Chirality Transfer in Gold Nanoclusters: Insights from Chiral Spectroscopy and Theoretical Modeling

In a recent study, we found that nematic liquid crystals (N-LCs) doped with chiral (S)-naproxen-functionalized dodecane thiolate protected gold nanoclusters (Au2, Au3) or non-chiral alkyl thiolate protected Au clusters (Au1) produce thin film textures with characteristic uniform stripe patterns separated by areas of homeotropic alignment. While these textures closely resemble textures commonly observed for chiral nematic (N*-)phases with large helical pitch, so-called cholesteric finger textures, they originate from local concentration differences of the nanoclusters in the N-LC solvent. While areas with higher particle content form linear particle aggregates (stripe domains) due to the surface anchoring of the N-LC molecules to the cluster surface, areas of lower particle concentration give homeotropic alignment as a result of particles residing at the glass–N-LC interfaces. To elucidate and confirm a chirality transfer from the chirally modified gold clusters to the non-chiral N-LC, despite the complex thin film textures, we here present detailed induced circular dichroism (ICD) studies of thin films of 5CB doped with the three different Au clusters. These experiments revealed that the chiral Au nanoclusters (Au2, Au3) successfully transfer chirality to the N-LC host producing a chiral nematic phase (N*) with the opposite helical sense in comparison to the pure, organic chiral dopant dispersed in the same N-LC host. Thus, these results provide the first experimental proof for the usefulness of gold nanoclusters as chiral dopants for N-LCs. In contrast, for the non-chiral Au cluster, at a macroscopic level, no relationship between the cholesteric finger-like textures and chirality was found.

Magnetic coordination complexes based on Schiff bases are promising new molecular materials for electronics. Two μ‐oxo FeIII dimeric complexes of enantiomers of Schiff base ligands (N,N′‐(1R,2R)‐1,2‐diphenylethylenebis(salicylideneimine) (H2salphen‐R, 1 a) and N,N′‐(1S,2S)‐1,2‐diphenylethylenebis(salicylideneimine) (H2salphen‐S, 1 b)) were synthesized; further reaction with 4‐salicylideneamino‐1,2,4‐triazole (Hsaltrz) led to enantiomers of two one‐dimensional (1D) FeIII coordination complexes. The structures of these complexes were determined by X‐ray diffraction. Magnetic susceptibility measurements revealed that μ‐oxo dimeric complexes displayed strong antiferromagnetic coupling, whereas the 1D complexes exhibited paramagnetic behavior. The chirality of the Schiff bases transferred to macromolecular chirality of the complexes, which could be monitored by both vibrational and electronic circular dichroism (CD) spectroscopic methods. The macroscopic handedness was manifested in CD signals attributed to exciton coupling between the ligands. Thus, the chiral spectroscopies were useful to probe the chirality of the complexes, their structure, and the polymerization degree.

Phase transfer of aqueous racemic penicillamine-protected gold nanoclusters into chloroform is achieved by hydrophobizing the anionic nanocluster surface with a chiral ephedrinium cation (that is, chiral phase transfer), yielding asymmetric transformation or symmetry breaking of the optically inactive gold nanoclusters: The phase-transferred nanoclusters display appreciable optical activity in the metal-based electronic transition region. The ephedrinium cation contains chiral substituents with R,S absolute configuration, so that “nanocluster diastereomers” also can be produced via the phase transfer of enantiopure (S)- or (R)-penicillamine-protected gold nanoclusters with the chiral cations. Upon phase transfer, absorption spectra of the gold nanoclusters are almost invariant, whereas their circular dichroism (CD) signals exhibit a significant change in the metal-based electronic transition region. On the basis of the optical/chiroptical responses of the asymmetrically transformed gold nanoclusters as well as model quantum chemical calculations, the induced optical activity is most likely due to the ligand dissymmetric field brought about by the surface stereostructures.

