Direct Spectrophotometric Measurement of Demetalation Kinetics of 5,10,15,20-Tetraphenylporphyrinatozinc(II) at the Liquid−Liquid Interface by a Centrifugal Liquid Membrane Method

Abstract

The equilibrium and kinetics of the protonation of 5,10, 15,20-tetraphenylporphyrin (H2TPP) and the demetalation of 5,10,15,20-tetraphenylporphyrinatozinc(II) (ZnTPP) at a dodecane−aqueous acid interface were investigated by means of a new in situ spectrophotometric method, the centrifugal liquid membrane method, which can provide the ultrathin two-phase liquid membrane system in a rotating glass cell. The consumption of H2TPP in the bulk dodecane phase and the production of the diprotonated aggregate, (H4TPP2+)n, adsorbed at the liquid−liquid interface were directly measured from the spectral change. The equilibrium constants of the interfacial aggregation of H4TPP2+ and the demetalation of ZnTPP were determined as log(Ke1/dm6 mol-2) = 2.14 ± 0.07 and log(Ke2/dm9 mol-3) = −6.05 ± 0.04 at 298 K, respectively. The observed rate constant of the demetalation of ZnTPP depended upon the first order of the acidity function, and it was suggested that the rate-determining step is the formation of the monoprotonat...