Expanded porphyrin incorporated solvent polymeric membrane electrodes: protonation and interaction with an analyte anion at organic/water interface as studied by optical second harmonic generation and Fourier transform infrared attenuated total reflectance spectrometry

Optical second harmonic generation (SHG) at the surfaces of poly(vinyl chlo ride) (PVC) supported liquid membranes based on expanded porphyrins was mea sured to better understand the relationship between protonation of ionophor es at the membrane surface and selective EMF responses by the membranes tow ard anionic analytes in low pH ranges, Sapphyrin and rubyrin were chosen as representative examples of the ionophores and 3,5-dinitrobenzoate as their primary analyte anion, The square root of SHG intensity rootI(2 omega), of the sapphyrin based membranes increased in sigmoidal two steps according t o the decrease in pH in the adjacent aqueous phase in the absence of 3,5-di nitrobenzoate. This result was attributed to the change of protonation form of sapphyrin, i.e. unprotonated-sapphyrin, monoprotonated-sapphyrin and di protonated-sapphyrin, respectively, apparent surface protonation constants (apparent surface pK(a1), pK(a2)) for sapphyrin at the water/membrane inter face were determined therefrom, Apparent surface pK(a1) thus estimated for sapphyrin was found to be close to the pHs at which the corresponding membr ane begun to exhibit EMF and SHG responses toward 3,5-dinitrobenzoate Fouri er transform infrared attenuated total reflectance (FTIR-ATR) measurements of the sapphyrin incorporated membrane showed that 3,5-dinitrobenzoate was transported into the membrane phase only in the pH range where sapphyrin is highly protonated at the water/membrane interface and formed a complex wit h protonated sapphyrin. From these SHG, FTIR-ATR and EMF results, it is con cluded that the high degree of protonation of sapphyrin at the membrane sur face is requisite for the generation of anionic EMF responses toward the pr imary analyte anion, Apparent surface pK(a1) values estimated for sapphyrin in the sapphyrin and KTpClPB based membrane and for rubyrin in the rubyrin based membrane were also found to match the pHs below which the correspond ing membranes exhibit EMF responses toward 3,5-dinitrobenzoate. (C) 2001 El sevier Science B.V. All rights reserved.