title: Strategic design and synthesis of osmium(II) complexes bearing a single pyridyl azolate pi-chromophore: achieving high-efficiency blue phosphorescence by localized excitation.
abstract: We present the strategic design and synthesis of Os(II) complexes bearing a single pyridyl azolate pi-chromophore with an aim to attain high efficiency blue phosphorescence by way of localized transition. It turns out that our proposal of localized excitation seems to work well upon anchoring a single pi-chromophore on the Os(II) complexes such that the control of MLCT versus pipi* (or even LLCT) transitions is more straightforward. Among the titled complexes, [Os(CO)3(tfa)(fppz)] (1) and [Os(CO)3(tfa)(fbtz)] (5) (tfa=trifluoroacetate, (fppz)H=3-(trifluoromethyl)-5-(2-pyridyl)pyrazole, and (fbtz)H=3-(trifluoromethyl)-5-(4-tert-butyl-2-pyridyl)-1,2,4-triazole) give the anticipated blue phosphorescence with efficiencies of 0.26 (lambdamax=460 nm) and 0.27 (lambdamax=450 nm), respectively. For their halide analogues [Os(CO)3(X)(fppz)] (2, X=Cl; 3, X=Br; 4, X=I) and phosphine-substituted isomeric derivatives [Os(tfa)(fppz)(PPh2Me)2(CO)] (6-8), the localization of the excitation energy seems to populate at certain vibrational modes with weak bonding strength and hence an associated shallow potential energy surface to induce a facile radiationless transition. Furthermore, their ancillary ligands play an important role in fine-tuning not only the energy gap but also the emission intensity, i.e., in manifesting the radiationless transition pathways. Our results clearly show that there is always a tradeoff upon varying the parameters in an aim to optimize the hue and efficiency of phosphorescence toward blue.
authors.affiliation: Department of Chemistry and Instrumentation Center, National Taiwan University, Taipei 106, Taiwan. ychi@mx.nthu.edu.tw
authors.forename: Yi-Ming
authors.initials: YM
authors.lastname: Cheng
authors.forename: Elise Y
authors.initials: EY
authors.lastname: Li
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
authors.forename: Kwun-Chi
authors.initials: KC
authors.lastname: Hwang
authors.forename: Yao-Lun
authors.initials: YL
authors.lastname: Chen
authors.forename: Yi-Hwa
authors.initials: YH
authors.lastname: Song
authors.forename: Yun
authors.initials: Y
authors.lastname: Chi