Ultrafast Electron Dynamics Theory of Photo-excited Ruthenium Complexes

TL;DR

This paper presents a quantum decay mechanism explaining ultrafast singlet to triplet electron transitions in Ruthenium complexes, resolving experimental contradictions and highlighting vibrational cooling's role in energy dissipation.

Contribution

It introduces a novel quantum decay model for photo-excited Ruthenium complexes, explaining ultrafast singlet-triplet decay without significant bond length change.

Findings

Decay occurs in about 300 femtoseconds.

Triplet metal-centered state mediates the decay.

Metal-ligand bond length remains nearly unchanged.

Abstract

An explanation is provided for the ultrafast photo-excited electron dynamics in low-spin Ruthenium (II) organic complexes. The experimentally-observed singlet to triplet decay in the metal-to-ligand charge-transfer (MLCT) states contradicts the expectation that the system should oscillate between the singlet and triplet states in the presence of a large spin-orbit coupling and the absence of a significance change in metal-ligand bond length. This dilemma is solved with a novel quantum decay mechanism that causes a singlet to triplet decay in about 300 femtoseconds. The decay is mediated by the triplet metal-centered state ($^3$MC) state even though there is no direct coupling between the $^1$MLCT and $^3$MC states. The coupling between the $^3$MLCT and $^3$MC via excited phonon states leads to vibrational cooling that allows the local system to dissipate the excess energy. In the relaxed state, the population of the $^3$MC state is low and the metal-ligand bond length is almost unchanged with respect to the initial photoexcited state, in agreement with experiment.