  Recent observations of destructive quantum interference in single-molecule
junctions confirm the role played by quantum effects in the electronic
conductance properties of molecular systems. We show here that the destructive
interference can be turned ON or OFF within the same molecular system by
mechanically controlling its conformation. Using a combination of ab-initio
calculations and single-molecule conductance measurements, we demonstrate the
existence of a quasi-periodic destructive quantum interference pattern along
the breaking traces of {\pi}-{\pi} stacked molecular dimers. The detection of
these interferences, which are due to opposite signs of the intermolecular
electronic couplings, was only made possible by a combination of wavelet
transform and higher-order statistical analysis of single-breaking traces. The
results demonstrate that it is possible to control the molecular conductance
over a few orders of magnitudes and with a sub-angstrom resolution by
exploiting the subtle structure-property relationship of {\pi}-{\pi} stack
dimers. These large conductance changes may be beneficial for the design of
single-molecule electronic components that exploit the intrinsic quantum
effects occurring at the molecular scale.
