Spectroscopic and Theoretical Investigations on Effective and Selective Interaction of Fullerenes C60 and C70 with a Derivatized Zn-phthalocyanine: Stabilization of Charge-Recombined State by Side-On Approach of C70

The photophysical aspects of noncovalently linked fullerenes C-60 and C-70 with a designed metallophthalocyanine, namely, zinc-2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (1) have been investigated employing various spectroscopic tools such as UV-vis absorption spectrophotometry, steady state and time-resolved fluorescence, along with solution state IR measurements in toluene medium. The ground state interaction between fullerenes and 1 is first evidenced from UV-vis measurements. Binding constants (K) for the complexes of C-60 and C-70 with 1 are determined to be 13 235 and 27 670 dm(3).mol(-1), respectively. Steady state fluorescence experiment reveals efficient quenching of the excited singlet state of 1 in presence of both C-60 and C-70. Emission studies reveal K values of 8945 and 19 175 dm(3).mol(-1) for C-60/1 and C-70/1 complexes, respectively. The magnitude of K suggests that 1 preferentially binds C-70 in comparison to C-60 as average selectivity in K of C-70 over C-60 is enumerated to be 2.0. Time resolved emission measurements establish that C-70/1 complex is stabilized much more in comparison to C-60/1 complex in terms of charge recombination. Liquid IR studies provide very good support in favor of ground state complexation between fullerenes and 1. Molecular mechanics calculations at force field model and semiempirical calculations employing third parametric method substantiate the strong binding between C-70 and 1, and at the same time, determine the orientation of bound guest (here C-70) within the cavity of 1.