The photochemistry of mono- and dinuclear cyclometalated bis(tridentate)ruthenium(II) complexes: dual excited state deactivation and dual emission.

The synthesis and characterization of a series of weakly emissive mononuclear cyclometalated Ru(dpb-R)(tpy) complexes with functional groups R of varying electron-donating characters at the dpb ligand are described (dpbH = 1,3-di(2-pyridyl)benzene, tpy = 2,2';6',2''-terpyridine, 1(+): R = NHCOMe, 2(+): R = NH2, 3(+): R = COOEt, 4(+): R = COOH). Steady-state emission spectroscopy in the temperature range between 298 K and 77 K revealed a previously unrecognized excited state deactivation pathway via low-lying triplet ligand-to-ligand ((3)LL'CT) charge transfer states in addition to the well-known pathway via(3)MC states. Thermal activation barriers for depopulation of the emissive metal-to-ligand charge transfer ((3)MLCT) states via the (3)MC (metal-centered) and (3)LL'CT states were determined experimentally for complexes 1(+) and 3(+). The experimental results were further corroborated by calculating the respective (3)MLCT-(3)LL'CT and (3)MLCT-(3)MC transition states and their energies with density functional theoretical methods. The R substituent modifies the energy difference between the (3)MLCT and (3)LL'CT states and the corresponding activation barrier but leaves the analogous (3)MLCT/(3)MC energetics essentially untouched. Additionally, the dinuclear complex (tpy)Ru(dpb-NHCO-dpb)Ru(tpy), 6(2+), containing a biscyclometalating bridge was devised. Despite the asymmetric nature induced by the amide bridge, the mixed-valent cation 6(3+) is ascribed to Robin-Day class II with a broad and intense intervalence charge-transfer (IVCT) absorption (λmax = 1165 nm). Upon optical excitation, the Ru(II)/Ru(II) complex 6(2+) exhibits dual emission in liquid solution from two independently emitting (3)MLCT states localized at the two remote [Ru(tpy)] fragments. No equilibration via Dexter energy transfer is possible due to their large distance and short excited state lifetimes.