The synthesis of cycloarenes in solution is challenging
because
of their low solubility and the often hindered cyclodehydrogenation
reaction of their nonplanar precursors. Using an alternative on-surface
synthesis protocol, we achieved an unprecedented double-stranded hexagonal
cycloarene containing 108 sp2 carbon atoms. Its synthesis
is based on hierarchical Ullmann coupling and cyclodehydrogenation
of a specially designed precursor on a Au(111) surface. The structure
and other properties of the cycloarene are investigated by scanning
tunneling microscopy/spectroscopy, atomic force microscopy, and density
functional theory calculations.
Allylboration of ketones with γ-disubstituted allylboronic acids is performed in the presence of chiral BINOL derivatives. The reaction is suitable for single-step creation of adjacent quaternary stereocenters with high selectivity. We show that, with an appropriate choice of the chiral catalyst and the stereoisomeric prenyl substrate, full control of the stereo- and enantioselectivity is possible in the reaction.
We
revisit the question of kekulene’s aromaticity by focusing
on the electronic structure of its frontier orbitals as determined
by angle-resolved photoemission spectroscopy. To this end, we have
developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene,
which allows us to prepare sufficient quantities of kekulene of high purity
directly on a Cu(111) surface, as confirmed by scanning tunneling
microscopy. Supported by density functional calculations, we
determine the orbital structure of kekulene’s highest occupied
molecular orbital by photoemission tomography. In agreement with a
recent aromaticity assessment of kekulene based solely on C–C
bond lengths, we conclude that the π-conjugation of kekulene
is better described by the Clar model rather than a superaromatic
model. Thus, by exploiting the capabilities of photoemission tomography,
we shed light on the question which consequences aromaticity holds
for the frontier electronic structure of a π-conjugated molecule.
Herein we report a versatile concept for the synthesis of fourfold functionalized, soluble pyrenes, peropyrenes, terropyrenes, and quarterropyrenes. They were obtained by a modular stepwise approach towards the rylene scaffold via Suzuki-Miyaura cross coupling, oxidative cyclodehydrogenation in the presence of caesium hydroxide under air, and finally zinc-mediated reductive silylation. The silylated reaction products were characterized by X-ray crystallography. The first example of a synthesized and crystallized quarterropyrene is presented and its oxidation reaction investigated. The functionalized ropyrenes were systematically characterized by means of UV/Vis-NIR and photoluminescence spectroscopy showing a bathochromic shift of 80 nm per naphthalene unit and a nearly linear increase of the extinction coefficients. Cyclic voltammograms and DFT calculations identify them as electron-rich dyes and show a narrowing of the electrochemically determined HOMO-LUMO gap and lower oxidation potentials for the higher homologues.
New ferrocenylsulfonium cation based ionic liquids were prepared by direct alkylation of the corresponding ferrocenyl-based thioethers with N-alkylbis(trifluoromethanesulfonyl)imides (R'TFSI). This convenient direct access to organometallic sulfonium bis(trifluoromethanesulfonyl)imide (TFSI) salts without the need for ion exchange was chosen in order to obtain highly pure and reversibly redox active room temperature ILs in many cases. In other cases the anion cation interaction in the solid state was studied by XRD analyses. Moreover a diferrocenylmethylsulfonium tetrafluoroborate with two redox active centers was synthesized. The redox chemistry of these sulfonium salts was investigated via cyclic voltammetry. Furthermore, UV-Vis spectra and thermoanalytical data are discussed. The electron-withdrawing sulfonium group is directly bonded to the ferrocenyl unit, therefore this cationic group influences the potential of these ionic liquids in a more pronounced way than being anchored to the ferrocenyl unit via an organic spacer. With their low absorbance in the visible light and reversible, tunable redox potential, these room temperature ILs open perspectives as redox mediators in dye sensitized solar cells (DSSCs), as redox electrolytes in supercapacitors or as overcharge protection additives in batteries.
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