Self-assembled monolayers of cavitand receptors for the binding of neutral molecules in water

Resorcin[4]arene cavitands 2-4 were synthesized starting from tetrol 5. With these cavitand tetrasulfides, self-assembled monolayers (SAMs) on gold were prepared and characterized by electrochemical capacitance and resistance measurements, contact angle (CA) measurements, grazing angle FT-IR spectroscopy, and X-ray photoelectron spectroscopy (XPS). In situ surface plasmon resonance (SPR) spectroscopy was employed to study the host-guest interactions at the monolayer-water interface, between receptor adsorbates 1-4 and p-toluic acid, benzoic acid, p-nitrobenzoic acid, p-hydroxybenzoic acid, p-cresol, p-nitrophenol, and p-methoxyphenol. SPR results show that the presence of functional groups on the upper rim of the adsorbate molecules induces selectivity in the guest recognition process. Furthermore, it is interesting to note that such interactions are not directly related to the degree of hydrophobicity of the receptor adsorbate or of the guest. Concentration-dependent experiments were performed with p-nitrophenol as guest for a monolayer of adsorbate 1. At a 3 mM concentration of p-nitrophenol, approximately five to eight guest molecules associate with each adsorbate molecule, implying the formation of 1-4 layers of guests on the receptor surface.