Synergistic regulation of conductance via functional groups and connection topology in cyclopentadienone molecular junctions

Abstract

Cyclopentadienone represents a class of antiaromatic five-membered ring systems whose conductance regulation by functional groups and connection topology remains unclear. Herein, we design and synthesize six cyclopentadienone derivatives by varying the central functional group (CO, CH(OH), CH₂) and the anchoring positions (3,4-: ortho; 2,5-: meta). Single-crystal structures of o-CO and m-CO reveal distinct packing modes. UV-vis spectra show that meta-isomers exhibit a ∼10 nm redshift relative to ortho-isomers, indicating a narrower energy gap. Conductance measurements via the STM-BJ technique show that all meta-isomers exhibit higher conductance than their ortho-counterparts, with the order: m-CHOH (10^−3.66 G₀) > m-CH₂ (10^−3.77 G₀) > m-CO (10^−3.99 G₀) > o-CO (10^−4.39 G₀) > o-CH₂ (10^−4.45 G₀) > o-CHOH (10^−4.72 G₀). This establishes connection topology as the primary factor governing conductance. Functional group modification further tunes the meta/ortho conductance ratio: the hydroxyl group increases it to ∼11.5, whereas the carbonyl group lowers it to ∼2.5, revealing a strong synergy. Theoretical calculations show that although the carbonyl group confers the strongest antiaromaticity and smallest gap, its cross-conjugation introduces destructive quantum interference (DQI) in the meta-pathway, suppressing conductance. The hydroxyl group exerts opposite effects depending on the pathway: in the meta-pathway with constructive quantum interference (CQI), it optimizes energy alignment, whereas in the ortho-pathway with DQI, it shifts the transmission minimum toward the Fermi level, further suppressing conductance. This study demonstrates that antiaromaticity alone is insufficient to predict conductance, which is synergistically regulated by quantum interference and electronic effects dictated by topology and functional groups.