Cation Effect on the Quenching of the Photoexcited State of Tetrakis(μ-pyrophosphito-P,P′)diplatinate(II) by Octacyanomolybdate(IV) in Aqueous Solution

The photoquenching reaction of [Pt₂(pop)₄]⁴⁻ (pop²⁻ = pyrophosphite(2−)) with [Mo(CN)₈]⁴⁻ was studied in 0.01 M HCl or HClO₄ in the presence of various added electrolytes (0.49 M) (LiCl, NaCl, KCl, CsCl, NH₄Cl, (C₂H₅)₄NCl, and NaClO₄) at 25 °C. The quenching rate constants (k_q) differ by nearly 1 order of magnitude from 1.17 × 10⁷ (LiCl) to 9.90 X 107 M⁻¹ s⁻¹ (CsCl). The phosphorescence lifetime (τ₀) in the absence of the quencher is practically constant (8.3–8.6 μs) regardless of the type of electrolyte added, k_q, shows a linear correlation with the reciprocal of the cation Stokes radii except for the case of (C₂H₅)₄N⁺. The cation effect was interpreted by considering a precursor ion triplet among two negatively charged reactants and a foreign cation. The role of the cation is to reduce the Coulombic barrier of the two reactants in order to form an encounter complex.