SELF-AGGREGATION OF CATIONIC PORPHYRINS IN WATER - CAN PI-PI STACKINGINTERACTION OVERCOME ELECTROSTATIC REPULSIVE FORCE

There has been a controversy about the self-aggregation of 0,15,20-tet rakis[4-(N-methyl)pyridinium]-porphyrin (TMPyP(4)) in water. In order to make clear whether TMPyP(4) forms a dimer in water, H-1 NMR and UV- vis spectroscopic studies of nyl-10,15,20-tris[4-(N-methyl)pyridinium] porphyrin (TriMPyP), iphenyl-15,20-bis[4-(N-methyl)pyridinium]porphyri n (5,10-DiMPyP), and diphenyl-10,20-bis[4(N-methyl)pyridinium]porphyri n (5,15-DiMPyP) have been carried out. The H-1 NMR spectra indicate th e formation of the self-aggregates of these phenylpyridiniumporphyrins in D2O when the porphyrin concentration is 1 x 10(-3) M. Comparison o f the H-1 NMR spectra or TMPyP(4) with those of TriMPyP and DiMPyPs cl early exhibits that TMPyP(4) does not form its dimer in D2O. Broadenin g of the signal due to the beta-pyrrole protons of the cationic porphy rin is ascribed to the tautomerism of the inner N-D deuterons with the rate slower than that of the N-H protons. At lower concentrations (si milar to 10(-5) M), both DiMPyP's form dimers in water in the presence of KNO3, while TMPyP(4) and TriMPyP exist as monomers. The associatio n constants for dimerization in water at 25 degrees C have been determ ined to be 7.36 x 10(5) M-1 for 5,10-DiMPyP in the presence of 0.05 M KNO3 and 1.12 x 10(6) M-1 for 5,15-DiMPyP in the presence of 0.01 M KN O3. Large and negative Delta H and Delta S values for dimerization of these DiMPyP's suggest the London's dispersion force as the main bindi ng force.