Tetrathiafulvalene Hetero Radical Cation Dimerization in a Redox-Active Catenane
journal contributionposted on 20.02.2016, 05:39 authored by Cheng Wang, Scott M. Dyar, Dennis Cao, Albert C. Fahrenbach, Noah Horwitz, Michael T. Colvin, Raanan Carmieli, Charlotte L. Stern, Sanjeev K. Dey, Michael R. Wasielewski, J. Fraser Stoddart
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 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 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 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 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 catenane have led to a deeper fundamental understanding of the mechanism of radical cation dimer formation between constitutionally different TTF units.