Chirality control of a single carbene molecule by tip-induced van der Waals interactions

Nanoscale Manipulation of Single-Molecule Conformational Transition through Vibrational Excitation

Controlling molecular actions on demand is a critical step toward developing single-molecule functional devices. Such control can be achieved by manipulating the interactions between individual molecules and their nanoscale environment. In this study, we demonstrate the conformational transition of a single pyrrolidine molecule adsorbed on a Cu(100) surface, driven by vibrational excitation through tunneling electrons using scanning tunneling microscopy. We identify multiple transition pathways between two structural states, each governed by distinct vibrational modes. The nuclear motions corresponding to these modes are elucidated through density functional theory calculations. By leveraging fundamental forces, including van der Waals interactions, dipole-dipole interactions, and steric hindrance, we precisely tune the molecule-environment coupling. This tuning enables the modulation of vibrational energies, adjustment of transition probabilities, and selection of the lowest-energy transition pathway. Our findings highlight how tunable force fields in a nanoscale cavity can govern molecular conformational transitions, providing a pathway to engineer molecule-environment interactions for targeted molecular functionalities.

Enantiopure molecules form apparently racemic monolayers of chiral cyclic pentamers

Ultra-high vacuum scanning tunneling microscopy (UHV-STM) was used to investigate two related molecules pulse-deposited onto Au(111) surfaces: indoline-2-carboxylic acid and proline (pyrrolidine-2-carboxylic acid). Indoline-2-carboxylic acid and proline form both dimers and C5-symmetric "pinwheel" pentamers. Enantiomerically pure S-(-)-indoline-2-carboxylic acid and S-proline were used, and the pentamer structures observed for both were chiral. However, the presence of apparently equal numbers of 'right-' and 'left-handed' pinwheels is contrary to the general understanding that the chirality of the molecule dictates supramolecular chirality. A variety of computational methods were used to elucidate pentamer geometry for S-proline. Straightforward geometry optimization proved difficult, as the size of the cluster and the number of possible intermolecular interactions produced an interaction potential with multiple local minima. Instead, the Amber force field was used to exhaustively search all of phase space for chemically reasonable pentamer structures, producing a limited number of candidate structures that were then optimized as gas-phase clusters using density functional theory (DFT). The binding energies of the two lowest-energy pentamers on the Au(111) surface were then calculated by plane-wave DFT using the VASP software, and STM images predicted. These calculations indicate that the right- and left-handed pentamers are instead two different polymorphs.

Chirality Determination of Nanocrystals by Electron Crystallography

Chirality is a common phenomenon in nature and plays an important role in the properties of matter. The rational synthesis of chiral compounds and exploration of their applications in various fields require an unambiguous determination of their handedness. However, in many cases, determinations of the chiral crystal structure and chiral morphology have been a challenging task due to the lack of proper characterization methods, especially for nanosized crystals. Therefore, it is crucial to develop novel and efficient characterization methods. Owing to the strong interactions between matter and electrons, electron crystallography has become a powerful tool for structural analysis of nanomaterials. In recent years, methods based on electron crystallography, such as high-resolution electron microscopy imaging and electron diffraction, have been developed to unravel the chirality of nanomaterials. This brings new opportunities to the design, synthesis, and applications of versatile chiral nanomaterials. In this perspective, we summarize the recent methodology developments and ongoing research of electron crystallography for chiral structure and morphology determination of nanocrystals, including inorganic and organic materials, as well as highlight the potential and further improvement of these methods in the future.

Structural and Electronic Chirality in Inorganic Crystals: from Construction to Application

Chirality represents a fundamental characteristic inherent in nature, playing a pivotal role in the emergence of homochirality and the origin of life. While the principles of chirality in organic chemistry are well-documented, the exploration of chirality within inorganic crystal structures continues to evolve. This ongoing development is primarily due to the diverse nature of crystal/amorphous structures in inorganic materials, along with the intricate symmetrical and asymmetrical relationships in the geometry of their constituent atoms. In this review, we commence with a summary of the foundational concept of chirality in molecules and solid states matters. This is followed by an introduction of structural chirality and electronic chirality in three-dimensional and two-dimensional inorganic materials. The construction of chirality in inorganic materials is classified into physical photolithography, wet-chemistry method, self-assembly, and chiral imprinting. Highlighting the significance of this field, we also summarize the research progress of chiral inorganic materials for applications in optical activity, enantiomeric recognition and chiral sensing, selective adsorption and enantioselective separation, asymmetric synthesis and catalysis, and chirality-induced spin polarization. This review aims to provide a reference for ongoing research in chiral inorganic materials and potentially stimulate innovative strategies and novel applications in the realm of chirality.

Formation and Manipulation of Diatomic Rotors at the Symmetry-Breaking Surfaces of a Kagome Superconductor

Construction of diatomic rotors, which is crucial for artificial nanomachines, remains challenging due to surface constraints and limited chemical design. Here we report the construction of diatomic Cr-Cs and Fe-Cs rotors where a Cr or Fe atom switches around a Cs atom at the Sb surface of the newly discovered kagome superconductor CsV3Sb5. The switching rate is controlled by the bias voltage between the rotor and scanning tunneling microscope (STM) tip. The spatial distribution of rates exhibits C2 symmetry, possibly linked to the symmetry-breaking charge orders of CsV3Sb5. We have expanded the rotor construction to include different transition metals (Cr, Fe, V) and alkali metals (Cs, K). Remarkably, designed configurations of rotors are achieved through STM manipulation. Rotor orbits and quantum states are precisely controlled by tuning the inter-rotor distance. Our findings establish a novel platform for the controlled fabrication of atomic motors on symmetry-breaking quantum materials, paving the way for advanced nanoscale devices.

Self-Assembly of the Chiral Donor-Acceptor Molecule DCzDCN on Cu(100)

Donor-acceptor (D-A) structured molecules are essential components of organic electronics. The respective molecular structures of these molecules and their synthesis are primarily determined by the intended area of application. Typically, D-A molecules promote charge separation and transport in organic photovoltaics or organic field-effect transistors. D-A molecules showing a larger twist angle between D and A units are, e.g., essential for the development of high internal quantum efficiency in organic light-emitting diodes. A prototypical molecule of this D-A type is DCzDCN (5-(4,6-diphenyl-1,3,5-triazine-2-yl)benzene-1,3-dinitrile). In most cases, these molecules are only investigated regarding their electronic and structural interaction in bulk aggregates but not in ultrathin films supported by a metallic substrate. Here, we present growth and electronic structure studies of DCzDCN on a Cu(100) surface. We used a complementary approach through the use of scanning tunneling microscopy/spectroscopy (STM and STS), ultraviolet/inverse photoemission spectroscopy (UPS and IPES), and single-molecule density functional theory (DFT) calculations. This method combination enabled us to investigate the adsorption geometry (STM) and the local electronic states near the Fermi energy (EF) of a single adsorbed molecule (using STS) and to compare these data with the integral overall electronic structure of the DCzDCN/Cu(100) interface (using UPS/IPES). The orientation of the molecules with the donor part toward the substrate results in a chiral resolution at the interface due to the molecular as well as the substrate symmetry and additional strong molecular electrostatic forces induced by the charge distribution of the twisted dicarbonitrile part. Thus, the formation of various bulk-unlike homochiral structures and the appearance of hybrid interface states modify the molecular electronic properties of the DCzDCN/Cu(100) system, e.g., the transport gap by -1.3 eV compared to that of a single DCzDCN molecule. This may be useful not only for optoelectronic applications but also in organic spintronics.