meso‐Tetratolylporphyrins substituted by pyridinium groups: aggregation, photophysical properties and complexation with DNA

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Abstract
The aggregation behavior, binding to DNA and photophysical properties of meso‐tetratolylporphyrins substituted with two (P2), three (P3) and four (P4) pyridinium groups were studied using UV–visible and transient spectroscopy, resonance light scattering, circular dichroism and time‐resolved luminescence of singlet oxygen. In aqueous solutions porphyrins P2–P4 were present as monomers and aggregates of H‐ and J‐type. The concentration ratio of monomer to aggregates was controlled by the number of cationic substituents, by ionic strength and by temperature. Porphyrin P4 was predominantly monomeric and was externally bound to calf thymus DNA with a binding constant of 1.2 × 106 M−1. Porphyrins P3 and P2 self‐assembled on the DNA backbone to form chiral assemblies not affecting the duplex melting point. The lifetimes of the triplet states of monomeric P3 and P4 increased after binding to DNA; however, this did not prevent the formation of singlet oxygen, 1O2. Copyright © 2004 John Wiley & Sons, Ltd.

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