Understanding room-temperature π-dimerisation of radical ions: intramolecular π-[TTF]₂²⁺ in functionalised...

Long, multicentre π-dimers of radical ions are weakly bound and can only be observed in solution at low temperature. However, recent supramolecular approaches induce the extra stabilisation required to preserve them at room temperature, by different means depending on the approach. In particular, π-[TTF]₂²⁺ dimers (TTF = tetrathiafulvalene) were detected upon oxidation of a TTF-based calix[4]arene in acetonitrile solution at room temperature, manifesting intramolecular [R-TTF]˙⁺[R-TTF]˙⁺ interactions (Chem. Commun. 2006, 2, 2233). In this work, the reasons behind the remarkable formation of these π-dimers in the calix[4]arene, [calix], molecule are unravelled by means of DFT calculations. We first demonstrate that the properties of the π-[R-TTF]₂²⁺ dimers are preserved in the [calix]²⁺. Most importantly, our results show that the π-dimerised and non-dimerised forms of the [calix]²⁺ are isoenergetic at room temperature, and that the activation energy for this process is ca. 9.5 kcal mol^-1. Hence, both forms coexist in equilibrium at 298 K, as the intramolecular nature of the interaction ensures a high reaction rate. The role of the Na⁺ cation in preventing the π-[R-TTF]₂²⁺ dimerisation of the [calix]²⁺ receptor is also examined, unveiling that this effect is mostly due to the electrostatic repulsion induced by the cation. Finally, we provide a revision on room-temperature stable supramolecular long, multicentre π-dimers of radical ions, a class of systems with great potential as molecular switches.