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

There has been a controversy about the self-aggregation of 5,10,15,20-tetrakis4-( N -methyl)pyridiniumporphyrin (TMPyP(4)) in water. In order to make clear whether TMPyP(4) forms a dimer in water, 1 H NMR and UV−vis spectroscopic studies of 5-phenyl-10,15,20-tris4-( N -methyl)pyridiniumporphyrin (TriMPyP), 5,10-diphenyl-15,20-bis4-( N -methyl)pyridiniumporphyrin (5,10-DiMPyP), and 5,15-diphenyl-10,20-bis4-( N -methyl)pyridiniumporphyrin (5,15-DiMPyP) have been carried out. The 1 H NMR spectra indicate the formation of the self-aggregates of these phenylpyridiniumporphyrins in D 2 O when the porphyrin concentration is 1 × 10 -3 M. Comparison of the 1 H NMR spectra of TMPyP(4) with those of TriMPyP and DiMPyPs clearly exhibits that TMPyP(4) does not form its dimer in D 2 O. 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 KNO 3, 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 × 10 5 M -1 for 5,10-DiMPyP in the presence of 0.05 M KNO 3 and 1.12 × 10 6 M -1 for 5,15-DiMPyP in the presence of 0.01 M KNO 3 . Large and negative Δ H and Δ S values for dimerization of these DiMPyP's suggest the London's dispersion force as the main binding force.