Excited -state coordination chemistry: A new quenching mechanism.

摘要

Changes in Bronsted acid-base behavior in the excited state have been well established among organic molecules as well a transition metal complexes. Though the ideas of Lewis acid-base chemistry are well established in the ground state, no clear examples as such, exist in the excited state. Reported here are some examples of such interactions. There are a number of reasons that this type of interaction is more difficult to observe, including the time constraint due to the unquenched lifetime of the excited state, competitive relaxation by quenching pathways, and in many cases the formation of only transient products. Despite these difficulties, we present a number of cases where this type of interaction is apparent.;Diimine complexes of Ru(II) have been reported to have values of DeltapKa as large as 7.0. In this work evidence is presented for an excited state acid-base interaction between [Ru(bpy)2dpp](ClO4)2 (bpy = 2,2'-bipyridine, dpp = 2,3-Bis(2-pyridyl)pyrazine), and a number of coordination complexes. In some cases the availability of a stable product allows an excited state interaction to lead to a net chemical change as in the case of PtCl62-. R*ubpy 2dpp2++PtCl 2-6&rarrr;Rubpy 2dppPtCl 2+4+2Cl- In other cases, such as the interaction of Ag+ with *Ru(bpy)2dpp2+, a product can form between the donor and quencher and it does readily by thermal pathways. Therefore, though the equilibrium shifts in the excited state, it recedes to the ground state equilibrium position upon relaxation, resulting in no net chemical change.;These processes have been characterized by measurement of the lifetime and intensity quenching, as well as the decline in emission intensity associated with product accumulation, and correlation of the rate of product accumulation measured directly. The latter two methods serve as an independent check of the Stern-Volmer quenching model used. The family of quenchers studied have been chosen for a variety of reasons including, their likelihood to undergo, or fail to undergo other mechanisms of quenching, as well as their character as a chemical family with known similarities and differences.