Ultrafast Cascading Theory in Intersystem Crossings in Transition-Metal Complexes

TL;DR

This paper presents a quantum-mechanical model explaining ultrafast intersystem crossing in transition-metal complexes, highlighting the role of energy differences and electron-phonon interactions in cascade decay mechanisms.

Contribution

It introduces a detailed quantum description of cascade decay in transition-metal complexes and identifies conditions for ultrafast intersystem crossing.

Findings

Ultrafast decay occurs mainly in antibonding metal-to-ligand charge-transfer states.

Energy differences and electron-phonon self energy determine cascade pathways.

Some intermediate states are bypassed during decay.

Abstract

We investigate the cascade decay mechanism for ultrafast intersystem crossing mediated by the spin-orbit coupling in transition-metal complexes. A quantum-mechanical description of the cascading process that occurs after photoexcitation is presented. The conditions for ultrafast cascading are given, which relate the energy difference between the levels in the cascading process to the electron-phonon self energy. These limitations aid in the determination of the cascade path. For $Fe^{2+}$ spin-crossover complexes, this leads to the conclusion that the ultrafast decay primarily occurs in the manifold of antibonding metal-to-ligand charge-transfer states. We also give an interpretation why some intermediate states are bypassed.