The reaction of tetra-p-formyltetra-O-propylcalix[4]arene with phenanthrenequinone in the presence of NH4OAc affords compound 2, a new class of calixarene with an expanded aromatic cavity, that could be stabilized by hydrogen-bonded bridges and/or ion pairing, thus preventing collapse into fully stacked pinched cone conformations as depicted. Two partially protonated calixarenes interdigitate in the solid state to give rise to a self-assembled face-to-face dimer, stabilized by pi-pi stacking interactions.
![p-(1H-Phenanthro[9,10-d]imidazol-2-yl)-substituted calix[4]arene, a deep cavity for guest inclusion](https://www.iciq.org/wp-content/uploads/2014/03/p-1H-Phenanthro910-dimidazol-2-yl-substituted-calix4arene-a-deep-cavity-for-guest-inclusion_eng_noticies_articles_2004_16.gif)