Supramolecular sensing of short chain alcohols with mixed-bridged thio-phosphonate cavitands

The possibility to enhance the selectivity of phosphonate cavitands toward short-chains alcohols, namely methanol and ethanol, reducing the size of the cavity with the introduction of bulky thio-phosphonate bridging units at the upper rim is here presented. Three new mixed bridged thio-phosphonate cavitands 3POiii1PSi[H, CH3, Ph], AB-2POii2PSii[H, CH3, Ph] and AC-2POii2PSii[H, CH3, Ph] were designed, synthesized and tested as QCM sensors. The measurements show that the presence of two bulky H-bond silent PS bridges makes the resulting AB/AC-2POii2PSii[H, CH3, Ph] cavitands effective in discriminating methanol and ethanol with respect to their longer analogs. The QCM measurements also show that the substitution of a single PO bridge with a PS one, as in 3POiii1PSi[H, CH3, Ph], is not sufficient, as it reduces the responses without affecting the selectivity pattern. The crystal structures of the AB-2POii2PSii[H, CH3, Ph]·MeOH and the AC-2POii2PSii[H, CH3, Ph]·MeOH complexes show that the key factors affecting the sensing performances toward alcohols of these new class of mixed-bridged phosphonate cavitands are synergistic CH–π and H-bonding interactions, as in the parent tetraphosphonate cavitands.