can be locked into either a right- or left-handed ''spiral staircase'' conformation . An intriguing variation on oligoparaphenylene helices are molecular ribbons reported by Fuji et al. (Fig. 1.2c). The locking mechanism in these configuration-ally defined oligonaphthalene derivatives relies on restricted rotation about single bonds due to unfavorable steric barriers (that is, atropisomerism) . Molecular rods are rigid ''unfoldable'' oligomers with well-defined molecular dimensions. For example, Semetey et al. have reported the synthesis of a series of water-soluble oligopiperidine molecular rods with as many as ten piperidine rings (Fig. 1.2d). There is NMR evidence for a chair conformation of each piperidine ring with each piperidine unit in an equatorial position with combined nitrogen inversion and chair-chair inversion throughout the well-defined backbone. According to the authors, there is rotation about the C-N bonds, but this results in only small deviations of the linear geometry of the molecule . A more rigid spiro-linked molecular rod was reported by Levins et al. This molecular scaffold was elegantly prepared by rigidifying a flexible oligomer via the formation of two diketopiperazine rings (Fig. 1.2e) . With regards to molecular rods, some molecules can have unrestricted rotations along the backbone but do not fold for geometric reasons - that is, if the backbone is linear (180° connectivity) no amount of rotation will cause molecular folding (however molecular ''twisting'' can occur). This is the case for oligo(para-phenylene ethylene) molecular wires reported by Schumm et al. .
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