Rhenium(i) and ruthenium(ii) complexes with a crown-linked methanofullerene ligand: synthesis, electrochemistry and photophysical characterization

Abstract

A cyclopropanation reaction has been used to prepare two methanofullerenes bearing a 2,2′-bipyridine (8) or pyridine (11) ligand separated from the fullerene through an oxyethylene macrocyclic spacer. Derivatives 8 and 11 were, in turn, employed to synthesize two fullerene-based ruthenium(II) and rhenium(I) donor–acceptor dyads whose molecular structure was confirmed by ¹H NMR, ¹³C NMR and exact mass determination. The UV-Vis spectrum of the dyads is the superimposition of those of appropriate model systems, indicating that ground-state electronic interactions between the constituent chromophores, in solution, are negligible, in line also with the electrochemical results. The complex voltammetric pattern was characterized by the superimposition of signals attributed to one moiety or another without significant shifts with respect to their models. Furthermore, both species undergo partial chemical degradation in the time scale of cyclic voltammetry upon their multiple reduction. Photophysical properties of 1 and 2, namely, excited state interactions between the ruthenium(II) or rhenium(I) complexes and [60]fullerene have been investigated by steady-state and time-resolved UV-Vis-NIR luminescence spectroscopy that was complemented by nanosecond laser flash photolysis in CH₂Cl₂ solutions. All experimental findings were set into relation with the corresponding reference compounds. More precisely, excitation of the metal complexes in 1 and 2 gives rise to a notable steady-state and time-resolved luminescence quenching of both metal to ligand charge transfer states (i.e., [Ru(bpy)₃]²⁺ and [(bpy)Re(CO)₃(py)]⁺). Conclusive evidence about the nature of the photoproducts came from nanosecond laser flash photolysis. In these experiments, only the long-lived and oxygen-sensitive [60]fullerene triplets were detected. Two pathways are envisioned for this [60]fullerene triplet formation. Firstly, intramolecular transduction of the triplet excited state energy evolving from the photoexcited metal complexes. Secondly, intersystem crossing of directly excited [60]fullerene.