para–Phenylenediamine Dimer as a Redox–Active Guest for Supramolecular Systems

Para–phenylenediamine dimers with the biphenyl centrepieces form supramolecular complexes with multiple units of α/β/γ-cyclodextrins and/or cucurbit[7/8]urils. Several redox agents were tested for switching between fully reduced colourless and partially oxidised deep blue diradical forms. The reducing effect of some agents was selectively inhibited by complexation with cucurbit[7]uril.

Abstract

Redox–active components are highly valuable in the construction of molecular devices. We combined two p-phenylenediamines (p-PDA) with a biphenyl (BiPhe) unit to prepare a supramolecular guest 4 consisting of three binding sites for cucurbit[7/8]uril (CBn) and/or cyclodextrins (CD). Supramolecular properties of 4 were investigated using NMR, UV-vis, mass spectrometry and isothermal titration calorimetry. Our analysis revealed that 4 forms higher–order host–guest complexes, wherein a CD unit occupies the central BiPhe site, secured by two CBn units at the terminal p-PDA sites. Additionally, 1 : 1 complexes with α-CD and β-CD, a 1 : 2 complex with γ-CD and 2 : 1 complexes with CB7 and CB8 were identified. Through UV-vis and cyclic voltammetry, redox processes leading to the formation of a stable, deep blue dication diradical of 4 are elucidated. Furthermore, it is demonstrated that CB7 selectively protects oxidised 4 from reduction in the presence of a reducing agent. The supramolecular and redox properties of the structural motif represented by 4 render it an interesting candidate for the construction of supramolecular devices.

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