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

Abstract

The photophysical aspects of noncovalently linked fullerenes C₆₀ and C₇₀ 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 C₆₀ and C₇₀ with 1 are determined to be 13 235 and 27 670 dm³·mol^-1, respectively. Steady state fluorescence experiment reveals efficient quenching of the excited singlet state of 1 in presence of both C₆₀ and C₇0. Emission studies reveal K values of 8945 and 19 175 dm³·mol^-1 for C₆₀/1 and C₇₀/1 complexes, respectively. The magnitude of K suggests that 1 preferentially binds C₇₀ in comparison to C₆₀ as average selectivity in K of C₇₀ over C₆₀ is enumerated to be 2.0. Time resolved emission measurements establish that C₇₀/1 complex is stabilized much more in comparison to C₆₀/1 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 C₇₀ and 1, and at the same time, determine the orientation of bound guest (here C₇₀) within the cavity of 1.