Classes of molecules are disclosed which can, for example, be used in molecular switches. The classes of molecules include at least three segments—an electronic donor (“D”), a switchable bridge (“B”), and an electronic acceptor (“A”)—chemically connected and linearly arranged (e.g., D-B-A). The electronic donor can be an aromatic ring system with at least one electron donating group covalently attached; an aromatic ring system with an electron withdrawing group covalently attached is usually employed as the electronic acceptor; and the switchable bridge can be a pi system that can be switched on or off using an external electric field.