Selective Photodissociation of Acetonitrile Ligands in Ruthenium Polypyridyl Complexes Studied by Density Functional Theory

Metal complexes that release ligands upon photoexcitation are important tools for biological research and show great potential as highly specific therapeutics. Upon excitation with visible light, [Ru(TQA)(MeCN)(2)](2+) [TQA = tris(2-quinolinylmethyl) amine] exchanges one of the two acetonitriles (MeCNs), whereas [Ru(DPAbpy)MeCN] 2+ [D PAbpy = N- (2,2' -bipyri di n- 6-y1) -N,N-b is (pyri din -2yl-methyl)amine] does not release MeCN. Furthermore, [Ru(TQA)(MeCN)(2)](2) is highly selective for release of the MeCN that is perpendicular to the plane of the two axial quinolines. Density functional theory calculations provide a clear explanation for the photodissociation behavior of these two complexes. Excitation by visible light and intersystem crossing leads to a sixcoordinate 3MLCT state. Dissociation of acetonitrile can occur after internal conversion to a dissociative (MC)-M-3 state, which has an occupied d sigma* orbital that interacts in an antibonding fashion with acetonitrile. For [Ru(TQA)(MeCN)(2)](2+), the dissociative 3MC state is lower than the (MLCT)-M-3 state. In contrast, the 3MC state of [Ru(DPAbpy)MeCN](2) that releases acetonitrile has an energy higher than that of the (MLCT)-M-3 state, indicating dissociation is unfavorable. These results are consistent with the experimental observations that efficient photodissociation of acetonitrile occurs for [Ru(TQA)(MeCN)(2)](2+) but not for [Ru(DPAbpy)MeCN]2+. For the release of the MeCN ligand in [Ru(TQA)(MeCN)(2)](2+) that is perpendicular to the axial quinoline rings, the 3MLCT state has an occupied quinoline pi* orbital that can interact with a do-* Ru-NCCH3 antibonding orbital as the Ru NCCH3 bond is stretched and the quinolines bend toward the departing acetonitrile. This reduces the barrier for the formation of the dissociative 3MC state, leading to the selective photodissociation of this acetonitrile. By contrast, when the acetonitrile is in the plane of the quinolines or bpy, no interaction occurs between the ligand g* orbital and the clo-* Ru NCCH3 orbital, resulting in high barriers for conversion to the corresponding 3MC structures and no release of acetonitrile.