Solvent Dependence of the Single Molecule Conductance of Oligoyne-Based Molecular Wires

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 behavior is rationalized through theoretical and computational 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.