Assembly of a Supramolecular Capsule on a Molecular Printboard

Abstract

A molecular capsule based on ionic interactions between two oppositely charged calix[4]arenes, 1 and 2, was assembled both in solution and on a surface. In solution, the formation of the equimolar assembly 1·2 was studied by ¹H NMR, ESI-MS, and isothermal titration calorimetry, giving an association constant (K_a) of 7.5 × 10⁵ M^-1. A β-cyclodextrin self-assembled monolayer (β-CD SAM) on gold was used as a molecular printboard to anchor the tetraguanidinium calix[4]arene (2). The binding of tetrasulfonate calix[4]arene 1 was monitored by surface plasmon resonance spectroscopy. Rinsing of the surface with a high ionic strength aqueous solution allows the removal of the tetrasulfonate calix[4]arene (1), while by rinsing with 2-propanol it is possible to achieve the complete desorption of the tetraguanidinium calix[4]arene (2) from the β-CD SAM. The K_a for the capsule formation on a surface is 3.5 × 10⁶ M^-1, thus comparing well with the K_a determined in solution.