ffi cient access to conjugated 4 , 4 ’ - bipyridinium oligomers using the Zincke reaction : synthesis , spectroscopic and electrochemical properties †

Bipyridinium residues are amongst the most intensively studied building blocks in supramolecular chemistry. They have found widespread application in molecular systems that exhibit controlled switching, rotational motion and potential for data storage. Many of these complex nanosystems harness the controllable electrochemical properties of 4,4′-bipyridinium ions (“viologens”) in either their synthesis or application. The reversible oneand two-electron reductions of 4,4′-bipyridinium derivatives (see ESI – Scheme 1†) are accompanied by a dramatic change in the UV-vis absorption spectra of the materials, an effect which has been widely studied in the context of electrochromic devices. The colour of cationic bipyridinium residues is highly dependent on the electronic ground state energy of the molecule, which can be finely tuned by varying the substituents. There have been a number of studies concerning the synthesis of oligomers and polymeric materials containing 4,4′bipyridine units in the backbone. Typically, the electroactive 4,4′-bipyridinium groups are separated by at least one methylene residue. This structural arrangement precludes electronic communication between the bipyridinium groups. In addition, there have been several reports of the synthesis of socalled “extended viologens” whereby two pyridinium residues are separated by increasing numbers of aromatic rings. These have rapidly found widespread application in the construction of supramolecular complexes. There are also examples of (di)phenyl viologen structures in which the conjugation length has been extended by the addition of an aromatic residue to the N-termini of the viologen. Small molecules containing 4,4′or 3,3′-bipyridine units, and polymers with 3,3′-bipyridine residues have been shown to exhibit interesting conductivity properties in the solid state. Our own work in this area has previously concentrated on the synthesis of 4,4′-bipyridinium-containing macrocycles such as 4 (Scheme 1). We synthesized a family of related macrocycles by exploiting the Zincke reaction, in which the condensation of nucleophilic amines with N-(2,4-dinitrophenyl)pyridinium salts resulted in efficient access to N-substituted pyridinium residues. These macrocycles underwent one-electron reduction both chemically [e.g., by triethylamine (TEA)] and electrochemically, to yield the corresponding radical cations such as 5. The unpaired spin density was found to be delocalized over four aromatic/heterocyclic rings,