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.
The pKa values and the 1H NMR spectra of tetraarylporphyrins in the presence of cyclodextrins suggest that the microscopic environment of the positively polarized cavity of cyclodextrin promotes the inclusion of the anionic guest and prohibits the penetration of the cationic guest into the cyclodextrin cavity.
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