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Sample Paper - 4938 Organometallics 2003, 22, 4938-4946...

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Synthesis and Characterization of Metal Complexes Possessing the 5-(2-Pyridyl) Pyrazolate Ligands: The Observation of Remarkable Osmium-Induced Blue Phosphorescence in Solution at Room Temperature Pei-Chi Wu, ² Jen-Kan Yu, Yi-Hwa Song, ² Yun Chi,* Pi-Tai Chou,* ,‡ Shie-Ming Peng, and Gene-Hsiang Lee Department of Chemistry, National Tsing Hua University, 300, Hsinchu, Taiwan, Republic of China, and Department of Chemistry and Instrumentation Center, National Taiwan University, 106, Taipei, Taiwan, Republic of China Received July 13, 2003 A total of three distinctive main group and transition metal complexes containing the 2-pyridyl pyrazolate (pypz) ligand were prepared, namely, [B(C 6 F 5 ) 2 (pypz)] ( 1 ), [Ru(CO) 2 - (pypz) 2 ]( 2 ), and [Os(CO) 2 (pypz) 2 3 ), where (pypz)H ) 3-trifluoromethyl-5-(2-pyridyl)pyrazole. Single-crystal X-ray diffraction studies were carried out on complexes 2 and 3 , revealing octahedral coordination geometry with two CO ligands located at cis dispositions. While the pypz ligand arrangement for complex 2 is in cis- (N py ,N py ) and trans- (N pz ,N pz ), complex 3 reveals a different configuration, cis- (N pz ,N pz ) and trans- (N py ,N py )(N py for pyridine-N and N pz for pyrazolate donor sites). Similar to that of the in-situ-prepared pypz anion, the boron complex [B(C 6 F 5 ) 2 (pypz)] ( 1 ) exhibits a strong emission centered at 380 nm, which is unambiguously assigned to fluorescence derived from the S 1 ( ππ *) f S 0 transition. In contrast to the nonluminescent behavior for Ru complex 2 , the Os complex 3 displays unique, strong room-temperature phosphorescence, showing vibronic progressions at 430, 457, and 480 nm. The remarkable differences in photophysical properties were rationalized by a combination of π -electron accepting CO ligand, relative pypz orientation, and heavy-atom-enhanced spin - orbit coupling effects. 1. Introduction Third-row transition metal complexes incorporating simple polypyridine ligands, 1 such as 2,2 -bipyridine (bpy) or 1,10-phenanthroline (phen), and cyclometalated ligands, 2 such as 2-phenylpyridine and benzoquinoline, have attracted a great deal of interest in recent years. Research in this area was principally motivated by the use of these complexes in the study of excited-state electron and energy transfer 3 as well as the potential applications in the fabrication of organic light-emitting diodes (OLEDs). 4 Isoelectronic transition metal ions such as Re(I), Os(II), and Ir(III), possessing a unique d 6 -electron configuration, are particularly attractive because of their strong metal - ligand interaction, long- lived excited states, and high luminescence efficiencies, which significantly improve the likelihood of energy transfer prior to radiative or nonradiative relaxation. The strong spin - orbit coupling expected for these heavy metal ions, with atomic numbers Z ) 75 - 77, would lead to an efficient intersystem crossing from the singlet excited state to the triplet manifold. Furthermore, mixing singlet and triplet excited states via spin - orbit coupling, to a great extent, would also remove the spin- forbidden nature of the T 1 f
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Sample Paper - 4938 Organometallics 2003, 22, 4938-4946...

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