title: Steric influence on the excited-state lifetimes of ruthenium complexes with bipyridyl-alkanylene-pyridyl ligands.
abstract: 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 Ru (II) complex with the isopropylene-bridged ligand 2 was found to be 50 times shorter than that of the corresponding homoleptic Ru (II) 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 Ru (II) 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 ( (3)MLCT) and metal-centered ( (3)MC) excited states. The large difference in measured excited-state lifetimes for the two homoleptic Ru (II) 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 (3)MLCT states to short-lived (3)MC states.
authors.affiliation: Department of Photochemistry and Molecular Science, Uppsala University, Uppsala, Sweden.
authors.forename: Maria
authors.initials: M
authors.lastname: Abrahamsson
authors.forename: Maria J
authors.initials: MJ
authors.lastname: Lundqvist
authors.forename: Henriette
authors.initials: H
authors.lastname: Wolpher
authors.forename: Olof
authors.initials: O
authors.lastname: Johansson
authors.forename: Lars
authors.initials: L
authors.lastname: Eriksson
authors.forename: Jonas
authors.initials: J
authors.lastname: Bergquist
authors.forename: Torben
authors.initials: T
authors.lastname: Rasmussen
authors.forename: Hans-Christian
authors.initials: HC
authors.lastname: Becker
authors.forename: Leif
authors.initials: L
authors.lastname: Hammarström
authors.forename: Per-Ola
authors.initials: PO
authors.lastname: Norrby
authors.forename: Björn
authors.initials: B
authors.lastname: Akermark
authors.forename: Petter
authors.initials: P
authors.lastname: Persson
mesh_terms: D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives*; D018360:Crystallography, X-Ray; D008024:Ligands; D008697:Methane / Q000031:analogs & derivatives; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010777:Photochemistry; D011407:Propane / Q000031:analogs & derivatives; D011725:Pyridines / Q000737:chemistry*; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D008024:Ligands; D009942:Organometallic Compounds; D011725:Pyridines; D015082:2,2'-Dipyridyl; D012428:Ruthenium; D008697:Methane; D011407:Propane