Thermodynamic Study of Ion-Pairing Effects between Reduced Double-Decker Lutetium(III) Phthalocyanines and a Cationic Matrix

The thermodynamics of ion paring between electrochemically reduced [(C ₆H₁₃S)₈Pc]₂Lu or [(C ₁₂H₂₅S)₄Pc]₂Lu and a cationic tetra-n-octylphosphonium bromide (4C₈P⁺Br^-) matrix that was deposited in a gel-like cast film on a basal plane pyrolytic graphite electrode was investigated in an aqueous KC1 solution that varied in concentration from 0.005 to 1.0 M. The purpose of this study was to elucidate ion-pairing effects between reduced double-decker lutetium(III) phthalocyanines with charges of -1 to -5 and cationic sites of the 4C₈P ⁺Br^- matrix. Both phthalocyanines undergo five one-electron reductions in the film, and a linear relationship was observed between the measured half-wave potentials (E_1/2) for these processes and the Cl^- concentration of the electrolyte in the film. The apparent number of electrons transferred in each reduction step was obtained from plots of E_1/2 versus ln[Cl^-], which gave Nernstian slopes of -23.5 ± 2.3 mV (or -54.1 ± 5.3 mV from plots of E _1/2 vs log[Cl^-]) over a chloride concentration range of 0.005 to 1.0 M. The E_1/2 versus ln[Cl^-] plots indicate that the stepwise addition of electrons to the phthalocyanines in the film is accompanied by a stepwise unit increase in the number of associated 4C ₈P⁺ cations. Thus, the ratio of phthalocyanine molecules to monocations increases from 1:1 for the singly reduced derivative to 1:5 in the case of {[(C₆H₁₃S)₈Pc]₂Lu} ^5- or {[(C₁₂H₂₅S)₄Pc] ₂Lu}^5-. This result leads to the conclusion that a binding of reduced [(C₅H₁₃S)₈Pc]₂Lu or [(C₁₂H₂₅S)₄Pc]₂ to the cationic sites of the 4C₈P⁺ matrix is responsible for the experimentally observed positive shift of the measured half-wave potentials as compared to E_1/2 values for the same electrode reactions in nonaqueous media. The changes in E_1/2 upon changing the electrolyte counter cation and/or the charge of the matrix are also described, and these results further support the observed ion-pairing effects that were examined by means of electrochemical and spectroscopic techniques.