Evidence for Tetraphenylporphyrin Monoacids

Abstract

Upon dilution from a concentrated solution in dichloromethane, the diacid form of tetraphenylporphyrin {H(4)TPP(X)(2)} (X = Cl, PF(6) and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, TFPB) affords eventually the unprotonated free base species H(2)TPP. At a difference of chloride, in the case of PF(6)(-) and TFPB(-) anions the conversion occurs with the intermediacy of a species, which has been assigned to a monoacid derivative on the basis of UV/vis absorption, fluorescence emission (static and dynamic), and resonance light scattering. Ground-state gas-phase geometries have been calculated both for the diacid {H(4)TPP(PF(6))(2)} and monoacid {H(4)TPP(PF(6))(2)} and {H(3)TPP(Cl)} species at the DFT/BP86 level of theory. TDDFT calculations using different functionals (BP86, SAOP, and B3LYP) have been exploited to provide electronic vertical excitation energies and oscillator strengths, yielding a remarkably good description of the optical spectra for these compounds and supporting the identification of the monoacid species. Gas-phase thermodynamic calculations on the chloride species provide an estimate of the Gibbs free energy changes associated with the two protonation steps, supporting the observed different behavior of this anion with respect to PF(6)(-) and TFPB(-).

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

“…1 . In agreement with the spectroscopic and DFT study of the acid-base behavior of H 2 TPP in dichloromethane, the occurrence of a large and non-coordinating counter-anion like TFPB À or TPBCl À should result in a stabilization of the monoacid, which is otherwise difficult to detect [14]. A similar voltammetric behavior was reported indicating the successive transfer of two protons facilitated by 5,10,15,20-tetraphenylporphyrinatocobalt(II) (CoTPP), which was obtained from commercial source [5].…”

meso-Aryl-substituted free-base porphyrins: formation, structure and photostability of diprotonated species

“…4). Such behavior is well known from the literature [37], and usually related to the solvent's polarity [20]. Nevertheless, it is worth highlighting the peculiar flexibility of the molecular system in diverse porphyrin compounds as reflected by these spectral changes, which has been particularly emphasized in Fig.…”

“…The reaction of porphyrins with TFA and/or other acceptor species (e.g., HCl, BF 3 , SbCl 3 etc.) in homogenous solutions can be monitored by electronic absorption spectra measurements [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33].…”

“…In consequence, the symmetry of the initially quasiflat molecule considerably changes [34][35][36][37][38]. Due to redistribution of electronic density within the bonding system of the core, the newly created adducts may reveal unique chemical and photochemical properties, unlike typical porphyrins, first reflected in their absorption and fluorescence spectra [20].…”

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

“…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][4][5][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.…”