Thermal and redox properties of Donor–Acceptor foldamers consisting of tetrathiafulvalene and viologen

Abstract

Donor–acceptor (DA)-type foldamers consisting of tetrathiafulvalene (TTF) and viologen (V2+) units were synthesized. In those foldamers, tri(ethylene glycol)-substituted DA units were alternatively connected via the folding point of catechol. The synthesis of TV2+-2mer, V2+TV2+-3mer, and V2+TV2+TV2+-5mer were confirmed by 1H and 13C NMR and high-resolution ESI-MS spectra. Thermal and electrochemical properties were characterized through temperature-dependent UV–Vis spectroscopy and cyclic voltammetry, respectively. Despite the weak molecular interaction between the tri(ethylene glycol)-substituted TTF and V2+ molecules, the foldamers formed the charge-transfer (CT) complex because of the tethered structure of the DA units on the catechol unit. The melting curve analysis of the polymer folding indicated that the degree of polymer folding was <50% at room temperature due to weak CT interactions. It was confirmed that the thermal stability of the folded state increased with the number of DA units. The oligomers were chemically oxidized, and the changes in their UV–Vis spectra were monitored. For V2+T•+V2+T•+V2+-5mer, intramolecular TTF radical dimer was formed. In the cyclic voltammogram, the redox reactions of the TTF and V2+ units were observed at the positive and negative potentials, respectively. When the V2+ unit was fully reduced to the neutral state, the oligomers became insoluble in acetonitrile. The stepwise conformation change of the V2+TV2+TV2+-5mer (folded, hooped, and extended states) in response to the oxidation states of the TTF unit (neutral, radical cation, and di-cation) was discussed with the help of computational modeling (ONIOM(M06:UFF) method).