title: Tunneling versus hopping in mixed-valence oligo-p-phenylenevinylene polychlorinated bis(triphenylmethyl) radical anions.
abstract: 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.
authors: Institut de Ciència de Materials de Barcelona (CSIC) and Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Campus Universitari de Bellaterra, E-08193 Cerdanyola, Spain.
authors: Vega
authors: V
authors: Lloveras
authors: José
authors: J
authors: Vidal-Gancedo
authors: Teresa M
authors: TM
authors: Figueira-Duarte
authors: Jean-François
authors: JF
authors: Nierengarten
authors: Juan J
authors: JJ
authors: Novoa
authors: Fernando
authors: F
authors: Mota
authors: Nora
authors: N
authors: Ventosa
authors: Concepció
authors: C
authors: Rovira
authors: Jaume
authors: J
authors: Veciana
mesh_terms: D000838:Anions / Q000737:chemistry; D004563:Electrochemistry; D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry; D054879:Halogenation; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011145:Polyvinyls / Q000737:chemistry*
chemical_list: D000838:Anions; D005609:Free Radicals; D011145:Polyvinyls; C530822:oligo(p-phenylene vinylene)