Peter C.M. Christianen, High Field Magnet Laboratory, University of Nijmegen, Toernooiveld 7, 6525 ED Nijmegen.
Supramolecular chemistry provides a means to construct well-defined nanoscale objects, such as molecular fibers, vesicles and helices, out of simple molecular building blocks, often inspired by natural systems. The formation of such entities makes use of the weak, non-covalent, molecule-specific interactions between organic molecules, like electrostatic interactions, hydrogen bonding and aromatic p-stacking. The physical properties of supramolecular assemblies depend not only on the type of molecule used as building block, but also on the internal molecular arrangement. At present, our main challenge is to design and synthesize novel materials, which self-organize in identical, perfectly ordered nanostructures, with tailor-made properties, that can be used in future devices.
In this talk I will discuss our efforts to fabricate, characterize and unravel the physical properties of molecular nano-assemblies, featuring a large variety of different building blocks, such as porphyrins, cyanines, thiophenes and phenylenevinylenes. We explore two main strategies:
Magnetic field alignment of molecular aggregates in solution
In general supramolecular architectures are formed in solution and subsequently transferred onto a substrate. It would therefore be very useful to already determine the molecular arrangement of aggregates in solution. The Brownian motion of the assemblies, however, hampers easy determination of almost any property, since it is averaged over many aggregates that are randomly oriented in space. I will show that this problem can be solved by magnetically aligning the supramolecular assemblies, enabling the determination of their internal molecular arrangement by measuring their polarized absorbance spectra.
spectroscopy on individual supramolecular assemblies
As an alternative approach we determine the properties of individual supramolecular assemblies deposited on a substrate by using atomic force microscopy (AFM), polarized fluorescence microscopy and scanning near-field optical microscopy (SNOM). I will demonstrate the formation of ring-shaped assemblies of porphyrin hexa- and dodecamers and molecular fibers consisting of oligo(p-phenylenevinylene molecules. The nearly perfect internal ordering is evidenced by the polarized fluorescence emission of single assemblies, which are therefore well suited to study the rate of energy transfer throughout the structure.