The exploring and understanding the electronic properties of molecules connected to metallic leads is a vital part of nanoscience if molecule is to have a future. This thesis documents a study for various families of organic and organometallic molecules, which offer unique concepts and new insights into the electronic properties of molecular junctions. Different families of molecules were studied using a combination of density functional theory (DFT) and non-equilibrium Green’s function formalism of transport theory. The main results of this thesis are as follows:
A quantum circuit rule for combining quantum interference effects in the conductive properties of oligo(phenyleneethynylene) (OPE)-type molecules possessing three aromatic rings was investigated both theoretically and experimentally. Molecules were of the type X-Y-X, where X represents pyridyl anchors with para (p), meta (m) or ortho (o) connectivities and Y represents a phenyl ring with p and m connectivities. The conductances Gxmx (Gxpx) of molecules of the form X-m-X (X-p-X), with meta (para) connections in the central ring, were predominantly lower (higher), irrespective of the meta, para or ortho nature of the anchor groups X, demonstrating that conductance is controlled by the nature of quantum interference in the central ring Y. The singlemolecule conductances were found to satisfy the quantum circuit rule Gppp/Gpmp=Gmpm/Gmmm. This demonstrates that the contribution to the conductance from the central ring is independent of the para versus meta nature of the anchor groups.
The conductance and the decay of conductance as a function of molecular length within a homologous series of oligoynes, Me3Si― (C≡C)n―SiMe3 (n = 2, 3, 4, or 5), is shown to depend strongly on the solvent medium. Single molecule junction conductance measurements have been made with the I(s) method for each member of the series Me3Si―(C≡C)n―SiMe3 (n = 2, 3, 4, and 5) in mesitylene (MES), 1,2,4- trichlorobenzene (TCB), and propylene carbonate (PC). In mesitylene, a lower conductance is obtained across the whole series with a higher length decay (β ≈ 1 nm−1). In contrast, measurements in 1,2,4-trichlorobenzene and propylene carbonate give higher conductance values with lower length decay (β ≈ 0.1 and 0.5 nm−1 respectively). This behaviour is rationalized through theoretical investigations, where β values are found to be higher when the contact Fermi energies are close to the middle of the HOMO−LUMO gap but decrease as the Fermi energies approach resonance with either the occupied or unoccupied frontier orbitals. The different conductance and β values between MES, PC, and TCB have been further explored using DFT-based models of the molecular junction, which include solvent molecules interacting with the oligoyne backbone. Good agreement between the experimental results and these “solvated” junction models is achieved, giving new insights into how solvent can influence charge transport in oligoyne-based single molecule junctions.
The single molecule conductances of a series of bis-2,2′:6′,2″-terpyridine complexes featuring Ru(II), Fe(II), and Co(II) metal ions and trimethylsilylethynyl (Me3SiC≡C−) or thiomethyl (MeS-) surface contact groups have been determined theoretically and experimentally.
The single molecule conductance of metal complexes of general form transRu(C≡CArC≡CY)2(dppe)2 and trans-Pt(C≡CArC≡CY)2(PPh3)2 (Ar = 1,4-C6H2-2,5- (OC6H13)2; Y = 4-C5H4N, 4-C6H4SMe) have been determined theoretically and experimentally. The complexes display high conductance (Y = 4-C5H4N, M = Ru (0.4±0.18 nS), Pt (0.8±0.5 nS); Y = 4-C6H5SMe, M = Ru (1.4±0.4 nS), Pt (1.8±0.6 nS)) for molecular structures of ca. 3 nm in length, which has been attributed to transport processes arising from tunneling through the tails of LUMO states.