Controlling quantum interference in tetraphenyl-aza-BODIPYs

This study presents systematic theoretical investigations employing an ab initio DFT approach combined with analysis of heuristic tight-binding models. We examine the effect of using conjugated and non-conjugated bridge on the electrical transport of tetraphenyl-aza-BODIPY derivatives. This work demonstrates that, substitution a conjugated bridging atom by non-conjugated one, causes the electrical conductance to switch from constructive quantum interference CQI to destructive DQI (on/off). This demonstration of switching behaviour means that if molecules with alternating structures (i.e., non-/conjugated), can be deposited on a metal surface, then they form a basis for enhancing the thermovoltage in nanoscale thermoelectric nanotechnology devices

Conductance behavior of tetraphenyl-Aza-Bodipys

We studied the electrical conductance of single-molecule junctions formed from molecular wires with four anchor groups. Three tetraphenyl-aza-BODIPYs with four or two thiomethyl anchor groups were synthesized, and their single-molecule conductance was measured using break-junction-STM. Using DFT based calculations these compounds were shown to display a combination of a high and low conductance, depending on the molecule's connectivity in the junction. A scissor correction is employed to obtain the corrected HOMO-LUMO gaps and a tight binding model (TBM) is used to highlight the role of transport through the pi system of the tetraphenyl-aza-BODIPY central unit. The three higher-conductance geometries follow the sequence 3 > 4 > 2, which demonstrates that their conductances are correlated with the number of anchors

Molecular-scale thermoelectricity:a worst-case scenario

This article highlights a novel strategy for designing molecules with high thermoelectric performance, which are resilient to fluctuations. In laboratory measurements of thermoelectric properties of single-molecule junctions and self-assembled monolayers, fluctuations in frontier orbital energies relative to the Fermi energyE(F)of electrodes are an important factor, which determine average values of transport coefficients, such as the average Seebeck coefficient & x3008;S & x3009;. In a worst-case scenario, where the relative value ofE(F)fluctuates uniformly over the HOMO-LUMO gap, a "worst-case scenario theorem" tells us that the average Seebeck coefficient will vanish unless the transmission coefficient at the LUMO and HOMO resonances take different values. This implies that junction asymmetry is a necessary condition for obtaining non-zero values of & x3008;S & x3009; in the presence of large fluctuations. This conclusion that asymmetry can drive high thermoelectric performance is supported by detailed simulations on 17 molecules using density functional theory. Importantly, junction asymmetry does not imply that the molecules themselves should be asymmetric. We demonstrate that symmetric molecules possessing a localised frontier orbital can achieve even higher thermoelectric performance than asymmetric molecules, because under laboratory conditions of slight symmetry breaking, such orbitals are 'silent' and do not contribute to transport. Consequently, transport is biased towards the nearest "non-silent" frontier orbital and leads to a high ensemble averaged Seebeck coefficient. This effect is demonstrated for a spatially-symmetric 1,2,3-triazole-based molecule, a rotaxane-hexayne macrocycle and a phthalocyanine

Single-molecule conductance oscillations in alkane rings

We investigate the single-molecule electrical conductance of alkane rings connected to gold electrodes and demonstrate that their logarithmic conductances are ocillatory functions of length. This contrasts with the logarithmic conductances of alkane chains, which decay linearly with length. This non-classical behaviour is attributed to conformational effects in the alkane rings, which tend to be more (less) planar when their branches contain even (odd) numbers of CH2 groups. Surprisingly the conductances of alkane rings with two parallel conductance paths are predicted to be lower then those of the corresponding linear chains with only one conductance path

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

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