Ultrasound-Directed Symmetry Breaking and Spin Filtering of Supramolecular Assemblies from only Achiral Building Blocks

Herein we describe the self-assembly of an achiral molecule into macroscopic helicity as well as the emergent chiral-selective spin-filtering effect. It was found that a benzene-1,3,5-tricarboxamide (BTA) motif with an aminopyridine group in each arm could coordinate with Ag^I and self-assemble into nanospheres. Upon sonication, symmetry breaking occurred and the nanospheres transferred into helical nanofibers with strong CD signals. Although the sign of the CD signals appeared randomly, it could be controlled by using the as-made chiral assemblies as a seed. Furthermore, it was found that the charge transport of the helical nanofibers was highly selective with a spin-polarization transport of up to 45 %, although the chiral nanofibers are composed exclusively from achiral building blocks. This work demonstrates symmetry breaking under sonication and the chiral-selective spin-filtering effect.     

Solvation-Mediated Self-Assembly from Crystals to Helices of Protic Acyclic Carbene AuI-Enantiomers with Chirality Amplification

Constructing chiral supramolecular assembly and exploring the underlying mechanism are of great significance in promoting the development of circularly polarized luminescence (CPL)-active materials. Herein, we report a solvation-mediated self-assembly from single-crystals to helical nanofibers based on the first protic acyclic (methoxy)(amino)carbenes (pAMACs) Au^I-enantiomers driven by a synergetic aurophilic interactions and H-bonds. Their aggregation-dependent thermally activated delayed fluorescence properties with high quantum yields (Φ_FL) up to 95 % were proved to be attributed to packing modes of Au⋅⋅⋅Au dimers with π-stacking or one-dimensional extended Au⋅⋅⋅Au chains. Via drop-casting method, supramolecular P- or M-helices were prepared. Detailed studies on the helices demonstrate that formations of extended helical Au⋅⋅⋅Au molecular chains amplify supramolecular chirality, leading to strong CPL with high dissymmetry factor (|g_lum|=0.030, Φ_FL=67 %) and high CPL brightness (B_CPL) of 4.87×10⁻³. Our findings bring new insights into the fabrication of helical structures to improve CPL performance by modifying aurophilic interactions.     

Switching Chirophilic Self-assembly: From meso-structures to Conglomerates in Liquid and Liquid Crystalline Network Phases of Achiral Polycatenar Compounds

Spontaneous generation of chirality from achiral molecules is a contemporary research topic with numerous implications for technological applications and for the understanding of the development of homogeneous chirality in biosystems. Herein, a series of azobenzene based rod-like molecules with an 3,4,5-trialkylated end and a single n-alkyl chain involving 5 to 20 aliphatic carbons at the opposite end is reported. Depending on the chain length and temperature these achiral molecules self-assemble into a series of liquid and liquid crystalline (LC) helical network phases. A chiral isotropic liquid (Iso₁^[*^]) and a cubic triple network phase with chiral I23 lattice were found for the short chain compounds, whereas non-cubic and achiral cubic phases dominate for the long chain compounds. Among them a mesoscale conglomerate with I23 lattice, a tetragonal phase (Tet_bi) containing one chirality synchronized and one non-synchronized achiral network, an achiral double network meso-structure with Ia d space group and an achiral percolated isotropic liquid mesophase (Iso₁) were found. This sequence is attributed to an increasing strength of chirality synchronization between the networks, combined with a change of the preferred mode of chirophilic self-assembly between the networks, switching from enantiophilic to enantiophobic with decreasing chain length and lowering temperature. These nanostructured and mirror symmetry broken LC phases exist over wide temperature ranges which is of interest for potential applications in chiral and photosensitive functional materials derived from achiral compounds.     

Chiral Cooperativity: The Effect of Distant Chiral Centers in Ferrocenylamine Ligands upon Enantioselectivity in the Gold(I)-Catalyzed Aldol Reaction

