Helical phenylenes

Many metal-catalyzed reactions are accelerated by light irradiation [88], In the case of the Vollhardt reaction [89], which is performed with conveniently available Co catalysts, the irradiation with visible light is included in the standard conditions. In a triple [2 + 2 + 2] cycloaddition, the nonaalkyne derivative 86 is transformed into the [7]Phenylene 87 (Scheme 5.17) [90]. Compound 87, containing six benzo-cyclobutene moieties, is a partial structure of the archimedene C12o 88. In this way a variety of similar benzocyclobutene structures such as helical phenylenes [91] can be built up. 

Molecular models indicate that, starting with angular [6]phenylene, the two ends of the angular phenylenes suffer steric interactions that render them helical, an expectation that was quantified theoretically [65]. The hexagonal squeezes of these helical phenylenes (heliphenes) are helicenes, a dass of PAHs that has received much scrutiny [66]. 

Apolar m-phenylene ethynylene oligomers 77 fold in apolar solvents, such as alkanes, into helices.117 Due to the strongly favored arene-arene interactions in this solvent type, the folding into a helical oligomer coincides with stacking of the folded oligomers into helical columnar polymers and a strong... 

In entry 12, which includes a />-phenylene unit in the bridge, the top ladder layer is twisted with respect to the bottom layer, resulting in a helical structure with no center of symmetry, and in which all eight tin atoms are chiral. In the OT-phenylene equivalent (entry 13), on the other hand, the top and bottom layers are parallel. Entry 14 can be obtained solvent free, but also forms a 4THF and a 3CH2Cl2 complex, and all three forms have a twist angle of 28-29°. 

Fig. 4 Lowest energy conformations resulting from a Monte Carlo conformational search on an unsubstituted phenylene ethynylene dodecamer. The ideal helical conformation is shown in the lower right-hand corner. All of the other conformations are destabilized by the addition of bulky side chains...

Mio MJ (2001) Hexagonally packed helical ohgo(m-phenylene ethynylene) nanotubules... 

The zero-shear viscosity r 0 has been measured for isotropic solutions of various liquid-crystalline polymers over wide ranges of polymer concentration and molecular weight [70,128,132-139]. This quantity is convenient for studying the stiff-chain dynamics in concentrated solution, because its measurement is relatively easy and it is less sensitive to the molecular weight distribution (see below). Here we deal with four stiff-chain polymers well characterized molecu-larly schizophyllan (a triple-helical polysaccharide), xanthan (double-helical ionic polysaccharide), PBLG, and poly (p-phenylene terephthalamide) (PPTA Kevlar). The wormlike chain parameters of these polymers are listed in Tables... 

The most easily identifiable characteristics are those related to the shape of the complexes, with their double-stranded helical cores. In this respect, the electrochemical and photochemical properties of Cu2(K-84)2+ are not much different from those of the open-chain helical precursor or its O-methylated version. The strong electronic coupling between the two copper centers is clearly a consequence of the 1,3-phenylene bridges between the two complex subunits and the topological properties of the ligand have virtually no influence. 

In addition to these helical polymers, phenylene-pyridinylenes have been investigated as linear oligomers held in place by intramolecular hydrogen bonds. This is demonstrated by the oligomer 62, and is expressed in terms of its optical properties <2005CEJ5889>. 

The introduction of angular connectors (e.g. 1,3-phenylenes, 2,7-naphthylenes) in the Jt-conjugated backbones may provide an oligomer with a preferred helical... 

Fig. 7 Schematic representation of the formation of a helical columnar structure by hierarchical self-assembly of a chiral oligo(p-phenylene vinylene) 10 bearing ureido-s-triazine units in M-dodecane. (Reproduced with permission from [51]. Copyright 2005 American Chemical Society)...

Even the distorted boat-like deck in [2.2]metacyclophanes can be constructed by an intramolecular version of the benzannulation. A suitable precursor bears a chromium vinylcarbene and an allcyne moiety linked to a meta-phenylene core by two-atom bridges, as shown for complexes 80. Benzannulation under the typical conditions affords hydro-quinonophanes 81 in fair yields (Scheme 31) [73]. Interestingly, the intramolecular benzannulation approach even tolerates heteroatom bridges, which impose both additional strain and helicity on the cyclophane skeleton [73b]. 

As an aside, it is noted that the latter may not be a hard and fast rule, at least when other ligand types are present. Sauvage, Balzani et alP used NMR data as well as absorption and fluorescence results to demonstrate the protonation-driven formation of a double-helical structure involving two ligand strands derived from a pair of 2-/ -anisyl-l,10-phenanthroline units linked in their 9-positions by a 1,3-phenylene spacer. In this case, the first protonation step does not simply represent mono-protonation of one of the phenanthroline units but rather appears to involve a co-operative interaction with a pair phenanthroline units (on different strands). 

The remarkable cylindrical supramolecular structure 56 forms by spontaneous assembly from 11 particles five ligand molecules (three 5,5 -linked bis-bipyridines and two hexa-azatriphenylene ligands) and six copper(I) ions. The hexa-azatri-phenylene ligands remain flat and are orientated almost parallel to each other they are not eclipsed as shown in 56 but are rotated with respect to each other by 27°. Reflecting this, the overall structure has a triple-helical twist. After allowing for van der Waals radii, the internal cavity is about 4 A in diameter and 4 A high. 

All the electrochemical data obtained for the phenylene bridged knots clearly show that the presence of a eonducting spacer inside a narrow rigid central helical core leads to electrochemical properties that are noticeably different from those of knotted complexes in which the two metal-based components are linked by nonconducting -(CH2)n- spacers [49]. 

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