Abstract
Understanding how quantum interference (QI) controls charge transport through metal | molecule | metal junctions is central to the progress of molecular electronics. In this work, scanning tunneling microscopy break junction (STM-BJ) measurements combined with charge transport calculations have been used to investigate, for the first time, the conductance of single-molecules that combine (i) multiple cross-conjugation points, (ii) nonalternant frameworks, and (iii) heteroatoms in the conductance pathway. The six studied molecules are symmetrical triaryl systems, each containing two terminal 1,3-difunctionalized pyrrole rings. Their structures vary in three aspects: (i) the anchoring groups (SMe or thiolate), (ii) the connectivity (para or meta central ring), and (iii) whether the central ring is benzene or pyridine. STM-BJ studies showed that para-connectivity afforded higher conductance than meta-connectivity, but in contrast to analogous conjugated systems, there was no appreciable difference between meta-connected benzene and pyridine species. These results offer an advanced testbed for QI interpretation via intuitive curly arrow rules, orbital analysis, "M-theory", and high-level computational techniques. By combining these methods to account for subtle modulation of destructive QI and antiresonances, the experimental trends have been successfully rationalized. These fundamental experimental and computational insights should be applicable to other heterocycles in molecular junctions.