Porphyrins. XXXVII. Absorption and emission of weak complexes with acids, bases, and salts

Abstract

Absorption and emission data are reported for species resulting from addition of acids, alkali, and metal salts to H₂(Etio) (etioporphyrin) and H₂(TPP) (tetraphenylporphyrin) in various organic solvents. With acid addition, dications of both TPP and Etio are observed in equilibrium with free base, and a monocation of Etio is also observed in acetone, but not in benzene. The fluorescence yield of H₄(TPP)Cl₂ was approximately 0.1, and that of H₄(TPP)Br₂, about 0.01 in acetone. The fluorescence yield of H₄(Etio)Cl₂ was approximately twice that of H₄(Etio)Br₂ in THF. With alkali addition, absorption spectra show a predicted variation with metal, and all species show strong fluorescence and weak phosphorescence, which is consistent with only a small heavy atom effect. In addition, three distinct species were observed in the fluorescence of Na₂(TPP) complexes in THF at 77 K. With metal salt addition, spectra closely resembling monocation and dication spectra are produced in acetone for Etio and TPP, respectively. For all cases studied, except perhaps uranyl salts with TPP, fluorescence yields agreed to within about 25% of that of H₃(Etio) acetate for Etio complexes and about 25% of that of H₄(TPP)Cl₂ for TPP complexes, indicating shielding of the metal salt from the porphyrin π cloud by solvent molecules.