Tuning Single-Molecule Conductance via Substituents at the 1-Position of a 2,5-Diarylpyrrole Core

Abstract

Whether a strong electronic effect can overcome a severely distorted conformation in determining single-molecule conductance remains unclear. Here we address this question through a series of 2,5-bis(4-(methylthio)phenyl)pyrrole (BTPP) derivatives functionalized at the 1‑position with substituents of varying electronic character (-NMe₂, -OMe, -H, -F, -CHO, -COOEt). Scanning tunneling microscopy break-junction (STM-BJ) measurements yield single-molecule conductance values ranging from 10-3.39 to 10-3.81 G₀, exhibiting a negative correlation with the para-Hammett constants (σp). Flicker noise analysis reveals that the phenyl group substituted at the 1-position of pyrrole can form a through-space conjugated transport pathway with the adjacent diaryl moiety, indicating a mixed transport mechanism of through-bond and through-space. Notably, the BTPP-NMe₂ molecule adopts a nearly perpendicular spatial conformation (θ₂ ≈ 89.5°), yet the strong electron-donating effect of -NMe₂ overcomes the detrimental influence of this unfavorable conformation, resulting in the highest conductance. This study reveals the coordinated mechanism in which electronic effects play a dominant role while conformation acts as a secondary modulating factor, establishing clear principles for the rational design of molecular wires.