Spectral and theoretical studies on effective and selective non-covalent interaction between tetrahexylporphyrins and fullerenes

The host–guest interaction of zinc(II) 5,10,15,20-tetrahexylporphyrin (Zn-THP) and its free base (H2-THP) with fullerenes (C60 and C70) has been studied in toluene medium. Binding constants (K) for H2- and Zn-THP complexes of fullerenes were determined by UV–vis, fluorescence and NMR spectroscopic techniques. Large K values of C70/THP complexes (KC70KC70) were obtained in the range of 1.4–2.5 × 104 M−1, while those of C60/THP complexes (KC60KC60) were smaller (1.0–3.2 × 103 M−1). These results show that the KC70KC70 is about 10 times as large as KC60KC60 in both THPs (KC70KC70/KC60KC60 = 10). Enthalpies of formation (ΔHf°) for various fullerene/THP complexes were estimated by ab initio   calculations; ΔHf° for C60/H2-THP, C70/H2-THP, C60/Zn-THP and C70/Zn-THP complexes are 5.82, 2.80, 2.31 and 1.54 kcal mol−1, respectively. The trends in ΔHf° support the experimental results of selective complexation of THPs towards C70 over C60 and fullerenes towards Zn-THP over H2-THP.