Intramolecular Diels-Alder Reaction of a Biphenyl Group in a Strained meta-Quaterphenylene Acetylene.

At elevated temperatures, a strained, cyclic meta-quaterphenylene acetylene undergoes an intramolecular cyclization reaction to form benz[e]indeno[1,2,3-hi]acephenanthrylene. This reaction represents an example of a Diels-Alder reaction at the 2-, 1-, 1'-, and 2'-positions of a biphenyl derivative, a region analogous to the bay regions of perylene and other periacenes. The reaction proceeds cleanly with high conversion. Kinetics studies of a methylated derivative reveal that the ΔG‡ for the reaction is ∼40-41 kcal/mol, and computational models predict a similar value of Grel for the transition state of a concerted [4 + 2]-cycloaddition.

Synthesis and Electronic Properties of Fluoreno[2,1-a]fluorenedione and Fluoreno[1,2-a]fluorenedione.

The [2,1-a]- and [1,2-a]-isomers of fluorenofluorenedione have been synthesized via intramolecular Friedel-Crafts acylations. DFT calculations indicate that the [1,2-a]-isomer adopts a twisted, helical C2-symmetric structure and that its protonated form is the thermodynamic product of the Friedel-Crafts acylation in hot sulfuric acid. Absorption spectroscopy and cyclic voltammetry measurements provide experimental estimations of frontier molecular orbital energy levels, which are reported and discussed.

Topical Developments in High-Field Dynamic Nuclear Polarization.

We report our recent efforts directed at improving high-field DNP experiments. We investigated a series of thiourea nitroxide radicals and the associated DNP enhancements ranging from ε = 25 to 82 that demonstrate the impact of molecular structure on performance. We directly polarized low-gamma nuclei including 13C, 2H, and 17O using trityl via the cross effect. We discuss a variety of sample preparation techniques for DNP with emphasis on the benefit of methods that do not use a glass-forming cryoprotecting matrix. Lastly, we describe a corrugated waveguide for use in a 700 MHz / 460 GHz DNP system that improves microwave delivery and increases enhancements up to 50%.

Molecular spur gears comprising triptycene rotators and bibenzimidazole-based stators.

Dynamic gearing of molecular spur gears, the most common type of mechanical gear, is elucidated. Molecular design and conformational analysis show that derivatives of 4,4-bis(triptycen-9-ylethynyl)bibenzimidazole represent suitable constructs to investigate gearing behavior of collateral triptycene (Tp) groups. To test this design, DFT calculations (B97-D/Def2-TZVP) were employed and the results suggest that these molecules undergo geared rotation preferentially to gear slippage. Synthesis of derivatives was carried out, providing a series of molecular spur gears, including the first desymmetrized spur gear molecules, which were subsequently subjected to stereochemical analysis.

Synthesis, X-ray crystal structures, and computational studies of 1,1'-bridged 4,4'-diaryl-2,2'-bibenzimidazoles: building blocks for supramolecular structures.

A series of C-shaped, 1,1'-alkyl-bridged 4,4'-diaryl-2,2'-bibenzimidazoles has been synthesized. The crystal structures of these compounds have been determined and packing diagrams demonstrate that these molecules either form linear intercalated molecular chains or include solvent molecules in the solid state. Crystal structures are compared to computational structures determined using density functional theory, with the BMK/DZV(2d,p) method. The C-shaped or tweezer-like geometry enables them to act as building blocks for supramolecular architectures.

Fluoreno[2,1-a]fluorene: an ortho-naphthoquinodimethane-based system with partial diradical character.

Fluoreno[2,1-a]fluorene, a molecule comprising fused ortho-quinodimethane units in a 1,5-napthoquinodimethane core, has been prepared and investigated with spectroscopy (UV-Vis-NIR, 1H-NMR and Raman), SQUID magnetometry, spectroelectrochemistry and quantum chemistry. While para-quinodimethanes with a 2,6-substitution pattern exist as closed-shell species and meta-quinodimethanes with 2,7-substitution favour a ground electronic state with very large diradical character, our 1,5-substituted ortho-naphthoquinodimethane-based system exhibits an intermediate degree of diradical character.

Synthesis of 4,4'-bisaryl-2,2'-bisbenzimidazoles as building blocks for supramolecular structures.

A series of 4,4'-bisaryl-2,2'-bisbenzimidazoles has been synthesized from the corresponding 4,4'-dibromo-2,2'-bisbenzimidazoles by Negishi coupling reactions. This procedure affords highly substituted bisbenzimidazoles. [reaction: see text]

Synthesis and properties of the 5,10,15-trimesityltruxen-5-yl radical.

The synthesis of a long-lived, truxene-based radical that is highly delocalized and exhibits a narrow EPR absorption is reported. The radical is stable for multiple hours in a solution exposed to air and remains for months in the solid state under inert gas. Characterization and properties are discussed.

Manisyl-substituted polypyridine coordination compounds: Metallo-supramolecular networks of interdigitated double helices assembled via CH...pi and pi-pi interactions.

A series of ML(2) coordination compounds of manisyl-substituted 2-pyridin-2-yl-1,10-phenanthrolines (pherpys) and 2,2':6',2''-terpyridines (terpys) with Fe(ii), Co(ii), Ni(ii), Cu(ii), Zn(ii), Ru(ii), and Os(ii), were synthesized. The pyridyl-phenanthroline-metal complexes form isomorphous crystals (space group Pcca), except for the zinc complex, which is isostructural but not isomorphous (space group P2(1)/c). Traces of the Ru complex induce the Zn complex to crystallize in the Pcca modification. Terpyridine complexes are isostructural but divided into two subgroups (Pnna and C2/c). Within the crystal lattice, the dications arrange into two-dimensional sheets of interdigitated left- and right-handed double helices via CHpi and pi-pi interactions of the pendant manisyl functionalities. All of the complexes exhibit weak ligand charge transfer (LCT) emission in solution at room temperature and the osmium complexes possess an emissive metal to ligand charge transfer (MLCT) state.