Tetrathiafulvalene Hetero Radical Cation Dimerization in a Redox-Active [2]Catenane

The electronic properties of tetrathiafulvalene (TTF) can be tuned by attaching electron-donating or electron-withdrawing substituents. An electron-rich macrocyclic polyether containing two TTF units of different constitutions, namely 4,4′-bis­(hydroxymethyl)­tetrathiafulvalene (OTTFO) and 4,4′-bisthiotetrathiafulvalene (STTFS), has been synthesized. On two-electron oxidation, a hetero radical dimer is formed between OTTFO^•+ and STTFS^•+. The redox behavior of the macrocyclic polyether has been investigated by electrochemical techniques and UV–vis and electron paramagnetic resonance (EPR) spectroscopies. The [2]­catenane in which the macrocyclic polyether is mechanically interlocked with the cyclobis­(paraquat-<i>p</i>-phenylene) (CBPQT⁴⁺) ring has also been prepared using template-directed protocols. In the case of the [2]­catenane, the formation of the TTF hetero radical dimer is prevented sterically by the CBPQT⁴⁺ ring. After a one-electron oxidation, a 70:30 ratio of OTTFO^•+ to STTFS^•+ is present at equilibrium, and, as a result, two translational isomers of the [2]­catenane associated with these electronically different isomeric states transpire. EPR titration spectroscopy and simulations reveal that the radical states of the two constitutionally different TTF units in the [2]­catenane still experience long-range electronic intramolecular coupling interactions, despite the presence of the CBPQT⁴⁺ ring, when one or both of them are oxidized to the radical cationic state. These findings in the case of both the free macrocyclic polyether and the [2]­catenane have led to a deeper fundamental understanding of the mechanism of radical cation dimer formation between constitutionally different TTF units