Synthesis and charge transfer interactions of cyclobis(4,4′-azopyridinium-p-phenylene)

A new tetracationic cyclophane, cyclobis(4,4′-azopyridinium-p-phenylene) (2·4PF₆), was synthesized via template-directed synthesis and characterized by MS and NMR spectroscopy. For the preparation of π-electron-deficient 2·4PF₆, the use of π-electron-rich templates such as 1,4-bis[2-(2-hydroxyethoxy)ethoxy]benzene (4) and 1,5-bis[2-(2-hydroxyethoxy)ethoxy]naphthalene (5) enhanced the cyclophane formation in 17 and 19% yields, respectively. ¹H NMR spectra of the reaction mixture revealed that the cyclophane precursor 1·2PF₆ formed supramolecular assembling complexes with hydroquinone derivatives 4 and 5 before the ring formation. The intermolecular charge transfer interaction and the stability constants (K_a) of complexes generated between the electron-acceptor 2·4PF₆, and electron-donors 1,4-dimethoxybenzene 3, 4, and 5 were evaluated using UV-vis, spectroscopy and spectroscopic titration. The absorption bands attributable to CT interactions were centred at 580 nm for 3, 635 nm for 4, and 646 nm for 5, with stability constants of 17±2, 2100±200, and 2400±300, respectively.