Gunther Brunklaus

Thiadiazoloquinoxaline–Acetylene Containing Polymers as Semiconductors in Ambipolar Field Effect Transistors

Two conjugated copolymers, PPTQT and PTTQT, were developed based on thiadiazoloquinoxalines connected via ethynylene π-spacer to thiophene units. PPTQT showed maximum hole and electron mobility of 0.028 and 0.042 cm(2)/V s, respectively, being the first example of an ambipolar semiconducting material bearing triple bonds in the polymer backbone.

Transient States in [2 + 2] Photodimerization of Cinnamic Acid: Correlation of Solid-State NMR and X-ray Analysis

13C-CPMAS and other solid-state NMR methods have been applied to monitor the solid-state reactions of trans-cinnamic acid derivatives, which are the pioneer and model compounds in the field of topochemistry previously studied by X-ray diffraction, AFM, and vibrational spectroscopy. Single-crystal X-ray analyses of photoirradiated alpha-trans-cinnamic acid where the monomers are arranged in a head-to-tail manner have revealed the formation of a centrosymmetric alpha-truxillic acid photodimer. For a centrosymmetric dimer, however, two cyclobutane carbon signals and one carbonyl carbon signal were expected apart from other aromatic carbon signals. Instead, four cyclobutane and two carbonyl carbon signals were observed suggesting the formation of a non-centrosymmetric photodimer. Removing hydrogen bonds from the system by esterfication of alpha-truxillic acid yield a centrosymmetric photodimer. Careful analysis of the obtained products via solid-state NMR clearly showed that the observed peak splittings in the 13C-CPMAS spectra did not originate from packing effects but rather result from asymmetric hydrogen bonds distorting the local symmetry. Further evidence of this rather dynamic hydrogen-bonding stems from high-temperature X-ray data revealing that only the joint approach of both X-ray analysis and solid-state NMR at similar temperatures allows for the successful characterization of dynamic processes occurring in topochemical reactions, thus, providing detailed insight into the reaction mechanism of organic solid-state transformations.

R sequences for the scalar-coupling mediated homonuclear correlation spectroscopy under fast magic-angle spinning

Abstract High efficiency of homonuclear polarization transfer via scalar spin–spin coupling is achieved using a pulse sequence designed based on R pulse symmetry. Numerical simulations show that this new pulse sequence is robust with respect to rf inhomogeneity and resonance offsets. The required rf nutation frequency is five times the spinning frequency. Experimental data obtained for Ag 4 P 2 S 6 -II indicate that the new pulse sequence has a maximum efficiency about 75% and therefore is very promising for the measurement of homonuclear correlation spectra.

Aromatic, microporous polymer networks with high surface area generated in Friedel–Crafts-type polycondensations

A series of novel, microporous polymer networks (MPNs) have been generated in a simple, acid catalysed Friedel–Crafts-type self-condensation of A2B2- and A2B4-type fluorenone monomers. Two A2B4-type monomers with 2,7-bis(N,N-diphenylamino) A or 2,7-bis[4-(N,N-diphenylamino)phenyl] D substitution of the fluorenone cores lead to MPNs with high SBET surface areas of up to 1400 m2 g−1. Two MPNs made of binary monomer mixtures showed the highest Brunauer–Emmett–Teller (BET) surface areas SBET of our series (SBET of up to 1800 m2 g−1) after washing the powdery samples with supercritical carbon dioxide. Total pore volumes of up to 1.6 cm3 g−1 have been detected. It is observed that the substitution pattern of the monomers is strongly influencing the resulting physicochemical properties of the microporous polymer networks (MPNs).

NMR studies of phosphorus chalcogenide–copper iodide coordination compounds

The local structures of the new phosphorus chalcogenide – copper iodide coordination compounds (CuI)P4Se4, (CuI)2P8Se3, (CuI)3P4Se4, and (CuI)3P4S4 are investigated using comprehensive 63Cu, 65Cu, and 31P magic angle spinning NMR techniques. Peak assignments are proposed on the basis of homo- and heteronuclear indirect spin–spin interactions, available from lineshape analysis and/or two-dimensional correlation spectroscopy. In particular, the 31P-63,65Cu scalar coupling constants have been extracted from detailed lineshape simulations of the 31P resonances associated with the Cu-bonded P atoms. In addition, the RNνn pulse symmetry concept of Levitt and coworkers has been utilized for total through-bond correlation spectroscopy (TOBSY) of directly-bonded phosphorus species. The resonance assignments obtained facilitate a discussion of the 31P and 63,65Cu NMR Hamiltonian parameters in terms of the detailed local atomic environments. Analysis of the limited data set available for this group of closely related compounds offers the following conclusions: (1) bonding of a special phosphorus site in a given P4Xn (X = S, Se) molecule to Cu+ ions shifts the corresponding 31P NMR signal upfield by about 50 ppm relative to the uncomplexed molecule, (2) the magnitude of the corresponding scalar 31P-63,65Cu spin–spin coupling constant tends to decrease with increasing Cu–P distance, and (3) the 63,65Cu nuclear electric quadrupolar coupling constants appear to be weakly correlated with the shear strain parameter specifying the degree of local distortion present in the four-coordinated [CuI2P2] and [CuI3P] environments. Overall, the results illustrate the power and potential of advanced solid state NMR methodology to provide useful structural information in this class of materials.

