Following a route described by Müllen et al. we improved the preparation of 1,2,3‐tris(3″‐iodobiphenyl‐2′‐yl)benzene (4). During the required iododesilylation with iodine monochloride the unexpected formation of a mono‐chlorinated compound 6 as major product was observed, when an excess of the iodination reagent was employed. The reaction mechanism for the formation of this compound involving an oxidizing process is discussed. X‐ray crystallography of the subsequent alkynylated product 7 proves the location of the chlorine atom at the central benzene ring. The related compound 5 was likewise investigated by X‐ray analysis revealing a preferred solid‐state conformation (“2‐up‐1‐down”) different from that of the chlorinated compound (“propeller” conformation). Compound 7 showed remarkable conformational dynamics in solution as observed by 1H NMR spectroscopy. Temperature‐dependent measurements allowed the calculation of an energy difference of two major conformers of ca. 2 kJ/mol and a rotational barrier ΔGrot of ca. 77 kJ/mol. These values were convincingly confirmed by DFT calculations. A subsequent Sonogashira reaction of 4 with a suitable bipyridine derivative provided a unique trivalent bipyridine derivative.
The self-assembly behavior of five star-shaped pyridyl-functionalized 1,3,5-triethynylbenzenes was studied at the interface between an organic solvent and the basal plane of graphite by scanning tunneling microscopy. The mono- and bipyridine derivatives self-assemble in closely packed 2D crystals, whereas the derivative with the more bulky terpyridines crystallizes with porous packing. DFT calculations of a monopyridine derivative on graphene, support the proposed molecular model. The calculations also reveal the formation of hydrogen bonds between the nitrogen atoms and a hydrogen atom of the neighboring central unit, as a small nonzero tunneling current was calculated within this region. The title compounds provide a versatile model system to investigate the role of multivalent steric interactions and hydrogen bonding in molecular monolayers.
Starting from easily accessible precursors we describe the preparation of a series of branched oligo(2-thienyl)-and oligo(2,2'-bithienyl)-substituted pyridine derivatives. With palladium-catalyzed crosscoupling reactions of pyridyl nonaflates/triflates as key steps we synthesized 2,6-di(2-thienyl)pyridines bridged by thiophene or benzene rings. By selective bromination of 2,6-di(2-thienyl)pyridine and 2,4,6-tri(2-thienyl)pyridine and subsequent coupling reactions an access to oligo(2,2'-bithien-5-yl)-substituted pyridine derivatives was gained. The constitution and solid state conformation of 2,6-bis(2,2-bithien-5-yl)-pyridine was determined by X-ray analysis. This series of new pyridine-thiophene conjugates was systematically investigated by UV/vis spectroscopy.Large Stokes shifts in the neutral and protonated form were observed. The electrochemical oxidation of two typical compounds was studied, however, these oxidations were irreversible forming a polymeric film at the anode. As a selected example, a thiophene-bridged 2,6-di(2-thienyl)pyridine derivative was also investigated by scanning tunneling microscopy showing an interesting self-assembly into a highly ordered monolayer on highly oriented pyrolytic graphite.
Flexible and straightforward syntheses of a series of D3h‐ or C3h‐symmetrical star‐shaped compounds with pyridine end groups are reported. In all cases, the acid‐mediated cyclocondensations of the corresponding aryl methyl ketone provided the central benzene ring. For the preceding preparation of the functionalized compound arms, Suzuki couplings were applied. The crucial introduction of the pyridine C‐2 and C‐6 substituents occurred by Fe(acac)3‐catalyzed alkylations (acac = acetylacetonate). The preparation of the C3‐symmetrical compound involved an alternating sequence of halogenations and coupling reactions. The self‐assembly behavior of the four resulting star‐shaped compounds at the interface between 1‐phenyloctane and the basal plane of highly oriented pyrolytic graphite (HOPG) was studied by scanning tunnelling microscopy (STM). We found self‐assembled monolayers with structures strongly dependent on the substitution patterns of the investigated compounds. The reduction of the symmetry from a D3h‐ to a C3h‐symmetrical compound led to an entirely different self‐assembly behavior with the change from a hexagonal to a lamellar arrangement.
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