What Can CISS Teach Us about Electron Transfer?

Electron transfer (eT) processes have garnered the attention of chemists and physicists for more than seven decades, and it is commonly believed that the essential features of the electron transfer mechanism are well understood─despite some open questions relating to the efficiency of long-range eT in some systems and temperature effects that are difficult to reconcile with the existing theories. The chiral induced spin selectivity (CISS) effect, which has been studied experimentally since 1999, demonstrates that eT through chiral systems depends on the electron's spin. Attempts to explain the CISS effect by adding spin-orbit coupling to the existing eT theories fails to reproduce the experimental results quantitatively, and it has become evident that the theory for explaining CISS must consider electron-vibration and/or electron-electron interactions. In this Perspective we identify some features of the CISS effect that imply that we should reconsider and refine the Marcus-Levich-Jortner mechanistic description for eT processes, especially for nonlinear systems and in the case of long-range eT.