Long-range chiral cooperativity in enantiomerically pure ferrocenylamine ligands containing both planar and multiple centers of chirality (multiple stereogenic C-atoms) was demonstrated in the Au^I-catalyzed reaction of aldehydes and isocyanoesters. Synthetic methodology was developed for the synthesis of ferrocenylamine ligands with two and three chiral centers of known absolute configuration in the C-side chain in addition to the planar chirality of the molecule. The diastereo- and enantioselectivity of the Au^I-catalyzed formation of the trans- and cis-dihydrooxazoles 5 and 6 , respectively, from benzaldehyde ( 1 ) and methyl isocyanoacetate ( 2 ) depend upon the sequence of chirality (absolute configuration of the chiral centers) in the side chain of the ferrocenylamine ligands. Particularly significant effects were observed upon the enantioselectivity for the minor cis-dihydrooxazole 6 , for which, in certain cases, resulted in a change in the enantiomeric dihydrooxazole 6 produced in excess with a change in the absolute configuration of a distant chiral center. Significant effects upon diastereo- and enantioselectivity were observed when chiral ferrocenylamine ligands containing free OH groups were utilized. Using ligands containing a free OH group gave 6 with an absolute configuration opposite to that produced by the corresponding ester and carbamate derivatives. The possible mechanisms for the transmission of chiral information in the proposed stereoselective transition state (TS) was discussed, including both the formation of a stereogenic N-atom and steric effects based upon Newman's rule of six.     

Organic–Inorganic Hybrid Gold Halide Perovskites: Structural Diversity through Cation Size

Crystal structures of a series of organic–inorganic hybrid gold iodide perovskites, formulated as A₂[Au^II₂][Au^IIII₄] [A=methylammonium (MA) ( 1 ) and formamidinium (FA) ( 2 )], A′₂[I₃]_1−x[Au^II₂]_x[Au^IIII₄] [A′=imidazolium (IMD) ( 3 ), guanidinium (GUA) ( 4 ), dimethylammonium (DMA) ( 5 ), pyridinium (PY) ( 6 ), and piperizinium (PIP) ( 7 )], systematically changed depending on the cation size. In addition, triiodide (I₃⁻) ions were partly incorporated into the AuI₂⁻ sites of 3 – 7 , whereas they were not incorporated into those of 1 and 2 . Such a difference comes from the size of the organic cation. Optical absorption spectra showed characteristic intervalence charge-transfer bands from Au^I to Au^III species, and the optical band gap increased as the size of the cation became larger.     

Controlling Metallophilic Interactions in Chiral Gold(I) Double Salts towards Excitation Wavelength-Tunable Circularly Polarized Luminescence

Materials exhibiting excitation wavelength-dependent photoluminescence (Ex-De PL) in the visible region have potential applications in bioimaging, optoelectronics and anti-counterfeiting. Two multifunctional, chiral [Au(NHC)₂][Au(CN)₂] (NHC=(4R,5R)/(4S,5S)-1,3-dimethyl-4,5-diphenyl-4,5-dihydro-imidazolin-2-ylidene) complex double salts display Ex-De circularly polarized luminescence (CPL) in doped polymer films and in ground powder. Emission maxima can be dynamically tuned from 440 to 530 nm by changing the excitation wavelength. The continuously tunable photoluminescence is proposed to originate from multiple emissive excited states as a result of the existence of varied Au^I⋅⋅⋅Au^I distances in ground state. The steric properties of the NHC ligand are crucial to the tuning of Au^I⋅⋅⋅Au^I distances. An anti-counterfeiting application using these two salts is demonstrated.     

A monomeric gold(I) carbanion complex with an uncoordinated thioether: [2‐(methylsulfanyl)phenyl](triphenylphosphine)gold(I)

The title compound, [Au(C₇H₇S)(C₁₈H₁₅P)], is conformationally chiral and crystallizes from benzene–hexane as individually enantiopure crystals. This mononuclear compound has the Au^I atom linearly bound to a triphenylphosphine P atom and to a phenyl C atom of a 2‐(methylsulfanyl)phenyl group. The angle at the Au^I atom is 175.9 (2)°. The linear ligand coordination about the Au^I atom has geometric parameters inside the remarkably narrow range found for gold complexes bound by a phosphine ligand and by the ortho‐C atom of a substituted phenyl group. This is the first example of gold(I) attached to a methylsulfanyl aromatic carbanion.     

Controlling Metallophilic Interactions in Chiral Gold(I) Double Salts towards Excitation Wavelength‐Tunable Circularly Polarized Luminescence

Materials exhibiting excitation wavelength‐dependent photoluminescence (Ex‐De PL) in the visible region have potential applications in bioimaging, optoelectronics and anti‐counterfeiting. Two multifunctional, chiral [Au(NHC)₂][Au(CN)₂] (NHC=(4R,5R)/(4S,5S)‐1,3‐dimethyl‐4,5‐diphenyl‐4,5‐dihydro‐imidazolin‐2‐ylidene) complex double salts display Ex‐De circularly polarized luminescence (CPL) in doped polymer films and in ground powder. Emission maxima can be dynamically tuned from 440 to 530 nm by changing the excitation wavelength. The continuously tunable photoluminescence is proposed to originate from multiple emissive excited states as a result of the existence of varied Au^I???Au^I distances in ground state. The steric properties of the NHC ligand are crucial to the tuning of Au^I???Au^I distances. An anti‐counterfeiting application using these two salts is demonstrated.     

