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In this thesis, a combination of density functional theory (DFT) based calculations and nonequilibrium Green’s functions are employed to investigate electron transport in molecular switches, molecular cords and nanoscale devices.   Molecular electronic devices have been proposed as an approach to complement today’s silicon based electronic devices. However, engineering of such miniature devices and design of functional molecular components still present significant challenges.   First, the way to connect a molecule to conductive electrodes has to be controlled. We study, in a nanoelectrode-nanoparticle platform, how structural changes affect the measured conductance and how current fluctuations due to these structural changes can be decreased. We find that, for reproducible measurements, it is important to have the molecules chemically bonded to the surfaces of adjacent nanoparticles. Furthermore, we show by a combination of DFT and theoretical modeling that we can identify signals from single-molecules in inelastic electron spectroscopy measurements on these devices.   Second, active elements based on molecules, some examples being switches, rectifiers or memory devices, have to be designed. We study molecular conductance switches that can be operated by light and/or temperature. By tuning the substituents on the molecules, we can optimize the shift of the most conducting molecular orbital and increase the effective coupling between the molecule and the electrodes when going from the OFF to the ON-state of the switches, giving high switching ratio (up to three orders of magnitude). We also study so called mechanoswitches that are activated by a mechanical force elongating the molecules, which means that these switches could operate as sensors.   Furthermore, we have studied two different classes of compounds that may function either as rigid molecular spacers with a well-defined conductance or as molecular cords. In both cases, we find that it is of great importance to match the conjugation of the anchoring groups with the molecular backbone for high conductance.   The last part of the thesis is devoted to another interesting semiconductor material, diamond. We have accurately calculated the band structure and effective masses for this material. Furthermore, these results have been used to calculate the Hall coefficient, the resistivity and the Seebeck coefficient.

This thesis focuses on the chemical concept of conjugation, i.e., electron delocalization, and the effect it has on electronic and optical properties of molecules. The emphasis is on electron delocalization across a saturated σ-bonded segment, and in our studies these segments are either inserted between π-conjugated moieties or joined together to form longer chains. The electronic and optical properties of these compounds are probed and compared to those of traditionally π-conjugated compounds. The investigations utilize a combination of qualitative chemical bonding theories, quantum chemical calculations, chemical syntheses and different spectroscopic methods. Herein, it is revealed that a saturated σ-bonded segment inserted between two π-systems can have optical and electronic properties similar to a cross-conjugated compound when substituents with heavy Group 14 elements (Si, Ge or Sn) are attached to the central atom. We coined the terminology cross-hyperconjugation for this interaction, and have shown it by both computational and spectroscopic means. This similarity is also found in cyclic compounds, for example in the 1,4-disilacyclohexa-2,5-dienes, as we reveal that there is a cyclic aspect of cross-hyperconjugation. Cross-hyperconjugation can further also be found in smaller rings such as siloles and cyclopentadienes, and we show on the similarities between these and their cross-π-conjugated analogues, the fulvenes. Here, this concept is combined with that of excited state aromaticity and the electronic properties of these systems are rationalized in terms of “aromatic chameleon” effects. We show that the optical properties of these systems can be rationally tuned and predicted through the choice of substituents and knowledge about the aromaticity rules in both ground and excited states. We computationally examine the relation between conjugation and conductance and reveal that oligomers of 1,4-disilacyclohexa-2,5-dienes and related analogues can display molecular cord properties. The conductance through several σ-conjugated silicon compounds were also examined and show that mixed silicon and carbon bicyclo[2.2.2]octane compounds do not provide significant benefits over the open-chain oligosilanes. However, cyclohexasilanes, a synthetic precursor to the bicyclic compounds, displayed conformer-dependent electronic structure variations that were not seen for cyclohexanes. This allowed for computational design of a mechanically activated conductance switch.