Kinetic Investigation on Tetrakis(4-Sulfonatophenyl)Porphyrin J-Aggregates Formation Catalyzed by Cationic Metallo-Porphyrins

Under mild acidic conditions, various metal derivatives of tetrakis(4-N-methylpyridinium)porphyrin (gold(III), AuT(4); cobalt(III), CoT(4); manganese(III), MnT(4) and zinc(II), ZnT(4)) catalytically promote the supramolecular assembling process of the diacid 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H(2)TPPS(4)) into J-aggregates. The aggregation kinetics have been treated according to a well-established model that involves the initial formation of a critical nucleus containing m porphyrin units, followed by autocatalytic growth, in which the rate evolves as a power of time. An analysis of the extinction time traces allows to obtain the rate constants for the auto-catalyzed pathway, k(c), and the number of porphyrins involved in the initial seeding. The aggregation kinetics have been investigated at fixed H(2)TPPS(4) concentration as a function of the added metal derivatives MT(4). The derived rate constants, k(c), obey a rate law that is first order in [MT(4)] and depend on the specific nature of the catalyst in the order AuT(4) > CoT(4) > MnT(4) > ZnT(4). Both resonance light scattering (RLS) intensity and extinction in the aggregated samples increase on increasing [MT(4)]. With the exception of AuT(4), the final aggregated samples obtained at the highest catalyst concentration exhibit a negative Cotton effect in the J-band region, evidencing the occurrence of spontaneous symmetry breaking. The role of the nature of the metal derivative in terms of overall charge and presence of axial groups will be discussed.