Molecular binding behavior of water-soluble calix[4]arenes with asymmetric 4,4'-bipyridinium guests in aqueous solution: regioselective recognition or not?

The molecular binding behavior of water-soluble calix[4]arenes (p-sulfonatocalix[4]arene (SC4A) and p-sulfonatothiacalix[4]arene (STC4A)) with two asymmetric 4,4'-bipyridinium guests (N-methyl-N'-adamantane carbomethyl-4,4'-bipyridinium dibromide (MVAd) and N-methyl-N'-(naphthalen-2-ylmethyl)-4,4'-bipyridinium bromide iodide (MVNp)) was systematically studied using NMR spectroscopy and microcalorimetry in a neutral aqueous solution. Either the methyl group or the adamantane moiety in MVAd could enter into the SC4A and STC4A cavities, without regioselectivity. The STC4A cavity can also accommodate MVNp, either through incorporation of the methyl group or the naphthalene moiety, without regioselectivity. However, we were surprised to find that MVNp could only be included within the SC4A cavity through incorporation of the methyl group, with regioselectivity, which is rare for a flexible host. Furthermore, both SC4A and STC4A can form stable inclusion complexes with the two investigated asymmetric 4,4'-bipyridinium guests, driven by very favorable enthalpy changes, and the thermodynamic origins of the host selectivities for MVAd and MVNp can be well explained through their binding modes. The finding of this novel regioselective recognition is promising for potential applications in the development of more sophisticated biomimetic materials.