Fractal growth of PAMAM dendrimer aggregates and its impact on the intrinsic emission properties.

The dynamics of aggregation between electrostatically bound carboxylate and amine terminated poly(amidoamine) {PAMAM} dendrimers have been investigated in aqueous medium. Unlike the covalently connected tecto (dendrimer) analogues, aggregates of electrostatically bound PAMAM dendrimers can self-assemble in a dendritic pattern, generating fascinating fractal structures. The kinetic studies of the aggregation in water by dynamic light scattering (DLS) experiments suggest that diffusion limited colloidal aggregation (DLCA) is the prevailing mechanism for the fractal growth between 3.5 generation carboxylate terminated PAMAM dendrimer and fourth generation amine terminated PAMAM dendrimer. The fractal aggregation observed in aqueous medium was further corroborated by scanning and transmission electron microscopic studies. Furthermore, unprecedented enhancement in the intrinsic emission intensity from PAMAM dendrimer was observed associated with the dendritic aggregation of the dendrimer units. Most importantly, the present study suggests that electrostatic self-assembly can be utilized as an effective synthetic strategy to generate highly stable, nano- to microscale higher order complex structures from PAMAM dendrimers, which significantly reduces the nonradiative pathways of the excitons, resulting in many fold enhancements in the intrinsic emission intensity from the system.