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Interactions between electrodes and pendant groups in cross-conjugated systems destroy destructive quantum interference resulting in an increase of G and a decrease of S.

Dialkynylferrocenes exhibit attractive electronic and rotational features that make them ideal candidates for use in molecular electronic applications. However previous works have primarily focussed on single-molecule studies, with limited opportunities to translate these features into devices. In this report, we utilise a variety of techniques to examine both the geometric and electronic structure of a range of 1,1’-dialkynylferrocene molecules, as either single-molecules, or as self-assembled monolayers. Previous single molecule studies have shown that similar molecules can adopt an ‘open’ conformation. However, in this work, DFT calculations, STM-BJ experiments and AFM imaging reveal that these molecules prefer to occupy a ‘hairpin’ conformation, where both alkynes point towards the metal surface. Interestingly we find that only one of the terminal anchor groups binds to the surface, though both the presence, and nature of the second alkyne affects the thermoelectric properties of these systems. First, the secondary alkyne acts to affect the position of the frontier molecular orbitals, leading to increases in Seebeck coefficient. Secondly, theoretical calculations suggested that rotating the secondary alkyne away from the surface acts to modulate thermoelectric properties. This work represents the first of its kind to examine the assembly of dialkynylferrocenes, providing valuable information about both their structure and electronic properties, as well as unveiling new ways in which both of these properties can be controlled.

We report an experimental and theoretical study of the single-molecule conductance of two separate families of regioisomeric phthalocyanine derivatives in which the binding groups are connected at opposite or at contiguous positions about the phthalocyanine moiety. We observe that compounds with the longest distance between the anchoring groups yield molecular junctions with a higher conductance compared to those with a shorter distance. We performed both density functional theory (DFT) and tight binding calculations to understand how interference effects explain the experimental results including the role of constructive and destructive quantum interference in five phthalocyanine derivatives.

The authors regret that the name of one of the authors (Troy L. R. Bennett) was shown incorrectly in the original article. The corrected author list is as shown above. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.

This thesis investigates the fundamental aspects of the molecular-scale junctions and their electrical properties. Experimental and theoretical studies assessed the importance of finding ways to get consistent and reproducible improvements in electronics devices fabrications techniques. In this context, I will start this thesis by introducing a general discussion about using the density functional theory (DFT), and Green's function to study transport calculations at a molecular scale. Then, I will present the theoretical and experimental benzo-(bis)imiadzol molecular studies in chapter 4. This study focusses on changes in the conductance as a consequence of chemical stimuli. I will demonstrate that benzo-bis(imidazole) conductance switching upon protonation depends on the lateral functional groups. The protonated H-substituted molecule shows a higher conductance than the neutral one (Gpro>Gneu), while the opposite (Gneu>Gpro) is observed for a molecule functionalized by amino-phenyl groups. Based on theoretical calculations, I conclude that these opposite behaviours depend on the electronic coupling between molecules and electrodes. Furthermore, quantum interference properties have recently attracted excessive interest in electron transport studies at the single-molecule scale. Within this framework, myself and collaborators have aimed to improve the efficiency of pi-stacked molecules in controlling quantum interference by carefully designing them. Chapter 5 introduces a novel strategy for designing folded carbazoles with low conductance in different structures. This strategy highlights the presence or absence of destructive quantum interference in different configurations. This project is part of a collaborations with the experimental group at the University of Madrid, which is ongoing at present.

The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up of QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on CQI effects within the core. We also demonstrate that for molecules with thioether anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ∼50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step toward functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications

The functionalities offered by single-molecule electrical junctions are yet to be translated into monolayer or few-layer molecular films, where making effective and reproducible electrical contact is one of the challenging bottlenecks. Here we take a significant step in this direction by demonstrating that excellent electrical contact can be made with a monolayer biphenyl-4,4′-dithiol (BPDT) molecular film, sandwiched between gold and graphene electrodes. This sandwich device structure is advantageous, because the current flows through the molecules to the gold substrate in a 'cross-plane' manner, perpendicular to the plane of graphene, yielding high-conductance devices. We elucidate the nature of the cross-plane graphene/molecule/Au transport using quantum transport calculations and introduce a simple analytical model, which captures generic features of the current-voltage characteristic. Asymmetry in junction properties results from the disparity in electrode electrical properties, the alignment of the BPDT HOMO-LUMO energy levels and the specific characteristics of the graphene electrode. The experimental observation of scalability of junction properties within the junction area, in combination with a theoretical description of the transmission probability of the thiol-graphene contact, demonstrates that between 10% and 100% of the molecules make contact with the electrodes, which is several orders of magnitude greater than that achieved to date in the literature. © 2018 The Royal Society of Chemistry.

