Perfluorocyclobutyl-linked hexa-peri-hexabenzocoronene networks

Hexa-peri-hexabenzocoronene (HBC) derivatives possess many interesting properties ranging from liquid crystallinity, to hole transport, to variable emission behavior, yet are plagued by insolubility and general accessibility of polymerizable substituents. The first synthesis of trifluorovinyl ether-substituted hexabenzocoronene and its polymerization to perfluorocyclobutyl (PFCB) polymers and copolymers is shown. Unlike hydrocarbon ethers, fluoro vinyl ethers are stable under HBC oxidation conditions. Discrete HBC units in PFCB polymers provide access to potentially processable HBC optical materials.     

The azine bridge as a conjugation stopper: an NMR spectroscopic study of electron delocalization in acetophenone azines

Dipole parallel-alignment of organic molecular crystals of azines has been achieved with a design that was based on the hypothesis that the azine bridge is a conjugation stopper. This hypothesis has now been tested in detail, and (1)H and (13)C NMR spectroscopic data of symmetric and asymmetric acetophenone azines are presented in support of this design concept. Previous structural, ab initio, and electrochemical studies have shown that the azine bridge largely inhibits through-conjugation in molecules with the general structure DPhC(Me) [double bond] N [bond] N [double bond] C(Me)PhA, where D is a donor group and A is an acceptor group. NMR spectroscopy is an excellent tool to probe the degree of conjugation through the azine bridge. The NMR results reported here for nine symmetrical and 18 asymmetrical azines show in a compelling fashion that the hypothesis holds. Varying the donor group does not change the chemical shifts of the aromatic hydrogen and carbon atoms on the acceptor-substituted phenyl ring. Likewise, varying the acceptor group does not change the chemical shifts of the atoms in the donor-substituted phenyl ring.     

Application of Optimal Control Theory to Fourier Transform Ion Cyclotron Resonance

We study the application of Optimal Control Theory to Ion Cyclotron Resonance. We test the validity and the efficiency of this approach for the robust excitation of an ensemble of ions with a wide range of cyclotron frequencies. Optimal analytical solutions are derived in the case without any pulse constraint. A gradient-based numerical optimization algorithm is proposed to take into account limitation in the control intensity. The efficiency of optimal pulses is investigated as a function of control time, maximum amplitude and range of excited frequencies. A comparison with adiabatic and SWIFT pulses is done. On the basis of recent results in Nuclear Magnetic Resonance, this study highlights the potential usefulness of optimal control in Ion Cyclotron Resonance.     

Rational Improvement of Single-Molecule Magnets by Enforcing Ferromagnetic Interactions

The anisotropy barrier of polynuclear single-molecule magnets is expected to be higher with less tunneling the better stabilized the spin ground state is so that less M_S mixing in the ground state and with excited spin states occur. We have realized this experimentally in two structurally related heptanuclear SMMs: the triplesalen-based [Mn^III ₆ Cr^III]³⁺ and the triplesalalen-based *[Mn^III ₆ Cr^III]³⁺ . The ligand system triplesalen was developed to enforce ferromagnetic interactions by the spin-polarization mechanism. However, we found weak antiferromagnetic couplings, that we assigned to an inefficient spin-polarization by a heteroradialene formation. To prevent this heteroradialene formation, the triplesalalen ligand H₆talalen was designed. Here, we present the building block [(talalen )Mn^III₃]³⁺ and its application for the assembly of [{(talalen )Mn^III₃}₂{Cr^III(CN)₆}]³⁺ (= *[Mn^III ₆ Cr^III]³⁺ ). Both the trinuclear and heptanuclear complexes are SMMs. The comparison to the related triplesalen complex [(feld )Mn^III₃]³⁺ proves the absence of heteroradialene character and the enforcement of ferromagnetic Mn^III-Mn^III interactions in the (talalen )⁶⁻ complexes. This results in an increase of the barrier for spin reversal U_eff from 25 K in the triplesalen-based [Mn^III ₆ Cr^III]³⁺ SMMs to 37 K in the triplesalalen-based *[Mn^III ₆ Cr^III]³⁺ SMM proving the success of our concept. Based on this study, the next step in the rational improvement of our SMMs is discussed.     

A critical evaluation of heteronuclear TOCSY (HEHAHA) experiments for 1H,6Li spin pairs

Heteronuclear TOCSY (HEHAHA) experiments for ¹H,⁶Li spin pairs in organolithium compounds with adjacent strongly coupled ¹H,¹H spin systems showed unexpected cross peak behaviour: for n‐butyllithium ¹H,⁶Li cross peaks were completely missing, whereas for the dimer of (Z)‐2‐lithio‐1‐(o‐lithiophenyl)ethane, a cross peak for remote protons was observed even at very short mixing times. It was assumed that strong magnetization transfer within the proton spin systems was responsible for these results, which prevented unambiguous chemical shift assignments. Selective experiments with the ⁶Li,¹H‐HET‐PLUSH‐TACSY sequence then showed the expected ⁶Li,¹H cross peaks for the transfer via the directly coupled ¹H and ⁶Li nuclei. For n‐butyllithium transfer to H(C_α) via an unresolved heteronuclear coupling constant below 0.1 Hz is unambiguously observed. Cross peaks in the 2D ⁶Li,¹H‐HET‐PLUSH‐TACSY spectra for the dimer of (Z)‐2‐lithio‐1‐(o‐lithiophenyl)ethane are readily explained by the measured coupling network and the corresponding active mixing conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Design and application of robust rf pulses for toroid cavity NMR spectroscopy

We present robust radio frequency (rf) pulses that tolerate a factor of six inhomogeneity in the B? field, significantly enhancing the potential of toroid cavity resonators for NMR spectroscopic applications. Both point-to-point (PP) and unitary rotation (UR) pulses were optimized for excitation, inversion, and refocusing using the gradient ascent pulse engineering (GRAPE) algorithm based on optimal control theory. In addition, the optimized parameterization (OP) algorithm applied to the adiabatic BIR-4 UR pulse scheme enabled ultra-short (50 μs) pulses with acceptable performance compared to standard implementations. OP also discovered a new class of non-adiabatic pulse shapes with improved performance within the BIR-4 framework. However, none of the OP-BIR4 pulses are competitive with the more generally optimized UR pulses. The advantages of the new pulses are demonstrated in simulations and experiments. In particular, the DQF COSY result presented here represents the first implementation of 2D NMR spectroscopy using a toroid probe.     

Nanoparticle Aggregation and Fractal Growth in Fluid Smectic Membranes

We observe directly the diffusion and aggregation of buckyball clusters dispersed in thin, freely-suspended films of smectic liquid crystal using reflected light microscopy. The buckyballs at early times are barely resolvable, nanoscale clusters, which diffuse stochastically in the film. Clusters eventually coalesce to form micron-scale, fractal aggregates whose effective radius increases approximately linearly with time, so that after several days, millimeter-size fractals extend across the entire film. The measured fractal dimension of these final networks suggests that the aggregation of the buckyball clusters in the film is a diffusion-limited process.     

Über die Einwirkung von Blausäure auf Methyloldimethylacetaldehyd

Ohne ZusammenfassungZum Schlusse meiner Arbeit erfülle ich die angenehme Pflicht, meinem hochverehrten Lehrer, Herrn Hofrat A. Lieben, für das warme Interesse, das er meiner Arbeit entgegenbrachte, meinen herzlichsten Dank auszusprechen, auch sehe ich mich veranlaßt, dem Herrn Dr. Cäsar Pomeranz für die freundliche Förderung meiner Arbeit zu danken.