Self-Aggregation of Cationic Porphyrins in Water. Can π−π Stacking Interaction Overcome Electrostatic Repulsive Force?

Abstract

There has been a controversy about the self-aggregation of 5,10,15,20-tetrakis[4-(N-methyl)pyridinium]porphyrin (TMPyP(4)) in water. In order to make clear whether TMPyP(4) forms a dimer in water, 1H NMR and UV−vis spectroscopic studies of 5-phenyl-10,15,20-tris[4-(N-methyl)pyridinium]porphyrin (TriMPyP), 5,10-diphenyl-15,20-bis[4-(N-methyl)pyridinium]porphyrin (5,10-DiMPyP), and 5,15-diphenyl-10,20-bis[4-(N-methyl)pyridinium]porphyrin (5,15-DiMPyP) have been carried out. The 1H NMR spectra indicate the formation of the self-aggregates of these phenylpyridiniumporphyrins in D2O when the porphyrin concentration is 1 × 10-3 M. Comparison of the 1H NMR spectra of TMPyP(4) with those of TriMPyP and DiMPyPs clearly exhibits that TMPyP(4) does not form its dimer in D2O. Broadening of the signal due to the β-pyrrole protons of the cationic porphyrin 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 (∼10-5 M), both DiMPyP's form dimers in water in the presence of KNO3, while TMPyP(4) and TriMPyP exist as monomers. The association constants for dimerization in water at 25 °C have been determined to be 7.36 × 105 M-1 for 5,10-DiMPyP in the presence of 0.05 M KNO3 and 1.12 × 106 M-1 for 5,15-DiMPyP in the presence of 0.01 M KNO3. Large and negative ΔH and ΔS values for dimerization of these DiMPyP's suggest the London's dispersion force as the main binding force.