Photophysical and Theoretical Investigations on Fullerene/Phthalocyanine Supramolecular Complexes

The present paper reports the photophysical aspects of a very interesting and unique host-guest interaction between fullerene and phthalocyanines, viz., free base phthalocyanine (H-2-Pc) and zinc-phthalocyanine (Zn-Pc), in toluene medium. Ground state electronic interaction between these two supramolecules has been evidenced from the observation of well-defined charge transfer (CT) absorption bands in the visible region. Vertical ionization potentials of the phthalocyanines have been determined utilizing CT transition energy. Magnitude of degrees of CT reveals that, in the ground state, 2-4% CT takes place. Binding constants (K) for the fullerene/phthalocyanine complexes were determined from the fluorescence quenching experiment. Large K values in the ranges similar to 4.7 x 10(4) to 7.3 x 10(4) and 2.3 x 10(4) to 2.5 x 10(4) dm(3)center dot mol(-1) were obtained for the 1: 1 fullerene complexes of Zn and H-2-Pc, respectively. Values of K suggest that both H-2- and Zn-Pc could not serve as an efficient discriminators between C-60 and C-70. Theoretical calculations as well as C-13 NMR studies establish that the orientation of C-70 toward phthalocyanine is favored in end-on orientation, which proves that interaction between fullerenes and phthalocyanines were governed by the electrostatic mechanism rather than dispersive forces associated with pi-pi interaction.