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.