Anharmonic Magnetic Deformation of Molecular Nanocapsules

The self-assembly of small molecular building blocks into larger nanostructures offers unique possibilities to make nano-sized objects with tailor-made chemical and physical properties. It is however extremely difficult to determine those properties at the nano-scale, in particular when the objects are floating in a solution. In a joint effort of researchers from the Institute for Molecules and Materials (High Field Magnet Laboratory and Condensed Matter Theory), the Laboratory of Macromolecular and Organic Chemistry (Eindhoven) and the Department of Chemistry of the University of Durham (UK), a new method was developed for determining the elasticity of supramolecular self-assemblies. The concept relies on the deformation of molecular nanocapsules in a strong magnetic field. The magnetic deformation is counteracted by the intermolecular forces that are responsible for the strength of the capsules. By measuring the deformation, via the optical anisotropy, it is possible to determine the elasticity of these novel nanocapsules. These findings were published in the journal Physical Review Letters.

The determination of the elasticity of capsules through magnetic deformation was proposed 30 years ago, but it was never demonstrated experimentally. We have used sexithiophene molecules (Fig. a) that form hollow capsules(Fig. b) in 2-propanol, with diameters of about 100 nm. Due to their strongly anisotropic diamagnetic susceptibility sexithiophene molecules tend to align along the magnetic field direction. A sufficiently strong magnetic field, therefore, deforms capsules to oblate spheroids, when more molecules are parallel (top and bottom of the capsule), than perpendicular (around the equator of the capsule) to the field (Fig. c). The deformation at a certain field is given by the balance of the magnetic and restoring elastic forces. By applying magnetic fields up to 20 T and measuring the shape of the deformed capsules, via linear birefringence (solid lines, Fig. d), we can relate the exerted magnetic force to the actual deformation. Using a newly developed theoretical model that calculates the birefringence (dashed lines Fig. d) from the free energy of the molecular membrane, these results measure the strength (elasticity) of the capsules. We find that for small fields the deformation scales quadratically with both the field and the capsule radius. Furthermore, our measurements also give access to the regime of large, i.e. anharmonic, deformations at high magnetic fields, where the capsules are flattened considerably (Fig. e).

Magnetic deformation of nanocapsules is thus a very elegant way to quantify the elastic constant of nanocapsules, resulting from the intermolecular interactions in the molecular membrane. Such a quantitative description of non-covalent interactions within molecular assemblies is an important step towards the detailed understanding of the rules of self-assembly.

Figure: Sexithiophene (6T) molecules (a) form hollow capsules in 2-propanol (b). To maximize the number of molecules along the magnetic field B, the capsules deform to oblate spheroids (c). (d) Measured (solid lines) and calculated (dashed lines) magnetic birefringence of 6T nanocapsules for various temperatures. (e) Calculated shapes at 20 T

This work was published in:

O.V. Manyuhina, I.O. Sklyarevskiy, P. Jonkheijm, P.C.M. Christianen, A. Fasolino, M.I. Katsnelson, A.P.H.J Schenning, E.W. Meijer, O. Henze, A.F.M. Kilbinger, W.J. Feast and J.C. Maan,
Anharmonic magnetic deformation of self-assembled molecular nanocapsules
Physical Review Letters 98, 146101 (2007)