Probing molecular recognition in the solid-state by use of an enolizable chromophoric barbituric acid

Complex formation of the enolizable chromophor 1-n-butyl-5-(4-nitrophenyl)barbituric acid 1 with multiple binding sites for supramolecular assemblies and its corresponding adducts produced with the Proton Sponge (1,8-bis(dimethylamino)naphthalene), PS) and the adenine-mimetic 2,6-diacetamidopyridine (DAC) have been studied by means of solid-state proton NMR spectroscopy under fast magic-angle spinning, X-ray analysis, and UV/vis spectroscopy. Both NMR data and X-ray results reveal that the enolic chromophor undergoes self-aggregation to hydrogen-bonded dimers which are involved in stacked arrangements. Depending on the nature of the added base, this dimeric assembly is preserved in the formed enolate anion but can be broken in the presence of complementary hydrogen-bonding pattern leading to supramolecular complexes. Molecular recognition of these structural different bases significantly influences the chromophoric pi-system of 1.

Ab-initio crystal structure analysis and refinement approaches of oligo p-benzamides based on electron diffraction data

Ab-initio crystal structure analysis of organic materials from electron diffraction data is presented. The data were collected using the automated electron diffraction tomography (ADT) technique. The structure solution and refinement route is first validated on the basis of the known crystal structure of tri-p-benzamide. The same procedure is then applied to solve the previously unknown crystal structure of tetra-p-benzamide. In the crystal structure of tetra-p-benzamide, an unusual hydrogen-bonding scheme is realised; the hydrogen-bonding scheme is, however, in perfect agreement with solid-state NMR data.

Solid-state NMR and X-ray analysis of structural transformations in O-H...N heterosynthons formed by hydrogen-bond-mediated molecular recognition.

The template-mediated, hydrogen-bond-driven co-crystallization of trans-1,2-bis(4-pyridyl)ethylene (bpe) and resorcinol together with 1,2-bis(4-pyridyl)ethane (bpet) yielded two new polymorphs of 2(bpe):2(res) and 2(bpet):2(res) molecular adducts, thereby exploiting the molecular specificity of resorcinol-pyridine O-H...N recognition in the presence of multiple dipyridines. Comprehensive understanding of the subsequent [2 + 2] photodimerization of the known polymorph of 2(bpe):2(res) complex was obtained by applying single-crystal X-ray analysis and (13)C CPMAS solid-state NMR at different levels of conversion, ranging from monomer to the dimer. In addition, removal of the resorcinol template from the 2(bpe):2(res) complex yields a distorted tetrakis(4-pyridyl)cyclobutane, revealing a rather different molecular geometry (orthorhombic, Pccn phase). Ambiguous peak splittings and the presence of unexpected resonances in the respective (13)C CPMAS NMR spectra have been successfully explained by the joint approach of X-ray analysis and density functional theory (DFT) chemical shift computations.

Heterosynthon mediated tailored synthesis of pharmaceutical complexes: a solid-state NMR approach

Based on crystal engineering principles, we have explored the predictability of resulting structures of a multi-component pharmaceutical model complex derived from 4-hydroxybenzoic acid (4HBA) and quinidine, an anti-malarial constituent of Cinchona tree bark. Though the obtained complex is stabilized by a slightly different set of charge-assisted heterosynthons as proposed, the applied concept was efficient in predicting the salt formation. The salt 1 crystallizes in a monoclinic space group [P21 (no. 4), Z = 8, a = 6.914 A, b = 36.197 A, c = 9.476 A and β = 92.126], where the asymmetric unit is comprised of two quinidine and two 4HBA molecules. In addition, a micro-crystalline, less-defined sample of 1 was obtained from rapid co-crystallization in ethanol and successfully identified via both infrared spectroscopy and multinuclear solid-state NMR. The interpretation of the obtained NMR data was supported by DFT quantum-chemical computations while illustrating options of “NMR crystallography”.

H-bonding schemes of di- and tri-p-benzamides assessed by a combination of electron diffraction, X-ray powder diffraction and solid-state NMR

The crystal structures of di- and tri-p-benzamides are solved by a combination of single crystal, electron and powder X-ray diffraction. Different hydrogen-bonding schemes observed in the two structures are described and classified. The hydrogen-bonding networks are correlated to complementary data obtained from multinuclear solid-state NMR.