Highly insulating alkane rings with destructive σ-interference

Abstract

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 CmCn 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 CnCn rings is lower than that of the corresponding single chains (Cn), 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.