Molecular electronics is a useful method for exploring nanoscale and discovering new organic materials that are both low-cost and environmentally friendly. This thesis presents the theoretical methods employed to support this process, starting in chapters 2 and 3, accordingly. I have discussed the fundamental equations and methods that underpin my work, such as the
Schrodinger equation, density functional theory (DFT), and the SIESTA program, which is responsible for implementing DFT and solving the equations that are underlying it. In addition, I present an explanation of the single particle transport theory, which is based on the Hamiltonian and Green's functions, as well as some examples of how it might be used.

Chapter 4. This chapter mainly discusses the influence of heteroatom including which position will alleviate destructive quantum interference (DQI), and which position will not. In addition, if we change linkers, the influence of heteroatom will change. These results are supported by my calculations.

Chapter 5. This chapter discusses the transport properties of stable organic radicals for electronic devices due to their half-filled orbitals near the Fermi energy. Also, see the systematic changes when we remove the hydrogen from the OH groups to produce the radicals, and how that affects the electrical conductance.