Absorption Spectrophotometric, NMR and Quantum Chemical Investigations of Ground State Non-Covalent Interactions between Fullerenes and a Designed Trihomocalix[6]arene in Solution

Abstract

Ground state non-covalent interactions between a newly designed macrocyclic 1,3,5-trihomo calix[6]arene receptor, designated as 1, and the C60 and C70 fullerenes have been studied in toluene solutions. It was observed that the absorbances of both C60 and C70 solutions increased upon the addition of increasing concentrations of compound 1. Job’s method of continuous variation established 1:1 stoichiometry for these fullerene-1 complexes. The binding constant (K) data reveal that compound 1 binds to C70 more strongly compared to C60, i.e., \(K_{C60\mbox{-}\boldsymbol{1}} = 230~\mathrm{dm}^{3}{\cdot}\mathrm{mol}^{-1}\) and \(K_{C70\mbox{-}\boldsymbol{1}}= 517~\mathrm{dm}^{3}{\cdot}\mathrm{mol}^{-1}\). Proton NMR analysis provides very good support for strong binding between C70 and 1. Estimations of the solvent reorganization energy (RS) suggest that the C70-1 complex is stabilized more than the corresponding C60-1 complex, with \(R_{S(C60\mbox{-}\boldsymbol{1})} = -1.970~\mathrm{eV}\) and \(R_{S(C70\mbox{-}\boldsymbol{1})}= -2.300~\mathrm{eV}\). Molecular mechanics force field method calculations established that the binding pattern of C70 towards 1 occurs in the side-on rather than end-on orientation, and that the C70-1 complex gains 5.23 kJ⋅mol−1 of extra stabilization energy with this side-on geometrical arrangement.