Quantum interference (QI) dominates the electronic properties
of
single molecules even at room temperature and can lead to a large
change in their electrical conductance. To take advantage of this
for nanoelectronic applications, a mechanism to electronically control
QI in single molecules needs to be developed. In this paper, we demonstrate
that controlling the quantum interference of each spin in a stable
open-shell organic radical with a large π-system is possible
by changing the spin state of the radical. We show that the counterintuitive
constructive spin interference in ameta-connected
radical changes to destructive interference by changing the spin state
of the radical from a doublet to a singlet. This results in a significant
change in the room temperature electrical conductance by several orders
of magnitude, opening up new possibilities for spin interference based
molecular switches for energy storage and conversion applications.