Ternary porphyrin aggregates and their chiral memory

In this paper we investigate the self-aggregation of tetracationic porphyri n meso-tetrakis(N-methplpyridinium-4-yl)porphinatocopper(II) (CuT4) onto a chiral polymeric matrix (polyglutamic acid, PG) as well as mixtures of tetr acationic (CuT4) and tetraanionic, meso-tetrakis(4-sulfonatophenyl)porphine (H4TPPS), porphyrins onto the same PG matrix as a function of pH, temperat ure, ionic strength, and reactant concentration. The systems have been stud ied by light absorption, fluorescence, circular dichroism (CD), and resonan ce light scattering techniques. PG chains undergo a pH-dependent phase tran sition from a chiral alpha -helix conformation (low charge density) to an a chiral random coil structure thigh charge density). By contrast, a pH incre ase also enhances cationic porphyrin adsorption and self-aggregation onto t he polymer matrix, the chirality of which induces strong dichroism in the a dsorbed porphyrins. In the case of binary systems (ionic polymer and opposi tely charged porphyrin) the competition between these opposite demands has been rationalized on the basis of a thermodynamic model for self-aggregatio n in a two-phase system (bulk solution and polymer adsorption sites) and gi ves 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 b een 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 s olution. 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 o f the PG alpha -helix conformation. By contrast, the stable aggregates made up of an equimolar mixture of anionic and cationic porphyrins retain "memo ry" of the original matrix chirality even after months. This behavior is re miniscent of that of a ferromagnetic material when the strong magnetic fiel d has been turned off: below a certain critical temperature the magnetizati on goes to zero, while above that critical value the magnetization slowly d ecreases, eventually reaching a constant nonzero value. Furthermore, tightl y bound anionic-cationic porphyrins to the chiral pc template do not show a ny CD change upon addition of a large excess of template bearing opposite c hirality, confirming a very slow exchange kinetics among the aggregates.