A molecular system is provided for nanometer-scale reversible electronic and optical switches, specifically, electric field-activated molecular switches that have an electric field induced band gap change that occurs via rotation of rotor units connected to immobile stator units. The molecular system has two branches on one side of an immobile junction unit and one or two branches on the opposite side to thereby provide "Y" and "X" configurations, respectively. The ends of the branches opposite the junction unit are connected to, or electrically associated with, other molecular systems or substrates, such as electrodes. The rotor units each rotate between two states as a function of an externally-applied field. Both multi-stable molecular mechanical devices and electric field-activated optical switches are provided.