Steric Influence on the Excited-State Lifetimes of Ruthenium Complexes with Bipyridyl−Alkanylene−Pyridyl Ligands
Journal article, 2008
The structural effect on the metal-to-ligand charge transfer (MLCT) excited-state lifetime has been investigated in bis-tridentate Ru(II)−polypyridyl complexes based on the terpyridine-like ligands [6-(2,2′-bipyridyl)](2-pyridyl)methane (1) and 2-[6-(2,2′-bipyridyl)]-2-(2-pyridyl)propane (2). A homoleptic ([Ru(2)2]2+) and a heteroleptic complex ([Ru(ttpy)(2)]2+) based on the new ligand 2 have been prepared and their photophysical and structural properties studied experimentally and theoretically and compared to the results for the previously reported [Ru(1)2]2+. The excited-state lifetime of the homoleptic RuII complex with the isopropylene-bridged ligand 2 was found to be 50 times shorter than that of the corresponding homoleptic RuII complex of ligand 1, containing a methylene bridge. A comparison of the ground-state geometries of the two homoleptic complexes shows that steric interactions involving the isopropylene bridges make the coordination to the central RuII ion less octahedral in [Ru(2)2]2+ than in [Ru(1)2]2+. Calculations indicate that the structural differences in these complexes influence their ligand field splittings as well as the relative stabilities of the triplet metal-to-ligand charge transfer (3MLCT) and metal-centered (3MC) excited states. The large difference in measured excited-state lifetimes for the two homoleptic RuII complexes is attributed to a strong influence of steric interactions on the ligand field strength, which in turn affects the activation barriers for thermal conversion from 3MLCT states to short-lived 3MC states.