Due to the interest in the origin of life and the need to synthesize new functional materials, the study of the origin of chirality has been given significant attention. The mechanism of chirality transfer at molecular and supramolecular levels remains underexplored. Herein, we study the mechanism of chirality transfer of N, N’-bis (octadecyl)-L-/D-(anthracene-9-carboxamide)-glutamic diamide (L-/D-GAn) supramolecular chiral self-assembled at the air/water interface by chiral sum-frequency generation vibrational spectroscopy (chiral SFG) and molecular dynamics (MD) simulations. We observe long-range chirality transfer in the systems. The chirality of Cα-H is transferred first to amide groups and then transferred to the anthracene unit, through intermolecular hydrogen bonds and π-π stacking to produce an antiparallel β-sheet-like structure, and finally it is transferred to the end of hydrophobic alkyl chains at the interface. These results are relevant for understanding the chirality origin in supramolecular systems and the rational design of supramolecular chiral materials.

NADH mimics on diacetone- d-glucose: Stereoselective biomimetic reduction of benzoylformate and interpretation of chirality transfer deduced by molecular orbital approach

In recent years, gold‐mediated reactions have attracted considerable interests and witnessed explosive developments. Along with this trend, corresponding theoretical modelings are playing increasingly important role in the mechanism study of gold‐mediated reactions. In this chapter, we briefly discuss some recent advances in understanding of gold‐mediated reactions from theoretical modeling. Relevant theoretical methods as well as some technical issues for computational treatment of heavy element gold are introduced. As a key point in many homogenous gold‐mediated reactions that involve single metal site, substrate‐gold interaction is examined from theoretical point of view. Then the mechanisms of some typical homogenous gold‐mediated reactions are discussed. Concerning gold‐mediated reactions involving more than single metal site, CO oxidation by gold nanoclusters is discussed, with focus on structures of gold clusters and influencing factors for catalytic activities towards CO oxidation.

An efficient strategy for high‐performance chiral materials is to design and synthesize host molecules with left‐ and right‐ (M‐ and P‐)twisted conformations and to control their twisted conformations. For this, a quantitative analysis is required to describe the chiroptical inversion, chiral transfer, and chiral recognition in the host‐guest systems, which is generally performed using circular dichroism (CD) and/or proton nuclear magnetic resonance (1H NMR) spectroscopies. However, the mass‐balance model that considers the M‐ and P‐twisted conformations has not yet been established. In this study, we derived the novel equations based on the mass‐balance model for the 1 : 1 host‐guest systems. Then, we further applied them to analyze the 1 : 1 host‐guest systems for the achiral calixarene‐based capsule molecule, achiral dimeric zinc porphyrin tweezer molecule, and chiral pillar[5]arene with the chiral and/or achiral guest molecules by using the data obtained from the CD titration, variable temperature CD (VT−CD), and 1H NMR experiments. The thermodynamic parameters (ΔH and ΔS), equilibrium constants (K), and molar CD (Δϵ) in the 1 : 1 host‐guest systems could be successfully determined by the theoretical analyses using the derived equations.

Chirality expression modifications occurring for a simple chiral amino-alcohol, d-Alaninol, adsorbed on Cu(100), when passing from low to high molecular coverage, are studied by means of scanning tunneling microscopy (STM) and theoretical modeling. At low coverage, d-Alaninol molecules organize in tetramers that are aligned to Cu(100) unit vectors whereas, increasing the molecular amount on the surface, the interplay of supramolecularity and chirality induces a rotation of the tetramers of 14° with respect to the substrate lattice vectors. This behaviour is analyzed by means of a theoretical approach that combines classical molecular dynamics (MD) and density functional theory (DFT). The force field adopted in MD calculations is parametrized by fitting it to results obtained at DFT level on a single adsorbed alaninol molecule on the Cu(100) surface. The configurations of isolated and surrounded tetramers most sampled by MD are extracted by Principal Component Analysis. Further DFT relaxation with semiempirical dispersion correction (DFT-D) and considering also the possible dehydrogenation of both functional groups of alaninol, give rise to a few structures that are discussed in terms of their energetics and by comparing the resulting simulated STM images with the experimental ones.

