Kinetics of Supramolecular Aggregation Investigated With Magnetic Fields

Self-assembly of organic molecules offers an attractive approach for the development of potentially functional nanostructures, using relatively simple molecular building blocks. In contrast to the numerous investigations of the structural properties of supramolecular aggregates, only a small number of studies focus on the time scale of self-assembly, disassembly and aggregate morphology transitions. The kinetics can give valuable information on the aggregation-route and the intermolecular interactions. In a collaboration between the Institute for Molecules and Materials (High Field Magnet Laboratory, Nijmegen), the Division of Molecular and Nanomaterials and Institute of Nanoscale Physics and Chemistry (Leuven), the Institute für Technische Chemie und Polymerchemie (Karslruhe) and the Kekulé-Institut für Organische Chemie und Biochemie (Bonn), we have investigated the kinetics of the temperature induced aggregation of macrocyclic molecules in solution using magnetic field induced birefringence. This technique is sensitive to the degree of molecular order of an entire aggregate, rather than the short range order probed by techniques like UV-Vis and circular dichroism spectroscopy. Therefore, we can distinguish ordered and disordered macrocycle aggregates in a quantitative manner.

We have found three consecutive stages in the aggregation process of the macrocycle system, all with their own typical time scale: disordered objects, ordered fibers and a network respectively. Right after cooling a solution to a temperature Tf, large disordered objects are formed. The transition of these objects to ordered fibers takes days or weeks, and the formation rate increases with decreasing Tf. Such a slow formation is unusual for individual conjugated aggregates, with typical time scales of a few seconds to several hours. We attribute this exceptionally slow behavior to the polystyrene tails of the macrocycles that hamper the aggregation of the rigid core. For low temperatures, the fibers form a network after several days.

Linking aggregation kinetics to molecular properties, will lead to a better understanding of the mechanisms by which molecules self-assemble, allowing for a more rational design of the organic molecular building blocks.


Figure: (a) Chemical structure of the macrocycle molecule (n~25). (b) Solution birefringence at B=20 T as a function of time for different temperatures Tf. Inset shows a full birefringence curve. (c) AFM image of a 10-6 M solution dropcast on mica showing the fibers. (d) Schematic representation of the situation at Tf=14 oC showing the transition from disordered objects (I) to ordered fibers (II) to a large network of fibers (for low Tf) (III).

This work was published in:

Jeroen C. Gielen, An Ver Heyen, Svetlana Klyatskaya, Willem Vanderlinden, Sigurd Hoger, J.C. Maan, Steven De Feyter and Peter C.M. Christianen
Aggregation Kineticsof Macrocycles Detected by Magnetic Birefringence
Journal of the American Chemical Society 131 (40), 14134-14135 (2009)