Teresa M. Figueira-Duarte

Molecular and supramolecular C60–oligophenylenevinylene conjugates

Fullerene derivatives are attractive building blocks for the preparation of molecular and supramolecular photoactive devices. As a part of this research, combination of C60 with oligophenylenevinylene (OPV) subunits has generated significant research efforts. These results are summarized in the present account to illustrate the current state-of-the-art of fullerene chemistry for the development of new photoactive materials.

Tunneling versus Hopping in Mixed-Valence Oligo-p-phenylenevinylene Polychlorinated Bis(triphenylmethyl) Radical Anions

Radical anions 1(-•)-5(-•), showing different lengths and incorporating up to five p-phenylenevinylene (PPV) bridges between two polychlorinated triphenylmethyl units, have been prepared by chemical or electrochemical reductions from the corresponding diradicals 1-5 which were prepared using Wittig-Horner-type chemistry. Such radical anions enabled us to study, by means of UV-vis-NIR and variable-temperature electron spin resonance spectroscopies, the long-range intramolecular electron transfer (IET) phenomena in their ground states, probing the influence of increasing the lengths of the bridges without the need of using an external bias to promote IET. The temperature dependence of the IET rate constants of mixed-valence species 1(-•)-5(-•) revealed the presence of two different regimes at low and high temperatures in which the mechanisms of electron tunneling via superexchange and thermally activated hopping are competing. Both mechanisms occur to different extents, depending on the sizes of the radical anions, since the lengths of the oligo-PPV bridges notably influence the tunneling efficiency and the activation energy barriers of the hopping processes, the barriers diminishing when the lengths are increased. The nature of solvents also modifies the IET rates by means of the interactions between the oligo-PPV bridges and the solvents. Finally, in the shortest compounds 1(-•) and 2(-•), the IET induced optically through the superexchange mechanism can also be observed by the exhibited intervalence bands, whose intensities decrease with the length of the PPV bridge.

Influence of bridge topology and torsion on the intramolecular electron transfer.

To study molecules able to act as good molecular wires, intramolecular electron transfer between two triphenylmethyl redox centers connected by bridges with different topologies and substituents have been studied in solution, both by UV-Vis-NIR and EPR spectroscopies. The synthetic methodology used allows a complete control of the geometry of polychlorotriphenylmethyl diradicals 1 and 2, which have para and meta topologies, respectively, as well as of their E/Z isomerism. This fact is used to show the influence of the different topologies in the ease of electron transfer, which is larger for the para than for the meta isomer where a small or negligible electronic coupling is observed. The related diradical 3 that have the same topology as the para isomer 1 but bearing two substituents on the central phenyl ring shows similar ease of electron transfer, that the para isomer 1.

Synthesis and electronic properties of fullerene derivatives substituted with oligophenylenevinylen–ferrocene conjugates

C60-bridge-Fc arrays (C60-Fc, C60-PV-Fc, C60-2PV-Fc, C60-3PV-Fc), bearing a fulleropyrrolidine and a ferrocene (Fc) unit connected via an oligophenylenevinylene (OPV) bridge have been prepared. Suitable dyads (PV-Fc, 2PV-Fc, 3PV-Fc) to be used as references for the study of the electronic properties of the largest arrays have been also synthesized. The electrochemical properties of all the dyad and triad multicomponent arrays have been studied by cyclic voltammetry, evidencing a rich and complex electrochemical pattern due to the presence of several electroactive moieties. The first reduction is always assigned to the fullerene moiety and the first oxidation is centred on the Fc group, making the triad systems suitable candidates for photoinduced electron transfer via the interposed bridge. Photophysical studies evidence a complete quenching of the fluorescence of organic conjugated moieties in PV-Fc, 2PV-Fc, 3PV-Fc, possibly via energy transfer to the Fc unit. In the more complex C60/Fc arrays the quenching of the C60 moieties is ultrafast in CH2Cl2 solution and most likely attributable to electron transfer via the OPV wire. In toluene, the dynamic process of singlet and triplet fullerene quenching can be traced via time resolved fluorescence and transient absorption spectroscopy and the values of the rate constants are smaller with increasing donor–acceptor distance. Definitive assignment of the intercomponent quenching mechanism between the fullerene and the ferrocene moiety (energy or electron transfer) can hardly be obtained. Several repeated attempts to detect the radical anion fingerprint of fulleropyrrolidine with transient absorption failed, even in bimolecular quenching experiments. This supports the view that distance-dependent C60 → Fc singlet–triplet and triplet–triplet energy transfer may compete successfully with the desired charge separation step.

Changes in electronic couplings of mixed-valence systems due to through-space intramolecular interactions

The strength of the electronic interactions between the two redox moieties in fullerene-substituted mixed-valence bis(ferrocenylethynyl)ethene derivatives is modulated by the through-space intramolecular electronic interactions of C(60) with the bridging conjugated system.