An efficient strategy for high-performance chiral materials is to design and synthesize host molecules with left- and right- (M- and P-)twisted conformations and to control their twisted conformations. For this, a quantitative analysis is required to describe the chiroptical inversion, chiral transfer, and chiral recognition in the host-guest systems, which is generally performed using circular dichroism (CD) and/or proton nuclear magnetic resonance (1H NMR) spectroscopies. However, the mass-balance model that considers the M- and P-twisted conformations has not yet been established. In this study, we derived the novel equations based on the mass-balance model for the 1 : 1 host-guest systems. Then, we further applied them to analyze the 1 : 1 host-guest systems for the achiral calixarene-based capsule molecule, achiral dimeric zinc porphyrin tweezer molecule, and chiral pillar[5]arene with the chiral and/or achiral guest molecules by using the data obtained from the CD titration, variable temperature CD (VT-CD), and 1H NMR experiments. The thermodynamic parameters (ΔH and ΔS), equilibrium constants (K), and molar CD (Δϵ) in the 1 : 1 host-guest systems could be successfully determined by the theoretical analyses using the derived equations.

The transfer of chirality from one set of molecules to another is fundamental for applications in chiral technology and has likely played a crucial role for establishing homochirality on earth. Here we show that an intrinsically chiral gold cluster can transfer its handedness to an achiral molecule adsorbed on its surface. Solutions of chiral Au38(2-PET)24 (2-PET=2-phenylethylthiolate) cluster enantiomers show strong vibrational circular dichroism (VCD) signals in vibrations of the achiral adsorbate. Density functional theory (DFT) calculations reveal that 2-PET molecules adopt a chiral conformation. Chirality transfer from the cluster to the achiral adsorbate is responsible for the preference of one of the two mirror images. Intermolecular interactions between the adsorbed molecules on the crowded cluster surface seem to play a dominant role for the phenomena. Such chirality transfer from metals to adsorbates likely plays an important role in heterogeneous enantioselective catalysis.

Insights into the imprint of biomolecular chirality on the hydration water of biomolecules are crucial for comprehending biomolecular function and the molecular origin of biological homochirality in life, yet they remain poorly understood. In this study, we successfully constructed chiral water superstructures templated by a chiral biomolecular assembly under ambient conditions and performed the first investigation of its ultrafast vibrational dynamics by utilizing femtosecond time-resolved chiral sum frequency generation vibrational spectroscopy (SFG-VS). Chiral biomolecular templates were prepared by assembling achiral amphiphilic molecules into chiral superstructures with C1 point group symmetry at the lipid membrane surface. The formation of chiral water superstructure is dictated by the amide moieties of the chiral biomolecular assembly, particularly by the N-H group with the C1 point group. The vibrational relaxation time of chiral water (0.8 ± 0.1 ps) is identical to that of the N-H group but significantly longer than that of both bulk water and typical interfacial achiral water (≤ 0.3 ps). This indicates that chiral water strongly couples with the N-H vibrational mode but decouples from nearby achiral interfacial water and bulk water. Chiral water exhibits distinctly different behavior from achiral water. Our findings highlight the significant role of chiral water in vibrational energy redistribution and propagation within biomolecules and contribute to understanding the mechanism of chirality generation and transfer.

The infrared vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of methyl lactate were measured in the 1000-1800 cm(-1) region in the CCl(4) and H(2)O solvents, respectively. In particular, the chirality transfer effect, i.e. the H-O-H bending bands of the achiral water subunits that are hydrogen-bonded to the methyl lactate molecule exhibit substantial VCD strength, was detected experimentally. A series of density functional theory calculations using B3PW91 and B3LYP functionals with 6-311++G(d,p) and aug-cc-pVTZ basis sets were carried out to simulate the VA and VCD spectra of the methyl lactate monomer and the methyl lactate-(H(2)O)(n) complexes with n = 1, 2, 3. The population weighted VA and VCD spectra of the methyl lactate monomer are in good agreement with the experimental spectra in CCl(4). Implicit polarizable continuum model was found to be inadequate to account for the hydrogen-bonding effect in the observed VA and VCD spectra in H(2)O. The methyl lactate-(H(2)O)(n) complexes with n = 1, 2, 3 were used to model the explicit hydrogen-bonding. The population weighted VA and VCD spectra of the methyl lactate-H(2)O binary complex are shown to capture the main spectral features in the observed spectra in aqueous solution. The theoretical modeling shows that the extent of chirality transfer depends sensitively on the specific binding sites taken by the achiral water molecules. The observation of chirality transfer effect opens a new spectral window to detect and to model the hydrogen-bonding solvent effect on VCD spectra of chiral molecules.

