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-p-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.