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 (3LL′CT) charge transfer states in addition to the well-known pathway via3MC states. Thermal activation barriers for depopulation of the emissive metal-to-ligand charge transfer (3MLCT) states via the 3MC (metal-centered) and 3LL′CT states were determined experimentally for complexes 1+ and 3+. The experimental results were further corroborated by calculating the respective 3MLCT–3LL′CT and 3MLCT–3MC transition states and their energies with density functional theoretical methods. The R substituent modifies the energy difference between the 3MLCT and 3LL′CT states and the corresponding activation barrier but leaves the analogous 3MLCT/3MC energetics essentially untouched. Additionally, the dinuclear complex [(tpy)Ru(dpb-NHCO-dpb)Ru(tpy)]2+, 62+, containing a biscyclometalating bridge was devised. Despite the asymmetric nature induced by the amide bridge, the mixed-valent cation 63+ is ascribed to Robin–Day class II with a broad and intense intervalence charge-transfer (IVCT) absorption (λmax = 1165 nm). Upon optical excitation, the RuII/RuII complex 62+ exhibits dual emission in liquid solution from two independently emitting 3MLCT 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.