REVERSIBLE AGGREGATION/DEAGGREGATION OF PORPHYRIN J-AGGREGATES INDUCED BY SILVER(I) CATIONS

Porphyrin J-aggregates have been largely investigated due to their remarkable structural, electronic and chiral properties. Much of these studies have been carried out on water soluble tetra-anionic tetrakis(4-sulfonato-phenyl)porphyrin (H2TPPS4). In the presence of added salts or at high acid concentration the diacid H4TPPS4 self-assembles into J-aggregates stabilized primarily by electrostatic interactions between the positive protonated core of the macrocycle and the negatively charged sulfonate groups of adjacent porphyrins. The kinetics of supramolecular assembling of these nanoaggregates are characterized by a sigmoidal profile with an initial lag period.1 Detailed kinetic studies on the reverse reaction for J-aggregates , i.e. the disassembly process, are far less frequent. Quite recently, we reported that for the metallation of the acidic form of H2TPPS4 (di-anionic H4TPPS4) by copper(II), the order of reagent mixing determines the rate and mechanism of CuTPPS4 formation. When copper salts are added last, the kinetic profile is fit as a (pseudo)-first order process. However, J-aggregates of the di-anionic H4TPPS4 porphyrin are rapidly formed at pH ~ 3 when copper(II) salts are incorporated in solution prior to porphyrin addition. The subsequent metallation of the porphyrin units leads to the disassembling of these arrays via a pseudo-zero order kinetic profile, suggesting an attack of the metal ion at the rims of the nanostructure. Here we report on the aggregation of H4TPPS4 under acidic pH in the presence of excess silver(I) ions. The formed J-aggregates under light irradiation undergo to the formation of the silver(II) metal derivative of the porphyrin, that in the dark slowly interconverts back to the J-aggregated species. Based on previous literature reports, a mechanism has been proposed.