Spectroscopic and theoretical insights into the origin of fullerene-calix[4]pyrrole interaction.

The present paper reports, for the first time, supramolecular interaction of meso-octamethyl calix[4]pyrrole (1) with fullerenes C(60) and C(70) in solutions having varying polarity (e.g., toluene, 1,2-dichlorobenzene and benzonitrile and chloroform). The interaction is facilitated through charge transfer (CT) transition as evidenced from well-defined CT absorption bands in the visible region of absorption spectroscopy. Utilizing the CT transition energy for the complexes of 1 with various electron acceptors, we have determined the ionization potential of 1. Estimation of degrees of CT, oscillator, and transition dipole strengths suggest that the complexes are almost of neutral character in ground state. Higher magnitude of electronic coupling element value for the C(70)-1 complex compared to C(60)-1 indicates strong binding between C(70) and 1. Binding constants (K) of the fullerene-1 complexes have been determined from UV-vis investigations, which indicate high selectivity of 1 toward C(70). Extraordinary large K value of the C(70)-1 complex in chloroform medium (K approximately 1.43 x 10(6) dm(3) x mol(-1)) establishes that a polar environment facilitates such interaction. Both proton NMR and liquid IR studies provide very good support in favor of strong binding between C(70) and 1. (13)C NMR study proves that C(70) binds 1 with its equatorial belt, which substantiates the role of pi-pi interaction behind such strong interaction (i.e., high K value). Semiempirical theoretical calculations at the third parametric level (PM3) explore the stability difference between C(60)- and C(70)-1 complexes. PM3 calculations also reveal that approach of C(70) toward 1 is directed in side-on manner rather than in a conventional end-on alignment.