Photophysical Investigations on Determination of Ionicity and Electronic Structures for the Non-covalent Complexes of Calix[4]resorcinarene with Fullerenes C60 and C70 in the Solution State

Ground state non-covalent interactions between a macrocyclic receptor, C-methylcalix[4]resorcinarene (1) and fullerenes (C₆₀ and C₇₀) have been studied in benzonitrile by an absorption spectrophotometric method. Absorption bands are located in the visible region due to the charge transfer (CT) transition between 1 and various electron acceptors (including fullerenes), namely, 2,3-dichloro-5,6-dicyano-p-benzoquinone, tetracyanoquinodimethane and p-chloranil. Utilizing the CT absorption bands, various important physicochemical parameters, including oscillator strength, resonance energy, transition dipole strength of all the acceptor-1 complexes and vertical ionization potential of 1 are determined. Job’s method of continuous variation reveals 1:1 stoichiometry between fullerenes and 1. The most fascinating feature of the present study is that 1 binds selectively to C₇₀ compared to C₆₀ as obtained from binding constant (K) data of C₆₀-1 (K_C60-1K_{mathrm{C}60mbox{-}mathbf{1}}) and C₇₀-1 (K_C70-1K_{mathrm{C}70mbox{-}mathbf{1}}) complexes, i.e., K_C60-1=190K_{mathrm{C}60mbox{-}mathbf{1}}=190 dm³⋅mol⁻¹ and K_C70-1=5,800K_{mathrm{C}70mbox{-}mathbf{1}}=5{,}800 dm³⋅mol⁻¹ and selectivity (K_C70-1 /K_C60-1 ) ∼30. Quantum chemical calculations based on hybrid density functional theory estimate the enthalpies of formation of the fullerene-1 complexes in vacuo and provide very good support for selectivity in the K values of the C₇₀ and C₆₀ complexes of 1. The exchange and correlation energies have been calculated using a hybrid DFT functional method. We have opted to use the hybrid DFT functional over the Hartree-Fock method, as it can account for correlation effects also. Molecular electrostatic potential map calculations give a clear picture on the electronic structures of the fullerene-1 complexes.