Anharmonic magnetic deformation of self-assembled molecular nanocapsules

TL;DR

This study investigates how high magnetic fields deform self-assembled molecular nanocapsules, confirming theoretical predictions at low fields and proposing a new model to explain high-field behavior.

Contribution

The paper experimentally confirms the quadratic scaling of deformation with magnetic field and capsule size, and introduces a modified free energy model for high-field behavior.

Findings

Deformation scales quadratically with magnetic field and capsule radius at low fields.

Bending rigidity of capsules determined as (2.6±0.8)×10^{-21} J.

High-field behavior indicates increased rigidity not explained by Helfrich model.

Abstract

High magnetic fields were used to deform spherical nanocapsules, self-assembled from bola-amphiphilic sexithiophene molecules. At low fields the deformation -- measured through linear birefringence -- scales quadratically with the capsule radius and with the magnetic field strength. These data confirm a long standing theoretical prediction (W. Helfrich, Phys. Lett. {\bf 43A}, 409 (1973)), and permits the determination of the bending rigidity of the capsules as (2.6$\pm$0.8)$\times 10^{-21}$ J. At high fields, an enhanced rigidity is found which cannot be explained within the Helfrich model. We propose a complete form of the free energy functional that accounts for this behaviour, and allows discussion of the formation and stability of nanocapsules in solution.