO/H2 O (5 : 1 v/v), S-trans-1 initially forms a fibrous self-assembled product, which then undergoes dynamic transformation into helical tubes (left-handed or M-type) through helical ribbons (right-handed or P-type). Interestingly, both helical supramolecular architectures are capable of emitting CPL signals. The metastable helical ribbons show CPL signals (glum =±4.7×10-2 ) at 570 nm. Meanwhile, the nanotubes, which are the thermodynamic products, show intense CPL signals (glum =±5.6×10-2 ) at 610 nm accompanied by helicity inversion. This study provides an efficient way to develop highly dissymmetric CPL nanomaterials by regulating the morphology of metallosupramolecular architectures.

Helix Induction and Inversion of Polymeric Foldamer Regulated by the Single Enantiomers.

Generally, a single enantiomer can induce a foldamer into a preferred-handed helix, while another condition is required for the helical inversion. Herein, it is found that the helix induction and subsequent inversion of poly(m-phenylene diethynylene)-based foldamer bearing aza-18-crown-6 pendants (Poly-1) can be realized by increasing the concentration of sterically hindered l-amino acid perchlorate salts. When the amount of chiral enantiomers is small, one enantiomer tends to complex with two non-adjacent aza-18-crown-6 rings via three N+ H···O hydrogen bonds in a sandwich mode. Notably, the transition dipole moment is perpendicular to the aza-18-crown-6 ring, so that the induced helical chirality in Poly-1 backbone is opposite to the chirality of enantiomers. When the amount of chiral enantiomers is large enough, each aza-18-crown-6 is occupied by one enantiomer, which causes the transition dipole moment in a parallel direction to aza-18-crown-6 ring. In this case, the increased steric hindrance can facilitate the inversion of screw sense of Poly-1 backbone, which is directed by chiral center of enantiomers. As a result, a helix inversion has been achieved successfully. This work not only provides a novel strategy for regulating the two-stage folded helical conformations by the single enantiomers, but opens a window to develop chiral recognition materials.

A Rotation-Inversion Dual-Motion Molecular Switch: Race for NMR Signaling.

The interplay between the thermal helical inversion (THI) of the stiff-stilbene moiety and the rotation of the dimethylamino (DMA) group in 1 results in a dependence of the DMA NMR signals on the THI kinetics in (E)-1 but the rotation kinetics in (Z)-1, because the faster motion mode is responsible. Consequently, the photochemical switching from (E)-1 to (Z)-1 illustrates the phenomenon of "switchable motion detection" by the same set of NMR signals in a dual-motion molecular system.

Consecutively fused single‐, double‐, and triple‐expanded helicenes

AbstractWe report the quantitative, chemoselective, and diastereospecific formation of the defect‐free C‐shaped single and S‐ and M‐shaped consecutively fused double‐ and triple‐expanded homochiral helicenes (all‐right (P) or all‐left (M) handed), in which two and three expanded helicene subunits were exclusively fused together in a helically winding manner, respectively, through the trifluoroacetic acid‐promoted intramolecular multistep cascade alkyne benzannulations of anthracene‐based cyclization precursors composed of a large number of complicated dynamic stereoisomers due to axial chirality. The resulting racemic expanded helicenes were all separated into enantiomers by chiral chromatography and showed intense circular dichroism and circularly polarized luminescence (CPL) with the absorption and luminescence dissymmetry factors of up to 1.5 × 10−2 and 0.94 × 10−2, respectively. Moreover, the triple‐expanded helicene showed the highest CPL brightness (255 M−1·cm−1) among all the reported CPL‐active carbo‐ and heterohelicenes and ‐helicenoids. The helicity inversion barriers remarkably increased with the increasing number of incorporated helicene subunits, and the racemization of the triple‐expanded helicene was completely suppressed even at 80°C, providing the first example of an optically active expanded helicene with a static helical chirality.Key Points Defect‐free synthesis of single‐, double‐, and triple‐expanded helicenes with C‐, S‐, and M‐shaped fully conjugated helical ladder structures, respectively. Quantitative and chemoselective acid‐catalyzed diastereospecific multistep cascade cyclizations. Circularly polarized luminescence of configurationally stable optically pure expanded helicenes.

