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.

We studied the interplay between quantum interference (QI) and molecular asymmetry in charge transport through a single molecule. Eight compounds with five-membered core rings were synthesized and their single-molecule conductances were characterized using the mechanically controllable break junction (MCBJ) technique. It is found that the symmetric molecules are more conductive than their asymmetric isomers and there is no statistically-significant dependence on the aromaticity of the core. In contrast, we find experimental evidence of destructive QI in fivemembered rings, which can be tuned by implanting different heteroatoms into the core ring. Our findings are rationalized by the presence of anti-resonance features in the transmission curves calculated using non-equilibrium Green"s functions. This novel mechanism for modulating QI effects in charge transport via tuning of molecular asymmetry will lead to promising applications in the design of single-molecule devices.

We demonstrate the bottom-up in-situ formation of organometallic oligomer chains at the single-molecule level. The chains are formed using the mechanically controllable break junction technique operated in a liquid environment, and consist of alternating isocyano-terminated benzene monomers coordinated to gold atoms. We show that the chaining process is critically determined by the surface density of molecules. In particular, we demonstrate that by reducing the local supply of molecules within the junction, either by lowering the molecular concentration or by adding side groups, the oligomerization process can be suppressed. Our experimental results are supported by ab-initio simulations, confirming that the isocyano terminating groups display a high tendency to form molecular chains, as a result of their high affinity for gold. Our findings open the road for the controlled formation of one-dimensional, single coordination-polymer chains as promising model systems of organometallic frameworks.

We report the synthesis of a series of oligophenylene-ethynylene (OPE) derivatives with biphenylene core units, designed to assess the effects of biphenylene antiaromaticity on charge transport in molecular junctions.

The influence of electron donors and electron acceptors of variable strength in the 4 and 4' position of 2 and 2' propyl-bridged axial chiral biphenyl cyclophanes on their atropisomerization process was studied. Estimated free energies ΔG(‡)(T) of the rotation around the central biphenyl bond which were obtained from (1)H-NMR coalescence measurements were correlated to the Hammett parameters σ(p) as a measure for electron donor and acceptor strength. It is demonstrated that the resulting nice linear correlation is mainly based on the influence of the different substituents on the π-system of the biphenyl cyclophanes. By lineshape analysis the rate constants were calculated and by the use of the Eyring equation the enthalpic and entropic contributions were evaluated. Density functional theory calculations show a planar transition state of the isomerization process and the calculated energy barriers based on this reaction mechanism are in good agreement with the experimentally obtained free energies.

We synthesized a series of CBP derivatives, using methyl groups as spatially demanding groups, locking the angle between the carbazole subunit and the biphenyl backbone as potential matrix material for blue organic light emitting diodes (OLEDs). The locked rotation was achieved by four methyl groups either in the 1,8-positions of the carbazole subunit (1) or in the 3,5,3',5'-positions of the biphenyl subunit (2) and the fixed spatial arrangement was confirmed by x-ray analysis. The physical properties of CBP derivatives based on the parent structure 2 were further tailored by electron withdrawing CF 3groups in 3,6-or 2,7-positions of the carbazole subunits (3 and 4) or alternatively by electron donating CH 3 O-groups in the 2,7-positions (5) of the same building blocks. Increased triplet energies (E T ) compared to the parent compound CBP were found for all synthesized CBP derivatives 1-5. Enhanced glass transition temperatures ranging between 129°C to 205°C further corroborate the application potential of these derivatives for matrix material in blue emitting OLEDs.

Seebeck coefficient measurements provide unique insights into the electronic structure of single-molecule junctions.

and molecular electronics, [11,12] over medical diagnostic devices [13][14][15] to applications tuning macroscopic and/or environmental features like, e.g., the wettability [16][17][18] or the biocompatibility [19][20][21] of material surfaces. In most cases, a bifunctional molecule, combining an anchor group attaching the molecular compound to the surface with an exposed subunit representing the active moiety with tailored physiochemical properties is used to tune the surface's terminal appearance. While such approaches are successfully applied for entire objects and surfaces of considerable dimensions and macroscopic separations, there are limitations as soon as spatial patterns of varying surface functionalities and microscopic separations are desired or a spatially constrained surface-access including buried microfluidic chips has to be overcome. For example, conventional wet chemical surface functionalization procedures are spatially limited to the size of the droplet deposited (and its subsequent surface interaction) and methods depositing the molecules from gas-phase in vacuum require advanced masking strategies. Challenging are devices with a variety of molecular features to be created in a parallel process and in a locally controlled manner, like, e.g., multidimensional sensing platforms where a multitude of nearby molecular receptors are screening analytes in parallel. Or microfluidic chips with constrained access to functional sites as the channels are locally enclosed and inaccessible for droplet deposition. Of great interest are devices analyzing molecular binding events electronically, due to their miniaturization potential and the analyte-selective binding without the requirement of labels. In such devices, each electrode is electrically addressable in a separated manner as they were contacted individually. Such a preexisting electrical wiring scheme is a unique feature for an immobilization strategy able to distinguish between the applied electrical potentials of each electrode and a common electrolyte. The electrochemical release of thiol-based self-assembled monolayers (SAMs) on gold electrodes has been reported. [22] This allows the disintegration of existing SAMs. The subsequent decoration of the liberated electrode with an alternative thiol-molecule, however, is handicapped by scrambling with the remaining SAMs. Electrochemical triggered constructive build-up of molecular Local functionalization of surfaces is a current technological challenge. An electrochemically addressable alkyne protection group is presented enabling the site-selective liberation of alkynes exclusively on electrified electrodes. This controlled deprotection is based on a mendione chromophore which becomes a strong enough nucleophile upon reduction to intramolecularly attack the trialkylsilane alkyne protection group. The site-selective liberation of the alkyne is demonstrated by immobilizing the protected alkyne precursor on a transparent TiO 2 electrode and subsequently immobilizing red and blue azide dyes by azide-alky...