PHOTOINDUCED ELECTRON-TRANSFER IN SUPRAMOLECULAR FULLERENE FERROCENE SYSTEMS/

In this paper we summarize studies regarding the deactivation of photo excited fullerenes via inter- and intramolecular quenching reactions w ith ferrocene as electron donor. Intermolecular reductive quenching of excited tripler fullerenes in heterogeneous media (gamma-cyclodextrin and surfactant solutions) gave rise to long-lived charge separated st ates. Intramolecular processes were followed after irradiation of full erene derivatives covalently linked to ferrocene through flexible spac ers or rigid unsaturated bridges. Steady-state fluorescence and time-r esolved flash photolysis of these fullerene-ferrocene based dyads have shown that, in all cases, electron transfer evolves from photoinduced bleaching of the fullerene excited singlet state by the ferrocenyl mo iety. However, the nature of the spacer between C-60 and ferrocene sug gests two different quenching mechanisms: through bond electron transf er for rigid spaced dyad 2, formation of a transient intramolecular ex cited stare complex (exciplex) for flexible spaced dyad 1. While in dy ad 2 fast charge recombination prevents enough stabilization, the satu rated hydrocarbon bridge in dyad 1 stabilizes a long-lived charge-sepa rated state in benzonitrile (tau(1/2) = 2.5 mu s). Steady-state and ti me-resolved photolytic experiments on a ferrocene-peptide/fulleropyrro lidine host-guest complex were carried out. In the proposed complex, i n which the peptide adopts in CHCl3 solution a helical conformation; t he two ferrocene groups react mainly with the excited singlet state of the electron accepting. fulleropyrrolidine guest. On the other hand, in a 1:1 CHCl3/hexafluoroisopropanol mixture weaker interactions of th e ferrocene with the fullerene core lead predominantly to quenching of the longer-lived excited triplet state. This suggests substantial cha nges of the ability of the ferrocene-peptide to complex the C-60 deriv ative.