Impact of π-electron delocalization on quantum interference in single-molecule junctions of benzene derivatives

Quantum interference (QI) is a key phenomenon in nanoscale charge transport through molecular junctions. In this study, we investigate how π-electron delocalization influences QI by comparing a series of π-conjugated molecules of varying lengths, specifically <i>para</i>- and <i>meta</i>-connected dipyridylbenzene derivatives. Conductance measurements using the break-junction technique reveal that <i>para</i>-connected molecules exhibit conductance values approximately one order of magnitude higher than their <i>meta</i>-connected counterparts, consistent with constructive and destructive QI, respectively. Furthermore, conductance is markedly suppressed in <i>meta</i>-connected molecules with extended π-conjugation, indicating that enhanced π-electron delocalization amplifies destructive interference. These experimental observations are supported by theoretical calculations based on the density functional theory-non-equilibrium Green's function (DFT-NEGF) method. Our findings highlight the critical role of π-electron delocalization in modulating QI and offer a design principle for molecular electronic devices that harness quantum interference.