Synthesis of Dibenzo[h,t]rubicene through Its Internally Dimethoxy-substituted Precursor

Abstract Dibenzo[h,t]rubicene was synthesized from 1,5-dichloroanthraquinone via an internally dimethoxylated intermediate. In contrast to the planar conformation of rubicene, dibenzo[h,t]rubicene adopted a double-helicene-like structure due to the intramolecular steric repulsion around the fused benzene-rings. Theoretical calculation suggests that the helical inversion occurs frequently with an inversion barrier of 4.8 kcal mol−1. The HOMO–LUMO gap of dibenzo[h,t]rubicene is smaller than that of rubicene, resulting in its red-shifted absorption tailing to 690 nm.

Unconventional assemblies of bisacylhydrazones: The role of water for circularly polarized luminescence

AbstractUnderstanding the precise molecular arrangement of chiral supramolecular polymers is essential not only to comprehend complex superstructures like proteins and DNA but also for the development of next‐generation optoelectronic materials, including materials displaying high‐performance circularly polarized luminescence (CPL). Herein, we report the first chiral supramolecular polymer systems based on hydrazone–pyridinium conjugates comprising alkyl chains of different lengths, which afforded control of the apparent supramolecular chirality. Although supramolecular chirality is governed basically by the remote chiral centers of alkyl chains, helicity inversion was achieved by controlling the conditions under which the hydrazone building blocks underwent aggregation (i.e., solvent compositions or temperature). More importantly, the addition of water to the system led to aggregation‐induced hydrazone deprotonation, which resulted in a completely different self‐assembly behavior. Structural water molecules played an essential role, forming the assembly's channel‐like backbone, around which hydrazone molecules gathered as a result of hydrogen bonding interactions. Further co‐assembly of an achiral hydrazone luminophore with the given supramolecular polymer system allowed the fabrication of a novel CPL‐active hydrazone‐based material exhibiting a high maximum value for the photoluminescence dissymmetry factor of −2.6 × 10−2.

Helicity-driven chiral self-sorting supramolecular polymerization with Ag+: right- and left-helical aggregates.

The study of chiral self-sorting is extremely important for understanding biological systems and for developing applications for the biomedical field. In this study, we attempted unprecedented chiral self-sorting supramolecular polymerization accompanying helical inversion with Ag+ in one enantiomeric component. Bola-type terpyridine-based ligands (R-L1 and S-L1) comprising R- or S-alanine analogs were synthesized. First, R-L1 dissolved in DMSO/H2O (1 : 1, v/v) forms right-handed helical fibers (aggregate I) via supramolecular polymerization. However, after the addition of AgNO3 (0.2–1.1 equiv.) to the R-L1 ligand, in particular, it was found that aggregate II with left-handed helicity is generated from the [R-L1(AgNO3)2] complex through the [R-L1Ag]+ complex via the dissociation of aggregate I by a multistep with an off pathway, thus demonstrating interesting self-sorting properties driven by helicity and shape discrimination. In addition, the [R-L1(AgNO3)2] complex, which acted as a building block to generate aggregate III with a spherical structure, existed as a metastable product during the formation of aggregate II in the presence of 1.2–1.5 equiv. of AgNO3. Furthermore, the AFM and CD results of two samples prepared using aggregates I and III with different volume ratios were similar to those obtained upon the addition of AgNO3 to free R-L1. These findings suggest that homochiral self-sorting in a mixture system occurred by the generation of aggregate II composed of the [R-L1Ag]+ complex via the rearrangement of both, aggregates I and III. This is a unique example of helicity- and shape-driven chiral self-sorting supramolecular polymerization induced by Ag+ starting from one enantiomeric component. This research will improve understanding of homochirality in complex biological models and contribute to the development of new chiral materials and catalysts for asymmetric synthesis.

