title: Ruthenium(II) complexes with improved photophysical properties based on planar 4'-(2-pyrimidinyl)-2,2':6',2' '-terpyridine ligands.
abstract: A series of new tridentate polypyridine ligands, made of terpyridine chelating subunits connected to various substituted 2-pyrimidinyl groups, and their homoleptic and heteroleptic Ru(II) complexes have been prepared and characterized. The new metal complexes have general formulas [(R-pm-tpy)Ru(tpy)]2+ and [Ru(tpy-pm-R)2]2+ (tpy = 2,2':6',2' '-terpyridine; R-pm-tpy = 4'-(2-pyrimidinyl)-2,2':6',2' '-terpyridine with R = H, methyl, phenyl, perfluorophenyl, chloride, and cyanide). Two of the new metal complexes have also been characterized by X-ray analysis. In all the R-pm-tpy ligands, the pyrimidinyl and terpyridyl groups are coplanar, allowing an extended delocalization of acceptor orbital of the metal-to-ligand charge-transfer (MLCT) excited state. The absorption spectra, redox behavior, and luminescence properties of the new Ru(II) complexes have been investigated. In particular, the photophysical properties of these species are significantly better compared to those of [Ru(tpy)2]2+ and well comparable with those of the best emitters of Ru(II) polypyridine family containing tridentate ligands. Reasons for the improved photophysical properties lie at the same time in an enhanced MLCT-MC (MC = metal centered) energy gap and in a reduced difference between the minima of the excited and ground states potential energy surfaces. The enhanced MLCT-MC energy gap leads to diminished efficiency of the thermally activated pathway for the radiationless process, whereas the similarity in ground and excited-state geometries causes reduced Franck Condon factors for the direct radiationless decay from the MLCT state to the ground state of the new complexes in comparison with [Ru(tpy)2]2+ and similar species.
authors.affiliation: Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, Département de Chimie, Université de Montréal, Canada.
authors.forename: Yuan-Qing
authors.initials: YQ
authors.lastname: Fang
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Taylor
authors.forename: François
authors.initials: F
authors.lastname: Laverdière
authors.forename: Garry S
authors.initials: GS
authors.lastname: Hanan
authors.forename: Frédérique
authors.initials: F
authors.lastname: Loiseau
authors.forename: Francesco
authors.initials: F
authors.lastname: Nastasi
authors.forename: Sebastiano
authors.initials: S
authors.lastname: Campagna
authors.forename: Hélène
authors.initials: H
authors.lastname: Nierengarten
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Leize-Wagner
authors.forename: Alain
authors.initials: A
authors.lastname: Van Dorsselaer