1,8-Oxybis(ethyleneoxyethyleneoxy)anthracene-9,10-dione with a reduced cavity size, due to the presence of a carbonyl oxygen within the crown, has been shown by X-ray crystallography to encapsulate a hydronium ion. Hydrogen bonds between the hydronium ion and the crown oxygens are particularly strong, averaging 2.56 Å. The polyether ring also exhibits a pronounced folding to optimize encapsulation of the hydronium ion. The hydronium hydrogen atoms have been unequivocally observed, indicating near ideal pyramidal geometry, due to the absence of hydrogen-bonding disorders previously observed in other [H3O·18-crown-6]+ structure analyses. Intense luminescence is concurrently observed with complexation of the hydronium ion within the polyether ring. Common oxoacids acids, such as HNO3, H2SO4, HClO4, and CF3SO3H, result in luminescent intensities approximately 40 times larger than that of 1,8-oxybis(ethyleneoxyethyleneoxy)anthracene-9,10- dione alone, which also correlates with the pK(a)s of these acids. This capability serves a unique pK(a) regimen as a possible sensor molecule for measuring acidities of potent oxo and superacids.