Helical Oligoenes: Conformations, Bond Alternation, and Competing Through‐Bond and Through‐Space Transmission

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.

Conformations of cyclopentasilane stereoisomers control molecular junction conductance

“…As the molecule is spatially crowded, intramolecular distances between atoms are relatively small, so that through-space interactions may give rise to a significant contribution to the total transmission. This contribution to the transmission has recently been thoroughly studied in π-conjugated helicenes, 44 – 46 and have also been demonstrated to be significant in cyclic and linear silanes. 12 , 13 , 43 We note the distinction between through-bond and through-space interactions is a bit arbitrary and no difference was found between the physical mechanisms of these two processes.…”

Exploring Electrical Currents through Nanographenes: Visualization and Tuning of the through‐Bond Transmission Paths

“…When the bonds are equalized, the preference for a single direction is lost and the current travels equally through both arms of the cycle to the other contact. [13,14] Even more recently,N ozakia nd Schmidt developed an alternative method to plot local currents, obtained in the same formalism as the Artaios code (NEGF), in ar eal space representation. [15] Ac ode, capable of producing similarr eal space currentm aps but workingi nt he Source-and-Sink Potential (SSP) method,h as been developed almost simultaneously by some of the authors of the current study and will be published soon.…”

“…only through one arm of the cycle). When the bonds are equalized, the preference for a single direction is lost and the current travels equally through both arms of the cycle to the other contact . Even more recently, Nozaki and Schmidt developed an alternative method to plot local currents, obtained in the same formalism as the Artaios code (NEGF), in a real space representation .…”

Crystal structure of tetramethylammonium 1,1,7,7-tetracyanohepta-2,4,6-trienide

“…Polymethines, being fully conjugated hydrocarbons, represent the simplest 'molecular wires' with potential uses in organic electronic applications thanks to their easily tuned band gaps, and their wide range of absorption covering the visible spectrum (Etemad & Heeger, 1982;Meisner et al, 2012;Jayamurugan et al, 2014). Crystallographic data for polymethines are rather scare because of their instability and low solubility (Chetkina & Bel'skii, 2002;Meisner et al, 2012;Tsuji & Hoffmann, 2016). A successful strategy to increase the chemical stability with respect to oxidative decomposition has been reported (Meisner et al, 2012) that includes the decoration of polyenes with cyano groups and which resulted in the synthesis of a library of odd-numbered members from three to thirteen linear conjugated olefins and the determination of their crystal structures.…”