Ternary porphyrin aggregates and their chiral memory

In this paper we investigate the self-aggregation of tetracationic porphyrin meso-tetrakis(N-methplpyridinium-4-yl)porphinatocopper(II) (CuT4) onto a chiral polymeric matrix (polyglutamic acid, PG) as well as mixtures of tetracationic (CuT4) and tetraanionic, meso-tetrakis(4-sulfonatophenyl)porphine (H4TPPS), porphyrins onto the same PG matrix as a function of pH, temperature, ionic strength, and reactant concentration. The systems have been studied by light absorption, fluorescence, circular dichroism (CD), and resonance light scattering techniques. PG chains undergo a pH-dependent phase transition from a chiral alpha -helix conformation (low charge density) to an achiral random coil structure thigh charge density). By contrast, a pH increase also enhances cationic porphyrin adsorption and self-aggregation onto the polymer matrix, the chirality of which induces strong dichroism in the adsorbed porphyrins. In the case of binary systems (ionic polymer and oppositely charged porphyrin) the competition between these opposite demands has been rationalized on the basis of a thermodynamic model for self-aggregation in a two-phase system (bulk solution and polymer adsorption sites) and gives rise to a maximum in the induced CD intensity and hypochromicity of the adsorbed porphyrins on varying the pH. An even more complex behavior has been observed in the ease of ternary systems (ionic polymer interacting with mixtures of anionic and cationic porphyrins) because of the (pH-modulated) possibility of self-aggregation both onto the polymer matrix and in bulk solution. The studied systems also show interesting effects depending on the strength of self-association among porphyrins, In fact, weakly aggregated porphyrins (CuT4 onto PG) lose their chirality upon pH-induced disruption of the PG alpha -helix conformation. By contrast, the stable aggregates made up of an equimolar mixture of anionic and cationic porphyrins retain "memory" of the original matrix chirality even after months. This behavior is reminiscent of that of a ferromagnetic material when the strong magnetic field has been turned off: below a certain critical temperature the magnetization goes to zero, while above that critical value the magnetization slowly decreases, eventually reaching a constant nonzero value. Furthermore, tightly bound anionic-cationic porphyrins to the chiral pc template do not show any CD change upon addition of a large excess of template bearing opposite chirality, confirming a very slow exchange kinetics among the aggregates.