Highly insulating alkane rings with destructive σ-interference

Destructive quantum interference(DQI)provides a unique approach to controlling the leakage current in the OFF state of molecular devices.However,the DQI in π-conjugated molecular building blocks cannot exhibit ultralow conductance due to the existence of covalently bonded σ-transport channels.Thus,suppressing the σ-contribution via σ-DQI is essential for the fabrication of molecular junctions with high insulation and effective modulation of conductance in single-molecule junctions.Here,we demonstrate the existence of σ-DQI even in a simple series of C_(m)C_(n) ring molecules,with parallel chains of m and n alkyl units(where m,n=6,8 or 10),by measuring their electrical conductance and Seebeck coefficients.Counterintuitively,the conductance of the symmetric C_(n)C_(n) rings is lower than that of the corresponding single chains(C_(n)),which is in contrast to the conductance superposition law in multi-channeled systems.Combined theoretical calculations reveal that the gauche conformation in a shorter chain fixed by another chain leads to the decreased conductance in alkyl rings,which originates from the phase-coherent tunneling and DQI in σ-conjugated systems.Our finding suggests that through appropriate conformation locking by cyclization,the covalent alkane system can exhibit DQI,which offers strategies for future designs of molecular electronic devices and materials.

Intermolecular coupling enhanced thermopower in single-molecule diketopyrrolopyrrole junctions

Sorting out organic molecules with high thermopower is essential for understanding molecular thermoelectrics.The intermolecular coupling offers a unique chance to enhance the thermopower by tuning the bandgap structure of molecular devices,but the investigation of intermolecular coupling in bulk materials remains challenging.Herein,we investigated the thermopower of diketopyrrolopyrrole(DPP)cored single-molecule junctions with different coupling strengths by varying the packing density of the self-assembled monolayers(SAM)using a customized scanning tunneling microscope break junction(STM-BJ)technique.We found that the thermopower of DPP molecules could be enhanced up to one order of magnitude with increasing packing density,suggesting that the thermopower increases with larger neighboring intermolecular interactions.The combined density functional theory(DFT)calculations revealed that the closely-packed configuration brings stronger intermolecular coupling and then reduces the highest occupied molecular orbital(HOMO)-lowest unoccupied molecular orbital(LUMO)gap,leading to an enhanced thermopower.Our findings offer a new strategy for developing organic thermoelectric devices with high thermopower.