Self-assembled supramolecular ferrocene-fullerene dyads and triad: Formation and photoinduced electron transfer

Supramolecular ferrocene-fullerene constructs in which the donor, ferrocene linked to a benzo-18-crown-6 entity (Fc-crown), was self-assembled with the acceptor, fullerene bearing one or two alkyl ammonium ions (NH3+-C-60), yielding dyads or a triad, respectively. The newly formed conjugates were characterized by spectroscopic (fluorescence, electospray ionization-mass, and H-1 NMR) and electrochemical methods. The adopted crown ether-alkyl ammonium ion binding strategy resulting in stable donor-acceptor conjugates was also supported by the computational studies performed at the DFT B3LYP/3-21G(*) level in addition to the binding, constants obtained from fluorescence quenching studies. The experimentally calculated free-energy changes indicated exothermic light-induced charge-separation process. Accordingly, efficient photoinduced charge-separation processes were confirmed by the combination of the time-resolved fluorescence and nanosecond transient absorption spectral measurements. The rates of charge recombination were found to be 2-3 orders of magnitude lower, yielding radical ion-pairs, Fc(+)-crown/NH3+-C-60(center dot-) with lifetimes in the 10-240 ns range. Generally, by increasing the donor-acceptor distance, a decrease in both k(CS) and k(CR) was observed for the supramolecular ferrocene-fullerene dyads; that is, the lifetimes of Fc(+)-crown/NH3+-C-60(center dot-) changed from 10 to 165 ns. However, for the triad, involving two ferrocene donors of varying donor-acceptor distances, the k(CR) originating from the far-side located ferrocene was found to be 240 ns while the k(CR) from the near-side located ferrocene was faster than the time duration of the nanosecond laser pulse (6 ns).