Pairing-up viologen cations and dications: a microscopic investigation of van der Waals interactions.

The microscopic origin of van der Waals- and magnetic-interactions in 4,4' methyl viologen cation-based units (MV+˙ and MV2+) was inspected using wave function (variational DDCI and perturbative MP2, CASPT2) and density functional theory (DFT) calculations. The analysis deepens the comprehension of the magnetic behavior of experimental bis-viologen cyclophanes ([CYC]2(+˙)), in which the MV+˙ units are connected through alkyl linkers of different lengths. The formation of the so-called long-multicenter bonds in such radical dimers, responsible for the quenching of the magnetic response, was analyzed in [MV2]2(+˙). Dynamical correlation effects, accessible from second-order perturbation corrections, were decisive in observing a bonding regime characterized by an equilibrium distance of 3.3 Šand a 45 kJ mol-1 dissociation energy. At larger intermolecular distances, our calculations on [MV2]2(+˙) indicate that the singlet and triplet states are energetically competing (i.e. weak exchange interactions, JAB). Despite the absence of any clear bonding regime at the MP2 level, the puzzling association of two di-cations into [MV2]4+ is anticipated at 3.3 Šusing weakly screened point charges (ε = 1.5) to account for the Coulomb interactions between the solvated subunits. The main conclusion is that both dispersion interactions and environment effects are required to overcome the Coulomb repulsion associated with doubly-charged species. All these data provide some complementary insights into the nature and amplitude of interactions between cation and dication units, and their relevance in various experimental manifestations.

Flexible Viologen Cyclophanes: Odd/Even Effects on Intramolecular Interactions.

The ability of three bis-viologen cyclophanes to act as redox-triggered contractile switches is investigated. Odd/even effects in the formation of cyclic bis-viologens are circumvented by the use of a Zincke salt intermediate and a tetrathiafulvalene template to prepare a flexible cyclophane with hexyl linkers. Comparative spectro-electrochemical studies of this macrocycle with two other pentyl- or heptyl-linked cyclic bis-viologens show that the development of intramolecular interactions in aqueous solution depends on the length of the bridges. This dependence is confirmed by EPR and DFT studies of the magnetic coupling in the diradical dication species. The anti-ferromagnetic or ferromagnetic nature of the coupling depend, respectively, on the odd or even number of methylene groups in the spacer.

Self-assembled viologens on HOPG: solid-state NMR and AFM unravel the location of the anions.

The controlled growth of self-assembled networks on surfaces based on viologen salts is a major scientific challenge due to their unique electronic properties. The combination of solid-state NMR spectroscopy and atomic force microscopy at ambient conditions can unravel the fine organization of the supramolecular network on a graphitic surface by positioning the counter-ions relative to the viologen cation.

A Highly Stable Organic Radical Cation.

Functionalization of a methylviologen with four methyl ester substituents significantly facilitates the first two reduction steps. The easily generated radical cation shows markedly improved air stability compared to the parent methylviologen, making this derivative of interest in organic electronic applications.

Viologen cyclophanes: redox controlled host-guest interactions.

Viologens can exist in three redox states varying from dicationic to neutral. This work emphasizes the control of the host-guest properties in bis-viologen cyclophanes. Two flexible cyclophanes were prepared by a cyclisation method sensitive to the odd/even number of carbons in the flexible chains linking two viologens. C5 and C7 cyclophanes were characterised by X-ray diffraction in their tetracationic state and their diradical dicationic state. In the presence of tetrathiafulvalene or methyl viologen as guests, inclusion complexes were obtained, including a mixed valence species.