  As the dimensions of a conductor approach the nano-scale, quantum effects
will begin to dominate its behavior. This entails the exciting possibility of
controlling the conductance of a device by direct manipulation of the electron
wave function. Such control has been most clearly demonstrated in mesoscopic
semiconductor structures at low temperatures. Indeed, the Aharanov-Bohm effect,
conductance quantization and universal conductance fluctuations are direct
manifestations of the electron wave nature. However, an extension of this
concept to more practical emperatures has not been achieved so far. As
molecules are nano-scale objects with typical energy level spacings (~eV) much
larger than the thermal energy at 300 K (~25 meV), they are natural candidates
to enable such a break-through. Fascinating phenomena including giant
magnetoresistance, Kondo effects and conductance switching, have previously
been demonstrated at the molecular level. Here, we report direct evidence for
destructive quantum interference in charge transport through two-terminal
molecular junctions at room temperature. Furthermore, we show that the degree
of interference can be controlled by simple chemical modifications of the
molecule. Not only does this provide the experimental demonstration of a new
phenomenon in quantum charge transport, it also opens the road for a new type
of molecular devices based on chemical or electrostatic control of quantum
interference.
