Sequential Deprotonation of meso-(p-Hydroxyphenyl)porphyrins in DMF:  From Hyperporphyrins to Sodium Porphyrin Complexes

Abstract

Sequential deprotonations of meso-(p-hydroxyphenyl)porphyrins (p-OHTPPH2) in DMF + H2O (V/V = 1:1) mixture have been verified to result in the appearance of hyperporphyrin spectra. However, when the deprotonations of these p-OHTPPH2 are carried out in DMF, the spectral changes differ considerably from those in the mixture mentioned above. At low [OH-], the optical spectra in the visible region are still considered to have characteristics of hyperporphyrin spectra. Further deprotonation at much higher basicity makes the optical spectra form three-banded spectra similar to those in the acidic solution. To clarify the molecular origins of these changes, UV-vis, resonance Raman (RR), proton nuclear magnetic resonance (1H NMR) experiments are carried out. Our data give evidence that p-OHTPPH2 in DMF can be further deprotonated of pyrrolic-H by higher concentrated NaOH, due to an aprotic medium like DMF effectively weakening the basicity of the porphyrin relative to that of the NaOH, and coordinates with two sodium ions (except the sodium ions that interact with the peripherial phenoxide anions) to form the sodium complexes of p-OHTPPH2 (Na2P, to lay a strong emphasis on the sodium ions that coordinate with the central nitrogen atom), which can be regarded as the porphyrin anions being perturbed by the sodium cations due to their highly ionic character. The negative centers generated by deprotonation of pyrrolic-H and phenolic-H are not thoroughly delocalized between the substituents and the porphyrin ring. Thus the negative centers generated by deprotonation of pyrrolic-H only act as electron-donating groups on the porphyrin pi system, and the negative charges of the phenoxide anion are also mainly localized on the peripheral substituents. As a result, the porphyrin pi orbitals cross over the phenoxide anion pi orbital and turn into HOMOs, which turns hyperporphyrin spectra of deprotonated phenolic-H of p-OHTPPH2 into three-banded spectra of regular metalloporphyrins.

Planar and Nonplanar Free‐Base Tetraarylporphyrins: β‐Pyrrole Substituents and Geometric Effects on Electrochemistry, Spectroelectrochemistry, and Protonation/Deprotonation Reactions in Nonaqueous Media

“…[25] A loss of protons from free-base porphyrins can occur in nonaqueous media containing added base, with the degree of deprotonation depending upon the porphyrin substituents, the solvent, the strength of the added base, and the planarity of the macrocycle. The deprotonation of H 2 (P) to give [H(P)] À and [(P)] 2À has been examined by UV/Vis spectroscopy in nonaqueous media [27,30,[37][38][39][40] but, to our knowledge, there has been no electrochemistry reported for the deprotonated porphyrins under these solution conditions. Thus, an electrochemical and spectroelectrochemical study of the protonated and deprotonated compounds 1-10 in CH 2 Cl 2 should provide new information on redox potentials and UV/Vis spectra for a series of planar and nonplanar free-base porphyrins.…”

Aggregation Properties of Hyperporphyrins with Hydroxyphenyl Substituents

“…In these cases, or in the presence of peripheral amino or hydroxyl substituent groups, lone pair orbitals with the correct symmetry are available for a charge-transfer transition from the donor atom to the porphyrin. [10][11][12][13][14][15][16] To the best of our knowledge, the formation of aggregates, which would also lead to broadened and red-shifted extinction features, has not been reported in the literature for porphyrins functionalized with hydroxyphenyl or aminophenyl groups. Following our interest in the behavior of porphyrins upon acidification in organic solvents, we have already investigated tetraphenylporphyrin (TPP), 29 the three isomers of tetrakis-(pyridyl)porphyrin (TpyP), 30 and the tetrabutylammonium salt of tetrakis(4-sulfonatophenyl)porphyrin (TPPS).…”

Spectroscopic characterization of free-base hydroxy(arylethynyl)porphyrins in acidic and basic media

“…This is attributed to the ability of water as solvent to weaken the basicity of the NaOH relative to that of the porphyrin. In contrast, in both DMF [30] and DMSO [37] solutions, sequential deprotonation of the OH groups, followed by those in the center of the porphyrin is observed. This full deprotonation of the porphyrin yields a spectrum that resembles that of a porphyrin dication [30].…”

“…In contrast, in both DMF [30] and DMSO [37] solutions, sequential deprotonation of the OH groups, followed by those in the center of the porphyrin is observed. This full deprotonation of the porphyrin yields a spectrum that resembles that of a porphyrin dication [30]. Figure 6 shows the spectral changes for H 2 THPP and 1 upon the addition of NaOH in 1:1 DMF:H 2 O, and the data is summarized in Table 2.…”

“…Porphyrin diacids may be obtained upon protonation of the imino nitrogen atoms and exhibit perturbed photophysical properties relative to the neutral derivatives [25−29]. Deprotonation of the pyrollic nitrogens can result in alkali metal complexes with distinct spectral signatures [30]. These unique properties make free-base porphyrins of interest, particularly for applications involving pH changes.…”