A Rational Approach Towards Determination of Optical Ionicity and Non-covalent Interactions in Fullerene-Calix[4]arene Host-Guest Complexes

Abstract

The present article reports the host-guest complexation of a calix[4]arene derivative, namely 4-iso-propyl-calix[4]arene (1), with fullerenes (both C60 and C70) in toluene and benzonitrile solutions. It is observed that the charge-transfer (CT) absorption bands are located in the ground state for the C60 and C70 complexes of 1. By utilizing the CT absorption bands, various important physicochemical parameters like the oscillator strength, resonance energy, transition dipole moment, electronic coupling element and solvent reorganization energies have been estimated for the C60-1 and C70-1 complexes. The CT transition energy is very helpful for determining the vertical ionization potential of 1 in solution. Jobs method of continuous variation was used to establish 1:1 stoichiometry for the fullerene complexes of 1. The most fascinating feature of the present study is that 1 binds C70 preferentially compared to C60 as obtained from binding constant (K) data. The effect of solvent on the complexation of fullerenes (C60 and C70) with 1 is clearly observed from the trend in the K values: in toluene Open image in new window and Open image in new window whereas in benzonitrile, Open image in new window and Open image in new window . Molecular mechanics force field (MMMF) calculations reveal fascinating features regarding the binding pattern of fullerenes towards 1 in vacuo in terms of enthalpy of formation. MMMF calculations establish that during C70-1 complexation, C70 is directed in an end-on manner rather than the traditional side-on pattern.