Evidence of tetraphenylporphyrin monoacids by ion-transfer voltammetry at polarized liquid|liquid interfaces

Abstract

We present a simple methodology to illustrate the existence of tetraphenylporphyrin monoacid based on ion-transfer voltammetry at a polarized water|1,2-dichloroethane interface and organic pK values are also estimated.The acid-base property of free-base porphyrins has been widely investigated, but the subject is still the matter of some controversy. 1 The tetrapyrrole ring of a free-base porphyrin contains two tertiary nitrogen atoms, which allows the gain of protons to form a monoacid and a diacid, as illustrated for the free-base 5,10,15,20-meso-tetrapehenylporphyrin (H 2 TPP) in Scheme 1, and the existence of monoacids has received considerable attention. By titrating H 2 TPP usually one isosbestic point due to H 2 TPP and H 4 TPP 2+ is observed, whereas the intermediate H 3 TPP + could not be clearly detected. 2 The existence of H 3 TPP + , as well as pK a1 and pK a2 values, has been reported in a few cases, 3-6 one of which is based on the ion-transfer voltammetry at a polarized water|nitro-benzene interface. 6 Since the pioneering work of Koryta et al., 7 ion transfer voltammetry at the liquid|liquid interface (also called the Interface between Two Immiscible Electrolyte Solutions (ITIES)) has become a well-established method to study the acid-base properties of molecules dissolved in an organic phase in contact with an aqueous electrolyte. As shown by Reymond et al. for the study of therapeutic molecules, this methodology allows the determination of pK values, and the drawing of ionic partition diagrams. 8 Herein, using this simple methodology, we illustrate the existence of H 3 TPP + in 1,2-dichloroethane (DCE) and pK a1 and pK a2 values are also estimated.An ITIES is formed when an aqueous solution containing hydrophilic ions is put in contact with an organic solution containing lipophilic ions. Electrochemical polarization supplied by an external voltage can give a polarizable potential window, the width of which is defined by the transfer reactions of the electrolyte ions across the interface. The potential dependence for ion transfer across the interface follows a Nernst equation. For example, using 10 mM LiCl and 100 mM HCl as the aqueous electrolytes and bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate (BTPPATPFB, 5 mM) as the lipophilic electrolyte in DCE, a potential window ranging from À0.5 to 0.4 V can be obtained, as shown by the dotted line in Fig. 1(a). This window is determined by the transfer of H + and Cl À from water to DCE at positive and negative potentials, respectively. With the help of an ionophore present in DCE that can complex with H + , the transfer of H + will be facilitated and a wave will appear in the middle of the potential window. Stated in another way, H + will transfer at less positive potentials in this case, due to the presence of a proton acceptor in the organic phase. This phenomenon is usually called facilitated ion transfer reaction (or Transfer by Interfacial Complexation, TIC) 9 and the shift of the transfer potential provides information...

Electrochemical evidence of catalysis of oxygen reduction at the polarized liquid–liquid interface by tetraphenylporphyrin monoacid and diacid

“…The same shift was reported for the voltammetric peaks of the facilitated proton transfer (Fig. 1, solid red line) [13]. Indeed, the halfwave potential difference D Fig.…”

Ionic partition diagram of tetraphenylporphyrin at the water|1,2-dichloroethane interface

“…1 in Ref. [28] and as confirmed in the present study, pK DCE a1 and pK DCE a2 were found to be equal to 6.0 and 9.8 using TB À as the organic counter anion, and 6.7 and 10.3 when using TPBCl À as the organic counter anion [29] showing the influence of the organic supporting electrolyte in the basic properties of the porphyrin with …”

“…To determine the apparent assisted ion transfer potential of H + , the scan rate dependence of the cyclic voltammetry together with the concentration dependence were extrapolated to zero scan rate and peak to peak separation of approximately 60 mV was observed (see SI of [28]). The diffusion coefficient obtained from the scan rate and the concentration dependence were both found to be equal to 6. , respectively, can be estimated by exploring the pH dependence presented in Fig.…”

Influence of Тetrakis(4’-decaoxyphenyl)porphyrin Addition on Electrochemical Reduction of Oxygen on Рt Electrode in Dimethylformamide

“…Therefore, the interest to the chemistry of natural and synthetic porphyrins is continuously increased. However, the many properties of porphyrin were investigated only on the qualitative level due to the low solubility of porphyrins in the most solvents, [1] the formation of supramolecular aggregates, [2] the specific of adsorbed behavior at the interface, [3] etc. The porphyrins can include the several reaction centers and different substituents which allow changing its chemical activity and other properties.…”