title: Understanding the Excited State Behavior of Cyclometalated Bis(tridentate)ruthenium(II) Complexes: A Combined Experimental and Theoretical Study.
abstract: The synthesis and characterization of the donor-acceptor substituted cyclometalated ruthenium(II) polypyridine complex isomers [Ru(dpb-NHCOMe)(tpy-COOEt)](PF6) 1(PF6) and [Ru(dpb-COOEt)(tpy-NHCOMe)](PF6) 2(PF6) (dpbH = 1,3-dipyridin-2-ylbenzene, tpy = 2,2';6,2"-terpyridine) with inverted functional group pattern are described. A combination of resonance Raman spectroscopic and computational techniques shows that all intense visible range absorption bands arise from mixed Ru → tpy/Ru → dpb metal-to-ligand charge transfer (MLCT) excitations. 2(PF6) is weakly phosphorescent at room temperature in fluid solution and strongly emissive at 77 K in solid butyronitrile matrix, which is typical for ruthenium(II) polypyridine complexes. Density functional theory calculations revealed that the weak emission of 2(PF6) arises from a (3)MLCT state that is efficiently thermally depopulated via metal-centered ((3)MC) excited states. The activation barrier for the deactivation process was estimated experimentally from variable-temperature emission spectroscopic measurements as 11 kJ mol(-1). In contrast, 1(PF6) is nonemissive at room temperature in fluid solution and at 77 K in solid butyronitrile matrix. Examination of the electronic excited states of 1(PF6) revealed a ligand-to-ligand charge-transfer ((3)LL'CT) as lowest-energy triplet state due to the very strong push-pull effect across the metal center. Because of the orthogonality of the participating ligands, emission from the (3)LL'CT is symmetry-forbidden. Hence, in this type of complex a stronger push-pull effect does not increase the phosphorescence quantum yields but completely quenches the emission.
authors.affiliation: Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz , Duesbergweg 10-14, 55128 Mainz, Germany.|Graduate School Materials Science in Mainz , Staudingerweg 9, 55128 Mainz, Germany.
authors.forename: Christoph
authors.initials: C
authors.lastname: Kreitner
authors.affiliation: Institute of Chemistry, University of Rostock , Albert-Einstein-Straße 3a, 18059 Rostock, Germany.
authors.forename: Elisa
authors.initials: E
authors.lastname: Erdmann
authors.affiliation: Institute of Chemistry, University of Rostock , Albert-Einstein-Straße 3a, 18059 Rostock, Germany.
authors.forename: Wolfram W
authors.initials: WW
authors.lastname: Seidel
authors.affiliation: Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg-University of Mainz , Duesbergweg 10-14, 55128 Mainz, Germany.
authors.forename: Katja
authors.initials: K
authors.lastname: Heinze