Salt and Specific Cation Effects in the Quenching of Triplet State Tetrakis(μ-pyrophosphite-P,P‘)diplatinate(II) by Acidopentacyanocobaltate(III) Anions

Abstract

In the presence of moderate to high concentrations of electrolytes, the emission of *[Pt2(pop)4]4- (where pop = μ-pyrophosphite-P,P‘) is quenched by the complexes [Co(CN)5X]3- (where X = N3 -, I-, Br-, Cl-, but not CN-). The salt effects on the emission decay lifetime quenching rate constants between these anionic species have been studied in the presence of MCl, M‘Cl2, or R n NH4 - n Cl (where M, M‘, and R represent alkali, alkaline earth metals, and alkyl respectively, n = 0−3) and K n X (X = Cl-, Br-, NO3 -, SO4 2-, [Co(CN)6]3-, n = 1−3). At 0.5 M cation concentration, second-order quenching rate constants, k q, are in the “nearly diffusion-controlled” range, 107−109 L mol-1 s-1, and k q decreases by an order of magnitude across the series of quenchers [Co(CN)5I]3- > [Co(CN)5N3]3- > [Co(CN)5Br]3- > [Co(CN)5Cl]3-. On the basis of a detailed study of [Co(CN)5I]3-, the quenching efficiency increases with background electrolyte concentration and the measured rate constants are in good agreement with predictions based on the Debye−Smoluchowski and Debye−Eigen equations for diffusion-controlled formation and dissociation in ionic solution of an encounter pair, together with a rate constant of 1.2 × 109 s-1 for the quenching step. However, the analysis provides further evidence for the Olson−Simonson effect; that is, in the presence of multivalent electrolyte ions, the salt effects are determined by the counterion concentration, here the cation, rather than by the ionic strength. Specific cation effects are observed such that the quenching rate constants increase in the following sequences: Li+ < Na+ < K+ < Cs+; Mg2+ < Ca2+ < Sr2+ < Ba2+; NH4 + < MeNH3 + < Me2NH2 + < Me3NH+; Et3NH+ < Et2NH2 + < EtNH3 +; n-PrNH3 + < EtNH3 + < MeNH3 +. For the alkali or alkaline-earth cations the large effects seen require participation of the cation in the transition state for the quenching step; the alkylammonium cations are also effective in this role, but the small differences in their efficiencies can be rationalized in terms of their effects on water structure.

Atom Transfer Processes in Oxidative Quenching of Triplet State Tetrakis(μ-pyrophosphite-P,P‘)diplatinate(II) by Acidopentacyanocobaltate(III) Anions

“…General Comments. As has been discussed in our earlier paper, Co(CN) 6 3- did not quench 3 Pt 2 (pop) 4 4- (in reality k q < 10 6 M -1 s -1 , based on the ±0.1 μs uncertainty of the lifetime measurements). For the acidopentacyanocobaltate quenchers studied, the quenching rate constants were in the nearly diffusion-controlled regime, 10 7 −10 8 L mol -1 s -1 , decreasing in the order Co(CN) 5 I 3- > Co(CN) 5 N 3 3- > Co(CN) 5 Br 3- > Co(CN) 5 Cl 3- .…”

“…Preparation of Materials. The complexes K 3 [Co(CN) 5 X], with X = I, Br, Cl, N 3 , CN, and K 4 [Pt 2 (μ-P 2 O 5 H 2 ) 4 ]·2H 2 O were prepared, purified, and characterized spectroscopically as detailed earlier . The spectra of the acidopentacyanocobaltates are available as Figure S1 in the Supporting Information.…”

“…This molecule has a long-lived triplet excited state, which phosphoresces with high quantum yield, and can participate in a variety of energy- and electron-transfer processes, with reports − of atom-transfer processes in reaction with organic halides. We have recently reported that this excited triplet state is also quenched by a series of anionic Co(III) complexes, where because of the 3−/4− charge repulsion, the quenching is assisted by electrolyte. The effect of electrolye concentration on the quenching rate constant was shown to be predicted well by the extended Debye−Hückel theory even up to electrolyte concentrations as high as 2 M. In addition, large specific cation effects were found using different electrolytes and these were interpreted in terms of direct participation of the cation in the transition state for the quenching.…”

Quenching of Triplet State Tetrakis(μ-pyrophosphito-PP‘)diplatinate(II) by Nickel(II) Macrocyclic and Tris-diimine Complexes

“…For comparison purposes, the diffusion-controlled rate constants k d , k -d , k′ d , and k′ -d defined in eq 4 were calculated using the Debye-Smoluchowski and Debye-Eigen equations for the diffusion of charged particles. This treatment has been outlined by Chiorboli and co-workers 23 who give the relevant equations in cgs units, and by Kirk and Cai 24 in SI units. The calculated value of 1.6 × 10 10 M -1 s -1 for the diffusional rate constant k d is in excellent agreement with the value of a 1 obtained from the fit to the quenching data.…”