Design, syntheses, and studies of supramolecular porphyrin-fullerene conjugates, using bis-18-crown-6 appended porphyrins and pyridine or alkyl ammonium functionalized fullerenes

Photoinduced electron-transfer processes in cis and trans functionalized bis-18-crown-6 porphyrin self-assembled with fullerene functionalized with pyridine or alkylammonium cation entities are reported. The structural integrity of the newly formed supramolecular conjugates was accomplished by optical absorption and emission, electron spray ionization mass, electrochemistry, and semiempirical PM3 calculations. A 1:2 stoichiometry of the supramolecular porphyrin:fullerene conjugates was deduced from these studies. The conjugates revealed stable "two-point"' binding involving metal-ligand coordination and alkylammonium cation-crown ether binding or only the latter type of binding depending upon the functionality of the fullerene and metal ion in the porphyrin cavity. The effect of the variation on free energy changes of charge separation and the charge recombination was achieved by varying the metal ion in the porphyrin cavity. The charge-separation rates (k_CS) determined from the picosecond time-resolved emission studies were generally higher for the cis biscrown functionalized porphyrins than those of the corresponding trans ones. A comparison of the k_CS values reported earlier for 1:1 porphyrin-fullerene conjugates with a similar self-assembly mechanism suggested , that employing a higher number of acceptor entities improves the electron-transfer rates. The calculated chargerecombination rates (k_CR) were 2-3 orders of magnitude smaller than the k_CS values, suggesting the occurrence of the charge recombination process in the Marcus inverted region. The lifetimes of the radical ion pair (TRIP) ranged between 46 and 233 ns indicating charge stabilization in the studied conjugates.