Light-induced aggregation of cationic porphyrins

The formation of ion-pairs between cationic porphyrins (5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP), metallocomplexes Zn(II)TMPyP, Pd(II)TMPyP, Au(III)TMPyP, 5,10,15,20-tetrakis(α-[trimethylphosphonium]-p-tolyl)porphyrin, 5,10,15,20-tetrakis(α-pyridinio-p-tolyl)porphyrin) and triiodide anion leads to an extensive porphyrin aggregation in neutral aqueous solutions. Triiodide counteranion can be produced in situ by a photochemical reaction of cationic porphyrin sensitizers with oxygen in the presence of I− since photoproduced singlet oxygen 1O2 oxidizes I− to I3−. The subsequent aggregation of the porphyrin/I3− ion-pairs causes fast quenching of the porphyrin triplet states and consequently restricts the formation of 1O2. As a result, the formation of 1O2 is stopped at a critical concentration of photoproduced triiodide. The presence of calf thymus DNA, cyclodextrin, or calixarene forming supramolecular assemblies with porphyrin and/or I3− prevents the formation of ion-pairs with I3− and preserves the effective production of 1O2 by porphyrins. Aggregation is also eliminated at higher temperatures. Porphyrin Sn(IV)TMPyP does not aggregate probably because of two axial ligands and preserves its photosensitizing ability.