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+−C60), yielding dyads or a triad, respectively. The newly formed conjugates were characterized by spectroscopic (fluorescence, electospray ionization-mass, and 1H 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+-C60•- with lifetimes in the 10−240 ns range. Generally, by increasing the donor−acceptor distance, a decrease in both kCS and kCR was observed for the supramolecular ferrocene−fullerene dyads; that is, the lifetimes of Fc+-crown/NH3+-C60•- changed from 10 to 165 ns. However, for the triad, involving two ferrocene donors of varying donor−acceptor distances, the kCR originating from the far-side located ferrocene was found to be 240 ns while the kCR from the near-side located ferrocene was faster than the time duration of the nanosecond laser pulse (6 ns).