The Electron-Rich and Nucleophilic N-Heterocyclic Imines on Metal Surfaces: Binding Modes and Interfacial Charge Transfer.

The strongly electron-donating N-heterocyclic imines (NHIs) have been employed as excellent surface anchors for the thermodynamic stabilization of electron-deficient species due to their enhanced nucleophilicity. However, the binding mode and interfacial property of these new ligands are still unclear, representing a bottleneck for advanced applications in surface functionalization and catalysis. Here, NHIs with different side groups have been rationally designed, synthesized, and analyzed on various metal surfaces (Cu, Ag). Our results reveal different binding modes depending on the molecular structure and metal surface. The molecular design enables us to achieve a flat-lying or upright configuration and even a transition between these two binding modes depending on the coverage and time. Importantly, the two binding modes exhibit different degrees of interfacial charge transfer between the molecule and the surface. This study provides essential microscopic insight into the NHI adsorption geometry and interfacial charge transfer for the optimization of heterogeneous catalysts in coordination chemistry.

On-surface synthesis of ballbot-type N-heterocyclic carbene polymers.

N-Heterocyclic carbenes (NHCs) are established ligands for metal complexes and surfaces. Here we go beyond monomeric NHCs and report on the synthesis of NHC polymers on gold surfaces, consisting of ballbot-type repeating units bound to single Au adatoms. We designed, synthesized and deposited precursors containing different halogens on gold surfaces under ultrahigh vacuum. Conformational, electronic and charge transport properties were assessed by combining low-temperature scanning tunneling microscopy, non-contact atomic force microscopy, X-ray photoelectron spectroscopy, first-principles calculations and reactive force field simulations. The confirmed ballbot-type nature of the NHCs explains the high surface mobility of the incommensurate NHC polymers, which is prerequisite for their desired spatial alignment. The delicate balance between mobility and polymerization rate allows essential parameters for controlling polymer directionality to be derived. These polymers open up new opportunities in the fields of nanoelectronics, surface functionalization and catalysis.

Thermal conductivity across transition metal dichalcogenide bilayers.

Cross-plane thermal conductivities of transition metal dichalcogenide M X 2 (M = Mo, W; X = S, Se) bilayers are determined by homogeneous nonequilibrium molecular dynamics. The best insulator is found to be W S e 2 : W S e 2 , closely followed by W S 2 : W S 2 and M o S e 2 : W S e 2 . Thermal conductivities of heterobilayers are close to the average of the two corresponding homobilayer values, despite the mass and lattice mismatch. To disentangle the effects of atomic mass, lattice constant, and interaction potential, these three parameters are systematically varied. Phonon spectral analysis provides further insight into their roles and reveals the weak influence of spectral overlap between the two layers and the dominance of boundary scattering. The observed trends can be rationalized using Slack's formula in terms of the average atomic mass and the Debye temperature. Accurate interlayer interaction potentials are developed based on experimental elastic constants. Their effect on the bilayer cross-plane thermal conductivities is found to be minor.

Quantum Chemical Study of the Pressure-dependent Phosphorescence of [Cr(ddpd)2 ]3+ in the Solid State.

The chromium(III) complex [Cr(ddpd)2 ][BF4 ]3 shows two spin-flip emission bands in the near-infrared spectral region. These bands shift bathochromically by -14.1 and -7.7 cm-1 kbar-1 under hydrostatic pressure (Angew. Chem. Int. Ed. 2018, 57, 11069). The present study elucidates the structural changes of the chromium(III) cations under pressure using density functional theory with periodic boundary conditions and the resulting effects on the excited state energies using high-level CASSCF-NEVPT2 calculations. The differences of the bands in pressure sensitivity are traced back to a different orbital occupation of the intraconfigurational excited states.

ReaxFF-based nonadiabatic dynamics method for azobenzene derivatives.

ReaxFF reactive force fields have been parameterized for the ground and first excited states of azobenzene and its derivatives. In addition, an extended set of ab initio reference data ensures wide applicability, including to azosystems in complex environments. Based on the optimized force fields, nonadiabatic surface hopping simulations produce photoisomerization quantum yields and decay times of azobenzene, both in the gas phase and in n-hexane solution, in reasonable agreement with higher level theory and experiment. The transferability to other azo-compounds is illustrated for different arylazopyrazoles as well as ethylene-bridged azobenzene. Moreover, it has been shown that the model can be easily extended to adsorbates on metal surfaces. The simulation of the ring-opening of cyclobutene triggered by the photoisomerization of azobenzene in a macrocycle highlights the advantages of a reactive force field model.

bcc superstructures: RE2RuIn with RE = Sc, Y, Dy-Tm and Lu.

The rare earth-rich intermetallic phases RE2RuIn with RE = Sc, Y, Dy-Tm and Lu were synthesized by reactions of the elements in sealed tantalum ampoules in an induction furnace. The samples were characterized through Guinier powder patterns and the structures of Sc2RuIn and Er2RuIn were refined from single crystal X-ray diffraction data. The indides crystallize with the Pt2ZnCd type space group P4/mmm. The RE2RuIn phases are superstructures of the bcc packing and can be explained as intergrowth variants of tetragonally distorted, CsCl derived slabs of compositions RERu and REIn. Chemical bonding is discussed for Sc2RuIn and Sc2RuMg in comparison with the binaries ScRu, ScMg and ScIn. The Ru/Mg respectively Ru/In ordering leads to an increase of Sc-Sc bonding for the slab with the shorter Sc-Sc distances, while the Sc-Ru bond strength values remain similar. The strongest bonding interactions occur within the magnesium and indium square nets. Magnetic susceptibility measurements reveal Pauli paramagnetism for Lu2RuIn while Dy2RuIn, Ho2RuIn, Er2RuIn and Tm2RuIn are Curie-Weiss paramagnets. Antiferromagnetic ordering occurs at 13.1, 5.3 and 2.9 K for Dy2RuIn, Er2RuIn and Tm2RuIn, respectively. Dy2RuIn and Er2RuIn show metamagnetic transitions at critical fields of 4.6 and 3.2 T.

One-step synthesis of indolizino[3,4,5-ab]isoindoles by manganese(I)-catalyzed C-H activation: structural studies and photophysical properties.

Herein, a regioselective synthesis of indolizino[3,4,5-ab]isoindoles, a valuable class of heterocycles with interesting luminescence properties, is described using manganese(I)-catalyzed C-H activation. The reported transformation proceeds in one-step and employs readily available 2-phenylpyridines as starting materials. Furthermore, the obtained single products exhibit blue-greenish fluorescence with high quantum yields.

Covalent photofunctionalization and electronic repair of 2H-MoS2via nitrogen incorporation.

A route towards covalent functionalization of chemically inert 2H-MoS2 exploiting sulfur vacancies is explored by means of (TD)DFT and GW/BSE calculations. Functionalization via nitrogen incorporation at sulfur vacancies is shown to result in more stable covalent binding than via thiol incorporation. In this way, defective monolayer MoS2 is repaired and the quasiparticle band structure as well as the remarkable optical properties of pristine MoS2 are restored. Hence, defect-free functionalization with various molecules is possible. Our results for covalently attached azobenzene, as a prominent photo-switch, pave the way to create photoresponsive two-dimensional (2D) materials.