Factors influencing self-aggregation tendencies of cationic porphyrins in aqueous solution

The self-aggregation of cationic porphyrins in aqueous solution has been studied by means of absorption, H-1 NMR, and resonance light scattering spectroscopy. The aim of the present study is clarification of the factors which most influence porphyrin self-aggregation in water. Cationic 5,10,15,20-tetrasubstituted porphyrins [PorSub(4): PC3Py (Sub = -(CH2)(3)-Py+Cl-), PC5Py (Sub = -(CH2)(5)-Py+Cl-), PC7Py (Sub -(CH?)7-Py+Cl-), TPPOC2Py (Sub = P-C6H4-O(CH2)(2)-Py+Br-), and TPPOC3Py (Sub = p-C6H4-O(CH2)(3)-Py+Br-), where Py+ = N-alkylpyridinium] were used. PC3Py forms a dimer in aqueous solution with or without an added inorganic salt. In the presence of KNO3, dimer formation of PC3Py is dominated by a large and negative enthalpy change. The entropy change for aggregation increases with increasing concentrations of added KNO3, while enthalpy changes are almost constant. The thermodynamic parameters suggest that dimer formation of PC3Py is the result of a large enthalpic gain due to extended van der Waals interactions, in cooperation with enhanced hydrophobic interactions. Electrostatic repulsion suppresses further association. However, other cationic porphyrins studied show evidence of higher self-aggregate formation. X-ray crystallographic studies of PC5Cl (Sub = -(CH2)(5)-Cl), a precursor of PC5Py, show the formation of a slipped face-to-face dimer as the basic unit for forming a crystal lattice, while that of TPP (Sub = -C6H5) indicates the absence of face-to-face stacking interaction in the crystals. Judging from these results, it is assumed that the slipped face-to-face dimer is the unit of the higher self-aggregates of PC5Py or PC7Py in water with or without KNO3. Elongated alkyl chains of PC5Py and PC7Py may be responsible by enhancing hydrophobic interaction of the porphyrins studied. TPPOC2Py alone forms a J-aggregate, characterized by a sharp and red-shifted Soret band and a strongly enhanced resonance light scattering signal. Higher self-aggregates having face-to-face arrangement are formed with TPPOC3Py; these provide no enhanced RLS feature. The present study reveals the dependence of aggregate formation of water-soluble porphyrins on the peripheral mesosubstituents.