On the role of ligand-field states for the photophysical properties of ruthenium(II) polypyridyl complexes

The role of ligand-field states for the photophysical properties of d(6) systems has been discussed in a large number of publications over the past decades. Since the seminal paper by Houten and Watts, for instance, the quenching of the (MLCT)-M-3 luminescence in ruthenium(II) polypyridyl complexes is attributed to the presence of the first excited ligand-field state, namely a component of the T-3(1)(t(2g)(5)e(g)(1)) state, at similar energies. If this state lies above the (MLCT)-M-3 state, the luminescence is quenched via thermal population at elevated temperatures only. If it lies well below, then the luminescence is quenched down to cryogenic temperatures. In this contribution we present transient absorption spectra on non-luminescent ruthenium polypyridyl complexes such as [Ru(m-bpy)(3)](2+), m-bpy=6-methyl-2,2'-bipyridine, in acetonitrile at room temperature, which reveal an ultra-rapid depopulation of the (MLCT)-M-3 state but a much slower ground state recovery. We propose that in this and related complexes the methyl groups force longer metal-ligand bond lengths, thus resulting in a lowering of the ligand-field strength such that the (3)dd state drops to below the (MLCT)-M-3 state, and that furthermore the population of this state from the (MLCT)-M-3 state occurs faster than its decay to the ground state. In addition we demonstrate that in this complex the luminescence can be switched on by external pressure, which we attribute to a destabilisation of the ligand-field state by the pressure due to its larger molecular volume compared to the ground state as well as the (MLCT)-M-3 state. (C) 2014 Elsevier B.V. All rights reserved.