Gold(I)‐Catalyzed Enantioselective Intermolecular Hydroarylation of Allenes with Indoles and Reaction Mechanism by Density Functional Theory Calculations

Chiral binuclear gold(I) phosphine complexes catalyze enantioselective intermolecular hydroarylation of allenes with indoles in high product yields (up to 90 %) and with moderate enantioselectivities (up to 63 % ee). Among the gold(I) complexes examined, better ee values were obtained with binuclear gold(I) complexes, which displayed intramolecular Au^I Au^I interactions. The binuclear gold(I) complex 4c [(AuCl)₂( L3 )] with chiral biaryl phosphine ligand (S)‐(−)‐MeO‐biphep ( L3 ) is the most efficient catalyst and gives the best ee value of up to 63 %. Substituents on the allene reactants have a slight effect on the enantioselectivity of the reaction. Electron‐withdrawing groups on the indole substrates decrease the enantioselectivity of the reaction. The relative reaction rates of the hydroarylation of 4‐X‐substituted 1,3‐diarylallenes with N‐methylindole in the presence of catalyst 4c [(AuCl)₂( L3 )] / AgOTf [ L3 =(S)‐(−)‐MeO‐biphep], determined through competition experiments, correlate (r²=0.996) with the substituent constants σ. The slope value is −2.30, revealing both the build‐up of positive charge at the allene and electrophilic nature of the reactive Au^I species. Two plausible reaction pathways were investigated by density functional theory calculations, one pathway involving intermolecular nucleophilic addition of free indole to aurated allene intermediate and another pathway involving intramolecular nucleophilic addition of aurated indole to allene via diaurated intermediate E2 . Calculated results revealed that the reaction likely proceeds via the first pathway with a lower activation energy. The role of Au^I Au^I interactions in affecting the enantioselectivity is discussed.     

Mirror Symmetry Breaking and Network Formation in Achiral Polycatenars with Thioether Tail

Mirror symmetry breaking in systems composed of achiral molecules is of importance for the design of functional materials for technological applications as well as for the understanding of the mechanisms of spontaneous emergence of chirality. Herein, we report the design and molecular self-assembly of two series of rod-like achiral polycatenar molecules derived from a π-conjugated 5,5’-diphenyl-2,2’-bithiophene core with a fork-like triple alkoxylated end and a variable single alkylthio chain at the other end. In both series of liquid crystalline materials, differing in the chain length at the trialkoxylated end, helical self-assembly of the π-conjugated rods in networks occurs, leading to wide temperature ranges (>200 K) of bicontinuous cubic network phases, in some cases being stable even around ambient temperatures. The achiral bicontinuous cubic Ia d phase (gyroid) is replaced upon alkylthio chain elongation by a spontaneous mirror symmetry broken bicontinuous cubic phase (I23) and a chiral isotropic liquid phase (Iso₁^[*^]). Further chain elongation results in removing the I23 phase and the re-appearance of the Ia d phase with different pitch lengths. In the second series an additional tetragonal phase separates the two cubic phase types.     

Integrating Achiral and Chiral Organic Ligands in Zero-Dimensional Hybrid Metal Halides to Boost Circularly Polarized Luminescence

Chiral zero-dimensional hybrid metal halides (0D HMHs) could combine excellent optical properties and chirality, making them promising for circularly polarized luminescence (CPL). However, chiral 0D HMHs with efficient CPL have been rarely reported. Here, we propose an efficient strategy to achieve simultaneously high photoluminescence quantum yield (PLQY) and large dissymmetry factor (g_lum), by integrating achiral and chiral ligands into 0D HMHs. Specifically, three pairs of chiral 0D hybrid indium-antimony chlorides are synthesized by combing achiral guanidine with three types of chiral methylbenzylammonium-based derivatives as the organic cations. These chiral 0D HMHs exhibit near-unity PLQY and large g_lum values up to around ±1×10⁻². The achiral guanidine ligand is not only essential to crystallize these hybrid indium-antimony chlorides to achieve near-unity PLQYs, but also greatly enhances the chirality induction from organic ligands to inorganic units in these 0D HMHs. Furthermore, the choice of different chiral ligands can modify the strength of hydrogen bonding interactions in these 0D HMHs, to maximize their g_lum values. Overall, this study provides a robust way to realize efficient CPL in chiral HMHs, expanding their applications in chiroptical fields.     

