Langmuir–Schäfer films of a new calix[4]pyrrole-based macrocycle exhibiting induced chirality upon binding with chiral alcohol vapours

Abstract

A new calix[4]pyrrole-based macrocycle, meso-tetra(methyloctyl)calix[4]pyrrole (MTP), has been synthesized and fully characterized. Thin films of MTP have been transferred onto hydrophobized quartz and silicon oxide sheets by using the Langmuir–Schäfer (LS) method (horizontal lifting). Scanning force microscopy (SFM) investigation showed that the LS films are almost homogeneous and are consistent with the X-type configuration with the hydrophobic tails of MTP disposed almost perpendicularly with respect to the substrate. Induced circular dichroism (ICD) measurements have been performed in the presence of chiral alcohol vapours, such as (−)-(R)-2-butanol, (−)-(R)-2-pentanol and (−)-(R)-2-hexanol. The results showed that although not chiral by themselves the MTP molecules within the film exhibit chirality induced by binding with the chiral guests. The appearance of ICD signals is accounted for by the close proximity of the alcohol molecules to the pyrrole moieties of MTP. The reversibility of the binding event, coupled with the differentiated response observed towards the alcohols investigated, makes the LS films promising active layers for the chemical recognition of short-chain polar neutral molecules.