Permittivity-Dependent Entropy Driven Complexation Ability of Cone and Paco Tetranitro-calix[4]arene toward para-Substituted Phenols

Considering the importance of the polarizability of the rings of calixarenes in the entropy-driven interaction processes, we examined the effect of entropy compensation on the complex formation of cone and partial cone (paco) conformers of tetranitro-calix [4]arene, possessing O-ethyl substituents at the lower rim. Both calixarene conformers were fully characterized including X-ray crystallography. Various para-substituted phenols were used as guest molecules. Photoluminescence (PL) measurements and quantum-chemical (QC) investigations were used. A permittivity dependence of the molecular interactions was obtained in different alcohols as solvents. It was found that the cone conformer of the title calixarene derivative forms stable complexes with all phenols of the p-substituted series. The free enthalpy changes show very high complex stability of cone calixarene with p-nitro and p-chloro-substituted phenols. In the cases of parent phenol, p-cresol and p-tBu-phenol, the stability is significantly lower; however, it slightly increases with the increasing electron density on the aromatic ring of guest molecules. Similarly, the entropy changes are significantly different for these two separated groups: the entropy changes obtained in the former cases are nearly the same, while large differences in the formation entropy were obtained in the latter cases. Both the experimental and theoretical investigations revealed that no considerable interaction exists between phenols and the paco conformer of the title calixarene. It is probably due to the locking of the calixarene cavity by the bent O-ethyl chain.