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 H 2(Etio) (etioporphyrin) and H 2(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 H 4(TPP)Cl 2 was approximately 0.1, and that of H 4(TPP)Br 2, about 0.01 in acetone. The fluorescence yield of H 4(Etio)Cl 2 was approximately twice that of H 4(Etio)Br 2 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 Na 2(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 H 3(Etio) acetate for Etio complexes and about 25% of that of H 4(TPP)Cl 2 for TPP complexes, indicating shielding of the metal salt from the porphyrin π cloud by solvent molecules.