Spectroscopic and electrochemical characterization of the interaction of nitrogen monoxide and cobalt tetrasulfonated phthalocyanine in aqueous solutions and surfactant films

Abstract

Cobalt(II) tetrasulfonated phthalocyanine (Co(II)TSPc) was attached to didodecyldimethylammonium (DDA) and dodecyltrimethylammonium (DTA) films. Spectroscopic data suggest that monomeric Co(II)TSPc is the major species in the surfactant films. Differential pulse voltammetry for CoTSPc/DTA films in phosphate buffer solution at pH 6 shows reduction peaks at +0.61, +0.06 and −0.64 V attributed to Co(III)/Co(II), Co(II)/Co(I) and TSPc reduction, respectively. In CoTSPc/DDA films, Co(II) is reduced at −0.03 V and TSPc at −0.68 and −0.84 V, and the peak for Co(III)/Co(II) is undetectable. The reaction of Co(II)TSPc with NO was studied for dissolved CoTSPc as well as for CoTSPc/DDA and CoTSPc/DTA films using spectroscopic and electrochemical techniques. From cyclic voltammetric data for Co(II)TSPc/DDA films in NO containing solution at pH 9, the stability constant of the Co(II)TSPc(NO) adduct was estimated to be 1.3×10 8 M −1. A slow shift of the FTIR band located at 1646–1700 cm −1 for solid Co(II)TSPc/DDA films exposed to NO is attributed to a conformation from a bent to linear configuration for the CoTSPc(NO) adduct. The rate of this transformation as obtained from UV–Vis experiments conducted for CoTSPc dissolved in aqueous solution as well as from FTIR and electrochemical data for CoTSPc/DDA films is 0.015±0.002 min −1. The Co(II)TSPc(NO) adduct formed in these films in solutions at pH 9 at NO concentrations <7×10 −5 M is reduced at −0.17 V. Higher NO concentrations cause reductive nitrosylation and the formation of a Co(I)TSPc(NO +) complex which is reduced at −0.72 and −0.46 V, in DDA and DTA films, respectively. The electroreduction of NO at these potentials is positively shifted by 100 and 450 mV relatively to that obtained for DDA and DTA films devoid of CoTSPc, respectively.