The nature of the AuI ... AuI Interactions between Cationic [AuL2]+ Complexes in the Solid State

The interaction energy of a [Au{C(NHMe)₂}₂]⁺ ... [Au{C(NHMe)₂}₂]⁺ dimer is investigated using the MP2 method and the LANL2DZ basis set when isolated or embedded in ionic an [Au{C(NHMe)₂}₂]₂anion₂ aggregate, a good model for the environment that these dimers feel in ionic crystals. A repulsive interaction energy is obtained when the dimer is isolated. However, it is possible to find short Au^I ... Au^I separations in [Au{C(NHMe)₂}₂]₂anion₂ aggregates, because in these aggregates the sum of the cation ... anion interactions overweight the sum of the cation ... cation plus anion...anion interactions. This explains why short Au^I ... Au^I separations are found in ionic crystals. The Au^I ... Au^I interaction found in [Au{C(NHMe)₂}₂]₂ anion₂ aggregates shows the same features observed in energetically stable dimers presenting Au^I... Au^I bonds. This makes appropriate to use the name counterion-mediated bonds for the Au^I... Au^I interactions found in [Au{C(NHMe)₂}₂]₂ anion₂ aggregates and ionic crystals.     

Chiroptical Amplification of Induced Circularly Polarized Luminescence in Nucleotide-Templated Supramolecular Polymer

Efficient circularly polarized luminescence (CPL) from purely organic molecules holds great promise for applications in displays, sensing, and bioimaging. However, achieving high dissymmetry values (g_lum) from organic chromophores remains a significant challenge. Herein, we present a bioinspired approach using adenosine triphosphate (ATP)-triggered supramolecular polymerization of a naphthalene diimide-derived monomer ( ANSG ) to induce CPL with a remarkable g_lum value of 1.1×10⁻². The ANSG molecules undergo a templated, chiral self-assembly through a cooperative growth mechanism in the presence of ATP, resulting in scrolled nanotubes with aggregation-induced enhanced emission (AIEE) and induced CPL. Furthermore, we demonstrate the concept of chiroptical amplification of induced CPL by efficiently amplifying asymmetry using a mixture of chiral ATP and achiral pyrophosphate. This innovative approach opens numerous opportunities in the emerging field of circularly polarized luminescence.     

Near-Infrared-Absorbing Chiral Open [60]Fullerenes

Though [60]fullerene is an achiral molecular nanocarbon with I_h symmetry, it could attain an inherent chirality depending upon a functionalization pattern. The conventional chiral induction of C₆₀ relies mainly upon a multiple addition affording a mixture of achiral and chiral isomers while their chiral function would be largely offset by the existence of pseudo-mirror plane(s). These are major obstacles to proceed further study on fullerene chirality and yet leave its understanding elusive. Herein, we showcase a carbene-mediated synthesis of C₁-symmetric chiral open [60]fullerenes showing an intense far-red to near-infrared absorption. The large dissymmetry factor of |g_abs|=0.12 was achieved at λ=820 nm for circular dichroism in benzonitrile. This is, in general, unachievable by other small chiral organic molecules, demonstrating the potential usage of open [60]fullerenes as novel types of chiral chromophores.     

A Chiral Gold Nanocluster Au20 Protected by Tetradentate Phosphine Ligands

The chirality of a gold nanocluster can be generated from either an intrinsically chiral inorganic core or an achiral inorganic core in a chiral environment. The first structural determination of a gold nanocluster containing an intrinsic chiral inorganic core is reported. The chiral gold nanocluster [Au₂₀(PP₃)₄]Cl₄ (PP₃=tris(2‐(diphenylphosphino)ethyl)phosphine) has been prepared by the reduction of a gold(I)–tetraphosphine precursor in dichloromethane solution. Single‐crystal structural determination reveals that the cluster molecular structure has C₃ symmetry. It consists of a Au₂₀ core consolidated by four peripheral tetraphosphines. The Au₂₀ core can be viewed as the combination of an icosahedral Au₁₃ and a helical Y‐shaped Au₇ motif. The identity of this Au₂₀ cluster is confirmed by ESI‐MS. The chelation of multidentate phosphines enhances the stability of this Au₂₀ cluster.     

