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 (3MC) state even though there is no direct coupling between the 1MLCT and 3MC states. The coupling between the 3MLCT and 3MC 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 3MC state is low and the metal-ligand bond length is almost unchanged with respect to the initial photoexcited state, in agreement with experiment.
@article{arxiv.1003.4752,
title = {Ultrafast Electron Dynamics Theory of Photo-excited Ruthenium Complexes},
author = {Jun Chang and A. J. Fedro and Michel van Veenendaal},
journal= {arXiv preprint arXiv:1003.4752},
year = {2012}
}