Extended curly arrow rules to rationalise and predict structural effects on quantum interference in molecular junctions

Abstract

An extension to curly arrow rules for the prediction of quantum interference behaviour in conjugated molecular wires widens the scope of this simple graphical method to wires containing heteroatoms, cross-conjugation and/or non-alternant structures.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

“…However, computational studies showed this was a steric rather than an electronic effect; enhanced conductance was seen in the transmission function if the methoxy substituent was constrained to an unfavourable position in which its lone pair could interact with the OPE π-system. 153 These 132,153 and other studies [34][35][36] have related the increased conductance associated with the addition of heteroatoms to meta-linked OAEs to the electron-withdrawing or -donating properties of the heteroatom(s). Charge transport simulations showed that the presence of donor or acceptor groups at certain positions shifted DQI antiresonances away from E F , meaning T (E) and hence conductance was higher in this region.…”

“…Certain structural features can cause antiresonances to be shifted away from E F and thereby reduce their impact on conductance. [34][35][36] Fig. 3 Cartoon illustrating the MCBJ method.…”

“…For polycyclic aromatic hydrocarbons, QI behaviour can be qualitatively predicted using e.g. graphical methods 34 or magic ratio rules. 117,126 More quantitative predictions rely on the calculation of transmission functions 31,32 (see Fig.…”

“…63 A theoretical study indicated that the connectivity of cross-conjugated anthraquinones can influence QI behaviour; 241 this observation has recently been rationalised using an approach based on curly arrows. 34 Zhang et al used OAE3s containing benzodithiophene (BDT) derivatives to investigate the relationship between conjugation and QI. 66 A linearly-conjugated BDT (17) was compared to its quinone analogue (35) and an isomeric cross-conjugated BDT (61), alongside an anthraquinone species (32) for reference.…”

“…137 Contrasting results were obtained for meta-connectivity (66, blue) where conductance followed the trend X = CMe 2 ≈ NEt > O ≈ S. Aromaticity alone cannot explain this conductance trend, and as for five-membered heterocycles 118 electronegativity and QI appear to be important factors. 137 In an alternative rationalisation, the heteroatoms can also be considered as cross-conjugating functionality 34 (cf. the discussion of the structurally-similar fluorenone-containing OAE 63b above 138 ).…”