Christian Merten

Coaxing solid-state phosphorescence from tellurophenes.

The synthesis of the first examples of tellurophenes exhibiting phosphorescence in the solid state and under ambient conditions (room temperature and in air) is reported. Each of these main-group-element-based emitters feature pinacolboronates (BPin) as ring-appended side groups. The nature of the luminescence observed was also investigated using computational methods.

Fast functionalization of multi-walled carbon nanotubes by an atmospheric pressure plasma jet.

The afterglow of an atmospheric pressure plasma has been used for the fast oxidative functionalization of multi-walled carbon nanotubes (MWCNTs). Scanning electron microscopy and Raman spectroscopy demonstrate that the MWCNT morphology is mostly preserved when the MWCNTs are dispersed in a solvent and injected as a spray into the plasma. Contact angle measurements show that this approach enhances the wettability of MWCNTs and reduces their sedimentation in an aqueous dispersion. X-ray photoelectron spectroscopy, IR spectroscopy, and electrokinetic measurements show that oxygen plasma incorporates about 6.6 at.% of oxygen and creates mainly hydroxyl and carboxyl functional groups on the MWCNT surface. The typical effective treatment time is estimated to be in the range of milliseconds. The approach is ideally suited for combination with the industrial gas phase CVD synthesis of MWCNTs.

Determining the structure of α-phenylethyl isocyanide in chloroform by VCD spectroscopy and DFT calculations—simple case or challenge?

The present work provides a comprehensive theoretical and experimental study of the vibrational circular dichroism of (S)-α-phenylethyl isocyanide in chloroform. The structure in solution is investigated systematically by density functional theory calculations starting from the isolated molecule and then by considering solvent effects implicitly using the PCM and explicitly by taking into account molecular complexes of the isocyanide molecule and chloroform. Furthermore, the influence of dimerisation is evaluated, and it is finally found that the structure of (S)-α-phenylethyl isocyanide in chloroform is best described assuming a solvated dimer. These results are underlined by a quantitative correlation of the experimentally and theoretically obtained rotational strengths and a robust modes analysis.

Vibrational Circular Dichroism Spectroscopy of Solid Polymer Films : Effects of Sample Orientation

Vibrational circular dichroism (VCD) spectra of anisotropic thin solid samples are often superimposed with large contributions of linear birefringence and linear dichroism. In this study a theoretical approach is given on how to extract the true VCD spectrum out of such superimposed spectra. To verify this approach, the VCD spectra of achiral polymer films were examined. The polymers are supposed to give a zero line as VCD spectrum after eliminating the linear contributions. Applying our approach, in which four VCD spectra in different but selected sample orientations are recorded, and calculating their average, leads to the expected result, i.e., a zero line for achiral polymers. The advantage of this method for the elimination of artifacts from solid-state VCD spectra is that no further measurements are required (e.g., linear dichroism measurements or the determination of the orientation with the maximum anisotropy).

Strong solvent-dependent preference of Δ and Λ stereoisomers of a tris(diamine)nickel(II) complex revealed by vibrational circular dichroism spectroscopy.

In the present study, we use vibrational circular dichroism (VCD) spectroscopy to investigate the metal-centered Δ and Λ chirality of a tris(diamine)nickel(II) complex. Chiral diphenylethylenediamine is chosen as the ligand, which puts the Δ and Λ isomers of the complex in a diastereomeric relationship. X-ray crystallography indicates an equal preference of both stereoisomers in the solid state. This equal preference is also supported by the related density functional theory calculations. A comparison between the experimental and calculated VCD spectra also proves the existence of both isomers in an acetonitrile solution. However, a significant shift of the equilibrium toward the Λ diastereomer is found for the complex in dimethyl sulfoxide. This solvent-induced preference for a particular absolute configuration is hypothesized to arise from a stronger and more effective solvation of the Λ isomer. The observation that the solvent can significantly influence and shift an equilibrium between two diastereomeric forms is expected to have important implications on structural analysis and on how reaction mechanisms are rationalized.

Synthesis and luminescent properties of Lewis base-appended borafluorenes.

