Determination of binding strength for the supramolecular complexation of a designed bisporphyrin with C60, C70 and their derivatives employing absorption spectrophotometric, fluorescence and quantum chemical calculations.

The present paper reports the synthesis of a designed bisporphyrin (1), and its supramolecular complexes with C60, C70 and their derivatives, namely, tert-butyl-(1,2-methanofullerene)-61-carboxylate (2) and [6,6]-phenyl C70 butyric acid methyl ester (3) in toluene medium. C60, C70 and their derivatives undergo ground state non-covalent interaction with 1 is evidenced from absorption spectrophotometric study in which it is observed that the intensity of the Soret absorption band of 1 decreases considerably in presence of C60, C70 and their derivatives. Steady state fluorescence studies reveal efficient quenching of fluorescence of 1 in presence of fullerenes. The binding constant (K) values of the fullerene/1 complexes follows the trend: 2/1<C(60)/1<3/1<C(70)/1. However, selectivity in K values of the bisporphyrin complexes is found to be higher for fullerene derivatives in comparison to C60 and C70. Time resolved emission studies establish relatively long-lived charge separated state for the C(70)/1 complex. Molecular mechanics calculations at force field model in vacuo evoke the single projection geometric structures of various fullerene/1 complexes and interpret their stability differences in terms of heat of formation values.