title: Luminescent osmium(II) complexes with functionalized 2-phenylpyridine chelating ligands: preparation, structural analyses, and photophysical properties.
abstract: Cyclometalated osmium complexes with the formulas [Os(ppy) 2(CO) 2] ( 1a, b), [Os(dfppy) 2(CO) 2] ( 2a, b), and [Os(btfppy) 2(CO) 2] ( 3a, b) have been synthesized, for which the chelating chromophores ppyH, dfppyH, and btfppyH denote 2-phenylpyridine, 2-(2,4-difluorophenyl)pyridine, and 2-(2,4-bis(trifluoromethyl)phenyl)pyridine, respectively. The isomers 1a- 3a, possessing an intrinsic C 2 rotational axis as determined by single-crystal X-ray diffraction analysis, underwent slow isomerization in solution at elevated temperature, giving the respective thermodynamic products 1b- 3b, which showed a distinctive coordination arrangement produced by a 180 degrees rotation of one cyclometalated ligand around the Os(II) metal center. In contrast to the case for 1a, b and 2a, which are inert to substitution, complexes 2b and 3b (or 3a) readily react with PPh 2Me to afford the products [Os(dfppy) 2(CO)(PPh 2Me)] ( 4) and [Os(btfppy) 2)(PPh 2Me)] ( 6), in which the incoming PPh 2Me replaced the CO located trans to the carbon atom of one cyclometalated ligand. UV-vis and emission spectra were measured, revealing the lowest excited state for all complexes as a nominally ligand-centered (3)pipi* state mixed with certain MLCT character. Introduction of the electron-withdrawing substituents on the cyclometalated chelates or replacement of one CO ligand with phosphine at the metal center increased the MLCT contribution in the first excited state, giving a broad and featureless emission with greatly enhanced quantum yields.
authors.affiliation: Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan.
authors.forename: Kwun-Chi
authors.initials: KC
authors.lastname: Hwang
authors.forename: Jing-Lin
authors.initials: JL
authors.lastname: Chen
authors.forename: Yun
authors.initials: Y
authors.lastname: Chi
authors.forename: Chun-Wei
authors.initials: CW
authors.lastname: Lin
authors.forename: Yi-Ming
authors.initials: YM
authors.lastname: Cheng
authors.forename: Gene-Hsiang
authors.initials: GH
authors.lastname: Lee
authors.forename: Pi-Tai
authors.initials: PT
authors.lastname: Chou
authors.forename: Sue-Yi
authors.initials: SY
authors.lastname: Lin
authors.forename: Ching-Fong
authors.initials: CF
authors.lastname: Shu