Mass-sensitive solvent vapor detection with calix[4]resorcinarenes: Tuning sensitivity and predicting sensor effects

Preorganized calix[4]resorcinarenes forming vaselike cavities are tested with respect to their chemical sensing capabilities as coatings on mass-sensitive devices. A synthetically modified vaselike host molecule with a definite cavity morphology forms densely packed layers for measurements with a SAW oscillator, In contrast to this detection of surface phenomena with ultrathin layers, QCM resonators require bulky microporous coatings with a high vapor permeability. Thus, short response times are attained even for layers using hulk effects, which are necessary to compensate the smaller sensitivity of the QCM. A 3-fold substitution of a basic calix[4]resorcinarene cavity with 2,3-dichloroquinoxaline provides a one-side-opened structure forming a molecular entrance for the easy access of analyte molecules. In combination with aliphatic spacers, this material fulfills the required demands of high sensitivity and short response times, and the detection of solvents in the gas phase to 2.5 ppm is realized. The sensor effects of the established coatings are correlated to host-guest stabilization enthalpies which were calculated by force field methods. An improved thermodynamic model that considers entropies of condensation is tested successfully for the prediction of the sensor behavior.