Surface-enhanced Raman optical activity (SEROA) is a new technique combining the sensitivity of the surface-enhanced Raman scattering (SERS) with the detailed information about molecular structure provided by the chiral spectroscopies. So far, experimental SEROA spectra have been reported in several studies, but the interpretation and theoretical background are rather limited. In this work, general expressions for the electromagnetic contribution to SEROA are derived using the matrix polarization theory and used to investigate the enhancement in model systems. The results not only reveal a strong dependence of the enhancement on the distance between the molecule and a metal part but also the dependence of the ratio of ROA and Raman intensities (circular intensity difference, CID) on the distance and rotational averaging. For a ribose model, an optimal molecule-colloid distance was predicted which provided the highest CIDs. However, the CID maximum disappeared after a rotational averaging. For cysteine zwitterion, the simulated SEROA and SERS spectra provided a qualitative agreement with previous experiments.

Amplificationof chirality across length scales is a key conceptpertinent to many models aiming to unravel the origin of homochirality.Tactoids of lyotropic chromonic liquid crystals formed by DNA, dyes,and other flat ionic molecules in water in the biphasic nematic +isotropic regime turn out to be a particularly relevant system toinvestigate chirality transfer and amplification. Herein, we presentexperiments to determine the amplification of chirality by luminescentgold nanoclusters decorated with adenosine monophosphate inducingchiral nematic tactoids formed by disodium cromoglycate in water.Polarized optical microscopy investigations of the induced homochiraltactoids reveal that adenosine monophosphate shows a higher opticalactivity when bound to the surface of such gold nanoclusters in comparisonto free adenosine monophosphate, despite a three-time lower overallconcentration. Free adenosine monophosphate also induces the oppositechiral twist both in the bulk nematic phase as shown by induced thinfilm circular dichroism spectropolarimetry and in the tactoids incomparison to adenosine monophosphate bound to the gold nanocluster.Overall, these experiments demonstrate that lyotropic chromonic liquidcrystal tactoids are powerful systems to image and quantify chiralityamplification by key biological chiral molecules that would have playeda role in the origin of homochirality.

Asymmetric arrangement of metal atoms is crucial for understanding the chirality origin of chiral metal nanoclusters and facilitating the design and development of new chiral catalysts and chiroptical devices. Here, we describe the construction of four asymmetric gold and gold-silver clusters by chirality transfer from diimido ligands. The acquired metal clusters show strong circular dichroism (CD) response with large anisotropy factors of up to 6 × 10(-3), larger than the values of most reported chiral gold nanoclusters. Regardless of the same absolute configuration of the applied three diimido ligands, sigmoidal and reverse-sigmoidal arrangements of gold atoms both can be achieved, which resultantly produce an opposite Cotton effect within a specific absorption range. On the basis of the detailed structural characterization via X-ray crystallography and contrast experiments, the chirality contribution of the imido ligand, the asymmetrically arranged metal cluster, and the chiral arrangement of aromatic rings of phosphine ligands have been qualitatively evaluated. Time-dependent DFT calculations reveal that the chiroptical property of the acquired metal clusters is mainly influenced by the asymmetrically arranged metal atoms. Correlation of asymmetric arrangements of metal atoms in clusters with their chiroptical response provides a viable means of fabricating a designable chiral surface of metal nanoclusters and opens a broader prospect for chiral cluster application.