Adaptive crystals of homothiacalix[4]arene capable of molecular recognition, with preferential uptake of benzene over cyclohexane

Abstract

Unique responsive and selective behaviour of homothiacalix[4]arene crystals is reported. A benzene disolvate adopting the 1,3-alternate conformation (HTCA-s_1,3) can be converted to a guest-free non-porous phase (HTCA-a_1,3) upon heating, with the host molecules retaining the same 1,3-alternate conformation. The process is reversible, with non-porous apohost crystals ‘opening up’ upon exposure to benzene vapour to reaccommodate the solvent molecules, as confirmed by SCXRD and sorption studies monitored in situ via PXRD analysis. The gate opening mechanism is selective, preventing cyclohexane molecules from passing through, which creates the opportunity to apply this system, for example, in benzene capture/separation. Furthermore, with a higher energy boost, the crystal of the HTCA-a_1,3 apohost undergoes a significant conformational change, resulting in HTCA-a_1,2, an apohost in which the host molecules adopt the 1,2-alternate conformation.