A series of Lewis base adducts of 9-bromo-9-borafluorene (BrBFl-LB, LB = IPr, IPrCH2, PPh3, and PCy3), parent borafluorenes (HBFl-IPr and HBFl-IPrCH2), and the bisadduct [(DMAP)2BFl]Br were prepared and structurally characterized (IPr = [(HCNDipp)2C:], IPrCH2 = [(HCNDipp)2C═CH2], Dipp = 2,6-i-Pr2C6H3, and DMAP = N,N-dimethylaminopyridine). The adducts BrBFl-IPr, BrBFl-PPh3, BrBFl-PCy3, [(DMAP)2BFl]Br, BrBFl-IPrCH2, and HBFl-IPrCH2 were found to exhibit bright blue luminescence with low to moderately high quantum efficiencies (19 to 63%). Selective irradiation at different excitation wavelengths revealed the presence of two distinct emission processes in the adducts BrBFl-LB, leading to a ligand-independent, presumably borafluorene-based, blue light emission at 435 nm and another less intense emission band in the ultraviolet region (315-324 nm); [(DMAP)2BFl]Br exhibits an emission profile that tails into the visible region. Time-dependent density functional theory studies are also included for representative borafluorene adducts. With a judicious choice of functional groups at boron, one can envisage the future generation of a whole library of 4-coordinate borafluorene-based luminogens that complement the efficient light-emitting behavior known for the widely studied boron-dipyrromethene analogues.

General Enantioselective C−H Activation with Efficiently Tunable Cyclopentadienyl Ligands

Cyclopentadienyl (Cp) ligands enable efficient steering of various transition-metal-catalyzed transformations, in particular enantioselective C-H activation. Currently only few chiral Cp ligands are available. Therefore, a conceptually general approach to chiral Cp ligand discovery would be invaluable as it would enable the discovery of applicable Cp ligands and to efficiently and rapidly vary and tune their structures. Herein, we describe the three-step gram-scale synthesis of a structurally diverse and widely applicable chiral Cp ligand collection (JasCp ligands) with highly variable and adjustable structures. Their modular nature and their amenability to rapid structure variation enabled the efficient discovery of ligands for three enantioselective RhIII -catalyzed C-H activation reactions, including one unprecedented transformation. This novel approach should enable the discovery of efficient chiral Cp ligands for various further enantioselective transformations.

Contrasting Reactivities of Silicon and Germanium Complexes Supported by an N-Heterocyclic Guanidine Ligand

We report the synthesis of an acyclic two-coordinate germylene supported by two bulky and electron donating N-heterocyclic guanidine [IPr═N](-) ligands (IPr = [(HCNDipp)2C:]; Dipp = [2,6-(i)Pr2C6H3]), and its reactivity with molecular hydrogen to form IPr═NH, which presumably proceeds via the unstable intermediate [H2Ge(N═IPr)2]. Our attempts to isolate the corresponding silylene [:Si(N═IPr)2] led to an unexpected ligand activation/rearrangement process involving N-C(aryl) bond cleavage within the N-heterocyclic guanidine ligand; this transformation was also studied by computational methods.

Stereochemical Communication within a Chiral Ion Pair Catalyst

Ionic interactions are increasingly appreciated as a key, asymmetry-inducing factor in enantioselective catalytic transformations, including those involving Brønsted acid or base catalysis, phase-transfer catalysis, and related processes. However, a detailed understanding of these interactions is often lacking. Herein, we show how an enantiopure anion enforces a chiral conformation onto a catalytically relevant achiral cation. Specifically, we use vibrational circular dichroism (VCD) spectroscopy to monitor the transmission of stereochemical information from a chiral phosphate anion to a flexible manganese(III)-salen cation. We show that VCD can be used to study solvent effects and that the obtained chiroptical data directly and quantitatively correlate with the experimentally observed enantioselectivity in an asymmetric olefin epoxidation reaction.

Determination of the helical screw sense and side-group chirality of a synthetic chiral polymer from Raman optical activity.

Splitting it up: excellent agreement between the experimental and the quantum-chemically simulated Raman optical activity (ROA) spectrum of (+)-poly(trityl methacrylate) shows that the polymer backbone adopts a left-handed helical conformation while the trityl side groups display a left-handed propeller conformation. Thus ROA can be used to determine the complete structure of synthetic chiral polymers in solution.