Fluorescence-based investigations of the host–guest inclusion of anilinonaphthalene sulfonic acids (1,8- and 2,6-ANS) by dimethoxypillar[5]arene in nonaqueous solvents

Abstract

Pillar[n]arenes are a relatively new class of macrocyclic host molecules which have not been extensively studied for their ability to form inclusion complexes. The properties and host–guest inclusion complexes of dimethoxypillar[5]arene (DMPill[5]) hosts with the polarity-sensitive fluorescent probes 8-anilinonaphthalene-1-sulfonic acid (1,8-ANS) and 2-anilinonaphthalene-6-sulfonic acid (2,6-ANS) as guests were investigated in nonaqueous solvents via fluorescence spectroscopy. The binding properties of DMPill[5] were found to depend significantly on the shape of the guest molecule, as well as on the properties of the solvent. Formation of host–guest inclusion complexes of DMPill[5] with these two fluorescent ANS guests occurred in the polar aprotic solvents acetonitrile, acetone, and tetrahydrofuran. Fluorescence titration experiments were performed to determine the nature and strength of the complexation. In the case of 1,8-ANS, 1:1 host–guest complexation was observed in all three solvents, with average binding constant K values of 408, 1000 and 5500 M−1 in acetone, acetonitrile, and tetrahydrofuran, respectively. This large dependence of the binding constant on solvent provides insight into the nature of the binding in these complexes. In the case of 2,6-ANS, 2:1 host–guest complexation was observed; this difference with 1,8-ANS was explained in terms of the shape and size of these two isomeric guests. These results show that DMPill[5] is an excellent host for these neutral aromatic guests, and shows strong binding abilities even in these nonaqueous solvents. The strength (or lack) of binding in different solvents was found to be dependent on a range of factors beyond solvent polarity, including guest and solvent shape and size, and most importantly, specific solvent–solute interactions.