Evidence for tetraphenylporphyrin monoacids

Upon dilution from a concentrated solution in dichloromethane, the diacid form of tetraphenylporphyrin {H4TPP(X)2} (X = Cl, PF6 and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, TFPB) affords eventually the unprotonated free base species H2TPP. At a difference of chloride, in the case of PF6- 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 {H4TPP(PF6)2} and monoacid {H3TPP(PF6)} and {H3TPP(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 PF6- and TFPB-.