Ligand-Selective Photodissociation from [Ru(bpy)(4AP)4]2+: aSpectroscopic and Computational Study

The new complex [Ru(bpy)(4AP)4]2+ (1), where bpy = 2,2′-bipyridine and 4AP = 4-aminopyridine, undergoes selective photodissociation of two 4APs upon light excitation of the metal-ligand-to-ligand charge-transfer (MLLCT) band at 510 nm. The photoproducts of the reaction are mer-[Ru(bpy)(4AP)3(H2O)]2+ (2a) and trans- (4AP)[Ru(bpy)(4AP)2(H2O)2]2+ (3a). Photodissociation occurs in two consecutive steps with quantum yields of 1 = 6.1 × 10-3 and 2 = 1.7 × 10-4, respectively. Complex 1 was characterized by combined spectroscopic and theoretical techniques. EXAFS experiments at the Ru K-edge (22117 eV) of 1 in an aqueous solution gave a Ru-N distance of 2.09 Å. Photoproducts were characterized by electronic spectroscopy, 1D and 2D NMR, and mass spectrometry. Singlet and triplet excited states of 1 were studied by density functional theory (DFT) and time-dependent DFT for characterizing the optical properties of the complex. In the singlet state, 1MC (metal-centered) dissociative states lie 0.65 eV above the main 1MLLCT transition in the visible region of the UV-vis absorption spectrum. In the triplet state, the energy difference between these states is not reduced. However, potential energy curves of singlet and triplet excited states of 1 along the Ru-N(axial 4AP) and Ru-N(equatorial 4AP) stretching coordinates show that the release of the first 4AP may occur from the triplet state by mixing of 3MLLCT and 3MC dissociative states. This mixing is favored when the Ru-N(equatorial 4AP) bond is elongated, explaining the formation of the photoproduct 2a.