Accelerating slow excited state proton transfer

Abstract

Visible light excitation of the ligand-bridged assembly [(bpy) 2RuaII(L) RubII(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-•)Ru bIII-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 kq = 4.1 × 108 M-1·s-1 and an estimated pKa*value of ∼5 ± 1 for the [(bpy) 2RuaII(L•-)RubIII(bpy) (OH2)4+]*excited state. Following proton loss and rapid excited-state decay to give [(bpy) 2RuaII(L)RubII(bpy)(OH) 3+] in a H2PO4-/HPO42- buffer, back proton transfer occurs from H2PO 4- to give [(bpy)2RuaII(L)Rub(bpy)(OH2)4+] with k PT,2 = 4.4 × 108 M-1·s -1. From the intercept of a plot of kobs vs. [H 2PO4-], k = 2.1 × 106 s -1 for reprotonation by water providing a dramatic illustration of kinetically limiting, slow proton transfer for acids and bases with pK a values intermediate between pKa(H3O +) = -1.74 and pKa(H2O) = 15.7.