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

Abstract

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 (λ_max = 1165 nm). Upon optical excitation, the Ru^II/Ru^II 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.