New Ru(II) Complexes for Dual Photoreactivity: Ligand Exchange and 1O2 Generation

Uncovering the factors that govern the electronic structure of Ru(II)-polypyridyl complexes is critical in designing new compounds for desired photochemical reactions, and strategies to tune excited states for ligand dissociation and (1)O2 production are discussed herein. The generally accepted mechanism for photoinduced ligand dissociation proposes that population of the dissociative triplet ligand field ((3)LF) state proceeds through thermal population from the vibrationally cooled triplet metal-to-ligand charge transfer ((3)MLCT) state; however, temperature-dependent emission spectroscopy provides varied activation energies using the emission and ligand exchange quantum yields for Ru(bpy)2(L)2(2+) (bpy = 2,2'-bipyridine; L = CH3CN or py). This suggests that population of the (3)LF state proceeds from the vibrationally excited (3)MLCT state. Because the quantum yield of ligand dissociation for nitriles is much more efficient than that for py, steric bulk was introduced into the ligand set to distort the pseudo-octahedral geometry and lower the energy of the (3)LF state. The py dissociation quantum yield with 500 nm irradiation in a series of Ru(tpy)(NN)(py)(2+) complexes (tpy = 2,2':6',2″-terpyridine; NN = bpy, 6,6'-dimethyl-2,2'-bipyridine (Me2bpy), 2,2'-biquinoline (biq)) increases by 2-3 orders of magnitude with the sterically bulky Me2bpy and biq ligands relative to bpy. Ultrafast transient absorption spectroscopy reveals population of the (3)LF state within 3-7 ps when NN is bulky, and density functional theory calculations support stabilized (3)LF states. Dual activity via ligand dissociation and (1)O2 production can be achieved by careful selection of the ligand set to tune the excited-state dynamics. Incorporation of an extended π system in Ru(II) complexes such as Ru(bpy)(dppn)(CH3CN)2(2+) (dppn = benzoidipyrido3,2-a:2',3'-cphenazine) and Ru(tpy)(Me2dppn)(py)(2+) (Me2dppn = 3,6-dimethylbenzoidipyrido3,2-a:2',3'-cphenazine) introduces low-lying, long-lived dppn/Me2dppn (3)ππ* excited states that generate (1)O2. Similar to Ru(bpy)2(CH3CN)2(2+), photodissociation of CH3CN occurs upon irradiation of Ru(bpy)(dppn)(CH3CN)2(2+), although with lower efficiency because of the presence of the (3)ππ* state. The steric bulk in Ru(tpy)(Me2dppn)(py)(2+) is critical in facilitating the photoinduced py dissociation, as the analogous complex Ru(tpy)(dppn)(py)(2+) produces (1)O2 with near-unit efficiency. The ability to tune the relative energies of the excited states provides a means to design potentially more active drugs for photochemotherapy because the photorelease of drugs can be coupled to the therapeutic action of reactive oxygen species, effecting cell death via two different mechanisms. The lessons learned about tuning of the excited-state properties can be applied to the use of Ru(II)-polypyridyl compounds in a variety of applications, such as solar energy conversion, sensors and switches, and molecular machines.