Accelerating slow excited state proton transfer

作者：Stewart David J; Concepcion Javier J; Brennaman M Kyle; Binstead Robert A; Meyer Thomas J^*

来源：Proceedings of the National Academy of Sciences, 2013, 110(3): 876-880.

DOI：10.1073/pnas.1220742110

• kinetics
• photochemistry
• luminescence
• transient absorption

Visible light excitation of the ligand-bridged assembly [(bpy)(2)Ru-a(II)(L) Ru-b(II)(bpy)(OH2)(4+)] (bpy is 2,2'-bipyridine; L is the bridging ligand, 4-phen-tpy) results in emission from the lowest energy, bridge-based metal-to-ligand charge transfer excited state (L-center dot)Ru-b(III)-OH2 with an excited-state lifetime of 13 +/- 1 ns. Near-diffusion-controlled quenching of the emission occurs with added HPO42- and partial quenching by added acetate anion (OAc-) in buffered solutions with pH control. A Stern-Volmer analysis of quenching by OAc- gave a quenching rate constant of k(q) = 4.1 x 10(8) M-1.s(-1) and an estimated pK(a)* value of similar to 5 +/- 1 for the [(bpy)(2)Ru-a(II)(L center dot-)Ru-b(III)(bpy) (OH2)(4+)]* excited state. Following proton loss and rapid excited-state decay to give [(bpy)(2)Ru-a(II)(L) Ru-b(II)(bpy)(OH)(3+)] in a H2PO4-/HPO42- buffer, back proton transfer occurs from H2PO4- to give [(bpy)(2)Ru-a(II)(L)Ru-b(bpy)(OH2)(4+)] with k(PT,2) = 4.4 x 10(8) M-1.s(-1). From the intercept of a plot of k(obs) vs. [H2PO4-], k = 2.1 x 10(6) s(-1) for re-protonation by water providing a dramatic illustration of kinetically limiting, slow proton transfer for acids and bases with pK(a) values intermediate between pK(a)(H3O+) = -1.74 and pK(a)(H2O) = 15.7.

• 出版日期2013-1-15