Protonation of porphyrins in liquid PVC membranes: Effects of anionic additives and application to pH-sensing

We report on a study on the protonation of representative prophyrins, metalloporphyrins, porphyrin-ketones and a Pd-porphyrin Schiff base in aqueous solution and when dissolved in plasticized PVC membranes. The respective protolytic forms were characterized by absorption and emission spectra as well as apparent pK_a values. It is shown that the protonation of porphyrins dissolved in PVC membranes is much more difficult than in solution, resulting in an extraordinarily large decrease in the apparent pK_a values which can be as large as 5 units when compared to the corresponding data for the porphyrins in solution. In certain cases, no protonation at all occurred within the pH 2-12 range. At the same time, N-MeEP which is much more basic (its intrinsic pK being 11.2) can be protonated in such membranes into the monocation at weakly acidic pH, but the apparent pK_a also is lowered by about 6 units. On addition of a tetraphenylborate anion to the PVC membranes, protonation was found to proceed much easier. Both the monocation and dication was identified in case for the porphyrins, while for the prophyrin-ketones direct protonation into dication took place, and N-methyletioporphyrin was present as the monocation over the whole pH 2-12 range. Membranes composed of solutions of porphyrin-ketones or the Pd-porphyrin Schiff base in plasticized PVC were identified as being useful for purposes of continuously sensing pH in the physiological range. Their response time, calibration curves, signal stability and effects of buffer ionic strength and temperature were investigated in more detail.