From Symmetry Breaking to Unraveling the Origin of the Chirality of Ligated Au13Cu2 Nanoclusters

A general method, using mixed ligands (here diphosphines and thiolates) is devised to turn an achiral metal cluster, Au₁₃Cu₂, into an enantiomeric pair by breaking (lowering) the overall molecular symmetry with the ligands. Using an achiral diphosphine, a racemic [Au₁₃Cu₂(DPPP)₃(SPy)₆]⁺ was prepared which crystallizes in centrosymmetric space groups. Using chiral diphosphines, enantioselective synthesis of an optically pure, enantiomeric pair of [Au₁₃Cu₂((2r,4r)/(2s,4s)‐BDPP)₃(SPy)₆]⁺ was achieved in one pot. Their circular dichroism (CD) spectra give perfect mirror images in the range of 250–500 nm with maximum anisotropy factors of 1.2×10^?3. DFT calculations provided good correlations with the observed CD spectra of the enantiomers and, more importantly, revealed the origin of the chirality. Racemization studies show high stability (no racemization at 70 °C) of these chiral nanoclusters, which hold great promise in applications such as asymmetry catalysis.     

Bimetallic Catalytic Asymmetric Tandem Reaction of β‐Alkynyl Ketones to Synthesize 6,6‐Spiroketals

The enantioselective tandem reaction of β,γ‐unsaturated α‐ketoesters with β‐alkynyl ketones was realized by a bimetallic catalytic system of achiral Au^ΙΙΙ salt and chiral N,N′‐dioxide‐Mg^ΙΙ complex. The cycloisomerization of β‐alkynyl ketone and asymmetric intermolecular [4+2] cycloaddition with β,γ‐unsaturated α‐ketoesters subsequently occurred, providing an efficient and straightforward access to chiral multifunctional 6,6‐spiroketals in up to 97 % yield, 94 % ee and >19/1 d.r. Besides, a catalytic cycle was proposed based on the results of control experiments.     

Temperature Control of Sequential Nucleation–Growth Mechanisms in Hierarchical Supramolecular Polymers

Upon cooling in solution, chiral triarylamine tris-amide unimers produce organogels by stacking into helical supramolecular polymers, which subsequently bundle into larger fibers. Interestingly, circular dichroism, vibrational circular dichroism, and AFM imaging of the chiral self-assemblies revealed that monocolumnar P-helical fibrils formed upon fast cooling, whereas bundled M-superhelical fibers formed upon slow cooling. The mechanistic study of this structural bifurcation reveals the presence of a strong memory effect, reminiscent of a complex stepwise combination of primary and secondary nucleation-growth processes. These results highlight the instrumental role of sequential self-assembly processes to control supramolecular architectures of multiple hierarchical order.     

Electronic Circular Dichroism-Circularly Polarized Raman (eCP-Raman): A New Form of Chiral Raman Spectroscopy

This Concept article summarizes recent work on the development of a new form of chiral Raman spectroscopy, e CP-Raman, which combines two spectroscopies: electronic circular dichroism (ECD) and circularly polarized Raman (CP-Raman). First, some puzzling observations while carrying out Raman optical activity (ROA) measurements of several transition metal complexes under resonance are described, as well as the search for the mechanisms responsible. Then an equation for quantifying the e CP-Raman contribution is presented, followed by several examples of how e CP-Raman influences the I_R−I_L spectra of achiral and chiral solvent molecules and of a number of chiral solutes under resonance. The conditions to extract resonance ROA, when the e CP-Raman contribution is minimized, are also discussed. Finally, we comment on the potential applications of e CP-Raman.     

Stereoselective Plasmonic Interaction in Peptide-tethered Photopolymerizable Diacetylenes Doped with Chiral Gold Nanoparticles

Designing polymeric systems with ultra-high optical activity is instrumental in the pursuit of smart artificial chiroptical materials, including the fundamental understanding of structure/property relations. Herein, we report a diacetylene ( DA ) moiety flanked by chiral D - and L -FF dipeptide methyl esters that exhibits efficient topochemical photopolymerization in the solid phase to furnish polydiacetylene ( PDA ) with desired control over the chiroptical properties. The doping of the achiral gold nanoparticles provides plasmonic interaction with the PDA s to render asymmetric shape to the circular dichroism bands. With the judicious design of the chiral amino acid ligand appended to the AuNPs, we demonstrate the first example of selective chiral amplification mediated by stereo-structural matching of the polymer-plasmonic AuNP hybrid pairs. Such ordered self-assembly aided by topochemical polymerization in peptide-tethered PDA provides a smart strategy to produce soft responsive materials for applications in chiral photonics.