Photoacidic and Photobasic Behavior of Transition Metal Compounds with Carboxylic Acid Group(s)

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• 作者：Ryan M. O’Donnell ; Renato N. Sampaio ; Guocan Li ; Patrik G. Johansson ; Cassandra L. Ward ; Gerald J. Meyer
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：March 23, 2016
• 年：2016
• 卷：138
• 期：11
• 页码：3891-3903
• 全文大小：819K
• ISSN：1520-5126

文摘

Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [Ru^II(btfmb)₂(LL)]²⁺, [Ru^II(dtb)₂(LL)]²⁺, and [Ru^II(bpy)₂(LL)]²⁺, where bpy is 2,2′-bipyridine, btfmb is 4,4′-(CF₃)₂-bpy, and dtb is 4,4′-((CH₃)₃C)₂-bpy, and LL is either dcb = 4,4′-(CO₂H)₂-bpy or mcb = 4-(CO₂H),4′-(CO₂Et)-2,2′-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long τ > 100 ns excited state lifetimes. The mcb compounds had very similar ground state pK_a’s of 2.31 ± 0.07, and their characterization enabled accurate determination of the two pK_a values for the commonly utilized dcb ligand, pK_a1 = 2.1 ± 0.1 and pK_a2 = 3.0 ± 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [Ru^III(btfmb^–)L₂]²⁺* localized excited state and a [Ru^III(dcb^–)L₂]²⁺* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent pK_a* values. The ability to make a single ionizable group photobasic or photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s).