Photoinduced Dissociative Electron Transfer: Is the Quantum Yield Theoretically Predicted to Equal Unity?

An attractive manner of fighting back-electron transfer to the ground state in photoinduced electron transfer reactions is to use a system in which the donor and/or the acceptor in the ion-pair undergoes a rapid fragmentation. Intuitively, it seems that an ideal situation in this respect, leading to a unity quantum yield, should be met when fragmentation and electron transfer are concerted. Accordingly, a quantum yield below 1 would be the signature of a nonconcerted two-step mechanism. It is shown, from first principles, that a purely dissociative photoinduced electron transfer is not necessarily endowed with a unity quantum yield. The reason is that the system partitions between fragmentation and back-electron transfer in the funnel offered by the upper first-order potential energy surface combining the ground state and fragments zero-order surfaces. A semiclassical model is presented, relating the quantum yield to the electronic matrix coupling element, H. Only in the case of a completely nonadiabatic ...