UV-Vis and NMR spectroscopic investigations on effective and selective non-covalent interactions between fullerenes and calix[6]arene

Ground state non-covalent interactions between a macro cyclic calixarene receptor, namely, 4-tert-butylcalix[6]arene (1), and fullerenes (C60 and C70) were studied in toluene medium by absorption spectrophotometric method. Absorption band due to the charge transfer (CT) transition have been observed in the visible region between fullerenes and 1. Utilizing the CT absorption bands, various important physicochemical parameters like oscillator strength, resonance energy, transition dipole strength of the fullerene-1 complexes and ionization potential of 1 is determined in present investigations. From Jobs method of continuous variation, it is observed that both C60 and C70 form stable 1:1 complexes with 1. The most fascinating feature of the present study is that 1 binds selectively C70 compared to C60 as obtained from binding constant (K) data of C60-1 (KC60-1) and C70-1 (KC70-1) complexes, i.e., KC60-1 = 32,400 dm3·mol− 1 and KC70-1 = 110,000 dm3.mol− 1 and selectivity (KC70-1/KC60-1) = 3.4. 1H NMR analysis provides very good support in favor of strong binding between C70 and 1.