Formation of Solvent-Separated Ion Pairs in Calixarene Ester–Alkali Picrate Complexes

Association constants (Ka) of calix[n]arene esters (1n: n=4, 6, 8) for alkali picrates have been determined in tetrahydrofuran and compared with the 1n-induced bathochromic shifts of the absorption maxima of alkali picrates. It has been established that (1) 1n’s mostly form the 1 : 1 complexes with alkali metal cations, (2) the Ka values for 1n are generally smaller than those for crown ethers, and (3) the certain M+–1n complexes are considerably solvent-separated because of the “encapsulation effect” of the calixarene cavity. This is the first example for the systematic investigation on the relation between Ka and the geometry of the calixarene ester complexes.