Combined QM/MM and classical molecular dynamics study of [Ru(bpy)3]2+ in water

The solvation of the [Ru(bpy)3]2+ (bpy=2,2'-bipyridyl) cation was studied by molecular dynamics simulations, using both a hybrid QM/MM Car-Parrinello scheme and a classical force field. The trajectory analysis reveals that the first solvation sphere consists of chains of H-bonded water molecules intercalating between the bpy ligands, which in turns induces an organization of the second solvation sphere. Comparison with a simple charged sphere model shows the influence of the geometry of the cation and its ligands on the surrounding water. A water residence time in the first solvation shell of [Ru(bpy)3]2+ of 12.4 ps is computed, which is significantly larger than the value of 4.5 ps found in bulk water. On the other hand, due to the one-dimensional arrangement of hydrogen bonds, the dipole of the water molecules can easily reorient, providing a way to explain the fast solvent dependent relaxation dynamics observed following photoexcitation.