Chiral Induced Spin Selectivity Gives a New Twist on Spin-Control in Chemistry

The electron’s spin, its intrinsic angular momentum, is a quantum property that plays a critical role in determining the electronic structure of molecules. Despite its importance, it is not used often for controlling chemical processes, photochemistry excluded. The reason is that many organic molecules have a total spin zero, namely, all the electrons are paired. Even for molecules with high spin multiplicity, the spin orientation is usually only weakly coupled to the molecular frame of nuclei and hence to the molecular orientation. Therefore, controlling the spin orientation usually does not provide a handle on controlling the geometry of the molecular species during a reaction. About two decades ago, however, a new phenomenon was discovered that relates the electron’s spin to the handedness of chiral molecules and is now known as the chiral induced spin selectivity (CISS) effect. It was established that the efficiency of electron transport through chiral molecules depends on the electron spin and that it changes with the enantiomeric form of a molecule and the direction of the electron’s linear momentum. This property means that, for chiral molecules, the electron spin is strongly coupled to the molecular frame. Over the past few years, we and others have shown that this feature can be used to provide spin-control over chemical reactions and to perform enantioseparations with magnetic surfaces.