Adaptive helicity and chiral recognition in bright europium quadruple-stranded helicates induced by host-guest interaction

Chiral information transfer is crucial to design new functions like, for instance, sensing through chiroptical probes in which the input is readout as a consequence of a chiral recognition event. Herein, we report lanthanide quadruple-stranded helicates [Ln 2 L 4 ] 2− that show a guest-to-host chirality transfer. As confirmed by X-ray diffraction (XRD) and density functional theory (DFT) calculations, these cages are present in solution as an equilibrating racemic mixture of left- and right-handed helicates. The helicates adapt their helical conformation by encapsulation of a chiral guest enabling also straightforward enantiomeric excess determination. Asymmetric induction is demonstrated by circular dichroism (CD) and circularly polarized luminescence (CPL), and a helicity inversion mechanism based on a Bailar twist is proposed and studied by DFT calculations. The presented research contributes to the design of helicates with a unique combination of confined cavities, adaptive chirality, and peculiar luminescent properties, finding applications toward the development of optical probes for selective chiral sensing via molecular recognition. • Helicates adapt in response to a recognition process in a confined space • Chiral recognition through host-guest events detected by CD and CPL • Very bright europium helicates with luminescence visible to naked eye • Helicity inversion mechanism, i.e., Bailar twist, is revealed by DFT calculations Rancan et al. report chirality transfer for chiroptical molecular recognition. Very bright europium helicates enable a chiroptical readout toward chiral guests. Helicates adapt their conformation with a guest-to-host hierarchical chirality transfer. The molecular recognition process is detected by induced circular dichroism (CD) and circularly polarized luminescence (CPL).

Merging Supramolecular and Covalent Helical Polymers: Four Helices Within a Single Scaffold.

Supramolecular and covalent polymers share multiple structural effects such as chiral amplification, helical inversion, sergeants and soldiers, or majority rules, among others. These features are related to the axial helical structure found in both types of materials, which are responsible for their properties. Herein a novel material combining information and characteristics from both fields of helical polymers, supramolecular (oligo(p-phenyleneethynylene) (OPE)) and covalent (poly(acetylene) (PA)), is presented. To achieve this goal, the poly(acetylene) must adopt a dihedral angle between conjugated double bonds (ω1) higher than 165°. In such cases, the tilting degree (Θ) between the OPE units used as pendant groups is close to 11°, like that observed in supramolecular helical arrays of these molecules. Polymerization of oligo[(p-phenyleneethynylene)n]phenylacetylene monomers (n = 1, 2) bearing L-decyl alaninate as the pendant group yielded the desired scaffolds. These polymers adopt a stretched and almost planar polyene helix, where the OPE units are arranged describing a helical structure. As a result, a novel multihelix material was prepared, the ECD spectra of which are dominated by the OPE axial array.

Synthesis, Structures, and Properties of Helically Fused Anthraquinones with Unusually Close Carbonyl-Carbonyl Contacts.

Electron-deficient aromatic ketones consisting of three fused anthraquinone units were synthesized by oxidation of the corresponding fused anthracenes. X-ray analysis revealed that these compounds had nonplanar helical structures with unusual contacts, C=O⋅⋅⋅C=O 2.467 Å, between the inner carbonyl groups. The role of n⋅⋅⋅π* interactions in the short contacts was evaluated using a noncovalent interaction plot and natural bond orbital analysis. The dynamic process involving helical inversion was observed by the variable temperature 1 H NMR measurement of a derivative with 2,4,6-trimethylphenyl groups, and the barrier was estimated to be 77 kJ mol-1 . DFT calculations indicated that the helical inversion proceeded via a multistep mechanism. The characteristic spectroscopic and electrochemical data due to the electron-deficient anthraquinone units and the sterically congested carbonyl groups are discussed with the aid of DFT calculations.

Five-Fold Symmetric Pentaindolo- and Pentakis(benzoindolo)Corannulenes: Unique Structural Dynamics Derived from the Combination of Helical and Bowl Inversions.

Peripherally π-extended corannulenes bearing quintuple azahelicene units, 10 and 11, were prepared and their dynamic behaviors were studied experimentally and theoretically. The fused corannulenes were synthesized from sym-pentabromocorannulene in three steps. X-Ray diffraction analysis for 10 displayed a conformer possessing a P(M) bowl chirality and a PPMPM (PMPMM) helical chirality, which was found to be the most stable conformer(s). Variable-temperature NMR measurements of 10 and 11 revealed that their structural isomers can be interconvertible in solution, depending on the steric congestion around the helical scaffolds. Automated search for conformers in the equilibrium and transition states by Artificial Force Induced Reaction (AFIR) method revealed their interconversion networks, including bowl-inversion and helical-inversion. This analysis indicated that the co-existing corannulene and azahelicene moieties influence the conformational dynamics, which leads to mitigation of the activation energy barriers for isomerization.