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 ((LL)-L-3'CT) charge transfer states in addition to the well-known pathway via (MC)-M-3 states. Thermal activation barriers for depopulation of the emissive metal-to-ligand charge transfer ((MLCT)-M-3) states via the 3MC (metal-centered) and (LL)-L-3'CT states were determined experimentally for complexes 1(+) and 3(+). The experimental results were further corroborated by calculating the respective (MLCT)-M-3-(LL)-L-3'CT and (MLCT)-M-3-(MC)-M-3 transition states and their energies with density functional theoretical methods. The R substituent modifies the energy difference between the (MLCT)-M-3 and (LL)-L-3'CT states and the corresponding activation barrier but leaves the analogous (MLCT)-M-3/(MC)-M-3 energetics essentially untouched. Additionally, the dinuclear complex [(tpy) Ru(dpb-NHCO-dpb) Ru(tpy)](2+), 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 (lambda(max) = 1165 nm). Upon optical excitation, the Ru-II/Ru-II complex 6(2+) exhibits dual emission in liquid solution from two independently emitting (MLCT)-M-3 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.