Quantum interference and heteroaromaticity of para- and meta-linked bridged biphenyl units in single molecular conductance measurements

Abstract

Is there a correlation between the (hetero)aromaticity of the core of a molecule and its conductance in a single molecular junction? To address this question, which is of fundamental interest in molecular electronics, oligo(arylene-ethynylene) (OAE) molecular wires have been synthesized with core units comprising dibenzothiophene, carbazole, dibenzofuran and fluorene. The biphenyl core has been studied for comparison. Two isomeric series have been obtained with 4-ethynylpyridine units linked to the core either at para-para positions (para series 1–5) or meta-meta positions (meta series 6–10). A combined experimental and computational study, using mechanically controlled break junction measurements and density functional theory calculations, demonstrates consistently higher conductance in the para series compared to the meta series: this is in agreement with increased conjugation of the π–system in the para series. Within the para series conductance increases in the order of decreasing heteroaromaticity (dibenzothiophene < carbazole < dibenzofuran). However, the sequence is very different in the meta series, where dibenzothiophene ≈ dibenzofuran < carbazole. Excellent agreement between theoretical and experimental conductance values is obtained. Our study establishes that both quantum interference and heteroaromaticity in the molecular core units play important and inter-related roles in determining the conductance of single molecular junctions.

Exploring antiaromaticity in single-molecule junctions formed from biphenylene derivatives

“…When the electron pathway passes directly through the antiaromatic core unit, the conductance of the 2,7-disubstituted biphenylene derivative 2 is 32 times lower than that of 1, which agrees with its fluorene analogue in a previous report. 32 Molecule 3 with SMe anchoring groups shows a slightly higher conductance value (10 −4.4±0.52 G 0 ) than that of its pyridyl analogue 1 and agrees well with 1,4-bis( p-methylthiophenylethynyl) Fig. 2 Typical conductance traces for compounds 1-4 (a) and 5-8 (b), respectively.…”

“…30 In contrast, a study on a comparable series of 2,5-disubstitutedfuran, -pyrrole, -thiophene and -cyclopentadiene derivatives with pyridyl anchors found no statistically significant dependence of the conductance on the aromaticity of the core. 31 For molecules with tricyclic cores, it has been shown that increasing aromaticity at the core decreases the conductance for the para-linked molecules (dibenzothiophene < carbazole < dibenzofuran < fluorene) with pyridyl anchors, 32 in agreement with Venkatarman et al 30 However, the sequence is different for the isomers where the tricyclic core is meta-linked into the backbone (dibenzothiophene ≈ dibenzofuran < carbazole ≈ fluorene). 32 It is therefore of particular interest to explore the effect of incorporating 4n π-electron antiaromatic rings into the backbone of molecules that are wired into junctions.…”

“…31 For molecules with tricyclic cores, it has been shown that increasing aromaticity at the core decreases the conductance for the para-linked molecules (dibenzothiophene < carbazole < dibenzofuran < fluorene) with pyridyl anchors, 32 in agreement with Venkatarman et al 30 However, the sequence is different for the isomers where the tricyclic core is meta-linked into the backbone (dibenzothiophene ≈ dibenzofuran < carbazole ≈ fluorene). 32 It is therefore of particular interest to explore the effect of incorporating 4n π-electron antiaromatic rings into the backbone of molecules that are wired into junctions. There are very few reported measurements on molecules of this type as they are usually demanding to synthesize and they are often unstable under ambient conditions.…”

Oscillating Seebeck coefficients in π-stacked molecular junctions

“…Recently it has been demonstrated both experimentally and theoretically that single molecules can display bi-thermoelectric behaviour, in which the sign of S changes in response to both geometric changes [10][11][12] and the application of pressure, 11 which shi the positions of the HOMO and LUMO transport resonances relative to E F . An alternative approach to the control of quantum transport through single molecules involves utilizing quantum interference (QI) of de Broglie waves associated with electrons passing through the molecule, [13][14][15][16][17][18][19][20][21][22][23][24][25] which can increase or decrease the electrical conductance depending on whether QI is constructive or destructive. One type of molecular junction that has shown QI involves two pi-stacked molecules, [26][27][28][29][30] as shown in Fig.…”

Unusual structure of a biphenyl fragment: the important role of weak interactions

“…One can tailor the properties of a device by modifying a small portion of the building unit. The backbone of the molecule will ultimately determine the specific function of the molecular device (McCreery, 2004;Michoff et al, 2013;Low et al, 2015;Xiang et al, 2016;Gantenbein et al, 2017;Bâ ldea, 2019;Lenfant et al, 2017). The molecular structure of biphenyl and its derivatives has been the subject of numerous studies.…”