Coherent electron transport in poly(p-phenylene).

Context: Conductive polymers like poly(p-phenylene) (PPP) exhibit polarons and bipolarons (radical cations/dications) in bulk. While individual π-conjugated molecules are generally conductive, the role of these charged states in single-molecule junctions, where deviations from bulk behavior are suggested, remains unclear. This study employs first-principles calculations to examine the relationship between charge state (neutral, radical cation, dication) and conductance in single oligo(p-phenylene) junctions. My results demonstrate significant conductance enhancement upon doping, yet reveal that the single-molecule charge transport mechanism deviates substantially from bulk expectations, highlighting differences between molecular and bulk electronic properties.

Methods: Geometry optimizations for mercapto-terminated octa(p-phenylene) (neutral, radical cation, and dication states) used Density Functional Theory (B3LYP functional, 6-31G(d,p) basis set, RHF/ROHF methods) via Gaussian 16. Coherent electron transport calculations for molecules bridging Au(111) electrodes employed the Non-Equilibrium Green's Function (NEGF)-DFT method with QuantumATK. These transport calculations utilized norm-conserving Troullier-Martin pseudopotentials, double-zeta plus polarization (DZP) basis sets for C, H, S, single-zeta plus polarization (SZP) for Au, and the Perdew-Burke-Ernzerhof (PBE) functional with the spin-polarized Generalized Gradient Approximation (SGGA).