Published: March 29, 2011 r2011 American Chemical Society 5818 dx.doi.org/10.1021/ja1083859 | J. Am. Chem. Soc. 2011, 133, 5818–5833 ARTICLE pubs.acs.org/JACS Tunneling versus Hopping in Mixed-Valence Oligo-p-phenylenevinylene Polychlorinated Bis(triphenylmethyl) Radical Anions Vega Lloveras, † Jos  e Vidal-Gancedo, † Teresa M. Figueira-Duarte, ‡ Jean-Franc - ois Nierengarten, ‡ Juan J. Novoa, § Fernando Mota, § Nora Ventosa, † Concepci o Rovira,* ,† and Jaume Veciana* ,† † Institut de Ci  encia de Materials de Barcelona (CSIC) and Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Campus Universitari de Bellaterra, E-08193 Cerdanyola, Spain ‡ Laboratoire de Chimie des Mat  eriaux Mol  eculaires, Ecole Europ eenne de Chimie, Polym eres et Mat  eriaux, Universit  e de Strasbourg et CNRS (UMR 7509), 25 rue Becquerel, 67087 Strasbourg, Cedex 2, France § Departament de Química Física, Facultat de Química, Universitat de Barcelona, Av. Diagonal 47, 08208 Barcelona, Spain b S Supporting Information ’ INTRODUCTION Long-range electron transfer (ET) between donor (D) and acceptor (A) molecules/groups where the separation between the donor and acceptor greatly exceeds their spatial extent is a subject of considerable interest for studies of ET mechanisms in biological systems and in nanoscale circuits based on single molecular devices. 13 Electron transfer in donorbridge acceptor (D-B-A) molecules has been actively investigated, in part to determine which aspects of the bridge structure control the intramolecular electron transfer (IET). 4 Both the electronic structure of the bridge and the effective distance between the D/A centers are known to play a critical role in determining the ease of the ET. 4 However, very little is known about the influence of external factors, like the solvent media and tem- perature-induced perturbations, on the IET of nonperturbed ground states of such systems. In most of the small D-B-A systems, the ET rate scales exponentially with the length of the bridge between the D/A moieties. The distance dependence of ET rate constants, k ET , is often described by k ET = k 0 exp(βd), where k 0 is a kinetic pre-exponential factor, d is the DA center-to-center distance, and β is the attenuation factor that depends primarily on the nature of the molecular bridge. Theory predicts that another regime may also exist wherein the distance dependence is very weak and in which the molecular bridge essentially acts as an incoherent molecular wire, yielding a reciprocal scaling of k ET with the distance. 411 Therefore, beyond the range of direct DA electronic overlap, IET may occur either through a bridge- mediated superexchange — also known as direct (nonresonant) tunneling — between D/A electronic states or through an inco- herent hopping. The relative contributions of these mechanisms depend in part on the electronic nature of the bridge as well Received: September 16, 2010 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) phenom- ena 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.