The lines in Fig. 8 are incorrectly labeled. The caption for Fig. 8 should read: "Fig. 8. DFT transmission curves. (a) The black curve shows the transmission coefficient for molecule b of Fig. 3. (b) The blue dashed curve shows the transmission coefficient for molecule a of Fig. 3.’’ Therefore, the first two paragraphs of Section III should read: "So far the discussion has focused on tight-binding representations of the molecules shown in Fig. 7. For molecules (a and b) containing two carbonyl groups, Fig. 8 shows DFT results for the transmission coefficients (see Methods for more details). In common with the tight binding results of Fig. 4, both a and b show alternating destructive interference features. In the case of a, such a feature occurs between the LUMO and LUMO+1, whereas in the case of b, they occur between the HOMO and LUMO and between LUMO+2 and LUMO+3. On the other hand for both molecules, destructive interference features appear near E = -2.4 eV, which are almost independent of connectivity, signaling the presence of a Fano resonance.’’ The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.

We have employed the scanning tunneling microscope break-junction technique to investigate the single-molecule conductance of a family of 5,15-diaryl porphyrins bearing thioacetyl (SAc) or methylsulfide (SMe) binding groups at the ortho position of the phenyl rings (S2 compounds). These ortho substituents lead to two atropisomers, cis and trans, for each compound, which do not interconvert in solution under ambient conditions; even at high temperatures, isomerization takes several hours (half-life 15 h at 140 degrees C for SAc in C2Cl4D2). All the S2 compounds exhibit two conductance groups, and comparison with a monothiolated (S1) compound shows the higher group arises from a direct Au-Porphyrin interaction. The lower conductance group is associated with the S-to-S pathway. When the binding group is SMe, the difference in junction length distribution reflects the difference in S-S distance (0.3 nm) between the two isomers. In the case of SAc, there are no significant differences between the plateau length distributions of the two isomers, and both show maximal stretching distances well exceeding their calculated junction lengths. Contact deformation accounts for part of the extra length, but the results indicate that cis-to-trans conversion takes place in the junction for the cis isomer. The barrier to atropisomerization is lower than the strength of the thiolate Au-S and.Au-Au bonds, but higher than that of the Au- SMe bond, which explains why the strain in the junction only induces isomerization in the SAc compound.

The design of thermoelectric materials for the efficient conversion of waste heat into electricity requires simultaneous tuning of their electrical and thermal conductance. A comparative theoretical study of electron and phonon transport in thiophene and ethylenedioxythiophene (EDOT) based molecular wires is performed. It is shown that modifying thiophene by substituting ethylenedioxy enhances the thermoelectric figure of merit ZT for molecules of the same length. Furthermore, it is demonstrated that the electrical conductance of EDOT-based wires decays more slowly with length than that of thiophene-based wires and that their thermal conductance is lower. The room-temperature ZT of undoped EDOT is found to be rather low. However, doping of EDOT by the electron acceptor tolunenesulfunate increases the Seebeck coefficient and electrical conductance, while decreasing the thermal conductance, leading to a thermoelectric figure of merit as high as ZT = 2.4.

In this thesis we use a combination of density functional theory and equilibrium Green’s function to study thermoelectricity in molecular scale. We have aimed to improve the efficiency of single molecules in converting heat to electricity by carefully designing them. We introduced a novel strategy for designing molecules with low thermal conductance. This strategy states that adding side branches of different length to the backbone of a molecule can eliminate phonon transport through the backbone over a wide range of frequencies. Moreover, we demonstrate that chemical modification of thiophene molecular wire to ethylenedioxy-thiophene molecular wire can improve their thermoelectric efficiency. Furthermore, to demon- strate that the adopted molecular designs will be effective when scaled up to a self-assembled monolayer (SAM) of molecules, we model three independent exper- iments on gold-SAM-graphene vertical transport devices. The agreement between our calculations and the experiments elucidates the survival of quantum interfer- ence effect on a single molecular level in SAM devices. The work presented in this thesis has received considerable attention from many experimental groups in the UK and overseas stimulating novel experimental studies which are ongoing at present.

Electrical conductance across a molecular junction is strongly determined by the anchoring group of the molecule. Here we highlight the unusual behavior of 1,4-bis(1H-pyrazol-4-ylethynyl)benzene that exhibits unconventional junction current versus junction-stretching distance curves, which are peak-shaped and feature two conducting states of 2.3 × 10-4 G0 and 3.4 × 10-4 G0. A combination of theory and experiments is used to understand the conductance of single-molecule junctions featuring this new anchoring group, i.e., pyrazolyl. These results demonstrate that the pyrazolyl moiety changes its protonation state and contact binding during junction evolution and that it also binds in either end-on or facial geometries with gold contacts. The pyrazolyl moiety holds general interest as a contacting group, because this linkage leads to a strong double anchoring of the molecule to the gold electrode, resulting in enhanced conductance values.