Dynamics of optical vortices in 2D materials

Optical vortices in planar geometries are a universal wave phenomenon, where electromagnetic waves possess topologically protected integer values of orbital angular momentum (OAM). The conservation of OAM governs their dynamics, including their rules of creation and annihilation. However, such dynamics remained so far beyond experimental reach. Here, we present a first observation of creation and annihilation of optical vortex pairs. The vortices conserve their combined OAM during pair creation/annihilation events and determine the field profile throughout their motion between these events. We utilize free electrons in an ultrafast transmission electron microscope to probe the vortices, which appear in the form of phonon polaritons in the 2D material hexagonal boron nitride. These results provide the first observation of optical vortices in any 2D material, which were predicted but never observed. Our findings promote future investigation of vortices in 2D materials and their use for chiral plasmonics, toward the control of selection rules in light-matter interactions and the creation of optical simulators of phase transitions in condensed matter physics.

Shifts in Backbone Conformation of Acetylcholinesterases upon Binding of Covalent Inhibitors, Reversible Ligands and Substrates

The influence of ligand binding to human, mouse and Torpedo californica acetylcholinesterase (EC 3.1.1.7; AChE) backbone structures is analyzed in a pairwise fashion by comparison with X-ray structures of unliganded AChEs. Both complexes with reversible ligands (substrates and inhibitors) as well as covalently interacting ligands leading to the formation of covalent AChE conjugates of tetrahedral and of trigonal-planar geometries are considered. The acyl pocket loop (AP loop) in the AChE backbone is recognized as the conformationally most adaptive, but not necessarily sterically exclusive, structural element. Conformational changes of the centrally located AP loop coincide with shifts in C-terminal α-helical positions, revealing interacting components for a potential allosteric interaction within the AChE backbone. The stabilizing power of the aromatic choline binding site, with the potential to attract and pull fitting entities covalently tethered to the active Ser, is recognized. Consequently, the pull can promote catalytic reactions or relieve steric pressure within the impacted space of the AChE active center gorge. These dynamic properties of the AChE backbone inferred from the analysis of static X-ray structures contribute towards a better understanding of the molecular template important in the structure-based design of therapeutically active molecules, including AChE inhibitors as well as reactivators of conjugated, inactive AChE.

Superior robustness of anomalous non-reciprocal topological edge states

Robustness against disorder and defects is a pivotal advantage of topological systems1, manifested by the absence of electronic backscattering in the quantum-Hall2 and spin-Hall effects3, and by unidirectional waveguiding in their classical analogues4,5. Two-dimensional (2D) topological insulators4–13, in particular, provide unprecedented opportunities in a variety of fields owing to their compact planar geometries, which are compatible with the fabrication technologies used in modern electronics and photonics. Among all 2D topological phases, Chern insulators14–25 are currently the most reliable designs owing to the genuine backscattering immunity of their non-reciprocal edge modes, brought via time-reversal symmetry breaking. Yet such resistance to fabrication tolerances is limited to fluctuations of the same order of magnitude as their bandgap, limiting their resilience to small perturbations only. Here we investigate the robustness problem in a system where edge transmission can survive disorder levels with strengths arbitrarily larger than the bandgap—an anomalous non-reciprocal topological network. We explore the general conditions needed to obtain such an unusual effect in systems made of unitary three-port non-reciprocal scatterers connected by phase links, and establish the superior robustness of anomalous edge transmission modes over Chern ones to phase-link disorder of arbitrarily large values. We confirm experimentally the exceptional resilience of the anomalous phase, and demonstrate its operation in various arbitrarily shaped disordered multi-port prototypes. Our results pave the way to efficient, arbitrary planar energy transport on 2D substrates for wave devices with full protection against large fabrication flaws or imperfections.

