Helical Oligoenes: Conformations, Bond Alternation, and Competing Through‐Bond and Through‐Space Transmission. Issue 14 (17th February 2016) - Tsuji, Yuta
Abstract: There is a consensus that long‐range electron transfer/transport occurs by a through‐bond rather than through‐space mechanism. In helical all‐ Z, all‐s‐ cis oligoenes, one can set up an interesting competition in the medium‐separation regime between a closer (in distance) through‐space path and a more distant through‐bond one. Although such oligoene conformations/isomers are unstable (by around 4 kcal mol −1 per double bond relative to all‐ E, all‐s‐ trans isomers), recent synthetic efforts on truncated helicenes and oligothiophenes have provided related molecules. On the way to transmission calculations with electrodes attached to the termini of helical oligoenes, we uncover an interesting conformational ambiguity in all‐ Z, all‐s‐ cis oligoenes, the existence of a broad conformational minimum for helical compression, with hints of end‐to‐end frontier‐orbital‐caused stabilization. There is relationship between helical oligoene structures and the corresponding substructure of a helicene, but there are also significant differences in the number of olefin subunits per helix turn. In Hückel transport calculations, the role of TB or TS mechanisms is obscured to an extent by variations in bond alternation and dihedral angle along the oligomer chain. However, the operation of a dominant through bond mechanism emerges clearly in local transmission plots. In moving the electrodes to carbon position related by quantum interference, it is possible to uncover a through space mechanism. Abstract : The structures of and electron transport through helical oligoenes were studied theoretically to determine whether the dominant transport mechanism is through‐space or through‐bond. A range of almost equal energy structures for all‐ Z all‐s‐ cis tetraene, pentaene, and hexaene varying in helix pitch was found. An intramolecular orbital interaction stabilizes a more compact conformation in the longer helical oligoenes.