Inclusion behavior of water-soluble thiacalix- and calix[4]arenes towards substituted benzenes in aqueous solution.

Inclusion abilities of thiacalix- and calix[4]arenetetrasulfonate (3 and 4) towards mono-substituted benzenes were investigated in neutral aqueous solution. In general, the hosts regioselectively encapsulated the guests from the aromatic moiety except the complexation of toluene by 4, in which the guest penetrated from either the aromatic or the methyl group. Stabilities of the inclusion complexes increased with the electron-withdrawing ability of the substituent on the guest, suggesting pi-pi electronic interaction between the host and guest. In spite of the lower electron density of the aromatic ring, thiacalix[4]arene 3 showed higher inclusion ability than calix[4]arene 4, suggesting that the size rather than the electron density of the calix framework is a more important factor in determining the inclusion ability.