Molecular Aggregate Structure Magnetically Unraveled

Chemical self-assembly of organic molecules into larger aggregates is a promising approach for constructing potentially functional nanostructures. The large variety of molecular building blocks permits the fabrication of supramolecular nanostructures with a large variation in composition, shape and functionality. In many cases, however, the precise internal molecular arrangement of the nanostructures is unknown, which hampers the development of a deeper understanding of the structure-property relationship, required to steer the self-assembly process towards molecular nanostructures with desired functionalities. Therefore, it is important to measure the molecular arrangement of aggregates, preferably in the solution in which they are formed.

In a collaboration between researchers from the Institute for Molecules and Materials (High Field Magnet Laboratory, Nijmegen), the Laboratory of Macromolecular and Organic Chemistry (Eindhoven), the DUBBLE group at the ESRF (Grenoble) and the Department of Chemistry of the University of Durham (UK), the virtues of Small Angle Xray Scattering (SAXS) and magnetic field alignment were combined to determine the molecular structure of sexithiophene self-assemblies in a solution of o-dichlorobenzene (ODCB) and n-butanol. This oligothiophene molecule is interesting as an organic semiconductor material, and a detailed understanding of the ordering is needed to develop protocols for the formation of well-defined supramolecular structures for use in future devices, such as field-effect-transistors.

To unravel the directionality of the ordering we performed SAXS and birefringence measurements on magnetically aligned sexithiophene aggregates. We found that the T6 molecules pack in a cylindrically shaped aggregate with the long molecular axis pointing in the radial direction and a π-π stacking in the tangential direction. Comparing this result with previous work demonstrates that the aggregate morphology of this semiconducting material can be tuned by using different solvents. A better understanding of such a versatile process is an important step towards the use of organic molecules as nano-sized building blocks in future devices, especially in combination with the alignment in a magnetic field.

Figure: (a) SAXS results of unaligned (left) and aligned (right) T6 aggregates. The ring, which contracts to two small spots in a magnetic field, indicates a periodicity of 4.8 nm (shown by the cross section). (b) Magnetic birefringence as a function of magnetic field for different temperatures. The arrows indicate the point where the birefringence is at 80% of the saturation value. Solid lines are fits to the data to obtain the aggregate size (expressed in terms of number of molecules per aggregate) shown in the inset. The line in the inset is a guide to the eye. (c) The determined aggregate structure with the alignment behavior in a magnetic field.

This work was published in:

Jeroen C. Gielen, Martin Wolffs, Giuseppe Portale, Wim Bras, Oliver Henze, Andreas F.M. Kilbinger, W. James Feast, J.C. Maan, Albertus P.H.J. Schenning, and Peter C.M. Christianen,
Molecular Organization of Cylindrical Sexithiophene Aggregates measured by X-ray Scattering and Magnetic Alignment
Langmuir 25(3), 1272-1276 (2009)