Electric-Field Gating of Dynamic through-Space Conjugation in sp2/sp3-Bridged Molecular Junctions

The precise identification and regulation of dynamic charge-transport pathways in multiaryl frameworks are pivotal for unlocking advanced optoelectronic functionalities and elucidating single-molecule transport phenomena. However, achieving reversible on-off switching of through-space conjugation (TSC) and quantitatively correlating this process with single-molecule conductance remain formidable challenges. Herein, using tetraphenylethylene (TPE-2S) and tetraphenylethane (TPA-2S) as model systems, we demonstrate that external electric fields can reversibly gate dynamic TSC channels, enabling in situ and quantitative modulation of single-molecule conductance via the scanning tunneling microscopy break-junction (STM-BJ) technique. Systematic concentration- and bias-dependent STM-BJ control measurements, flicker-noise spectroscopy, and comprehensive DFT-NEGF transport calculations collectively validate that elevated applied bias drives reversible intramolecular torsional rearrangement and pairwise π-π dimerization of the flexible sp3-bridged TPA-2S, triggering a distinct conductance transition from weak monomeric through-bond tunneling to efficient intermolecular through-space charge transport. Comprehensive single-crystal and spectroscopic analyses reveal that the central C═C bond in TPE-2S sustains efficient through-bond conjugation, whereas the sp3-hybridized C-C bridge in TPA-2S disrupts covalent connectivity, confining electronic delocalization. Complementary transmission eigenchannel, local current density, and bias-dependent projected device density of states (PDDOS) calculations directly visualize charge delocalization across the phenyl-phenyl stacking interface in TPA-2S dimers. BSSE-corrected intermolecular binding energies, field-amplified dipole polarization, and diabatic electronic coupling matrix elements further provide quantitative thermodynamic evidence that an external electric bias thermodynamically stabilizes the π-stacked dimer assembly. Flicker noise spectroscopy and theoretical transport calculations corroborate the reversible interconversion between single-channel monomer tunneling and multichannel dimer through-space conduction. This study establishes electric-field gating as a robust strategy for manipulating dynamic TSC and precisely tuning single-molecule conductance, positioning STM-BJ as a powerful platform to resolve transient single-junction transport behavior inaccessible via ensemble spectroscopy, and laying a foundational framework for designing stimuli-responsive single-molecule devices.