Radiative Deactivation of Lowest Singlet and Triplet Excited States of Water-Soluble Porphyrins

Radiative deactivation of lowest excited triplet and singlet states for series of free bases and Zn2+-complexes of water-soluble 5,10,15,20-tetrakis-(4-sulfonatophenyl)porphyrin (H2T4SPP); 5,10,15,20-tetrakis-(4-N-alkylpyridyl) porphyrin (H2T4AlkPyP); and 5,10,15,20-tetrakis-(3-N-alkylpyridyl)porphyrin (H2T3AlkPyP) was studied in solutions at 77 and 293 K. The fluorescence and phosphorescence quantum yields were found to have small but detectable differences for derivatives of the same porphyrin ligand with different counterions for the ionized aryl substituents. It was shown that these differences were due to spin–orbit coupling effects and variation of the electron donor/acceptor properties of the peripheral substituents that changed the electronic energy levels in the porphyrin–counterion system.