Isolation and X-ray Structural Characterization of a Dicationic Homotrimer of 2,3,6,7-tetramethoxy-9,10-dimethylanthracene cation radical

Electrochemical oxidation of 2,3,6,7-tetramethoxy-9,10-dimethylanthracene (1) showed that it undergoes a highly reversible electrochemical oxidation (Eox = 0.81 V vs SCE) and forms a modestly stable cation-radical salt in solution. X-ray crystallography established that 1+SbCl6- crystallizes as a (centrosymmetric) dicationic homotrimer via a close cofacial association of a pair of cationic and one neutral molecule of 1 with an interplanar separation of ∼3.2 Å. The structure of the dicationic homotrimer was also reproduced by DFT calculations. Furthermore, the structure of a dicationic spiro adduct, formed by a slow decomposition of a solution of 1+SbCl6- was also established by X-ray crystallography

Charge Delocalization in Self-Assembled Mixed-Valence Aromatic Cation Radicals

The spontaneous assembly of aromatic cation radicals (D+•) with their neutral counterpart (D) affords dimer cation radicals (D2+•). The intermolecular dimeric cation radicals are readily characterized by the appearance of an intervalence charge-resonance transition in the NIR region of their electronic spectra and by ESR spectroscopy. The X-ray crystal structure analysis and DFT calculations of a representative dimer cation radical (i.e., the octamethylbiphenylene dimer cation radical) have established that a hole (or single positive charge) is completely delocalized over both aromatic moieties. The energetics and the geometrical considerations for the formation of dimer cation radicals is deliberated with the aid of a series of cyclophane-like bichromophoric donors with drastically varied interplanar angles between the cofacially arranged aryl moieties. X-ray crystallography of a number of mixed-valence cation radicals derived from monochromophoric benzenoid donors established that they generally assemble in 1D stacks in the solid state. However, the use of polychromophoric intervalence cation radicals, where a single charge is effectively delocalized among all of the chromophores, can lead to higher-order assemblies with potential applications in long-range charge transport. As a proof of concept, we show that a single charge in the cation radical of a triptycene derivative is evenly distributed on all three benzenoid rings and this triptycene cation radical forms a 2D electronically coupled assembly, as established by X-ray crystallography

Design, synthesis and study of the bridged and cofacially-arrayed poly-p-phenylene molecular wires

Two novel series of bridged and cofacially-arrayed poly- p -phenylenes have been designed synthesized and studied. The bridged poly-p -phenylenes have been synthesized from a readily available diacetylenic precursor utilizing three high yielding steps, and their structures were determined by 1 H/13 C NMR spectroscopy as well as X-ray crystallography. The racemization barrier between the two atropoisomers was found to be ∼12 Kcal mol-1 ; the versatility of the synthesis employed was extended to synthesis a triply bridged tetra- p -phenylene and a quadruply bridged penta-p -phenylene. The cofacially-arrayed poly-p -phenylenes have shown that the X-ray crystal structures of the neutral compounds are largely dominated by C-H--π-interactions interactions while the dicationic species display an almost perfect parallel arrangement of the cofacially-arrayed poly- p -phenylene moieties. Electrochemistry of the cofacially-arrayed poly-p -phenylenes and their model compounds consistently met the reversibility criteria. Electronic absorption spectra show that the two series are strikingly similar; however the emission spectra show that the cofacially-arrayed poly-p -phenylenes are significantly broader and bathochromically shifted in comparison to the model compounds. Electrochemical oxidation of 2,3,6,7-tetramethoxy-9,10-dimethylanthra-cene showed that it undergoes a highly reversible electrochemical oxidation ( Eox = 0.81 V vs. SCE) and forms a modestly stable cation-radical salt in solution. The X-ray crystal structure showed the presence of a dicationic homotrimer that decomposes in the spiro adduct when allowed to sit at ambient temperatures

Induktion axialer Chiralität in einem Geländer-Oligomer durch Längendiskrepanz der Oligomerstränge

Helikale Moleküle zeigen einzigartige physikalische Eigenschaften, die auf ihre Chiralität zurückgehen. Wir beschreiben hier einen neuen Ansatz zum Aufbau von “Geländer”-Oligomeren durch Verbindung zweier unterschiedlich langer Oligomerstränge. Dabei windet sich ein längerer Oligobenzylether-Strang um ein kürzeres Oligophenyl-Rückgrat, um die Längendiskrepanz auszugleichen. Das neue Geländer-Oligomer (1) wurde durch Umwandlungen funktioneller Gruppen und Kreuzkupplungsschritte sowie interne nukleophile Cyclisierungen aufgebaut und vollständig charakterisiert, darunter durch Röntgenstrukturanalyse. Die Isolierung der reinen Enantiomere ermöglichte die Verfolgung des Racemisierungsprozesses mittels CD-Spektroskopie

A Versatile Preparation of Geländer-Type \u3cem\u3ep\u3c/em\u3e-Terphenyls from a Readily Available Diacetylenic Precursor

A series of doubly bridged p-terphenyls (4) have been synthesized utilizing a facile three-step synthesis starting from the readily available diacetylenic precursor (1) in excellent overall yields, and their structures were confirmed by 1H/13C NMR spectroscopy as well as by X-ray crystallography. The racemization barrier between the meso and chiral atropisomers of one of the derivatives of 4 was found to be ∼12 kcal/mol by variable-temperature NMR spectroscopy. The versatility of the protocol developed herein was further demonstrated by the preparation of a quadruply bridged penta-p-phenylene derivative

Tuning Helical Chirality in Polycyclic Ladder Systems

Conceptually and experimentally, a new set of helical model compounds is presented herein that allow correlations between structural features and their expression in the secondary structure to be investigated. A cross-linked oligomer with two strands of mismatching lengths connected in a ladder-type fashion serves as a model system. Compensation for the dimensional mismatch leads to the adoption of a helical arrangement. A strategically placed relay ensures the continuity and uniformity of the helix. Upon exchanging the heteroatomic linkage, the helix responds by increasing or decreasing the torsion of the backbone. Inversion of the relay’s substitution pattern causes a distortion of the structure, while maintaining the directionality of the helix. Based on a short synthetic protocol with a modular precursor, four closely related “Geländer” oligomers (Geländer is the German word for bannister) were accessed and fully characterized. XRD analysis for one representative of each helical arrangement and complementary computational studies for the remaining derivatives allowed the impact of the alterations on the secondary structures to be studied. Isolation of pure enantiomers of all new Geländer oligomers provided insight into the racemization kinetics and estimation of the racemization barrier. In silico simulation of the electronic circular dichroism spectra of the model compounds enabled the helicity of the isolated samples to be assigned