Solvent effects on spectrophotometric titrations and vibrational spectroscopy of 5,10,15-triphenyl-20-(4-hydroxyphenyl)porphyrin in aqueous DMF

The spectrophotometric titration by sodium hydroxide of 5,10,15-triphenyl-20-(4-hydroxyphenyl)porphyrin ((OH)₁PH₂) is studied as a function of solvent composition of DMF-H₂O binary solvent mixture ([OH^-] = 0.04 M). Combining the structure changes of the porphyrin and the "four orbital" model of Gouterman, many features of the optical spectra of this deprotonated para-hydroxy-substituted tetraphenylporphyrin in different composition of binary solvent mixtures can be rationalized. In highly aqueous solvents, the changes of the titration curves are shown to be mainly due to hydrogen-bonding of the oxygen of the phenoxide anion group by the hydroxylic solvent, Which decreases the energy of the phenoxide anion π orbital. Thus the phenoxide anion π orbital cannot cross over the porphyrin π orbital being a different HOMO. However, its energy is close to that of the porphyrin π orbitals. As a result, in the visible region, no charge-transfer band is observed, while in the visible-near region, the Soret peak split into two components. In nonaqueous solvents, the changes are mainly attributed to further deprotonation of pyrrolic-Hs of (OH)₁PH₂ by NaOH and coordination with two sodium ions to form the sodium complex of (OH)₁PH₂, which turns hyperporphyrin spectra of deprotonated of phenolic-H of (OH)₁PH₂ into three-banded spectra of regular metalloporphyrin.