Dissociation reactions of free tetrapyridinium tetracations and of their catenanes

Electrospray ionization from methanolic solution can be used for the generation of the free cyclophane tetracations 1(4+) -3(4+) from the corresponding hexafluorophosphates. In the idealized gas phase, these tetracations are long-lived and can easily be handled for further spectroscopic studies. Collision-induced dissociation of the free tetracations brings about charge separation via cleavage of the pyridinium bonds, leading to a pair of dications. Subsequently, these dications undergo another charge separation reaction to finally afford singly charged cations. In addition to the free tetracations, also the corresponding trications having one PF6- counterion are examined. Collision-induced dissociation of the trications leads to a formal substitution reaction concomitant with C-F bond formation. Further, the catenanes of the tetracations 1(4+) -3(4+) with bis-p-phenylene-34-crown-10 (4) are investigated. For the parent compound 1, also the gas-phase infrared spectrum is reported for the first time.     

Selective NO(x) optical sensing with surface-confined osmium polypyridyl complexes

Optical detection of parts-per-million (ppm) levels of NO2 (1-10 ppm) and NO(x) (800-2550 ppm) by a structurally well-defined monolayer consisting of osmium polypyridyl complexes on glass substrates has been demonstrated.     

Molecule and electron transfer through coordination-based molecular assemblies

This study provides insight into the internal structure of surface-confined molecular assemblies. The permeability of the layer-by-layer grown thin films can be controlled systematically by varying their composition and the structure of their molecular components. Moreover, the thickness can be used to control molecule permeation versus electron transfer.     

Coordination‐Based Molecular Assemblies of Oligofurans and Oligothiophenes

Molecular assemblies (MAs) of oligofurans and oligothiophenes were formed from solutions on various substrates. These films were obtained by alternating deposition of organic chromophores (oligofurans or oligothiophenes) and a palladium salt. These coordination‐based MAs were characterized by UV/Vis spectroscopy, spectroscopic ellipsometry, atomic force microscopy (AFM), X‐ray reflectivity (XRR), X‐ray photoelectron spectroscopy (XPS), and electrochemistry. The MAs exhibit similar electrochemical behavior and their growth and structure are apparently not affected when different organic template layers are used. The density of the MAs is a function of the structure of the molecular component. The oligothiophene density is approximately 50 % higher than that observed for the oligofuran‐based assemblies. The optical and electrochemical properties of the MAs scale linearly with their thickness. The UV/Vis data indicate that upon increasing the film thickness, there is no significant conjugation between the metal‐separated organic chromophores. DFT calculations confirmed that the HOMO–LUMO gap of the surface‐bound oligofuran and oligothiophene metal oligomers do not change significantly upon increasing their chain length. However, electrochemical measurements indicate that the susceptibility of the MAs towards oxidation is dependent on the number of chromophore units.     

Raman spectroscopy of M3B7O13X boracites (M = Cr,Co,Ni,Cu,Zn,Cd; X = Cl,Br,I)

We report results of a Raman study on single crystals of 16 boracites M₃B₇O₁₃X (M = Cr,Co,Ni,Cu,Zn,Cd; X = Cl,Br,I) over a broad temperature range. The Raman modes for all boracites in their high‐temperature prototype cubic (F3c) phase are compared. With decreasing temperature, most (but not all) compounds present a transition to the low‐temperature orthorhombic phase (Pca2₁) or to a sequence of orthorhombic, monoclinic (Pa), and trigonal (R3c) phases. The variations of the Raman spectra through different phases are studied in detail. Special attention is paid to the temperature hysteresis near the transitions and the dependence of transition temperature on the direction of crystal growth for the same material. Copyright © 2014 John Wiley & Sons, Ltd.     

Site affinity effects upon charge injection into siloxane-based monolayers

Submolecular electrical information is successfully derived by applying element-specific, chemically resolved electrical measurements to a covalently bound stilbazole-based monolayer on a silicon substrate. Pronounced affinity effects are found in the response of adjacent atomic sites to external charge injection, accompanied by intramolecular polarization variations. These noncontact electrical read-out capabilities may provide a first entry toward the realization of organic devices based on submolecular electrical units.     

Dissociation routes of protonated toluene probed by infrared spectroscopy in the gas phase

The structures of C(7)H(9)(+) ions generated by protonation of toluene are investigated by means of gas-phase infrared spectroscopy in conjunction with labeling experiments and complementary mass spectrometric studies. In full consistency with previous studies, the unimolecular as well as the multiphoton-induced dissociation of mass-selected C(7)H(9)(+) ions lead to losses of molecular hydrogen and methane. Labeling data clearly imply the occurrence of skeletal rearrangements of protonated toluene to isomeric structures in the course of fragmentation. Complementary reactivity studies indicate, however, that the C(7)H(7)(+) ions generated upon dehydrogenation of C(7)H(9)(+) bear the benzylium structure, rather than that of the more stable tropylium ion. Combination of labeling data and extensive theoretical studies lead to a scheme for the fragmentation of protonated toluene, which can account for all experimental findings reasonably well. As far as infrared spectroscopy of gaseous ions is concerned, the present results confirm the structural predictions derived from theory and provide evidence for the existence of protonated cycloheptatriene but also pose some questions about the comparability of intensities in multiphoton dissociation and linear absorption spectra.     

Electrochemical studies on Zn deposition and dissolution in sulphate electrolyte

The electrochemical deposition and dissolution of Zn on Pt electrode in sulphate electrolyte was investigated by electrochemical methods in an attempt to contribute to the better understanding of the more complex Zn–Cr alloy electrodeposition process. A decrease of the Zn electrolyte pH (from 5.4 to 1.0) so as to minimise/avoid the formation of hydroxo-products of Cr in the electrolyte for deposition of alloy coatings decreases the current efficiency for the Zn reaction, but the rate of the cathode reaction increases significantly due to intense hydrogen evolution. The results of the investigations in Zn electrolytes with pH 1.0–1.6 indicate that Zn bulk deposition is preceded by hydrogen evolution, stepwise Zn underpotential deposition (UPD) and formation of a Zn–Pt alloy. Hydrogen evolution from H₂O starts in the potential range of Zn bulk deposition. Data obtained from the electrochemical quartz crystal microbalance (EQCM) measurements support the assumption that electrochemical deposition of Zn proceeds at potentials more positive than the reversible potential of Zn. Anodic potentiodynamic curves for galvanostatically and potentiostatically deposited Zn layers provide indirect evidence about the dissolution of Zn from an alloy with the Pt substrate. The presumed potential of co-deposition of Cr (−1.9 V vs. Hg/Hg₂SO₄) is reached at a current density of about 300 mA cm⁻².