In Silico Studies on Selected Neutral Molecules, CGa2Ge2, CAlGaGe2, and CSiGa2Ge Containing Planar Tetracoordinate Carbon

Density functional theory (DFT) was used to study the structure, stability, and bonding in some selected neutral pentaatomic systems, viz., CGa2Ge2, CAlGaGe2, and CSiGa2Ge containing planar tetracoordinate carbon. The systems are kinetically stable, as predicted from the ab initio molecular dynamics simulations. The natural bond orbital (NBO) analysis showed that strong electron donation occurs to the central planar carbon atom by the peripheral atoms in all the studied systems. From the nucleus independent chemical shift (NICS) analysis, it is shown that the systems possess both σ- and π- aromaticity. The presence of 18 valence electrons in these systems, in their neutral form, appears to be important for their stability with planar geometries rather than tetrahedral structures. The nature of bonding is understood through the adaptive natural density partitioning analysis (AdNDP), quantum theory of atoms in molecules (QTAIM) analysis, and also via Wiberg bond index (WBI) and electron localization function (ELF).

Magnetic Shielding Study of Bonding and Aromaticity in Corannulene and Coronene

Bonding and aromaticity in the bowl-shaped C5v and planar D5h geometries of corannulene and the planar D6h geometry of coronene are investigated using 3D isosurfaces and 2D contour plots of the isotropic magnetic shielding σiso(r) and, for planar geometries, of the out-of-plane component of the shielding tensor σzz(r). Corannulene and coronene both feature conjoined shielded “doughnuts” around a peripheral six-membered carbon ring, suggesting strong bonding interactions and aromatic stability; a deshielded region inside the hub ring of corannulene indicates that this ring is antiaromatic, more so in planar corannulene. The switch from the planar to the bowl-shaped geometry of corannulene is shown to enhance both bonding and the local aromaticities of the five- and six-membered rings; these factors, in addition to ring strain reduction, favour the bowl-shaped geometry. The most and least shielded bonds in both corannulene and coronene turn out to be the spoke and hub bonds, respectively. The higher π electron activity over spoke bonds in planar corannulene and coronene is supported by σzz(r) contour plots in planes 1 Å above the respective molecular planes; these findings about spoke bonds are somewhat unexpected, given that ring current studies indicate next to no currents over spoke bonds.

Design of Functionalized Lobed Particles for Porous Self-Assemblies

Colloidal particles fabricated with anisotropic interactions have emerged as building blocks for designing materials with various nanotechnological applications. We used coarse-grained Langevin dynamics simulations to probe the morphologies of self-assembled structures formed by lobed particles decorated with functional groups. We tuned the interactions between the functional groups to investigate their effect on the porosity of self-assembled structures formed by lobed particles with different shapes (snowman, dumbbell, trigonal planar, tetrahedral, square planar, trigonal bipyramidal, and octahedral) at different temperatures. The dumbbell, trigonal planar, and square planar shaped particles, with planar geometries, form self-assembled structures including elongated chains, honeycomb sheets, and square sheets, respectively. The particles with non-planar geometries (tetrahedral, trigonal bipyramidal, and octahedral) self-assemble into random aggregate morphologies. The structures formed by trigonal bipyramidal and octahedral particles exhibit smaller and homogeneous pores compared to the structures formed by trigonal planar and square planar particles. The porosity in self-assembled structures is substantially enhanced by the functionalization of particles.

Gold Derivatives Development as Prospective Anticancer Drugs for Breast Cancer Treatment

Commonly used anticancer drugs are cisplatin and other platinum-based drugs. However, the use of these drugs in chemotherapy causes numerous side effects and the onset of frequent drug resistance phenomena. This review summarizes the most recent results on the gold derivatives used for their significant inhibitory effects on the in vitro proliferation of breast cancer cell models and for the consequences deriving from morphological changes in the same cells. In particular, the study discusses the antitumor activity of gold nanoparticles, gold (I) and (III) compounds, gold complexes and carbene-based gold complexes, compared with cisplatin. The results of screening studies of cytotoxicity and antitumor activity for the gold derivatives show that the death of cancer cells can occur intrinsically by apoptosis. Recent research has shown that gold (III) compounds with square planar geometries, such as that of cisplatin, can intercalate the DNA and provide novel anticancer agents. The gold derivatives described can make an important contribution to expanding the knowledge of medicinal bioorganometallic chemistry and broadening the range of anticancer agents available, offering improved characteristics, such as increased activity and/or selectivity, and paving the way for further discoveries and applications.