Single-molecule electron transport near a charge-trapping orbital-level alignment

The transport properties of single-molecule junctions are fundamentally governed by the energy alignment of molecular frontier orbitals relative to the electrode Fermi level. Although charging-induced reorganization is known to significantly shift this alignment, precisely how these shifts give rise to distinct transport behaviors remains elusive. Here, we use combined scanning tunneling microscopy (STM) and non-contact atomic force microscopy (AFM) at 5.5 K to investigate individual copper phthalocyanine (CuPc) molecules on a bilayer NaCl film supported by Cu(100) substrate. We identified three distinct transport phenotypes on the same substrate-characterized by behaviors ranging from elastic tunneling and dynamic charging to stable charge trapping. These phenotypes depend mainly on the reorganized orbital energy of the singly occupied molecular orbital (SOMO) relative to the substrate Fermi energy. We find that when the SOMO lies close to the substrate Fermi level, the molecule enters a sensitive charge-trapping regime in which an energy shift of only ~100 meV can change the charged-state lifetime by several orders of magnitude. This result is well captured by a theoretical model. Furthermore, we demonstrate that a pentacene molecule-functionalized tip can switch the transport behaviors. Our findings reveal how atomic-scale dielectric disorder within the moiré superlattice amplifies subtle electronic inhomogeneities into discrete transport regimes, underscoring the extreme sensitivity of molecular conductance to orbital-level alignment near the charge trapping regime and to local environmental engineering.