Structure and Thermodynamics of Ionic Dendrimer-dye Assemblies

In this study, the association ("electrostatic self-assembly") of cationic generation 4 poly(amidoamine) dendrimer with a set of oppositely charged aromatic and aliphatic organic sulfonate counterions was investigated. Aggregate size and shape depend on the counterion structure. Formation of defined assemblies was possible with stiff, mutually interacting aromatic azo-dye counterions (acid red dyes Ar26, Ar44, Ar27, Ar18) but not with flexible or small aliphatic disulfonates and sulfate. Dynamic light scattering yielded sizes in the order of 100 nm. Particle shapes as investigated by small-angle neutron scattering were spherical, ellipsoidal, cylindrical, or of core-shell type. Results revealed that the molar charge ratio of the components is a crucial factor in the aggregation. UV-vis showed a cooperative binding process and yielded twist angles of dye molecules of 15 degrees to 30 degrees according to exciton theory. Isothermal titration calorimetry on dendrimer-dye assembly formation and comparison systems gave information on thermodynamics and contributing interaction energies. The main enthalpic contribution was provided by the mutual interaction of the dye molecules, while electrostatic interaction contributes a maximum of about 1/3 of the total interaction enthalpy.