Spectroscopic and theoretical investigations on effective and selective complexation between fullerenes (C60 and C70) and calix[5]arene

Ground state interactions between a macrocyclic calixarene receptor, namely, 4-tert-butyl calix[5]arene (1) with fullerenes C60 and C70, were studied by absorption spectrophotometric method in toluene medium. Well defined charge transfer (CT) absorption bands were located for the complexes of C60 and C70 with 1. Utilizing the CT absorption bands, various important physico-chemical parameters like oscillator strength, resonance energy and electronic coupling elements were determined for fullerene-1 complexes. It was observed that 1 binds preferentially C70 compared to C60 as obtained from binding constant (K) data, i.e., K C 60 - 1 = 23600 dm3 mol−1 and K C 70 - 1 = 94460 dm3 mol−1. Proton NMR investigations also provided a very good support in favor of strong complexation between C70 and 1. PM3 calculations established that binding pattern of C70 towards 1 was end-on rather than side-on as C70/1 complex gained 2.43 kJ mol−1 of extra stabilization energy in case of its end-on orientation rather than traditional side-on approach.